Airway receptor activity in the developing opossum.

Characteristics of airway receptor discharge were evaluated in pentobarbital-anesthetized, gallamine-paralyzed, artificially ventilated, open-chest suckling and weanling opossums (Didelphis marsupialis). Animals were tested with single-unit vagal recordings at 20, 30, 55, and 90 days of age; results were compared with previous data from adults. Rapidly adapting airway receptors ( RARs ) comprised a smaller percentage of the sampled population at 20 and 30 days of age than in older groups. The static firing properties of slowly adapting airway receptors (SARs) were examined at 0-20 cmH2O transpulmonary pressure (Ptp). At the higher Ptp levels, average receptor discharge rates increased with increasing age; these results are similar to findings in placental mammals. At 5 cmH2O, however, only the 20-day-old animals exhibited reduced rates of SAR discharge. The dynamic response of SARs in 20- and 30-day-old animals was reduced compared with adult values, as measured by the adaptation of their discharge frequencies; however, the reduction was proportional to the lower receptor firing rates. About 80% of tested SARs were inhibited by CO2 at 20 and 55 days; similar results were obtained in adults. Other results have shown that morphologic development of the vagi in opossums at 50 days of age compares with placental mammals similarly studied at birth. In contrast, the static SAR discharge in the 50-day-old opossum at low Ptp (i.e., 5 cmH2O) is similar to that observed in adults, whereas the newborn placental mammal has a greatly reduced SAR firing rate. This suggests that early utilization of the lungs can contribute to functional maturation of airway receptors.