摘要:

The activation and inactivation relations of several ion channel currents overlap, suggesting the existence of a steady-state or “window” current. We studied L-type Ca2+channel window current in single cardiac Purkinje cells using a voltage-clamp protocol by which channels were first inactivated nearly completely during a long-duration depolarizing step, and then the recovery of Ca2+current was observed during repolarizing steps into the L-type Ca2+window voltage range. With these conditions, a small-amplitude inward Ca2+current gradually developed after repolarization to voltages within the window but not after steps to voltages positive or negative to it. Window current was suppressed by Cd2+(50 μM), nifedipine (1 μM), and nicardipine (1 μM), and it was augmented by isoproterenol (5 μM) and Bay K 8644 (1 μM). At voltages at which window current developed, L-type Ca2+channels also recovered to a closed state from which they could be reopened by an additional depolarizing step. At voltages positive to the window range, channel recovery to a closed state(s) was absent, whereas at voltages negative to the window range, channel recovery to a closed state(s) increased, as expected from the “steady-state” inactivation relation. Our results provide direct measurement of L-type Ca2+window current and distinguish it from other processes, such as slow inactivation. Our findings support the postulate that within a window there occur channel transitions from inactivated to closed states, and these channels (re)open, and this process may occur repetitively. Some physiological and pathophysiological roles for L-type Ca2+window current are discussed. (Circulation Research1992;70:445–455)

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Canine cardiac Purkinje cells contain both L- and T-type calcium currents, yet the single Ca2+channels have not been characterized from these cells. Additionally, previous studies have shown an overlap between the steady-state inactivation and activation curves for L-type Ca2+currents, suggesting the presence of L-type Ca2+“window” current. We used the on-cell, patch-clamp technique to study Ca2+channels from isolated cardiac Purkinje cells. Patches contained one or more Ca2+channels 75% of the time. L-type channels were seen in 69% and T-type channels in 73% of these patches. With 110 mM Ba2+as the charge carrier, the conductances of the L- and T-type Ca2+channels were 24.2±0.8 pS (n=9) and 9.0±0.5 pS (n=8), respectively (mean±SEM). With 110 mM Ca2+as the charge carrier, the conductance of the L-type Ca2+channel decreased to 9.7±1.2 pS (n=4), whereas the T-type Ca2+channel conductance was unchanged. Voltage-dependent inactivation was shown for both L- and T-type Ca2+channels, although for L-type Ca2+channel with Ba2+as the charge carrier, inactivation took at least 30 seconds at a potential of +40 mV. After channel inactivation was complete, L-type Ca2+channel reopenings were observed following repolarizing steps into the window voltage range. Thus, our data identify both L- and T-type Ca2+channels in cardiac Purkinje cells and demonstrate, at the single-channel level, L-type channel transitions expected for a window current. Window current may play an important role in shaping the action potential and in arrhythmogenesis. (Circulation Research1992;70:456–464)

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The goals of this study were 1) to quantitate the effects of atherosclerosis on physiological and pharmacological endothelium-dependent vasoactive responses in coronary arterioles downstream from arterial lesions and 2) to determine if administration of l-arginine, the precursor for endothelium-derived relaxing factor, would restore normal endothelium-dependent function. Dietary-induced atherosclerosis was induced in pigs, and vasomotor responses of isolated, cannulated coronary arterioles (30–70 μm in diameter) were assessed by measuring diameter changes in vitro. To assess pharmacological alterations of endothelium-dependent responses, dose-response curves were constructed to ADP, serotonin, and histamine. To assess physiological alterations in endothelial function, different flow rates were established across the vessel. Arteriolar diameters were measured in vessels from normal and atherosclerotic pigs under control conditions, after administration of l-arginine, and after endothelial denudation. In arterioles from normal pigs, administration of serotonin, histamine, or ADP produced dose-dependent vasodilation, which was abolished by endothelial denudation. In arterioles from atherosclerotic pigs, administration of histamine, serotonin, and ADP produced dilation at only the highest doses (10−6-10−7M), and the extent of dilation was only 20–30% of that observed in arterioles from normal pigs. Initiation of flow also produced vasodilation in arterioles from normal pigs that was completely abolished after endothelial denudation. In arterioles from atherosclerotic pigs, flow-induced responses were absent. These abnormal physiological and pharmacological responses (i.e., blunted vasodilation to pharmacological stimulation and to flow) were restored after administration of l-arginine for 40 minutes. The vascular responses after administration of l-arginine were not different from those observed under control conditions in arterioles from normal pigs. In addition, l-arginine did not restore vasodilation to the endothelium-dependent agonists in denuded segments. From these data in arterioles downstream from atherosclerotic lesions, we conclude that 1) the ED50and maximal responses of endothelium-dependent vasodilation to ADP, histamine, and serotonin are attenuated; 2) the physiological response to flow, that is, flow-mediated endothelium-dependent vasodilation, is absent; and 3) the abnormality in arteriolar responsiveness during large vessel disease involves an impairment of the synthesis and/or release of endothelium-derived relaxing factor. (Circulation Research1992;70:465–476)

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

xtracellular ATP modulates cardiac contraction through P2-purinoceptors on cardiac myocytes. To elucidate the molecular mechanism of this response, we examined the effects of P2-purinoceptor activation on phosphoinositide (PI) hydrolysis and the cAMP system in cultured ventricular myocytes of fetal mice. In a concentration-dependent manner, ATP stimulated accumulations of [3H]inositol monophosphate, bisphosphate, and trisphosphate with the half-maximum effective concentration of ∼1 μM in the myocytes labeled with [3H] inositol. The order of efficacy of a series of adenyl compounds for stimulation of PI hydrolysis was adenosine 5′-O-(3-thiotriphosphate) (ATPγS), ATP>ADP, 5′-adenylylimidodiphosphate (APPNP) > α,β-methyleneadenosine 5′-triphosphate (APCPP) > β,γ-methyleneadenosine 5′-triphosphate, AMP > adenosine. On the other hand, 100 μM ATPγS inhibited isoproterenol-induced accumulation of cAMP by ∼70% without decreasing the time to maximal cAMP levels, as measured by radioimmunoassay. This response was also concentration dependent, with a half-maximum inhibitory concentration (IC50) of ∼1 μM. All of the tested ATP, ADP, and ATP analogues inhibited the cAMP system, and the responses to ATPγS, APPNP, and APCPP were insensitive to an A1-purinoceptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine. Pertussis toxin attenuated the ATP-induced PI hydrolysis by no more than 25% at 100 ng/ml but completely suppressed the ATPγS-induced inhibition of the cAMP system. Protein kinase C-activating phorbol ester, 4β-phorbol 12β-myristate 13α-acetate, inhibited the ATP-induced PI hydrolysis with an IC50of ∼1 nM and also attenuated the ATPγS-induced inhibition of the cAMP system at ≥1 nM, although a biologically inactive phorbol ester, 4α-phorbol 12,13 -didecanoate, did not. From these data, P2-purinoceptor activation stimulates PI hydrolysis by activating phospholipase C primarily through pertussis toxin-insensitive G proteins and attenuates cAMP accumulation by inhibiting adenylate cyclase through pertussis toxin-sensitive G proteins. Protein kinase C is likely to negatively regulate both the signal transduction systems. (Circulation Research1992;70:477–485)

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

lthough numerous studies have implicated accelerated phospholipid catabolism during myocardial ischemia as an important contributor to ischemic membrane dysfunction, no information is currently available on the subcellular distribution, physical properties, or kinetic characteristics of human myocardial phospholipase A2. In this report, we demonstrate that the overwhelming majority (98%) of total phospholipase A2activity in human myocardium (obtained from transplant recipients) is calcium independent, plasmalogen selective, and is distributed between the microsomal (60–70% of total activity) and cytosolic (30–40% of total activity) fractions. Both human myocardial microsomal and cytosolic phospholipase A2enzymes 1) preferentially hydrolyze plasmalogen molecular species containing arachidonic acid at thesn-2 position, 2) are recalcitrant to chemical inactivation by the indole-reactive agent parabromophenacyl bromide, 3) are irreversibly inhibited by covalent modification of an essential thiol residue by 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB), and 4) are exquisitely sensitive to mechanism-based inhibition by (E)-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one (bromoenol lactone). In sharp contrast, human mitochondrial phospholipase A21) accounts for only a diminutive amount of total myocardial phospholipase A2activity (1–2%), 2) is augmented by calcium ion, 3) exhibits a higher reaction velocity using phosphatidylcholine in comparison with plasmenylcholine substrate, and 4) is not substantially inhibited by either DTNB or bromoenol lactone. Collectively, these results demonstrate that the majority of phospholipase A2activity in human myocardium is catalyzed by a novel class of calcium-independent plasmalogen-selective phospholipases A2and underscore the potential importance of this class of enzymes in mediating membrane dysfunction during myocardial infarction in humans. (Circulation Research1992;70:486–495)

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Positron emission tomography offers the ability to noninvasively assess regional myocardial perfusion in absolute terms (i.e., milliliters per gram per minute). Accurate estimates have been difficult to achieve with generator-produced82Rb because of the complex behavior of this tracer in the myocardium. The aim of the present study was to determine whether regional myocardial blood flow could be assessed quantitatively with82Rb and positron emission tomography by using a two-compartment kinetic model. Regional perfusion in milliliters per gram per minute was estimated from dynamic tomographic scans after intravenous administration of82Rb in 18 studies in 13 intact dogs studied without intervention, after 2 and 24 hours of induced ischemia, during reperfusion after transient occlusion, or at rest and after pharmacological hyperemia after induced coronary artery stenosis. Regional flow was estimated along with the forward and backward rates of transport (k1and k2[minutes−1]) after the relative volume of distribution of the first compartment was fixed to 0.53 ml/ml and the tomographic parameters, the recovery and spillover fractions, were fixed to averaged values obtained in previous studies. In 36 comparisons, estimates of regional flow with82Rb correlated well with flow measured with concomitantly administered radiolabeled microspheres (r=0.91,p<0.05) over the flow range from 0.14 to 4.25 ml/g/min. A putative index of viability, k2, increased significantly in regions with severe ischemia. The results suggest that quantification of regional myocardial perfusion is possible in centers using82Rb for estimates of myocardial perfusion when a physiologically appropriate, two-compartment model is used. (Circulation Research1992;70:496–507)

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

hole-cell calcium current (ICa) and inwardly rectifying potassium current (IK1) were studied in 21-day fetal, 28-day fetal (total gestation, 31 days), 2–5-day neonatal, and adult rabbit ventricular myocytes isolated by enzymatic dissociation. Whole-cell peak ICaand IK1at −100 mV increased significantly after birth. Cell size approximated from cell membrane capacitance also increased with age, with the most significant increase occurring after birth. When normalized to cell surface area, peak ICa. density increased from day 21 of gestation to the neonate and then increased again from neonate to adult. In all age groups, peak ICaoccurred at a test potential of +10 mV, and the shape of the Ca2+current-voltage relation did not change with age. These findings suggest that there are no significant developmental changes in the voltage dependence of ICa-Therefore, the measured age-related increase in Ca2+current density may result from increased channel expression. IK1also exhibited a pattern of increasing current density with age. For IK1, the increase in current density was most rapid between day 21 and the perinatal period and much slower after birth. These results demonstrate that ICaand IK1undergo significant changes during late fetal and postnatal development. (Circulation Research1992;70:508–515)

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The contributions of changes in primary systolic and diastolic properties, limitations of contractile reserve, and alterations in energy efficiency to the left ventricular dysfunction seen with chronic pacing tachycardia were investigated. Seven dogs (heart failure group) were ventricularly paced at 250 beats per minute for 26.3±2.9 days and compared with a separate control group (n=8). Studies were performed with isolated, metabolically supported hearts coupled to a computer-controlled loading system. Pressure-volume relations and myocardial oxygen consumption (MVo2) were measured to assess chamber systolic and diastolic properties and efficiency (relation between MVo2and pressure-volume area [PVA]). Systolic function was reduced in failure hearts versus controls as assessed by the slope of the end-systolic pressure-volume relation (1.29±0.94 versus 2.71±0.98 mm Hg/ml,p<0.01) and lowered end-systolic stiffness at a matched stress (956.1±123.5 versus 1,401.7±431.7 g/cm2,p< 0.05). Diastolic chamber and myocardial stiffness were unaltered in failure hearts, but the unstressed diastolic-arrested volume was significantly larger (33.3±3.9 versus 21.9±7.6 ml,p< 0.01). Inotropic response to increased heart rate and exogenous β-adrenergic stimulation (dobutamine HCI) was significantly impaired in failure compared with control hearts. Most interestingly, failure hearts had a lowered slope of the MVo2-PVA relation (2.1±1.1 versus 2.9±1.4 ml O2· mm Hg−1· ml−l· 100 g left ventricle−1,p<0.001), indicating increased effliciency of chemomechanical energy conversion. The y intercept of the MVo2-PVA relation, which reflects oxygen costs of basal metabolism and excitation-contraction coupling, was unchanged in the two groups despite decreased contractility of the heart failure hearts. These results demonstrate reduced chamber and myocardial contractility, dilatation without alteration of passive myocardial properties, impaired contractile reserve, and novel alterations in cardiac efficiency in this model of heart failure. (Circulation Research1992;70:516–529)

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

Animal studies have shown that the power output of the left ventricle is related to afterload by a bell-shaped curve. Furthermore, the normal ventricle operates at the maximum power point, whereas the diseased ventricle operates off the maximum power point because of increased afterload. We studied this relation in eight patients with dilated cardiomyopathy. A high-fidelity catheter with micromanometer pressure and electromagnetic velocity transducers was used to measure blood pressure and flow velocity in the ascending aorta. The latter was converted into volumetric flow by calibrating with the simultaneously determined thermodilution cardiac output. Ventricular power was calculated by dividing the integral of the aortic blood pressure-flow product by the duration of the cardiac cycle. Intravenous nitroprusside was used to alter afterload and the power-afterload relation was obtained by plotting power against mean aortic blood pressure. In all patients, as blood pressure was lowered initially, the power output of the left ventricle increased. As the dose of nitroprusside was increased further, the total power either plateaued or actually decreased. The averaged power and the mean blood pressure at control were 1.00±0.62 W and 92±9.3 mm Hg, respectively. The averaged maximum increase in power with nitroprusside was 22%, to 1.22±0.73 W, and this occurred at a mean pressure of 80±8.8 mm Hg. This study showed that the power-afterload relation in the human ventricle has a maximum power point at some intermediate level of afterload, similar to that found in animal studies. Instead of operating at the maximum power point, the failing ventricle operates far to the right of the peak because of inappropriately elevated afterload. This mismatch between the left ventricle and the arterial circulation resulted in a submaximal transfer of power. Afterload reduction improves power output by an improvement in matching between the left ventricle and the periphery. (Circulation Research1992;70:530–535)

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID

摘要:

The patch-clamp technique was used to study the electrophysiological properties of single smooth muscle cells obtained from the human cystic artery. These cells contracted on exposure to high K+and had a mean resting potential of −36±7 mV. Under current clamp, regenerative responses could not be elicited when depolarizing pulses were applied. Voltage-clamp measurements demonstrated that a large fraction of the outward current was inhibited by tetraethylammonium (5–10 mM) or Ca2+channel blockers and that it was enhanced by increasing [Ca2+], suggesting that it is a Ca2+-activated K+current. In addition, spontaneous transient outward currents that were sensitive to extracellular Ca2+were observed in some cells. In cell-attached patch-clamp recordings, Ca2+-activated K+channels that had a conductance of 117 pS were consistently identified. At negative potentials (approximately −60 mV), these single-channel events deactivated completely and very quickly, suggesting that they do not control the resting membrane potential in healthy cystic artery cells. Ca2+currents that were recorded using Ba2+(10 mM) as the charge carrier were enhanced by the dihydropyridine agonist, Bay K 8644, and blocked by nifedipine (0.1 μM). Only one type of Ca2+current, the L-type, could be identified in these cells. These results demonstrate that the major ionic currents in the human cystic artery are similar to other mammalian arteries and indicate that this tissue will be a useful model for studying the metabolic and pharmacological modulation of ionic currents in human vascular smooth muscle. (Circulation Research1992;70:536–545)

ISSN:0009-7330    

出版商:OVID    

年代:1992

数据来源: OVID