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 and higher than 200 nM for ET-3, suggesting that the ET(A) 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+]i and muscle tension, an an indicator of Ca2+ sensitivity of contractile elements, in the presence of ET-1 was identical to that in the presence of high K+ in non-pregnant and pregnant rat uteri. The ET-1-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+]i nor muscle tension. These results suggest that ET-1 acts on ET(A) receptors, increase [Ca2+]i and induces contraction without changing Ca2+ sensitivity of contractile elements. The increase in [Ca2+]i seemed to be mediated by opening of L-type Ca2+ channels in non-pregnant rat uterus and also of non-L-type Ca2+ channels in pregnant rat uterus, but not by Ca2+ release from intracellular storage sites.
Sakata
K
K
Department of Veterinary Pharmacology, Faculty of Agriculture, University of Tokyo, Japan.
Karaki
H
H
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects; D003600:Cytosol / Q000378:metabolism; D016232:Endothelins / Q000494:pharmacology*; D005260:Female; D066298:In Vitro Techniques; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D014590:Uterine Contraction / Q000187:drug effects*; D014599:Uterus / Q000187:drug effects* / Q000502:physiology; D014700:Verapamil / Q000494:pharmacology
D015220:Calcium Channels; D016232:Endothelins; D014700:Verapamil; D011188:Potassium; D002118:Calcium