Potentiation of Responses to Noradrenaline and Reversal of Sympathetic Nerve Blockade in the Guinea‐pig Vas Deferens

The block of sympathetic nerves caused by guanethidine in various isolated organs may be reversed by a number of substances (Day, 1962; Bentley, 1962; Day & Rand, 1963; Bentley & Sabine, 1963). These antagonizing agents include sympathomimetic and parasympathomimetic drugs, histamine and 5-hydroxytryptamine. The mode of action of amphetamine has been investigated by Day & Rand (1963) who showed that this drug competes with guanethidine for absorption sites on sympathetic nerves. They found, however, that noradrenaline does not show this competitive antagonism. Matsumoto & Horita (1962) have suggested that the ability of various sympathomimetic amines to antagonize the effects of bretylium on the nictitating membrane involved potentiation of the diminished amounts of transmitter released from the sympathetic nerve. This hypo- thesis was investigated in more detail in the present study using guinea-pig isolated vas deferens preparations. The ability of various substances to potentiate the effects of nor- adrenaline was compared with their action in reversing the sympathetic block caused by guanethidine, low calcium, procaine and previous treatment with reserpine.