Abstract
Recent reports indicate that the presynaptic negative feedback mechanism involved in regulating sympathetic neurotransmission is only functional within a narrow range of neuronal activity. The specific aim of this report is to examine the release of sympathetic transmitter at varying frequencies of stimulation, and the effects of α-adrenoceptor antagonists on the overflow. The experiments were carried out in the3H-noradrenaline-labeled heart of the guinea pig. The overflow of tritium (per pulse) increased from a stimulation frequency of 0.125 to 10 Hz and declined at 30 Hz. Phentolamine or yohimbine produced facilitation of the overflow from 0.125 to 10 Hz; the effect was more pronounced at lower frequencies of stimulation. We conclude that the presynaptic α-adrenoceptor-mediated negative feedback mechanism operates over a wide range of cardiac sympathetic nerve activity.
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