Mechanisms of the thapsigargin-induced Ca2+ entry in in situ endothelial cells of the porcine aortic valve and the endothelium-dependent relaxation in the porcine coronary artery

1 The mechanisms of the thapsigargin (TG)-induced capacitative Ca2+ entry in in situ endothelial cells and its role in the regulation of arterial tone were investigated using front-surface fluorimetry and fura-2-loaded strips of porcine aortic valve and coronary artery. 2 In the presence of extracellular Ca2+, TG induced an initial rapid and a subsequent sustained elevation of cytosolic Ca2+ concentration ([Ca2+](i)) in valvular strips. In the absence of extracellular Ca2+, TG induced only a transient increase in [Ca2+](i). 3 The TG-induced sustained elevation of [Ca2+](i) in endothelial cells was inhibited completely by 1 mM Ni2+ and partly by 10 mu M econazole and 30 mu M ML-9, but not by 900 ng ml(-1) pertussis toxin or 100 mu M wortmannin. Therefore, cytochrome P450 and protein phosphorylation are suggested to be involved in the TG-induced Ca2+ influx in in situ endothelial cells. 4 TG induced an endothelium-dependent large relaxation consisting of an initial and a late sustained relaxation in coronary arterial strip precontracted with U46619 (a thromboxane A2 analogue). Indomethacin alone had no effect, while indomethacin plus N-omega-nitro-L-arginine (L-NOARG) markedly inhibited the sustained phase and slightly inhibited the initial phase of the TG-induced relaxation. 5 TG induced a smaller but sustained relaxation during the 40 mM K+-induced precontraction than that seen during the U46619-induced precontraction. This relaxation was completely abolished by the pretreatment with indomethacin plus L-NOARG. 6 In conclusion, both nitric oxide (NO) and endothelium-derived hyperpolarizing factor were suggested to mediate the TG-induced relaxation, while NO plays a major role in the sustained relaxation. The TG-induced sustained [Ca2+]i elevation in endothelial cells was thus suggested to be mainly linked to the sustained production of NO.