Inhibitory effects of 6 cardiac glycosides on sodium pump in sheep Purkinje fibers

The inhibitory effects of 6 cardiac glycosides (digoxin, meproscillarin, acrihellin, deslanoside, ouabain, K-strophanthin) on sodium pump in sheep Purkinje fibers were studied. Isolated Purkinje fibers were used and stimulated at 0.4 Hz regularly. Intracellular sodium activity (aiNa), potassium activity (aiK) and membrane potential (Em) were monitored with single barrel normal and ion-selective micro-electrodes. It was shown that aiNa rose definitely from the concentration of 30 nmol.L-1, and became more significant until the dose of 0.4 mumol.L-1. aiNa rose very much faster within the range of toxic doses (from 1-20 mumol.L-1 to 1 mmol.L-1). The occurrence time of delay afterdepolarization (DAD) after adding drugs varied with various drugs used, or with different experiments, hence the time points of DAD and delta aiNa-time, concentration relation curves were quite scattered. But when basing on the appearance of DAD as a common time point we calculated the temporal variation of aiNa before and after toxic effect, the relation curves in a same sort of glycoside were kept uniform. When the toxic effects of glycosides appeared, the inhibition degree of sodium pump activity was quite close (between 52.8% and 65.5%), suggesting that there existed close relationship between sodium pump inhibition and toxic effects of glycosides. Intracellular potassium ions lost progressly within the scope of subtoxic doses (10-30 nmol.L-1) and lost more significantly within the scope of toxic doses (1-20 mumol.L-1). Based on the results of calculation, the inward current induced by the sodium pump inhibition might be responsible for the abnormal automaticity.