CHARACTERIZATION OF THE ADRENORECEPTOR ACTIVITIES OF ISOPRENALINE IN THE FIELD STIMULATED RAT VAS DEFERENS: SELECTIVE SUPERSENSITIVITY TO β2‐MEDIATED RESPONSES FOLLOWING RESERPINE TREATMENT

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Abstract
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 β2‐adreno‐receptor antagonist timolol, but not affected by the prejunctional α2 or postjunctional α1‐adrenoreceptor antagonists rauwolscine and prazosin, respectively.2 In vas deferens of rats previously treated with reserpine, the postjunctional β2‐adrenoreceptor‐mediated inhibitory response to isoprenaline was markedly potentiated.3 High concentrations of isoprenaline (EC50= 1.5 uM) also inhibited contractility in tissues in which postjunctional β2‐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 α2‐adreno‐receptors.4 Rauwolscine pretreatment unmasked an ability of isoprenaline (EC50= 17.1 juM) 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 α1‐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 β2>α2>α1. Pretreatment with reserpine produces rapid and selective development of supersensitivity to the postjunctional β2‐mediated inhibitory response of isoprenaline in this preparation.