A phosphorus-31 nuclear magnetic resonance study of the metabolic, contractile, and ionic consequences of induced calcium alterations in the isovolumic rat heart

Sites and modes of action of proctolin and the FLP F2 on lobster cardiac muscle

At the threshold concentration (1-10 pmol l-1), the neuropeptide hormones proctolin (PR) and the FLRFamide-like peptide (FLP) F2cause an increase in amplitude of electrically evoked contractions (each contraction is a brief tetanus) of lobster heart ostial muscle. At higher concentrations each peptide also induces an increase in tonus (contracture). The PR-induced contracture and augmentation of tetani are proportional to increases in [Ca2+]i. The rate of onset and recovery of peptide-induced effects on both tetani and contracture appeared to reduced by Ca2+ storage by the sarcoplasmic reticulum (SR). Enhanced tetani following a contracture may be due to enhanced voltage-gated Ca2+current and sarcoplasmic reticular (SR) Ca2+ loading. The SR Ca2+ loading appears to be specific for PR and F2, since glutamic-acid-induced contractures are not followed by increased tetani. The prolonged elevation of [Ca2+]i during contracture causes a right-ward shift in the force-pCa curve indicating a decrease in myofibrillar sensitivity to Ca2+. Blocking voltage-gated Ca2+ channels with Cd2+, nifedipine or verapamil, while reducing tetani, does not prevent peptide-induced contracture and enhanced tetani. Opening SR Ca2+ channels and depleting SR Ca2+with either caffeine or ryanodine blocked tetani but permitted accelerated peptide-induced contractures. We conclude that PR and F2 at low concentration enhance voltage-dependent Ca2+ induced Ca2+ release from the SR, while higher hormone levels directly gate Ca2+ entry across the sarcolemma.

Strain softening behaviour in nonviable rat right-ventricular trabeculae, in the presence and the absence of butanedione monoxime

Strain softening is commonly reported during mechanical testing of passive whole hearts. It is typically manifested as a stiffer force-extension relationship in the first deformation cycle relative to subsequent cycles and is distinguished from viscoelasticity by a lack of recovery of stiffness, even after several hours of rest. The cause of this behaviour is presently unknown. In order to investigate its origins, we have subjected trabeculae to physiologically realistic extensions (5-15% of muscle length at 26 degrees C and 0.5 mm Ca(2+)), while measuring passive force and dynamic stiffness. While we did not observe strain softening in viable trabeculae, we found that it was readily apparent in nonviable (electrically inexcitable) trabeculae undergoing the same extensions. This result was obtained in both the presence and absence of 2,3-butanedione monoxime (BDM). Furthermore, BDM had no effect on the passive compliance of viable specimens, while its presence partly inhibited, but could not prevent, stiffening of nonviable specimens. Loss of viability was accompanied by a uniform increase of dynamic stiffness over all frequencies examined (0.2-100 Hz). The presence of strain softening during length extensions of nonviable tissue resulted in a comparable uniform decrease of dynamic stiffness. It is therefore concluded that strain softening is neither intrinsic to viable rat right ventricular trabeculae nor influenced by BDM but, rather, reflects irreversible damage of tissue in partial, or full, rigor.

The steady-state force-Ca2+ relationship in intact lobster (Homarus americanus) cardiac muscle

The heart of the decapod crustacean is activated by regular impulse bursts from the cardiac ganglion. The cardiac pump function depends on ganglionic burst frequency, burst duration, and burst impulse frequency. Here, we activated isolated lobster cardiac ostial muscle (Orbicularis ostii muscle, OOM) by stimulus trains in vitro in order to characterize the response of the contractile apparatus to [Ca2+]i. We employed stimulus trains that generate a steady state between the [Ca2+]i and force in order to estimate the Ca2+ sensitivity of myofilaments. Force and [Ca2+]i transients were simultaneously recorded using a silicon strain gauge and the fluorescence of iontophoretically microinjected fura-2 salt. We examined the effects of tetanus duration (TD), the interval between trains, and 6 microM cyclopiazonic acid, an inhibitor of the SR Ca2+ pump, on the steady-state force-[Ca2+]i relationship. The instantaneous force-[Ca2+]i relationships appeared sigmoidal (EC50 and Hill coefficient, 98.8+/-32.7 nM and 2.47+/-0.20, mean +/- SD, respectively), as did the curves superimposed after 500 ms following the start of stimulation, indicating that the force-[Ca2+]i relationship had reached a steady state at that time. Also, the maximum activated force (Fmax) was estimated using the steady-state force-[Ca2+]i relationship. Prolonged stimulus trains, decreasing the interval between recurrent trains from 5 to 2.5 s, and cyclopiazonic acid each increased the measured EC50 without changing Fmax. The EC50 correlated strongly with averaged [Ca2+]i over time. We conclude that the steady-state force-[Ca2+]i relationships in the OOM indicate cooperation between force generation and Ca2+ binding by the myofilaments. Our data also suggest the existence of a novel Ca2+-dependent mechanism which reduces Ca2+ sensitivity and accelerates relaxation of lobster cardiac muscle myofilaments.

Species-independent metabolic response to an increase of [Ca(2+)](i) in quiescent cardiac muscle

1. The aim of the present investigation was to contrast the Ca2+ dependence of cardiac energy metabolism in two species with differential reliance on extracellular Ca2+ for excitation-contraction coupling. 2. We measured energy expenditure as the rate of oxygen consumption (Vo2) of isolated, Langendorff-perfused hearts of rats and guinea-pigs during KCl arrest. In parallel experiments, we indexed intracellular Ca2+ concentration ([Ca2+]i) of isolated right-ventricular trabeculae, using the Ca2+ fluorophore fura-2 and ratiometric spectrofluorometry. By varying extracellular Na+ concentration ([Na+]o), Vo2-[Na+]o and [Ca2+]i-[Na+]o relationships were constructed for each species. 3. Reduction of [Na+]o during K+ arrest caused pronounced species-dependent elevations of both Vo2 and [Ca2+]i. Despite the species dependence of both Vo2 and [Ca2+]i on [Na+]o, a single species-independent Vo2-[Ca2+]i relationship obtained. 4. We infer that elevation of the metabolic rate of the arrested heart above its basal value is determined primarily by [Ca2+]i and is not species dependent.

The effects of nifedipine on ventricular fibrillation mean frequency in a porcine model of prolonged cardiopulmonary resuscitation

We assessed the effects of a calcium channel blocker versus saline placebo on ventricular fibrillation mean frequency and hemodynamic variables during prolonged cardiopulmonary resuscitation (CPR). Before cardiac arrest, 10 animals were randomly assigned to receive either nifedipine (0.64 mg/kg; n = 5) or saline placebo (n = 5) over 10 min. Immediately after drug administration, ventricular fibrillation was induced. After 4 min of cardiac arrest and 18 min of basic life support CPR, defibrillation was attempted. Ninety seconds after the induction of cardiac arrest, ventricular fibrillation mean frequency was significantly (P < 0.01) increased in nifedipine versus placebo pigs (mean +/- SD: 12.4 +/- 2.1 Hz versus 8 +/- 0.7 Hz). From 2 to 18.5 min after the induction of cardiac arrest, no differences in ventricular fibrillation mean frequency were detected between groups. Before defibrillation, ventricular fibrillation mean frequency was significantly (P < 0.05) increased in nifedipine versus placebo animals (9.7 +/- 1.2 Hz versus 7.1 +/- 1.3 Hz). Coronary perfusion pressure was significantly lower in the nifedipine than in the placebo group from the induction of ventricular fibrillation to 11.5 min of cardiac arrest; no animal had a return of spontaneous circulation after defibrillation. In conclusion, nifedipine, but not saline placebo, prevented a rapid decrease of ventricular fibrillation mean frequency after the induction of cardiac arrest and maintained ventricular fibrillation mean frequency at approximately 10 Hz during prolonged CPR; this was nevertheless associated with no defibrillation success.

IMPLICATIONS

This study evaluates the effects of a calcium channel blocker on ventricular fibrillation mean frequency, hemodynamic variables, and resuscitability during prolonged cardiopulmonary resuscitation (CPR) in pigs. Nifedipine, but not saline placebo, prevented a rapid decrease of ventricular fibrillation mean frequency after the induction of cardiac arrest and maintained ventricular fibrillation mean frequency at approximately 10 Hz during prolonged CPR but did not improve resuscitability.

Extracellular Ca2+ is obligatory for ouabain-induced potentiation of cardiac basal energy expenditure

1. The method of action of cardiac glycosides is commonly explained by the 'pump-inhibition hypothesis': inhibition of the Na+/K+-ATPase allows [Na+]i to rise, eventually reversing Na+/Ca2+ exchange. The resulting influx of Ca2+o increases [Ca2+]i, thereby activating intracellular Ca2+-dependent ATPases and, hence, energy demand. This sequence has been presumed to occur during diastole as well as systole. However, it has been reported that dihydro-ouabain-induced potentiation of heat production by quiescent ventricular trabeculae persists in the absence of Ca2+o. This implies that the pump-inhibition hypothesis is inapplicable during diastole. 2. We tested this implication by: (i). measuring the rate of oxygen consumption (Vo2) of arrested guinea-pig whole-hearts; (ii). measuring[Ca2+]i in quiescent ventricular trabeculae; and (iii). mathematical modelling using software (Oxsoft Heart, Oxford Software, Oxford, UK) based on DiFrancesco-Noble formalism. 3. Upon induction of arrest, whole heart Vo2 fell to one-quarter of its 'beating' value. Subsequent perfusion with ouabain (20 micromol/L), in the presence of Ca2+o, increased Vo2 fourfold. This increase was prevented by withholding Ca2+o. Comparable results were obtained in quiescent trabeculae: ouabain increased [Ca2+]i only if Ca2+o was present. Mathematical modelling readily simulated these experimental results. 4. We conclude that influx of Ca2+o is mandatory for potentiation of cardiac basal metabolism by cardiac glycosides.

Na-dependent changes in intracellular Ca in spontaneously hypertensive rat hearts

To determine whether Na/Ca exchange is altered in primary hypertension, Na-dependent changes in intracellular Ca, ([Ca]i), were measured in isolated perfused hearts from Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Intracellular Na, (Nai, mEq/kg dry wt), and [Ca]i were measured by NMR spectroscopy. Control [Ca]i was less in WKY than SHR (176 +/- 18 vs 253 +/- 21 nmol/l; mean +/- S.E., P < 0.05), whereas Nai was not significantly different. One explanation for this is that net Na/Ca exchange flux is decreased in SHR. If this hypothesis is correct, the rate of Ca uptake in SHR should be less than WKY when Na/Ca exchange is reversed by decreasing the transmembrane Na gradient. The Na gradient was reduced by decreasing extracellular Na, ([Na]o) and/or by increasing [Na]i. To increase [Na]i, Na uptake was stimulated by acidification while Na extrusion by Na/K ATPase was inhibited by K-free perfusion. Seventeen minutes after acidification, Nai had increased but was not significantly different in SHR and WKY (18.0 +/- 2.3 to 57.4 +/- 7.6 vs 20.3 +/- 0.6 to 66.5 +/- 4.8 mEq/kg dry wt, respectively). Yet [Ca]i was greater in WKY than SHR (1768 +/- 142 vs 1201 +/- 90 nmol/l; P < 0.05). [Ca]i was also measured after decreasing [Na]o from 141 to 30 mmol/l. Fifteen minutes after reducing [Na]o, [Ca]i was greater in WKY than SHR (833 +/- 119 vs 425 +/- 94 nmol/l; P < 0.05). Thus for both protocols, decreasing the transmembrane Na gradient led to increased [Ca]i in both SHR and WKY, but less increase in SHR. The results are consistent with the hypothesis that Na/Ca exchange activity is less in SHR than WKY myocardium.

Acute and chronic hypokalemia sensitize the isolated heart to hypoxic injury

We examined the effects of acute and/or chronic hypokalemia on responses to 30 min of hypoxia and recovery in the isolated, perfused heart model. We found that both acute hypokalemia and chronic hypokalemia impaired contractility [expressed as maximum slope of pressure increase over time (dP/dt): 501 +/- 49 and 529 +/- 48 vs. 1,302 +/- 118 mmHg/s, P < 0.01] and recovery of ATP concentrations (determined with 31P NMR spectroscopy: 30 +/- 6 and 40 +/- 10 vs. 67 +/- 5% initial, P < 0.05) at 30 min of recovery. Moreover, the combination of acute hypokalemia and chronic hypokalemia had additive effects (dP/dt 166 +/- 15 mmHg/s and ATP 21 +/- 7% initial, both P < 0.01). We also measured cytosolic calcium with surface fluorescence spectroscopy after indo 1 loading. Acute hypokalemia and acute hypokalemia + chronic hypokalemia increased cytosolic calcium (averaged throughout the cardiac cycle) during and after hypoxia (390- to 460-nm ratio at 30 min of recovery: 0.46 +/- 0.07 and 0.65 +/- 0.07 vs. 0.18 +/- 0.03, P < 0.01), whereas control and chronic hypokalemia hearts had only small changes with hypoxia and recovery. Finally, when we examined mitochondria isolated from hearts perfused under experimental conditions, we found that chronic hypokalemia-alone mitochondria and chronic hypokalemia + acute hypokalemia mitochondria had marked impairment of state 3 respiration compared with control hearts (52 +/- 13 and 50 +/- 9 vs. 128 +/- 10 natm.min-1.mg protein-1 with succinate as substrate, P < 0.01), whereas acute hypokalemia mitochondria demonstrated only subtle changes. These data suggest that both acute hypokalemia and chronic hypokalemia impair cardiac responses to hypoxia. The mechanism may involve impairment of calcium metabolism, but cytosolic calcium alterations do not explain all of the metabolic and functional effects of acute hypokalemia and chronic hypokalemia in the setting of hypoxia.

Effects of creatine phosphate and inorganic phosphate on the sarcoplasmic reticulum of saponin-treated rat heart

1. Ventricular trabeculae from rat heart were permeabilized by treatment with saponin. In the presence of 150 nM Ca2+, application of 20 mM caffeine released Ca2+ from the sarcoplasmic reticulum (SR), resulting in a transient contracture. Ca2+ released from the SR was detected using fura-2 fluorescence. The amplitudes of the caffeine-induced Ca2+ transients were used to assess SR Ca2+ content. 2. In the absence of creatine phosphate (CP), introduction of 5-30 mM inorganic phosphate (Pi) caused a net release of Ca2+ from the SR. Subsequent caffeine-induced Ca2+ and tension transients were smaller in the presence of Pi. Under these conditions, 30 mM Pi decreased the caffeine-induced Ca2+ transients by 45 +/- 3.1% (mean +/- S.D., n = 14). On removal of Pi, the [Ca2+] transiently decreased and the caffeine-induced Ca2+ transients returned to control levels over 4-6 min. 3. In the presence of CP (5-15 mM), the Ca2+ transients were unaffected by the introduction of Pi (5-30 mM) or slightly increased in amplitude. Pi (30 mM) significantly increased the caffeine-induced Ca2+ transients by 7 +/- 8.8% (mean +/- S.D., n = 19, P < 0.05) in the presence of 15 mM CP. The release of Ca2+ on addition of Pi and decrease in [Ca2+] on Pi withdrawal was less pronounced or absent completely in the presence of CP. The inhibitory effects of Pi on caffeine-induced Ca2+ release became apparent as the [CP] was decreased from 5 to 0 mM. 4. In the presence of the creatine phosphokinase inhibitor dinitro-fluorobenzene (DNFB) the effects of Pi (in the presence of CP) were qualitatively similar to the results obtained in the absence of CP, although the decrease in caffeine-induced Ca2+ release was less pronounced. 5. These results suggest that the rise in [Pi]i during ischaemia or anoxia will have little effect on the regulation of Ca2+ by the SR while the [CP]i remains above 5 mM. However, as the [CP] decreases below 5 mM, the accumulation of Pi within the cytosol will progressively reduce the SR Ca2+ content. CP may act in conjunction with endogenous creatine phosphokinase to modify the response of the SR to Pi, and possible mechanisms are considered.

Release of adenine nucleotide metabolites by toxic concentrations of cardiac glycosides

In isolated perfused guinea-pig hearts the effect of toxic concentrations of cardiac glycosides on the release of the adenine nucleotide metabolites adenosine, inosine, hypoxanthine, xanthine, and uric acid was investigated. Digoxin concentrations of 0.03-1 mumol.l-1 produced moderate to severe tachyarrhythmias. Large amounts of metabolites were released by concentrations of 0.1 mumol.l-1, and higher. Occurrence of glycoside-induced ventricular fibrillation was associated with a particularly high release. Metabolite release was also obtained when fibrillation was elicited electrically in normal control hearts, or in hearts receiving simultaneously a marginally toxic digoxin concentration (0.03 mumol.l-1). Digoxin-induced tachyarrhythmias and metabolite release were almost completely prevented by a high potassium concentration in the coronary perfusion fluid (8.1 mmol.l-1). The antiarrhythmic effect was also obtained with lidocaine (60 mumol.l-1), but the release was only partially antagonized. Similar results concerning arrhythmias and metabolite release as with digoxin were obtained with ouabain. The findings suggest that the decrease in myocardial ATP observed in glycoside-intoxicated heart preparations is partly due to the loss of nucleotide precursor substances. Moreover, it appears likely that liberated adenosine in the interstitium of severely intoxicated heart preparations reaches pharmacologically effective concentrations.

31P nuclear magnetic resonance study of the effects of the calcium ion channel antagonist fantofarone on the rat heart

The biochemical and mechanical effects of a new calcium ion channel antagonist, fantofarone ((2-isopropyl-1-((4-(3-(N-methyl-N-(3,4-dimethoxy-beta-phenethyl)-amino) propyloxy)benzenesulfonyl))-indolizine), on isovolumic perfused rat heart have been assessed by using 31P nuclear magnetic resonance (NMR) spectroscopy together with simultaneous monitoring of myocardial mechanical function. Cytosolic pH and phosphocreatine, adenosine triphosphate and inorganic phosphate contents were monitored by using 31P NMR. Heart rate, coronary flow and left ventricular developed pressure were measured routinely to assess mechanical function. Perfusion with 10 nM, 100 nM or 1 microM fantofarone for a period of 48 min did not cause any measurable metabolic changes. However, coronary vasodilatation and a partial positive inotropic effect were noted. A 15-min pretreatment with 100 nM did not protect against the deleterious effects of an 18-min period of normothermic, zero-flow ischemia. In contrast, a 20-min pretreatment period with 1 microM fantofarone significantly improved the recovery of mechanical performance, metabolic activity and pH after the same 18 min of ischemia. While only a slight protection of the ATP pool was noted during the ischemic period, major beneficial effects were observed during the reperfusion period, such that reflow was characterized by high recoveries of left ventricular pressure and rate pressure product (70-80%), low end diastolic pressure (< 10 mm Hg), significant recovery of ATP content (to 55%), a complete repletion of the phosphocreatine pool and a fast return of cytosolic pH to normal value.

The metabolic consequences of an increase in the frequency of stimulation in isolated ferret hearts

1. The metabolic consequences of an increase in the frequency of stimulation were examined in isolated ferret hearts. Intracellular pH (pHi) and the intracellular concentrations of phosphocreatine ([PCr]i), inorganic phosphate ([Pi]i) and ATP were measured by 31P nuclear magnetic resonance (NMR) spectroscopy. 2. Increasing the stimulus rate from 0.1-0.7 to 2 Hz caused an increase in [Pi]i and a decrease recovery of both [PCr]i and [Pi]i during continued stimulation. There was no change in [ATP]i during stimulation at 2 Hz. Increasing the stimulus rate caused an intracellular acidosis of around 0.1 pH units. 3. Increasing the stimulus rate generally caused an initial increase in developed pressure, followed by a decrease over 1-2 min to a steady level slightly lower than developed pressure at the low (control) stimulus rate. The increase in stimulus rate caused a 4- to 6-fold increase in time-averaged muscle activity. 4. Both oxygen uptake and production of lactate increased on 2 Hz stimulation. Lactate production accounted for less than 5% of ATP production at low or high stimulus rates, suggesting that significant anoxia was not occurring during stimulation. The observed lactate production was, however, sufficient to explain most of the intracellular acidosis observed when the stimulus rate was raised. When glycolysis was prevented by removal of glucose and depletion of glycogen stores, 2 Hz stimulation was accompanied by an intracellular alkalosis rather than an acidosis, suggesting that lactate production by glycolysis was the cause of the intracellular acidosis. 5. Reducing the rate of glycolysis increased the size of changes in [PCr]i and [Pi]i evoked by stimulation at 2 Hz. Furthermore, there was now no partial reversal of the changes in [PCr]i and [Pi]i during 2 Hz stimulation. 6. When oxidative phosphorylation was inhibited by replacing O2 with N2, increasing the rate of stimulation from 0.1-0.7 to 1-2 Hz caused an initial increase followed by a large fall in developed pressure, which declined to a level well below that at the control stimulus rate. The increase in stimulus rate was accompanied by a large fall in [PCr]i, an increase in [Pi]i, and an intracellular acidosis of 0.1-0.3 pH units. The fall in developed pressure was consistent with the known effects of the rise in [Pi]i and the fall in pHi on the contractile apparatus.

Hypoxia and metabolic acidosis in the isolated heart: evidence for synergistic injury

Although hypoxia and metabolic acidosis have both been shown to impair cardiac function, some workers have suggested that acidosis during a period of hypoxia will actually accelerate physiologic recovery from this insult. To address the interactions of metabolic acidosis and hypoxia further, isolated isovolumic rat hearts were exposed to normal perfusion conditions for 30 min to establish baseline conditions, then either continued normal conditions, metabolic acidosis, hypoxia, or combined acidosis and hypoxia for 30 min and subsequently reperfused under normal perfusion conditions for an additional 30 min. We observed that acidosis + hypoxia impaired recovery of cardiac contraction more than acidosis or hypoxia alone following experimental perfusion. The combination of acidosis and and hypoxia also impaired cardiac energy metabolism more than acidosis or hypoxia alone as assessed by increases in tissue inorganic phosphate during experimental perfusion as well as during reperfusion. These data suggest that during hypoxia, acidosis appears to primarily impair cardiac energy production as we have previously observed in the normoxic isolated rat heart. Therefore, in the intact beating heart, acidosis may not protect from hypoxic injury as has been suggested in simpler systems but may not protect from hypoxic injury as has been suggested in simpler systems but rather may exacerbate at.

Sodium ion transport in rat hearts during cold ischemic storage: 23Na and 31P NMR study

The success of heart transplantation is limited by the negative correlation between the length of the cold ischemic storage period and the quality of functional recovery. We use 23Na, 31P NMR spectroscopy, and hemodynamic parameters to describe temperature-dependent changes in sodium influx and the concentration of phosphorus high-energy compounds during different storage periods. Perfusion with Krebs-Henseleit solutions containing Dy(TTHA)3- permitted discrimination of intra- and extracellular sodium during cold ischemic storage. The 23Na NMR visibilities under the acquisition and processing parameters used in our experiments were 40 +/- 4% for the intracellular compartment and 97 +/- 11% for the extracellular compartment. At 4 degrees C, the intracellular Na+ accumulation exceeded that observed at 15 and 22 degrees C. The ATP and PCr depletion rates were much lower at 4 degrees C and the left ventricular contractility was higher after longer periods of storage, as the storage temperature decreased. The intracellular Na+ concentration cannot serve as a marker for the postischemic recovery probability. The relative activity of the Na/K ATPase pumps is not correlated with the preservation success. However, intracellular sodium ion accumulation is a major factor in the time lag of the reperfusion recovery.

Contractile, metabolic and arrhythmogenic effects of ionic and nonionic contrast agents in the isolated rat heart

Intracoronary administration of contrast agents may be associated with contractile dysfunction and arrhythmias. To further establish the mechanisms of these alterations, we studied high-energy phosphate metabolism, developed pressure, the occurrence of arrhythmias, and the effects of verapamil during infusion of ionic and nonionic agents in isovolumic, retrogradely perfused rat hearts using 31P nuclear magnetic resonance imaging (NMR). Diatrizoate meglumine (Renografin) infusion reduced developed pressure (DP) to 17.1 +/- 3.4% (p less than 0.001) of the control level, and immediately following termination of the infusion, sudden ventricular tachycardia (VT) was observed in four of six hearts. In the presence of verapamil, meglumine reduced DP to 13 +/- 1.9% of control values and none of these six hearts developed VT. Iopamidol infusion in the presence of verapamil (n = 6) and alone (n = 6) resulted in a decrease in DP to 87% of control value, and no arrhythmias, significant change in high-energy phosphate levels, or changes in pH were observed. These results suggest that contrast-induced contractile depression is not mediated by changes in high-energy phosphate metabolism or pH. Arrhythmias associated with meglumine administration alone and suppressed by verapamil are probably related to calcium loading.

Myocardial Na+ during ischemia and accumulation of Ca2+ after reperfusion: a study with monensin and dichlorobenzamil

The intracellular cation contents were determined in isolated perfused rat heart using cobaltic EDTA as a marker of the extracellular space. In hearts in which Na+ accumulation was induced with monensin, a Na+ ionophore, during 20 min-ischemia which otherwise did not result in accumulation of Na+, the levels of Na+ and Ca2+ were significantly higher after reperfusion with a significant decrease in K+. While the recovery of the cardiac mechanical function (CMF) was complete after reperfusion in control hearts, the recovery was incomplete in monensin-hearts. Dichlorobenzamil (DCB), the most specific inhibitor of Na(+)-Ca2+ exchanger, infused for 10 min before induction of ischemia in a dose of 10(-5) M, which produced a definite suppression of CMF (over 80%), inhibited the accumulation of Ca2+ and Na+ and the loss of K+ and ATP after 40 min-ischemia and reperfusion. The same dose of DCB given for 3 min before induction of ischemia and after reperfusion, which produced a less than 20% inhibition of CMF, failed to prevent the Ca2+ accumulation after 40 min-ischemia and reperfusion. These findings are at variance with the idea that the accumulation of Na+ during ischemia and the consequent augmented operation of Na(+)-Ca2+ exchange is responsible for accumulation of Ca2+ after reperfusion.

Dehydrogenase activation by Ca2+ in cells and tissues

The activation of intramitochondrial dehydrogenases by Ca2+ provides a link between the intensity of work performance by a tissue and the activity of pyruvate dehydrogenase and the tricarboxylate cycle, and hence the rate of ATP production by the mitochondria. Several aspects of this model of the control of oxidative phosphorylation are examined in this article, with particular emphasis on mitochondrial functioning in situ in cardiac myocytes and in the intact heart. Recent use of the fluorescent Ca2+ chelating agents indo-1 and fura-2 has allowed a more quantitative description of the dependence of dehydrogenase activity upon concentration of free intramitochondrial Ca2+, in experiments with isolated mitochondria. Further, a novel technique developed by Miyata et al. has allowed description of free intramitochondrial Ca2+ within a single cardiac myocyte, and the conclusion that this parameter changes in response to electrical excitation of the cell over a range which would be expected to give substantial modulation of dehydrogenase activity.

Contractile, metabolic and electrophysiologic effects of ethanol in the isolated rat heart

The metabolic, functional and electrical effects of ethanol were studied in the isolated isovolumic rat heart retrogradely perfused at constant flow using phosphorus-31 nuclear magnetic resonance spectroscopy and surface electrogram recordings. Ethanol (0.75 to 6.0 vol%; 128 to 1024 mM) caused a concentration-dependent decline in developed pressure without a change in adenosine triphosphate, phosphocreatine, inorganic phosphate or pH. Ethanol (6%) caused abolition of electrical activity. The functional decline could be rapidly and completely reversed by perfusing with ethanol-free solution and, significantly although not completely, reversed by increasing perfusate calcium to 4 mM. Furthermore, ethanol shifted the perfusate calcium-tetanic pressure relationship in the presence of ryanodine (1 microM) downwards and to the right. The results suggest ethanol's acute effects in this model are not mediated by changes in energy metabolism or cellular pH, but rather by sarcolemmal effects and by a decrease in both myofilament calcium sensitivity and maximal force generating ability.

Consequences of acute ischemia for the electrical and mechanical function of the ventricular myocardium. A brief review

Reduction or interruption of the blood supply to the myocardium leads to marked disturbances of electrical and mechanical function within a few seconds. Electrical dysfunction is characterized by an initial depolarization of the resting membrane, and a decrease of the amplitude, the upstroke velocity and the duration of the action potential. Both depolarization and depression of the action potential are closely associated with intracellular metabolic acidosis. After this initial phase, electrical cell-to-cell uncoupling develops, probably as a consequence of increased cytosolic free [Ca++]. Mechanical dysfunction is characterized by a dissociation of the initial decrease of active force development from the subsequent ischemic contracture. Active force development in acute ischemia is inhibited by the accumulation of ischemic metabolic products (H+, inorganic phosphate (Pi), Mg++) but not by a marked decrease of [ATP]. The subsequent ischemic contracture is probably initiated by release of Ca++ from intracellular stores. This release causes rapid consumption of ATP and the development of rigor within 1-2 minutes.

Calcium oscillations index the extent of calcium loading and predict functional recovery during reperfusion in rat myocardium

Delayed recovery of contractile function after myocardial ischemia may be due to prolonged recovery of high-energy phosphates, persistent acidosis, increased inorganic phosphate, and/or calcium loading. To examine these potential mechanisms, metabolic parameters measured by 31P nuclear magnetic resonance spectroscopy, and spontaneous diastolic myofilament motion caused by sarcoplasmic reticulum-myofilament calcium cycling indexed by the scattered light intensity fluctuations (SLIF) it produces in laser beam reflected from the heart, were studied in isolated atrioventricularly blocked rat hearts (n = 10) after 65 min of ischemia at 30 degrees C. All metabolic parameters recovered to their full extent 5 min after reperfusion. Developed pressure evidenced a small recovery but then fell abruptly. This was accompanied by an increase in end diastolic pressure to 37 +/- 5 mm Hg and a fourfold increase in SLIF, to 252 +/- 58% of baseline. In another series of hearts initial reperfusion with calcium of 0.08 mM prevented the SLIF rise and resulted in improved developed pressure (74 +/- 3% vs. 39 +/- 13% of control), and lower cell calcium (5.9 +/- 3 vs. 10.3 +/- 1.4 mumol/g dry wt). Thus, during reperfusion, delayed contractile recovery is not associated with delayed recovery of pH, inorganic phosphate, or high-energy phosphates and can be attributed, in part, to an adverse effect of calcium loading which can be indexed by increased SLIF occurring at that time.

Post-extrasystolic potentiation and the force-frequency relationship: differential augmentation of myocardial contractility in working myocardium from patients with end-stage heart failure

We studied post-extrasystolic potentiation (PESP) and frequency potentiation (FP) of human working myocardium isolated from the ventricles of 10 patients with end-stage heart failure (CHF) and 17 non-failing controls (CTL). The contractility index was peak isometric tension developed in vitro by trabeculae carneae (CTL n = 34, CHF n = 31); programmed electrical stimulation was used to initiate as well as alter the timing relationship of the contractile events. While holding constant the total number of contractions per unit of time, we compared the augmentation of contractile performance that occurred upon doubling the stimulation frequency (FP) to that of changing the stimulation pattern (PESP). In the CTL group we found that FP and PESP differed in their ability to augment cardiac contractile performance, PESP being more effective; 105 +/- 13% for PESP vs. 34 +/- 11% (mean +/- S.E.M.) for FP. In the CHF group, the inotropic response to PESP was similar to CTL; in contrast, the relative efficacy of FP (3 +/- 3%) compared to PESP (81 +/- 14%) was markedly diminished. Studies with positive inotropic agents revealed that the percent change in contractility induced by FP and PESP depends upon the relative inotropic state of the heart; however, the contractile response to PESP always equaled or exceeded those produced by clinically relevant concentrations of inotropic agents, particularly in CHF muscles. Agents that increase intracellular levels of adenosine 3',5'-cyclic monophosphate reversed the FP abnormalities seen in the CHF group, suggesting that deficient production of this second messenger in heart failure may cause the abnormal force-frequency relationship in failing myocardium. We conclude that the differential responses of myopathic muscle to PESP and FP may be caused by abnormal restitution processes during diastole. Our results suggest that PESP may be an effective therapeutic modality for patients with severe heart failure who have failed to adequately respond to inotropic drugs, and may serve as a useful indicator of cardiac contractile reserve in these patients.

Decrease in internal H+ and positive inotropic effect of heptaminol hydrochloride: a 31P n.m.r. spectroscopy study in rat isolated heart

1. The cardiotonic effect of heptaminol hydrochloride (Hept-a-myl, Delalande) was studied using 31P-nuclear magnetic resonance (n.m.r.) spectroscopy and left ventricular pressure (LVP) measurements in rat isolated hearts. The possibility of this effect being mediated by an intracellular realkalinisation was tested. 2. Isolated hearts were perfused at 10 ml min-1 by the Langendorff method with Krebs-Henseleit solution at 37 degrees C and stimulated at 5 Hz. Mechanical activity was measured as variations of left ventricular pressure (LVP). 31P-n.m.r. spectra were recorded every 2 min. Changes in cardiac adenosine triphosphate (ATP), phosphocreatine (PCr) and inorganic phosphate (Pi) were followed and intracellular pH (pHi) was estimated from the chemical shift of Pi. 3. The effects of heptaminol were tested in different conditions: normoxia, moderate ischaemia, severe ischaemia, and moderate ischaemia in the presence of amiloride or guanidinium chloride as inhibitors of the Na-H exchange. 4. In normoxia, heptaminol induced a cyclic increase of systolic LVP, associated with an increase in Pi. No significant effect on pHi was observed. In changing from normoxia to moderate ischaemia, PCr and systolic LVP decreased; a mild intracellular acidification (pHi 6.96) was obtained. Heptaminol induced a restoration of pHi and increased LVP. In severe ischaemia, the realkalinization effect and the restoration of LVP induced by heptaminol were no longer observed. During moderate ischaemia, Na-H exchange inhibitors decreased pHi and LVP. Heptaminol applied in the presence of these inhibitors was unable to restore pHi and LVP. In severe ischaemia, the realkalinization effect and the restoration of LVP induced by heptaminol were no longer observed. During moderate ischaemia, Na-H exchange inhibitors decreased pHi and LVP. Heptaminol applied in the presence of these inhibitors was unable to restore pHi and LVP. 5. These results suggest that the positive inotropic effect of heptaminol during moderate ischaemia could be related to a restoration of internal pH, possibly mediated by a stimulation of the Na-H exchange.

Direct evidence for a role of intramitochondrial Ca2+ in the regulation of oxidative phosphorylation in the stimulated rat heart. Studies using 31P n.m.r. and ruthenium red

1. The concentrations of free ATP, phosphocreatine (PCr), Pi, H+ and ADP (calculated) were monitored in perfused rat hearts by 31P n.m.r. before and during positive inotropic stimulation. Data were accumulated in 20 s blocks. 2. Administration of 0.1 microM-(-)-isoprenaline resulted in no significant changes in ATP, transient decreases in PCr, and transient increases in ADP and Pi. However, the concentrations of all of these metabolites returned to pre-stimulated values within 1 min, whereas cardiac work and O2 uptake remained elevated. 3. In contrast, in hearts perfused continuously with Ruthenium Red (2.5 micrograms/ml), a potent inhibitor of mitochondrial Ca2+ uptake, administration of isoprenaline caused significant decreases in ATP, and also much larger and more prolonged changes in the concentrations of ADP, PCr and Pi. In this instance values did not fully return to pre-stimulated concentrations. Administration of Ruthenium Red alone to unstimulated hearts had minor effects. 4. It is proposed that, in the absence of Ruthenium Red, the transmission of changes in cytoplasmic Ca2+ across the mitochondrial inner membrane is able to maintain the phosphorylation potential of the heart during positive inotropic stimulation, through activation of the Ca2+-sensitive intramitochondrial dehydrogenases (pyruvate, NAD+-isocitrate and 2-oxoglutarate dehydrogenases) leading to enhanced NADH production. 5. This mechanism is unavailable in the presence of Ruthenium Red, and oxidative phosphorylation must be stimulated primarily by a fall in phosphorylation potential, in accordance with the classical concept of respiratory control. However, the full oxidative response of the heart to stimulation may not be achievable under such circumstances.

Chronic sodium depletion increases myocardial calcium content in normotensive rats

Increased myocardial contractility was found in the perfused heart isolated from sodium depleted Sprague-Dawley rats. Previously, it was reported that in vitro exposure of different cardiac preparations to low Na+ buffers was associated with both an increased contractility and an increased Ca2+ content in the cells. Therefore, this study was designed to examine increases in ventricular Ca2+ content in the hearts of chronically sodium depleted rats. Two groups of 12-week-old Sprague-Dawley rats were studied. One group (n = 5) received furosemide (5 mg/kg/day IP for 4 days), a low Na+ diet and distilled drinking water for 6 weeks (low sodium plus diuretics group = LSD). The other group (n = 5) received the same low Na+ diet, but 0.5% NaCl was supplemented in drinking water (regular sodium group = RS). Body weight and blood pressure were measured weekly during the dietary period in all rats. At the end of the 6 weeks, heart weight as well as water and electrolyte contents of the heart were measured in all animals. Results showed that both body weight and heart weight were significantly lower in LSD than in RS. Moreover, ventricular Na+ content was reduced while ventricular Ca2+ content was doubled in LSD compared to RS (8.2 +/- 0.2 units vs. 9.2 +/- 0.3 units, p less than .05 and 0.45 +/- 0.13 units vs. 0.23 +/- 0.01 units, p less than .01, respectively). We conclude that in vivo sodium depletion induces an increase in ventricular calcium content; this increased myocardial calcium may be related to the increased in vitro cardiac contractility observed after chronic in vivo sodium depletion, but its distribution between intracellular and extracellular compartments needs to be determined.

Mechanism of rate-dependent pH changes in the sheep cardiac Purkinje fibre

1. The mechanism of the rate-dependent decrease in intracellular pH (pHi) and its recovery were studied in isolated sheep cardiac Purkinje fibres. Intracellular Na+ activity (aiNa) and pHi were measured using ion-selective microelectrodes. Twitches were elicited by field stimulation or by depolarizing pulses applied using a two-microelectrode voltage clamp. 2. A 3 Hz train of short (50 ms) depolarizing voltage-clamp pulses induced a reversible fall in pHi which was accompanied by a reversible increase in aiNa. A train of longer (200 ms) pulses also produced a fall in pHi which was now paralleled by a decrease in aiNa. These observations indicate that the rate-dependent acidosis is not dependent upon a rise in aiNa. 3. Neither the fall in pHi nor the increase in aiNa seen upon an increase in action potential frequency was inhibited by amiloride (1 mmol l-1) which indicates that Na+-H+ exchange is not involved in the generation of the acidosis. Furthermore, the rate-dependent acidosis was not abolished in Na+-free solution (Li+ or N-methyl glucamine substituted) indicating that other Na+-requiring processes (such as Na+-Ca2+ exchange) are not a necessary requirement. Rate-dependent pHi changes were also unaffected by the stilbene compound DIDS indicating no participation by Cl--HCO-3 exchange. 4. The rate-dependent acidosis was inhibited by the organic calcium antagonist D600 (20 mumol l-1) which also inhibited twitch tension. This suggests that the acidosis is related to the activation by Ca2+ of developed tension. D600 also inhibited the rate-dependent rise in aiNa (field stimulation). 5. The rate-dependent acidosis was not inhibited by cyanide (2 mmol l-1) but it was blocked by iodoacetate (0.5 mmol l-1) and by 2-deoxyglucose (DOG) (10 mmol l-1, applied in glucose-free solution). These results suggest that the acidosis is generated metabolically via stimulation of glycolysis, following an increase in contraction. Contributions from aerobic metabolism are likely to be small. 6. Twitch tension was inhibited by ryanodine (10 mumol l-1) but the drug had little inhibitory effect on the rate-dependent acidosis. A tonic component of tension was observed, however, in the presence of ryanodine. The lack of effect of ryanodine upon the rate-induced acidosis is discussed. 7. The half-time of pHi recovery from the frequency-dependent acidosis was consistently shorter than that from an intracellular acid load induced by adding and then removing external NH4Cl (10 mmol l-1).(ABSTRACT TRUNCATED AT 400 WORDS)

Sustained function of normoxic hearts depleted in ATP and phosphocreatine: a 31P-NMR study

A model of high-energy phosphate depletion was developed in the normoxic isovolumic rat heart perfused with acetate, 2-deoxy-D-glucose (2DG), and insulin. Intracellular phosphorylation of 2DG abstracts phosphorus from its normal pathways. This results in a decrease of high-energy phosphates without any increase in Pi. During the first 15 min of 2DG phosphorylation, the changes in ATP, Pi, and intracellular pH (pHi) were slight, and work was unaltered, although phosphocreatine (PCr) concentration dropped by 50%. After 45 min, the heart reached a new steady state characterized by a drastic reduction in both PCr and ATP: PCr was 15% of control, and in most hearts ATP became invisible on the nuclear magnetic resonance (NMR) spectra. Nevertheless, the heart still developed 65% of its original systolic pressure, whereas diastolic pressure was unchanged. Oxygen consumption per unit work remained constant during 2DG perfusion. This is, to our knowledge, the first experimental model of sustained cardiac contractility at such low contents of both ATP and PCr. However, our results are compatible with present knowledge of the cytosolic energy transfer by PCr and of the control of force in myofilaments.

High-energy phosphates in quiescent, beating and contracted cardiac cells

Isolated myocytes from ventricles are quiescent in the presence of 0.9 mM calcium. However, it is possible to induce beating by adding 0.5 mM BaCl2 to the media or to induce a contracture by elevating the external concentration of potassium (72.5 mM K). During the viable stage of contracture, which is up to 1 h, the sarcomere spacing is 1.7 +/- 0.1 micron and no leakage of intracellular components is observed. The metabolic properties of the cells in quiescent, beating and contracted states were compared. The O2 consumption (natom per mg cell protein per min) increased from 10-11 in quiescent cells to 60-66 in beating cells and 90-99 in contractured cells. In contrast no significant difference was found in the metabolite levels in the three cellular states: (nmol per mg cell protein +/- S.E.M.) ATP, 20.9 +/- 1.5; CrP, 22.3 +/- 2.2; ADP, 6.03 +/- 0.67; Cr, 10.8 +/- 1. It is proposed that the combined action of myosine ATPase, ATP synthase and cytosolic and mitochondrial creatine kinases serves to buffer the metabolite levels during periods of enhanced oxygen consumption.

Relationship between ionic perturbations and electrophysiologic changes in a canine Purkinje fiber model of ischemia and reperfusion

Standard and ion-sensitive microelectrodes were used to identify the basis of electrophysiologic changes that occur in canine cardiac Purkinje fibers superfused with "ischemic" solution (40 min) and then returned to standard Tyrode's solution. Maximum diastolic potential (EMDP) decreased (-92.6 +/- 2.4 to -86.0 +/- 4.0 mV; n = 19; P less than 0.001) during exposure to "ischemia," and after reperfusion, rapidly hyperpolarized to -90.0 +/- 4.7 (2 min) and then depolarized to -47.0 +/- 7.5 mV (10 min; P less than 0.001). No significant change in intracellular K activity (alpha ik) was noted throughout. Extracellular K activity (alpha ek) changed only during reperfusion, reaching a nadir at 5 min (3.5 +/- 0.4 to 2.6 +/- 0.5 mM, P less than 0.03), and thus can not account for the decrease in EMDP during reperfusion. Mean alpha iNa increased (8.7 +/- 1.3 to 10.9 +/- 1.9 mM; n = 10; P less than 0.01) during ischemia, but rapidly declined during reperfusion to 5.1 +/- 2.2 mM (10 min; P less than 0.01). Exposure to acetylstrophanthidin (4-5 x 10(-7) M) during the final 10 min of ischemia increased alpha iNa to 19.9 +/- 3.8 mM (n = 5), which was unchanged at 5 min of reperfusion. This suggests that Na-K pump inhibition during ischemia was minimal and that the pump was stimulated early during reperfusion, accounting for the initial transient hyperpolarization. Resting tension did not change significantly during exposure to ischemia; however, return to control Tyrode's solution caused a marked rise to 11.3 +/- 9.9 mg/mm2 (n = 13, P less than 0.001). This is consistent with a calcium overload state during reperfusion. The depolarization seen during reperfusion may result from activation of a Ca-activated, nonselective cation channel or enhanced electrogenic Na/Ca exchange.

Control of mitochondrial respiration in muscle

Control of mitochondrial respiration depends on ADP availability to the F1-ATPase. An electrochemical gradient of ADP and ATP across the mitochondrial inner membrane is maintained by the adenine nucleotide translocase which provides ADP to the matrix for ATP synthesis and ATP for energy-dependent processes in the cytosol. Mitochondrial respiration is responsive to the cytosolic phosphorylation potential, ATP/ADP.Pi which is in apparent equilibrium with the first two sites in the electron transport chain. Conventional measures of free adenine nucleotides is a confounding issue in determining cytosolic and mitochondrial phosphorylation potentials. The advent of phosphorus-31 nuclear magnetic resonance (P-31 NMR) allows the determination of intracellular free concentrations of ATP, creatine-P and Pi in perfused muscle in situ. In the glucose-perfused heart, there is an absence of correlation between the cytosolic phosphorylation potential as determined by P-31 NMR and cardiac oxygen consumption over a range of work loads. These data suggest that contractile work leads to increased generation of mitochondrial NADH so that ATP production keeps pace with myosin ATPase activity. The mechanism of increased ATP synthesis is referred to as 'stimulus-response-metabolism' coupling. In muscle, increased contractility is a result of interventions which increase cytosolic free Ca2+ concentrations. The Ca2+ signal thus generated increases glycogen breakdown and myosin ATPase in the cytosol. This signal is concomitantly transmitted to the mitochondria which respond to small increases in matrix Ca2+ by activation of Ca2+-sensitive dehydrogenases. The Ca2+-activated dehydrogenase activities are key rate-controlling enzymes in tricarboxylic acid cycle flux, and their activation by Ca2+ leads to increased pyridine nucleotide reduction and oxidative phosphorylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous Ca2+ release from the sarcoplasmic reticulum limits Ca2+-dependent twitch potentiation in individual cardiac myocytes. A mechanism for maximum inotropy in the myocardium

We hypothesized that the occurrence of spontaneous Ca2+ release from the sarcoplasmic reticulum (SR), in diastole, might be a mechanism for the saturation of twitch potentiation common to a variety of inotropic perturbations that increase the total cell Ca. We used a videomicroscopic technique in single cardiac myocytes to quantify the amplitude of electrically stimulated twitches and to monitor the occurrence of the mechanical manifestation of spontaneous SR Ca2+ release, i.e., the spontaneous contractile wave. In rat myocytes exposed to increasing bathing [Ca2+] (Cao) from 0.25 to 10 mM, the Cao at which the peak twitch amplitude occurred in a given cell was not unique but varied with the rate of stimulation or the presence of drugs: in cells stimulated at 0.2 Hz in the absence of drugs, the maximum twitch amplitude occurred in 2 mM Cao; a brief exposure to 50 nM ryanodine before stimulation at 0.2 Hz shifted the Cao of the maximum twitch amplitude to 7 mM. In cells stimulated at 1 Hz in the absence of drugs, the maximum twitch amplitude occurred in 4 mM Cao; 1 microM isoproterenol shifted the Cao of the maximum twitch amplitude to 3 mM. Regardless of the drug or the stimulation frequency, the Cao at which the twitch amplitude saturated varied linearly with the Cao at which spontaneous Ca2+ release first occurred, and this relationship conformed to a line of identity (r = 0.90, p = less than 0.001, n = 25). The average peak twitch amplitude did not differ among these groups of cells. In other experiments, (a) the extent of rest potentiation of the twitch amplitude in rat myocytes was also limited by the occurrence of spontaneous Ca2+ release, and (b) in both rat and rabbit myocytes continuously stimulated in a given Cao, the twitch amplitude after the addition of ouabain saturated when spontaneous contractile waves first appeared between stimulated twitches. A mathematical model that incorporates this interaction between action potential-mediated SR Ca2+ release and the occurrence of spontaneous Ca2+ release in individual cells predicted the shape of the Cao-twitch relationship observed in other studies in intact muscle. Thus, the occurrence of spontaneous SR Ca2+ release is a plausible mechanism for the saturation of the inotropic response to Ca2+ in the intact myocardium.

Influence of sodium-hydrogen exchange on intracellular pH, sodium and tension in sheep cardiac Purkinje fibres

1. The influence of sarcolemmal Na+-H+ exchange upon intracellular Na+ activity (aiNa), intracellular pH (pHi), extracellular surface pH (pHs) and tonic tension was investigated in sheep cardiac Purkinje fibres. Intracellular ion activities were measured with liquid sensor ion-selective micro-electrodes. A two-micro-electrode voltage-clamp was also used to control membrane potential while simultaneously recording tonic tension. 2. Inhibition of the sarcolemmal Na+-K+ pump by strophanthidin (10 mumol/l) produced a rise in aiNa, an increase in [Ca2+]i as evidenced by a rise in tonic tension, and a fall in pHi of 0.1-0.3 units. The intracellular acidosis has been shown previously to be linked to the rise in [Ca2+]i (Vaughan-Jones, Lederer & Eisner, 1983). 3. Amiloride (1-2 mmol/l), an inhibitor of Na+-H+ exchange, produced a small reversible decrease in pHi and aiNa. Both effects became more pronounced in strophanthidin-exposed fibres. In addition, pHi decreased during application of strophanthidin and this decrease was reversibly inhibited by amiloride. It is concluded that sarcolemmal Na+-H+ exchange is stimulated following inhibition of the Na+-K+ pump. 4. In strophanthidin-exposed fibres, a rise in [Ca2+]i resulted in an intracellular acidosis which could still be observed in the presence of amiloride (1 mmol/l). This suggests that the fall in pHi was not caused by a modulatory effect of [Ca2+]i on sarcolemmal Na+-H+ exchange. 5. Tetrodotoxin (TTX) produced a small fall in aiNa (ca. 0.5 mmol/l) which was not augmented in the presence of strophanthidin. Furthermore, the effects on aiNa of TTX and amiloride were additive. Thus the influence of amiloride on aiNa does not involve blockade of voltage-gated Na+ channels. 6. The stoicheiometry of Na+-H+ exchange, estimated from the rates of change of pHi and aiNa in amiloride, appeared to be electroneutral (1:1). The stoicheiometry was unaffected by changes in pHi. 7. In strophanthidin-exposed fibres (i.e. aiNa is elevated), the recovery of pHi from an intracellular acidosis (brought about by brief exposure to NH4Cl) was slowed greatly by amiloride (1-2 mmol/l). The rise in aiNa that occurred during pHi recovery was also reduced by amiloride. It is concluded that Na+-H+ exchange can be stimulated by a fall in pHi under conditions where aiNa is elevated. However, at a given pHi, its rate of recovery was slower in the presence than in the absence of strophanthidin.(ABSTRACT TRUNCATED AT 400 WORDS)

Relations among sodium pump inhibition, Na-Ca and Na-H exchange activities, and Ca-H interaction in cultured chick heart cells

Sodium pump inhibition in cardiac muscle cells is associated with changes in intracellular sodium, calcium, and hydrogen concentrations as well as in membrane ion transport activity. We examined further the functional relations among these entities using cultured chick ventricular cells. [Ca]i and pHi were determined from fluorescence signals obtained from cells loaded with fura-2 or BCECF, respectively. Ouabain (100 microM) elevated [Ca]i eightfold and decreased pHi by 0.11 unit (a 30% increase in [H+]). In the presence of 10 microM ethylisopropylamiloride, a potent inhibitor of Na-H exchange, ouabain elevated [Ca]i 3.5-fold and reduced pHi by 0.16 unit (a 48% increase in [H+]). Exposure to sodium-free (sodium replaced with potassium) medium produced a twelvefold increase in [Ca]i and a 0.12 pH unit decrease in pHi. In cells treated with 100 microM ouabain, exposure to sodium-free (lithium) medium resulted in a 22-fold sustained increase in [Ca]i and a rapid intracellular acidification (pH 7.15 to 6.60). The effect of ouabain or sodium-free medium on pHi was abolished in calcium-free medium; addition of 1 mM Ca rapidly increased [Ca]i and decreased pHi. In cells treated with subtoxic (3 microM) or toxic (100 microM) concentrations of ouabain, initial 24Na uptake rates were significantly greater than in control cells and were significantly reduced in the presence of 10 microM ethylisopropylamiloride. We conclude that ouabain (100 microM) produces intracellular acidification as a result of sodium pump inhibition; calcium accumulation via Na-Ca exchange, and subsequent Ca-H interaction within the cell.(ABSTRACT TRUNCATED AT 250 WORDS)