bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonuniform loss of regional flow reserve during myocardial ischemia in dogs.
Authors: Coggins D L , Flynn A E , Austin R E , Aldea G S , Muehrcke D , Goto M , Hoffman J I
Published: June 2012

To determine whether coronary vasodilator reserve that persists during myocardial ischemia is present in all left ventricular regions, we measured regional blood flow in 192 left ventricular pieces (mean weight, 201 mg) in each of eight dogs by using radioactive microspheres while perfusing the left main coronary artery at 70, 50, 40, and 30 mm Hg. Flows were measured before and during adenosine infusion to determine flow reserve. Perfusion at 40 and 30 mm Hg produced ischemia in all dogs. At 70 mm Hg, 100% of left ventricular regions had significant flow reserve, compared with 92%, 55%, and 8% during perfusion at 50, 40, and 30 mm Hg, respectively. A greater amount of flow reserve and a greater number of regions responded to adenosine in the subepicardium than in the subendocardium at 50, 40, and 30 mm Hg. We conclude that coronary flow reserve persists in only a subset of left ventricular regions during ischemia and that the number of regions with persistent flow reserve decreases with perfusion pressure. These findings may best be explained by a model in which regional ischemia is a maximal coronary vasodilator and persistent pharmacological vasodilator reserve seen when global markers indicate ischemia simply reflects persistent endogenous flow reserve in myocardial regions not yet ischemic.
Dynamic and static conditioning pressures evoke equivalent rapid resetting in rat aortic baroreceptors.
Authors: Andresen M C , Yang M Y
Published: June 2012

A recent study suggested that exposure of carotid sinus baroreceptors to pulsatile pressures for a period of minutes can decrease the static threshold pressure at which they begin to discharge; that is, the exposure can sensitize baroreceptors. Another study found that the rapid resetting of carotid sinus baroreceptors to elevated conditioning pressures was reduced or eliminated by pulsatile conditioning. In the present study, we tested the responses of aortic baroreceptors in an in vitro preparation to a range of static and dynamic conditioning pressures lasting 15 minutes. Slow test ramps of increasing pressure were used to assess static discharge properties (threshold and gain). To be accepted for analysis (n = 12), each baroreceptor had to successfully complete static and dynamic test sequences for at least three different conditioning mean arterial pressure levels. We measured aortic diameter simultaneously with pressure and baroreceptor discharge. Generally, we found no significant difference between the static pressure threshold measured before and after dynamic conditioning. The static pressure threshold was linearly related to the mean level of the conditioning pressure, and no differences in the slopes of these relations (a measure of the ability of a baroreceptor to rapidly reset) were found between static and dynamic conditioning. After dynamic conditioning, discharge rates returned to near control levels in all cases within a few seconds of the return to static conditioning. Two baroreceptors exposed to the vasodilator nitroprusside throughout the experiment showed similar results. Diameter measurements indicated no role of vessel diameter changes during dynamic or static conditioning. In conclusion, we found no evidence of a long-lasting sensitization of aortic baroreceptors by dynamic pressure inputs.(ABSTRACT TRUNCATED AT 250 WORDS)
Effects of thrombosis on vascular tone in rat mesenteric arteries with endothelium in vivo.
Authors: Araki H , Nishi K , Jougasaki M
Published: June 2012

We examined the effects of thrombosis on vascular tone in vivo, using small mesenteric arteries of rats (about 300 microns i.d.) and a video recording system attached to a microscope. To induce thrombosis we damaged the vessel wall over a short segment by compression and exposed the media to the bloodstream. A thrombus mainly consisting of platelets gradually enlarged at the damaged site and caused 50-100% narrowing of the lumen. The diameter at the site downstream from the thrombus did not change as long as the blood flow was not interrupted. Instead, vasoconstrictor response to serotonin (3.0 and 30 micrograms/ml) or norepinephrine (0.1, 0.3, and 1.0 microgram/ml) given topically was significantly inhibited at the site downstream from the thrombus compared with that at the upstream site (p less than 0.01). Nonthrombotic mechanical narrowing of the vascular lumen did not affect vascular constriction by these agents. After damage to the endothelium by intra-arterial infusion of a detergent (0.3% CHAPS), the inhibition of the constrictor response by the thrombus was no longer observed. Once the thrombus occluded the vascular lumen and the blood flow was interrupted, the diameter at the downstream site declined markedly, from 291 +/- 22 to 77 +/- 21 microns (mean +/- SEM, n = 7, p less than 0.01) even without endothelial damage. Mechanical nonthrombotic occlusion of the vascular lumen induced only a small diameter reduction to 241 +/- 16 microns. We conclude that a partially occlusive thrombus may release a material that inhibits vascular constriction by serotonin or norepinephrine through an endothelium-dependent mechanism and that an occlusive thrombus induces constriction of the downstream vascular bed even with endothelium.
Isolated guinea pig coronary smooth muscle cells. Acetylcholine induces hyperpolarization due to sarcoplasmic reticulum calcium release activating potassium channels.
Authors: Ganitkevich V , Isenberg G
Published: June 2012

Smooth muscle cells, dispersed from the circumflex coronary artery of the guinea pig, were studied with the whole-cell configuration of the patch-clamp. The resting potential of about -40 mV was superimposed by spikelike hyperpolarizations (SLHs) up to -20 mV amplitude. The SLHs resulted from spontaneous transient outward currents (spontaneous TOCs) measured under voltage-clamp (-40 or -50 mV). Acetylcholine (ACh; 10 microM) increased SLHs and TOCs in amplitude and frequency. Atropine blocked the ACh effects. ACh-induced SLHs or TOCs were suppressed by bath application of tetraethylammonium (1 or 10 mM) or by cell dialysis with cesium, suggesting that they result from induction of potassium currents. In cell-attached patches, induction of currents through 130-pS potassium channels was recorded when ACh was bath-applied. An ACh-induced increase in intracellular [Ca2+] is suggested as a second messenger since SHLs and TOCs were suppressed by cell dialysis of 10 mM EGTA. ACh induced SHLs and TOCs in the absence of extracellular calcium. Intracellular application of 5 mg/ml heparin blocked ACh-induced TOCs. When the intracellular calcium stores were depleted by pretreatment with caffeine, the ACh effects were suppressed. Similarly, ACh pretreatment reduced the caffeine-induced outward currents. The results suggested that ACh augments calcium release from the sarcoplasmic reticulum, and the released calcium activates maxi potassium channels. In the single cell, calcium-activated potassium channels generate TOCs and SLHs that sum up to a hyperpolarization of the multicellular tissue.
Differential effects of cardiac hypertrophy and failure on right versus left ventricular calcium activation.
Authors: Perreault C L , Bing O H , Brooks W W , Ransil B J , Morgan J P
Published: June 2012

We studied calcium responsiveness of skinned muscle preparations from the right and left ventricles of rats with cardiac hypertrophy and cardiac hypertrophy plus failure. To test the hypothesis that differences in contractile function are due to changes in myofilament calcium responsiveness, we compared preparations from spontaneously hypertensive rats with cardiac failure, spontaneously hypertensive rats without cardiac failure, and age-matched normotensive Wistar-Kyoto control rats 18-24 months of age. Rats with failure had pleural/pericardial effusions, left atrial thrombi, and right and left ventricular hypertrophy. Muscles were skinned by saponin (250 micrograms/ml) and activated with a series of calcium buffers. Data were plotted as pCa (-log[Ca2+]) versus isometric force and then fit to a modified Hill equation. Values for 50% maximal activation (calcium sensitivity), maximal calcium-activated force, and the slope of the calcium-force relation were compared. Our data indicate that with the development of hypertrophy, calcium sensitivity of left ventricular muscles remains unaffected, but maximal calcium-activated force is increased. In contrast, maximal calcium-activated force declines toward control levels with the development of left ventricular failure, despite the continued presence of significant hypertrophy. In the normotensive rats, the left ventricle is more sensitive to calcium than the right ventricle (pCa50 = 6.0 +/- 0.05 versus 5.7 +/- 0.09; p less than 0.05); however, both the calcium sensitivity and maximal calcium-activated force of the right ventricle increase with the development of compensatory hypertrophy secondary to left ventricular failure. These changes that occur in rats with cardiac hypertrophy and failure may represent important physiological adaptive mechanisms.
Sinus and atrioventricular nodal distribution of sympathetic fibers that contain neuropeptide Y.
Authors: Warner M R , Levy M N
Published: June 2012

Neuropeptide Y and norepinephrine are localized in sympathetic nerve terminals throughout the heart. We sought to determine the functional distribution of the neuropeptide Y-containing sympathetic fibers to the sinus and atrioventricular (AV) nodal regions. We recorded cycle length, AV interval, and arterial pressure in 14 anesthetized dogs. We assessed the release of neuropeptide Y from sympathetic nerve terminals by measuring the attenuation of the vagal effects on cycle length and AV interval that occurred after unilateral ansa subclavia stimulation. Three-minute trains of right or left ansa stimulation, each applied at frequencies of 2, 5, and 10 Hz, produced a frequency-dependent inhibition of the vagal effects on cycle length and AV interval. After right ansa stimulation (10 Hz), however, the percent inhibition of the vagal effects on cycle length was 21 +/- 5% greater (p less than 0.001) than the percent inhibition of the vagal effects on AV interval. Conversely, after left ansa stimulation (10 Hz), the percent inhibition of the vagal effects on AV interval was 54 +/- 7% greater (p less than 0.001) than the percent inhibition of the vagal effects on cycle length. The vagal stimulus characteristics (frequency or voltage) did not significantly alter the percent inhibition, nor did the percent inhibition depend on the vagus stimulated (right or left vagus). We conclude that most of the neuropeptide Y-containing sympathetic fibers at the sinus node originate in right-sided ganglia, whereas most of those at the AV node originate in left-sided ganglia.
Age-related decrease of calcitonin gene-related peptide-containing vasodilator innervation in the mesenteric resistance vessel of the spontaneously hypertensive rat.
Authors: Kawasaki H , Saito A , Takasaki K
Published: June 2012

We previously demonstrated that the mesenteric resistance blood vessels have nonadrenergic, noncholinergic vasodilator innervation in which calcitonin gene-related peptide (CGRP) is a possible neurotransmitter. The role of CGRP-containing vasodilator nerves in hypertension was investigated in perfused mesenteric vascular beds isolated from spontaneously hypertensive rats (SHR). The adrenergic vasoconstrictor responses to perivascular nerve stimulation in both SHR (8-, 15-, and 30-week-old) and age-matched Wistar-Kyoto (WKY) rat preparations increased with aging, but the response was greater in SHR than in WKY rats at all ages. The preparation isolated from SHR and WKY rats was precontracted by continuous perfusion of Krebs' solution containing 7 x 10(-6) M methoxamine plus 5 x 10(-6) M guanethidine. In both SHR and WKY rats, perivascular nerve stimulation (1-8 Hz) produced frequency-dependent vasodilation, which was blocked by 1 x 10(-7) M tetrodotoxin, pretreatment with 5 x 10(-7) M capsaicin, and denervation by cold storage (4 degrees C for 72 hours). The vasodilation induced by perivascular nerve stimulation in SHR greatly decreased with age, whereas a slight decrease in the response with age was found in WKY rats. The neurogenic vasodilation in the young SHR preparation was similar in magnitude to the vasodilation in age-matched WKY rats, whereas the vasodilation in 15- and 30-week-old SHR was significantly smaller than that in age-matched WKY rats. In both SHR and WKY rats, perfusion of rat CGRP (1 x 10(-10) to 3 x 10(-8) M) produced marked vasodilation in a concentration-dependent manner. The CGRP-induced vasodilation in SHR increased with age, whereas an age-related decrease in vasodilation was found in WKY rats. Perivascular nerve stimulation (4 and 8 Hz) of the perfused mesenteric vascular bed evoked an increased release of CGRP-like immunoreactive substance in the perfusate, which was significantly less in 15-week-old SHR than in age-matched WKY rats. Immunohistochemical studies showed an age-related decrease in CGRP-like immunoreactive fibers in SHR but not in WKY rats. These results suggest that CGRP-containing vasodilator innervation is greatly decreased when SHR develop and maintain hypertension. It is also suggested that the decreased vasodilator mechanism by CGRP-containing nerves contributes to the development and maintenance of hypertension.
Swimming causes myosin adaptations in the rat cardiac isograft.
Authors: Advani S V , Geenen D , Malhotra A , Factor S M , Scheuer J
Published: June 2012

To investigate the contributions of humoral and hemodynamic factors to cardiac adaptations associated with chronic exercise, female Fischer 344 rats were subjected to chronic swimming, infrarenal cardiac transplantation, or both. Swimming resulted in hypertrophy (11-12%) of the in situ hearts in both the unoperated and operated animals compared with the matched sedentary controls. The cardiac isograft exhibited atrophy (32-35%), which was not attenuated by swimming. The cardiac isograft was also associated with a decrease in the percent of V1 myosin isoenzyme, which was attenuated by swimming (45 +/- 5% versus 66 +/- 6%). Swimming also increased the percent of this isomyosin in the in situ hearts of operated rats. These data suggest that hemodynamic load and/or neural innervation are necessary for hypertrophy associated with chronic conditioning by swimming, whereas myosin isoenzyme control is significantly mediated by humoral factors.
Diabetes-induced alterations in atrial natriuretic peptide gene expression in Wistar-Kyoto and spontaneously hypertensive rats.
Authors: Matsubara H , Mori Y , Yamamoto J , Inada M
Published: June 2012

We investigated the effects of streptozotocin-induced diabetes on atrial natriuretic peptide (ANP) synthesis, hemodynamic parameters, blood volume, and histopathology, as well as the reversibility of such effects with insulin therapy in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). The biatrial ANP messenger RNA (mRNA) levels in the diabetic WKY rats increased by 16-17% compared with those in the age-matched WKY rats at 12 weeks after the onset of diabetes, whereas their ventricular ANP mRNA levels showed increases of 190% in left ventricles and 160% in right ventricles at 8 weeks. In the diabetic SHRs, the left atrial ANP mRNA levels increased by 36% compared with those in the age-matched SHRs, as early as 4 weeks after diabetes onset. Their ventricular ANP mRNA levels also showed 80-82% increases in left and right ventricles at 4 weeks. In proportion to changes in cardiac ANP synthesis, the biventricular end-diastolic pressures were significantly elevated at 8 weeks in the diabetic WKY rats and at 4 weeks in the diabetic SHRs. The blood volume significantly increased at 8 weeks in the diabetic WKY rats and remained higher thereafter, whereas it did not change in the diabetic SHRs throughout the experimental period. The left ventricular peak dP/dt was depressed in the 8-week diabetic SHRs, whereas in the diabetic WKY rats, its depression was observed at 12 weeks after diabetes onset. Histopathological studies showed that diabetic changes in ANP synthesis and hemodynamic parameters described above occurred before the cardiomyopathic histological changes. Cardiac ANP synthesis in the diabetic rats completely reverted to control levels after insulin therapy, accompanied by normalization of hemodynamic parameters. The present study indicates that 1) ANP synthesis is significantly augmented in the streptozotocin-induced diabetic rat compared with that in the normal rat, and the combination of diabetes and hypertension produces an earlier and greater effect in stimulating cardiac ANP synthesis than does either disease alone; 2) an elevation in the intraventricular filling pressure that occurs before observable cardiomyopathic histopathological alterations might be involved partially in the augmented ANP synthesis; and 3) the reversibility with insulin therapy suggests that the streptozotocin-induced alterations observed in cardiac ANP synthesis and hemodynamics result from insulin-deficient diabetes mellitus, not from cardiac toxicity of streptozotocin.
Myocyte cell loss and myocyte cellular hyperplasia in the hypertrophied aging rat heart.
Authors: Anversa P , Palackal T , Sonnenblick E H , Olivetti G , Meggs L G , Capasso J M
Published: June 2012

To determine the effects of age on the myocardium, the functional and structural characteristics of the heart were studied in rats at 4, 12, 20, and 29 months of age. Mean arterial pressure, left ventricular pressure and its first derivative (dP/dt), and heart rate were comparable in rat groups up to 20 months. During the interval from 20 to 29 months, elevated left ventricular end-diastolic pressure and decreased dP/dt indicated that a significant impairment of ventricular function occurred with senescence. In the period between 4 and 12 months, a reduction of nearly 19% in the total number of myocytes was measured in both ventricles. In the subsequent ages, similar decreases in myocyte cell number were found in the left ventricle, whereas in the right ventricle, the initial loss was fully reversed by 20 months. Moreover, from 20 to 29 months, a 59% increase in the aggregate number of myocytes occurred in the right ventricular myocardium. In the left ventricle, a 3% increment was also seen, but this small change was not statistically significant. These estimations of myocyte cellular hyperplasia, however, were complicated by the fact that cell loss continued to take place with age. The volume fraction of collagen in the tissue, in fact, progressively increased from 8% and 7% at 4 months to 16% and 22% at 29 months in the left and right ventricles, respectively. In conclusion, myocyte cellular hyperplasia tends to regenerate the ventricular mass being lost with age in the adult mammalian rat heart.
NG-monomethyl L-arginine inhibits endothelium-derived relaxing factor-stimulated cyclic GMP accumulation in cocultures of endothelial and vascular smooth muscle cells by an action specific to the endothelial cell.
Authors: Johns R A , Peach M J , Linden J , Tichotsky A
Published: June 2012

The effect of NG-monomethyl L-arginine (LNMMA), an analogue of L-arginine (a proposed precursor of endothelium-derived relaxing factor [EDRF]), on EDRF release from bovine pulmonary artery endothelial cells was investigated using endothelial cell-vascular smooth muscle cocultures and a superfused column containing endothelial cells grown on microcarrier beads. Cocultures were stimulated with control buffer, ATP, bradykinin, melittin, A23187, or nitroprusside in the presence and absence of varying concentrations of LNMMA (30-300 microM). LNMMA caused significant, concentration-dependent decreases in cyclic GMP accumulation in response to the endothelium-dependent dilators bradykinin, ATP, melittin, and A23187 but had no effect on control or nitroprusside-stimulated cocultures. The inhibitory effect of LNMMA on cyclic GMP accumulation was partially reversed by treatment with L-arginine, but was unaffected by D-arginine. To determine the specific site of action of LNMMA, endothelial cells on microcarrier beads were placed in a column and superfused with buffer. The effluent from the column was collected in 30-second (1.5-ml) fractions into 2-cm2 monolayer wells of vascular smooth muscle cells before and after addition of agonists (bradykinin, A23187) to the column inflow. The cyclic GMP content of each well of smooth muscle cells was determined as an index of EDRF activity. LNMMA superfused through the endothelial cell column inhibited cyclic GMP accumulation in vascular smooth muscle cells induced by bradykinin and A23187. LNMMA introduced into the effluent from the endothelial cell column had no effect on smooth muscle cyclic GMP levels. We conclude that LNMMA is an effective, specific inhibitor of EDRF production or release, and its action is specific to the endothelial cell.
Dual action of FRC8653, a novel dihydropyridine derivative, on the Ba2+ current recorded from the rabbit basilar artery.
Authors: Oike M , Inoue Y , Kitamura K , Kuriyama H
Published: June 2012

Actions of FRC8653 on the macroscopic and unitary Ba2+ currents were studied using the rabbit basilar artery. Application of (+/-)-FRC8653 (less than 1 microM) increased the amplitude of the inward current when depolarization pulses more negative than -10 mV were applied but inhibited it when depolarization was more positive than 0 mV (in each case from a holding potential of -80 mV). At a holding potential of -40 mV, (+/-)-FRC8653 (greater than 0.1 nM) consistently inhibited the inward current. (-)-FRC8653 (greater than 1 nM) inhibited the amplitude of the inward current evoked by a depolarizing pulse more positive than -10 mV (the holding potential being -80 mV). At the holding potential of -80 mV, but not at -40 mV, (+)-FRC8653 (1 microM) enhanced the current amplitude evoked by a depolarizing pulse more negative than -10 mV but inhibited the current evoked by a pulse more positive than 0 mV. (+/-)-FRC8653 shifted the voltage-dependent inhibition curves to the left, and the slope of the curve became steeper (test pulse of +10 mV). Two types of single Ca2+ channel currents (12 and 23 pS) were recorded from the basilar artery by the cell-attached patch-clamp method. Opening of the 12-pS channel occurred with a depolarizing pulse (-20 mV) from a holding potential of -80 mV, but not from one of -60 mV. (+)-FRC8653 activated, and (-)-FRC8653 inhibited, the 23-pS channel.(ABSTRACT TRUNCATED AT 250 WORDS)
Contractile effect of the chemotactic factors f-Met-Leu-Phe and C5a on the human isolated umbilical artery. Role of cyclooxygenase products and tissue macrophages.
Authors: Marceau F , deBlois D , Laplante C , Petitclerc E , Pelletier G , Grose J H , Hugli T E
Published: June 2012

Factors that are chemotactic for phagocytic leukocytes are known to elicit important acute circulatory changes, and the role of circulating leukocytes in these models is controversial. To evaluate the role of the blood vessel wall in the absence of circulating cells, spiral strips of human umbilical artery were exposed in vitro to the chemotactic peptides f-Met-Leu-Phe (1-100 nM) or C5a (2.5-25 nM). Contractile responses were observed for both peptides. Certain agonist analogues and a selective antagonist of the chemotactic action of f-Met-Leu-Phe behaved correspondingly as agonists and antagonist of the contractile effect on umbilical artery. The anaphylatoxin C3a also exerted a contractile effect on the tissue (25 nM and above), but this effect was highly tachyphylactic. Inhibitory drugs were used to examine the contributions of secondary mediators in eliciting the effects of C5a and f-Met-Leu-Phe. The contractile effect of both peptides was massively inhibited either by indomethacin or the thromboxane A2/prostaglandin H2 antagonist SQ 29548. Dazmegrel, a thromboxane A2 synthetase inhibitor, had partial inhibitory effects on contractions induced by either peptide. The contractile effect of C3a was prevented by indomethacin pretreatment. Vascular strips did not release measurable histamine in the bathing fluid after challenge with C5a or f-Met-Leu-Phe. The tissue apparently contains neither histamine nor mast cells. Autoradiography of 125I-labeled C5a or f-Met-Leu-Phe analogue showed specific binding of the peptides to cells dispersed in the vessel wall, but more frequently at the periphery. Cells stained positively for alpha-naphthyl acetate esterase showed a similar distribution. Pure cultures of smooth muscle cells derived from the umbilical artery failed to release prostanoids when exposed to f-Met-Leu-Phe or C5a, whereas fresh strips of this artery released more thromboxane B2 than the baseline in response to these peptides. We conclude that macrophagelike cells, present in the vessel wall, are the likely target cells for the chemotactic peptides. These cells trigger a contractile effect of the smooth muscle by generating cyclooxygenase products.
5,6-epoxyeicosatrienoic acid, a novel arachidonate metabolite. Mechanism of vasoactivity in the rat.
Authors: Carroll M A , Garcia M P , Falck J R , McGiff J C
Published: June 2012

We have reported that 5,6-epoxyeicosatrienoic acid (5,6-EET) was the only cytochrome P-450-dependent arachidonic acid (AA) epoxide to dilate the isolated, perfused caudal artery of the rat. We have investigated the mechanisms by which 5,6-EET dilates the rat-tail artery by studying the effect of deendothelialization and inhibition of AA metabolic pathways (cyclooxygenase, lipoxygenase, and cytochrome P-450 monooxygenase) on the vascular action of the epoxide. Rat isolated caudal arteries were perfused with Krebs-Henseleit solution at 37 degrees C, pH 7.4, and gassed with 95% O2-5% CO2. Arterial tone was elevated with phenylephrine; acetylcholine (0.5 nmol) was used to detect the presence of intact, functional endothelium. Doses of 5,6-EET, from 6.25 to 25.0 nmol, were injected close-arterially. After obtaining control responses, the same doses were randomly retested after deendothelialization or in the presence of inhibitors of AA metabolism. Removal of the endothelium decreased by 70% the vasodilator responses to 5,6-EET. The endothelial dependency was a function of the epoxide interacting with cyclooxygenase of the endothelium, because indomethacin (3 microM) and aspirin (50 microM) prevented the vasodilator response to 5,6-EET while not affecting the response to acetylcholine. SKF-525A (1.1 microM) and metyrapone (150 microM) did not affect the responses to the 5,6-EET, whereas clotrimazole (0.7 microM) and nordihydroguaiaretic acid (2.5 microM) had nonspecific effects, decreasing responses to 5,6-EET and acetylcholine. Because 5,6-EET failed to stimulate detectable release of prostanoids into the effluent from the caudal artery, we conclude that 5,6-EET requires conversion by cyclooxygenase for expression of its vasoactivity.
Energy demand, supply, and utilization in hypoxia, and force recovery after reoxygenation in rabbit heart muscle.
Authors: Dietrich D L , Mast F , Elzinga G
Published: June 2012

In rabbit papillary muscle contracting at 20 degrees C in nitrogen at 0.2 Hz, glycolytic ATP formation is just enough to support the diminished contractile activity. Basal metabolism, important to maintain cellular function and integrity, is strongly inhibited. In the present study, we address the question of whether the inhibition of basal processes in hypoxia determines redevelopment of force in reoxygenation. By not stimulating the muscle during hypoxia, we try to make more ATP available for basal processes. Isometric force of papillary muscles (0.2-Hz stimulation) is measured before, during, and after 40 minutes of hypoxia. ATP formation and utilization in hypoxia are estimated from lactate production and changes in nucleotides and creatine compounds. After reoxygenation, muscles stimulated during hypoxia produce a steady-state force of 78% of the aerobic control; resting muscles recover to 94%. In contrast to expectation, lactate production in hypoxic resting muscles is only 30% of that in contracting ones. The findings indicate that basal metabolic rate of hypoxic muscles at rest is 14% of that of quiescent, well-oxygenated myocardium. We conclude that in hypoxic myocardium little ATP is available for basal metabolism, irrespective of the energy demand of the contractile system. It is therefore unlikely that the lower force found after reoxygenation in muscles stimulated during hypoxia is related to the degree of inhibition of basal processes.
Reperfusion and readmission of oxygen. Pathophysiological relevance of oxygen-derived free radicals to arrhythmogenesis.
Authors: Yamada M , Hearse D J , Curtis M J
Published: June 2012

We have examined the pathophysiological role of readmission of oxygen (and hence production of oxygen-derived free radicals) in the initiation of reperfusion-induced arrhythmias by separating, on a temporal basis, readmission flow from readmission of oxygen. Isolated rat hearts (n = 12/group) were subjected to 10 minutes of regional ischemia and 10 minutes of reperfusion. In controls reperfused with oxygenated solution (Po2 greater than 600 mm Hg), 92% of hearts developed ventricular fibrillation (VF) during the first 20 seconds of reperfusion, whereas in hearts reperfused with hypoxic solution (Po2 9.3-12.2 mm Hg), the incidence of VF was only 17% (p less than 0.05). Subsequent readmission of control solution (Po2 greater than 600 mm Hg) to the latter group led, within 20 seconds, to the appearance of VF in seven of the 10 hearts (70%) that had not previously fibrillated. To examine whether hypoxic reperfusion had prevented VF or merely delayed its onset, the studies were repeated in separate groups of hearts with the duration of hypoxic reperfusion extended to 5 minutes. In addition, to examine the partial pressure dependence of the relation, the Po2 in the reperfusion solution was set at one of five different levels: greater than 600, 150-192.7, 69-85.6, 9.2-14.8, or 0.0 mm Hg. It was found that hypoxia merely delayed VF onset by 20-40 seconds and did not significantly reduce the incidence of VF, which was 83%, 92%, 67%, 58%, and 58%, respectively. This indicated that readmission of oxygen is unnecessary for the initiation of VF during reperfusion. The hearts that reverted to sinus rhythm during the ensuing 5 minutes (n = 8, 4, 5, 9, and 8, respectively) were used to assess the arrhythmogenic consequences of readmission of oxygen. When control solution (Po2 greater than 600 mm Hg) was readmitted, new episodes of VF were elicited within 20 seconds in a manner that was inversely proportional to the preceding Po2 (p less than 0.05), the incidence of new episodes of VF being 0%, 0%, 40%, 67%, and 86%, respectively. The arrhythmogenic effect of readmission of oxygen was not the result of a sudden increase in heart rate, because a similar arrhythmogenic effect of readmission of oxygen was seen in separate groups of hearts that were paced (350 beats/min) throughout hypoxia and readmission of oxygen. In conclusion, readmission of flow and readmission of oxygen are independent determinants of reperfusion-induced arrhythmias.(ABSTRACT TRUNCATED AT 400 WORDS)
Beta-adrenergic regulation of the muscarinic-gated K+ channel via cyclic AMP-dependent protein kinase in atrial cells.
Authors: Kim D
Published: June 2012

Cholinergic and beta-adrenergic stimulations of ionic currents are major physiological mechanisms in the regulation of heart rate and contractility. Muscarinic receptor stimulation is known to reduce beta-adrenergic effects on calcium current via reduction of cyclic AMP. Whether the beta-adrenergic stimulation affects the muscarinic response is not known. I report here that the beta-adrenergic agonist isoproterenol enhanced the muscarinic-activated K+ channel activity in rat atrial cells. Application of cyclic AMP-dependent protein kinase or its catalytic subunit to the cytoplasmic side of the membrane augmented the acetylcholine-activated K+ channel activity twofold to threefold. Increases in channel activity produced by isoproterenol or cyclic AMP-dependent protein kinase were associated with fourfold to fivefold and approximately twofold increases in the mean open and closed time durations, respectively. Alkaline phosphatase treatment reversed these effects. These results suggest that cyclic AMP-dependent phosphorylation of the K+ channel or associated regulatory proteins modulates the gating kinetics of the channel. This mechanism may be important in the regulation of pacemaker activity and, thus, the heart rate during beta-adrenergic stimulation.
Vasoactive intestinal polypeptide facilitates atrioventricular nodal conduction and shortens atrial and ventricular refractory periods in conscious and anesthetized dogs.
Authors: Rigel D F , Lathrop D A
Published: June 2012

Our study was designed to determine the cardiac electrophysiological influence of vasoactive intestinal polypeptide (VIP) in conscious dogs. Dogs (n = 8) were chronically instrumented with arterial and venous catheters, cervical vagal cooling coils, and right atrial and right ventricular bipolar epicardial pacing and recording electrodes. After autonomic blockade (10 mg/kg i.v. hexamethonium, 0.11 mg/kg i.v. atropine, and vagal cold blockade), VIP (50 and 100 pmol/kg/min i.v.) or isoproterenol (ISO) (250 and 500 pmol/kg/min i.v.) increased heart rate (maximum increases: VIP, 81.1 +/- 4.2 beats/min; ISO, 61.3 +/- 8.5 beats/min), decreased the atrial-ventricular interval (during constant atrial pacing) (VIP, -41.9 +/- 6.3 msec; ISO, -34.6 +/- 7.4 msec), shortened the atrial effective refractory period (VIP, -24.4 +/- 2.1 msec; ISO, -30.6 +/- 4.4 msec) and ventricular effective refractory period (VIP, -4.2 +/- 0.7 msec; ISO, -10.0 +/- 2.4 msec), and decreased mean arterial pressure (VIP, -51.9 +/- 4.0 mm Hg; ISO, -26.1 +/- 2.4 mm Hg). beta-Adrenergic blockade with propranolol (1 mg/kg i.v.) eliminated the positive chronotropic and atrioventricular nodal dromotropic responses to bolus doses of ISO (30, 100, 300, and 1,000 pmol/kg i.v.) but did not affect the responses to VIP (10, 30, 100, and 300 pmol/kg i.v.). Comparable blood pressure decreases produced by sodium nitroprusside caused only minimal changes in heart rate, atrial-ventricular conduction times, and atrial and ventricular refractory periods. In three additional anesthetized dogs, after vagotomy and beta-adrenergic blockade (1 mg/kg i.v. propranolol), VIP (100 pmol/kg/min i.v.) shortened the atrial-His interval but did not alter intra-atrial, intraventricular, or His-Purkinje conduction. Our findings combined with the demonstration by others of VIP-immunoreactive nerves innervating canine sinus nodal cells, atrioventricular nodal cells, and atrial and ventricular myocardial cells suggest that endogenous VIP may directly alter the electrical properties of the heart.
Evaluation of left ventricular mechanical restitution in closed-chest dogs based on single-beat elastance.
Authors: Freeman G L , Colston J T
Published: June 2012

The mechanical restitution of the left ventricle of closed-chest dogs was modeled as a monoexponential relation, using peak single-beat elastance as a measure of contractile strength. Data were obtained from nine dogs chronically instrumented with three sets of piezoelectric diameter gauges to assess ventricular volume and high-fidelity manometers to measure pressure. Mechanical restitution curves were obtained during both atrial and ventricular pacing in six dogs. The time constant of mechanical restitution was 64.3 +/- 11.4 msec for atrial and 122.2 +/- 26.3 msec for ventricular paced runs (p less than 0.01). These values are smaller than those previously reported from isolated hearts or isolated muscle segments. Although the time of onset of mechanical restitution was longer for atrial than for ventricular runs (255.1 +/- 14.3 versus 225.1 +/- 9.6 msec, p less than 0.05), the plateau to which mechanical function rose was not different. During ventricular pacing, fusion beats were noted in four dogs. The magnitude of the mechanical contribution of these fusion beats fell to a nadir at approximately 250 msec, suggesting that intracellular calcium available for crossbridge interaction is dropping during this time. In three additional dogs, the time constant of postextrasystolic restitution was found to vary depending on the preceding extrasystolic interval. Thus, mechanical restitution of the ventricle is a dynamic process that can be assessed using an elastance-based approach in the in situ heart.
Intracellular pH regulation in ferret ventricular muscle. The role of Na-H exchange and the influence of metabolic substrates.
Authors: Blatter L A , McGuigan J A
Published: June 2012

Aspects of pH regulation in ferret ventricular cells have been investigated by using pH- and sodium-selective microelectrodes in bicarbonate-free Tyrode's solution. An acid load was produced by the transient application of NH4Cl (10 or 20 mmol/l). A complete recovery from an acid load was still observed after multiple applications of NH4Cl, but amiloride (0.75 or 1 mmol/l), a blocker of the Na-H exchanger, increased the acidification and inhibited the recovery. Measurements of intracellular sodium concentration showed a transient decrease during the application of NH4Cl and a transient increase above control values during recovery from acidification. This increase was inhibited by amiloride. Intracellular sodium loading (strophanthidin [low calcium-low potassium Tyrode's solution]) did not initially cause an intracellular pH (pHi) change, but the acidification induced by amiloride under those circumstances was larger. Reducing extracellular sodium concentration from 155 to 5 or to 1.5 mmol/l caused an acidification. Changing extracellular pH (pHo) from 6.4 to 8.4 caused an average linear change in pHi in the same direction of 0.085 pHi units/pHo units. The mean intracellular buffering capacity measured with the NH4Cl method and with the proton extrusion mechanism blocked by amiloride was 36 +/- 15 mmol pH-1.l-1 (mean +/- SD), approximately half that of previous estimations. Changing the metabolic substrate from glucose to pyruvate in the superfusing solution caused an acidification of 0.21 pH units. This could be partially blocked by alpha-cyano-4-hydroxycinnamate, a finding consistent with a pyruvate-H+ cotransport and/or a pyruvate-OH- countertransport system being present in ventricular cells. The results of the present study show that ventricular cells can effectively buffer hydrogen ions and that an Na-H exchange system plays a major role in the regulation of pHi.
Ventricular pressure-area loop characteristics in the stage 16 to 24 chick embryo.
Authors: Keller B B , Hu N , Serrino P J , Clark E B
Published: June 2012

The accurate description of embryonic cardiovascular function requires the adaption of standard measurement techniques to the small scale of the developing heart. In the mature heart, the analysis of ventricular pressure and volume accurately defines function. Because in vivo measures of volume are not feasible in the embryonic heart, we tested the hypothesis that ventricular pressure-area loops accurately define ventricular function in the stage 16 to stage 24 white Leghorn chick embryo. We simultaneously measured ventricular pressure with a servo-null pressure system and recorded video images at 60 Hz. The pressure waveform was superimposed onto the video image in real time. Video fields were planimetered for epicardial ventricular cross-sectional area and ventricular pressure. Pressure and area data were smoothed using a fast Fourier transform filter and plotted. Data are reported as mean +/- SEM, n greater than or equal to 4, and were tested by regression analysis and analysis of variance (p less than 0.05). Heart rate increased from 90 +/- 7 beats/min at stage 16 to 130 +/- 13 beats/min at stage 24. All pressure-area loops displayed diastolic filling, isometric contraction, ejection, and isometric relaxation, similar to pressure-volume loops of the mature heart. Isometric contraction time increased from 42 +/- 5 to 62 +/- 4 msec (p less than 0.05), while isometric relaxation time was 124 +/- 12 and 120 +/- 10 msec (p greater than 0.05) between stages 16 and 24, respectively. The maximum ratio of instantaneous ventricular pressure to area identified end systole better than peak ventricular pressure or minimum ventricular area. Thus, pressure-area relations define ventricular function in the embryonic chick heart.
Mechanism of early ischemic contractile failure. Inexcitability, metabolite accumulation, or vascular collapse?
Authors: Koretsune Y , Corretti M C , Kusuoka H , Marban E
Published: June 2012

The basis of early ischemic contractile failure was investigated in perfused ferret hearts at 27 degrees C. Isovolumic left ventricular developed pressure fell by more than 50% within 30 seconds of the onset of total global ischemia and reached zero by 5 minutes. Monophasic action potential recordings revealed no decrease in excitability during this period. Phosphorus nuclear magnetic resonance spectra obtained at 30-second resolution showed no significant changes in inorganic phosphate or phosphocreatine during the first 30 seconds of ischemia. Intracellular pH (pHi) and ATP changed even more slowly; therefore, none of these metabolites could account for the rapid fall in force. To gauge the contribution of intravascular pressure, we compared ordinary aortic flow occlusion with tissue-level ischemia induced by massive coronary microembolization at the level of the precapillary arterioles. Functional depression developed significantly more slowly in the microembolized hearts, despite accumulation of inorganic phosphate and protons comparable with that in ordinary ischemia. After microembolization, the time course of functional depression reflected much more closely the concomitant inorganic phosphate and pHi changes. Thus, our results provide novel evidence supporting the importance of vascular collapse in the mechanism of early ischemic contractile failure.
Putative mechanisms of the impairment of endothelium-dependent relaxation of the aorta with atheromatous plaque in heritable hyperlipidemic rabbits.
Authors: Tagawa H , Tomoike H , Nakamura M
Published: June 2012

Attenuation of acetylcholine-induced endothelium-dependent relaxation of thoracic aortas excised from Watanabe heritable hyperlipidemic (WHHL) rabbits linearly correlated with the percent area coated with atheromatous plaque. To elucidate mechanisms related to this reduced endothelium-dependent relaxation in the presence of atherosclerosis, the acetylcholine-induced release of endothelium-derived relaxing factor (EDRF) was assessed functionally as a percent relaxation of the precontracted detector strips obtained from the tunica media beneath the intact intima or the atheromatous plaque in the same aortic ring preparation. Relaxations of the normal detectors to effluents containing EDRF of thoracic aortas during stimulation by acetylcholine (3 x 10(-6) M) in heterozygous and homozygous WHHL rabbits were 73 +/- 5% and 59 +/- 9% (p less than 0.01) of the phenylephrine-induced precontraction, respectively. Relaxations of the atherosclerotic detectors to effluents (EDRF) through the aortas during stimulation by acetylcholine (3 x 10(-6) M) in heterozygous and homozygous WHHL rabbits were 16 +/- 4% and 14 +/- 5%, respectively--values significantly smaller than those seen in the normal detectors. When superoxide dismutase was added to the perfusate of the donors from homozygous and heterozygous WHHL rabbits, atherosclerotic detectors relaxed by effluents stimulated by acetylcholine to 73% and 65% (p less than 0.01 versus before the addition of superoxide dismutase) of the normal detector, respectively. Relaxations induced by sodium nitroprusside as well as the contractions by acetylcholine, phenylephrine, and KCl (118 mM) were comparable in detector strips from the normal and atherosclerotic portions. Thus, not only is the amount of EDRF released by acetylcholine reduced in the presence of atherosclerosis, the tunica media beneath the atheromatous plaque is also to some extent responsible for the superoxide-induced inactivation of EDRF.
Virtual cathode effects during stimulation of cardiac muscle. Two-dimensional in vivo experiments.
Authors: Wikswo J P , Wisialowski T A , Altemeier W A , Balser J R , Kopelman H A , Roden D M
Published: June 2012

We have found that when suprathreshold cathodal stimuli were applied to the epicardium of canine ventricle, impulse propagation originated at a "virtual cathode" with dimensions greater than those of the physical cathode. We report the two-dimensional geometry of the virtual cathode as a function of stimulus strength; the results are compared with the predictions of an anisotropic, bidomain model of cardiac conduction recently developed in our laboratories. Data were collected in six pentobarbital-anesthetized dogs by using a small plaque electrode sewn to the left ventricular epicardium. Arrival times at closely spaced bipolar electrodes oriented radially around a central cathode were obtained as a function of stimulus strength and fiber orientation. The dimensions of the virtual cathode were determined by linear back-extrapolation of arrival times to the time of stimulation. The directional dependence of the conduction velocity was consistent with previous reports: at 1 mA, longitudinal (0 degree) and transverse (90 degrees) velocities were 0.60 +/- 0.03 and 0.29 +/- 0.02 m/sec, respectively. At 7 mA, the longitudinal velocity was 0.75 +/- 0.05 m/sec, whereas there was no significant change in the transverse velocity. In contrast to conduction velocity, the virtual cathode was smallest in the longitudinal orientation and largest between 45 degrees and 60 degrees. Virtual cathode size was dependent on both orientation and stimulus strength: at 0 degree, the virtual cathode was small (approximately 1 mm) and relatively constant over the range of 1-7 mA; at oblique orientations (45 degrees-90 degrees), it displayed a roughly logarithmic dependence on stimulus strength, approximately 1 mm at 1 mA and approximately 3 mm at 7 mA. The bidomain, anisotropic model reproduced both the stimulus strength and the fiber-orientation dependence of the virtual cathode geometry when the intracellular and extracellular anisotropies were 10:1 and 4:1, respectively, but not when the two anisotropies were equal. We suggest that the virtual cathode provides a direct measure of the determinants of cardiac activation; its complex geometry appears to reflect the bidomain, anisotropic nature of cardiac muscle.
Modulation of crossbridge kinetics by myosin isoenzymes in skinned human heart fibers.
Authors: Morano I , Bletz C , Wojciechowski R , Rüegg J C
Published: June 2012

Skinned fibers from the normal human heart with the beta-myosin heavy chain (ventricular fibers) revealed both a higher force generation per cross section and a higher Ca2+ sensitivity than skinned fibers with the alpha-myosin heavy chain (atrial fibers). The relation between isometric ATPase activity and isometric tension of atrial fibers was higher than that of ventricular fibers. Since the ATPase-tension relation equals the rate constant for the transition from force-generating into non-force-generating crossbridge states (g(app)), myosin heavy chain isoenzymes seem to have different crossbridge turnover kinetics. Modulation of g(app) by myosin heavy chain isoenzymes could explain the different contractile behavior of atrial and ventricular fibers. g(app) was independent of Ca2+.