Comparison of alpha₂ adrenoreceptors on arterial smooth muscle and brain homogenates from spontaneously hypertensive and wistar-kyoto normotensive rats

Alpha₂adrenoreceptors are located on vascular smooth muscle of the rat tail artery. In the present study this receptor was studied in spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Adrenergic agonists were used to produce isometric contractions of helically-cut tail artery strips from SHR and WKY. Clonidine and guanabenz, alpha2 agonists, were more potent in the SHR than in the WKY (e.g. clonidine: EC₅₀SHR = 3.5 ± 0.6 × 10^-8M; EC₅₀WKY = 17.0 ± 0.2 × 10^-8M; P < 0.0005). There was no difference in potency between the alpha! agonists, phenylephrine and methoxamine. Yohimbine, an alpha₂antagonist, was more potent in inhibiting the clonidine-induced contraction in the SHR (pA₂= 7.66 versus 7.14). To determine the number of alpha2 adrenoreceptors, the specific binding of³H-clonidine to homogenates of tail artery and of five brain areas was also measured. The maximum number of high-affinity sites on the tail artery was threefold greater in SHR than in WKY (31 ± 5 versus 11 ± 3 fmol/mg protein, P < 0.0005). No differences in the number or affinity of alpha2 receptor sites was found in the hypothalamus, hippocampus, locus coeruleus or parietal cortex of the two strains of rat. There was a difference in the amygdala (SHR: 163 ± 16 versus WKY: 108 ± 14, P < 0.05). The larger number of alpha₂adrenoreceptors on the vascular smooth muscle in SHR may provide an explanation for the supersensitivity of SHR to adrenergic agonists.