A comparison of the blocking effect of alpha-adrenoceptor antagonist drugs towards directly and indirectly acting agonists on the rat vas deferens.

The antagonistic effect of dihydroergocristine and phentolamine towards the directly acting amine noradrenaline and the indirectly acting amine tyramine on the rat vas deferens was evaluated. It has been demonstrated that both alpha-adrenoceptor blocking drugs reduced the effect of the indirectly acting amine at concentrations lower than those needed to antagonize the action of noradrenaline. When concentrations of the alpha-adrenoceptor blocking drugs effective against noradrenaline were employed a qualitative difference in the antagonism towards the directly and indirectly acting amines has also been shown. It appears that in the rat vas deferens the effects induced by tyramine-released noradrenaline are more easily affected by alpha-adrenoceptor antagonist drugs than those induced by exogenously applied noradrenaline.