The Role of Arterial Baroreceptors in the Undivided Circulation of Anuran Amphibians

Abstract

Baroreceptors with afferent fibers in branches of the glossopharyngeal nerve or the vagus have been identified in all three arterial arches in anuran amphibians. Baroreceptors in the pulmocutaneous arch (PCA) appear to provide the principal feedback of arterial blood pressure to the cardiovascular centers of the medulla. An additional contribution arises from receptors within the aortic arch. The receptors are slowly adapting mechanoreceptors and, at least in the case of the PCA baroreceptors, transmit baroreceptive information via nonmyelinated afferents. Pressure thresholds coincide with the range of systolic arterial pressures associated with undisturbed toads. Therefore, increases in systolic pressure above resting result in both increases in firing frequency and the recruitment of previously inactive receptors. Denervation experiments demonstrate that feedback from PCA baroreceptors tonically inhibits the cardiovascular system even at rest. Experimental elevation of PCA pressure above resting values causes bradycardia and reduces systemic vascular resistance, resulting in a compensatory reduction in systemic arterialpressure. In addition, PCA resistance increases, which probably reflects a constriction of the extrinsic pulmonary artery. This response opposes the negative feedback regulation of arterial pressure but may protect the pulmonary microvasculature by increasing upstream resistance, thereby reducing intravascular pressures in pulmonary vessels. This is consistent with the view that the primary role of anuran PCA baroreceptors is to protect the delicate vasculature of the gas-exchange organ against high central driving pressures transmitted by the incompletely divided anuran circulation. Future studies should be directed toward an investigation of the central pathways involved in the PCA baroreflex.