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

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Abstract
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 μ M), a known blocker of ATP-sensitive K channels, or precontraction with a high concentration of KCl (50 m M) had no effect on ACh-induced relaxation. Similarly, inhibition of the Na,K–ATPase with ouabain (10 μ M) 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 μ M), 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 μ M), 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.