Inhibition of a somato-sympathetic reflex via peripheral presynaptic α-adrenoceptors

Abstract

In cats anaesthetized with chloralose—urethane (70/100 mg/kg) a somato-sympathetic reflex increase in blood pressure and heart rate was evoked by electrical stimulation of sensory fibres running within the tibial nerve. I.v. injection of 30 μg/kg/ of the α-adrenoceptor agonist clonidine cauased a reflex inhibition in intact and vagotomized animals. In contrast, injection of 2 μg/kg clonidien into the vertebral artery elicited hypotension but not reflex inhibition. In small doses, phenylephrine which is know from in vitro experiments to prefer the postsynaptic α-adrenoceptor, potentiated the reflex increase in blood pressure by an additive effect on the vascular α-adrenoceptor, while oxymetazoline and tramazoline, which preferentially activated 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 α-adrenolytic drug yohimbine. The results suggests that peripheral presynaptic α-adrenoceptors are responsible for somato-sympathetic reflex inhibition. The importance of peripheral presynaptic α-adrenoceptors in the in vivo regulation of blood pressure is discussed.