ADAPTATION OF CAROTID CHEMORECEPTORS TO STEP INCREASES IN PACO2 IN ANESTHETIZED CATS

1. The rate of change sensitivity of some carotid chemoreceptors to wi thin-breath changes in PaCO2 would suggest that the half-life of adapt ation of these receptors to a step increase is shorter than the 5-10 s previously reported. 2. In six anesthetized cats, step increases in P aCO2 (10 in each cat) were produced by injection of CO2 into the inspi red gas during high-frequency jet ventilation. Chemoreceptor discharge was recorded from single-fiber preparations of the divided carotid si nus nerve, and the changes in PaCO2 were followed with the use of an i n vivo pH electrode. 3. The adaptation half-lives were 0.3, 0.8, 1.2, 1.3, 1.6, and 8.6 s. The physiological significance of these findings in terms of respiratory control and the mechanism of chemotransduction are discussed. The receptors with the shortest and longest half-lives showed corresponding differences in response to sine-wave oscillation s in PaCO2. 4. In a further group of five cats, chemoreceptor response s to step increases in PaCO2 were tested before and during infusion of KCl to produce a mean arterial [K+] of 6.8 +/- 0.2 (SE) mM. Under the se conditions the hyperkalemia caused no further increase in discharge . 5. We conclude that adaptation of the chemoreceptor response to incr eases in PaCO2 is much faster than previously reported and that this f inding is consistent with observations of chemoreceptor responses to r espiratory PaCO2 oscillations.