The evaluation of the α-adrenoceptor blocking action of indoramin, phentolamine and thymoxamine on the rat and guinea-pig isolated mesenteric vasculature and aortic spiral preparations

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Abstract
The perfused mesenteric vasculature of the rat is a specific, sensitive and stable preparation for the rapid determination of α-adrenoceptor blocking pA2 values. Both types of pA2 determination—the direct determination (Schild, 1947) and the Arunlakshana & Schild (1959) analysis—can be made on this tissue. Indoramin, phentolamine and thymoxamine were evaluated and gave respectively α-block pA2 values of 8ṁ05, 7ṁ84 and 6ṁ47. The pA2 values made on the rat perfused mesentery were compared with those obtained on the rat aorta (indoramin 7ṁ68, phentolamine 8ṁ29, thymoxamine 6ṁ50), the guinea-pig aorta (indoramin 7ṁ38, phentolamine 7ṁ64, thymoxamine 6ṁ93) and the perfused mesenteric bed of the guinea-pig (indoramin 8ṁ48, phentolamine 7ṁ51, thymoxamine 6ṁ97). Statistical analyses of Kb (dissociation constant) values suggest that indoramin has an additional blocking action on resistance vessels, which is not possessed by phentolamine or thymoxamine.