Electrogenic Li+/Li+ exchange mediated by the Na+-K+ pump in rabbit cardiac myocytes.

The membrane Na+-K+ pump can be activated at extracellular sites by several different monovalent cations, including Li+, while being highly selective for the physiological activator Na+ at intracellular sites. We examined whether Li+ can replace Na+ as an activator of the Na+-K+ pump at intracellular sites. Single cardiac myocytes were voltage clamped at 0 mV with wide-tipped patch pipettes filled with a K+- and Na+-free solution containing 160 mM Li+. Ouabain induced an inward shift of membrane current when the myocytes were superfused with Na+-Tyrode solution containing 5.6 mM K+. The shift was dependent on the presence of intracellular Li+ (LiCl in pipette filling solution replaced with tetramethylammonium chloride) and extracellular K+. When we replaced Na+ and K+ in the superfusate with Li+ and voltage clamped myocytes using Li+-containing filling solutions in patch pipettes, ouabain induced an inward shift in membrane current similar to that recorded when myocytes were superfused with K+-containing Na+-Tyrode solution. These findings indicate that the Na+-K+ pump can mediate electrogenic exchange of intracellular Li+ for extracellular K+ or Li+.

Regulation of intracellular pH in cardiac muscle during cell shrinkage and swelling in anisosmolar solutions.

The effect on intracellular pH (pHi) of exposure to solutions of progressively increasing osmolarity from 418 to 620 mosM and to hyposmolar solutions (240 mosM) was examined in guinea pig ventricular muscle using ion-selective microelectrodes. Exposure of tissue to 418 mosM Tyrode solution (100 mM sucrose added) produced an intracellular alkalosis of approximately 0.1 U, whereas exposure to 620 mosM solution (300 mM sucrose added) caused an intracellular acidosis of approximately 0.1 U. The maximal rate of recovery of pHi from acidosis induced by an NH4Cl prepulse increased progressively as extracellular osmolarity was raised from 310 to 620 mosM. This suggests that the acidosis observed at steady state in 620 mosM solution is not due to inhibition of the Na(+)-H+ exchanger. In the presence of 10 microM ryanodine, exposure to 620 mosM solution produced a sustained intracellular alkalosis. We suggest that the decrease in pHi during exposure to 620 mosM solution is due, at least in part, to the acidifying influence of Ca2+ release from the sarcoplasmic reticulum. This decrease in pHi is expected to contribute to the negative inotrop reported in studies of cardiac contractility in markedly hyperosmolar solutions. There was no change in pHi when tissue was exposed to hyposmolar solution. However, the maximal rate of recovery of pHi from acidosis was slower in hyposmolar than in isosmolar solution, despite a concomitant decrease in the intracellular buffer capacity. This suggests that osmotic cell swelling results in inhibition of the sarcolemmal Na(+)-H+ exchanger.

Sodium-hydrogen exchange in guinea-pig ventricular muscle during exposure to hyperosmolar solutions.

1. The effect on intracellular pH (pHi) and intracellular Na+ activity (aNai) of exposure to hyperosmolar solutions was investigated in guinea-pig ventricular muscle using ion-sensitive microelectrodes. 2. Exposure of tissue to solution made hyperosmolar by the addition of 100 mM-sucrose produced an intracellular alkalinization of 0.10 pH units and hyperpolarization of the membrane potential. 3. When extracellular Na+ was reduced to 15 mM by substitution of NaCl with choline chloride, exposure to hyperosmolar solutions caused a decrease in pHi. Identical experiments using LiCl as the sodium substitute resulted in an increase in pHi of a magnitude similar to that seen at physiological Na+ levels. 4. In the presence of 50 microM-5-(N,N-dimethyl)amiloride (DMA), an inhibitor of Na(+)-H+ exchange, pHi decreased upon exposure to hyperosmolar solution. 5. The recovery of pHi from an intracellular acidosis (induced by brief exposure to NH4Cl) was enhanced in hyperosmolar solution when compared to recovery in isosmolar solution. This enhancement was observed even when aNai was markedly elevated (greater than 25 mM) by inhibition of the Na(+)-K+ pump. 6. There was an increase in aNai during exposure to hyperosmolar solutions. When the Na(+)-K+ pump was inhibited with dihydro-ouabain a component of this increase in aNai was sensitive to DMA. 7. We conclude that exposure of cardiac tissue to hyperosmolar solutions results in an intracellular alkalosis due to activation of the sarcolemmal Na(+)-H+ exchanger. Such changes should be considered when exposure to hyperosmolar solutions is used in the study of excitation-contraction coupling and cardiac muscle mechanics.

Effects of hyperkalemia, acidosis, and hypoxia on the depression of maximum rate of depolarization by class I antiarrhythmic drugs in guinea pig myocardium: differential actions of class Ib and Ic agents.

Standard microelectrode methods were used to record intracellular action potentials from strips of guinea pig right ventricular myocardium superfused with either standard physiological saline (pH 7.3; PO2 greater than 650 mm Hg; [K+] = 5.6 mM) or the same solution modified to produce either hyperkalemia ([ K+] = 11.2 mM), acidosis (pH = 6.3), or hypoxia (PO2 = 60 mm Hg). The effects on action potential parameters of three therapeutic concentration of lidocaine, flecainide, and encainide were studied under all four conditions at four different drive rates (interstimulus interval = 2,400, 1,200, 600, and 300 ms). Hyperkalemia in the absence of drugs produced reductions in resting potential (-87.9 +/- 3.8 to -74.6 +/- 3.3 mV), maximum rate of depolarization (316 +/- 68 to 240 +/- 12 V/s), and action potential duration (178 +/- 21 to 165 +/- 27 ms). All three drugs produced increased depression of Vmax in hyperkalemia compared to control conditions but, at all three concentrations and all four rates, this enhancement of effect was greater for lidocaine than for either of the other two agents (which did not differ significantly from each other; p less than 0.001). Similar though less marked effects were produced by acidosis (3.5 mV depolarization and 19% reduction in Vmax), and once again the depression of Vmax by lidocaine was enhanced more by this intervention than were the actions of encainide or flecainide (p less than 0.01). Hypoxia had no effect on action potential parameters other than duration and no significant modulation of drug actions was seen for this intervention.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective depression of maximum rate of depolarization of guinea-pig ventricular action potentials by amiodarone and lignocaine in simulated ischaemia: comparison with encainide.

1. Standard microelectrode techniques were used to record intracellular action potentials from guinea-pig ventricular myocardium superfused with either control physiological saline (pH 7.5; pO2 500 mmHg; [K+] 5.6 mmol/L) or 'simulated ischaemic' solution (pH 6.4; pO2 90 mmHg; [K+] 11.2 mmol/L). 2. The effects on action potential parameters of therapeutic concentrations of lignocaine, amiodarone and encainide were studied under both conditions. 3. Simulated ischaemia, in the absence of drugs, produced marked reductions in resting potential (-86.6 +/- 2.3 to -64.7 +/- 3.5 mV), maximum rate of depolarization (Vmax; 263 +/- 66 to 106 +/- 36 V/s) and action potential duration (164 +/- 24 to 97 +/- 26 ms). No drug produced any additional effect on resting potential. 4. All three drugs produced enhanced depression of Vmax in ischaemia compared to control conditions (class I effect). This was much more marked for lignocaine and amiodarone (inactivated channel blockers) than for encainide (open channel blocker). 5. In addition the prolongation of action potential duration seen with acute exposure to amiodarone (174 +/- 12 to 192 +/- 17 ms; class III effect) was abolished under simulated ischaemic conditions. 6. It is concluded that lignocaine and amiodarone exert greater selectivity for ischaemic tissue than does encainide and that amiodarone may function primarily as a class I agent under ischaemic conditions.

Cardiac electrophysiological actions of captopril: lack of direct antiarrhythmic effects.

1. Standard microelectrode techniques were used to study the effects of captopril (1, 10 and 100 microM) on action potentials recorded from guinea-pig ventricular cells and sinoatrial node cells. 2. Captopril had no effect on the maximum rate of depolarization (Vmax) of ventricular action potentials in cells exposed to either normal Locke solution or 'simulated ischaemic' solution (K1 11.2 mM; pH-6.4; PO2 less than 80 mmHg), nor was there any augmentation of the normal small decline in Vmax with increasing stimulation rate (range of interstimulus intervals = 2400 ms to 300 ms). 3. Captopril had no effect on the duration of ventricular action potentials, nor did it alter the shortening seen on exposure to simulated ischaemia. 4. Captopril did not alter spontaneous sinus cycle length or any recorded parameter of sinus node action potentials. 5. It is concluded that any antiarrhythmic effects observed during clinical use of captopril are most unlikely to be due to direct actions of the drug on cardiac cell membrane properties.

Depression of maximum rate of depolarization of guinea-pig ventricular action potentials by metabolites of encainide.

1. Standard microelectrode methods have been used to record action potentials from guinea-pig ventricular myocardium and dog Purkinje fibres, and to study the effects of the two major metabolites of encainide, O-desmethyl encainide (ODE) and 3-methoxy-O-desmethyl encainide (MODE). 2. In concentrations similar to those found in patients during chronic encainide therapy, neither ODE nor MODE produced significant depression of maximum rate of depolarization (Vmax) of action potentials in unstimulated tissue. Repetitive stimulation, however, was associated with depression of Vmax which increased with increasing driving rates (rate-dependent block, RDB). At the fastest rate studied (interstimulus interval = 300 ms) ODE 1 microM depressed Vmax by 47.5 +/- 5.7% and MODE 1 microM, reduced Vmax by 52.2 +/- 12%. 3. The onset and offset kinetics of this rate-dependent block were very slow. Full development of RDB during a train required over 100 action potentials and the time constants of recovery of Vmax from RDB were 86.4 +/- 37 s for ODE and 100.4 +/- 18 s for MODE. The amount of RDB and its rate of onset increased with drug concentration. The recovery time constants were independent of inter-stimulus interval or drug concentration. Both metabolites also produced rate-dependent depression of conduction velocity in canine Purkinje fibres, but no evidence of selective depression of conduction of interpolated premature potentials was seen. 4. Early afterdepolarizations occurred spontaneously in three preparations in the presence of MODE, 1 microM and one preparation in ODE, 1 microM. 5. It is concluded that these metabolites of encainide may play a role in producing both its antiarrhythmic and its proarrhythmic effects.