Uptake of instilled radiolabeled lamellar bodies from alveolar compartment of the rat

Following the instillation of lamellar bodies containing dipalmitoyl phosphatidyl-[³H]choline (DPPC) down the trachea of adult rats, we found that the half-life of alveolar [³H]-DPPC was 85 min and the time constant was 120 min. As much as 85% was recycled. When we labeled the DPPC with both [³H]choline and [¹⁴C]acetate, the ratio ³H/¹⁴C increased in the alveolar compartment and then increased further in the lamellar body fraction of the recipient lungs, suggesting that some deacylation-reacylation was occurring. Further evidence of degradation was an increase in free [³H]choline in the microsomal fraction. Whereas hyperpnea induced by breathing 5% CO₂-13% O₂-82% N₂ increased the reuptake of DPPC, reuptake did not appear to be enhanced in the rest period immediately after hyperpnea induced by swimming, when alveolar DPPC was still markedly elevated. Propranolol did not affect reuptake, suggesting that β-adrenoceptors were not essential. We suggest that reuptake is coupled more to release than to the amount of surfactant in the alveolar compartment.