ADRENALINE ACTIVATION OF PREJUNCTIONAL β‐ADRENOCEPTORS IN GUINEA‐PIG ATRIA

Abstract

1 Adrenaline in a concentration of 1.0 μm 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 μm) and thus appears to be mediated by β-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 β-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 α-adrenoceptor blocking activity of oxprenolol.