Stereologic analysis of the in vivo alveolar type II cell response to isoproterenol or saline administration

Previous studies have demonstrated enhanced secretion of pulmonary surfactant from type II alveolar epithelial cells following beta-adrenergic stimulation. The present study was undertaken in order to provide quantitative morphologic data supporting this effect in vivo. Adult male Sprague-Dawley rats were injected subcutaneously with 150 mg/kg L-isoproterenol, a wide-range beta-adrenergic agonist, and killed at times 0.25-12 hours post-injection. Other rats were similarly injected with saline, and killed at times 0.25-6 hours post-injection. A third group of animals was not injected, nor handled, prior to the time of death, and served as baseline controls. Stereologic analysis of the intracellular organelles of the type II cells in the animals treated with L-isoproterenol revealed a significant decrease in lamellar body volume density, indicating increased secretion of surfactant, at 0.5-4 hours. The rough endoplasmic reticulum volume density increased significantly at 2-6 hours, indicating increased synthetic activity. In contrast, the type II cells of saline-injected animals showed no significant evidence of increased secretion, but did demonstrate a large increase in synthetic activity, resulting in many large lamellar bodies at 2 and 4 hours post-injection. The results of this study provide quantitative morphological evidence of beta-adrenergic stimulation of the secretion and synthesis of pulmonary surfactant secretion by type II cells of the adult rat lung in vivo. In addition, they suggest an enhancement of surfactant synthesis following saline injection, which is perhaps based on endogenous catecholamine release.