Oxytocin pretreatment of pregnant rat uterus inhibits Ca 2+ uptake in plasma membrane and sarcoplasmic reticulum

Abstract

Myometrium from rats in varying stages of pregnancy (from 17 to 22 days) was treated with oxytocin (0.1–10 μM) and plasma membranes and sarcoplasmic reticulum were isolated using a Percoll gradient. When the myometrium had been treated with oxytocin, Ca 2+ uptake was reduced by 29.4% in the plasma membrane and by 32.6% in the sarcoplasmic reticulum. The inhibitory action of oxytocin was highly dependent upon the stage of gestation: Only membranes from rats close to term (21–22 days) exhibited reduced Ca 2+ transport activity after hormone treatment. This effect correlated highly with a significant decrease in the serum progesterone level of these animals. In plasma membrane vesicles, oxytocin reduced the maximal velocity of the Ca 2+ pump without significantly affecting the affinity for Ca 2+. Oxytocin did not affect the passive permeability of the plasma membranes, nor their proportion of sealed inside-out vesicles nor the amount of Ca 2+-pump protein in these membranes. In addition, oxytocin caused no change in the passive permeability of the membrane nor in the rate of inositol triphosphate-induced Ca 2+ release from the sarcoplasmic reticulum. These results suggest that there is a specific action of oxytocin on the activity of the myometrial plasma membrane and sarcoplasmic reticulum Ca 2+ pumps which may contribute to the maintenance of an elevated intracellular calcium level during parturition.