Localization of Baroreceptors and Gain of the Baroreceptor-Heart Rate Reflex in the Lizard Trachydosaurus Rugosus

1. The location of arterial baroreceptors was studied in unanaesthetized Trachydosaurus rugosus by inflation of strategically placed perivascular cuffs. There was no evidence for baroreceptors at the site homologous with the carotid sinus of mammals or at the site homologous with the aortic arch baroreceptors of mammals and birds. The baroreceptors of T. rugosus probably lie in the truncus arteriosus. 2. When the sensitivity or gain of the cardiac response to changes in systemic arterial pressure was expressed as change in heart period per unit change in mean aortic blood pressure, the value for 6 animals was 1370 ms/kPa (100 ms/mmHg). This is very much greater than reported values for rabbits. If, instead, gain was calculated as the percentage increase in heart rate per unit fall in mean aortic pressure, a mean gain of 98%/kPa (7.2%/mmHg) was obtained. It is suggested that the latter method provides the better comparative value. 3. Adrenergic neurone blockade with bretylium and muscarinic blockade with atropine or hyoscine indicated that sympathetic fibres probably play the dominant role in generating the gain of the cardiac baroreflex.