Effect of Labetalol on the Uptake of [3h]‐(‐)‐noradrenaline Into the Isolated Vas Deferens of the Rat

1 The effects of the combined alpha- and beta-adrenoceptor blocking drug, labetalol, on the uptake of [3H]-(-)-noradrenaline into the isolated vas deferens of the rat have been determined and compared with those of some other alpha-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 microgram/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.