[Effect of potassium ions on electrogenesis and contraction of ureter smooth muscle].

In the ureter smooth muscle cells, high--K solution produced a depolarization at the onset of which the action potentials (AP) and contraction occur; the latter consists of an initial phasic (Ph) and subsequent tonic (T) components. Ph component is initiated by the AP, T component--by a stable potassium depolarization. In Ca--free solution the AP, Ph and T components are blocked, although a sustained potassium depolarization is preserved. Basing on these results it is suggested that: 1) phasic contraction is initiated by calcium ions influxed via fast potential--dependent calcium channels participating in the AP generation; 2) tonic contraction is also initiated mainly by extracellular calcium influx through slow potential--dependent calcium channels of plasmatic membrane that appear to be similar to calcium channels of sarcoplasmic reticulum membrane in skeletal muscles and of the membrane of presinaptic nerve terminals.