Effects of Histamine on the Resting and Stimulation‐induced Release of [3h]‐noradrenaline in Guinea‐pig Isolated Atria

1 The sympathetic transmitter stores of guinea-pig isolated atria were labelled with [3H]-noradrenaline. The effects of histamine (0.3 to 100 mumol/l) on resting and stimulation-induced (S-I, 2 Hz for 10 s) release of radioactivity were investigated. 2 Histamine, in low concentrations (0.3 and 1 mumol/l) had no effect on resting release but inhibited S-I release of radioactivity. The inhibition was abolished by the H2-receptor antagonist, cimetidine (10 mumol/l) and also by the H1-receptor antagonist, mepyramine (1 mumol/l). 3 The inhibitory actions of histamine on S-I release were not due to indirect effects involving alpha-adrenoceptors, beta-adrenoceptors, muscarinic cholinoceptors or prostaglandin synthesis. 4 Histamine in a high concentration (100 mumol/l) increased the resting and S-I release of radioactivity. The increase in resting release was abolished by the neuronal uptake blocking drug cocaine (30 mumol/l) but the increase in S-I release was only partially blocked by cocaine.