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 microM-1 mM) induced contraction in a concentration-dependent manner. However, when the endothelium was kept intact, cumulative application of UTP (0.1-100 microM) (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 microM N omega-nitro-L-arginine (L-NOARG) but not by 10 microM indomethacin. In the presence of intact endothelium, UTP (30 microM) induced a transient relaxation of the strips precontracted with 30 nM U-46619 (a stable analogue of thromboxane A2), 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 (P2U receptor agonists), but not by desensitization to alpha, beta-methylene-ATP (P2x receptor agonist) or to 2-methylthio-ATP (P2Y 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 P2U purinoceptor and [Ca2+]i elevation without Ca(2+)-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.
