Circadian pattern of ventilation during acute and chronic hypercapnia in conscious adult rats

Abstract

Because metabolism is a determinant of the ventilatory chemosensitivity, we tested the hypothesis that the ventilatory response to acute and prolonged hypercapnia is adjusted to the circadian oscillations in oxygen consumption (V˙o 2). Adult rats were instrumented for measurements of body temperature (Tb) and activity by telemetry. Pulmonary ventilation (V˙e) was measured by the barometric method and V˙o 2 by the flow-through method. In the acute experiments, 16 conscious rats entrained to a 12:12-h light (L)-dark (D) cycle (lights on 7:00 AM) were exposed to air, 2%, and then 5% CO2 in normoxia (30–45 min each) at 11:00 AM and 11:00 PM. In a separate group of seven rats, simultaneous recordings of all variables were made continuously for 3 consecutive days in air followed by 3 days in 2% CO2 in normoxia, in a 12:12-h L-D cycle (lights on 7:00 AM). In air, all variables were significantly higher at night, whether rats were studied acutely or chronically. Acute CO2exposure had similar significant effects at 11:00 AM and 11:00 PM onV˙e (∼25 and 100% increase with 2 and 5% CO2, respectively) and V˙o 2(∼8% drop with 5% CO2), such that the hyperventilatory response (% increase inV˙e/V˙o 2 from air) was similar at both times. Chronic CO2 breathing increasedV˙e at all times of the day, but less so during the L phase (∼15 vs. 22% increase in L and D, respectively), when activity was lower. However, V˙o 2 was reduced from the air level (∼10% drop) in the L, such that theV˙e/V˙o 2 response was similar between L and D. The same result was obtained when theV˙e/V˙o 2 response was compared between the L and D phases for the same level of activity. These results suggest that, throughout the day, the hypercapnic hyperpnea, whether during acute or prolonged CO2, is perfectly adjusted to the metabolic level.