[Cardio-vascular reactions to respiratory hypercapnia in rats]

1. Two populations of albino rats : normal and treated by phentolamine, or normal and vagotomized, have inhaled normoxic or hyperoxic mixtures with 3, 15 or 30% CO2 during 5 min. 2. Bradycardia is proportional to CO2 concentration, from 3% CO2. It is not suppressed by vagotomy. 3. Systemic arterial pressures, both systolic and diastolic, are decreased by 14% CO2. This hypotension is long lasting. At 30% CO2, immediate hypotension is followed by progressive recovery. After phentolamine, the return to normal is abolished. 4. Bradycardia is caused by CO2 or acidosis acting directly on the pace-maker cells. Variations of the systemic blood pressure are explained by an interference between a direct vasodilatation and a vasoconstriction due to sympathetic centres stimulation by CO2.