Effect of exogenous dopamine on the hypercapnic ventilatory response in cats during normoxia

The effects of exogenous dopamine on the normoxic hypercapnic ventilatory response were assessed in nine chloralose-urethane anesthetized cats using the technique of dynamic end-tidal forcing. The ventilatory responses to step changes in end-tidal PCO2 (PETCO2) were measured before (control), during and after intravenous infusion of dopamine (420 micrograms X kg-1 X h-1). Each response was separated into a slow central and a fast peripheral chemoreflex loop by fitting two exponential functions to the measured ventilation. Both loops were described by a CO2 sensitivity, time constant, time delay and a single off-set B (extrapolated PETCO2 of the steady-state response curve at zero ventilation). Dopamine infusion only caused a significant increase of B (mean 0.3 kPa, P less than 0.0001) compared to control; the other model parameters were not significantly affected. After dopamine infusion B returned to significantly lower values (mean 0.2 kPa, P = 0.006) than in control. In two additional cats the dopamine administered to the blood which was artificially perfusing the brainstem, did not affect ventilation. We conclude that in normoxic cats the effect of exogenous dopamine on the ventilatory response to CO2 is due to a CO2 independent inhibition of the ventilatory drive which originates outside the brainstem.