Lack of modulation by presynaptic α2-adrenoceptors of adrenergic transmitter release evoked by activation of 5-hydroxytryptamine and nicotine receptors

The determination of presynaptic pA2 values of yohimbine and phentolamine on the perfused rat heart under conditions of negligible autoinhibition

“…Thus in the isolated heart of the rabbit, phentolamine enhanced the noradrenaline overflow evoked by stimulation of the sympathetic nerves with 300 pulses at 5 Hz at concentrations (1 riM) that did not block the neuronal reuptake (Starke, Montel & Wagner, 1971). The frequencyresponse of the rabbit isolated perfused heart to nerve stimulation with 15 pulses at 3.2 Hz was potentiated by yohimbine, but not that with about 5 pulses at 0.32Hz (Carr & Fozard, 1981). However, the modulation of the frequency-response reflects, at best, the degree of autoinhibition occurring on nerve fibres innervating the sinus node.…”