The effect of the benzothiazepine diltiazem on force and Ca2+ current in isolated frog skeletal muscle fibres

Abstract

1. The action of the D-cis and L-cis isomers of the benzothiazepine derivative diltiazem on isometric force and L-type Ca2+ inward current in short muscle fibres from toe muscles of the frog (Rana esculenta and R. temporaria) was investigated under voltage clamp control with two internal microelectrodes. The experiments were performed at 10 degrees C in TEA-sulphate solution, in which the concentration of ionized Ca2+ was about 4 mM. 2. In the presence of diltiazem (D-cis, 0.25-30 microM; L-cis, 5-100 microM) normal Ca2+ currents and phasic contractures could be elicited by a depolarizing voltage step. However, after long-lasting depolarizations both the Ca2+ channel and the force controlling system (voltage sensor) remained refractory for minutes instead of seconds as under control conditions, i.e. they were 'paralysed'. L-cis diltiazem was about 20 times less effective in comparison with the D-cis isomer. 3. Speed of restoration of force and Ca2+ current was retarded by up to two orders of magnitude, depending on the applied concentration of diltiazem. 4. The steady state potential dependence of force restoration (V0.5 = -49 mV; the potential at which half of the voltage sensors are restored) and that of Ca2+ current restoration (V0.5 = -57 mV) were not altered by diltiazem. 5. The time course of the transition to paralysis, which was quite slow under control conditions, could be accelerated by diltiazem in a concentration-dependent manner. 6. Paralysis developed at the same speed when the fibre was depolarized from -90 mV to -30 mV or to +30 mV. 7. The voltage dependence of Ca2+ current inactivation (inactivation curve; conditioning pulses of 2 min duration) was shifted by about 14 mV to more negative potentials (V0.5 = -31 mV; V'0.5 = -45 mV) under the influence of 1 microM-D-cis diltiazem. It is assumed that no steady state was reached during the conditioning pulse. 8. The voltage dependence of force activation (V0.5 = -45 mV) was not altered by diltiazem. 9. Since the effects of both isomers of diltiazem could only be disclosed during the adjustment of a new steady state and since the drugs acted in a similar way on the inactivation kinetics of force and Ca2+ current, it is concluded that, in depolarized fibres, the drug facilitates the transition to a stabilized inactivated form of the voltage sensor/Ca2+ channel molecule.