A Mechanistic Study of the Postjunctional α-Adrenoceptors of the Femoral Vein of the Rat

Abstract

Contractile responses of the femoral vein of the rat to a-adrenoceptor agonists were investigated. The non-selective α-adrenoceptor agonist noradrenaline, the α1-selective agonists cirazoline and phenylephrine and the α2-selective agonists BHT 920 and UK 14,304 all elicited contractile responses. Maximum contractions to cirazoline, phenylephrine, BHT 920 and UK 14,304 were of similar size and were smaller than contractions to noradrenaline. Contractions to noradrenaline were inhibited by either the α1-antagonist corynanthine or the α2-antagonist idazoxan; those to cirazoline were antagonised by corynanthine but not idazoxan and those to BHT 920 by idazoxan but not corynanthine. These results indicate that the femoral vein of the rat possesses both postjunctional α1- and α2-adrenoceptors. In calcium-free EGTA PSS, noradrenaline or cirazoline elicited transient contractions suggesting release of intracellular calcium; a calcium release component could not be demonstrated to BHT 920. Contractions to noradrenaline in calcium-free EGTA PSS were significantly greater in the presence of idazoxan than those in the presence of corynanthine. These results suggest that stimulation of «,-adrenoceptors causes release of calcium from intracellular stores while stimulation of postjunctional α2- adrenoceptors can not. Study of receptor mediated changes in second messenger production revealed that stimulation of α1-adrenoceptors by cirazoline or by noradrenaline in the presence of idazoxan produced a significant increase in polyphosphoinositide hydrolysis while stimulation of α2-adrenoceptors by BHT 920 or noradrenaline in the presence of corynanthine did not. Conversely, stimulation of α2-adrenoceptors had no effect upon tissue cAMP levels while stimulation of α2-adrenoceptors caused a significant decrease in the intracellular cAMP content. These results suggest that postjunctional α1- and α2-adrenoceptors of the femoral vein of the rat differ in their calcium mobilisation processes and post receptor coupling mechanisms.