P-2u receptor-mediated release of endothelium-derived relaxing factor nitric oxide and endothelium-derived hyperpolarizing factor from cerebrovascular endothelium in rats

Background and Purpose-Stimulation of P-2u purinoceptors by UTP on endothel ium dilates the rat middle cerebral artery (MCA) through the release of end othelium-derived relaxing factor/nitric oxide (EDRF/NO) and an unknown rela xing factor, The purpose of this study was to determine whether this unknow n relaxing factor is endothelium-derived hyperpolarizing factor (EDHF). Methods-Rat MCAs were isolated, cannulated, pressurized, and luminally perf used. UTP was added to the luminal perfusate to elicit dilations. Results-Resting outside diameter of the MCAs in one study was 209+/-7 mu m (n = 10). The MCAs showed concentration dependent dilations with UTP admini stration. Inhibition of NO synthase with N-G-nitro-L-arginine methyl ester (L-NAME) (1 mu mol/L to 1 mmol/L) did not diminish the maximum response to UTP but did shift the concentration-response curve to the right, Scavenging NO with hemoglobin (1 or 10 mu mol/L) or inhibition of guanylate cyclase w ith ODQ (1 or 10 mu mol/L) had effects on the UTP-mediated dilations simila r to those of L-NAME. In the presence of L-NAME, dilations induced by 10 mu mol/L UTP were accompanied by 13+/-2 mV (P<0.009) hyperpolarization of the vascular smooth muscle membrane potential (-28+/-2 to -41+/-1 mV). Iberiot oxin (100 nmol/L), blocker of the large-conductance calcium-activated K cha nnels, sometimes blocked the dilation, but its effects were variable, Chary bdotoxin (100 nmol/L), also a blocker of the large-conductance calcium-acti vated K channels, abolished the L-NAME-insensitive component of the dilatio n to UTP. Conclusions-Stimulation of P-2u purinoceptors on the endothelium of the rat MCA released EDHF, in addition to EDRF/NO, and dilated the rat MCA by open ing an atypical calcium-activated K channel.