Dual regulation of cerebrovascular tone by UTP: P-2U receptor-mediated contraction and endothelium-dependent relaxation

1 The mechanisms of vascular tone regulation by extracellular uridine 5'-triphosphate (UTP) were investigated in bovine middle cerebral arterial strips. Changes in cytosolic Ca2+ concentration ([Ca2+](i)) and force were simultaneously monitored by use of front-surface fluorometry of fura-2. 2 In the arterial strips without endothelium, UTP (0.1 mu M-1 mM) induced contraction in a concentration-dependent manner. However, when the endothelium was kept intact, cumulative application of UTP (0.1-100 mu M) (and only at 1 mM) induced a modest phasic contraction in arterial strips. This endothelium-dependent reduction of the UTP-induced contraction was abolished by 100 mu M N-omega-nitro-L-arginine (L-NOARG) but not by 10 mu M indomethacin. In the presence of intact endothelium, UTP (30 mu M) induced a transient relaxation of the strips precontracted with 30 nM U-46619 (a stable analogue of thromboxane A(2)), which was completely inhibited by pretreatment with L-NOARG but not with indomethacin. 3 In the endothelium-denuded strips, the contractile response to UTP was abolished by desensitization to either ATP gamma S or ATP (P-2U receptor agonists), but not by desensitization to alpha,beta-methylene-ATP (P-2X receptor agonist) or to 2-methylthio-ATP (P-2Y receptor agonist). Desensitization to UTP abolished the contractile response to ATP. 4 In the endothelium-denuded artery, a single dose application of UTP induced an initial transient, and subsequently lower but sustained increase in [Ca2+](i) and force. In the absence of extracellular Ca2+, UTP induced only the initial transient increases in [Ca2+](i) and force, while the sustained increases in [Ca2+](i) and force were abolished. UTP (1 mM) had no effect on the basic [Ca2+](i)- force relationship obtained on cumulative application of extracellular Ca2+ at steady state of 118 mM K+-depolarization-induced contraction. 5 We conclude that in the presence of an intact endothelium, UTP-induced relaxation of preconstricted middle cerebral artery is mainly mediated indirectly, by the production of an endothelium-derived relaxing factor, but at high doses of UTP, vascular smooth muscle contraction is mediated directly via activation of P-2U purinoceptor and [Ca2+](i) elevation without Ca2+-sensitization of the contractile apparatus. UTP may thus exert a dual regulatory effect upon cerebrovascular tone, but in cases where the endothelium is impaired, it may also act as a significant vasoconstrictor.