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.

Date of Award	1989
Original language	English