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

Abstract

1 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). 2 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. 3 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. 4 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.