Metabolic-ventilatory interaction in conscious rats: effect of hypoxia and ambient temperature

Previous studies have indicated that the hypometabolic response to hypoxia depends on ambient temperature (Ta), being more pronounced in the cold. If metabolic rate were an important contributor to the level of ventilation (VE), the magnitude of the hyperpneic response to hypoxia should also depend on Ta. We tested this hypothesis on adult conscious male rats. In normoxia, a drop in Ta from 25 to 10 degrees C increased O2 consumption and CO2 production (VO2 and VCO2, respectively, measured by an open-flow technique) and VE (measured with the barometric method) by 80 and 60%, respectively, with no changes in blood gases. At both Ta, hypoxia (10% inspired O2, 33-35 Torr arterial PO2) induced the same degree of hyperventilation, i.e., the same drop in arterial PCO2 (about -13 Torr). The hyperventilation at 25 degrees C Ta was achieved exclusively by an increase in VE, whereas at 10 degrees C Ta the hyperpnea was minimal (+15%) and accompanied by a drop (-30%) in VO2 and VCO2. Diaphragmatic electromyograms confirmed the VE results. Changes in blood pressure were similar at both Ta. Addition of 3% CO2 to the inspired air further increased VE, indicating that the hypoxic rat was not breathing at its maximal VE at either Ta. We conclude that, in the rat, changes in metabolic rate play an important role in the VE response to hypoxia and that Ta influences the response because of its effect on the degree of hypoxic hypometabolism.