ACTIVATION OF PREJUNCTIONAL β‐ADRENOCEPTORS IN RAT ATRIA BY ADRENALINE APPLIED EXOGENOUSLY OR RELEASED AS A CO‐TRANSMITTER

Abstract

Adrenaline (10 nM) significantly enhanced the stimulation‐induced efflux of radioactivity from rat atria previously incubated with [3H]‐noradrenaline ([3H]‐NA). This effect was abolished by metoprolol (0.1 μm). Adrenaline in a higher concentration (1μm) and NA (1μm) significantly reduced the stimulation‐induced efflux of radioactivity. However, in the presence of phenoxybenzamine (10μm), adrenaline (1μm) enhanced the efflux, whereas NA (1μm) had no effect. In rat isolated atria pre‐incubated with adrenaline and then incubated with NA, both catecholamines were taken up and were released by field stimulation. When pre‐incubation was with adrenaline and incubation was with [3H]‐NA, metoprolol decreased the stimulation‐induced efflux of radioactivity. This effect did not occur if the atria were pre‐incubated with NA instead of adrenaline, suggesting that neuronally released adrenaline activates prejunctional β‐adrenoceptors. In conscious rats, intravenously administered adrenaline (6.0 and 0.6 nmol/kg) was taken up and retained in the atria and could be released by field stimulation. The release was calcium‐dependent from these rats up to 24 h after administration. 1981 British Pharmacological Society