Neuronal NO Mediates Cerebral Vasodilator Responses to K+ in Hypertensive Rats

Abstract

Potassium ion (K+) normally causes cerebral vasodilatation by activating inwardly rectifying K+ (KIR) channels. We tested whether chronic hypertension affects the magnitude and/or mechanism of K+-induced cerebral vasodilatation in vivo. Basilar artery responses were examined in anesthetized Wistar-Kyoto (WKY; mean arterial pressure, 114±4 mm Hg) and spontaneously hypertensive (SHR; 176±3 mm Hg) rats. In WKY, elevating cerebrospinal fluid K+ concentration from 3 mmol/L to 5 and 10 mmol/L caused vasodilatation (percent maximum, 12±1 and 48±7, respectively). The response to 5 mmol/L K+ was greater in SHR (percent maximum, 17±2 [P <0.05 versus WKY] and 49±4). The KIR channel inhibitor, barium ion (Ba2+, 100 &mgr;mol/L) selectively inhibited dilator responses to 5 and 10 mmol/L K+ by ≈75% in WKY. In SHR, Ba2+ had no effect on the response to 5 mmol/L K+, and only partially inhibited (by ≈40%) the response to 10 mmol/L K+. The nonselective NO synthase (NOS) inhibitor N&ohgr;-nitro-l-arginine methyl ester, the neuronal NOS (nNOS) inhibitor 1-(2-trifluromethyl-phenyl)imidazole, and the N-type calcium channel inhibitor &ohgr;-conotoxin GVIA, were all without effect in WKY, but markedly inhibited the response to 5 mmol/L K+ in SHR. When applied together with Ba2+, each of these inhibitors also profoundly reduced responses to 10 mmol/L K+ in SHR. Immunostaining of basilar arteries revealed that the perivascular nNOS-containing nerve plexus was denser in SHR. Thus, K+ dilates the normotensive basilar artery predominantly via KIR channel activation. During chronic hypertension, small physiological elevations in K+ dilate the basilar artery by an nNOS-dependent mechanism that appears to be upregulated in a compensatory manner.