Voltage clamp of single freshly dissociated smooth muscle cells: current-voltage relationships for three currents

Voltage-clamp experiments on single freshly dissociated (i.e. uncultured) vertebrate smooth muscle cells were carried out under conditions where the initial inward current, as well as various phases of outward current, could be studied. Current-voltage relationships were obtained for the initial current, the peak outward current, and a later, steady-state current, over a potential range of approximately -130 mV to +50 mV. Evidence is presented that the initial current is carried by Ca+++ ions and is responsible for the rising phase of the action potential and that the peak in the outward current is due to Ca++ activation of K+ conductance.