Circulatory Responses to Graded Stimulation of the Carotid Chemoreceptors in the Dog

The vascular effects of graded stimulation of the carotid chemoreceptors were studied in 13 anesthetized and artificially ventilated dogs, and stimulus-response curves were defined. The carotid bifurcations were isolated and perfused at constant pressure, and the vagi were cut. Autologous blood collected in a reservoir was equilibrated at Po2 varying from 104 to 34 mm Hg, Pco2 from 39 to 81 mm Hg, and pH from 7.46 to 6.87 and was used to stimulate the carotid chemoreceptors. Systemic arterial Po2 , Pco2 , and pH were kept normal. Reflex increases in aortic blood pressure and in hind-limb perfusion pressure (constant-flow perfusion) were first seen with Po2 of 70 mm Hg or Pco2 > 39 mm Hg. The vascular responses increased linearly with lower Po2 or higher Pco2 , until tensions of 34 and 71 mm Hg, respectively, were reached. These stimuli caused a reflex dilatation of the perfused saphenous vein that was also proportional to the degree of stimulation. Increasing the pH of the blood perfusing the chemoreceptors decreased these changes by more than 50%; decreases in pH, with normal Po2 and Pco2 , caused marked vascular responses, indicating an important role of pH in the activation of the chemoreflex mechanism.