Control of pulmonary surfactant secretion: an evolutionary perspective

Pulmonary surfactant, a mixture consisting of phospholipids (PL) and protei ns, is secreted by type II cells in the lungs of all air-breathing vertebra tes. Virtually nothing is known about the factors that control the secretio n of pulmonary surfactant in nonmammalian vertebrates. With the use of type II cell cultures from Australian lungfish, North American bullfrogs, and f at-tailed dunnarts, we describe the autonomic regulation of surfactant secr etion among the vertebrates. ACh, but not epinephrine (Epi), stimulated tot al PL and disaturated PL (DSP) secretion from type II cells isolated from A ustralian lungfish. Both Epi and ACh stimulated PL and DSP secretion from t ype II cells of bullfrogs and fat-tailed dunnarts. Neither Epi nor ACh affe cted the secretion of cholesterol from type II cell cultures of bullfrogs o r dunnarts. Pulmonary surfactant secretion may be predominantly controlled by the autonomic nervous system in nonmammalian vertebrates. The parasympat hetic nervous system may predominate at lower body temperatures, stimulatin g surfactant secretion without elevating metabolic rate. Adrenergic influen ces on the surfactant system may have developed subsequent to the radiation of the tetrapods. Furthermore, ventilatory influences on the surfactant sy stem may have arisen at the time of the evolution of the mammalian bronchoa lveolar lung. Further studies using other carefully chosen species from eac h of the vertebrate groups are required to confirm this hypothesis.