Experimental conditions required for the enhancement by alpha-adrenoceptor antagonists of noradrenaline release in the rabbit ear artery

Abstract

1 Segments of the rabbit ear artery were preincubated with (-)-[3H]-noradrenaline and then perfused/superfused and stimulated by transmural electrical pulses. The outflow of [3H]-noradrenaline, separated from its metabolites by column chromatography, was determined. 2 Tetrodotoxin abolished, cocaine increased, and clonidine reduced the overflow of [3H]-noradrenaline elicited by 10 shocks at 0.2 Hz, 10 shocks at 2 Hz or 100 shocks at 2 Hz. 3 The effects of yohimbine (0.1 or 1 microM), phentolamine (1 microM) and piperoxan (1 or 10 microM) depended on the stimulation conditions. No antagonist increased the overflow of [3H]-noradrenaline evoked by 10 pulses at 0.2 Hz, but all markedly increased the overflow evoked by 100 pulses at 2 Hz. Only piperoxan (10 microM) slightly enhanced the overflow at 10 pulses, 2 Hz. The effects of yohimbine and piperoxan were similar in arteries not exposed to cocaine and in those that were perfused/superfused with medium containing cocaine (10 microM) after preincubation. 4 It is concluded that yohimbine, phentolamine and piperoxan increase the release of noradrenaline only when the concentration of noradrenaline in the biophase of the ear artery is sufficiently high. The effect is, hence, an anti-noradrenaline effect and due to the blockade of presynaptic alpha-adrenoceptors. A second prerequisite for the release-enhancing effect appears to be a sufficient length of the pulse train, indicating that the alpha-adrenergic auto-inhibition develops relatively slowly.