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Beta-adrenoceptors in femoral and mesenteric arteries from 13-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were studied using radioligand binding assays and were compared with in vitro measurements of β-adrenoceptor-mediated relaxation. The relaxant responses to noradrenaline via β -adrenoceptors were significantly decreased in the SHR femoral artery when compared with the WKY femoral artery. However, under the same conditions, arterial relaxant responses to forskolin, an activator of adenylate cyclase, were not significantly different between SHR and WKY rats. Specific binding of 125Hodocyanopindolol to membranes prepared from femoral arteries of SHR and WKY rats was saturable and of high affinity. Neither the equilibrium dissociation constant of 125Hodocyanopindolol, nor the maximum number of binding sites were significantly different between SHR and WKY rats. Similar results were obtained in the case of mesenteric arteries from SHR and WKY rats. These results indicate that the decreased responsiveness to β -adrenoceptor stimulation in SHR arteries is not associated with alterations in β -adrenoceptors and further support the hypothesis that a reduced function of the stimulatory guanosine triphosphate-binding protein is responsible for the decreased responsiveness to a variety of receptor agonists whose mechanism of action involves adenylate cyclase activation.