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Actions of the digitalis analogue strophanthidin on action potentials and L‐type calcium current in single cells isolated from the rabbit atrioventricular node
Author(s) -
Hancox Jules C.,
Levi Allan J.
Publication year - 1996
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1996.tb15559.x
Subject(s) - atrioventricular node , sinoatrial node , digitalis , calcium , chemistry , rabbit (cipher) , pharmacology , medicine , computer science , heart rate , tachycardia , heart failure , blood pressure , computer security
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 μ 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 KCl‐based internal dialysis solution, exposure to 50 μ m strophanthidin caused a large reduction of I Ca,L at all potentials between −30 and +40 mV ( n = 4). At +10 mV, the mean I Ca,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,L more selectively, cells were dialysed with a Cs‐based pipette solution. A short strophanthidin exposure reduced I Ca,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,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,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.