Control of interbreath interval in the African lungfish

We have performed studies to examine the effect of variations in intrapulmonary pressure on the interval between lung breaths in the African lungfish. Studies were performed in two different preparations. In the first we produced changes in lung pressure using a controlled-infusion pump. Increases in intrapulmonary pressure prolonged the interval between lung breaths. At a pressure of 2.5 cmH2O the average interval was 2.6 +/- 1.8 min (mean +/- SD); at 5.0 cmH2O, 8.1 +/- 3.5 min; and at 7.5 cmH2O, 16.2 +/- 3.8 min. Inflations of the lung early in the interbreath interval had less of an effect on its duration than inflations later in the interval. In the second preparation we used a system in which gas flowed continuously through both lungs. Intrapulmonary pressure was varied by changing outlet pressure and O2 concentration by changing the composition of the gas mixture. This allowed separate control of both O2 concentration and intrapulmonary pressure. At a fixed O2 concentration intrapulmonary pressure increased the duration of the interval between lung breaths. At a fixed pressure, reductions in O2 concentration shortened the interval. There was no significant interactive effect of O2 and pressure. Lung inflation did not alter the frequency of gill ventilation. These results imply that a reflex highly similar to the Hering-Breuer expiratory-promoting reflex was already present in the African lungfish.