The histamine-like effects of tolazoline and clonidine: Evidence against direct activity at histamine receptors

Abstract

Both tolazoline and clonidine produce agonist effects sensitive to histamine H2-receptor antagonism in whole animals and a variety of isolated tissues. In guinea-pig right atria, tolazoline concentration-response curves were shifted to the right and progressively depressed by cimetidine. Clonidine produced a submaximal histamine-like tachycardia which was converted by cimetidine to a depression of sinus rate. Neither tolazoline nor clonidine stimulated histamine H2-receptors in rabbit right atria. Instead, these agents were very weak inhibitors of histamine with a pA2 for tolazoline of 3.3 and for clonidine of 3.7. No appreciable histamine H1-receptor activity was evident for either drug in ..