Actions of the digitalis analogue strophanthidin on action potentials and L-type calcium current in single cells isolated from the rabbit atrioventricular node

1 The atrioventricular node (AVN) of the heart is vital to normal cardiac function and is a major site of antiarrhythmic drug action. This study describes the effects of the cardiac glycoside analogue strophanthidin on spontaneous action potentials and L-type calcium current recorded from single AVN cells isolated from the rabbit heart. 2 With a standard KCl-based internal dialysis solution, exposure to 50 mu M strophanthidin produced a progressive depolarization of the maximum diastolic potential and a reduction in action potential amplitude and upstroke velocity. Sustained application resulted in the loss of action potentials and occurrence of spontaneous 'bell-shaped' depolarizations. 3 Cells were whole-cell voltage clamped at -40 mV and depolarizing voltage clamps applied. With a standard KC1-based internal dialysis solution, exposure to 50 mu M strophanthidin caused a large reduction of I-Ca,I-L at all potentials between -30 and +40 mV (n=4). At + 10 mV, the mean I-Ca,I-L amplitude was reduced from -232+/-65 pA to -48+/-26 pA (P<0.05; t test; n=5 cells). 4 To record I-Ca,I-L more selectively, cells were dialysed with a Cs-based pipette solution. A short strophanthidin exposure reduced I-Ca,I-L amplitude from -250+/-31 pA to -88+/-19 pA (P<0.001; n=8 cells). For both KCl and CsCl-based solutions it was observed that sustained exposure to strophanthidin for several minutes caused spontaneous inward fluctuations in the membrane current record similar to the 'I-TI' (arrhythmogenic oscillatory transient inward) current shown for other cardiac cells. 5 When the calcium chelator BAPTA was added to the pipette solution (10 mM), the reduction in I-Ca,I-L by strophanthidin was largely eliminated (P>0.1), and no spontaneous inward current fluctuations were observed after sustained exposure to strophanthidin (n=8 cells). 6 When external Ca in the perfusate was replaced with Ba, strophanthidin did not significantly reduce the Ba current through L-type calcium channels (n=5 cells). 7 We conclude that strophanthidin reduces I-Ca,I-L by an indirect action, mediated by the rise in intracellular calcium (Ca-i) which follows inhibition of the Na/K( pump caused by cardiac glycosides. The appearance of spontaneous I-TI With strophanthidin would also seem to be mediated by a rise in Ca-i, and may contribute to the spontaneous oscillations in membrane potential observed after prolonged strophanthidin exposure.