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

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Abstract
The washout of 22Na, 45Ca and 58CoEDTA- was observed in arterially perfused rabbit interventricular septa both in the presence and absence of intoxicating doses of the aglycone, 3 acetylstrophanthidin (ACS). The washout of 22Na from paced septa (21.0/min) was rapid with approximately 99% of the exchange occurring in 30 min. Septa labeled with 22Na and washed out in the presence of 10-5 M-ACS were quiescent, exhibited marked contracture and a slowed 22Na exchange with 99% of the initial 22Na leaving the tissue in approximately 200 min. Comparison of the washout of the extracellular marker 58CoEDTA- 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. Simultaneous measurements of the distribution of 22Na and 58CoEDTA- in the presence of 10-5 M-ACS revealed an apparent cellular Na concentration of 54.8 .+-. 10.4 (SE). 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. The slowest component of 22Na 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% (SE). Increases in the concentration of Mg from 1.0 to 16.0 mM were without effect on 22Na efflux. Application of 10-5 M-ACS produced marked depolarization of the membrane to new stable levels that lay between -10 and -20 mV. Washout of ACS after several hours of intoxication resulted in a partial decline in contracture tension, an acceleration of Ca efflux, little change in 22Na efflux and a repolarization of the cell membrane. Some of the Na and Ca movements reported were interpreted by supposing that there is, in this preparation, a coupled exchange of 3 Na+ for a single Ca2+.