Adrenaline activation of prejunctional beta-adrenoceptors in guinea-pig atria

1. Adrenaline in a concentration of 1.0 microM depressed the stimulation-induced efflux of tritium from the guinea-pig atria incubated with [3H]-noradrenaline, whereas adrenaline in a concentration of 0.5 nM significantly enhanced the stimulation-induced efflux of tritium. This enhancement was blocked by metoprolol (0.1 microM) and thus appears to be mediated by beta-adrenoceptors. 2. In guinea-pig atria incubated with unlabelled adrenaline and then with [3H]-noradrenaline, both catecholamines were released by field stimulation. In such atria metoprolol, practolol, oxprenolol or propranolol decreased the stimulation-induced efflux of tritium. These effects did not occur if the atria were incubated with unlabelled noradrenaline and then with [3H]-noradrenaline, suggesting that neuronally released adrenaline activates prejunctional beta-adrenoceptors. 3. The effect of oxprenolol in decreasing the release of tritium from guinea-pig atria, incubated with unlabelled adrenaline and then with [3H]-noradrenaline was greater in the presence of phentolamine. This may reflect the alpha-adrenoceptor blocking activity of oxprenolol.