RAT VENA-CAVA ALPHA(1B)-ADRENOCEPTORS - CHARACTERIZATION BY [H-3] PRAZOSIN BINDING AND CONTRACTION EXPERIMENTS

We examined which subtypes of alpha1-adrenoceptors are expressed in ra t vena cava by using both functional and [H-3]prazosin binding experim ents. Pretreatment with chloroethylclonidine inactivated about 80% of the speCifiC [H-3]prazosin binding sites and reduced the maximal norad renaline-induced contraction to the same extent. Competition with subt ype-selective agonists and antagonists showed primarily the alpha1B-ad renoceptor subtype in vena cava. The number of al-adrenoceptors estima ted with [H-3]prazosin binding and the maximal noradrenaline-induced c ontraction were dose-dependently inhibited by phenoxybenzamine, indica ting the absence of receptor reserve for noradrenaline in vena cava. A s the noradrenaline-induced contraction was largely inhibited in Ca2+- free solution, these results suggest that alpha1B-adrenoceptors can be mainly linked to Ca2+ influx in rat vena cava.