Functional significance and control of release of pulmonary surfactant in the lizard lung

The amount of pulmonary surfactant in the lungs of the bearded dragon (Pogona vitticeps) increases with increasing body temperature. This increase coincides with a decrease in lung compliance. The relationship between surfactant and lung compliance and the principal stimuli for surfactant release and composition (temperature, ventilatory pattern, and autonomic neurotransmitters) were investigated. We chose to investigate ventilatory pattern (which causes mechanical deformation of the type II cells) and adrenergic agents, because they are the major stimuli for surfactant release in mammals. To examine the effects of body temperature and ventilatory pattern, isolated lungs were ventilated at either 18 or 37 degrees C at different ventilatory regimens. An isolated perfused lung preparation at 27 degrees C was used to analyze the effects of autonomic neurotransmitters. Ventilatory pattern did not affect surfactant release, composition, or lung compliance at either 18 or 37 degrees C. An increase in temperature increased phospholipid reuptake and disproportionately increased cholesterol degradation/uptake. Epinephrine and acetylcholine stimulated phospholipid but not cholesterol release. Removal of surfactant caused a decrease in compliance, regardless of the experimental temperature. Temperature appears to be the principal determinant of lung compliance in the bearded dragon, acting directly to increase the tone of the smooth muscle. Increasing the ambient temperature may result in greater surfactant turnover by increasing cholesterol reuptake/degradation directly and by increasing circulating epinephrine, thereby indirectly increasing phospholipid secretion. We suggest that changing ventilatory pattern may be inadequate as a mechanism for maintaining surfactant homeostasis, given the discontinuous, highly variable reptilian breathing pattern.