Nitric oxide in the potassium-induced response of the rat middle cerebral artery: a possible permissive role

In the middle cerebral artery (MCA), the presence of nitric oxide (NO) is responsible for maintaining a more dilated state than in its absence during increases in extracellular K+ and osmolality. The purpose of the present study was to determine whether the involvement of NO was due to (a) a direct effect of the K+/osmolality (Khyper) on the endothelium or (b) a ‘permissive’ role of NO. MCAs (approximately 210 μm o.d.) were isolated, cannulated with glass micropipettes, and pressurized to 85 mmHg. When K+ (KCl) in the extraluminal bath was increased to 21 mM, the diameter increased by 15–20% with the magnitude of dilation diminishing with further increases in Khyper. The addition of NG-nitro-l-arginine methyl ester (l-NAME, 10−5 mM), an inhibitor of nitric oxide synthase, had no significant effect on dilations at lower Khyper concentrations but constricted the arteries relative to the control at 51, 66, and 81 mM Khyper. In the presence of l-NAME, the addition of an exogenous NO donor, S-nitroso-N-acetylpenicillamine (SNAP, 10−8 M) or an analog of cGMP, 8-bromo-cGMP (6×10−5 M), tended to restore the response of Khyperto near the original response. We conclude that the basal release of NO from the endothelium plays a permissive role in the Khyper-induced response.