CA-VALVE(+ MOBILIZATION MEDIATED BY ENDOTHELIN ET(A) RECEPTOR IN ENDOTHELIUM OF RABBIT AORTIC)

The mechanism of Ca++ mobilization induced by endothelins (ETs) and th e receptor subtype responsible for this effect were examined in the en dothelium of rabbit aortic valve. In the endothelium loaded with fura- 2, ET-1 (1-100 nM) induced large transient increase followed by small sustained increase in cytosolic Ca++ level ([Ca++](i)) in a concentrat ion-dependent manner. ET-3 induced only a small increase in [Ca++](i) at higher concentrations (100-300 nM) than ET-1, whereas a selective E T(B) agonist, 100 nM IRL 1620 (succinyl-[Glu(9), Ala(11,15)]ET-1 (8-21 )), was ineffective. A selective ET(A) antagonist, 3 mu M BQ-123, (cyc lo [-Asp-Pro-Val-Leu-Trp-]) but not a selective ET(B) antagonist, 10 m u M RES-701-1 [cyclic (Gly(1)-Asp(9)) -Gly-Thr-Ala-Pro-Asp-Trp-Phe-Phe -Asn-Tyr-Tyr-Trp)] inhibited the effects of ET-1 and ET-3. The sustain ed increase in [Ca++](i) induced by ET-1 was abolished by 30 mu M La++, although 100 nM nicardipine was ineffective. In the absence of exte rnal Ca++ (with 0.5 mM EGTA), ET-1 induced only a transient increase i n [Ca++](i), which was inhibited by an inhibitor of Ca++-ATPase in end oplasmic reticulum, 1 mu M thapsigargin. However, an inhibitor and an activator of Ca++-induced Ca++-release channel, 10 mu M ryanodine and 10 mM caffeine, did not change [Ca++](i). These results suggest that, in the endothelium of rabbit aortic valve, only the ET(A) receptor med iates the effects of ETs to increase [Ca++](i) which is attributable t o the release of Ca++ from thapsigargin-sensitive and ryanodine-insens itive Ca++ stores and also to the Ca++ influx through La+++-sensitive and dihydropyridine-insensitive Ca++ channels.