Effect of pH on calcium accumulation and release by isolated fragments of cardiac and skeletal muscle sarcoplasmic reticulum

Effects of intracellular acidosis on [Ca2+]i transients, transsarcolemmal Ca2+ fluxes, and contraction in ventricular myocytes

We examined the effects of intracellular acidosis produced by washout of NH4Cl on [Ca2+]i transients (indo-1 fluorescence), cell contraction (video motion detector), and 45Ca and 24Na fluxes in cultured chick embryo ventricular myocytes. Exposure of cells to 10 mM NH4Cl produced intracellular alkalosis (pH 7.6), and subsequent washout resulted in a transient acidosis (pH 6.5). Exposure to 10 mM NH4Cl slightly decreased [Ca2+]i transients but increased the amplitude of cell contraction. Subsequent washout of NH4Cl initially increased diastolic [Ca2+]i and decreased the peak positive and negative d[Ca2+]i/dt, while the amplitude of cell contraction was markedly decreased. Subsequently, peak systolic [Ca2+]i increased with partial recovery of contraction. A similar increase in [Ca2+]i and decrease in contraction after washout of NH4Cl was observed in single paced adult guinea pig ventricular cells. Acidosis decreased 45Ca uptake by sarcoplasmic reticulum vesicles isolated from chick embryo ventricle. However, the [Ca2+]i increase caused by intracellular acidosis was also observed in the presence of 10 mM caffeine, suggesting that altered sarcoplasmic reticulum handling of calcium is not the only mechanism involved. Intracellular acidosis only slightly increased total 24Na uptake under these conditions, an effect resulting from the combination of a stimulation of amiloride-sensitive sodium influx (Na(+)-H+ exchange) and inhibition of sodium influx via Na(+)-Ca2+ exchange, manifested by a significant decrease in 45Ca efflux. Further support for a lack of involvement of an increased [Na+]i in the observed increase in [Ca2+]i during acidosis was low-sodium, nominal 0-calcium extracellular solution, an experimental condition that minimizes the possible effects of Na(+)-H+ exchange and Na(+)-Ca2+ exchange. We conclude that the [Ca2+]i increase caused by intracellular acidosis in cultured ventricular cells is primarily due to changes in [Ca2+]i buffering and [Ca2+]i extrusion, rather than to an increase in transsarcolemmal calcium influx. Intracellular acidosis also markedly decreases the sensitivity of the contractile elements to [Ca2+]i in cultured chick embryonic and adult guinea pig ventricular myocytes.

The effects of cyanide on intracellular ionic exchange in ferret and rat ventricular myocardium

The effects of cyanide on Ca2+ exchange in isolated ventricular myocytes and on the intracellular concentrations of Ca2+, Na+ and H+ have been investigated to assess the contribution that mitochondria might play in cellular Ca2+ metabolism. Ionic levels were measured with ion-selective electrodes. KCN (2.5 mM) inhibited a component of Ca2+ exchange in myocytes that could be attributed to mitochondrial exchange, but was without effect on non-mitochondrial Ca2+ exchange. NaCN (2.5 mM) caused a transient reduction of [H+]i, [Na+]i and [Ca2+]i when applied to the superfusate bathing ventricular trabeculae or papillary muscles. The transient changes of [Na+]i were accentuated when the preparation was exposed to a solution which would be expected to increase the cellular calcium content. The reduction of [Na+]i which accompanies a reduction of the extracellular sodium concentration, [Na]o, was attenuated in the presence of NaCN, but the intracellular acidosis resulting from a reduction of [Na]o was unaffected by NaCN. A small, but significant, rise of [Ca2+]i accompanied a reduction of [Na]o but only when NaCN was present in the superfusate. It is concluded that cyanide ions have a reasonably specific action on cardiac cellular ionic metabolism. Its primary action is to prevent mitochondrial Ca2+ sequestration. It is postulated that a Na+/H+ exchange, possibly at the sarcolemma, could account for some of the changes to sarcoplasmic ionic levels observed. In a solution of low [Na]o, it is concluded that mitochondria could sequester at least 30% of the calcium accumulated by the cell even though the sarcoplasmic [Ca2+] does not exceed 0.3 microM.

The effects of sodium, hydrogen and magnesium ions on mitochondrial calcium sequestration in adult rat ventricular myocytes

The effect of Na+, H+ and Mg2+ ions on net calcium exchange induced in digitonin-treated myocytes has been investigated. Raising the [Na] from 1.4 to 31.4 mM revealed a sodium-sensitive fraction of net calcium exchange with a K1/2 for Na+ ions of 12 mM, alongside the respiration-dependent accumulation of calcium. An acidosis, but not an alkalosis, was found to depress both of these processes. Mg2+ ions exerted an effect solely on the respiration-dependent calcium sequestration. A simple semi-empirical model based on the experimental data was formulated to assess the effects that altering sarcoplasmic [Na+] and [H+] would have on the calcium-handling properties of cardiac mitochondria. It is concluded that part of the inotropic effects of these ions could be mediated via this organelle.

Mammalian skeletal muscle: long-lasting contractures and potentiated tetani produced by conditioning with weak acid anions

Reversible contractures can be induced in slow mammalian muscles by manipulations that probably generate a long-lasting alkalinization of the muscle cell interior. Such contractures reach about 1/4 of the tetanic force, P0, and last 10 times longer than potassium contractures. While in contracture, the muscle fibers have high resting potentials so that they can be electrically stimulated. Tetanic force is then increased and added to that of the contracture so that total force may reach 2 P0. This level of potentiation has not been reached by any previously-known method.

Role of sodium in ADP- and thrombin-induced megakaryocyte spreading

We investigated the role of sodium in megakaryocyte spreading induced by thrombin and ADP. We found that if extracellular sodium was replaced by lithium, potassium, or choline, spreading was inhibited. When extracellular sodium was present, amiloride or tetrodotoxin inhibited spreading. Using intracellular recording we found spreading to be associated with a permanent membrane depolarization. The extent and rate of thrombin-induced depolarization was reduced when lithium replaced sodium. Unspread cells had an average membrane potential of -44.8 mV. Spread cells had an average membrane potential of -18.46 mV. When choline replaced sodium, or when in the presence of tetrodotoxin and amiloride, the spread cells repolarized, indicating that the depolarization is due to an increase in sodium permeability. Similar treatments did not change the membrane potential of unspread cells. Incubation of megakaryocytes with A23187 together with monensin or methylamine induced spreading. Methylamine occasionally caused spreading by itself, but neither ionophore alone caused spreading. These results indicate that megakaryocyte spreading induced by ADP and thrombin depends on an increase in sodium conductance.

Effects of acid-base changes on excitation--contraction coupling in guinea-pig and rabbit cardiac ventricular muscle

1. Respiratory and metabolic acid-base changes caused similar steady-state changes in the contractility of cardiac ventricular muscle, but the rate of response was more rapid with the former intervention. 2. Variations in extracellular pCO2 and [HCO3-] at constant pH caused only a transient change in contractility. 3. An intracellular pH change can describe the above events. 4. The changes in contractility caused by extracellular acid-base changes could be explained by competition between Ca2+ and H+ ions for a single process. 5. Assuming an electroneutral scheme whereby one extracellular Ca2+ ion or two intracellular H+ ions compete for a binding site, the interior of ventricular cells must be better buffered than the extracellular fluid. 6. H+ ions evoked a release of Ca2+ ions from a mitochondrial suspension with a time course similar to the partial recovery of tension observed during a respiratory acidosis. 7. Respiratory and metabolic acidosis depressed the action potential plateau and prolonged repolarization. 8. The resting potential and the maximum rate of depolarization were unaffected by the above acid-base changes. 9. An acidosis depressed Ca2+ influx through the slow inward channel by an amount sufficient to account for the observed contractility changes. 10. It is concluded that between pH 7.6 and 6.6 the major physiological effect of an acidosis is to depress the slow inward current as a result of an intracellular pH change.