Journal of Applied Physiology, Vol 49, Issue 4 708-714, Copyright © 1980 by American Physiological Society
Effect of plasma carbonic anhydrase on ventilation in exercising dogs
S. M. Lewis and E. P. Hill
Recent studies suggest pH sampled by arterial chemoreceptors may not equal
that sampled by external pH electrodes, because the uncatalyzed hydration
of CO2 in plasma is a slow reaction (t 1/2 approximately 9 S). The
importance of this reaction rate to ventilatory control (particularly
during exercise) is not known. We studied the effect of catalyzing the
CO2-pH reaction in three awake exercising dogs with chronic tracheostomies
and carotid loops; the dogs were trained to run on a treadmill. Respiration
frequency, tidal volume, total ventilation, and end-tidal partial pressure
of CO2 (PCO2) were continuously monitored. Periodically, carotid artery
blood was drawn and analyzed for partial pressure of O2 (PO2), PCO2, pH,
and plasma carbonic anhydrase (CA) activity. Measurements were made during
steady-state exercise (3 mph and 10% grade), during a control period, after
injection of a 5 ml bolus of saline, and after injection of 5 mg/kg of
bovine CA dissolved in 5 ml of saline. This dose of CA increased the
reaction rate by more than 80-fold. Neither the control nor the CA
injections significantly altered the ventilatory parameters. Saline and CA
date differed by less than 5% in ventilation, 1 Torr in arterial PCO2, 0.01
in pH units, and 1.5 Torr in end-tidal PCO2. Thus the of CO2 hydration in
plasma is not a significant factor in ventilatory control.
