Venous return as stimulus for respiration in rabbit

The "exercise hyperpnoea" problem was studied in 44 anaesthetized rabbits by increasing the cardiac output by: a) tilting by 12 degrees the hindlimbs up; b) injecting into the right heart 15 ml of dextran; c) normocapnic or d) hypocapnic blood. In all cases there was an increase in minute ventilation, VE, independently of the presence or absence of an increase in CO2 flux to the lungs. We decreased the cardiac output by tilting up the head by 10 degrees; this elicited a decrease in VE accompanied by an increase in the fractional end-tidal CO2. For elucidating the role played by carotid body chemoreceptors in eleven rabbits the legs were tilted up before and while the animals breathed pure oxygen. The results were very similar. When the hindlimbs were tilted up after cervical bilateral vagotomy the VE response diminished about fifty per cent. We conclude that: a) hyperpnoea and hypopnoea elicited by a change in cardiac output are due to stimulation or depression of mechanoreceptors localized in the pulmonary circulation, b) carotid body chemoreceptors are not required for these responses, c) fifty per cent of the VE response depends on the integrity of the vagus nerves, d) it is probable that the increase in venous return plays an important role in the hyperpnoea at the onset of muscular exercise.