Clonidine and noradrenaline fail to inhibit vagal induced bradycardia - Evidence against prejunctional alpha-adrenoceptors on vagal varicosities in guinea pig right atria

Abstract

Guinea pig isolated right atria were used to test for inhibitory prejunctional α-adrenoceptors on terminal cardiac vagal varicosities. These receptors have been considered to be involved in reciprocal inhibition of transmitter release. Electrical field pulses delivered during the atrial refractory period caused biphasic changes in atrial period. An immediate bradycardia was followed by a more gradual tachycardia. An estimation of both parasympathetic and sympathetic transmitter release following a single stimulation was made possible by the tempral separation of the peak brahycardia and tachycardia. In one set of atria two stimulus-response curves were constructed with a 30 min interval. The..