Selective blocking properties of chloro acetyl catechol (CAC) on the alpha-adrenergic receptor

In anesthetized rats and cats chloro acetyl catechol (CAC, 10 mg/kg i.v. and 10-15 mg/kg i.v., respectively) selectively blocked the arterial hypertensive responses to epinephrine. No adrenergic blockade was produced by CAC in rats pretreated with 20 mg/kg i.p. of the compound 24 hr before the experiment. Norepinephrine-induced contractions of the rat vas deferens and the rat seminal vesicles in vitro were blocked by CAC 10(-4) M. The blockade was prevented by the simultaneous addition of norepinephrine or cysteine. The data are discussed in relation to : (a) the nature of the adrenergic blockade produced by CAC and (b) the possible existence of thiol groups in the vicinity of the active site of the alpha-adrenergic receptor.