DOPAMINERGIC INFLUENCE ON THE VENTILATORY RESPONSE TO SUSTAINED HYPOXIA IN THE CAT

We examined the effects of the dopamine blocker haloperidol (H) on the ventilatory response to sustained hypoxia in awake cats. Ventilation (VE) was measured plethysmographically, and endtidal PO2 (PET(O2)) and PCO2 were measured by sampling tracheal gas. Measurements were made before, during, and after 30 min of isocapnic hypoxia after pretreatment with 0.1 mg/kg of H and placebo (P). Experimental runs were conducted on separate days with the P day being first. Two levels of hypoxia were employed. Six cats were studied at a PET(O2) of similar to 61 Torr, and six cats were studied at a PET(O2) of similar to 41 Torr. In all experiments, isocapnia was maintained and end-tidal PCO2 did not differ on the H and P days. With mild hypoxia after P, VE increased to 137% of air breathing control in the first 5 min of hypoxia and then declined to 102% of control at 25 min of hypoxia. Results on the H day were similar. With more severe hypoxia after P, VE increased to 173% of control at 5 min and then declined to 127% of control. With H, VE increased to 172% of control in the first 5 min and then declined to 149% of control at 25 min. After 25 min of more severe hypoxia, both VE and tidal volume were significantly greater with H than with P (P < 0.05). H reduced the ventilatory decline observed with more severe hypoxia but had no effect on the ventilatory decline in milder hypoxia. These results indicate that dopaminergic mechanisms are involved in the ventilatory decline at some levels of hypoxia and that the mechanism of the decline varies with the level of hypoxia.