Role of autonomic nervous system controlling surface tension in fetal rabbit lungs

After the maternal abdomen was opened under methoxyflurane anesthesia, fetal rabbits of 27.5 days gestation were given injections through the intact uterine wall of saline, pilocarpine, isoxsuprine, muscarine, phenylephrine, atropine, phenoxybenzamine, or propranolo, alone or in appropriate combinations. Fetal rabbits were delivered by hysterotomy and killed without breathing 2.5 h later. Static pressure-volume curves with air showed improved retention on deflation in fetal rabbits that had injections of pilocarpine, or isoxsuprine, but not of muscarine or phenylephrine. The effect of pilocarpine on the pressure-volume curve was blocked by atropine, phenoxybenzamine, and propranolol, and the effect of isoxsuprine was blocked by propranolol but not phenoxybenzamine. The data suggest that pilocarpine produces secretion of surfactant into lung air spaces by exciting the sympathetic nervous system, a known function of pilocarpine, rather than the parasympathetic nervous system. This may result in stimulation of the same beta-adrenergic receptors affected by isoxsuprine which is also thought to stimulate surfactant secretion.