Ca45 and Sr89 movements with contraction of depolarized smooth muscle

Abstract

Previous findings showed that contractions of depolarized muscle bypass excitation-contraction coupling and that Sr replaces Ca in the contractile machinery but not in coupling. Polarized and K-depolarized thin strips of circular layer of cat intestine were subjected to electric fields. Washout curves for Ca⁴⁵ from resting muscles were composed of two exponentials with half-times of 8 min and 60 min. Ca influxes were increased in contracting muscles in both polarized and depolarized conditions. Resting influx was greater in K-Tyrode than in normal Tyrode's solution. With Sr the ratio of influxes of stimulated to resting polarized muscles was not significantly different from 1.0; contractions of polarized muscles were weak due to uncoupling. Uncoupling produced by 20 mm/liter Mg resulted in a ratio of Ca influxes of stimulated to resting polarized muscles not significantly different from 1.0. It is suggested that Ca entrance is not the sole coupling event and that increased Ca or Sr influx is more closely related to contraction than to changes in membrane potential.