AFFERENT DISCHARGES FROM CORONARY ARTERIAL AND VENTRICULAR RECEPTORS IN ANESTHETIZED DOGS

1. Previous work has shown that increases in aortic root pressure resu lt in reflex vasodilatation, and that this response is likely to resul t mainly from stimulation of receptors in the coronary arteries, altho ugh contribution from left ventricular receptors was not excluded. Thi s investigation was undertaken to resolve this question and to determi ne the afferent nerve fibres likely to be involved in this reflex. 2. In chloralose-anaesthetized dogs a perfusion circuit was used which al lowed us to change the pressures in: (a) the aortic root, coronary art eries and the left ventricle; (b) aortic root and coronary arteries at constant ventricular pressure; and (c) the left ventricle with mean ( although not pulse) aortic pressure constant. Electrophysiological rec ordings were made from slips dissected from the vagus nerve which resp onded with an increase in discharge to either combined increases in th e pressures, or to aortic root injections of veratridine. 3. Recording s were made from twenty-one vagal afferents. On the basis of their con duction velocities, eleven were classified as non-myelinated and ten a s myelinated. 4. Three non-myelinated afferents responded to veratridi ne injections only, three to both veratridine and combined aortic root and ventricular pressure changes, and five to pressure changes only. Responses to pressure occurred only when ventricular systolic pressure exceeded 30 kPa. 5. None of the myelinated afferents responded to ver atridine. All showed increases in discharge to combined increases in m ean aortic root, coronary arterial and left ventricular systolic press ures, which would be graded over a range similar to that which caused reflex changes. All were more sensitive to changes in mean coronary pr essure than to changes in ventricular systolic pressure. 6. We conclud e that myelinated vagal afferent nerve fibres, which respond predomina ntly to changes in mean coronary arterial pressure, are likely to be r esponsible for the vasodilatation to the changes in mean aortic root p ressure previously reported. These fibres are probably attached to cor onary arterial mechanoreceptors.