Carotid Body Reflexes in the Dog

Abstract

In vagotomized dogs anesthetized with morphine and chloralose the following results have been obtained:

1. Contrary to our previous belief 1 , 2 carotid chemoreceptors of some of these animals were continuously active under normal experimental conditions because—

(a) Respiration was often momentarily depressed when both sinus nerves were inactivated by cooling or by injection of procaine. This occurred also in decerebrated animals or anesthetized dogs breathing oxygen, and when rise in blood pressure upon blocking the nerves was obviated by preliminary selective denervation of the pressure receptors.

(b) Respiration was temporarily depressed when the fluid perfusing both carotid bodies was cooled from 38° to 26° C or less (confirming Bernthal and Weeks 3 ). Similar results were obtained when the perfusing fluid was phosphate-buffered Locke's solution at pH 7.5, free of CO₂ (less than 1 mm tension) and saturated with O₂ (more than 150 mm tension)—in brief, when the carotid chemoreceptors were subjected to much smaller amounts of the materials known to stimulate them than could ever be the case under physiological conditions. Part or all of the continuous activity revealed by these experiments may therefore have been contributed by something other than ordinary chemoreceptors; temperature receptors are suggested as a possibility.

2. The sensitivity of the carotid chemoreceptors was tested by perfusion of both carotid bodies with oxygenated Locke's solution of which either pH was kept constant and CO₂ tension was varied (according to the Hasselbalch equation) or CO₂ was removed and the pH varied by means of phosphate buffers. In those animals whose carotid bodies showed unusually great reactivity, the following results were obtained:

(a) CO₂ tension: The smallest change by which breathing was reflexly affected was 5 mm Hg, which was effective in one trial out of 8 on 5 animals.