Thapsigargin-induced endothelium-dependent triphasic regulation of vascular tone in the porcine renal artery

1 To elucidate the role of thapsigargin-induced Ca2+ entry in endothelial c ells in the regulation of vascular tone, changes in Ca2+ and force of smoot h muscle were simultaneously monitored in fura-2-loaded strips of porcine r enal artery. 2 During phenylephrine-induced sustained contraction, thapsigargin caused a n endothelium-dependent triphasic response; an initial relaxation, a subseq uent transient contraction, and a sustained relaxation. The initial relaxat ion and the contraction were associated with a decrease and an increase in [Ca2+](i), respectively. There was no apparent [Ca2+](i) decrease during th e 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 (1 0(-5) M indomethacin), a thromboxane A(2) (TXA(2))/prostagrandin H-2 (PGH(2 )) receptor antagonist (10(-5) M ONO-3708), and a TXA(2) synthase inhibitor (10(-5) M OKY-046). 4 During the phenylephrine-induced contraction in the presence of indometha cin, thapsigargin caused an initial, but not a sustained relaxation, in the presence of N-infinity-nitro-L-arginine methylester (L-NAME). During the c ontraction induced by phenylephrine plus 40 mM K+-depolarization in the pre sence of indomethacin, thapsigargin induced both a transient and a sustaine d relaxation. However, these relaxations were completely abolished in the p resence 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 t o that for force development in the arterial strips. 6 In conclusion, thapsigargin-induced Ca2+ entry in endothelial cells led t o triphasic changes in the tone of the porcine renal artery. The endotheliu m-dependent contraction was mediated mainly by TXA(2). Nitric oxide and hyp erpolarizing factor are both involved in the initial relaxation. However, a sustained relaxation was observed which mainly depended on nitric oxide.