Sarcolemmal L-type Ca2 Research Articles

Effects of volatile anesthetics on sarcolemmal calcium transport and sarcoplasmic reticulum calcium content in isolated myocytes.

The surface membrane Ca(2+)-adenosine triphosphatase and Na(+)-Ca(2+) exchanger transport Ca(2+) out of the ventricular myocyte, competing for cytosolic Ca(2+) with the Ca(2+)-adenosine triphosphatase located in the sarcoplasmic reticulum. In this study the authors examined the effects of halothane, isoflurane, and sevoflurane on Ca(2+) extrusion from the cell and sarcoplasmic reticulum Ca(2+) content. Single myocytes from the right ventricular free wall of adult male ferret hearts were isolated, loaded with the acetoxymethyl ester of the fluorescent Ca(2+) indicator fluo-3, and electrically stimulated at 0.25 Hz to reach a steady state level of intracellular Ca(2+) stores. The effects of halothane, isoflurane, and sevoflurane (1 minimum alveolar concentration) on the peak and rate of decline of the Ca(2+) transient induced by 10 mm caffeine were examined. The peak was used as an index of sarcoplasmic reticulum Ca(2+) content, and the rate of decline was used to monitor Ca(2+) extrusion from the cell. During control conditions, halothane reduced the Ca(2+) content of the sarcoplasmic reticulum, isoflurane maintained it, and sevoflurane caused it to increase. Halothane did not affect Ca(2+) extrusion from the cell, but both isoflurane and sevoflurane inhibited it. When Na(+)-Ca(2+) exchange was inhibited by ionic substitution, isoflurane and sevoflurane still reduced the rate of Ca(2+) efflux from the cell. However, when the sarcolemmal Ca(2+)-adenosine triphosphatase was inhibited by carboxyeosin, isoflurane and sevoflurane had no effect on Ca(2+) efflux. These results suggest that isoflurane and sevoflurane inhibit Ca(2+) transport from the cell via the sarcolemmal Ca(2+)-adenosine triphosphatase. This effect seems to counteract the decrease in Ca(2+) influx through sarcolemmal L-type Ca(2+) channels and maintains sarcoplasmic reticulum Ca(2+) stores.

Altered regulation of cardiac contraction and relaxation by Ca2+ in heart failure

Myocardial contractile dysfunction in congestive heart failure is characterized by a decrease in force developed and retardation of relaxation. These alterations are mainly due to those in intracellular Ca(2 +) transients (CaT) . CaT are regulated by a number of functional proteins, including sarcolemmal L-type Ca(2 +) channels, Na(+)/Ca(2 +) exchanger and Ca(2 +) ATPase, sarcoplasmic reticulum Ca(2 +) ATPase (SERCA 2 ) , phospholamban and ryanodine receptors, and mitochondrial Ca(2 +) uniporter. Changes in expression and function of these regulatory proteins that occur in the course of increasing severity of heart failure are responsible for the characteristic changes in force development and relaxation observed under pathophysiological conditions in congestive heart failure.

G i-Dependent Localization of β2-Adrenergic Receptor Signaling to L-Type Ca 2+ Channels

A plausible determinant of the specificity of receptor signaling is the cellular compartment over which the signal is broadcast. In rat heart, stimulation of β 1-adrenergic receptor ( β 1-AR), coupled to G s-protein, or β 2-AR, coupled to G s- and G i-proteins, both increase L-type Ca 2+ current, causing enhanced contractile strength. But only β 1-AR stimulation increases the phosphorylation of phospholamban, troponin-I, and C-protein, causing accelerated muscle relaxation and reduced myofilament sensitivity to Ca 2+. β 2-AR stimulation does not affect any of these intracellular proteins. We hypothesized that β 2-AR signaling might be localized to the cell membrane. Thus we examined the spatial range and characteristics of β 1-AR and β 2-AR signaling on their common effector, L-type Ca 2+ channels. Using the cell-attached patch-clamp technique, we show that stimulation of β 1-AR or β 2-AR in the patch membrane, by adding agonist into patch pipette, both activated the channels in the patch. But when the agonist was applied to the membrane outside the patch pipette, only β 1-AR stimulation activated the channels. Thus, β 1-AR signaling to the channels is diffusive through cytosol, whereas β 2-AR signaling is localized to the cell membrane. Furthermore, activation of G i is essential to the localization of β 2-AR signaling because in pertussis toxin-treated cells, β 2-AR signaling becomes diffusive. Our results suggest that the dual coupling of β 2-AR to both G s- and G i-proteins leads to a highly localized β 2-AR signaling pathway to modulate sarcolemmal L-type Ca 2+ channels in rat ventricular myocytes.

Propofol Inhibits Ca2+ Transients but Not Contraction in Intact Beating Guinea Pig Hearts

We investigated whether propofol inhibits Ca(2+) transients and left ventricular pressure (LVP) in intact beating guinea pig hearts at clinical concentrations and whether an inhibition of Ca(2+) transients by propofol results from an impairment of sarcolemmal or of sarcoplasmic reticulum (SR) function. By using a Langendorff's preparation, transmural left ventricular phasic intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured by the fluorescence ratio of indo-1 emission at 385 nm and 456 nm and was calibrated to Ca(2+) transients (in nM). The Ca(2+) transients during each contraction were defined as available [Ca(2+)](i). Sixty hearts were perfused with modified Krebs-Ringer's solution containing lipid vehicle and propofol (1 and 10 microM) in the absence and presence of ryanodine, thapsigargin, and nifedipine, while developed LVP and available [Ca(2+)](i) were recorded. Propofol (10 microM) decreased available [Ca(2+)](i) by 11.0% +/- 1.3% without decreasing developed LVP (% of control, P < 0.05). Propofol (10 microM) caused a leftward shift in the curve of developed LVP as a function of available [Ca(2+)](i). Propofol (10 microM) with nifedipine (1 microM), but not with ryanodine (1 microM) or thapsigargin (1 microM), decreased available [Ca(2+)](i) by 15.5% +/- 1.7% (P < 0.05). Propofol decreases available [Ca(2+)](i) without decreasing cardiac contraction, and it enhances myofilament Ca(2+) sensitivity in intact beating hearts at clinical concentrations. The inhibition of available [Ca(2+)](i) by propofol may be mainly mediated by an impairment of sarcoplasmic reticulum Ca(2+) handling rather than the sarcolemmal L-type Ca(2+) current. This is the first study of the effects of propofol on intracellular Ca(2+) concentration and myofilament Ca(2+) sensitivity under physiologic conditions in intact isolated beating guinea pig hearts.

The effect of ryanodine on isometric tension development in isolated ventricular trabeculae from Pacific mackerel ( Scomber japonicus)

An isometric muscle preparation was used to study the inhibitory effect of ryanodine on contractile function in isolated ventricular trabeculae of the Pacific mackerel ( Scomber japonicus). Ryanodine (an inhibitor of sarcoplasmic reticulum (SR) function) caused a 20% reduction in peak tension at 20°C, but not 15°C, over the range of frequencies (0.2–3.0 Hz) tested. This indicates that in the absence of a functional SR, the mackerel ventricle can maintain most of its contractile strength utilizing other modes of Ca 2+ delivery to the myofilaments. Ca 2+ flux through the sarcolemmal (SL) L-type Ca 2+-channels is most likely the predominant pathway for Ca 2+ activation of the myofilaments, although reverse mode Na +/Ca 2+ exchange could potentially contribute to a significant extent. High levels of adrenergic stimulation overwhelmed the negative inotropy caused by ryanodine, returning tension to pre-ryanodine levels, further suggesting that the mackerel ventricle can maintain contractile function without Ca 2+ contribution from the SR. These results are discussed within the context of what is known about SR Ca 2+ utilization in rainbow trout and tuna hearts.

The structure and function of the cardiac myocyte: A review of fundamental concepts

The structure and function of the cardiac myocyte: A review of fundamental concepts

Palmitoyl Carnitine Increases Intracellular Calcium in Adult Rat Cardiomyocytes

Earlier studies have demonstrated that palmitoyl carnitine (PC), a long chain acyl carnitine, accumulates in the ischemic myocardium. Although perfusion of hearts with PC is known to induce contractile dysfunction which resembles ischemic contracture, the mechanisms underlying this derangement are not clear. In this study, we examined the effect of exogenous PC on the intracellular concentration of calcium ([Ca2+]i) in freshly isolated cardiomyocytes from adult rat hearts. The results showed that PC elevated [Ca2+]iin a dose-dependent (5–20μm) manner; 15μm PC evoked a marked and reversible increase in [Ca2+]iwithout having any significant action on cell viability. The PC (15μm)-induced increase in [Ca2+]iwas slightly depressed but delayed in the absence of extracellular Ca2+. Pre-incubation of cardiomyocytes with sarcolemmal (SL)l-type Ca2+-channel blockers, verapamil or diltiazem, and inhibitors of SL Na+-Ca2+exchanger such as Ni2+or amiloride, depressed the PC-evoked increase in [Ca2+]isignificantly. Ouabain, a Na+-K+ATPase inhibitor, and low concentrations of extracellular Na+enhanced the PC-induced increase in [Ca2+]i. Depletion of the sarcoplasmic reticulum (SR) Ca2+stores by low micromolar concentrations of ryanodine (a SR Ca2+-release channel activator) or by thapsigargin (a SR Ca2+-pump ATPase inhibitor) depressed the PC-mediated increase in [Ca2+]i. Combined blockade of thel-type Ca2+channel, Na+-Ca2+exchanger and the SR Ca2+-pump had an additive inhibitory effect on the PC response. These observations suggest that the PC-induced increase in [Ca2+]iis dependent on both Ca2+-influx from the extracellular space and Ca2+-release from the SR stores. Thus, the accumulation of PC in the myocardium may be partly responsible for the occurrence of intracellular Ca2+overload in ischemic heart.

Regional Expression and Functional Characterization of the L-type Ca2+-Channel in Myocardium from Patients with End-stage Heart Failure and in Non-failing Human Hearts

The purpose of the present study was to investigate the expression and functional relevance of sarcolemmal L-type Ca2+-channels in failing and non-failing human myocardium. The protein expression of sarcolemmal L-type Ca2+-channels was determined with3H-(+)-PN 200-110-binding experiments and Western blot analysis using a specific antibody against theα1-subunit in membrane preparations of ventricular and atrial myocardium from both failing (n=15) and non-failing hearts (n=8). The gene expression of the ion conducting pore of the L-type Ca2+-channel was examined with Northern blot technique in human failing and non-failing RNA. For normalization the RNA expression of calsequestrin was used. In electrically driven ventricular papillary muscle strips and auricular trabeculae, the responses to nifedipine and Ca2+as parameters of myocardial function were studied. The protein expression as measured by3H-(+)-PN 200-110-binding (Bmax) and Western Blot analysis with calsequestrin as reference was similar in left ventricular failing and non-failing myocardium. However, both were reduced in atrial compared to ventricular tissue in failing and non-failing hearts. The KDremained unchanged. Calsequestrin levels were unaltered in failing and non-failing hearts. The gene expression of theα1-subunit was similar in human failing and non-failing hearts. The L-type Ca2+-channel antagonist nifedipine reduced force of contraction with the same potency and efficiency in ventricular failing and non-failing myocardium. In contrast, the potency of nifedipine was higher in atrial than in ventricular tissue. Consistently, atrial myocardium from patients with dilated cardiomyopathy was more sensitive towards Ca2+than those of the control group. In conclusion, the altered Ca2+-homeostasis in failing human myocardium may be less due to changes in sarcolemmal L-type Ca2+-channel expression or function than due to an altered intracellular Ca2+-handling.

Comparative Myocardial Depression of Sevoflurane, Isoflurane, and Halothane in Cultured Neonatal Rat Ventricular Myocytes

In this study, we compared the direct myocardial depressant effects of sevoflurane, isoflurane, and halothane and determined whether an L-type Ca2+ channel agonist, Bay K 8644, could attenuate the myocardial depression induced by these anesthetics in cultured neonatal rat ventricular myocytes. Ventricular myocytes were obtained from neonatal rats by enzymatic digestion with collagenase and then cultured for 6-7 days. The myocytes were stabilized in serum-free medium, and the spontaneous beating rate and contractile amplitude were measured by using a fiberoptic sensor. Each anesthetic decreased the beating rate and amplitude in a concentration-dependent manner (1%-4% vol/vol) (P < 0.001), with halothane decreasing the beating rate and amplitude the most (P < 0.01). Isoflurane caused larger decreases in the beating rate than sevoflurane at 3% and 4% (P < 0.05). Potency for suppression of contractile amplitude was in the order of halothane >> isoflurane > sevoflurane. However, the myocardial depressant effects of the anesthetics were not different when their concentrations were corrected for minimum alveolar anesthetic concentration values. Bay K 8644 significantly prevented the anesthetic-depressed amplitude (P < 0.05). We conclude that sevoflurane, isoflurane, and halothane have direct myocardial depressant effects on cultured neonatal rat ventricular myocytes and that the reduction of sarcolemmal L-type Ca2+ channel current levels mediates the myocardial depression observed in these immature hearts. Implications: Sevoflurane, isoflurane, and halothane have a direct cardiodepressant effect on cardiac excitation-contraction coupling in the immature heart, which is mediated by an interaction with the L-type Ca2+ channel. (Anesth Analg 1998;87:1041-7)

Ca2+ channel modulation alters halothane-induced depression of ventricular myocytes.

This study examined the direct myocardial depressant effect of halothane and determined whether an L-type Ca2+ channel agonist and antagonists altered the myocardial depression induced by halothane in cultured rat ventricular myocytes. Ventricular myocytes were obtained from neonatal rats by enzymatic digestion with collagenase and then cultured for 6 to 7 days. The myocytes were stabilized in a serum-free medium, and the spontaneous beating rate and amplitude were measured. To assess the halothane-induced conformational changes in L-type Ca2+ channel, receptor binding study was performed using a dihydropyridine derivative, [3H] PN 200-110, in cardiac membrane preparation. Halothane (1%, 2%, 3%, 4%) decreased the beating rate and amplitude in a concentration-dependent manner (P < 0.05). The myocardial depressant effects of halothane were potentiated by nifedipine or verapamil (P < 0.05). Bay K 8644, an L-type Ca2+ channel agonist, completely prevented the halothane-induced depression in amplitude (P < 0.05), but affected the beating rate less. Adding halothane (2%) decreased (P < 0.05) the maximum binding site density for [3H] PN 200-110 (from 198.6 +/- 23.7 fmol.mg-1 protein to 115.3 +/- 21.6 fmol.mg-1 protein) but did not affect binding affinity (from 0.461 +/- 0.077 nM to 0.307 +/- 0.055 nM). The reduction of Ca2+ current via sarcolemmal L-type Ca2+ channel, probably due to conformational changes in dihydropyridine binding sites, plays an important role in halothane-induced myocardial depression in living heart cells.

Effects of verapamil and ryanodine on activity of the embryonic chick heart during anoxia and reoxygenation.

Perturbations of the trans-sarcolemmal and sarcoplasmic Ca2+ transport contribute to the abnormal myocardial activity provoked by anoxia and reoxygenation. Whether Ca2+ pools of the extracellular compartment and sarcoplasmic reticulum (SR) are involved to the same extent in the dysfunction of the anoxic-reoxygenated immature heart has not been investigated. Spontaneously contracting hearts isolated from 4-day-old chick embryos were submitted to repeated anoxia (1 min) followed by reoxygenation (5 min). Heart rate, atrioventricular propagation velocity, ventricular shortening, velocities of contraction and relaxation, and incidence of arrhythmias were studied, recorded continuously. Addition of verapamil (10 nM), which blocks selectively sarcolemmal L-type Ca2+ channels, was expected to protect against excessive entry of extracellular Ca2+, whereas addition of ryanodine (10 nM), which opens the SR Ca2+ release channel, was expected to increase cytosolic Ca2+ concentration. Verapamil (a) had no dromotropic effect by contrast to adult heart, (b) attenuated ventricular contracture induced by repeated anoxia, (c) shortened cardioplegia induced by reoxygenation, and (d) had remarkable antiarrhythmic properties during reoxygenation specially. On the other hand, ryanodine potentiated markedly arrhythmias both during anoxia and at reoxygenation. Thus despite its immaturity, the SR seems to be functional early in the developing chick heart and involved in the reversible dysfunction induced by anoxia-reoxygenation. Moreover, Ca2+ entry through L-type channels appears to worsen arrhythmias especially during reoxygenation. These findings show that the Ca2+-handling systems involved in irregular activity in immature heart, such as the embryonic chick heart, may differ from those in the adult.

The direct effects of diazepam and midazolam on myocardial depression in cultured rat ventricular myocytes.

We examined the direct myocardial depressant effects of diazepam and midazolam and determined whether a benzodiazepine receptor antagonist, flumazenil, or an L-type Ca2+ channel agonist, Bay K 8644, affects the myocardial depression induced by diazepam and midazolam in cultured rat ventricular myocytes. Ventricular myocytes of neonatal rats were obtained by enzymatic digestion with collagenase and cultured for 6-7 days. The myocytes were stabilized in serum-free medium, and the spontaneous beating rate and amplitude were measured by determining displacement with a fiberoptic sensor. Myocytes were exposed to either diazepam or midazolam at concentrations of 0.1, 1, 10, and 100 microM. The beating rate and amplitude were measured 4 min after diazepam or midazolam administration. In other cells, either diazepam or midazolam was administered at each concentration in the presence of flumazenil or Bay K 8644. Midazolam and diazepam decreased the beating rate and amplitude concentration-dependently. These myocardial depressant effects were prevented by Bay K 8644 and, to a lesser degree, by flumazenil. Thus, the L-type Ca2+ channel is important in the direct myocardial depression caused by diazepam and midazolam. This study describes the direct effect of midazolam and diazepam on intrinsic myocardial contraction using cultured rat ventricular heart cells. Both of these drugs have a direct myocardial depressant effect at the cellular level, which is mainly mediated by an inhibition of the sarcolemmal L-type Ca2+ channel.

The effect of Ca(2+)-calmodulin-dependent protein kinase II on cardiac excitation-contraction coupling in ferret ventricular myocytes.

1. The effect of Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) on excitation-contraction coupling (E-C coupling) was studied in intact ferret cardiac myocytes using the selective inhibitor KN-93, KN-93 decreased steady-state (SS) twitch [Ca2+]i (by 51%), resting Ca2+ spark frequency (by 88%) and SS sarcoplasmic reticulum (SR) Ca2+ content evaluated by eaffeine application (by 37.5%). 2. Increasing extracellular Ca2+ concentration ([Ca2+]o) to 5 mM in KN-93 restored SR Ca2+ load and Ca2+ spark frequency towards that in control (2 mM Ca2+o), but SS twitch [Ca2+]i was still significantly depressed by KN-93. 3. KN-93 decreased Ca2+ transient amplitude of SS twitches much more strongly than the amplitude of post-rest (PR) twitches. In the control, the time constant (Tau) of [Ca2+]i decline of SS twitches was faster than that for PR twitches. This stimulation-dependent acceleration of [Ca2+]i decline was abolished by KN-93. 4. Voltage-clamp experiments demonstrated that KN-93 significantly inhibited sarcolemmal L-type Ca2+ current (ICa) during repetitive pulses by slowing recovery from inactivation. This may explain the preferential action of KN-93 to suppress SS vs. PR twitches. 5. In KN-93, even when both ICa and SR Ca2+ load were matched to the control levels by manipulation of conditioning voltage-clamp pulses, contraction and twitch Ca2+ transients were still both significantly depressed (to 39 and 49% of control, respectively). 6. Since KN-93 reduced SR Ca2+ release channel (RyR) activity during E-C coupling, even for matched SR Ca2+ load and trigger ICa, we infer that endogenous CaMKII is an important modulator of E-C coupling in intact cardiac myocytes. Effects of KN-93 on ICa and SS twitch [Ca2+]i decline also indicate that endogenous CaMKII may have stimulatory effects on ICa and SR Ca2+ uptake.

Opioid peptide receptor stimulation reverses beta-adrenergic effects in rat heart cells.

Opioid peptide receptor (OPR) agonists are co-released with the beta-adrenergic receptor (beta-AR) agonist norepinephrine (NE) from nerve terminals in the heart during sympathetic stimulation. Whereas recent studies indicate that OPR and beta-AR coexist on the surface of cardiac myocytes, whether significant "cross talk" occurs between OPR and beta-AR signaling cascades within heart cells is unknown. In the present study we demonstrate a marked effect of delta-OPR stimulation to modulate beta-adrenergic responses in single isolated rat ventricular myocytes. Nanomolar concentrations (10(-8) M) of the OPR agonist leucine enkephalin (LE), a naturally occurring delta-opioid peptide, inhibited NE-induced increases in sarcolemmal L-type Ca2+ current, cytosolic Ca2+ transient, and contraction. The antiadrenergic effect of LE was pertussis toxin sensitive and abolished by naloxone, an opioid receptor antagonist. In contrast, LE was unable to inhibit the positive inotropic effects induced by equipotent concentrations of 8-(4 chlorophenylthio)-adenosine 3',5'-cyclic monophosphate, a cell-permeant adenosine 3',5'-cyclic monophosphate analog, or by the non-receptor-induced increase in contraction by elevated bathing Ca2+ concentration. These results indicate that an interaction of the OPR and beta-AR systems occurs proximal to activation of the adenosine 3',5'-cyclic monophosphate-dependent protein kinase of the beta-AR intracellular signaling pathway. This modulation of beta-adrenergic effects by OPR activation at the myocyte level may have important implications in the regulation of cardiac Ca2+ metabolism and contractility, particularly during the myocardial response to stress.

Pharmacological differentiation of synergistic contribution of L-type Ca2+ channels and Na+/H+ exchange to the positive inotropic effect of phenylephrine, endothelin-3 and angiotensin II in rabbit ventricular myocardium.

The present study was designed to delineate pharmacologically the role of sarcolemmal L-type Ca2+ channels and Na+/H+ exchange in the positive inotropic effect (PIE) of phenylephrine mediated by alpha-1 adrenoceptors, endothelin (ET) and angiotensin II (Ang II) that stimulate phosphoinositide (PI) hydrolysis in the rabbit ventricular muscle. The PIE of these receptor agonists was compared with the PIE of isoprenaline that accumulates cyclic AMP. For this purpose, we investigated the influence of a Ca2+ antagonist, verapamil, and of an inhibitor of Na+/H+ exchange, 5-(N-ethyl-N-isopropyl) amiloride (EIPA), alone or in combination, on the cumulative concentration-response curve (CRC) for phenylephrine (with 0.3 microM bupranolol). ET-3 and Ang II in isolated right ventricular papillary muscles of the rabbit, which were electrically stimulated at 1 Hz in Krebs-Henseleit solution at 37 degrees C. Verapamil at 0.3 and 1 microM decreased the basal force of contraction to 37.0 +/- 4.0% and 13.2 +/- 1.1% of the control, respectively, while EIPA event at 10 microM affected the basal force to much less extent and decreased it to 87.0 +/- 1.4%. Verapamil (0.3 and 1 microM) and EIPA (1 and 10 microM), when used alone, each significantly attenuated but did not abolish the PIEs induced by phenylephrine, ET-3 and Ang II, while the simultaneous administration of verapamil (1 microM) and EIPA (10 microM) consistently and almost completely inhibited the PIE induced by these receptor agonists. By contrast, the PIE of isoprenaline was retained even in the presence of verapamil and EIPA. These results indicate that both the influx of Ca2+ ions through L-type Ca2+ channels and activation of Na+/ H+ exchange contribute synergistically to the PIE that is mediated by alpha-1 adrenergic, ET and Ang II receptor agonists, while these mechanisms are not essential for the beta-adrenoceptor-mediated PIE.

Force-frequency response in isoproterenol-induced hypertrophied rat heart

Rate-dependent force production was investigated using small trabecular muscle from control and hypertrophied rat cardiac muscle. Cardiac hypertrophy was induced by daily subcutaneous injection of isoproterenol (0.3 mg/kg body weight) for 12 days. The force-frequency relationship, for the control rat myocardium, is clearly biphasic. A stepped increase in stimulation frequency from 0.1 to 0.5 Hz results in a decrease in contractile force (negative phase). However, at higher stimulation frequency above 0.5 Hz, an increased contractile force is revealed (positive phase). Membrane action potential duration (APD 50) was used to reflect sarcolemmal Ca 2+ influx. The frequency-dependent increase in APD 50 and the ability of nifedipine, a sarcolemmal L-type Ca 2+ channel blocker, to eliminate the positive-force frequency response, indicate that sarcolemmal Ca 2+ influx is important for force development at high stimulation frequency. Relative Ca 2+ content of sarcoplasmic reticulum is estimated from rapid cooling contractures. The parallel change of rapid cooling contractures and twitch force suggests that the sarcoplasmic reticulum Ca 2+ content alters with varying frequencies of stimulation. Isoproterenol-induced hypertrophied muscle shows a greater contractile force, increased nifedipine-sensitive force development and prolonged APD 50 compared to controls. These data suggest a greater availability of intracellular Ca 2+ to activate contraction in hypertrophied muscle, possibly by amplified Ca 2+ influx via L-type channel.

Submicroscopic calcium signals as fundamental events of excitation--contraction coupling in guinea-pig cardiac myocytes.

1. Subcellularly localized Ca2+ signals have been proposed to represent elementary events of cardiac Ca2+ signalling (Ca2+ sparks), whereby an individual sarcolemmal L-type Ca2+ channel locally controls opening of a single (or a few) Ca2+ release channels in the sarcoplasmic reticulum (SR). 2. To investigate directly the elementary nature of this Ca(2+)-induced Ca2+ release mechanism we used flash photolysis of caged Ca2+ while simultaneously measuring the intracellular Ca2+ concentration ([Ca2+]i) with a laser-scanning confocal microscope. 3. Power spectral analysis of the confocal images performed in the spatial domain revealed that only Ca2+ signalling events involving the L-type Ca2+ channel pathway gave rise to Ca2+ sparks. In contrast, SR Ca2+ release triggered by photolytic [Ca2+]i jumps resulted in Ca2+ transients that were always spatially homogeneous. 4. From these findings we conclude that the fundamental event of Ca2+ signalling in cardiac muscle may be smaller in size or amplitude than a Ca2+ spark. 5. We term this event a 'Ca2+ quark' possibly resulting from gating of a single SR Ca2+ release channel. It is proposed that concerted activation of several 'Ca2+ quarks' may be required for a Ca2+ spark. The 'Ca2+ quark' could also be the fundamental event in other cell types implementing a hierarchical Ca2+ signalling concept.

Calcium channels and cation transport ATPases in cardiac hypertrophy induced by aortic constriction in newborn rats.

Cardiac enlargement due to gradual pressure overload was induced by abdominal aortic constriction in 2-day-old rats. On day 90, the functional performance of the left ventricle was assessed by acute load test (ligation of ascending aorta) in open-chest anaesthetized animals. Two subgroups, designated compensated and decompensated hypertrophy (CH and DH), were distinguished on the basis of the functional reserve of left ventricle, which was significantly impaired in DH but not in CH, and of right ventricle weight, which was markedly increased in DH but not significantly modified in CH. In total particulate fractions prepared from hypertrophied left ventricles, the levels (per g tissue) of sarcoplasmic reticulum Ca(2+)-transport systems were decreased, either slightly (by 13-16%: [3H]ryanodine binding) or moderately (by 28%: thapsigargin-sensitive Ca(2+)-ATPase activity). The number of sarcolemmal L-type Ca2+ channels ([3H]PN200-110 binding) was not modified significantly, while that of beta 1-adrenoceptors ([3H]CGP-12177 binding) was reduced, especially in the DH group (by 39%). Na+,K(+)-ATPase activity was reduced by 28% in CH and 41% in DH. [3H]Ouabain binding experiments (saturation and dissociation) indicated the existence of two high-affinity binding sites, attributable to the Na+, K(+)-ATPase alpha 3 and alpha 2 subunit isoforms; while the relatively minor alpha 3 component did not change significantly in hypertrophied ventricles, the alpha 2 component was markedly down-regulated, decreasing by 57% in CH and 82% in DH.

Early afterdepolarization formation in cardiac myocytes: analysis of phase plane patterns, action potential, and membrane currents.

Early afterdepolarizations (EADs) are among the mechanisms proposed to underlie ventricular arrhythmias. Sea anemone toxin, ATXII, known to delay Na inactivation and to induce plateau level voltage oscillations, was used to study the formation of EADs. Action potential and membrane currents were studied in rat ventricular myocytes using whole cell current and voltage clamp techniques. Phase plane trajectories were generated by plotting membrane potential (V) versus the first time derivative of membrane potential (dV/dt). Under current clamp conditions, ATXII (40 nM) consistently prolonged the action potential and induced EADs. The EADs developed at a plateau voltage between -10 and -40 mV. Calcium channel blockers, verapamil 10 microM and cobalt 4 mM, and the sarcoplasmic reticulum modulator, ryanodine (1 microM), did not antagonize ATXII effects on the action potential and EADs. However, Na channel blockers, tetrodotoxin 0.3 microM and lidocaine 40 microM, and rapid stimulation consistently shortened the prolonged action potential and suppressed EADs. Under voltage clamp conditions in the presence of ATXII, a slowly decaying inward current followed the fast inward current during depolarizing pulses. Membrane currents flowing at or later than 100 msec after the test pulse were analyzed. The control isochronal current-voltage (I-V) curves showed no late inward currents. In the presence of ATXII, all the isochronal I-V curves showed an inward current that was more prominent between -40 and 0 mV. The ATXII-induced current at the 100-msec isochrone activated at a potential of approximately -60 mV, peaked at about -20 mV, and reversed at +40 mV consistent with the Na current I-V curve. The isochronal I-V curves obtained after lidocaine superfusion resembled those of the control. The phase plane trajectory of the action potential obtained with ATXII showed an oscillatory behavior corresponding to the EAD range of potential; within this voltage range, the isochronal I-V curves were shown to cross the abscissa three times instead of once. These results suggest that, in this experimental model, neither sarcolemmal L-type Ca current nor sarcoplasmic reticulum Ca release plays a significant role in the genesis of ATXII-induced EADs. EADs are generated by a voltage-dependent balance between a markedly prolonged Na inward current and K outward currents within the voltage plateau range of the action potential but not by Ca current reactivation and inactivation.

Regulation and modulation of calcium channels in cardiac, skeletal, and smooth muscle cells

Regulation and modulation of calcium channels in cardiac, skeletal, and smooth muscle cells