EFFECT OF LABETALOL ON THE UPTAKE OF [3H]‐(‐)‐NORADRENALINE INTO THE ISOLATED VAS DEFERENS OF THE RAT

Abstract

1 The effects of the combined a- and, B-adrenoceptor blocking drug, labetalol, on the uptake of [3H]4-)-noradrenaline into the isolated vas deferens of the rat have been determined and compared with those of some other ca-adrenoceptor blocking drugs and cocaine. 2 Labetalol, like cocaine, produced a simple competitive inhibition of [3H]-(-)-noradrenaline uptake and was about 4 times less potent than cocaine. It is concluded that labetalol is a potent inhibitor of uptake1. Phentolamine and thymoxamine also inhibited [3H]-(-)-noradrenaline uptake, and were respectively 8 and 14 times less potent than cocaine. Tolazoline, piperoxan and yohimbine were inactive in concentrations up to 30 ig/ml. 3 The uptake1 blocking action of labetalol could explain, at least in part, the previously reported difference in its ability to block noradrenaline and phenylephrine vasopressor responses in the anaesthetized dog. 4 The possibility that uptake1 inhibitory concentrations of labetalol could be present in the blood of subjects receiving normal antihypertensive doses of the drug is discussed.