Previous works have shown that the alpha(2)-adrenoceptor agonist UK 14,304 induced the relaxation and hyperpolarization of the rat aorta, mediated by alpha(2)-adrenoceptors present in the smooth muscles, through small-conductance, ATP-sensitive K(+) channels. We now report that in spontaneously hypertensive rat (SHR) aortic rings, UK 14,304 induced concentration-dependent hyperpolarizing responses, which were inhibited by yohimbine, an alpha(2)-adrenoceptor inhibitor, and by glibenclamide, a specific inhibitor of small-conductance, ATP-sensitive K(+) channels. The responses were also partially inhibited by iberiotoxin and by apamin. Treatment with N(omega)-nitro-L-arginine (L-NNA) did not affect the response to UK 14,304. These results indicate that alpha(2)-adrenoceptors are present in SHR aortic smooth muscle cell membranes, but differ from those of normotensive animals regarding the K(+) channels involved in their responses. Moreover, the resting membrane potential (RMP) was significantly more negative in SHR than in normotensive rats. This relative hyperpolarized state is probably due to Ca(2+)-dependent K(+) channels being constitutively open in SHR, since the addition of iberiotoxin caused a significant depolarization of the aortic smooth muscle membranes in this strain.
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