beta2adrenoceptormediated prejunctional facilitation and postjunctional inhibition of sympathetic neuroeffector transmission in the guinea pig vas deferens this study examines the role of prejunctional and postjunctional betaadrenoceptors in the modulation of sympathetic cotransmission in the guinea pig vas deferens the prejunctional involvement of betaadrenoceptors was evaluated by testing the effects of several agonists and antagonists on the nerve stimulationevoked overflow of atp and norepinephrine ne from the in vitro vas deferens the nonsubtypeselective betaadrenoceptor agonist isoproterenol and the beta2subtypeselective agonist clenbuterol increased to a similar degree the overflow of atp and ne while the beta1subtypeselective agonist xamoterol and the beta3subtypeselective agonist brl 37 344 had no effect pretreatment with ici 118 551 a beta2subtypeselective antagonist abolished the facilitation of cotransmitter release by isoproterenol and clenbuterol while the beta1subtypeselective antagonist atenolol had no effect activation of betaadrenoceptors 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 dosedependent reduction of the purinergic but not the adrenergic phase of nerve stimulationinduced contraction of the guinea pig vas deferens when applied in concentrations greater than 1 microm isoproterenol but not clenbuterol actually produced a concentrationdependent facilitation of contractions evoked by both nerve stimulation and exogenously applied atp antagonists of alphaadrenoceptors blocked these facilitatory effects together these results demonstrate that beta2adrenoceptors 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