Studies Concerning the Source of Calcium for Contraction of Rat Myometrium

Abstract

Uptake and efflux of ⁸⁹Sr in pregnant rat myometrium were not different from uptake and efflux of ⁴⁵Ca although tension studies indicated that Sr²⁺ does not become sequestered. The effects of Ca⁺-free solution on uterine responses to acetylcholine (ACh) and K⁺ suggest that at 37 °C, these agents cause release of calcium from the same pool. Substitution of Na⁺ by Li⁺ in the medium caused a contraction which was dependent upon extracellular Ca²⁺, and which was accompanied by increased ⁴⁵Ca uptake and increased Ca²⁺ content. Three inhibitory drugs lanthanum, SKF 525-A, and chlorpromazine inhibited responses of myometrium to ACh and K⁺, but did not demonstrate selective inhibition of one of these stimulants. Effects of these inhibitory drugs were at least partially due to decreased frequency of action potential discharge. Of these three inhibitory agents only lanthanum inhibited (partially) ⁴⁵Ca and ⁸⁹Sr uptake into myometrium.The lanthanum technique for measurement of cellular ⁴⁵Ca influx failed to demonstrate differences in pools of calcium utilized for ACh and K⁺ induced contractions. The effects of lanthanum in contractility and ⁴⁵Ca experiments are discussed.