Inositol phosphates (IP) production and contraction in isolated but otherwise intact rat tail artery were measured in response to stimulation by vasopressin agonists. We have previously studied similar α-adrenoceptor responses. Identical rank orders of vasopressin agonists' potency were found for IP accumulation and contraction. The vasopressin analogue [1-(β-mercapto-β,β-cyclopentamethylene proprionic acid)-2-(O-methyl)tyrosine,D-arginine8] vasopressin, was shown to be a specific, reversible antagonist for both IP accumulation and contraction by all vasopressin agonists tested. No antagonism of vasopressin induced increases in IP accumulation or contraction were found using phenoxybenzamine. Therefore, (i) in this tissue vasopressin receptors mediate both contraction and IP accumulation; and (ii) vasopressin mediated responses appear to be direct effects not mediated via the activation of α-adrenoceptors. Demonstration of two entirely different receptors, mediating the same functional response, and which both promote IP production, is consistent with a general obligatory role for phosphoinositide catabolism in receptor mediated vascular smooth muscle contraction.
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