As part of our studies of the role of changes of intracellular pH (pH_i) in central chemosensitive neurons, we began studying the in vivo ventilatory response to increased inspired CO₂. Our hope was to identify distinct sub-populations of rats with widely different CO₂ sensitivity of in vivo ventilation so that we could study the cellular responses to hypercapnia (pH_i changes and increased chemosensitive neuron firing rate) and look for a correlation with the in vivo ventilatory patterns.

Ventilation in neonatal Sprague-Dawley rats (P1-P21) was studied using a dual chamber Respiromax Plethysmograph. Weight specific minute ventilation (V_E) was measured as a function of inspired CO₂ over the range of 0-5%. V_E increased linearly with increasing CO₂ due entirely to increased tidal volume (no change in respiratory frequency) [1]. The V_E response to increased CO₂ varied among individuals and from day to day in a given individual. The CO₂ sensitivity of ventilation (determined as the slope of the V_E vs.inspired % CO₂ curve) showed a biphasic relationship with age. CO₂ sensitivity was high in newborn rats (P1) and decreased to a minimum value at P7-P8. It increased again to reach a level at P14 that was very similar to the CO₂ sensitivity in adults [1]. The basis for this developmental pattern is unknown but it is not due to changes in the CO₂ responsiveness of chemosensitive neurons from the locus coeruleus, since these neurons show a constant increased firing rate of about 44% in response to hypercapnia (10% CO₂).

In an attempt to vary CO₂ sensitivity, we reared neonatal rats in a chronic hypercapnic (CH) environment [2] and studied the effect of this treatment on V_E and on CO₂ sensitivity of VE. We reared time pregnant mothers in an environment of constant CH (7% CO₂) for 1week prior to the birth of the pups and maintained the mother and pups in the CH environment until the neonates were tested (at least 6additional days). These neonates exhibited retarded growth (smaller by 1-2g) for the first 2weeks of life but attained the same weight as control rats (reared in room air) by P16. These CH rats exhibited higher CO₂ sensitivity than control rats at days P6-P9 and then showed lower values that were indistinguishable from control rats from P10->P19. Other litters were exposed to severe CH (10%) using the same protocol as for the 7% CH rats. Litters were culled in these severe CH rats (1/3 of the litter sacrificed at birth), but even so these rats showed marked growth retardation that got worse with increasing age. These rats exhibited an even higher CO₂ sensitivity than control or 7% CH rats at P6-P9 and then showed values that were similar to control rats at P10->P19. V_E increased with increasing exposure to CH from (mean ± SE; n = 14) 892 ± 100 (control) to 960 ± 109 (7% CH) to 1127 ± 108 (10% CH)ml-min^-1-kg^-1. We suggest that CH results in a slowing of development so that CO₂ sensitivity of V_E remains elevated longer after birth.

The biphasic developmental pattern suggests that after birth rats display a neonatal pattern of chemosensitivity that decreases during the first week of life and is replaced after the second week by a more adult form of chemosensitivity. A critical period of low in seen between these two periods. The CO₂ sensitivity of V_E mechanistic basis for this biphasic pattern is unknown as is the effect of CH on the properties of central chemosensitive neurons and both of these questions should be fruitful areas of future work.