Characterization of the adrenoreceptor activities of isoprenaline in the field stimulated rat vas deferens: selective supersensitivity to beta 2-mediated responses following reserpine treatment.

1 Low concentrations of isoprenaline (EC50 = 45.6 nM) inhibited contractions in the isolated field stimulated rat vas deferens. This inhibitory effect was markedly attenuated by the postjunctional beta 2-adrenoreceptor antagonist timolol, but not affected by the prejunctional alpha 2 or postjunctional alpha 1-adrenoreceptor antagonists rauwolscine and prazosin, respectively. 2 In vas deferens of rats previously treated with reserpine, the postjunctional beta 2-adrenoreceptor-mediated inhibitory response to isoprenaline was markedly potentiated. 3 High concentrations of isoprenaline (EC50 = 1.5 microM) also inhibited contractility in tissues in which postjunctional beta 2-adrenoreceptors were maximally blocked by high concentrations of timolol. This contractile inhibition produced by isoprenaline was abolished by rauwolscine but not significantly altered by prazosin or pretreatment of the rats with reserpine indicating stimulation of prejunctional alpha 2-adrenoreceptors. 4 Rauwolscine pretreatment unmasked an ability of isoprenaline (EC50 = 17.1 microM) to produce enhancement of field stimulation-induced contractions. This response was abolished by prazosin but was unaffected by timolol or reserpinization indicating an action upon postjunctional alpha 1-adrenoreceptors. 5 The data indicate isoprenaline activates adrenoreceptor mechanisms in the field stimulated rat vas deferens by a direct action not dependent upon endogenous catecholamines and with an order of activity of beta 2 much greater than alpha 2 greater than alpha 1. Pretreatment with reserpine produces rapid and selective development of supersensitivity to the postjunctional beta 2-mediated inhibitory response of isoprenaline in this preparation.