Involvement of IP₃-receptor activation in endothelin-1-induced Ca²⁺ influx in rat pulmonary small artery

We examined the endothelin-1 (ET-1)-induced increase in the intracellular free Ca²⁺ concentration ([Ca²⁺]_i) in fura-2-loaded rat pulmonary small arteries. ET-1 (30 nM) elicited a long-lasting increase in [Ca²⁺]_i in physiological salt solution (PSS). In subsequent experiments, arteries were pretreated with BQ-788, an ET _B-specific blocker, to allow us to focus on responses mediated via the ET_A receptor, the existence of which was confirmed by immunohistochemistry. In Ca²⁺-free PSS, ET-1 evoked a small transient increase in [Ca²⁺]_i, indicating Ca²⁺ release from the SR (sarcoplasmic reticulum). After a switch to PSS (containing 2 mM CaCl₂), ET-1 elicited a long-lasting increase in [Ca ²⁺]_i that was not inhibited by 1 μM nicardipine, an L-type Ca²⁺-channel inhibitor, suggesting involvement of a Ca ²⁺-influx pathway independent of that channel. In arteries preincubated with 30 μM cyclopiazonic acid (CPA) or 2 μM thapsigargin (TG), the ET-1-induced Ca²⁺-release was greatly reduced, and the induced Ca²⁺-influx was attenuated. U-73122, a phospholipase C (PLC) inhibitor, had inhibitory effects similar to those of CPA and TG on the ET-1-induced Ca²⁺-release and Ca²⁺-influx, whereas U-73343, an inactive analogue of U-73122, had no such effects. Two putative membrane-permeable IP₃-receptor blockers, 2-aminoethoxydiphenyl borate (2APB, 50 μM) and Xestospongin C (20 μM), (a) almost completely inhibited the ET-1-induced Ca²⁺-release and Ca²⁺-influx, and (b) reduced the ET-1-induced contraction. These results indicate that in rat pulmonary small arteries, ET-1 induces receptor-operated Ca²⁺ influx via the ET_A receptor, and that this influx interacts with InsP ₃-receptor activation.