Beta2-adrenoceptor-mediated prejunctional facilitation and postjunctional inhibition of sympathetic neuroeffector transmission in the guinea pig vas deferens

Abstract

This study examines the role of prejunctional and postjunctional beta-adrenoceptors in the modulation of sympathetic cotransmission in the guinea pig vas deferens. The prejunctional involvement of beta-adrenoceptors was evaluated by testing the effects of several agonists and antagonists on the nerve stimulation-evoked overflow of ATP and norepinephrine (NE) from the "in vitro" vas deferens. The nonsubtype-selective beta-adrenoceptor agonist isoproterenol and the beta2-subtype-selective agonist clenbuterol increased, to a similar degree, the overflow of ATP and NE, while the beta1-subtype-selective agonist xamoterol and the beta3-subtype-selective agonist BRL 37 344 had no effect. Pretreatment with ICI 118, 551, a beta2-subtype-selective antagonist, abolished the facilitation of cotransmitter release by isoproterenol and clenbuterol, while the beta1-subtype-selective antagonist atenolol had no effect. Activation of beta-adrenoceptors by either isoproterenol or clenbuterol, but not by xamoterol and BRL 37 344, reduced the amplitude of contractions evoked by exogenously applied ATP. Pretreatment with propranolol or ICI 118, 551, but not atenolol, prevented these inhibitory effects. Isoproterenol in lower concentrations produced dose-dependent reduction of the purinergic but not the adrenergic phase of nerve stimulation-induced contraction of the guinea pig vas deferens. When applied in concentrations greater than 1 microM, isoproterenol, but not clenbuterol, actually produced a concentration-dependent facilitation of contractions evoked by both nerve stimulation and exogenously applied ATP. Antagonists of alpha-adrenoceptors blocked these facilitatory effects. Together, these results demonstrate that beta2-adrenoceptors can influence sympathetic neuroeffector transmission both prejunctionally, where they facilitate equally well the release of sympathetic cotransmitters and postjunctionally, where they inhibit smooth muscle contractions evoked by ATP.