Chicken intrapulmonary chemoreceptor discharge frequency reduced by increasing rate of repetitive PCO2 changes

We have recently reported that repetitive changes of PCO2 given to innervated non-perfused chicken lungs reflexly inhibit respiratory efforts. The degree of respiratory inhibition is rate-dependent, decreasing as repetition rate is increased to 3 . 2 Hz (Barker, Burger & Nye, 1981) to nearly that expected from the average PCO2 of the two levels given. Intrapulmonary chemoreceptors (i.p.c.s), whose discharge frequencies are inversely related to PCO2, are presumably responsible for these effects. Here we report the activity of twenty single i.p.c.s in the left, non-perfused lungs of twelve thoractomized cockerels as PCO2 of the gas ventilating those lungs was repetively changed between 7 and 27 or 7 and 40 torr. Maximal discharge rates of individual receptors after a PCO2 decrease were related to the location of receptors within the gas exchange region. From 0 . 2 to 1 . 6 Hz both average and maximal discharge frequencies decreased. The inhibition of discharge in this range was greater than that predicted by the steady-state relationship between reflex responses and receptor discharge, suggesting that the reflex effects of i.p.c. discharge are not a simple function of the mean discharge frequency of all i.p.c.s. Either average i.p.c. discharge frequency per PCO2 cycle over-estimates their respiratory reflex inhibition or only the less rate-sensitive i.p.c.s, those in the middle of the gas exchange region, may dominate this rate-dependent respiratory reflex.