We tested whether activation of inwardly rectifying K(+) (Kir) channels, Na(+)-K(+)-ATPase, or nitric oxide synthase (NOS) play a role in K(+)-induced dilatation of the rat basilar artery in vivo. When cerebrospinal fluid [K(+)] was elevated from 3 to 5, 10, 15, 20, and 30 mM, a reproducible concentration-dependent vasodilator response was elicited (change in diameter = 9 +/- 1, 27 +/- 4, 35 +/- 4, 43 +/- 12, and 47 +/- 16%, respectively). Responses to K(+) were inhibited by approximately 50% by the Kir channel inhibitor BaCl(2) (30 and 100 microM). In contrast, neither ouabain (1-100 microM, a Na(+)-K(+)-ATPase inhibitor) nor N(G)-nitro-L-arginine (30 microM, a NOS inhibitor) had any effect on K(+)-induced vasodilatation. These concentrations of K(+) also hyperpolarized smooth muscle in isolated segments of basilar artery, and these hyperpolarizations were virtually abolished by 30 microM BaCl(2). RT-PCR experiments confirmed the presence of mRNA for Kir2.1 in the basilar artery. Thus K(+)-induced dilatation of the basilar artery in vivo appears to partly involve hyperpolarization mediated by Kir channel activity and possibly another mechanism that does not involve hyperpolarization, activation of Na(+)-K(+)-ATPase, or NOS.