Control of alveolar surfactant in rats at rest and during prolonged hyperpnoea: pharmacological evidence for two tissue pools of surfactant

Propranolol, atropine and indomethacin (i.p.) affect neither the amount (PLalv), nor the specific activity (PLalv_sp.act.) of alveolar surfactant‐type phospholipids lavaged from the lungs of unanaesthetized rats, either at rest or made hyperpnoeic for 24 h with 5%CO₂/13%O₂/82%N₂. Whereas salbutamol (280 μg kg⁻¹ body weight, i.p.) consistently increased PLalv and PLalv_sp.act., pilocarpine (1.5, 3, 10 and 50 mg kg⁻¹, i.p.) and labetalol (1 and 5 mg kg⁻¹, i.p.) had no effect. The dose of pilocarpine reported by others to release surfactant (150 mg kg⁻¹) induced marked salivation, diarrhoea, chromodacryorrhoea and a three‐fold increase in tidal volume. In the isolated perfused lung of the rat, salbutamol (1.5 μm) consistently increased PLalv_sp.act, whereas pilocarpine (0.1 and 1 μM) had no effect on these variables. In the isolated perfused lung, the maximum amount of surfactant that could be released by salbutamol (0.5 mM) was smaller than that which could be released in response to an increase in tidal volume (peak inflation pressure 28 cmH₂O). When the concentration of salbutamol in the isolated perfused lung was adjusted to produce the same increase in PLalv as did a single simulated deep breath, the PLalv_sp.act was significantly increased by salbutamol, but not by the simulated deep breath. We conclude, that neither the autonomic nervous system nor the prostaglandin system is essential for the release of surfactant at rest or during hyperpnoea. Furthermore, we suggest that two pools of surfactant, with different release mechanisms, exist in lung tissue. 1988 British Pharmacological Society