RESPIRATORY RESPONSES TO HYPOXIA AND HYPERCAPNIA IN ANESTHETIZED RABBITS WITH INDUCED HYPOTHYROIDISM

We have investigated the effects of hypothyroidism on hypoxic and hype rcapnic ventilatory drive. Experiments were carried out in control and hypothyroid rabbits. Hypothyroidism was developed by giving methimazo le, hn iodide-blocker, in food (75 mg/100 g food) for six weeks. When control group of animals were allowed to breathe a hypoxic gas mixture , there were significant increases in tidal volume (V-T), respiratory frequency (f) and ventilation minute volume (V-E). Injection of KCN (4 0 mu g kg(-1)) caused marked increases in these parameters. The V-T, V -E and PaO2 of the hypothyroid rabbits breathing air were lower and Pa CO2 was higher than that of the control group. During breathing of the hypoxic gas mixture by the hypothyroid rabbits V-T and V-E were signi ficantly decreased. In the hypothyroid animals, i.v. injection of KCN (40 mu g/kg(-1)), produced no significant changes in V-T f or V-E. Whe n the hypothyroid rabbits were allowed to breathe the hypercapnic gas mixtures, V-T, V-E, PaCO2 and PaO2 increased significantly. f was unaf fected. The increases in VT and VE in hypothyroid group were less in m agnitude as compared with the control group. The air breathing hypoven tilation in the hypothyroid animals is due to decreased or abolished s ensitivity of preipheral chemoreceptors. The further decline in ventil ation in hypothyroid animals in hypoxia indicates central depression i n the absence of peripheral chemoreceptor impulses.