Abstract
Effects of external ions (Na, K, Mg) on the recovery from K-contracture of the pregnant rat myometrium were investigated in Ca-free media. Relaxation was faster, when the external concentration of Na and K ions was higher. The time course of relaxation within a limited range of 60-20% of the mechanical output followed a single exponential function. The rate of relaxation, given by the reciprocal of the period during which the contraction decayed to 1/e, was 0.3-0.5min-1 at 34-35°C in isotonic Na solution. Low temperature caused prolongation of the relaxation, and the activation energy estimated on the rate of relaxation was 3-4kcal·mol-1. Relaxation became faster when Mg (0.5-20mM) was added to the relaxation media. Relaxation in isotonic K media was slower than in isotonic Na media, and the addition of Mg caused a slight acceleration of the relaxation. Low temperature caused an acceleration of the relaxation, irrespective of the presence or absence of external Mg ions. In low K media, where K ions were replaced by isosmolar sucrose, addition of Mg caused a significant acceleration of the relaxation. Activation energy ranged between 13-15kcal·mol-1 in the solutions containing Mg. This favors the existence of a relaxing system in pregnant rat myometrium that is Mg-and metabolismlinked.
