Role of InsP3 and ryanodine receptors in the activation of capacitative Ca2+ entry by store depletion or hypoxia in canine pulmonary arterial smooth muscle cells

Background and purpose: Experiments were performed to determine if capacitative Ca2+ entry (CCE) in canine pulmonary arterial smooth muscle cells ( PASMCs) is dependent on InsP(3) receptors or ryanodine receptors as induction of CCE is dependent on simultaneous depletion of the functionally separate InsP(3)- and ryanodine-sensitive sarcoplasmic reticulum (SR) Ca2+ stores in these cells. Experimental approach: Myocytes were isolated from canine pulmonary arteries using enzymatic procedures and were used within 8 h of preparation. Measurements of cytosolic Ca2+ were made by imaging fura-2 loaded individual myocytes that were perfused with physiological buffered saline solution with or without Ca2+. Key results: Treating myocytes with 10 mu M cyclopiazonic acid (CPA), removing extracellular Ca2+, and briefly applying 10 mu M caffeine and 10 mu M 5-hydroxytryptamine (5-HT) depleted SR Ca2+ stores. Extracellular Ca2+ reintroduction caused cytosolic [Ca2+] to elevate above baseline signifying CCE. The InsP(3) receptor inhibitors 2-aminobiphenylborate (50-75 mu M; 2-APB) and xestospongin-C (20 mu M; XeC) abolished CCE. Yet, CCE was unaffected by 10 mu M or 300 mu M ryanodine or 10 mu M dantrolene, which modify ryanodine receptor activity. Higher dantrolene concentrations (50 rho M), however, can inhibit both ryanodine receptors and InsP3 receptors, did reduce CCE. In contrast, CCE activated by hypoxia was unaffected by XeC (20 mu M). Conclusions and implications: The results provide evidence that CCE activated by depletion of both InsP3 and ryanodine SR Ca2+ stores in canine PASMCs is dependent on functional InsP(3) receptors, whereas the activation of CCE by hypoxia appears to be independent of functional InsP(3) receptors.