Changes in the sodium, potassium and chloride of rabbit auricles treated with ouabain

Regulation of intracellular and mitochondrial sodium in health and disease

The transmembrane sodium gradient is essential for both excitability of the cardiac cell and the regulation of the cytoplasmic concentrations of Ca and protons. In addition, movements of Na across the mitochondrial membrane affect matrix protons and calcium. In the first part of the review, we discuss the most important pathways responsible for sarcolemmal and mitochondrial sodium movements. The bulk of the review considers the changes of intracellular Na concentration ([Na(+)](i)) that occur in disease, specifically, ischemia, reperfusion, and heart failure. We review evidence implicating the increase of intracellular sodium to either increased influx of sodium (via either sodium channels or sodium/hydrogen exchange) or, alternatively, to decreased efflux on the Na/K pump. Although much has been learned about sodium regulation in the heart, there are still many unanswered questions, particularly concerning mitochondrial Na regulation.

Adrenaline, anti-adrenaline drugs and potassium movements in rabbits auricles

Adrenaline (2x10(-6) M) or isoprenaline (7.5x10(-8) M) increased the rate of (42)K uptake and the potassium content of right (spontaneously beating) auricles, but had no effect on potassium movements in quiescent left auricles. Although faster beating induced by electrical stimulation increased the rate of (42)K uptake, the actions of adrenaline were also apparent in auricles which were electrically stimulated so that they beat at a constant rate. The increase in (42)K uptake produced by adrenaline accounted entirely for the increase in potassium content of the tissue. Adrenaline, in concentrations ranging from 2x10(-6) M to 2x10(-4) M, had no effect on (42)K loss from electrically stimulated auricles. The action of adrenaline on (42)K uptake was blocked by dichloroisoprenaline (4x10(-6) M) but not by phenoxybenzamine (1.6x10(-6) M).

CALCIUM IN RELATION TO THE ACTIONS OF OUABAIN AND ADRENALINE ON THE HEART

Isolated pairs of rabbit auricles have been observed in media containing 6, 24 or 75 mM-potassium, with corresponding reductions in sodium concentration. In 24 mM-potassium, adrenaline restored beating and excitability, as did calcium chloride, but ouabain had no effect. In 75 mM-potassium, adrenaline had no effect; calcium chloride caused a contracture; ouabain had no direct effect, but auricles which had been beating in the presence of ouabain contracted promptly on transfer to 75 mM-potassium. Left auricles, which do not beat spontaneously, were less sensitive to calcium and to ouabain. The results showed a membrane stabilizing action of calcium and an action on muscular contraction, and suggested that cardiac glycosides acted by causing accumulation of calcium at the activator site in the tissue.

200 years of digitalis: the emerging central role of the sodium ion in the control of cardiac force

Digitalis has been used therapeutically for two centuries, but the mechanism by which it enhances the ability of cardiac muscle to produce force (the positive inotropic effect) has not been fully elucidated. The major controversy concerns the question of whether the inhibition of the Na+-K+ pump by digitalis, particularly at low (therapeutic) concentrations, increases the intracellular Na+ concentration and thus is causally related to the positive inotropic effect. Na+-selective microelectrodes, introduced recently, have made it possible to measure small changes in intracellular Na+ activity (aiNa) in beating preparations of cardiac muscle and, in particular, to follow the exact time course of change in both aiNa and contractile force during the positive inotropic effect of digitalis. It has been demonstrated that digitalis at low and high concentrations produces a parallel increase in aiNa and in contractile force during the onset of its effect; washout of the drug results in a parallel and complete recovery of aiNa and contractile force. Additional strong evidence for a correlation between the pump inhibition and digitalis inotropy is the fact that the magnitude of increase in aiNa and contractile force produced by digitalis depends on the level of aiNa and therefore on the rate of Na+ extrusion by the Na+-K+ pump. The study on the quantitative relationship between aiNa and contractile force reveals that the force of contraction is a power function of aiNa, such that a small rise in aiNa produces a significant increase in contractile force. Direct measurements of aiNa and intracellular free Ca2+ during digitalis inotropy strongly support the hypothesis that an increase in aiNa raises intracellular Ca2+ via Na+-Ca2+ exchange, thus producing the positive inotropic effect. In conclusion, the recent data available from the simultaneous and continuous measurements of aiNa and contractile force strongly indicate that the inhibition of the Na+-K+ pump is causally related to the positive inotropic effect of digitalis on cardiac muscle.

Synthesis and use of radio cobaltic EDTA as an extracellular marker in rabbit heart

A new gamma-labeled marker for extracellular space is the cobaltic form of 58Co-ethylenediaminetetraacetic acid (58Co-EDTA). The cobaltic ion has a much higher affinity for EDTA than the cobaltous ion; it is prepared as a potassium salt, K+(58Co3+-EDTA4-), and is apparently biologically inert. Testing by equilibration in intact rabbits and comparing the myocardial content with that of [14C]sucrose give values of the volume of distribution in the myocardium of 0.294 +/- 0.052 ml/g for 58Co-EDTA and 0.303 +/- 0.051 ml/g for [14C]sucrose (SD, n = 130, for two hearts), with the ratios of 58Co-EDTA/sucrose averaging 0.973 +/- 0.043 (n = 130). The average value of the extracellular fluid measured in isolated rabbit interventricular septum using Co-EDTA was 0.51 +/- 0.05 ml/g (SD, n = 16) and 0.46 +/- 0.04 ml/g using [14C]sucrose as an extracellular fluid space (ECF) marker. Flushing with a high concentration of nontracer Co-EDTA does not reveal any release from binding sites. The gamma-energy (811 KeV), long half-life (71.4 days), stability, and lack of binding to tissue components make 58Co-EDTA a useful marker for ECF.

Sodium efflux in rabbit myocardium: relationship to sodium-calcium exchange

1. The washout of (22)Na, (45)Ca and (58)CoEDTA(-) has been observed in arterially perfused rabbit interventricular septa both in the presence and absence of intoxicating doses of the aglycone, 3 acetylstrophanthidin (ACS).2. The washout of (22)Na from paced septa (21.0/min) was rapid with approximately 99% of the exchange occurring in 30 min.3. Septa labelled with (22)Na and washed out in the presence of 10(-5)m-ACS were quiescent, exhibited marked contracture and a slowed (22)Na exchange with 99% of the initial (22)Na leaving the tissue in approximately 200 min.4. Comparison of the washout of the extracellular marker (58)CoEDTA(-) from paced and intoxicated septa revealed no differences, suggesting that regions of the extracellular space had not become inexchangeable as a result of the contracture that these septa exhibited.5. Simultaneous measurements of the distribution of (22)Na and (58)CoEDTA(-) in the presence of 10(-5)m-ACS revealed an apparent cellular Na concentration of 54.8+/-10.4 (s.e.). This was considerably in excess of the maximum amount of Na contributing to the slowest component of Na efflux from intoxicated septa. Origins of this discrepancy are considered.6. The slowest component of (22)Na efflux from intoxicated septa could be markedly stimulated by increasing the external Ca concentration from a nominal 1.5 to 16 mm. The magnitude of stimulation was 44.3+/-7.8% (s.e.). Increases in the concentration of Mg from 1.0 to 16.0 mm were without effect on the (22)Na efflux.7. Application of 10(-5)m-ACS produced marked depolarization of the membrane to new stable levels that lay between -10 and -20 mV.8. Washout of ACS after several hours of intoxication resulted in a partial decline in contracture tension, an acceleration of Ca efflux, little change in (22)Na efflux and a repolarization of the cell membrane.9. Some of the Na and Ca movements reported in this paper have been interpreted by supposing that there is, in this preparation, a coupled exchange of three Na ions for a single Ca ion.

The effect of heart rate on the membrane responsiveness of rabbit atrial muscle

The maximum rate of rise of action potentials in myocardial fibers of the rabbit atrium decreases with an increase in heart rate. This decrease of the dV/dt max is accompanied by a decrease of the diastolic transmembrane potential prior to the moment of activation (take-off potential). Comparison of the membrane responsiveness curve (relation between dV/dt max and take-off potential) as measured by varying the extracellular potassium concentration at a fixed rate of stimulation, with the effect of changes in the frequency of stimulation on dV/dt max and take-off potential made clear that the fall in dV/dt max after a sudden increase in heart rate was stronger than could be explained by the concomitant decrease of the take-off potential alone. This implicates that the membrane responsiveness itself is heart rate dependent. A possible explanation for this observation is that when heart rate is increased the active Na/K pump is not able to maintain the intracellular concentration of Na and K at the original level. Acceleration of the heart will lead to an intracellular loss of potassium and a gain of sodium. The first causes a diminishment of the diastolic membrane potential which according to the membrane responsiveness curve is attended with a decrease of the dV/dt max. The second results in a decrease of the sodium concentration gradient and therefore in a further reduction of the dV/dt max. This hypothesis was confirmed by experiments with ouabain added to the perfusion fluid. Ouabain, which is known to inhibit the Na/K pump, caused a decrease of both the take-off potential and dV/dt max that was completely comparable with the effects of an increase of the frequency of stimulation. In addition, observation of the time course of the changes in dV/dt max and membrane "resting" potential after a sudden change in the rate of stimulation, gave support to the electrogenic concept of the active Na/K pump in cardiac muscle.

Choline permeability in cardiac muscle cells of the cat

Permeability of the cardiac cell membrane to choline ions was estimated by measuring radioactive choline influx and efflux in cat ventricular muscle. Maximum values for choline influx in 3.5 and 137 mM choline were respectively 0.56 and 9 pmoles/cm(2).sec. In 3.5 mM choline the intracellular choline concentration was raised more than five times above the extracellular concentration after 2 hr of incubation. In 137 mM choline, choline influx corresponded to the combined loss of intracellular Na and K ions. Paper chromatography of muscle extracts indicated that choline was not metabolized to any important degree. The accumulation of intracellular choline rules out the existence of an efficient active pumping mechanism. By measuring simultaneously choline and sucrose exchange, choline efflux was analyzed in an extracellular phase, followed by two intracellular phases: a rapid and a slow one. Efflux corresponding to the rapid phase was estimated at 16-45 pmoles/cm(2).sec in 137 mM choline and at 1.3-3.5 pmoles/cm(2).sec in 3.5 mM choline; efflux in 3.5 mM choline was proportional to the intracellular choline concentration. The absolute figures for unidirectional efflux were much larger than the net influx values. The data are compared to Na and Li exchange in heart cells. Possible mechanisms for explaining the choline behavior in heart muscle are discussed.

Ions and the action of digitalis

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