Postnatal developmental changes in CO2 sensitivity in rats

Ventilatory sensitivity to CO2 in awake adult Brown Norway (BN) rats is 50-75% lower than in adult Sprague-Dawley (SD) and salt-sensitive Dahl S (SS) rats. The purpose of the present study was to test the hypothesis that this difference would be apparent during the development of CO2 sensitivity. Four litters of each strain were divided into four groups such that rats were exposed to 7% inspired CO2 for 5 min in a plethysmograph every third day from postnatal day (P) 0 to P21 and again on P29 and P30. From P0 to P14, CO2 exposure increased pulmonary ventilation (V-E) by 25-50% in the BN and SD strains and between 25 to over 200% in the SS strain. In all strains beginning around P15, the response to CO2 increased progressively reaching a peak at P19-21 when V-E during hypercapnia was 175-225% above eucapnia. There were minimal changes in CO2 sensitivity between P21 and P30, and at both ages there were minimal between-strain differences. At P30, the response to CO2 in the SS and SD strains was near the adult response, but the response in the BN rats was 100% greater at P30 than in adults. We conclude that 1) CO2-sensing mechanisms, and/ or mechanisms downstream from the chemoreceptors, change dramatically at the age in rats when other physiological systems are also maturing (similar to P15), and 2) there is a high degree of age-dependent plasticity in CO2 sensitivity in rats, which differs between strains.