Analysis of acetylcholine-induced relaxation of rabbit isolated middle cerebral artery: effects of inhibitors of nitric oxide synthesis, Na,K-ATPase, and ATP-sensitive K channels.

The functional importance of membrane hyperpolarization through activation of ATP-sensitive K channels, or activation of the Na,K-ATPase, was investigated for acetylcholine (ACh)-induced relaxation of the rabbit isolated middle cerebral artery (MCA) precontracted with uridine triphosphate. Incubation with glibenclamide (1 microM), a known blocker of ATP-sensitive K channels, or precontraction with a high concentration of KCl (50 mM) had no effect on ACh-induced relaxation. Similarly, inhibition of the Na,K-ATPase with ouabain (10 microM) or incubation with a potassium-free solution had either no or only a small effect on ACh-induced relaxation. In contrast, NG-nitro-L-arginine (NOLAG) (1 to 10 microM), a structural analogue of L-arginine and an inhibitor of nitric oxide synthesis, produced concentration-dependent although apparently noncompetitive inhibition of ACh-induced relaxation. This inhibition was partially reversed by application of L-arginine (100 microM), a putative precursor for nitric oxide synthesis. It is concluded that membrane hyperpolarization induced by activation of ATP-sensitive K channels or Na,K-ATPase does not play a major functional role in ACh-induced relaxation of rabbit MCA. The potent inhibitory actions of NOLAG would suggest that the major mechanism of ACh-induced relaxation is by release of nitric oxide as in other cerebral and peripheral arteries.