Inhibition of somato-sympathetic reflex via peripheral presynaptic alha-adrenoceptors.

In cats anaesthetized with chloralose--urethane (70/100 mg/kg) a somato-sympathetic reflex increase of blood pressure and heart rate was evoked by electrical stimulation of sensory fibres running within the tibial nerve. I.V. injection of 30 microgram/kg of the alpha-adrenoceptor agonist clonidine caused a reflex inhibition in intact and vagotomized animals. In contrast, injection of 2 microgram/kg clonidine into the vertebral artery elicited hypotension but no reflux inhibition. In small doses, phenylephrine which is known from in vitro experiments to prefer the postsynaptic alpha-adrenoceptor, potentiated the reflex increase in blood pressure by an additive effect on the vascular alpha-adrenoceptor, while oxymetazoline and tramazoline, which preferentially activate the presynaptic receptor, were inhibitory. The inhibitory effect of tramazoline on the reflex increase in blood pressure and heart rate was antagonized by the presynaptic alpha-adrenolytic drug yohimbine. The results suggest that peripheral presynaptic alpha-adrenoceptors are responsible for somato-sympathetic reflex inhibition. The importance of peripheral presynaptic alpha-adrenoceptors in the in vivo regulation of blood pressure is discussed.