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Dihydropyridine-sensitive ion currents and charge movement in vesicles derived from frog skeletal muscle plasma membranes

Javier Camacho, Alejandro Carapia, Jorge Calvo, María C. García and Jorge A. Sánchez

Whole-cell voltage clamp experiments were performed in vesicles derived from frog skeletal muscle plasma membranes to characterize the electrophysiological properties of dihydropyridine (DHP) receptors. This preparation allows control of the composition of the internal medium and the recording of currents, without the influence of the sarcoplasmic reticulum (SR).

In solutions containing Ba²⁺, Bay K 8644-sensitive, L-type inward currents were recorded. Peak Ba²⁺ currents (I_Ba) averaged 3·0 µA µF^-1 and inactivated in a voltage-dependent manner. Half-maximal steady-state inactivation occurred at -40 mV. No major facilitation of tail currents was observed.

The time course of activation of L-type Ca²⁺ channels was voltage dependent and 10 times faster than that in muscle fibres; the current density values were also much lower.

Lowering [Mg²⁺]_i from 2 to 0·1 mM shifted the time to peak of I_Ba versus voltage relation by -13 mV.

In solutions that contained mostly impermeant ions, non-linear capacitive currents were recorded. Charge movement with properties resembling charge 1 was observed in polarized vesicles. The charge movement depended on voltage with Boltzmann parameters: Q_max (maximum charge), 45·6 nC µF^-1; V (potential at which Q = 0·5Q_max), -58·4 mV; and k (slope factor), 22·3 mV. There was no indication of the presence of Q (the 'hump' component of charge movement).

In depolarized vesicles, non-linear currents were observed during hyperpolarizing pulses. The currents produced an excessive charge during 'on' transients only. Charge during 'off' transients was linear from -180 to +60 mV. There was no evidence of the presence of charge 2.

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