Carotid chemoreceptor response to natural stimuli in the newborn kitten

The activities of carotid chemoreceptors at three levels of inspired PO2 (55, 145 and 690 Torr) and at two levels of inspired PCO2 (35 and 70 Torr in O2) were studied in 28 anesthetized, mechanically ventilated kittens aged 0-17 days. A biphasic response to hypoxia was found in 46% of them: the chemosensory activity increased to a peak within 30 sec after the initial response to hypoxia and thereafter declined slowly to a stable value. The steady-state single-fiber chemosensory activity at an inspired PO2 of 55 Torr was significantly lower in kittens less than 10 days old (mean +/- SE: 5.8 +/- 0.6 impulses.sec-1) than in the older ones (8.8 +/- 1.3 impulses.sec-1, P less than 0.03). The response curves to arterial PO2 were hyperbolic in both groups but the curve for the younger kittens was displaced to the left of the curve for the older ones. The response to hypercapnia was a progressive increase in chemosensory activity with little evidence of rapid or slow adaptation. The response to hypercapnia was significantly stronger in the older kittens than in the young ones. It is concluded that, in the kitten, the carotid chemoreceptor response to hypoxia may be biphasic. The responses to hypoxia and hypercapnia are already developed but are weak at birth and continue to develop further during the first weeks of postnatal life.