SINGLE PULSE STIMULATION OF GUINEA‐PIG VAS DEFERENS AND THE PRESYNAPTIC RECEPTOR HYPOTHESIS

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Abstract
1 The effect of phenoxybenzamine on the efflux of [3H]-noradrenaline and the mechanical response to single pulse excitation of superfused guinea-pig vas deferens was determined to examine the validity of the currently accepted hypothesis of a presynaptic negative feedback system on adrenergic nerve terminals. 2 The adrenoceptor antagonist enhanced both the outflow of tritium and the mechanical response to single pulse stimulation. The efflux of labelled material and the responses to 4 pulses were also enhanced, as expected. 3 Blockade of neuronal and extraneuronal uptake did not by itself increase nerve-induced outflow or the mechanical response nor did it prevent phenoxybenzamine from doing so. 4 The present observations cannot be accommodated within the framework of a hypothesis that proposes that the enhancement of response and tritium efflux by phenoxybenzamine results from blockade of a feedback system whereby noradrenaline released by previous impulses inhibits its own subsequent release.