bluuebunny/crossref_metadata_2025 huggingface dataset:
Activation-dependent cumulative depletions of extracellular free calcium in guinea pig atrium measured with antipyrylazo III and tetramethylmurexide.
Authors: Hilgemann D W , Delay M J , Langer G A
Published: June 2012

We have used a spectrophotometric method to monitor mean free extracellular calcium concentrations in isolated left atria of guinea pigs via extracellular application of the calcium-sensitive absorption dyes, antipyrylazo III and tetramethylmurexide. Exchange of extracellular free calcium with the bathing medium takes several minutes and closely parallels contractile response in this preparation. Under conditions favoring a rapid positive force staircase response during repetitive stimulation after a long rest period (2-10 minutes), cumulative depletions of extracellular calcium can clearly be differentiated from motion artifact due to muscle movement by multiple-wavelength spectrophotometry. Responses of similar magnitude and characteristics are obtained with both dyes employed. In the presence of 10(-7) M isoproterenol, the mean extracellular calcium concentration falls by at least 5% (0.25-0.8 mM total calcium concentration) in four beats at 0.5 Hz; extracellular calcium replenishes during rest with an apparent t1/2 of 25-60 seconds. A 10-minute pretreatment with 10(-8) M ryanodine greatly reduces the contraction force and motion artifact of the first beat after a rest period, whereby the magnitude of depletion response to one post-rest stimulation is increased 2- to 3-fold. With further ryanodine treatment, the magnitude of depletion responses remains stable, and the rate of calcium replenishment during rest increases many-fold. After ryanodine treatment and 10(-7) M isoproterenol, at least 10% of total dye accessible calcium (0.25-1.0 mM) is lost during two to five rapid stimulations, and returns to the extracellular space within 20 seconds of rest. Cumulative extracellular calcium depletion responses are strongly suppressed by 10(-6) M nifedipine. Cumulative depletion responses are also inhibited by 10 mM caffeine, whereby contraction and corresponding motion artifacts are increased at post-rest stimulation.
Depression of contractility following stretches and releases applied during contraction to single frog atrial cardiac cells.
Authors: Tarr M , Trank J W , Goertz K K
Published: June 2012

The effects of stretches and releases on the contractile performance of isolated single frog atrial cells (Rana catesbeiana) were investigated. A stretch or release was imposed on the cell--either during a contraction (test) or before the onset of contraction (control)--and the contractile performance (length, velocity and force) of the test contraction was compared with that of the control contraction to determine whether the stretch or release imposed on the contracting cell altered the contractility of the cell. We found that the velocity of cell (and sarcomere) shortening for the remainder of the test contraction following either a stretch or release was markedly less than that occurring at the same time in the control contraction. This decreased velocity occurred even though the force in the test contraction was less than that in the control contraction and the sarcomere length was longer in the test contraction than in the control contraction. These results indicate that after a stretch or release imposed on the contracting cell, the force-velocity relationship at any given length and time is depressed than had the stretch or release not been imposed on the contracting cell. Thus, stretches and releases applied to the contracting single cardiac cell either produce a long-term depression in the contractility of the cell, or that the contractility at any given time and sarcomere length depends markedly on the history of the contraction.
Myosin phosphorylation and cyclic adenosine 3',5'-monophosphate in relaxation of arterial smooth muscle by vasodilators.
Authors: Gerthoffer W T , Trevethick M A , Murphy R A
Published: June 2012

Recent evidence indicates that contraction of vascular smooth muscle may be regulated by two calcium-dependent mechanisms: activation of myosin kinase, and calcium binding to a second, unknown regulatory site. This hypothesis implies that vasodilators could modify vascular tone by several mechanisms, including inactivation of myosin kinase. Since relaxation of the carotid artery following agonist removal may occur when myosin phosphorylation is at resting levels, we could determine whether dephosphorylation of myosin is necessarily involved in the molecular mechanisms mediating relaxation in response to vasodilators. The relaxant effects of adenosine, 3-isobutyl-1-methylxanthine, forskolin, sodium nitroprusside, and 8-bromo-cGMP were tested under conditions where myosin phosphorylation was at basal levels (0.08 +/- 0.02 mol Pi/mol light chain). All of these agents increased the rate of relaxation in nonsteady state experiments where relaxation was induced by stimulus washout. Steady state dose-response curves were obtained for forskolin and 8-bromo-cGMP in the presence of basal myosin phosphorylation. Forskolin caused a dose-dependent increase in cAMP levels at a rate consistent with a cause and effect relationship between relaxation and total tissue cAMP content. Both drugs relaxed the muscles, with no detectable change in myosin phosphorylation. Therefore, dephosphorylation of myosin is not a necessary event in the molecular mechanism of several vasodilators, including some which presumably act via cyclic nucleotides.
Responses of renal hemodynamics and function to acute volume expansion in the conscious dog.
Authors: Sit S P , Morita H , Vatner S F
Published: June 2012

The renal vascular and functional responses to acute volume expansion were determined in conscious dogs with all reflexes intact, sinoaortic arterial baroreceptor denervation, arterial baroreceptor denervation plus bilateral stellectomy, and arterial baroreceptor denervation plus bilateral vagotomy. In the intact dogs, when an isotonic saline infusion increased right atrial pressure by 6 mm Hg, arterial pressure increased by 15 +/- 3 from 95 +/- 3 mm Hg (P less than 0.01) and heart rate rose from 87 +/- 4 to 135 +/- 6 beats/min (P less than 0.01), while renal blood flow rose only slightly (4 +/- 2%), and calculated renal resistance was not altered significantly. After arterial baroreceptor denervation, volume expansion with saline induced a greater rise in blood flow (16 +/- 2%), but did not alter arterial pressure, and calculated resistance fell by 19 +/- 3% from 0.72 +/- 0.05 mm Hg/ml per min (P less than 0.01), while heart rate still increased. After arterial baroreceptor denervation and either bilateral stellectomy or vagotomy, volume expansion reduced renal vascular resistance by similar amounts. When intact animals were volume expanded by 20% of estimated blood volume with isooncotic, isotonic 3% dextran in saline solution, and with renal perfusion pressure held constant, central venous pressure increased by 4.5 +/- 0.6 from 1.8 +/- 0.4 mm Hg (P less than 0.01), renal blood flow increased significantly by 16 +/- 5 ml/min (P less than 0.05) from 191 +/- 30 ml/min, while calculated renal vascular resistance decreased significantly by 0.08 +/- 0.02 from 0.62 +/- 0.09 mm Hg/ml per min (P less than 0.05). Average urine flow rate and sodium excretion 10-60 minutes after expansion increased markedly by 1.85 +/- 0.27 ml/min and 9.84 +/- 1.13 muEq/min per kg, respectively (P less than 0.01). After arterial baroreceptor denervation, volume expansion induced a similar rise in central venous pressure and renal blood flow. The diuretic and natriuretic responses were not attenuated by arterial baroreceptor denervation. After arterial baroreceptor denervation plus bilateral vagotomy, there was a significant and similar rise in renal blood flow, whereas diuretic (urine flow rate rose by only 0.50 +/- 0.10 from 0.35 +/- 0.08 ml/min) and natriuretic (sodium excretion rose by only 4.83 +/- 0.95 from 1.50 +/- 0.48 muEq/min per kg) responses were significantly attenuated (P less than 0.01).(ABSTRACT TRUNCATED AT 400 WORDS)
Electron probe analysis of sodium and other elements in hypertrophied and sodium-loaded smooth muscle.
Authors: Junker J L , Wasserman A J , Berner P F , Somlyo A P
Published: June 2012

The composition of normal, hypertrophied, or sodium-loaded rabbit portal anterior mesenteric vein and of normal and sodium-loaded guinea pig taenia coli smooth muscle was measured in cryosections with electron probe analysis, and the effects of wash with cold sodium-free (Lithium) solutions were determined. There was no significant difference in the cytoplasmic, nuclear, or mitochondrial concentrations of any of the measured elements (sodium, potassium, chloride, magnesium, calcium, phosphorus, sulfur) between hypertrophied, sham-operated, or control veins. The cytoplasmic potassium:sodium:chloride ratio in rabbit portal anterior mesenteric vein was 1:0.26:0.46, and the average sodium concentration (198 mmol/kg dry cytoplasmic weight) was nearly twice as high as estimated from ion flux measurements. The cytoplasmic sodium concentration of normal guinea pig taenia coli was 61 mmol/kg dry weight. The existence of a rapidly exchanging, relatively low affinity, and temperature-insensitive component of cytoplasmic sodium efflux was indicated by the reduction in cytoplasmic sodium after washout in cold, sodium-free (lithium or Tris-substituted) solutions. This reduction, by 62% in normal, 71% in sodium-loaded portal anterior mesenteric vein, and 36% in sodium-loaded guinea pig taenia coli smooth muscle, suggests that the lithium wash method can underestimate cell sodium. In sodium-loaded guinea pig taenia coli and portal anterior mesenteric vein smooth muscles, the cytoplasmic sodium analyzed in individual cells showed a bimodal distribution; in cryosections, the cells having the highest sodium and lowest potassium and phosphorus content also had a more electronlucent (light cell) appearance.
Canine myocardial reperfusion injury. Its reduction by the combined administration of superoxide dismutase and catalase.
Authors: Jolly S R , Kane W J , Bailie M B , Abrams G D , Lucchesi B R
Published: June 2012

Therapy directed against the toxic effects of reactive oxygen species may reduce the final extent of ischemic injury in otherwise viable tissue irreversibly injured by the abrupt reoxygenation of reperfusion. In four groups of dogs, superoxide dismutase plus catalase (groups I-III) or saline (controls) (group IV) was infused into the left atrium. Group I received the infusion for 2 hours, beginning 15 minutes before occlusion of the left circumflex coronary artery (90 minutes) and ending 15 minutes after reperfusion. Group II received the infusion for 1 hour starting 15 minutes before reperfusion. Group III received the infusion for 1 hour beginning 40 minutes after reperfusion. Dogs were killed the next day, and infarct size was determined by dissection and weighing, and confirmed histologically. Infarct size expressed as percent of the anatomic area at risk was: group I, 19.4 +/- 5.0; group II, 21.8 +/- 3.3; group III, 47.6 +/- 10.3; group IV, 43.6 +/- 3.5 (mean +/- SEM). Analysis of variance followed by Duncan's multiple range test showed that ultimate infarct size as assessed in groups I and II differed significantly (P less than 0.05) from that observed in the control animals in group IV, whereas infarct size between groups III and IV did not differ significantly (P greater than 0.05). The percent of left ventricle at risk did not differ between the four groups. The beneficial effects of superoxide dismutase plus catalase could not be explained by hemodynamic differences. Similar protection of jeopardized myocardium in groups I and II suggest that potentially viable tissue is salvaged by scavenging free radicals during early reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)
Transsarcolemmal calcium movements in arterially perfused rabbit right ventricle measured with extracellular calcium-sensitive dyes.
Authors: Hilgemann D W , Langer G A
Published: June 2012

Activation-dependent changes of mean extracellular calcium concentrations were monitored spectrophotometrically in arterially perfused right ventricle of rabbit via extracellular application of the calcium-sensitive absorption dyes, antipyrylazo III and tetramethylmurexide. After rest periods of 3 minutes or longer, 10% of total dye-accessible calcium (300-500 microM) is depleted cumulatively from the extracellular space by four to eight beats at 2 Hz stimulation. During continued stimulation, or during quiescence following a depletion response, mean extracellular calcium concentrations return toward the prestimulatory level in the course of 2-4 minutes. A single stimulation placed 2-30 seconds subsequent to rapid pacing results in a potentiated beat with an accompanying net increment of mean extracellular calcium. Total dye-accessible calcium can increase by at least 4% at such a beat, and depletion responses can be repeated immediately thereafter. During a sequence of such responses, extracellular calcium lost cumulatively during three to five rapid beats is replenished, for the most part, by a single post-stimulatory beat. These results demonstrate that most of the "activator pool" of cellular calcium can turn over to the extracellular space during a single contraction cycle in rabbit myocardium, and suggest that, in mammalian myocardium, diastolic calcium efflux may be quantitatively negligible in relation to efflux during excitation-contraction coupling.
Purkinje and ventricular activation sequences of canine papillary muscle. Effects of quinidine and calcium on the Purkinje-ventricular conduction delay.
Authors: Veenstra R D , Joyner R W , Rawling D A
Published: June 2012

We have studied in vitro preparations of canine right and left papillary muscles to determine the excitation sequences of the Purkinje and ventricular cells, using both monopolar surface electrodes and intracellular microelectrodes. Our results show, for right papillary muscles, that the Purkinje layer covers the basal part of the muscle, and that activation from the right bundle branch propagates over all of the Purkinje layer, but directly activates the underlying ventricular layer only at specific junctional sites. Left papillary muscles have attachments to both apical and basal Purkinje strands and the Purkinje layer covers the entire muscle, but, as for right papillary muscles, activation from the Purkinje layer to the ventricular layer occurs only at basal junctional sites. Antidromic conduction in papillary muscles (propagation from the ventricular layer to the Purkinje layer) can occur at regions other than the specific sites through which the Purkinje layer activates the ventricular layer. At the identified junctional sites, the Purkinje cell action potential duration is significantly shorter than in the free-running strand, but it remains longer than that of the ventricular cells. The time delay at the junctional sites is increased by quinidine, increased calcium concentration, and increased pacing frequency.
Histamine H1 receptor antagonists inhibit autoregulation of renal blood flow in the dog.
Authors: Banks R O , Inscho E W , Jacobson E D
Published: June 2012

Histamine H1 and H2 receptors are present in the canine renal circulation. We have examined the effects of H1 and H2 receptor antagonists on autoregulation of renal blood flow in the dog. Renal arterial pressure was reduced in a step-wise fashion to 80 mm Hg by means of an adjustable aortic clamp positioned above the left renal artery. Infusion of H2 antagonists, cimetidine or ranitidine, into the left renal artery at 10(-5) mol/min had no effect on autoregulation of renal blood flow or on the reactive hyperemia that occurred when the aortic constriction was released. By contrast, intrarenal infusion of 10(-5) mol/min chlorpheniramine, an H1 receptor antagonist, reversibly attenuated reactive hyperemia and the ability of the kidney to autoregulate renal blood flow. Similar results were obtained with other, chemically dissimilar H1 antagonists (terfenadine, diphenhydramine, and pyrilamine). The effects of chlorpheniramine on autoregulation of glomerular filtration rate also were evaluated. Before chlorpheniramine was infused (at 10(-5) mol/min), the reduction of renal arterial pressure to 90 mm Hg had no effect on the glomerular filtration rate, whereas, during infusion of the H1 antagonist, the glomerular filtration rate fell significantly when renal arterial pressure was reduced to 90 mm Hg. Infusion of histamine (1 microgram/kg per min) with increasing amounts of cimetidine, chlorpheniramine, diphenhydramine, or pyrilamine resulted in virtually identical dose-dependent decreases in histamine-induced renal vasodilation. However, even with 10(-5) mol/min cimetidine or 10(-5) mol/min chlorpheniramine, diphenhydramine, or pyrilamine, a significant histamine-induced renal vasodilation was observed.(ABSTRACT TRUNCATED AT 250 WORDS)
The effects of membrane potential, extracellular potassium, and tetrodotoxin on the intracellular sodium ion activity of sheep cardiac muscle.
Authors: January C T , Fozzard H A
Published: June 2012

The intracellular sodium ion activity was measured using liquid ion-exchange microelectrodes with rapid response times in sheep Purkinje fibers and ventricular muscle under voltage control. The mean sodium ion activity in quiescent Purkinje fibers was 8.5 mM at a holding potential of -80 mV. With maintained hyperpolarizing (-110 mV) or depolarizing (-40 and 0 mV) voltage steps, sodium ion activity increased or decreased, respectively. At 0 mV, the mean steady state value for the sodium ion activity was 3.8 mM. Following a voltage step to 0 mV, or back to -80 mV, the time course of the sodium ion activity change could be fitted by single exponentials, with similar half-times. Increasing the extracellular potassium ion concentration from 5.4 to 15 mM did not alter the steady state value of the sodium ion activity at clamped voltages of -80 or 0 mV, which suggests that the external potassium ion activating site of the Na-K pump was saturated. With the extracellular potassium concentration 0 mM (holding potential -80 mV), the sodium ion activity increased. When maintained depolarizing steps to 0 mV were applied, the sodium ion activity decreased by up to 20 mM. This large fall in sodium ion activity is assumed to represent partial reactivation of the Na-K pump due to potassium ion accumulation in clefts. We also studied the stimulation-dependent change in sodium ion activity. Trains of action potentials or short duration depolarizing voltage clamp steps caused a frequency dependent rise in sodium ion activity. The magnitude of the rise of sodium ion activity was not altered by lengthening the duration of each voltage clamp step, but was inhibited by tetrodotoxin or by holding the membrane potential at -50 mV between depolarizing steps. These results show that sodium ion activity is a complex function of membrane voltage, depolarization frequency, and time. The rise in sodium ion activity with stimulation appears to depend on sodium ion entry regulated by the sodium channel, and may be important in the modulation of intracellular calcium and tension through the Na+-Ca++ exchange mechanism.
Carotid sinus baroreceptor reflex control and the role of autoregulation in the systemic and pulmonary arterial pressure-flow relationships of the dog.
Authors: Shoukas A A , Brunner M J , Frankle A E , Greene A S , Kallman C H
Published: June 2012

To understand more fully the role of the arterial baroreceptor reflex on systemic and pulmonary vascular resistance, we studied the influence of the carotid sinus baroreceptor reflex control system on the entire systemic and pulmonary arterial pressure-flow relationships. Ten pentobarbital-anesthetized dogs, whose carotid sinuses were isolated, were used in a preparation in which the right and left hearts were bypassed to control systemic and pulmonary blood flows. At intrasinus pressures of 50, 125, and 200 mm Hg, systemic and pulmonary arterial pressures were measured in response to step changes in systemic and pulmonary blood flows from 0 up to 200 ml/min per kg. The systemic arterial pressure-flow relationship exhibited a marked nonlinearity, especially at either high or low flows. A third-order polynomial fit was found to represent the steady state systemic arterial pressure-flow relationship best. Blood flow autoregulation was reflected as a secondary change in systemic arterial pressure at constant flow approximately 15 seconds after the initial response. Blood flow autoregulation was seen in the entire systemic vascular bed over the entire range of flows studied. The degree of autoregulatory significantly contributed to the shape of the systemic arterial pressure-flow relationship. The steady state arterial pressure-flow relationship shifted upward and toward the pressure axis, increasing the calculated incremental resistance and total peripheral resistance as intrasinus pressure was decreased. The systemic zero-flow arterial pressure was found to increase with decreases in intrasinus pressure. The pulmonary arterial pressure-flow relationship was found to be linear in the range of flows studied from 25 up to 200 ml/min per kg.(ABSTRACT TRUNCATED AT 250 WORDS)
Role of prejunctional alpha 2-adrenergic receptors in the regulation of myocardial performance during exercise in conscious dogs.
Authors: Heyndrickx G R , Vilaine J P , Moerman E J , Leusen I
Published: June 2012

We examined, in conscious dogs, the potential role of prejunctional alpha 2-adrenergic receptors for the regulation of heart rate and contractility response to exercise through modulation of the neurotransmitter release. Changes in heart rate and left ventricular pressure with time during comparable exercise levels, together with changes in norepinephrine concentration in the coronary sinus, were compared before and after the intravenous administration of: prazosin (0.5 mg/kg), a preferential postjunctional alpha 1-adrenergic receptor blocking agent; phentolamine (1 mg/kg), a nonselective alpha-adrenergic blocking agent; and yohimbine (0.3 mg/kg), a preferential prejunctional alpha 2-adrenergic receptor blocking agent. During exercise after phentolamine or yohimbine, changes in heart rate and left ventricular dP/dt were markedly potentiated compared to the control exercise, as well as to exercise after prazosin, whereas the norepinephrine concentration in the coronary sinus was substantially elevated. After intracoronary administration of phentolamine (0.1 mg/kg) or yohimbine (0.03 mg/kg), heart rate and contractility response to exercise were also potentiated, compared to the control exercise. These observations indicate that, in the intact conscious animal, prejunctional alpha 2-adrenoreceptors are stimulated during exercise, thereby modulating the norepinephrine release through a negative feedback inhibitory mechanism. Blocking these receptors by phentolamine or yohimbine results in an uncontrolled norepinephrine release during exercise associated with an augmented beta-adrenergic receptor-mediated end organ response, i.e., a potentiation in heart rate and contractility response.
Adaptation of canine saphenous veins to grafting. Correlation of contractility and contractile protein content.
Authors: Seidel C L , Lewis R M , Bowers R , Bukoski R D , Kim H S , Allen J C , Hartley C
Published: June 2012

Saphenous veins are used extensively to replace stenotic coronary arteries. However, the contractile and biochemical adaptations of grafted veins are unknown. The three purposes of this work were to characterize the contractile properties of grafted veins, to determine whether altered contractile characteristics were associated with quantitative changes in actin, myosin and collagen, and to determine which changes were associated with the surgical procedure and which with placement in the arterial circulation. Canine saphenous veins were removed and returned to their original location (venous autograft), while others were used to replace a segment of femoral artery (arterial graft). The grafts were removed 1, 4, and 8 weeks later and compared with the contralateral saphenous vein. Both graft types exhibited an increase in sensitivity to norepinephrine but not to potassium chloride. The venous autograft exhibited a reversible reduction in myosin content and in maximum contractile response (force/cross-sectional area) to potassium chloride and norepinephrine. In contrast, the arterial graft exhibited increased wall thickness and content of all measured proteins and decreased maximum contractile response. The latter occurred even though there was an increase in the net production of actin and myosin. Expressing the maximum contractile response in terms of the myosin content did not normalize the contractile response. These results suggest that, except for the elevated sensitivity to norepinephrine, the vein is capable of recovering from the effects of surgery within 8 weeks; however, placement of the vein in the arterial circulation delays this recovery and initiates a hypertrophic response that includes an attenuation of contractile function.
A direct excitatory action of catecholamines on rat aortic baroreceptors in vitro.
Authors: Goldman W F , Saum W R
Published: June 2012

A persistent question with regard to the effects of catecholamines on baroreceptor activity has been whether the adrenergic action is indirectly mediated through an alteration in vessel wall tone or whether it is a direct action on the excitability of baroreceptor nerve endings. The effects of norepinephrine on the pressure-impulse frequency relationship of rat single aortic baroreceptor fibers were studied in vitro. Perfusion of the aortic arch with norepinephrine produced an excitatory response which consisted of: a drug-evoked stimulation of previously quiescent fibers at 0 mm Hg, and a sensitization of the response to pressure. The norepinephrine-induced facilitation was blocked by prazosin but not by yohimbine. Phenylephrine mimicked the effects of norepinephrine, whereas clonidine had no excitatory effects on baroreceptor activity. Baroreceptor activity was unaffected by beta-adrenergic agonists or antagonists. Norepinephrine had no effect on the pressure-volume relationship of the aortic arch, but produced a contraction of helical aortic strips. However, there was no correlation between the concentration-response relationship of norepinephrine on aortic smooth muscle and its excitatory action on baroreceptors. 5-Hydroxytryptamine and angiotensin II produced a contraction of helical strips, but had no effect on baroreceptor activity. Sodium nitrate reduced the norepinephrine-induced contractile response by 75%, but did not antagonize the baroreceptor facilitation. It is proposed that the excitatory action of norepinephrine is independent of smooth muscle activity, and is mediated by activation of alpha 1-adrenergic receptors located on the aortic baroreceptor nerve endings.
Origin of the background sodium current and effects of sodium removal in cultured embryonic cardiac cells.
Authors: Mead R H , Clusin W T
Published: June 2012

Cardiac automaticity is partly due to a diastolic sodium current. Possible mediators of this include tetrodotoxin-sensitive "fast" channels, cesium-sensitive time-dependent pacemaker current channels, calcium-gated nonspecific channels, and electrogenic sodium-calcium exchange. We have studied the effects of abrupt sodium removal on membrane current and conductance in voltage-clamped chick embryonic myocardial cell aggregates, in the presence of various sodium flux inhibitors. Total replacement of sodium by lithium, Tris, or tetraethylammonium ions in aggregates clamped in the pacemaker range caused a brief outward current followed by a sustained net inward current. The outward current reached a peak value of 1.1 +/- 0.5 microA/cm2 at a mean latency of 5.4 +/- 1.2 sec. (n = 6; V = -70.5 +/- 8.9 mV; Tris). Conductance often decreased during the outward current. The inward current developed exponentially (t = 19 +/- 5 sec) and reached a steady state value of -1.6 +/- 0.4 microA/cm2. This current was reversed by depolarization (mean reversal potential = -13 +/- 13 mV), and was accompanied by increased conductance and spontaneous mechanical activity. Neither of the sodium-removal currents was affected by 20 microM tetrodotoxin. Cesium (up to 20 mM) had no effect on the late inward current or the mechanical activity, but decreased the early outward current by 80 +/- 12%. Manganese (25 mM), which blocks sodium-calcium exchange, abolished the late inward current and the mechanical activity. Manganese also reduced the early outward current by 27 +/- 10%. Manganese and cesium together blocked all the effects of sodium removal. We conclude that removal of extracellular sodium interrupts a cesium-sensitive "background" current, that may be related to the time-dependent pacemaker current, If. Sodium removal also causes gradual activation of a nonspecific conductance, which can ultimately depolarize the cells, and which may be gated by cytoplasmic calcium.
Left ventricular diastolic pressure-segment length relations and end-diastolic distensibility in dogs with coronary stenoses. An angina physiology model.
Authors: Momomura S , Bradley A B , Grossman W
Published: June 2012

Isovolumic relaxation abnormalities have been noted in the ischemic left ventricle, but altered end-diastolic distensibility, as well as the role of right ventricular distention, is debated. Accordingly, left ventricular end-diastolic pressure and myocardial segment length were studied in the open-chest dogs with critical (90% diameter reduction) stenoses on both left anterior descending and circumflex coronary arteries. Regional segment length was measured with ultrasonic crystals placed subendocardially, and ischemia was induced by pacing tachycardia for 3 minutes. Transient vena caval occlusion was done to unload the right ventricle and to produce a series of left ventricular end-diastolic pressure and left ventricular end-diastolic segment length points before and after pacing tachycardia. After pacing tachycardia, left ventricular end-diastolic pressure (9.3 +/- 0.9 to 16.9 +/- 1.5 mm Hg, P less than 0.001) and time constant T of left ventricular isovolumic pressure decline (46 +/- 3 to 60 +/- 5 msec, P less than 0.01) increased, with an increase in left ventricular end-systolic segment length (9.8 +/- 0.3 to 10.5 +/- 0.3 mm, P less than 0.001), and a decrease in fractional shortening (17.6 +/- 1.7 to 14.5 +/- 1.3%, P less than 0.01) in the ischemic region, although right ventricular end-diastolic pressure was unchanged. With vena caval occlusion, right ventricular diastolic pressure fell promptly to near zero, followed by decrease in left ventricular pressure and segment length. In each dog, the left ventricular end-diastolic pressure-end-diastolic segment length relation shifted upward after pacing tachycardia. Pacing tachycardia was performed again in six dogs without stenoses. In this group, fractional shortening was preserved after pacing tachycardia (15.7 +/- 2.3 to 15.3 +/- 2.3%, NS), and left ventricular end-diastolic pressure (9.4 +/- 1.8 to 9.8 +/- 1.8 mm Hg, NS) was unchanged. The left ventricular end-diastolic pressure-segment length relation did not shift upward after pacing tachycardia. These data indicate that extrinsic compression of left ventricle by right ventricle is unlikely to be responsible for the upward shift in this model, and the upward shift in end-diastolic left ventricular pressure-segment length relations, as well as dynamic left ventricular diastolic pressure-segment length, supports the concept that persistent myosin-actin interaction throughout diastole plays an important role in the diastolic abnormalities in this angina physiology model.
Receptor mechanisms in the human epicardial coronary artery. Heterogeneous pharmacological response to histamine and carbachol.
Authors: Ginsburg R , Bristow M R , Davis K
Published: June 2012

We studied isolated ring segments from a number of sites along the course of epicardial coronary arteries from 24 human hearts in order to characterize regional responsiveness to vasoactive agents. Concentration-response curves revealed heightened sensitivity to histamine in the proximal portion of the coronary arteries, and increased sensitivity to carbachol in the distal portion of these same vessels. In contrast, the response to stimulation by calcium or phenylephrine was uniform throughout the length of the vessels examined. These data suggest that regional variations in agonist response reflect heterogeneity in receptor populations along the course of the human epicardial coronary artery.
Rapid recovery of cardiac beta-adrenergic receptors after isoproterenol-induced "down"-regulation.
Authors: Limas C J , Limas C
Published: June 2012

Desensitization of beta-receptor-linked adenylate cyclase occurs after prolonged occupancy of the beta-receptors by their agonists. We have followed the development and recovery from "down"-regulation of beta-receptors in enzymatically dissociated cardiac myocytes by using the hydrophilic antagonist [3H]-CGP-12177 to identify surface-bound beta-receptors. After in vitro incubation with (-)-isoproterenol, almost 50% of the beta-receptors are lost within 10 minutes. Isoproterenol-mediated cyclic adenosine monophosphate accumulation by isolated myocytes was also decreased after a 15-minute preincubation with isoproterenol. "Lost" beta-receptors can, however, be recovered when isoproterenol-pretreated, washed cardiac myocytes are incubated at 37 degrees C, 85 +/- 7% of the lost beta-receptors have returned to the cell surface after 20 minutes of incubation. The requirements for such recycling were investigated. Receptor recovery does not depend on de novo protein synthesis, since it is unaffected by prior exposure to cycloheximide. It is, however, dependent on cellular energy, because it is prevented by adenosine triphosphate depletion and involves a lysosomal step since it is inhibited by the lysomotropic agent, chloroquine. In addition, the Golgi apparatus and the microtubules are involved in the beta-receptor recycling to the cell surface, as evidenced by the inhibitory effects of monensin and colchicine, respectively. The mechanism of isoproterenol-induced down-regulation of cardiac beta-receptors involves a rapid, reversible cycling to and from the cytosol and the cell membrane. This intracellular receptor traffic is energy dependent, requires several structures, including lysosomes and microtubules, and may be modified by pathological processes involving the heart.
Regulation of cardiac contractile proteins. Correlations between physiology and biochemistry.
Authors: Winegrad S
Published: June 2012

Modulation of the functional properties of the contractile proteins of mammalian heart muscle plays a significant role in the response of the heart to beta-adrenergic stimulation. The most well understood modification is a change in the concentration of calcium ions that is required to activate the contractile system. By means of a cAMP-sensitive phosphorylation of the inhibitory subunit of troponin (TNI), the threshold concentration for activation can be increased as much as 5-fold without changing the maximum calcium-activated force. The protein kinase involved in this regulation is located in the sarcolemma. Cholinergic stimulation causes a dephosphorylation of TNI by a cGMP-sensitive phosphatase. The concentration of calcium ions required to activate contraction also decreases as muscle length increases. This response of the contractile proteins does not involve phosphorylation of TNI. Regulation of the maximum calcium-activated force can take place by a cAMP-sensitive reaction involving a different protein kinase that is located inside the cell. This mechanism involves at least two sequential reactions, one a cAMP-controlled phosphorylation of a protein bound to an intracellular membrane to release an active factor, and the second, an interaction between the active factor and the contractile proteins to enhance the capacity for generating force in the presence of calcium. Phosphorylation of the light chain of myosin is produced by a calmodulin-regulated kinase. The light chain of myosin is partially phosphorylated in the intact heart, but beta-adrenergic stimulation of the heart does not increase the decrease of phosphorylation in parallel with the increase in contractility.
Sodium-sensitive calcium binding to sarcolemma-enriched preparations from canine ventricle.
Authors: Frankis M B , Lindenmayer G E
Published: June 2012

Calcium binding to sarcolemma-enriched preparations from canine ventricle was evaluated. The preparation was exposed to calcium and 45Ca at physiological ionic strength, pH 7.4, for 15-18 hours at 5 degrees C. Bound calcium was separated from free by filtration and washing of the filter with solutions containing calcium and LaCl3. After equilibration at 5 degrees C, exposure to 37 degrees C caused an irreversible loss of binding. Monovalent cations (157 mM) reduced calcium binding: Na+ much greater than Li+ greater than Cs+ greater than K+ greater than Rb+ approximately equal to choline. In 1 microM calcium, divalent cations (3 mM) reduced binding: Sr++ greater than Ba++ greater than Mg++ approximately equal to Mn++. At 1-300 microM calcium, inhibition of the sodium-sensitive component of binding was characterized by I50's of 3.2-9.5 mM sodium. Comparison of binding by centrifugation versus filtration suggested that the sodium-sensitive component resided on constituents within the membrane vesicles. Calcium binding in 1 mM ethyleneglycol-bis-(beta-aminoethylether)N,N'-tetraacetic acid at pH 7.1 and 5 degrees C, revealed a single species of sodium-sensitive calcium-binding sites: Kd = 0.052 microM and Bmax = 6.73 nmol/mg. In 3 mM magnesium, the Kd was 0.205 microM and the Bmax was 9.03 nmol/mg. Nearly complete inhibition of binding was observed as sodium was raised from 1 to 10 mM. Thus, a substantial number of calcium-binding sites were detected at 5 degrees C in 3 mM magnesium at physiological ionic strength and pH 7.1. The affinity of these sites was in the range necessary to modulate intracellular free calcium. The sensitivity to sodium was at the lower end of the range estimated for intracellular sodium.
Pulmonary artery constriction produces a greater right ventricular dynamic afterload than lung microvascular injury in the open chest dog.
Authors: Calvin J E , Baer R W , Glantz S A
Published: June 2012

Investigators model noncardiogenic pulmonary hypertension by constricting the pulmonary artery to increase right ventricular afterload. To investigate this model's validity, we compared the right ventricular afterload, quantified as pulmonary input impedance, created by constricting the pulmonary artery and by inducing a pulmonary microvascular injury (with glass beads infused into the pulmonary circulation). The pulmonary injury constriction produced a different right ventricular afterload than the microvascular injury. The constriction increased both the input resistance and the characteristic impedance. Microvascular injury increased only input resistance. Physiological levels of lung inflation did not influence pulmonary impedance, but lung hyperinflation increased input resistance both before and while constricting the pulmonary artery or after producing microvascular injury. Total right ventricular power output and stroke work were unchanged during each vascular intervention. Pulmonary artery constriction did not affect power output distribution, whereas microvascular injury decreased oscillatory power and its relative contribution to total power. Lung hyperinflation dramatically reduced right ventricular power and left ventricular stroke work. These effects appeared mediated by right ventricular afterload increase uncompensated for by right ventricular preload increase. These observations help explain the hemodynamic consequences of acute pulmonary hypertension and the effects of lung hyperinflation with positive end-expiratory pressure respiration in such patients.
The role of free radical-mediated processes in oxygen-related damage in cultured murine myocardial cells.
Authors: Scott J A , Khaw B A , Locke E , Haber E , Homcy C
Published: June 2012

A new approach to quantifying myocyte cell death utilizing fluorescence-activated sorting of antimyosin antibody-labeled cells was used to study the effects of oxygen-generated free radicals on cell survival. Uptake of antimyosin, reflecting sarcolemmal damage, increased under conditions which promoted elevated free radical formation and decreased in the presence of increased levels of free radical-scavenging agents. Superoxide dismutase decreased antimyosin uptake at pH 6.7 and 7.5. Mannitol decreased antimyosin uptake at pH 6.5 and 6.7 but not at pH 7.5, and dimethyl sulfoxide decreased antimyosin uptake at pH 6.4 but not at pH 7.5. These data suggest that a greater portion of hydroxyl radicals are produced at higher concentrations of hydrogen ion. Mannitol, a scavenger of hydroxyl radicals, was effective in reducing antimyosin uptake at pH 7.5 in the presence of ferrous sulfate, but had no effect on antimyosin uptake in the absence of ferrous sulfate, suggesting possible iron-mediated catalysis of hydroxyl radical formation. The data suggest that oxygen-derived free radicals can cause significant loss of membrane integrity in cultured myocytes, that the species of radical formed is dependent both on pH and the concentration of iron salts, and that this injury is, at least in part, preventable by the administration of exogenous radical scavenging agents.
Fate of intercellular junctions in isolated adult rat cardiac cells.
Authors: Mazet F , Wittenberg B A , Spray D C
Published: June 2012

Freeze fracture and thin section techniques have revealed morphological changes in gap junctions and intercalated discs of adult rat myocytes following enzymatic dissociation. Cell separation leaves behind small vesicular remnants of formerly adjacent cells connected to the intact cell by gap junctions; in contrast, desmosomes cleave at the region of intercellular contact. Apparently, the next step in gap junction breakdown is internalization of the remnants. In thin section, lanthanum penetration reveals that the cleft of some apparently internalized gap junctions is in contact with the sarcolemma, while that of others is not. In freeze fracture replicas, cytoplasmic gap junctions frequently possess hexagonally packed domains of E-face pits separated by smooth regions that may correspond to separations of membranes of internalized junctions found in thin section. Study of material maintained overnight at 37 degrees C showed no surface junctional remnants; topologies of cytoplasmic gap junctions were generally complex, and concentric membrane vesicles were common. These observations suggest that enzymatic dissociation initiates a progressive, defined sequence of junctional internalization that begins with attached cell remnants and may have as the last determinable step the separation into single membranes inside the intact cell.
The effects of ryanodine on calcium-overloaded sheep cardiac Purkinje fibers.
Authors: Valdeolmillos M , Eisner D A
Published: June 2012

Prolonged exposure to high concentrations of strophanthidin produces an initial increase followed by a subsequent decrease of twitch tension. The slow decrease is termed calcium overload. The aim of the present work was to investigate the effects of ryanodine (an inhibitor of calcium release from the sarcoplasmic reticulum) on calcium-overloaded sheep cardiac Purkinje fibers. The fibers were voltage-clamped, and tension was measured while monitoring the intracellular calcium concentration with the photoprotein aequorin. When strophanthidin (10 microM) was applied to produce calcium overload, a depolarizing pulse produced twitch, and tonic components of tension and repolarization produced an aftercontraction. These components of tension were accompanied by corresponding increases of aequorin light. Ryanodine (1 microM) gave a transient increase of twitch tension. The twitch then decreased to very low levels. The aftercontraction and its corresponding aequorin light signal decreased monotonically on application of ryanodine. It has been suggested that the fall of force in calcium overload may be due to random diastolic release of calcium from the sarcoplasmic reticulum interfering with subsequent systolic calcium release. We suggest that the positive inotropic effect of ryanodine can be explained if ryanodine decreases the diastolic release of calcium. The transient positive inotropic effect of ryanodine reported here is therefore consistent with the hypothesis that the fall of force in calcium overload is due to diastolic calcium oscillations.
Elevated plasma norepinephrine concentrations in decompensated cirrhosis. Association with increased secretion rates, normal clearance rates, and suppressibility by central blood volume expansion.
Authors: Nicholls K M , Shapiro M D , Van Putten V J , Kluge R , Chung H M , Bichet D G , Schrier R W
Published: June 2012

Plasma norepinephrine concentrations are elevated in patients with decompensated cirrhosis, and correlate inversely with urinary sodium and water excretion. Increased plasma norepinephrine concentrations may result from a decreased metabolic clearance rate or an increased secretion rate, possibly in response to a decreased "effective arterial blood volume." If the latter hypothesis is correct, plasma norepinephrine might be expected to be suppressed when central blood volume is expanded by head-out water immersion. In the present study, plasma norepinephrine secretion and clearance rates were determined by infusion of tritiated norepinephrine. Norepinephrine secretion rates were elevated in eight cirrhotic patients as compared to control subjects (1.50 +/- 0.25 vs. 0.26 +/- 0.08 micrograms/m2 per min, P less than 0.001), whereas clearance rates were similar (3.13 +/- 0.48 vs. 2.60 +/- 0.28 liters/min, NS). Baseline plasma norepinephrine concentrations were markedly elevated in the cirrhotic patients (830 +/- 136 vs. 185 +/- 12 pg/ml, P less than 0.001). Head-out water immersion significantly suppressed plasma concentrations of both norepinephrine (704 +/- 72 to 475 +/- 70 pg/ml, P less than 0.005) and epinephrine (121 +/- 33 to 57 +/- 10 pg/ml, P less than 0.05) in all seven patients studied. We conclude that the high circulating catecholamine concentrations in cirrhosis are secondary to increased secretion, rather than to decreased metabolic clearance, and are suppressible by central blood volume expansion.
Role of adenosine in coronary blood flow regulation after reductions in perfusion pressure.
Authors: Dole W P , Yamada N , Bishop V S , Olsson R A
Published: June 2012

We employed intracoronary infusion of adenosine deaminase to test the hypothesis that endogenous adenosine contributes to regulation of coronary blood flow following acute reductions in coronary artery pressure. In 16 closed-chest anesthetized dogs, we perfused the left circumflex coronary artery from a pressurized arterial reservoir and measured coronary blood flow following changes in perfusion pressure before and 10 minutes after the start of intracoronary adenosine deaminase, 5 U/min per kg body weight. Parallel studies showed that this dose of enzyme resulted in cardiac lymph adenosine deaminase concentrations of 3.2 +/- 0.4 U/ml. Adenosine deaminase abolished the vasodilator response to intracoronary adenosine, 4 and 8 micrograms, but had no effect on the vasodilator response to intracoronary papaverine, 200 and 300 micrograms, demonstrating enzyme efficacy and specificity. Additional experiments demonstrated that adenosine deaminase reversibly attenuated myocardial reactive hyperemia following 5- and 10-second coronary occlusions by 30% (P less than 0.05), evidence that the infused enzyme effectively degraded endogenous adenosine. However, adenosine deaminase did not alter the time course for coronary autoregulation or the steady state autoregulatory flow response over the pressure range between 125 and 75 mm Hg. Further, adenosine deaminase did not alter steady state coronary flow when perfusion pressure was reduced below the range for effective autoregulation (60-40 mm Hg). Such results show that adenosine is not essential for either coronary autoregulation or for the maintenance of coronary vasodilation when autoregulatory vasodilator reserve is expended.