Calcium Dependency of Pregnant Rat Myometrium: Comparison of Circular and Longitudinal Muscle 1

Abstract

Whether the changes in spontaneous contractions of circular uterine muscle during pregnancy were related to alterations in Ca sensitivity or dependence was determined. Circular muscle (CM) and longitudinal muscle (LM) segments from rats on Days 16-17 of gestation and at term were compared with respect to: sensitivity of K-induced contractions to changes in extracellular Ca; and rate and magnitude of decrease of K- and acetylcholine (ACh)-induced contractions in Ca-free medium and in methoxyverpamil (D-600). The effects of low Ca and D-600 on spontaneous electrical activity of CM were also studied. Ca sensitivity was no different in CM and LM and did not change between Day 16 and term. There was no difference in the Ca-dependence of K- or ACh-induced contractions during this time. K contractions declined more rapidly than ACh contractions in Ca-free media, especially in CM. Spontaneous action potentials in CM were Ca-dependent and disappeared in low Ca or D-600 on Days 16-17 and at term. The changes in contractions of CM during pregnancy are not related directly to Ca sensitivity or dependence, but indirectly via Ca modulation of the action potentials.