bluuebunny/crossref_metadata_2025 huggingface dataset:
Spontaneous sarcomeric oscillations at intermediate activation levels in single isolated cardiac myofibrils.
Authors: Linke W A , Bartoo M L , Pollack G H
Published: June 2012

Spontaneous oscillations observed in various heart muscle preparations are widely thought to be triggered by spontaneous release of Ca2+ from the sarcoplasmic reticulum (SR). Here, we report undamped propagated oscillations that occur in the absence of SR. In single cardiac myofibrils treated with Triton X-100 to remove SR and held isometrically, partial activation initiated periodic fluctuations of sarcomere length persisting up to 1 hour. Oscillation characteristics could be readily quantitated by virtue of the small size of the preparation. In an individual sarcomere, the oscillation cycle generally consisted of a slow shortening phase, followed by a phase of rapid lengthening. Oscillations usually propagated along the myofibril--frequently along the entire specimen--in a wavelike fashion (average velocity, 12.3 microns/s at 10 degrees C; Q10, approximately 1.3). The oscillation period was 2.30 and 1.72 seconds at 10 degrees and 20 degrees C, respectively, and was insensitive to stretch. The average oscillation amplitude, which was temperature independent, decreased with stretch from more than 20% of the mean sarcomere length at lengths below 2 microns to zero beyond a sarcomere length of 3 microns. Stiffening of the Z line by labeling with anti-alpha-actinin resulted in a dose-dependent decrease of oscillation amplitude, while the period was not affected. Tension oscillations could not be detected in single myofibrils but were frequently detectable in myofibril doublets, where the oscillation magnitude (approximately 1 microgram) was above the noise floor. Addition of 10 mumol/L ryanodine to the activating solution did not alter oscillation characteristics, as expected, since the oscillations are unrelated to SR calcium release. On the basis of our results, we consider a mechanism for the oscillations in which a length dependence of myofibrillar Ca2+ sensitivity and a dynamic Z-line structure are essential.
Contractile and morphological impairment of cultured fetal mouse myocytes induced by oxygen radicals and oxidants. Correlation with intracellular Ca2+ concentration.
Authors: Nakamura T Y , Goda K , Okamoto T , Kishi T , Nakamura T , Goshima K
Published: June 2012

There is evidence that reperfusion injury of cardiac tissue may be caused by the generation of oxygen-derived free radicals and oxidants and by the induction of intracellular calcium overload, although the relation between these two mechanisms of injury is uncertain. In addition, the relation between the types of cellular injury and specific active species is unclear. In an attempt to resolve these problems, we investigated the effects of oxygen radicals and oxidants, which are purportedly generated during reperfusion after prolonged ischemia, and various antioxidants on contractility and morphology of cultured fetal mouse cardiac myocytes. Xanthine oxidase in the presence of xanthine, H2O2, HOCl, and NH2Cl induced cessation of spontaneous beating followed by cessation of electrical stimulation-elicited beating but did not induce an increase in [Ca2+]i. After prolonged incubation with xanthine oxidase + xanthine and H2O2, the cardiac myocytes showed morphological degeneration (at least 80% of the cells developed hypercontraction) with a concomitant increase in [Ca2+]i. These observations suggest that contractile impairment does not result in an increase of [Ca2+]i, but hypercontraction does. Catalase, but not superoxide dismutase, protected the cultured cardiac myocytes against xanthine oxidase + xanthine- and H2O2-induced contractile and morphological impairment. In the light of this observation, we hypothesize that the superoxide anion is not responsible for these types of impairment. Addition of dimethylthiourea (an .OH scavenger) and intracellular preloading with deferoxamine (an iron chelator) protected the myocytes against H2O2-induced contractile and morphological damage, but intracellular preloading with iron enhanced it. These observations led us to hypothesize that intracellularly generated .OH may be a mediator of H2O2-induced injury to cultured cardiac myocytes. In addition, we observed that H2O2 itself induced cessation of spontaneous but not electrical stimulation-elicited beating.
Active oxygen species play a role in mediating platelet aggregation and cyclic flow variations in severely stenosed and endothelium-injured coronary arteries.
Authors: Yao S K , Ober J C , Gonenne A , Clubb F J , Krishnaswami A , Ferguson J J , Anderson H V , Gorecki M , Buja L M , Willerson J T
Published: June 2012

A canine model with cyclic flow variations (CFVs) in stenosed and endothelium-injured coronary arteries was used to examine the role of active oxygen species in platelet aggregation in vivo. We studied 90 anesthetized dogs in which the pericardial cavity was opened and the heart was exposed. The velocity of blood flow in the left anterior descending coronary artery (LAD) was monitored by a pulsed Doppler flow probe. In 67 dogs, the LADs were stenosed by applying external constrictors at the site where the endothelium was mechanically injured. CFVs developed in all 67 dogs. Treatment with the antioxidants recombinant human copper-zinc superoxide dismutase (r-h-CuZnSOD), recombinant human manganese superoxide dismutase (r-h-MnSOD), and catalase eliminated platelet aggregation-associated coronary CFVs in 63%, 62%, and 64% of animals, respectively. Intravenous infusion of epinephrine restored CFVs in most dogs. Ketanserin, a serotonin (5-hydroxytryptamine2) receptor antagonist, abolished epinephrine-restored CFVs and eliminated CFVs in dogs in which CFVs had not been eliminated by free radical scavengers. In an additional 23 dogs, the LADs were stenosed but not mechanically injured. For control studies, saline was infused into the LADs of 5 dogs. Xanthine/xanthine oxidase was infused into the LADs of 8 dogs and induced CFVs in 4. Hydrogen peroxide was infused into the other 10 dogs and induced CFVs in 9. Histological analysis of the coronary artery revealed that the intima was significantly injured by the infusion. In ex vivo platelet aggregation studies, the in vivo treatment with r-h-CuZnSOD, r-h-MnSOD, and catalase significantly inhibited platelet aggregation induced by platelet-activating factor. Thus, active oxygen species are involved in mediating platelet aggregation and cyclic flow variations in stenosed and endothelium-injured canine coronary arteries in vivo.
Subunit-dependent modulation of recombinant L-type calcium channels. Molecular basis for dihydropyridine tissue selectivity.
Authors: Welling A , Kwan Y W , Bosse E , Flockerzi V , Hofmann F , Kass R S
Published: June 2012

At least four calcium channel subtypes (P, T, N, and L) have now been classified on the basis of their biophysical and/or pharmacological properties. L-type channels, a channel family particularly important to physiological function of the cardiovascular system, are identified by their slow voltage- and calcium-dependent inactivation as well as their sensitivity to dihydropyridine (DHP) calcium channel antagonists. In this study, we report the results of experiments in which we have measured the DHP modulation of recombinant calcium channel activity in cells transfected with alpha 1 subunits of cardiac and smooth muscle L-type calcium channels. We find subunit-dependent differences in the voltage and concentration dependence of channel modulation. Our results provide evidence for a molecular basis for DHP sensitivity of heart and smooth muscle calcium channels and, additionally, indicate that, even within one family of calcium channels, slight differences in channel structure can cause marked differences in channel pharmacology.
Depressed beta-adrenergic receptor- and endothelium-mediated vasodilation in conscious dogs with heart failure.
Authors: Kiuchi K , Sato N , Shannon R P , Vatner D E , Morgan K , Vatner S F
Published: June 2012

Peripheral vasodilator responsiveness was examined in pacing-induced heart failure (HF) in 11 conscious dogs chronically instrumented for measurement of systemic (total peripheral resistance [TPR]) and local (iliac blood flow) vascular resistance. Dose responses to isoproterenol (ISO), acetylcholine (ACh), and nitroglycerin (NTG) were examined in the same dogs before pacing (control) and after 4 to 7 weeks of rapid ventricular pacing, which induced congestive HF, characterized by increased left ventricular end-diastolic pressure (6.7 +/- 0.4 [control] versus 28 +/- 1.5 [HF] mm Hg) and decreased cardiac output (-30 +/- 5%) and left ventricular dP/dt (-53 +/- 3%), as well as ascites and peripheral edema. In the control state, TPR fell by 57 +/- 2% in response to ISO (100 ng/kg), by 61 +/- 3% in response to ACh (3 micrograms/kg), and by 55 +/- 2% in response to NTG (10 micrograms/kg). In HF, smaller decreases (P < .05) in TPR were observed with the same doses of ISO (-50 +/- 2%) and ACh (-49 +/- 2%) but not with NTG (-58 +/- 3%). Depressed responses to systemic ISO and ACh, but not NTG, were observed in HF in the presence of ganglionic blockade and also after local administration of smaller doses of the drugs in the absence of ganglionic blockade, but where systemic effects were not elicited. Inhibition of nitric oxide synthase increased TPR to a greater degree before HF (+154 +/- 28% [control]) than after (+80 +/- 22% [HF]) and eliminated the depressed responses to ACh but not to ISO. beta-Adrenergic receptor density, as determined by 125I-cyanopindolol binding in membrane preparations from mesenteric vessels was significantly decreased after HF (130 +/- 3 [control] versus 100 +/- 8 [HF] fmol/mg, P < .05) without any change in affinity. Thus, peripheral vascular beta-adrenergic receptor downregulation occurs in HF, independent of altered endothelium-mediated peripheral vasodilation.
Assessment of smooth muscle contribution to descending thoracic aortic elastic mechanics in conscious dogs.
Authors: Barra J G , Armentano R L , Levenson J , Fischer E I , Pichel R H , Simon A
Published: June 2012

Early investigators found contradictory evidence that vascular smooth muscle activation reduces the elastic modulus of the arterial wall under isotonic conditions but increases it under isometric conditions, concomitant with increased pulse-wave velocity. We examined the individual contributions of aortic constituents to the elastic modulus of the aortic wall to determine if isobaric analysis produces an accurate assessment of vascular smooth muscle activation. We used a modified Maxwell model assuming an incremental elastic modulus (Einc) composed of the elastic modulus of elastin fibers (EE), the elastic modulus of collagen fibers (EC) affected by the fraction of collagen fibers (fC) recruited to support wall stress, and the elastic modulus of the vascular smooth muscle (ESM) according to the following formula: Einc = EE+EC x fC+ESM.Einc was assessed in eight conscious dogs using descending thoracic aortic pressure (microtransducer) and diameter (sonomicrometry) measurements. Stress-strain relations in the control state and during activation of smooth muscle by continuous administration of phenylephrine (5 micrograms.kg-1 x min-1) were obtained by transient occlusions of the descending aorta and inferior vena cava. Results were as follows: EE was 4.99 +/- 1.58 x 10(6) dynes/cm2 (mean +/- SD), and EC was 965.8 +/- 399.8 x 10(6) dynes/cm2, assessed during the control state. Phenylephrine administration increased the theoretical pulse-wave velocity (Moens-Korteweg equation) from 5.25 +/- 1.03 m/s during the control state to 7.57 +/- 2.53 m/s (P < .005). Active muscle exhibited a unimodal stress-strain curve with a maximum stress of 0.949 +/- 0.57 x 10(6) dynes/cm2 at a corresponding strain value of 1.299 +/- 0.083. The maximum value observed corresponded, on the pressure-diameter curve of the active artery, to a pressure of 234.28 +/- 46.6 mm Hg and a diameter of 17.94 +/- 1.6 mm. The maximum ESM derived from the stress-strain relation of the active muscle was 8.345 +/- 7.56 x 10(6) dynes/cm2 at a strain value of 1.283 +/- 0.079. This point was located at 208.01 +/- 40.8 mm Hg and 17.73 +/- 1.41 mm on the active pressure-diameter curve. During activation of vascular smooth muscle, Einc decreased (P < .05) when plotted against internal pressure but increased (P < .05) when plotted against strain, over the operative range.(ABSTRACT TRUNCATED AT 400 WORDS)
Differences in cardioprotective efficacy of adrenergic receptor antagonists and Ca2+ channel antagonists in an animal model of dilated cardiomyopathy. Effects on gross morphology, global cardiac function, and twitch force.
Authors: Glass M G , Fuleihan F , Liao R , Lincoff A M , Chapados R , Hamlin R , Apstein C S , Allen P D , Ingwall J S , Hajjar R J
Published: June 2012

Turkey poults fed furazolidone (Fz) in high concentrations (700 ppm) develop dilated cardiomyopathy (Fz-DCM). We tested whether five cardioactive agents were cardioprotective in this model of heart failure, ie, whether they prevented dilatation and wall thinning and improved contractile performance. We compared the effects of chronic administration of a beta 1-selective and a nonselective beta-receptor antagonist, an alpha-receptor antagonist, and two Ca2+ channel antagonists in the presence of Fz administration. The greatest cardioprotection was found with treatment with either propranolol or nifedipine. At the gross morphological level, the effect of propranolol (a nonselective beta-adrenergic antagonist) was greater than the effect of atenolol (a selective beta 1-adrenergic antagonist), and the effect of nifedipine was greater than that of verapamil (Ca2+ channel antagonists), with all agents more cardioprotective than phenoxybenzamine (an alpha 1-adrenergic > alpha 2-adrenergic antagonist). Differences in cardioprotective efficacy of each agent increased with increased concentration. These data indicate that the dose and choice of a specific type of Ca2+ channel antagonist or beta-receptor antagonist might be important in the treatment of dilated cardiomyopathy. All agents that were cardioprotective caused similar functional improvements at both the whole heart and isolated muscle levels. Compared with control animals, Fz-DCM animals showed a significant reduction in peak left ventricular (LV) developed pressure (92 +/- 17 versus 143 +/- 24 mm Hg, P < .05), +dP/dt (1151 +/- 219 versus 2454 +/- 549 mm Hg/s), and -dP/dt (1128 +/- 291 versus 1875 +/- 396 mm Hg/s), with a significant increase in LV end-diastolic volumes (2.8 +/- 0.7 versus 0.16 +/- 0.1 mL for control animals, P < .05). In contradistinction, LV + dP/dt and -dP/dt values for animals receiving Fz plus a cardioactive agent that demonstrated cardioprotection were not significantly different from control values. Peak LV developed pressures were also similar for Fz animals receiving an agent that demonstrated cardioprotection and control animals not receiving any pharmacologic agent. Isolated muscles from Fz-DCM animals as well as animals receiving Fz plus cardioprotective pharmacologic agents responded normally with regard to increasing extracellular Ca2+ concentrations. Peak twitch forces were greater for animals receiving cardioprotective agents plus Fz than control animals not receiving any pharmacologic agents or Fz alone. At higher stimulation rates, Fz-DCM muscles demonstrated a significantly reduced peak twitch force (4 +/- 0.5 versus 1.5 +/- 0.4 g/mm2 for control muscles versus Fz-DCM muscles, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)
Vascular smooth muscle response to ouabain. Relation of tissue Na+ to the contractile response.
Authors: Stewart L , Hamilton C , Ingwall J , Naomi S , Graves S , Canessa M , Williams G , Hollenberg N
Published: June 2012

Smooth muscle responses to Na+ pump inhibition are thought to reflect two elements: a neurogenic contribution, involving catecholamine release from nerve terminals, and a myogenic response, attributed to relations between pump activity, [Na+]i, and [Ca2+]i. In the present study, we describe the time course and magnitude of cell Na+ changes, assessed by two methods, atomic absorption and nuclear magnetic resonance spectroscopy during the myogenic contractile response of rabbit aorta strips to ouabain. A threshold concentration of 3 x 10(-7) mol/L induced a gradual rise in [Na+]i. Both methods showed an essentially identical monotonic rise over 4 to 8 hours from a baseline level of 8 to 10 mmol/L water to a peak, which was approximately fivefold higher. The neurogenic (rapid) and myogenic (delayed and gradual) contractile responses were temporally distinct. Ouabain at 10(-7) mol/L, a concentration 10- to 100-fold lower than the threshold for catecholamine-dependent rapid-onset responses, induced only a delayed and gradual contractile response, which reached a maximum at 6 to 8 hours. With 10(-6) mol/L ouabain, the delayed response of 1.6 +/- 0.2 g peaked at 7.3 +/- 1.1 hours and was sustained for 16 hours. The time course was similar to that for change in [Na+] but somewhat later. Ouabain at 10(-5) and 10(-4) mol/L induced a delayed response that was identical in magnitude but also induced an early rapid contractile response, which was prevented by reserpine or phentolamine pretreatment. These agents did not influence the delayed response.(ABSTRACT TRUNCATED AT 250 WORDS)
Importance of metabolic inhibition and cellular pH in mediating preconditioning contractile and metabolic effects in rat hearts.
Authors: de Albuquerque C P , Gerstenblith G , Weiss R G
Published: June 2012

The pathophysiological mechanisms by which brief periods of flow interruption before a prolonged ischemic period, ischemic preconditioning (IPC), increase myocardial tolerance to ischemia and improve myocardial function during reperfusion are not completely understood. To test whether short periods of metabolic inhibition in the absence of a flow reduction induce similar protective effects, we studied cardiac function and metabolism using 31P nuclear magnetic resonance spectroscopy in isolated isovolumic rat hearts. Fifteen hearts underwent IPC, consisting of two 5-minute ischemia-reperfusion cycles (IPC group); 18 hearts underwent brief metabolic inhibition by exposure to two 5-minute infusions of 10 mmol/L sodium cyanide (CN group); and 15 hearts served as controls. Subsequently all hearts were subjected to 30 minutes of total global ischemia at 37 degrees C followed by reperfusion. At the end of the ischemic period, creatine phosphate and ATP levels did not differ among the groups. Cellular pH, however, plateaued at a higher level in the CN group (6.51 +/- 0.03) and IPC group (6.12 +/- 0.06) than in the control group (5.84 +/- 0.01, P < .001). IPC and CN hearts had better functional and metabolic recovery than the control hearts. Improved contractile recovery correlated with coronary flow rates at reperfusion (r = .7, P < .001) and with pHi values at 30 minutes of ischemia (r = .8, P < .001) but not with increased ATP levels during ischemia. Additional control hearts were reperfused at 15 mL/min so as to match the flow rates of IPC and CN groups, but this did not result in improved performance. To test the hypothesis that the preconditioning effect was related to pHi during ischemia, additional IPC and CN hearts underwent the same preconditioning protocol, except that the cellular pH at the end of the ischemic period was lowered by the use of hypercarbic superfusion during ischemia or by the use of bicarbonate-free perfusate just before sustained ischemia. Both of these interventions resulted in significantly lower contractile and metabolic recoveries than those observed in other IPC and CN hearts. Therefore, the preconditioning effect does not require reduced coronary flow but can be effectively elicited by metabolic inhibition per se in this model. The protective effect is not dependent on preservation of global myocardial energy stores but, rather, on reduced acidosis during the prolonged ischemic period.
A functional ryanodine-sensitive intracellular Ca2+ store is present in vascular endothelial cells.
Authors: Ziegelstein R C , Spurgeon H A , Pili R , Passaniti A , Cheng L , Corda S , Lakatta E G , Capogrossi M C
Published: June 2012

The presence of the ryanodine receptor was recently demonstrated in vascular and endocardial endothelium, but its function has not been established. We investigated whether functional ryanodine-sensitive Ca2+ stores are present in cultured endothelial cells from rat aorta (RAECs), human aorta (HAECs), human umbilical vein (HUVECs), and bovine pulmonary artery (BPAECs) and what role these may play in intracellular Ca2+ regulation. Under resting conditions, HAECs, BPAECs, and HUVECs demonstrated a slow increase in intracellular Ca2+ (indexed by indo 1 fluorescence) on exposure to 5 mumol/L ryanodine, whereas RAECs did not. However, after an initial bradykinin exposure in RAECs, ryanodine markedly blunted the rapid increase in Ca2+ on a second exposure to bradykinin. In HUVECs, ryanodine in buffer with 1.5 mmol/L Ca2+ did not inhibit the agonist-sensitive Ca2+ increase, whereas it blunted the rapid increase in Ca2+ on histamine exposure in buffer with 5 mmol/L Ca2+, suggesting that increasing [Ca2+] enhances the binding of ryanodine to its receptor. Thus, functional ryanodine-sensitive Ca2+ stores are present in vascular endothelial cells. These appear to be involved in regulation of Ca2+ storage and release from agonist-sensitive intracellular compartments.
Myocyte cellular hyperplasia and myocyte cellular hypertrophy contribute to chronic ventricular remodeling in coronary artery narrowing-induced cardiomyopathy in rats.
Authors: Kajstura J , Zhang X , Reiss K , Szoke E , Li P , Lagrasta C , Cheng W , Darzynkiewicz Z , Olivetti G , Anversa P
Published: June 2012

To determine whether cardiac failure produced by chronic coronary artery stenosis was associated with the activation of myocyte cellular hyperplasia in the myocardium, the changes in number and size of left ventricular myocytes were measured in rats 3 months after surgery. The hypertrophied left ventricle was found to possess 44%, 32%, 49%, and 48% fewer mononucleated, binucleated, trinucleated, and tetranucleated myocytes, respectively. In contrast, the hypertrophied right ventricle contained 1.49 x 10(6) more myocytes as a result of a 2.1-fold, 1.4-fold, and 1.4-fold increase in mononucleated, binucleated, and tetranucleated myocytes. Myocyte cell volume was seen to increase 49% and 21% in left and right ventricular myocytes, respectively. The process of myocyte cellular hyperplasia in the right ventricular myocardium was accompanied by capillary proliferation, and these events were responsible for the parallel addition of newly formed cells and capillaries within the wall and mural thickening. Moreover, the in-series insertion of new myocytes contributed to right ventricular dilatation after coronary artery stenosis. In view of the fact that extensive myocardial damage and cell loss may have masked the phenomenon of myocyte cellular hyperplasia in the left ventricle, the presence of DNA synthesis in myocyte nuclei was evaluated at 3 days, 1 week, 2 weeks, 1 month, and 3 months after coronary artery stenosis. Bromodeoxyuridine (BrdU) labeling markedly increased in myocyte nuclei of both ventricles, reaching its peak at 1 and 2 weeks. BrdU labeling of nonmyocyte nuclei also increased but mostly at 2 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)
Importance of sympathetic innervation in the positive inotropic effects of bradykinin and ramiprilat.
Authors: Minshall R D , Yelamanchi V P , Djokovic A , Miletich D J , Erdös E G , Rabito S F , Vogel S M
Published: June 2012

Isolated rat left atria or right ventricular strips were electrically stimulated at a constant frequency. The amplitude of twitch contractions, thus elicited, rose as a function of stimulation intensity because of increases in the evoked release of sympathetic catecholamines. Bradykinin had no effect on contractile force in preparations paced at a minimal intensity (threshold). By contrast, bradykinin (1 nmol/L to 1 mumol/L) markedly increased twitch contractile force when the preparations were paced at a high intensity (two to three times threshold). The EC50 for the positive inotropic action of bradykinin averaged 42 nmol/L. Ramiprilat (1 mumol/L), an angiotensin I-converting enzyme/kinase II inhibitor, shifted the EC50 for bradykinin to approximately 2 nmol/L. Ramiprilat (1 mumol/L) per se also produced a modest positive inotropic effect. The effects of bradykinin and/or ramiprilate were inhibited by HOE 140 (300 nmol/L), a bradykinin B2-receptor antagonist. Propranolol (1 mumol/L), a beta-adrenoceptor blocker, abolished the effects of bradykinin. After the destruction of sympathetic nerve endings by use of 6-hydroxydopamine, bradykinin no longer exerted a positive inotropic action. Cocaine (10 micrograms/mL), an inhibitor of catecholamine reuptake, potentiated the effect of bradykinin. Bradykinin did not affect the positive inotropic response to tyramine (10 mumol/L), whereas cocaine blocked it. Furthermore, bradykinin did not modify the dose-response curves for added norepinephrine. omega-Conotoxin (100 nmol/L) inhibited the positive inotropic effect of intensified stimulation and bradykinin potentiation. Bradykinin is suggested to facilitate the evoked release of sympathetic catecholamines and thereby cause a positive inotropic effect.
Subthreshold stimulation of Purkinje fibers interrupts ventricular tachycardia in intact hearts. Experimental study with voltage-sensitive dyes and imaging techniques.
Authors: Salama G , Kanai A , Efimov I R
Published: June 2012

The effects of subthreshold stimulation (STS) delivered during right atrial pacing and ventricular tachycardia (VT) were investigated in Langendorff-perfused guinea pig hearts. The hearts were stained with a voltage-sensitive dye (RH 421) to map the propagation of optical action potentials. Sustained VT was reliably induced by 5-second trains (cycle length [CL], 25 to 50 milliseconds; duration, 0.5 to 10 milliseconds; and voltage, 2x threshold voltage) of impulses (n = 12 hearts) or a single premature beat (n = 6). The location of extrastimuli was not critical to the induction of VT, but the diameter of the heart had to be > or = 14.5 mm. During VT, heart rate increased from 200 to 600 beats per minute; action potential durations decreased from 112 to 175 milliseconds to 60 to 105 milliseconds, with no diastolic interval. Activation on the epicardium spread anisotropically, but VT decreased the "apparent" maximum conduction velocity (theta max) by 68% and altered the orientation of the major axis from beat to beat. Activation patterns and theta max measured during VT were similar to patterns recorded during direct pacing of the ventricle and indicated that Purkinje fibers no longer propelled ventricular excitation. STS (CL, 25 to 50 milliseconds; duration, 0.5 to 25 milliseconds; and voltage, 0.5x to 0.8x threshold; trains of 2.0 to 2.5 seconds) interrupted VT when applied to Purkinje fibers lining the endocardium (n = 6) but failed to interrupt VT when applied to the epicardium (n = 8). In atrial pacing, STS delivered to the endocardium increased theta max from 2.44 +/- 0.32 (mean +/- SEM) to 3.63 +/- 0.21 m/s in a local region surrounding the first activation sites (n = 4). Alternatively, VT could be terminated by reducing theta max (approximately 55%) with procainamide (10 mumol/L) (n = 6). STS terminates VT by synchronizing ventricular excitation most likely by increasing local conduction and/or improving the coupling between Purkinje and ventricular cells.
Short-term diabetes alters K+ currents in rat ventricular myocytes.
Authors: Shimoni Y , Firek L , Severson D , Giles W
Published: June 2012

The electrophysiological properties of single ventricular myocytes from control rats and from rats made diabetic by streptozotocin (STZ) injection (100 mg/kg body weight) have been investigated using whole-cell voltage-clamp measurements. Our major goal was to define the effects of diabetes on rate-dependent changes in action potential duration and the underlying outward K+ currents. As early as 4 to 6 days after STZ treatment, significant elevation of plasma glucose levels occurs, and the action potential duration increases. In both control and diabetic rats, when the stimulation rate is increased, the action potential is prolonged, but this lengthening is considerably more pronounced in myocytes from diabetic rats. In ventricular myocytes from diabetic rats, the Ca(2+)-independent transient outward K+ current (I(t)) is reduced in amplitude, and its reactivation kinetics are slowed. These changes result in a smaller I(t) at physiological heart rates. The steady-state outward K+ current (IK) also exhibits rate-dependent attenuation, and this phenomenon is more pronounced in cells from diabetic rats. These STZ-induced changes in I(t) and IK also develop when a lower dose (55 mg/kg) of STZ is used and measurements are made after 7 weeks of treatment. These electrophysiological effects are not related to the hypothyroid conditions that accompany the diabetic state, since they cannot be reversed by replacement of the hormone L-triiodothyronine to physiological levels. Direct effects of STZ could be ruled out, since preceding the STZ injection with a bolus injection of 3-O-methylglucose, which prevents development of hyperglycemia, prevents the electrophysiological changes.(ABSTRACT TRUNCATED AT 250 WORDS)
Phasic secretion of newly synthesized atrial natriuretic factor from unstimulated atrial myocytes in culture.
Authors: Iida H , Shibata Y
Published: June 2012

We have examined kinetics and composition of newly synthesized proteins secreted from cultured atrial myocytes from adult rats. Under unstimulated conditions, noncontracting cultured atrial myocytes, which were pulse-labeled for 10 minutes with [35S]methionine, rapidly released a considerable portion of newly synthesized atrial natriuretic factor (ANF) in a phasic secretion with a peak at 40 to 80 minutes of chase time. The phasic secretion almost ceased after 80 minutes of chase, after which relatively slow release of the hormone was observed. The ability to stimulate the phasic secretion with secretagogues and a marked resemblance of the radiochemical composition of released proteins in the unstimulated phasic secretion to that in stimulated secretion suggest that the proteins discharged from the cells during the phasic secretion might be derived from secretory granules. Examination of the quantitative change of intracellular ANF showed that approximately 60% of newly synthesized labeled ANF was still retained in the cells after the termination of the phasic secretion, indicating that the termination of the phasic secretion was not due to depletion of the labeled protein in the cells. These results suggest that a proportion of newly synthesized ANF was rapidly released from the unstimulated atrial myocytes via a secretory route that shares certain features with both the regulated and the constitutive secretory pathway and that a part of newly synthesized ANF is processed for rapid release while the remainder is destined for slow release or storage within the cells.
Effects of phosphorylation of troponin I and C protein on isometric tension and velocity of unloaded shortening in skinned single cardiac myocytes from rats.
Authors: Hofmann P A , Lange J H
Published: June 2012

Effects on isometric tension generation and maximum velocity of unloaded shortening after exposure to cAMP-dependent protein kinase (PKA) were investigated in rat enzymatically isolated, tritonized ventricular myocytes. Exposure of myocytes to PKA in the presence of [32P]ATP resulted in phosphorylation of troponin I and C protein. Ca2+ sensitivity of isometric tension was assessed as pCa50, ie, the [Ca2+] at which tension was 50% of maximum, and was lower after PKA treatment (pCa50 5.58) than before PKA treatment (pCa50 5.74). This suggests beta-adrenergic stimulation of the heart and subsequent increases in PKA activity and phosphorylation of troponin I and C protein lead to a significant decrease in tension-generating ability at a given submaximum [Ca2+]. Unloaded shortening velocity was determined by measuring the time required to take up various amounts of slack imposed at one end of the cardiac myocyte preparation. Unloaded shortening velocity during maximum activation was 2.88 +/- 0.11 muscle lengths per second (mean +/- SEM) before PKA exposure and 2.86 +/- 0.13 muscle lengths per second after PKA exposure. Unloaded shortening velocity during 40% of maximum activation was 1.91 +/- 0.25 muscle lengths per second before PKA exposure and 2.17 +/- 0.15 muscle lengths per second after PKA exposure. The absence of an effect of PKA on unloaded shortening velocity in skinned ventricular myocytes suggests that beta-adrenergic stimulation of myocardium either does not affect myofilament velocity of shortening or alters velocity of shortening by a non-PKA-dependent process.
Angiotensin II induces fibronectin expression associated with cardiac fibrosis in the rat.
Authors: Crawford D C , Chobanian A V , Brecher P
Published: June 2012

Fibronectin expression was studied in the heart of rats given a continuous infusion of angiotensin II (Ang II). Northern blot analysis showed that left ventricular fibronectin steady-state mRNA increased fivefold to eightfold in response to pressor doses of Ang II after 24 hours. Accumulation of immunodetectable fibronectin in the ventricles occurred after the mRNA levels increased. The changes in fibronectin expression were reversible when Ang II treatment was withdrawn. The Ang II-induced increase in fibronectin mRNA accompanied similar increases for collagen type I, collagen type IV, and atrial natriuretic factor steady-state mRNA. Interstitial and perivascular fibrosis was identified in both ventricles of angiotensin-treated rats within 3 days. In situ hybridization identified cells associated with areas of fibrosis as the principal site of fibronectin mRNA accumulation in treated animals. By comparison, normal hearts showed fibronectin expression primarily within ventricular vascular tissue and the atrial endocardium. A dose-dependent reduction of fibronectin expression followed treatment with losartan, indicating an Ang II type 1 receptor-mediated effect. Normalization of blood pressure during Ang II infusion by either hydralazine or prazosin had different effects on the level of fibronectin steady-state mRNA, indicating that blood pressure elevation was not the principal factor responsible for fibronectin induction. Concurrent administration of angiotensin-converting enzyme inhibitors with Ang II attenuated the increased fibronectin expression. Our data indicate that Ang II induces an acute fibrotic response within the heart and suggests that Ang II stimulates fibronectin expression within nonmyocytic cardiac cells by a direct action.
Characterization of muscarinic acetylcholine receptors expressed by an atrial cell line derived from a transgenic mouse tumor.
Authors: Morton M E , Brumwell C , Gartside C L , Hauschka S D , Nathanson N M
Published: June 2012

The properties of muscarinic acetylcholine receptors in the cell line MCM1, derived from an SV40 T-antigen-induced atrial tumor in a transgenic mouse, were determined. Binding studies using the nonselective muscarinic antagonist [3H]quinuclidinyl benzilate, the M1-selective antagonist pirenzepine, and the M2-selective antagonist AFDX-116 indicate that the receptors have the pharmacological properties of the cardiac (M2) receptor subtype. The receptors could be immunoprecipitated with a monoclonal antibody specific for the cardiac receptor, thus confirming the identity of the receptors expressed in these cells. The types of G proteins expressed in the cells were determined by Northern blot analyses: mRNA encoding the alpha subunits of Gs, G(o), and Gi-2, but not Gi-1 or Gi-3, were detected, consistent with previous observations of neonatal mammalian atria. The muscarinic receptors were functionally active, as demonstrated by the ability of the agonist to stimulate phosphoinositide turnover and to inhibit adenylyl cyclase activity. The availability of a mammalian atrial cell line that continues to express the appropriate functionally coupled subtype of muscarinic receptor may provide a useful system for the investigation of the regulation of expression and function of cardiac muscarinic receptors.
Stunned myocardium after rapid correction of acidosis. Increased oxygen cost of contractility and the role of the Na(+)-H+ exchange system.
Authors: Hata K , Takasago T , Saeki A , Nishioka T , Goto Y
Published: June 2012

Left ventricular (LV) contractile dysfunction during acidosis has been reported to be almost reversible in crystalloid-perfused hearts after correction of acidosis. In contrast, we have found that, in blood-perfused hearts, contractile function is paradoxically depressed after correction of acidosis with a transient overshoot of contractility during the recovery of pH. To clarify the mechanism of this phenomenon, we measured the LV contractility index (Emax) and the relation between myocardial oxygen consumption (VO2) and systolic pressure-volume area (PVA, a measure of the LV total mechanical energy) before and after induction and rapid correction of acidosis by CO2 loading (pH 7.00) and unloading in 13 excised cross-circulated canine hearts. During the rapid correction of acidosis in six control hearts, a severe transient overshoot of Emax (404% of acidosis) occurred. However, after correction of acidosis, Emax and PVA were lower than the preacidosis values by 46% (P < .01) and 44% (P < .01) at the same LV volume. When the preacidosis Emax level was restored by Ca2+ infusion, the VO2 intercept (PVA-independent VO2) of the linear VO2-PVA relation exceeded the control value by 18% (P < .05) with an unchanged slope. In addition, the oxygen cost of contractility, defined as the slope of the relation between PVA-independent VO2 and Emax, increased by 83% (P < .01) after correction of acidosis, indicating that postacidosis myocardium requires higher VO2 for nonmechanical activities for a unit increase in Emax. Then, we hypothesized that these mechanoenergetic disorders after rapid correction of acidosis would result from Ca2+ overload via accelerated Na(+)-Ca2+ exchange due to the heavily operating Na(+)-H+ exchange system at the time of rapid pH recovery. To examine this hypothesis, dimethylamiloride, a selective Na(+)-H+ exchange inhibitor, was administered just before the correction of acidosis in the other seven hearts. The administration of dimethylamiloride completely prevented both the mechanical and energetic disorders after correction of acidosis. We conclude that rapid recovery of pH paradoxically depresses myocardial contractility and increases the oxygen cost of contractility through an activation of the Na(+)-H+ exchange system.
Lactate transport in mammalian ventricle. General properties and relation to K+ fluxes.
Authors: Shieh R C , Goldhaber J I , Stuart J S , Weiss J N
Published: June 2012

Net cellular L-lactate efflux associated with accelerated anaerobic glycolysis has been implicated as a potential cause of the marked cellular K+ loss contributing to lethal cardiac arrhythmias in ischemic heart and to impaired function of fatigued skeletal muscle. To examine the mechanisms of transsarcolemmal L-lactate movement in the heart, isolated guinea pig ventricular myocytes were loaded with the fluorescent H+ or K+ indicators, carboxy SNARF-1 or PBFI, respectively, under whole-cell patch-clamp conditions. With H+ as the only permeable monovalent cation, a rapid increase in extracellular L-lactate concentration ([L-]o) from 0 to 30 mmol/L at constant pHo (7.35) caused an intracellular acidification averaging 0.18 +/- 0.02 pH units in 60 seconds (n = 7), reflecting L-lactate influx in association with H+ influx (or OH- efflux). Under voltage-clamp conditions, no significant electrogenic current was associated with H(+)-coupled L-lactate influx, and membrane potential (-75 to +75 mV) had no effect on the degree of acidification produced by 30 mmol/L [L-]o, indicating that L-lactate influx was predominantly nonelectrogenic. Acidification in response to increased [L-]o was saturable (Km, approximately 5 mmol/L), partially stereospecific for L-lactate over D-lactate, and inhibited by 55 +/- 7% and 82 +/- 7% by the monocarboxylate carrier inhibitors alpha-cyano-4-hydroxycinnamate and mersalyl acid, respectively, consistent with a carrier-mediated transport mechanism. Extracellular K+ inhibited H(+)-coupled L-lactate influx by 36 +/- 2%, suggesting that K+ either inhibited or substituted for H+ in cotransport with L-lactate. However, in myocytes loaded with PBFI, no significant increase in [K+]i was detected during exposure to 30 mmol/L [L-]o, suggesting that only a minor component, if any, of L-lactate influx was cotransported or codiffused with K+.
Reoxygenation-induced relaxation of coronary arteries. A novel endothelium-dependent mechanism.
Authors: Close L A , Bowman P S , Paul R J
Published: June 2012

Coronary artery contractility is well known to be modulated by oxygen partial pressure. Both smooth muscle and the endothelium contribute to coronary artery oxygen sensitivity. Mechanisms underlying endothelium-dependent effects of oxygen include the sensitivity of the nitric oxide/endothelium-derived relaxing factor (EDRF), hydrogen peroxide, and eicosanoid pathways. In the present study, we characterize a novel endothelium-dependent component of porcine coronary artery oxygen sensitivity that is independent of these known pathways. Porcine coronary arteries were stimulated with either KCl or U46619. Hypoxia elicited a transient increase in force that was much greater in endothelium-intact arteries. This effect was abolished by nitric oxide/EDRF pathway inhibitors NG-monomethyl-L-arginine and N-nitro-L-arginine. In the steady state, hypoxia reduced isometric force to a similar degree in both intact and denuded arteries. Reoxygenation elicited a rapid and transient relaxation only in intact arteries. In contrast, this endothelium-dependent relaxation was not inhibited by nitric oxide/EDRF pathway inhibitors nor inhibitors of other potential oxygen-sensitive pathways, such as indomethacin, aminotriazole, superoxide dismutase, catalase, propranolol, or ouabain. The reoxygenation relaxation was, however, sensitive to very low levels of oxygen and was inhibited by cyanide and rotenone, suggesting an involvement of mitochondrial metabolism. Interestingly, the relaxation response to reoxygenation, similar to that for substance P, could be restored in denuded arteries by coupling with an endothelium-intact donor artery. This "sandwich" experiment suggests that the endothelium dependence is mediated by a transmissible factor. Our results indicate that a novel class of endothelium-dependent factors may contribute to coronary artery responses to changes in oxygen partial pressure.
Activation of cardiac vagal afferents by oxygen-derived free radicals in rats.
Authors: Ustinova E E , Schultz H D
Published: June 2012

Myocardial ischemia and reperfusion can evoke excitation of cardiac vagal afferent nerve endings and activation of a cardiogenic depressor reflex (Bezold-Jarisch effect). We postulate that oxygen-derived free radicals, which are well known to be produced during prolonged ischemia and reperfusion, contribute to this excitation. Hydroxyl radicals derived from hydrogen peroxide (H2O2) activate abdominal sympathetic afferents and produce reflex excitation of the cardiovascular system. However, it is not known whether inhibitory vagal cardiac afferents are activated by oxygen-derived free radicals. We recorded activity from 52 single vagal afferent fibers in 29 rats; the endings of these fibers were located in the walls of all four chambers of the heart. Thirty-three (63%) of these fibers were classified as chemosensitive C-fiber endings because of their irregular discharge under resting conditions, their activation in response to the topical application of capsaicin (1 to 10 micrograms) to the surface of the heart encompassing the receptive field, and their conduction velocities. Fourteen (27%) of the remaining fibers were found to be mechanoreceptors. Topical application of H2O2 to the heart activated 50% of the chemosensitive endings and did not directly affect cardiac mechanoreceptors. Activity increased by 498% at a dose of 3 mumol (P < .001). This effect was reproducible and dose dependent and was not due to [H+]. Topical application of xanthine/xanthine oxidase (20 mmol/0.03 mU) activated 8 of the 12 chemosensitive fibers tested and had no direct effect on mechanosensitive fibers. Activity increased by 287% (P < .001).(ABSTRACT TRUNCATED AT 250 WORDS)
In vivo characterization of vasodilating muscarinic-receptor subtypes in humans.
Authors: Bruning T A , Hendriks M G , Chang P C , Kuypers E A , van Zwieten P A
Published: June 2012

The role of muscarinic (M)-receptor subtypes in the regulation of vascular tone has not yet been defined in humans. To analyze the role of M-receptor subtypes in the forearm resistance vasculature of normotensive volunteers (n = 20), we infused acetylcholine (ACh) and methacholine (MCh) in the presence of saline and the antagonists atropine (nonselective), pirenzepine (M1 selective), and AF-DX 116 (M2 selective), using automated R-wave-triggered venous occlusion plethysmography. Schild analysis was applied by calculating plasma concentrations of the infused compounds and determining EC50 values. ACh and MCh both caused dose-dependent vasodilation, with EC50 values of 537 and 52 nmol/L, respectively. The apparent 10-fold higher potency of MCh compared with ACh may be explained by rapid degradation of ACh in plasma. The concentration-response curve of MCh was shifted to the right by atropine, pirenzepine, and AF-DX 116, with apparent pA2 values of 8.03 +/- 0.03, 6.71 +/- 0.08, and 5.32 +/- 0.05, respectively, and slopes not different from unity. The present technique enabled us to perform M-receptor characterization by Schild analysis in humans. The affinity constants and rank order of potency--atropine > pirenzepine > AF-DX 116-suggest that cholinergic vasodilation in this vascular bed is predominantly mediated by the M3-receptor subtype. The EC50 value of MCh and the pA2 values of pirenzepine and AF-DX 116 are comparable to values reported for in vitro experiments.
Methylene blue reverses endotoxin-induced hypotension.
Authors: Keaney J F , Puyana J C , Francis S , Loscalzo J F , Stamler J S , Loscalzo J
Published: June 2012

Hypotension in septic shock is a reflection of unregulated nitric oxide (NO) production and vascular smooth muscle guanylyl cyclase activation. We examined the effect of methylene blue on lipopolysaccharide (LPS)-induced shock in anesthetized rabbits. Shock was induced with 150 micrograms/kg LPS after measurement of mean arterial pressure, platelet cGMP, and total plasma NO (nitrogen monoxide+S-nitrosothiol) content. Measurements were repeated before and after the intravenous administration of 1, 5, and 10 mg/kg methylene blue in response to a 55% reduction in mean arterial pressure. At baseline, mean +/- SEM arterial pressure was 88 +/- 3 mm Hg, which fell to 51 +/- 3 mm Hg after LPS (P < .05). Methylene blue at doses of 1, 5, and 10 mg/kg produced a prompt dose-dependent increase in mean arterial pressure to 69 +/- 2, 77 +/- 3, and 81 +/- 2 mm Hg, respectively (P < .05 versus mean arterial pressure after LPS) in association with normalization of plasma total NO content (P < .05); however, methylene blue did not significantly affect intraplatelet cGMP levels. Thus, methylene blue restores normal arterial pressure in rabbits with septic shock. This effect is associated with persistent elevation of intraplatelet cGMP levels and normalization of total plasma NO content. These data are consistent with methylene blue-mediated inhibition of NO synthase and/or degradation of NO in this model and suggest a novel therapeutic approach to the treatment of septic shock.