Na+-Ca2+ exchange contributes to increase of cytosolic Ca2+ concentration during depolarization in heart muscle

Mitochondrial and cytosolic calcium in rat hearts under high-K(+) cardioplegia and pyruvate: mechano-energetic performance

High-K(+)-cardioplegia (CPG) and pyruvate (Pyr) are used as cardioprotective agents. Considering that mitochondria play a critical role in cardiac dysfunction, we investigated the effect of CPG on mitochondrial Ca(2+) uptake and sarcorreticular (SR) calcium handling. Cytosolic and mitochondrial Ca(2+), as well as mitochondrial membrane potential (ΔΨm) were assessed in rat cardiomyocytes by confocal microscopy. Mechano-calorimetrical correlation was studied in perfused hearts. CPG did not modify JC-1 (ΔΨm), but transiently increased, by up to 1.8 times, the Fura-2 (intracellular Ca concentration, [Ca(2+)]i) and Rhod-2 (mitochondrial free Ca concentration [Ca(2+)]m) fluorescence of resting cells, with exponential decays. The addition of 5 µmol·L(-1) thapsigargin (Tpg) increased the Rhod-2 fluorescence in a group of cells without any effect on the Fura-2 signal. In rat hearts perfused with CPG, 1 µmol·L(-1) Tpg decreased resting heat rate (ΔH(r): -0.44 ± 0.07 mW·g(-1)), while the addition of 5 µmol·L(-1) KB-R7943 increased resting pressure (ΔrLVP by +5.26 ± 1.10 mm Hg; 1 mm Hg = 133.322 Pa). The addition of 10 mmol·L(-1) Pyr to CPG increased H(r) (+3.30 ± 0.24 mW·g(-1)) and ΔrLVP (+2.2 ± 0.4 mm Hg), which are effects potentiated by KB-R7943. The results suggest that under CPG, (i) there was an increase in [Ca(2+)]i and [Ca(2+)]m (without changing ΔΨm) that decayed by exothermic removal mechanisms; (ii) mitochondrial Ca(2+) uptake contributed to the removal of cytosolic Ca(2+), in a process that was potentiated by inhibition of sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA), and reduced by KB-R7943; (iii) under these conditions, SERCA represents the main energetic consumer; (iv) Pyr increased the energetic performance of hearts,mainly by inducing mitochondrial metabolism.

Persistent sodium current is a target for cAMP-induced neuronal plasticity in a state-setting modulatory interneuron

We have identified a TTX-resistant low-threshold persistent inward sodium current in the cerebral giant cells (CGCs) of Lymnaea, an important state-setting modulatory cell type of molluscan feeding networks. This current has slow voltage-dependent activation and de-activation kinetics, ultra-slow inactivation kinetics and fast de-inactivation kinetics. It activates at approximately -90 mV, peaks at approximately -30 mV, reverses at approximately +35 mV and does not show full voltage-dependent inactivation even at positive voltage steps. Lithium-sodium replacement experiments indicate that the persistent sodium current makes a significant contribution to the CGC membrane potential. Injection of cyclic adenosine monophosphate (cAMP) into the CGC cell body produces a large increase in the persistent sodium current that lasts for several hours. cAMP injection also leads to increased bursting, a significant decrease in the resistance and a significant depolarization of the soma membrane, indicating that cAMP-dependent mechanisms induce prolonged neuronal plasticity in the CGCs. Our observations provide the first link between cAMP-mediated modulation of a TTX-resistant persistent sodium current and prolonged neuronal plasticity in an identified modulatory cell type that plays an important role in behavioral state setting.

Imidapril treatment improves the attenuated inotropic and intracellular calcium responses to ATP in heart failure due to myocardial infarction

1. Adenosine 5'-triphosphate (ATP) is known to augment cardiac contractile activity and cause an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in isolated cardiomyocytes. However, no information regarding the ATP-mediated signal transduction in the myocardium in congestive heart failure (CHF) is available. 2. CHF due to myocardial infarction (MI) in rats was induced by the occlusion of the left coronary artery for 8 weeks. The positive inotropy due to ATP was depressed in failing hearts. Treatment of 3 weeks infarcted animals with imidapril (1 mg kg(-1) day(-1)) for a period of 5 weeks improved the left ventricle function and decreased the attenuation of inotropic response to ATP. 3. ATP-induced increase in [Ca(2+)](i) was significantly depressed in cardiomyocytes isolated from the failing heart and this change was partially attenuated by imidapril treatment. However, the binding characteristics of (35)S-labeled adenosine 5'-(gamma-thio) triphosphate in sarcolemma isolated from the failing heart remained unaltered. 4. ATP-induced increase in [Ca(2+)](i) was depressed by verapamil and cibacron blue in both control and failing heart cardiomyocytes; however, the ATP response in the failing hearts, unlike the control preparations, was not decreased by ryanodine. This insensitivity to ryanodine was attenuated by imidapril treatment. 5. Treatment of infarcted rats with enalapril and losartan produced effects similar to imidapril. 6. These findings indicate that the positive inotropic response to ATP and ATP-induced increase in [Ca(2+)](i) in cardiomyocytes are impaired in heart failure. Furthermore, blockade of renin angiotensin system prevented the impairment of the ATP-mediated inotropic and [Ca(2+)](i) responses in the failing heart.

Distribution of a persistent sodium current across the ventricular wall in guinea pigs

A tetrodotoxin-sensitive persistent sodium current, I(pNa), was found in guinea pig ventricular myocytes by whole-cell patch clamping. This current was characterized in cells derived from the basal left ventricular subendocardium, midmyocardium, and subepicardium. Midmyocardial cells show a statistically significant (P<0.05) smaller I(pNa) than subendocardial and subepicardial myocytes. There was no significant difference in I(pNa) current density between subepicardial and subendocardial cells. Computer modeling studies support a role of this current in the dispersion of action potential duration across the ventricular wall.

Functional role of ionic regulation of Na+/Ca2+ exchange assessed in transgenic mouse hearts

Na+/Ca2+ exchange is the primary mechanism mediating Ca2+ efflux from cardiac myocytes during diastole and, thus, can prominently influence contractile force. In addition to transporting Na+ and Ca2+, the exchanger is also regulated by these ions. Although structure-function studies have identified protein regions of the exchanger subserving these regulatory processes, their physiological importance is unknown. In this study, we examined the electrophysiological and mechanical consequences of cardiospecific overexpression of the canine cardiac exchanger NCX1.1 and a deletion mutant of NCX1.1 (Delta680-685), devoid of intracellular Na+ (Na+i)- and Ca2+ (Ca2+i)- dependent regulatory properties, in transgenic mice. Using the giant excised patch-clamp technique, normal ionic regulation was observed in membrane patches from cardiomyocytes isolated from control and transgenic mice overexpressing NCX1.1. In contrast, ionic regulation was nearly abolished in mice overexpressing Delta680-685, indicating that the native regulatory processes could be overwhelmed by expression of the transgene. To address the physiological consequences of ionic regulation of the Na+/Ca2+ exchanger, we examined postrest force development in papillary muscles from NCX1.1 and Delta680-685 transgenic mice. Postrest potentiation was found to be substantially greater in Delta680-685 than in NCX1.1 transgenic mice, supporting the notion that ionic regulation of Na+/Ca2+ exchange plays a significant functional role in cardiac contractile properties.

Digoxin's Minimal Inotropic Effect Is Not Limited by Sodium-Calcium Exchange in the Intact Immature Rabbit Heart

BACKGROUND: In the intact immature heart, how much digoxin can drive sodium-calcium exchange has not been studied in the context of sodium-calcium exchanger abundance. METHODS AND RESULTS: The effects of digoxin and low potassium on contractility in the intact, paced and isovolumically contracting immature rabbit heart were studied in both the absence and presence of L-type calcium channel blockade. Without calcium channel blockade, digoxin increased contractility minimally and only at 10(_6) M/L. In contrast, low potassium (2.2 mM/L) substantially increased contractility in all experiments, a result indicating abundant sodium-calcium exchanger activity. During nifedipine-induced calcium channel blockade, digoxin (10(_6) M/L) allowed modest recovery of contractility, whereas digoxin and low potassium together allowed complete recovery as assessed by dP/dt(max); however, all hearts so perfused subsequently developed ventricular fibrillation, presumably because of calcium overload. CONCLUSIONS: In intact immature rabbit heart, digoxin can drive sodium-calcium exchange and thus increase contractility to only a minimal extent. This effect does not appear to be limited by intrinsic exchanger activity, which appears abundant in this preparation. Rather, digoxin's inability to drive the sodium-calcium exchanger may be due to developmental differences in binding to the sodium pump. The sodium-calcium exchanger itself seems capable not only of providing enough intracellular calcium for normal contraction, but also of overloading the myocardium with calcium, despite L-type calcium channel blockade.

Inhibition of [3H]-U69593 binding and the cardiac effects of U50, 488H by calcium channel blockers in the rat heart

1. The calcium channel blockers (CCBs), nifedipine, nicardipine, diltiazem and verapamil, were used to displace the binding of [3H]-U69593 ((5a, 7a,8b)-(+)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro[4,5] dec-8-yl)-benzeneacetamide), a specific kappa-opioid agonist, in the rat cardiac sarcolemma. The CCBs competed with the binding of [3H]-U69593 (4 nM) in a dose-dependent manner. The displacing potency of verapamil was 55 times greater than that of nifedipine. 2. The effects of two CCBs, verapamil and nifedipine, on the arrhythmogenic action of kappa-receptor stimulation by a specific kappa-receptor agonist, U50,488H (trans-(+/-(-3),4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl] cyclohexyl) benzeacetamide methanesulphonate), were also studied in the rat isolated perfused heart. U50,488H 80-800 nmol dose-dependently induced arrhythmias, which were completely abolished by a selective kappa-receptor antagonist, nor-BNI (nor-binaltorphimine, 17,17'-(dicyclopropylmethyl)-6,6',7,7'-6,6'-imino-7,7'-binorphinan -3,4',14, 14'-tetrol), at 100 nmol. The arrhythmogenic effect was also attenuated by both verapamil and nifedipine in a dose-dependent manner. The ED50 values for verapamil and nifedipine were 2.75 and 63.7 nmol, respectively. The antiarrhythmic potencies of these two CCBs were correlated to their displacing potencies and inversely related to their well known potencies in inhibiting transmembrane Ca2+ influx in the cardiac muscle. 3. Measurement of [Ca2+]i in the absence of free extracellular Ca2+ by a spectrofluorometric method, with fura-2 as Ca2+ indicator, showed that U50,488H 5 x 10(-5) M slowly increased [Ca2+]i in single ventricular myocytes and this effect was abolished by pretreatment with nor-BNI (5 microM), or ryanodine (5 microM). Verapamil 1 and 10 microM abolished the effect of U50,488H in 37.5% (3 out of 8) and 100% (12 out of 12) of the cells studied, respectively. On the other hand, nifedipine 10 and 100 microM had no effect at all. Neither verapamil nor nifedipine exerted any significant effect on the caffeine-induced Ca2+ transient. 4. The observations suggest that CCBs may inhibit the actions of kappa-receptor stimulation at the level of the kappa-receptor.

Pharmacological evidence for the persistent activation of ATP-sensitive K+ channels in early phase of reperfusion and its protective role against myocardial stunning

BACKGROUND

The activation of cardiac ATP-sensitive potassium channels is reported to protect myocardium during ischemia. However, the behavior and role of this channel during reperfusion remain uncertain.

METHODS AND RESULTS

Guinea pig right ventricular walls were studied by use of microelectrodes and a force transducer. Each preparation was perfused via the coronary artery at a constant flow rate and was stimulated at 3 Hz. In the first protocol, the preparation was subjected to 10 minutes of no-flow ischemia, which was followed by 60 minutes of reperfusion. Introduction of ischemia shortened the action potential duration (APD) to 58.7 +/- 3.1% of the preischemic values, in association with a decrease in the resting membrane potential (by 12 +/- 0.8 mV) and action potential amplitude (by 34.6 +/- 1.8 mV). On reperfusion, although the APD was restored, it remained shortened for up to approximately 30 minutes of reperfusion. In the presence of glibenclamide (10 mumol/L), the shortening of the APD during ischemia was significantly attenuated and the restoration of APD after reperfusion was significantly facilitated. When glibenclamide was applied from the onset of reperfusion, the persistent APD shortening was significantly suppressed. The developed tension decreased during ischemia and recovered after 60 minutes of reperfusion (up to 92.0 +/- 6.4% of preischemic values) in the untreated preparations. The application of glibenclamide that was started before ischemia or from the onset of reperfusion significantly suppressed the recovery of contractility (P < .05 versus untreated preparations). In the second series of experiments, 20 minutes of no-flow ischemia and 60 minutes of reperfusion were applied. This protocol produced a sustained contractile dysfunction after reperfusion (to 34.0 +/- 3.2% of preischemic values). In the presence of cromakalim (2 mumol/L), the APD shortening was enhanced during both ischemia and the early reperfusion period. Cromakalim significantly improved the contractile recovery (to 79.3 +/- 4.1% of preischemic values, P < .05 versus untreated preparations). The application of cromakalim that was started from the onset of reperfusion also improved the contractile recovery during this phase and this effect was associated with enhanced APD shortening. However, the cromakalim-treated preparations demonstrated a higher incidence of ventricular fibrillation during reperfusion.

CONCLUSIONS

Cardiac ATP-sensitive potassium channels are activated by ischemia, and a fraction of these channels remains activated during the early reperfusion phase. The resulting shortening of the APD prevents the heart from developing myocardial stunning.

Role of calcium loading in early afterdepolarizations generated by Cs+ in canine and guinea pig Purkinje fibers

INTRODUCTION

Our previous observations indicate that the Na+:Ca2+ exchange current (INa:Ca) plays an important role in early afterdepolarizations occurring at more negative Vm (L-EAD). The purpose of these studies was to examine the role of Ca(2+)-loading, which stimulates INa:Ca, in generation of L-EAD.

METHODS AND RESULTS

Purkinje strands and preparations of ventricular myocardium from dogs and guinea pigs were superfused with oxygenated physiologic buffer solutions at 37 degrees C. To induce EADs, [K+]o was reduced to 2.0 to 3.0 mM and [Cs+]o (3.6 to 4.0 mM) was added at slow rates of < or = 0.3 Hz. Isometric contraction in canine Purkinje strands and guinea pig papillary muscles doubled in 1-hour exposure to Cs+ and low [K+]o at slow rates and the uptake of 45Ca2+ was approximately doubled after 30 minutes. Forty-three percent of Purkinje fibers developed L-EAD after a latent period of 17 to 123 minutes of exposure. Ouabain (0.2 microM) suppressed L-EAD within 10 minutes reversibly. Ca(2+)-loading (low [Na+]o or high [Ca2+]o) for 5 to 10 minutes before exposure to Cs+, low [K+]o, and slow rates resulted in rapid development of L-EAD in all preparations during subsequent exposure. In Ca(2+)-loaded preparations, delayed afterdepolarizations (DADs) as well as L-EADs developed.

CONCLUSIONS

Reduction of K+ currents with Cs+, low [K+]o, and slow rates induced L-EAD in a fraction of Purkinje fibers after a latent period during which Ca(2+)-loading of the sarcoplasmic reticulum occurred, while fibers preloaded with Ca2+ developed L-EAD rapidly and uniformly. These findings indicate that Ca(2+)-loading is a critical condition for the development of L-EAD. Early suppression of L-EAD by ouabian suggests a dependence of L-EAD on low [Na+]i. These findings implicate INa:Ca in the generation of L-EAD.

Contribution of Na(+)-Ca2+ exchange to stimulation of transient inward current by isoproterenol in rabbit cardiac Purkinje fibers

Cellular mechanisms underlying beta-adrenergic stimulation of the arrhythmogenic transient inward current (TI) were investigated by using a two-microelectrode voltage-clamp technique in rabbit cardiac Purkinje fibers. TI induced by elevating [Ca2+]o to 30 mmol/L and substituting [Na+]o with N-methyl-D-glucamine (NMG) chloride had a distinct reversal potential (EREV) of -25 mV, suggesting that Na(+)-Ca2+ exchange was not the charge carrier for TI. In the absence of [Na+]o, isoproterenol (ISO, 0.01 to 5.0 mumol/L) had no effect on either inward or outward TI or on the current-voltage relation of TI. However, ISO (0.1 mumol/L) significantly increased both inward and outward TIs without affecting the EREV of TI, if [Na+]o was present. Pretreatment with propranolol (0.2 mumol/L) or atenolol (0.2 mumol/L) abolished the stimulatory effects of ISO. Addition of propranolol (0.2 to 0.5 mumol/L) after the effects of ISO had developed caused only partial reversal of TI stimulation. This indicates persistence of stimulatory effects downstream from the initial agonist-receptor interaction. Forskolin (1 mumol/L), a direct adenylate cyclase activator, also strongly increased both inward and outward TI in the presence of [Na+]o. These effects also were abolished when [Na+]o was substituted by NMG. Inward and outward TIs enhanced by either ISO or forskolin were reversed by two putative Na(+)-Ca2+ exchange blockers, dodecylamine (20 mumol/L) and quinacrine (20 mumol/L). These results suggest that beta-adrenergic stimulation of TI is mediated by the Na(+)-Ca2+ exchange; stimulation likely involves phosphorylation of the exchanger or some factor that modulates exchanger activity.

Rate-dependent prolongation of action potential duration in single ventricular myocytes obtained from hearts of rats with streptozotocin-induced chronic diabetes sustained for 30-32 weeks

We examined the characteristics of the action potentials of single ventricular myocytes obtained from the hearts of rats with chronically-induced diabetes. Male Wistar rats were made diabetic by injecting streptozotocin (65 mg/kg) and 30-32 weeks later the hearts were excised and used for an electrophysiological study. Action potentials were recorded from isolated right ventricular myocytes by an electrode fabricated for patch clamp in the whole-cell recording configuration. The action potential durations (APDs) of steady state chronic diabetic rat myocytes were longer than those of age-matched normal rat myocytes at all levels of repolarization (APD25, APD50, APD75, and APD90). As the stimulation frequency was increased (0.2-2 Hz), the APDs were lengthened in both diabetic and normal rats, and the difference of APDs between the groups was greater when the stimulation frequency was higher. When we examined alterations of APDs under conditions of train stimulation (2Hz, 20 stimuli), (1) the APDs in both groups were prolonged, and (2) the degree of prolongation of APD was significantly greater and the rate of APD prolongation was significantly faster in myocytes from the diabetic rats. The prolongation of APD in these heart cells is probably secondary to alteration of the transient outward current Ito, and sheds light on repolarization abnormality in cases of diabetic cardiomyopathy.

Biosynthesis and initial processing of the cardiac sarcolemmal Na(+)-Ca2+ exchanger

Based on the deduced amino-acid sequence of the cardiac Na(+)-Ca2+ exchanger, there are six potential N-linked glycosylation sites and a potential cleaved signal sequence. To study the post-translational modifications of the exchanger, in vitro translation was examined in the presence and absence of canine pancreatic microsomes. Glycosylation, detected as endoglycosidase H induced shifts in molecular size, was examined for proteins having different numbers of potential N-linked glycosylation sites by using full and partial length RNA transcripts. In the presence of microsomes, the molecular mass of the full-length clone increased from 110 to 113 kDa. Endoglycosidase H treatment led to a reduction to 108 kDa, indicating that glycosylation increases the molecular mass by approx. 5 kDa and a signal sequence of approx. 2 kDa is cleaved during processing. Analysis of molecular-mass shifts obtained with partial transcripts suggested that glycosylation occurs at position N-9. This was confirmed by site-directed mutagenesis studies. A molecular mass of approx. 120 kDa was measured for Western blots of cardiac sarcolemmal membrane or oocytes expressing the wild-type exchanger. The molecular mass was reduced by approx. 10 kDa for the N9Y mutant or from exchanger obtained from a baculovirus-infected insect cell line where glycosylation does not occur. The giant excised patch technique was used to determine the functional consequences of glycosylation. Na(+)-Ca2+ exchange current was examined in patches from oocytes expressing either the wild-type or N9Y mutant. The non-glycosylated mutant exhibited the same properties as the native exchanger with respect to voltage, sodium dependence, and the effects of chymotrypsin. The results indicate that glycosylation does not affect exchanger function in Xenopus oocytes and help to define exchanger topology.

Ouabain. A stimulator of atrial natriuretic peptide secretion and its mechanism of action

Ouabain increases atrial natriuretic peptide (ANP) secretion. When isolated superfused rat left atria were paced at 2 Hz, ouabain at concentrations of 50, 100, and 200 microM increased ANP secretion by 2.0 +/- 0.3-, 3.2 +/- 0.5-, and 4.2 +/- 0.5-fold, respectively. In this study, we examine the mechanism of ouabain-stimulated ANP secretion using the dose of 100 microM. To determine whether calcium played a role, atria were superfused with the calcium antagonist lanthanum. Superfusion with 2 mM LaCl3 completely inhibited ouabain-stimulated secretion, suggesting that calcium influx and/or sarcoplasmic reticulum (SR) calcium release provide essential sources of calcium for the stimulatory pathway. To determine the contribution of calcium from the SR, atria were superfused with ryanodine, an agent that depletes the SR of calcium. Superfusion with 1 microM ryanodine inhibited ouabain-stimulated secretion by 47%. Inhibition of Na+,K(+)-ATPase allows sodium to accumulate in the cell. A rise in intracellular sodium alters Na(+)-Ca2+ exchange, leading to an increase in cytosolic calcium. To determine the mechanism of sodium entry, atria were superfused with 5-(N,N-hexamethylene)amiloride (HMA), an inhibitor of Na(+)-H+ exchange, or with bumetanide, an inhibitor of Na(+)-K(+)-Cl- cotransport. Superfusion with 25 microM HMA inhibited ouabain-stimulated secretion by 71%; however, 100 microM bumetanide had no significant effect on secretion. Ouabain failed to stimulate ANP secretion by nonpaced (nonbeating) atria. Likewise, superfusion with the combination of ryanodine (1 microM) and the calcium channel antagonist israpidine (10 microM) totally blocked ouabain-stimulated ANP secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage dependence of force- and slow inward current restitution in ventricular muscle

This study was aimed to assess the relationship among the voltage-dependent processes underlying the excitation-contraction coupling, viz. force restitution (FR), transmembrane Ca fluxes and Ca release. The experiments (n = 22) were performed on voltage-clamped dog trabeculae in which force and slow inward current were measured. Standard steady-state was achieved by clamp driving at 0.5 Hz, 300 ms, 70 mV depolarizing pulses from holding = resting potential at 30 degrees C. Voltage and duration of single pulses and intervals in between were varied according to five protocols. The voltage dependence of Ca release was tested by varying single pulses at equal steady-state, i.e., at equal Ca availability. Contractions could be elicited in absence of ICa (20-30 mV step) and in the presence of disproportionately small ICa (above 80 mV). The voltage dependence of Ca availability for the release was tested by constant test pulses following either a variable conditioning clamp pulse or a period of rest at a variable voltage. After a low voltage pulse and, hence, depressed or absent ICa, the test contraction is diminished in presence of normal or even augmented Isi at any test interval (i.e., FR is depressed). Diminished Ca influx thus reduces the Ca availability of the subsequent beat. During prolonged depolarization (by 60 mV and more) a tonic response appears, but a phasic response cannot be elicited (FR is inhibited). Upon subsequent repolarization FR starts from zero and is significantly enhanced. It is concluded that, during depolarization, Ca release channels are in an open state, thus allowing free recirculation of Ca, but no build-up of a sufficient Ca gradient at the release site.

Mechanism of extracellular ATP-induced increase of cytosolic Ca2+ concentration in isolated rat ventricular myocytes

1. Changes in the cytosolic Ca2+ concentration ([Ca2+]i) of isolated rat ventricular myocytes in suspension were measured in response to extracellular ATP using the fluorescent Ca2+ indicators Quin-2 and Fura-2. 2. ATP produced a concentration-, time- and Mg(2+)-dependent, biphasic increase of [Ca2+]i whereas slowly hydrolysable ATP analogues produced a slow, monophasic increase of [Ca2+]i and the non-hydrolysable ATP analogues were without effect. 3. Extracellular Ca2+ was required for the ATP-induced increase of [Ca2+]i and pre-treatment of the cells with caffeine, ryanodine, verapamil or nimodipine partially inhibited the [Ca2+]i increase. 4. Whole-cell patch-clamp experiments revealed that ATP activated an ionic current that had a linear current-voltage relationship with a reversal potential near O mV. Quinidine, a putative P2 purinergic receptor blocker, abolished the ATP-activated current. The ATP-activated current was Mg2+ dependent. 5. Associated with the ATP-activated current was cellular depolarization. In a physiological solution, ATP depolarized cells to the threshold for the firing of action potentials. In the presence of the voltage-activated ion channel blockers tetrodotoxin, 4-aminopyridine, caesium and nitrendipine, ATP depolarized cells to -44 +/- 6 mV from a resting potential of -66 +/- 4 mV (n = 11). 6. Sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis and autoradiography demonstrated that extracellular ATP stimulated the phosphorylation of several extracellular membrane-bound proteins. The phosphorylation of these proteins was concentration, time and Mg2+ dependent. Pre-treatment of cells with the slowly hydrolysable ATP analogues inhibited the ATP-induced phosphorylation. Adenosine 5'-O-3-thiotriphosphate (ATP gamma S) thiophosphorylated proteins with the same apparent molecular weight as the proteins phosphorylated by ATP. 7. These results suggest that the ATP-induced increase of [Ca2+]i is a result of the activation, possibly by protein phosphorylation, of a novel ion channel carrying inward current. The ATP-activated channel may be permeable to Na+ and Ca2+ and causes [Ca2+]i to rise. More importantly, this inward current depolarizes the cell to the threshold of inducing spontaneous firing of action potentials. The firing of action potentials results in the influx of Ca2+ through L-type Ca2+ channels which would trigger Ca2+ release from the sarcoplasmic reticulum and lead to the increase in [Ca2+]i.

Control of the Na-Ca exchanger in isolated heart cells. II. Beat-dependent activation in normal cells by intracellular calcium

Electrical stimulation of isolated adult rat heart cells in suspension at 4 Hz resulted in a fourfold increase in the rate of sodium influx and efflux across the sarcolemma, with no change in total cell sodium, as measured with 22Na. The magnitude of stimulation-dependent sodium fluxes under these conditions averaged 17 nmol/min/mg protein. The increased rate of efflux was inhibited by tetrodotoxin, verapamil, or dichlorobenzamil and required extracellular calcium. The inhibition by tetrodotoxin was overcome by Bay K 8644. The basal rate of 22Na efflux in cells at rest was inhibited only slightly by dichlorobenzamil. The stimulation-induced efflux was not inhibited by ouabain, but in the presence of ouabain, stimulation increased the rate of accumulation of total sodium by 4 nmol/min/mg. This increase was inhibited by tetrodotoxin or verapamil. A calcium-dependent increase in rate of 22Na influx and efflux could also be induced by KCl addition. This was inhibited by verapamil and dichlorobenzamil but not by tetrodotoxin and was reversed by EGTA, but only after a delay. We conclude the following. 1) The Na-Ca exchanger in cells at rest is no more than 10% activated. 2) The exchanger becomes activated directly or indirectly by calcium that enters the cell through calcium channels during excitation. 3) In this preparation the major part of excitation-induced sodium fluxes are mediated by the Na-Ca exchanger, with only a relatively small direct participation of sodium channels. These channels participate indirectly by promoting calcium channel activation. 4) If all the calcium-dependent sodium fluxes were Na-Ca exchange, then calcium flux through the exchanger per beat would be about sevenfold larger than that through the calcium channels. An undetermined part of the calcium-dependent sodium fluxes, however, could be a direct Na-Na exchange through the activated Na-Ca exchanger.

Control of the Na-Ca exchanger in isolated heart cells. I. Induction of Na-Na exchange in sodium-loaded cells by intracellular calcium

Isolated adult rat heart cells in suspension were loaded with sodium by incubation with ouabain in the absence of calcium for 30 minutes. Addition of low levels of calcium induced accelerated rates of sodium influx and efflux, as measured with 22Na. The magnitude of calcium-induced 22Na efflux was 50-fold greater than the net rate of calcium uptake and required extracellular sodium, but not extracellular calcium, once some calcium was taken up. Calcium did not induce 86Rb efflux. The accelerated rate of 22Na efflux was prevented by verapamil, but verapamil was ineffective when added after calcium. Addition of EGTA after calcium reversed the effect of calcium, but only after incubation. Dichlorobenzamil, unlike verapamil, both prevented and reversed the induction of sodium fluxes by calcium. We conclude 1) that intracellular calcium induces Na-Na exchange through the Na-Ca exchanger in sodium-loaded cells exposed to calcium; and 2) that Na-Na exchange can be activated by calcium that enters the cell through calcium channels. We propose that this Na-Na exchange reflects the intrinsic activity of the Na-Ca exchanger.

The control of the contraction of myocytes from guinea-pig heart by the resting membrane potential

1. The influence of different holding potentials (-120 to -70 mV) on the contraction of enzymatically dispersed myocytes from guinea-pig hearts was evaluated. Contractions were elicited by repetitive depolarizations to 0 mV at 0.5 Hz. 2. While ineffective at 140 and 5 mmol l-1 [Na+]o and pipette Na+, respectively, depolarization of the resting membrane with the holding potential increased myocyte shortening at reduced Na+ gradients ([Na+]o 70 or [Na+]i 10-15 mmol l-1). Elevated intracellular Na+ after Na(+)-pump inhibition with ouabain 1-10 mumol l-1 was similarly effective with regard to the inotropic response to different holding potentials. 3. At -70 mV holding potential, reduction of [Na+]o from 140 to 70 mmol l-1 increased myocyte shortening and induced an inwardly directed component of the holding current which peaked at -44 +/- 10 pA and declined thereafter in parallel with the inotropic effect. The relation of this inward current to [Ca2+]i was confirmed by experiments at high Ca2+ buffer capacity where [Na+]o reduction induced a Ni(2+)-insensitive, outwardly directed component (36 +/- 15 pA) of the holding current. The observed inward current is suggested to reflect the extrusion of [Ca2+]i in exchange for [Na+]o as a counter-regulatory mechanism which limits the increase of [Ca2+]i. 4. The interventions which increased the strength of the contraction also enhanced the transient tail current after repolarization, suggesting its close relation to [Ca2+]i. This finding confirmed the pattern found with cell shortening. 5. It is concluded that under certain conditions, voltage-dependent and Na(+)-dependent Na(+)-Ca2+ exchange during the interval between the contractions is relevant to the diastolic concentration of [Ca2+]i which in turn determines the accumulation of Ca2+ in the sarcoplasmic reticulum and the magnitude of the subsequent contraction.

Comparison between the cardiac effects induced by muzolimine and furosemide in guinea-pig atria

Muzolimine (10-500 microM) induced a concentration-dependent reduction of both the contractile force and frequency in spontaneously beating atria and in electrically driven left atrium from reserpine-treated guinea pigs. This negative inotropic response was unaffected by the addition of atropine to the perfusion fluid, and it was highly sensitive to changes in external Ca2+ concentration. Both in spontaneously beating and in electrically driven atrium, muzolimine (50-400 microM) antagonized, in an apparently competitive manner, the increase in contractile force induced by cumulative addition of CaCl2 (0.68-9.59 mM) to the bathing fluid. Muzolimine (50-100 microM) reduced the inotropic response to low (5-30 nM), but not high (50-100 nM) concentrations of Bay K 8644, a calcium-channel agonist. The inotropic effects of 8-phenyltheophylline and of ouabain were antagonized by muzolimine (10-100 microM) in a noncompetitive manner, while the response to noradrenaline was not altered. Similar to muzolimine, verapamil at a concentration suitable to block calcium channels inhibited, in a noncompetitive way, the inotropic effect induced by 8-phenyltheophylline and by ouabain without altering the contractile response to noradrenaline. Furosemide (10 and 100 microM) did not influence the contractile force or the frequency of spontaneously beating atria, nor the inotropic effect induced by CaCl2, 8-phenyltheophylline, ouabain, or noradrenaline. These results indicate that the influence of muzolimine on guinea-pig atria originates from an inhibition of Ca2+ influx into cardiac cells and that furosemide does not mimic the effect of muzolimine at this level.

Sustained subthreshold-for-twitch depolarization in rat single ventricular myocytes causes sustained calcium channel activation and sarcoplasmic reticulum calcium release

Single rat ventricular myocytes, voltage-clamped at -50 to -40 mV, were depolarized in small steps in order to define the mechanisms that govern the increase in cytosolic [Ca2+] (Cai) and contraction, measured as a reduction in myocyte length. Small (3-5 mV), sustained (seconds) depolarizations that caused a small inward or no detectable change in current were followed after a delay by small (less than 2% of the resting length), steady reductions in cell length measured via a photodiode array, and small, steady increases in Cai measured by changes in Indo-1 fluorescence. Larger (greater than -30 and less than -20 mV), sustained depolarizations produced phasic Ca2+ currents, Cai transients, and twitch contractions, followed by a steady current and a steady increase in Cai and contraction. Nitrendipine (or Cd, verapamil, or Ni) abolished the steady contraction and always produced an outward shift in steady current. The steady, nitrendipine-sensitive current and sustained increase in Cai and contraction exhibited a similar voltage dependence over the voltage range between -40 and -20 mV. 2 microM ryanodine in the presence of intact Ca2+ channel activity also abolished the steady increase in Cai and contraction over this voltage range. We conclude that when a sustained depolarization does not exceed about -20 mV, the resultant steady, graded contraction is due to SR Ca2+ release graded by a steady ("window") Ca2+ current. The existence of appreciable, sustained, graded Ca2+ release in response to Ca2+ current generated by arbitrarily small depolarizations is not compatible with any model of Ca2(+)-induced Ca2+ release in which the releasing effect of the Ca2+ channel current is mediated solely by Ca2+ entry into a common cytosolic pool. Our results therefore imply a distinction between the triggering and released Ca2+ pools.

Role of ventriculovascular coupling in cardiac response to increased contractility in closed-chest dogs

While both dobutamine and pacing tachycardia augment left ventricular (LV) contractility, whether overall cardiovascular response to these stimuli is comparable is not known. To address this question we studied seven dogs previously instrumented with three LV diameter gauges and LV pressure manometers. After ganglionic blockade and sedation, caval occlusions were performed at heart rates of 120, 160, and 200 bpm before (C), and 160 and 200 bpm after administration of 10 micrograms/kg per min dobutamine, i.v. (D). The effective arterial elastance (Ea) went up from 14.2 +/- 4.5 mmHg/ml at C120 to 19.6 +/- 8.8 (P less than 0.025 vs C120) and 24.2 +/- 10.4 (P less than 0.001 vs C120) mmHg/ml at C160 and C200. Ees, the slope of the end-systolic pressure-volume relation, increased with pacing from 9.7 +/- 4.6 to 11.7 +/- 4.3 (P less than 0.02), and 13.2 +/- 5.7 (P less than 0.02) mmHg/ml at 160 and 200 bpm. With dobutamine infusion Ea went down, and Ees was further increased to 37.0 +/- 20.9 mmHg/ml at 160 bpm (P less than 0.002 vs C160), and 53.0 +/- 22.6 mmHg/ml at 200 bpm (P less than 0.002 vs C200). Comparison of stroke work and pressure-volume area from single beats with matched LV end-diastolic volumes showed that these were both increased by dobutamine, but not by pacing tachycardia. While increased heart rate after dobutamine markedly increased contractility, Ea was not changed, and neither stroke work nor pressure-volume was further increased. Thus, how well an increase in contractility is transmitted to the periphery is determined in part by arterial behavior. Assessment of both the arterial system and cardiac contractility is necessary to fully evaluate the overall impact of an inotropic stimulus.

Effects of KT-362, a new calcium release blocker, on vascular selectivity and hemodynamic actions in anesthetized dogs

Pharmacological properties of 5-(3-((2-(3,4-dimethoxyphenyl)ethyl)-amino)-1- oxopropyl)-2,3,4,5-tetrahydro-1,5-benzothiazepine fumarate (KT-362), a newly synthesized calcium release blocker, were studied by comparing its vascular selectivity and cardiovascular actions with those of verapamil, a calcium entry blocker. The relaxing effect of KT-362 in rabbit femoral and basilar artery strips contracted with norepinephrine was greater than that in aortic and coronary artery strips. In anesthetized mongrel dogs, KT-362 (0.1-3.0 mg/kg, i.v.) decreased the mean blood pressure, heart rate and total peripheral resistance in a dose-dependent manner, while cardiac output increased slightly despite a decrease in left ventricular pressure. This is consistent with the data on verapamil. Both i.a. and i.v. injections of KT-362 produced a marked dose-dependent increase in vertebral and femoral blood flow. Pretreatment of atropine, propranolol or diphenhydramine exerted no significant effect on the KT-362-induced vasodilation. Verapamil caused a marked increase in the vertebral and coronary blood flows after the injections, but only a slight increase in femoral blood flow. KT-362 at the dose of 10 mg/kg, i.v., had no significant effect on the PQ interval on the electrocardiogram in anesthetized dogs, but 0.1 mg/kg of verapamil increased this interval significantly. These results suggest that KT-362 has properties similar to calcium entry blockers such as verapamil on systemic hemodynamic actions except for the reactivity of vasculatures.

The effect of increasing extracellular potassium concentration on the resting heart rate of the isolated rat papillary muscle

The effect of elevating extracellular K+ concentration on the basal metabolism of the isolated rat left ventricular papillary muscle has been investigated. The preparation was mounted on a thermopile and connected to a force transducer, to allow simultaneous measurement of muscle heat production and force. The resting heat rate (RHR) of the quiescent preparation was measured as an index of basal metabolism. Throughout all of the experiments, the muscles were maintained under a resting force of 10 mN and all measurements of RHR were made at times when there was no active force present above this passive level. Elevating the extracellular K+ concentration from 5.9 to 20, 40, then 80 mM produced graded increases in the RHR. The increase in RHR produced by 40 mM K+ was observed to be time-dependent, its effect being significantly greater at 5-7 h than at 2-4 h after cardiectomy. Averaged over all times, the percentage increases in RHR produced by 20, 40, and 80 mM K+ in the presence of 2 mM Ca2+ were 6.4 +/- 2.0%, 28.7 +/- 2.3%, and 51.3 +/- 8.9% (mean +/- SEM) respectively. The high K(+)-induced increase in basal metabolism was also shown to be Ca2(+)-dependent, the increase in RHR produced by 40 mM K+ being greater the higher the extracellular Ca2+ concentration (0.5-8.0 mM). The addition of verapamil was found to partially inhibit the K(+)-induced increase in resting metabolism. These results show that elevation of the extracellular K+ concentration produces a graded increase in the RHR that is Ca2(+)-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of palytoxin on the calcium current and the mechanical activity of frog heart muscle

1. The effect of palytoxin (PTX) on the Ca current (ICa) and the mechanical activity of frog atrial fibres was studied by use of the double sucrose gap voltage clamp technique. 2. In normal Ringer solution, PTX transiently increased the electrically-evoked peak tension which then decreased while a major contracture developed. PTX slowed the time course of the relaxation phase of the evoked tension. 3. Evidence is presented which suggests that the toxin also increased the entry of Ca and Sr via the Na-Ca exchange mechanism. It also induced the development of a Ca-dependent outward current which was inhibited by Sr. 4. In Na-free solution, PTX increased ICa and shifted the reversal potential for Ca towards more negative membrane potentials, thus suggesting that the internal Ca concentration had increased. Current-voltage, tension-voltage, time to peak-voltage and inactivation time constant-membrane potential curves were all shifted towards more negative membrane potentials in the presence of PTX. 5. These effects of PTX are similar to those caused by the increase in internal Ca concentration induced by Na ionophores by way of voltage-dependent Ca influx of the Na-Ca exchange mechanism.

Egg jelly triggers a calcium influx which inactivates and is inhibited by calmodulin antagonists in the sea urchin sperm

Sea urchin sperm must undergo the acrosome reaction to fertilize eggs. The natural inducer of this reaction is the most external coat of the egg, named 'jelly'. The ionic composition of the extracellular and intracellular media and the permeability properties of the sperm plasma membrane are fundamental in this reaction. As Ca2+ is required for the acrosome reaction to occur, its intracellular concentration ([Ca2+]i) was measured with fura-2. In 10 mM Ca2+, egg jelly induced the acrosome reaction and an increase in [Ca2+]i that lasted for several minutes. However, at 0.5 or 2 mM Ca2+, it became evident that the Ca2+-influx pathway activated by jelly opened only for a few seconds; this prevented both the full increase in [Ca2+]i and the acrosome reaction even after the concentration of Ca2+ was raised to 10 mM. In the presence of jelly, the time this permeability pathway remained open was inversely related to the extracellular concentration of Ca2+ ([ Ca2+]e). Using Bisoxonol (a permeant fluorescent membrane potential probe), it was found that the jelly-induced depolarization depended on [Ca2+]e and was proportional to the increase in [Ca2+]i. Since [Ca2+]i could affect the jelly-induced Ca2+ influx through calmodulin, two of its antagonists, trifluoperazine and W-7, were tested. Both compounds blocked the acrosome reaction by inhibiting the jelly-induced increase in [Ca2+]i. W-5 at the same concentration had no effect. The results suggest that one of the jelly-activated Ca2+-influx pathways, probably a channel, is the target of the calmodulin antagonists.

'Slow' K+-stimulated Ca2+ influx is mediated by Na+-Ca2+ exchange: a pharmacological study

K+-stimulated 45Ca2+ influx was measured in rat brain presynaptic nerve terminals that were predepolarized in a K+-rich solution for 15 s prior to addition of 45Ca2+. This 'slow' Ca2+ influx was compared to influx stimulated by Na+ removal, presumably mediated by Na+-Ca2+ exchange. The K+-stimulated Ca2+ influx in predepolarized synaptosomes, and the Na+-removal-dependent Ca2+ influx were both saturating functions of the external Ca2+ concentration; and both were half-saturated at 0.3 mM Ca2+. Both were reduced about 50% by 20 microM Hg2+, 20 microM Cu2+ or 0.45 mM Mn2+. Neither the K+-stimulated nor the Na+-removal-dependent Ca2+ influx was inhibited by 1 microM Cd2+, La3+ or Pb2+, treatments that almost completely inhibited K+-stimulated Ca2+ influx in synaptosomes that were not predepolarized. The relative permeabilities of K+-stimulated Ca2+, Sr2+ or Ba2+ influx in predepolarized synaptosomes (10:3:1) and the corresponding selectivity ratio for Na+-removal-dependent divalent cation uptake (10:2:1) were similar. These results strongly suggest that the K+-stimulated 'slow' Ca2+ influx in predepolarized synaptosomes and the Na+-removal-dependent Ca2+ influx are mediated by a common mechanism, the Na+-Ca2+ exchanger.

Depolarization of macrophage polykaryons in the absence of external sodium induces a cyclic stimulation of a calcium-activated potassium conductance

Macrophage polykaryons associated with the foreign body granuloma display several electrophysiological properties when studied with intracellular microelectrodes. One of the most evident properties is the slow hyperpolarization (2-5 s long, 10-60 mV amplitude), due to transient openings of Ca2+-dependent K+ channels, that is similar to those observed in macrophages. How this oscillation of membrane potential is triggered is not well known and the only way to repeatedly activate it under experimental control is through the intracellular injection of Ca2+. Although this technique is important for understanding the properties of the K+ channels, no information has been obtained about the way Ca2+ levels are raised and controlled in the cytosol. Slow hyperpolarizations can also be triggered by electrical stimulation, but reproducibility is low with cells bathed in physiological solutions. We then decided to investigate the effect of depolarization on the electrophysiological properties of macrophage polykaryons exposed to bathing solutions of several ionic compositions. We show in this paper that cell membrane depolarization induced by a long current pulse can trigger several patterns of membrane potential changes and that, in the absence of extracellular Na+, repetitive oscillations of decaying amplitudes are observed in almost all the cells. They are very similar to the slow hyperpolarizations, are dependent on the presence of extracellular Ca2+, and are blocked by quinine and D-600. Whole-cell patch clamp recording under voltage clamp conditions showed an outward current that oscillates and that also exhibits decaying amplitudes. The data presented here indicate that these oscillations are a consequence of the cyclic opening of the Ca2+-activated K+ channels and support the hypothesis that favors the participation of Ca2+ channels and of the Ca2+/Na+ exchange system in their triggering. These two mechanisms are not enough to explain either why the K+ channels close or why the membrane potential returns to the original level at the end of each cycle. The possibility of using these oscillations as a model to study the slow hyperpolarization is discussed.

Defects in sarcolemmal Ca2+ transport in hearts due to induction of calcium paradox

Na+-Ca2+ exchange and Ca2+-pump activities were studied in sarcolemmal vesicles isolated from rat hearts subjected to "calcium paradox" on perfusion with Ca2+-free medium followed by reperfusion with medium containing 1.25 mM Ca2+. Perfusion of hearts with Ca2+-free medium for 5 minutes did not affect the Na+-dependent Ca2+ uptake, ATP-dependent Ca2+ uptake, or Ca2+-stimulated ATPase activities in sarcolemma. Reperfusion of the Ca2+-deprived hearts with medium containing Ca2+ for 1-2 minutes increased Na+-dependent Ca2+ uptake, whereas reperfusion for 5-10 minutes decreased Na+-dependent Ca2+ uptake in sarcolemmal vesicles. Both ATP-dependent Ca2+ uptake and Ca2+-stimulated ATPase activities in sarcolemma were depressed on reperfusion of Ca2+-deprived hearts for 2-10 minutes. Reperfusion of Ca2+-deprived hearts for 5 minutes, which failed to generate contractile force, resulted in contracture without any recovery of the contractile force development. These changes in sarcolemmal Ca2+ transport and contractile function were prevented when hearts were perfused with Ca2+-free medium either in the presence of low sodium (35 mM) or at a low temperature (21 degrees C) before starting the reperfusion. No alterations in the purity of the preparation or permeability of sarcolemmal vesicles with respect to Na+ or Ca2+ were detected in hearts perfused with Ca2+-free medium or on reperfusion with medium containing calcium. The results indicate abnormalities in sarcolemmal Na+-Ca2+ exchange and Ca2+-pump mechanisms on reperfusion of Ca2+-deprived hearts with medium containing Ca2+, and such changes may partly account for the occurrence of intracellular Ca2+ overload during the development of calcium paradox.

Stimulation of muscarinic receptors raises free intracellular Ca2+ concentration in rat ventricular myocytes

The effect of carbachol on free intracellular calcium concentration, ([Ca2+]i) and on intracellular hydrogen concentration (pHi) was determined from fluorescence signals obtained from rat ventricular myocytes. Application of carbachol (300 mumol/l) to quin2-loaded myocytes bathed in 2 mmol/l Ca2+-containing solution caused [Ca2+]i to increase within 7-10 minutes from 182 +/- 9 to 212 +/- 11 nmol/l (n = 4). Carbachol acted via stimulation of muscarinic receptors because atropine (1 mumol/l) either prevented or abolished the increase in [Ca2+]i. Carbachol also produced a positive inotropic effect in rat papillary muscles contracting isometrically at a frequency of 0.5 Hz and enhanced contracture in resting preparations in the presence of high extracellular Ca2+ concentration ([Ca2+]o) (20 mmol/l). The effect of carbachol on [Ca2+]i was dependent on [Ca2+]o. In the presence of 10 mmol/l [Ca2+]o, the increase in [Ca2+]i was about two times that elicited by carbachol when bath [Ca2+]o was 2 mmol/l. Reduction of [Ca2+]o to 50 mumol/l abolished the carbachol effect but did not prevent caffeine-induced Ca2+ release. The carbachol-induced rise in [Ca2+]i remained unchanged in the presence of either 10 mmol/l caffeine or 1 mumol/l ryanodine. In the absence of extracellular Na+ concentration [( Na+]o), carbachol no longer produced an increase in [Ca2+]i of cardiomyocytes and failed to enhance Na+-withdrawal contracture of the rat papillary muscle. In contrast to the effect on [Ca2+]i, carbachol did not produce any change in pHi as determined from fluorescence signals obtained from rat ventricular myocytes loaded with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of the cytosolic sodium ion concentration in rat brain synaptosomes by a fluorescence method

A method for the measurement of the cytosolic Na+ concentration in intact synaptosomes is described. This method makes use of a pH sensitive dye (BCECF) that can be loaded into the cytosol and a relatively specific ionophore (monensin) that can exchange Na+ for H+ across the synaptosomal membrane. By setting conditions such that there is no electrochemical potential difference for H+ across the membrane (no membrane potential and pHi = pHo), addition of ionophore would induce a H+ flux only if there is a concentration difference for Na+. Thus, when there is no fluorescence change (no cytosolic pH change) extracellular [Na+] equals intrasynaptosomal [Na+]. The intrasynaptosomal [Na+] concentration was determined to be 7 +/- 3 mM (n = 5; mean +/- S.E.). The results obtained with this fluorescence method are compared with estimates obtained by atomic absorption spectrometry. Limitations and applications of the method are discussed.

Effect of thyroid state on cytosolic free calcium in resting and electrically stimulated cardiac myocytes

The effects of the thyroid state on the cytosolic free Ca2+ concentration, [Ca2+]i, of resting and K+-depolarized cardiomyocytes were studied using the fluorescent Ca2+ indicator fura2. The mean resting [Ca2+]i in euthyroid myocytes (89 +/- 8 nM) was not significantly different from that in hyperthyroid myocytes (100 +/- 14 nM). The resting O2-consumption rate was identical for both groups when expressed per mg protein, but a 35% higher value was observed in the hyperthyroid group when expressed per cell on account of the cellular hypertrophy induced by thyroid hormone. Potassium induced depolarization (50 mM [K+]0) raised the level of [Ca2+]i by 50% in both groups. When ATP-coupled respiration was blocked with oligomycin, the 50 mM K+-induced rise in [Ca2+]i was accompanied in both groups by a 40% rise in glycolytic activity as inferred from measurement of lactate production. Ca2+-fluorescence transients were recorded from electrically stimulated myocytes of euthyroid, hyperthyroid and hypothyroid rats. The time taken to reach peak fluorescence (TPL) and that to 50% decay of peak fluorescence (RL0.5) decreased in the direction hypothyroid----hyperthyroid, indicating an increase in Ca2+ fluxes in the same direction. Isoproterenol (1 microM) enhanced the peak Ca2+ fluorescence in electrically stimulated hypothyroid and euthyroid myocytes but not in hyperthyroid myocytes. Both the TPL and RL0.5 were decreased by isoproterenol in euthyroid, but more so in hypothyroid myocytes. None of these parameters were influenced by isoproterenol in the hyperthyroid group. We conclude that (1) thyroid hormone increases neither the O2-consumption rate nor the level of [Ca2+]i of resting cardiomyocytes and (2) the effects of the beta-receptor-agonist isoproterenol on Ca2+ transients of electrically stimulated myocytes, are inversely related to the documented changes in beta-receptor density in heart tissue occurring with alterations in the thyroid state.

Heat production of quiescent ventricular trabeculae isolated from guinea-pig heart

1. A new calorimetric technique has been developed which allows continuous measurement of the rate of energy expenditure in superfused preparations of cardiac muscle. Thin trabeculae of guinea-pig ventricular muscle were mounted in a Perspex tube of 0.8 mm inner diameter and the temperature difference of the perfusate upstream and downstream of the preparation was measured. 2. The resting heat rate of trabeculae of 240-575 microns diameter from guinea-pig heart was determined repeatedly for up to 6 h after cardiectomy. It did not vary with time during the course of the experiment. 3. The average resting heat rate measured in HEPES-buffered Tyrode solution containing 20 mM-glucose and 2 mM-pyruvate as substrates was 130 +/- 29 mW/g dry weight or 36 +/- 8 mW/cm3 of tissue (n = 15). This is an order of magnitude larger than the resting heat rate reported in the literature for isolated cardiac preparations. 4. After omitting the pyruvate from the superfusate the resting heat rate decreased to 60-70% of its steady value within 4 min. After readmission of pyruvate this effect was reversed. The average resting heat rate with glucose as sole substrate was 23 +/- 4 mW/cm3. 5. Uncoupling of the mitochondria by 50 microM-2,4-dinitrophenol (DNP) increased the heat rate up to 170 mW/cm3. This effect could be maintained for several minutes and was fully reversible. Raising the external K+ concentration to 150 mM (NaCl replaced by KCl) induced a transient rise in the rate of heat production up to 115 mW/cm3. 6. The heat production during uncoupling of the mitochondria and during potassium contractures was inversely related to the diameter of the preparation. Calculation based on Hill's equation (Hill, 1928) indicated that this was caused by the development of anoxia at the core of the preparation. 7. In contrast, the rate of heat production of quiescent preparations was not correlated with diameter and calculation indicated that at rest there was no anoxic core. The high value of resting heat rate found in the present study is discussed within the context of the large variation of 1.7-25 mW/g reported in the literature for resting metabolic rate of cardiac muscle.

Low-sodium contractures indicating sarcolemmal Na/Ca-exchange in the frog heart

1. In the frog heart, Ca2+ enters the cell by the slow inward current (Isi) and by an electrogenic, carrier-mediated, and passive Na-out/Ca-in-exchange. 2. The latter reverses to Na-in/Ca-out-exchange during depolarization and thereby controls relaxation. 3. The exchange ratio is 3 Na+ for 1 Ca2+. 4. The Na/Ca-exchange is not inhibited by organic Ca-antagonists in frog myocardium, indicating that the initiation of the heart beat may mainly depend on Isi. 5. This is not necessarily in contradiction with the Na-Ca-antagonism, since there also exists an antagonism between Na+ and Ca2+ in the slow channel. 6. However, the contractures caused by a decrease of NaO+ are mediated by the Na/Ca-exchange.

Ryanodine releases calcium from sarcoplasmic reticulum in calcium-tolerant rat cardiac myocytes

1. The hypothesis tested in this study is that ryanodine depletes sarcoplasmic reticulum (s.r.) Ca2+ loading in suspensions of single adult rat cardiac myocytes by effecting Ca2+ release into the myoplasm resulting in an increase in myoplasmic free [Ca2+] ([Ca2+]i). The latter was monitored by the fluorescent dye, quin2. 2. The competency of the technique to detect s.r. Ca2+ release was tested by using caffeine to induce Ca2+ release. The addition of 5-10 mM-caffeine to myocytes loaded with quin2 and incubated in a medium containing 1 mM-Ca2+ gives a large, transient increase in fluorescence, which is interpreted as indicating an increase in [Ca2+]i. If the chelating agent EGTA is added to the cell suspension 1-5 min prior to the caffeine, to a concentration sufficient to decrease extracellular Ca2+ to 0.1-0.15 microM, then caffeine again gives a large, transient increase in fluorescence, indicative of the fact that sarcolemmal Ca2+ transport is not necessary for this response. The ionophore ionomycin also raises [Ca2+]i in a transient manner when added after EGTA. The addition of caffeine prior to ionomycin largely diminishes the response to the latter; however, addition of ionomycin prior to caffeine totally abolishes its effect to increase [Ca2+]i. This is taken to indicate that the intracellular store which is releasable by caffeine--and which presumably reflects the s.r.--is also releasable by ionomycin: ionomycin, however, also gives access to another, minor intracellular pool. 3. The plant alkaloid, ryanodine, at concentrations of 10(-8) to 10(-6) M, consistently causes a slow and prolonged increase in [Ca2+]i when added to cell suspensions incubated with 1 mM-extracellular Ca2+. Under conditions precluding net entry of Ca2+ into the cell, viz. 0.1 microM-extracellular Ca2+, ryanodine causes a more limited, partially reversible, increase in [Ca2+]i. 4. When added prior to EGTA, ryanodine attenuates, or prevents, the subsequent response to caffeine: efficacy depends upon the time of pre-incubation (1-10 min) and the concentration of ryanodine (10(-8) to 10(-6) M). When the response to caffeine is largely prevented by ryanodine, the response to ionomycin is also severely attenuated, i.e. there is no evidence that ryanodine causes sequestration of Ca2+ within an ionomycin-sensitive pool.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of calcium influx in isolated adult rat heart cells by ATP depletion

Using 45Ca, indo1, and quin2, calcium uptake was measured in isolated quiescent adult rat heart cells under different metabolic conditions. Exposure of cells in a medium containing 1 mM CaCl2 to rotenone and uncoupler resulted in adenosine triphosphate (ATP) depletion from 17.08 +/- 2.26 to 0.63 +/- 0.11 nmol/mg within 8 minutes, and the cells went into contracture. In this time, the cells lost 1.65 +/- 0.1 nmol Ca/mg of total rapidly exchangeable cellular calcium, and the level of free cytosolic calcium as measured by indo1 rose from 47.4 +/- 16.3 nM to 79.8 +/- 27.6 nM. The subsequent rate of rise of intracellular free calcium concentration was just 4 nM/min for at least 40 minutes. Therefore, we investigated the effect of ATP depletion on the rate of calcium entry. In cells loaded with sodium by ouabain treatment without calcium, the initial rate of calcium influx on calcium addition was inhibited by 82-84% when cellular ATP was depleted, as measured by 45Ca or indo1. Quin2 also showed a strong inhibition of calcium influx by ATP depletion, but itself also caused a strong inhibition of calcium influx. The rate of calcium influx declined even further in ATP-depleted cells after the initial influx: Between 1 and 12 minutes after calcium addition, the residual 45Ca uptake rate of the first minute was inhibited by an additional 90%. We conclude that ATP depletion per se does not quickly elevate cytoplasmic free calcium and that such an elevation is prevented by a very strong inhibition of the rate of calcium entry.

Effects of ryanodine and caffeine on contractility, membrane voltage, and calcium exchange in cultured heart cells

To investigate the mechanisms of action of ryanodine and caffeine, changes in mechanical and electrical activity caused by these agents were correlated with alterations in 45Ca fluxes and cell Ca contents in chick embryo ventricular cell monolayer cultures. Ryanodine (10(-10)-10(-5) M) irreversibly decreased contraction amplitude by 10-70% relative to control in a concentration-dependent manner with minimal effects on electrical activity. Ryanodine caused a slight decrease in rapid 45Ca uptake, but no change in total exchangeable calcium content or rapid 45Ca efflux. Caffeine (1-20 mM) caused a transient (less than 10 seconds) 5-12% increase in contraction amplitude followed by a sustained 9-76% decrease in contraction amplitude and a 10 mV decrease in diastolic membrane voltage. Caffeine caused a decrease in rapid 45Ca uptake, a decrease in total exchangeable calcium content, and an increase in rapid 45Ca efflux. These results suggest that caffeine produces a decrease in sarcoplasmic reticulum (SR) Ca2+ uptake, and/or an increase in SR Ca2+ release that eventually depletes the SR of Ca2+, presumably accounting for the negative inotropic effect. The ryanodine effects on contraction are more difficult to account for solely in terms of alterations of transsarcolemmal Ca2+ fluxes and Ca2+ contents. Our data indicate an important role for the SR in excitation-contraction coupling in cultured chick embryo ventricular cells and suggest that SR Ca2+ is part of the rapidly exchanging Ca2+ compartment noted in 45Ca flux studies.

Relation between cytosolic free Ca2+ concentration and the control of pyruvate dehydrogenase in isolated cardiac myocytes

The proportion of pyruvate dehydrogenase existing in the active form (PDHA) in suspensions of unstimulated cardiac myocytes oxidizing glucose is approx. 30%. Depolarization of the cells with concentrations of K+ above physiological values leads to an increase in the content of PDHA. Overloading of the cells with Na+ by treatment with veratridine and ouabain gives the same result. Each of these interventions is shown in experiments with Quin 2-loaded myocytes to lead to an increase in cytosolic free Ca2+ concentration ([Ca2+]c). Treatment of the cells with Ruthenium Red, an inhibitor of Ca2+ transport into mitochondria, largely prevents an increase in PDHA in response to addition of KCl or of veratridine plus ouabain. Ruthenium Red does not attenuate the increase in [Ca2+]c that occurs under these conditions. By contrast, treatment of the cells with ryanodine, an inhibitor of sarcoplasmic-reticulum Ca2+ transport and therefore of contraction, does not diminish the response of PDHA content to agents which raise [Ca2+]c; nor does loading of the cells with the Ca2+-chelating agent Quin 2, which also prevents contraction, at appropriate concentrations. It is concluded that an increase in [Ca2+]c causes an increase in PDHA content of cardiac myocytes independently of an increase in mechanical work. In the normal physiological situation the activation of dehydrogenases by Ca2+ is thought to help to maintain the balance of energy supply and demand during periods of increased work-load, which are associated with an increased myoplasmic [Ca2+]c.

Cardiac energetics: significance of mitochondria

The mitochondrial activity as the energy producing step during biological oxidation was observed at rest and its regulation by the energy consuming auxotonic contractile work, depending on the preload, afterload and beat rate in isolated superfused left guinea pig atria. The mitochondrial activity was measured by (1) continuous determination of the O2 uptake rate, (2) the rate of 14CO2 production from labelled glucose or FFA and (3) separate measurements of the atrial ATP-, ADP-, AMP-, CP- and NAD-concentrations, for determination of the energy state. Some results, with points of general interest, are reported and discussed, including this model, former studies about cardiac energetics and the efficiency of cardiac work, reviewed recently.

Membrane electrical properties of vesicular Na-Ca exchange inhibitors in single atrial myocytes

Na-loading single frog atrial cells produce changes in membrane currents that are similar to the creep currents originally observed in Na-loaded cardiac Purkinje fibers. Exposure to the Na ionophore, monensin, was used to induce creep currents in isolated atrial cells. The sensitivity of myocardial creep currents to three compounds that have been shown to be inhibitors of Na-Ca exchange flux activity in isolated sarcolemmal vesicles was assessed. Dodecylamine, quinacrine, and the amiloride analog, 3',4'-dichlorobenzamil block creep currents at concentrations well below those required to block Na-dependent Ca uptake in sarcolemmal vesicles. The estimated Ki's for inhibition of myocardial creep currents were 3 microM for dodecylamin, 10 micron for quinacrine, and 4 microM for 3',4'-dichlorobenzamil. The sensitivity of creep currents to these compounds is consistent with the hypothesis that creep currents may represent the electrogenic activity of a Na-Ca exchange carrier. In an additional series of experiments, the relative specificity of these compounds was tested by examining their effects on myocardial membrane channels. Both dodecylamine and 3',4'-dichlorobenzamil were found to inhibit myocardial Ca and K currents over the same range of concentrations in which block of exchange activity occurs. These results seriously question the use of these exchange carrier inhibitors as selective experimental probes for defining the role of Na-Ca exchange in various physiological processes.