1. To elucidate the role of thapsigargin-induced Ca2+ entry in endothelial cells in the regulation of vascular tone, changes in Ca2+ and force of smooth muscle were simultaneously monitored in fura-2-loaded strips of porcine renal artery. 2. During phenylephrine-induced sustained contraction, thapsigargin caused an endothelium-dependent triphasic response; an initial relaxation, a subsequent transient contraction, and a sustained relaxation. The initial relaxation and the contraction were associated with a decrease and an increase in [Ca2+](i), respectively. There was no apparent [Ca2+](i) decrease during the sustained relaxation. Thapsigargin-induced responses were observed at 10-8 M and higher concentrations, with the maximum response observed at 10-6 M. 3. The transient contraction was inhibited by a cyclo-oxygenase inhibitor (10-5 M indomethacin), a thromboxane A2 (TXA2)/prostaglandin H2 (PGH2) receptor antagonist (10-5 M ONO-3708), and a TXA2 synthase inhibitor (10-5 M OKY-046). 4. During the phenylephrine-induced contraction in the presence of indomethacin, thapsigargin caused an initial, but not a sustained relaxation, in the presence of Nω-nitro-L-arginine methylester (L-NAME). During the contraction induced by phenylephrine plus 40 mM K+-depolarization in the presence of indomethacin, thapsigargin induced both a transient and a sustained relaxation. However, these relaxations were completely abolished in the presence of L-NAME. 5. Thapsigargin caused a large Ca2+ elevation in cultured endothelial cells of the renal artery. The concentration-response relation was thus similar to that for force development in the arterial strips. 6. In conclusion, thapsigargin-induced Ca2+ entry in endothelial cells led to triphasic changes in the tone of the porcine renal artery. The endothelium-dependent contraction was mediated mainly by TXA2. Nitric oxide and hyperpolarizing factor are both involved in the initial relaxation. However, a sustained relaxation was observed which mainly depended on nitric oxide.
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