Respiratory effects of carbon dioxide-induced changes of medullary extracellular fluid pH in cats.

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Abstract
Acute and steady-state responses to hypercapnia of respiratory output, measured as integrated phrenic nerve activity, and medullary extracellular fluid (e.c.f.) pH, measured directly, were determined in paralyzed, vagotomized and glomectomized cats. Steady-state studies in 26 cats show that the respiratory response to progressive hypercapnic stimulation of the central chemoreceptors is curvilinear; that the relationship between increasing end-tidal PCO2 and medullary [H+] or changes of pH is linear (r = 0.995); a doubling of PCO2 causes 0.260 units pH changes; there is a curvilinear relationship between e.c.f. [H+] and the respiratory response that is the same as that found with CO2. Medullary e.c.f. pH measured by means of a surface electrode accurately reflects the CO2-induced [H+] stimulus to respiration. The decreasing respiratory responses to identical changes of central chemoreceptor input are due to progressive neuronal saturation of a central pathway between the chemoreceptors and the respiratory controller.

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