Ca2+-channel current and its modification by the dihydropyridine agonist BAY k 8644 in isolated smooth muscle cells

The electrophysiological properties of single smooth muscle cells isolated from the longitudinal layer of the guinea-pig ileum were studied with the whole-cell patch-clamp technique. The finding of resting potentials between -45 and -50 mV and the occurrence of spontaneous electrical activity when K+ was the predominant intracellular cation indicated that the cells were not leaky or hyperpermeable. The existence of an inward Ca2+ current overlapping in time with an outward rectifying K+ current was demonstrated. The latter could be selectively blocked by replacing internal K+ with Cs+ and external Ca2+ with Ba2+. Depolarizations to potentials between -40 and +50 mV evoked time-dependent inward currents, with a maximum peak value between -20 and 0 mV. For depolarizations beyond +50 mV time-dependent outward currents appeared. These currents were inhibited by 0.1 mM CdCl2. The activation of the inward current showed a sigmoidal time course, and the rate of onset of the current increased at more positive potentials. Inactivation could be described by two exponentials. The threshold for activation was about -40 mV, and full activation was reached at 0 mV. Inactivation was complete near 0 mV, whereas the channels were fully available at -80 mV. The fully-activated Ca2+-channel current was strongly voltage dependent. The conductance decreased for potentials close to the reversal potential, and showed rectification for hyperpolarizing potentials. The Ca2+ agonist BAY k 8644 enhanced the Ca2+-channel current without a significant effect on its kinetics. The fully-activated current and the steady-state activation were enhanced in a rather voltage-independent way.