The sympathetic transmitter stores of guinea‐pig isolated atria were labelled with [3H]‐noradrenaline. The effects of histamine (0.3 to 100 μmol/l) on resting and stimulation‐induced (S‐I, 2 Hz for 10 s) release of radioactivity were investigated.
Histamine, in low concentrations (0.3 and 1 μmol/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 μmol/l) and also by the H1‐receptor antagonist, mepyramine (1μmol/l).
The inhibitory actions of histamine on S‐I release were not due to indirect effects involving α‐adrenoceptors, β‐adrenoceptors, muscarinic cholinoceptors or prostaglandin synthesis.
Histamine in a high concentration (100 μmol/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 μmol/l) but the increase in S‐I release was only partially blocked by cocaine.