Respiratory inhibition from chicken intrapulmonary chemoreceptors reduced by increasing rate of repetitive PCO2 changes.

There is no functional residual capacity in the avian lung, so during inspiration the PCO2 of the gas entering the exchange region abruptly changes from its maximum, the dead space PCO2, to its minimum, the inspired PCO2. Dynamic responses of intrapulmonary chemoreceptors (i.p.c.s) to positive and negative going steps in PCO2 are asymmetrical and have maxima occurring at times comparable to inspiratory duration. Thus, the respiratory inhibition that i.p.c.s produce could depend on respiratory frequency. We tested this hypothesis by changing intrapulmonary PCO2 repetitively between 7 and 42 torr in the non-perfused left lungs of eight unidirectionally ventilated thoracotomized chickens. Right intrapulmonary and arterial PCO2 were kept constant. Respiratory movements were calibrated in terms of steady state intrapulmonary PCO2 levels. At PCO2 repetition rates comparable to normal respiratory frequency, respiratory movements were equal to those expected for an intrapulmonary PCO2 2--3 torr lower than the average of the two PCO2 levels used. This difference increased linearly as the logarithm of the repetition rate decreased. We conclude that respiratory inhibition by i.p.c.s is inversely related to respiratory rate as well as to average intrapulmonary PCO2 levels. Because respiratory frequency itself is negatively related to i.p.c. discharge frequency, i.p.c.s may provide control over respiratory frequency even if average PCO2 levels do not change.