Adaptation of carotid chemoreceptors to step increases in PaCO2 in anesthetized cats.

1. The rate of change sensitivity of some carotid chemoreceptors to within-breath changes in PaCO2 would suggest that the half-life of adaptation of these receptors to a step increase is shorter than the 5-10 s previously reported. 2. In six anesthetized cats, step increases in PaCO2 (10 in each cat) were produced by injection of CO2 into the inspired gas during high-frequency jet ventilation. Chemoreceptor discharge was recorded from single-fiber preparations of the divided carotid sinus nerve, and the changes in PaCO2 were followed with the use of an in 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 oscillations in PaCO2. 4. In a further group of five cats, chemoreceptor responses 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 these conditions the hyperkalemia caused no further increase in discharge. 5. We conclude that adaptation of the chemoreceptor response to increases in PaCO2 is much faster than previously reported and that this finding is consistent with observations of chemoreceptor responses to respiratory PaCO2 oscillations.