The effect of cold blockade of the medullary chemoreceptors on the CO2 modulation of vascular tone and heart rate

Publisher Website
Google Scholar
Abstract
A direct relationship between neurogenic vascular tone in the cat hindlimbs and arterial CO2 tension (PaCO2) was previously reported. Since this relationship is not appreciably changed by deafferentiation of peripheral chemoreceptors, the possible role of structures on the ventral surface of the medulla in mediating the CO2 sensitivity of neurogenic tone was studied. The vagosympathetic trunks and the carotid sinus nerves were sectioned bilaterally in cats under sodium pentobarbital anesthesia. The ventral surface of the medulla was perfused with artificial CSF of physiological PCO2, pH and temperature. The animals were hyperventilated with different mixtures of CO2 (0, 4 and 10% in O2), so as to change PaCO2 from 12.6 .+-. 1.2 to 57.4 .+-. 1.8 mmHg (mean .+-. SE). The perfusion pressure of the hindlimbs, vascularly isolated and perfused at constant flow, was positively correlated with PaCO2. Bilateral chemosensitive areas of the ventral surface of the medulla oblongata (7 mm caudal to the foramen cecum of the frontal bone and 3 mm lateral to the midline) were cooled to 12.degree. C by small thermodes. This maneuver decreased heart rate, perfusion pressure at each level of PaCO2 and the sensitivity of the perfusion pressure to changes in PaCO2. Cold blockade (12.degree. C) of the rest of the ventral medullary surface induced a further decrease in heart rate and perfusion pressure but not in the slope of the PaCO2-perfusion pressure relation. Similar results were obtained in a group of adrenalectomized animals. The medullary areas which mediate the central CO2-sensitivity of respiratory control systems apparently also play a role in regulation of the cardiovascular system.