THE EFFECT OF STROPHANTHIDIN ON ACTION-POTENTIAL, CALCIUM CURRENT AND CONTRACTION IN ISOLATED GUINEA-PIG VENTRICULAR MYOCYTES

1. A method is described for producing high yields of calcium-tolerant ventricular myocytes from guinea-pig hearts (73.4% rod-shaped cells, n = 19). Their action potential (AP) and membrane currents were recorded using conventional microelectrodes and cell shortening was measured optically using a linear photodiode array. 2. The sensitivity of the guinea-pig Na+-K+ pump to strophanthidin (a rapidly acting digitalis analogue) was determined by measuring the inhibition of outward pump current by different doses. The pump was found to have a dissociation constant (K(D)) for strophanthidin of 1.11 x 10(-5) M, and 5 x 10(-4) M-strophanthidin inhibited the pump maximally. 3. Exposure to strophanthidin resulted in an initial lengthening followed by a shortening of the AP, and an increased contraction. Initial AP lengthening was associated with a more positive AP plateau which became more negative as the AP shortened. 4. There was a reversible reduction of Ca2+ current (I(Ca)) during exposure to strophanthidin. I(Ca) changed reciprocally with contraction and with a similar time course. 5. Strophanthidin exposure caused a reduction of I(Ca) at all activating voltages, suggesting that it resulted in a reduction of Ca2+ conductance with little change of its voltage dependence. 6. The role of an increase of intracellular calcium (Ca(i)2+) was investigated by impaling myocytes with microelectrodes containing BAPTA 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, a calcium chelator) to increase Ca(i)2+ buffering. Strophanthidin still shortened the AP when BAPTA was present, suggesting that a rise of Ca(i)2+ is not a major cause of AP shortening. 7. Although AP shortening was little affected, the decline of I(Ca) with strophanthidin was markedly reduced when BAPTA was present, suggesting that a rise of Ca(i)2+ was the cause of the I(Ca) decline with strophanthidin. 8. When barium ions carried the current through Ca2+ channels, strophanthidin did not reduce Ca2+ channel current, suggesting that this compound does not have a direct inhibitory effect on the channel. 9. The results suggest that strophanthidin causes a reduction of I(Ca) by increasing Ca(i)2+, via the mechanism of Ca(i)2+-dependent inactivation of I(Ca). The reduction of I(Ca) at least partially explains the AP shortening and more negative plateau with strophanthidin. 10. The shortening of the AP, more negative plateau and reduced I(Ca) have negative inotropic effects which oppose the direct positive inotropic effect of strophanthidin.