Effects of endothelin on cytosolic Ca2+ level and mechanical activity in rat uterine smooth muscle

The effects of endothelin (ET) on cytosolic Ca2+ level ([Ca2+]i,) and mechanical activity were examined in isolated rat uterine smooth muscle. ET-1, ET-2, ET-3 and sarafotoxin S6b (STX) induced rhythmic contractions superimposed on an increased muscle tone. The concentration needed to induce a half-maximum contraction (EC50) was 1.6–3.3 nM for ET-1. ET-2 and STX ind higher than 200 nM for ET-3, suggesting that the ETA receptor is responsible for these contractions. The sensitivity to ET-1 of uterus at day 20 of gestation was higher than that of non-pregnant rat uterus. Contraction induced by ET-1 followed an increase in [Ca2+]i. The relation between [Ca2+], and muscle tension, an indicator of Ca2+ sensitivity of contractile elements, in :hc presence of ET-1 was identical to that in the presence of high K+ in non-pregnant and pregnant rat uteri. The ET-i-induced increases in [Ca2+]i, and muscle tension were strongly inhibited by verapamil in non-pregnant rat uterus. In pregnant rat uterus. however, verapamil only partially inhibited the increases. The verapamil-insensitive portions of [Ca2+]i and contraction were inhibited by EGTA. In the absence of external Ca2+ ET changed neither [Ca2+], nor muscle tension. These results suggest that ET-1 acts on ETA receptors, increase [Ca2+], and induces contraction without changing Ca2+ sensitivity of contractile elements. The increase in [Ca2i], seemed to be mediated hy opening of L-type Ca2+ channels in non-pregnant rat uterus and also of non-L-type Ca2+ channels in pregnant rat uterus, hut not hy Ca2+ release from intracellular storage sites.