Postjunctional alpha 2-adrenoceptors mediate venoconstriction in the hindquarters circulation of anaesthetized cats.

1 A study was made of the subtypes of postjunctional alpha-adrenoceptors which mediate arterial and venous constriction in the hindquarters circulation of anaesthetized cats, as measured by changes in perfusion pressure and vena cava blood flow, respectively. 2 It was found that, while noradrenaline caused constriction in both the arterial and venous compartments, methoxamine caused only arterial constriction. Clonidine and B-HT 920 also caused arterial and venous constriction although autodesensitization to both drugs occurred. 3 The ability of either prazosin or yohimbine to antagonize the constrictor effects of noradrenaline was also examined. It was found that the combination of both antagonist drugs abolished both the arterial and venous constrictor effects of noradrenaline. However, there was a greater prazosin-resistant response to noradrenaline in the venous compartment as compared with the arterial effects of noradrenaline. Yohimbine caused approximately equal reductions in the effect of noradrenaline in both arteries and veins, which was greater than that observed with prazosin. 4 These results suggest that, in the cat hindquarters, both alpha 1- and alpha 2-adrenoceptors are present in the arterial circulation, whereas there are mainly alpha 2-adrenoceptors in the venous circulation.