Oxytocin regulates the plasma membrane Ca2+ transport in rat myometrium

Abstract

Development of myometrium in young female rats was stimulated by administration of diethylstilboestrol. Plasma membrane and sarcoplasmic reticulum from rat myometrium were separated by a new and rapid method using a Percoll gradient. Calcium uptake was inhibited in plasma membrane vesicles isolated from oxytocin-treated myometrium, while no consistent effect of oxytocin was found on the Ca2+ uptake in the sarcoplasmic reticulum. Oxytocin regulated the plasma membrane Ca2+ pump by decreasing its apparent affinity for Ca2+ without affecting its maximal velocity. The K1/2 for Ca2+ in the absence of calmodulin was 0.41 +/- 0.04 microM in normal membranes; this was increased to 0.93 +/- 0.12 microM in oxytocin-treated membranes. Calmodulin decreased the K1/2 for Ca2+ to 0.27 +/- 0.027 microM and oxytocin also increased this, to 0.46 +/- 0.061 microM. The effect of oxytocin on the plasma membrane Ca2+ pump was highly dependent on the hormonal status of the animals. When the diethylstilboestrol was administered together with progesterone, the inhibitory action of oxytocin was totally suppressed, consistent with the expected action of this agent. The results suggest that regulation of the plasma membrane Ca2+ pump may be important in the prolonged elevation of intracellular Ca2+ caused by oxytocin.

Plasma Membrane Ca²⁺ Pump: Recent Developments

“…Up until recently, the effect of agonists on this pump has usually been as sessed by treating intact tissues, isolating plas ma membranes and measuring the Ca2+ up take by that fraction of the membrane vesicles which are inside out. By such techniques, it was possible to show, for instance, an oxyto cin-dependent inhibition of the Ca2+ pump in myometrial smooth muscle [33,34], The re cent availability of thapsigargin, a specific inhibitor of the sarco/endoplasmic reticulum Ca2+ pump (SERCA) has made possible the measurement of the contribution of PMCA in intact cells. In cells which lack the Na+/Ca2+ exchanger, SERCA can be inhibited by thap sigargin and the action of the plasma mem brane Ca2+ pump can be observed.…”

Ca2+ pumps in smooth muscle cells

“…The main difficulty involved in ATPase measurements on plasma membranes is the presence of a high background Ca 2+-independent Mg 2* ATPase activity which is not constant with time and which may itself be influenced by agonists or by calmodulin (Missiaen et al, 1988a,b). This activity may be particularly high in plasma membranes of vascular (Eggermont et al, 1988) and uterine smooth muscle (Missiaen et al, 1988b;Enyedi et al, 1989a). In many earlier studies the relation between the measured ATPase activity and the activity of the Ca 2+ pump was not firmly established (Akerman & Wikstrom, 1979;Soloff & Sweet, 1982;Popescu et al, 1985).…”

“…Equivalent results were obtained on the myometrium of rats primed with diethylstilbestrol. Pretreatment of these tissues with oxytocin induces a reduction of the Ca z÷ uptake in subsequently isolated membranes (Missiaen et al, 1988c;Enyedi et al, 1989a). Oxytocin did not affect the maximal velocity of the Ca 2+ pump but decreased the K~ for Ca z+ as well in 2 the presence as in the absence of calmodulin (Enyedi et al, 1989).…”