Calcium ion is thought to play a role in the structure and function of pulmonary surfactant after secretion into the alveolar space. Since fetal lung liquid calcium concentration is inadequate for this hypothesized role, at a time when optimal surfactant function is necessary for survival, we speculated that the necessary calcium is secreted with the surfactant material, i.e., in the lamellar body. Lungs from rat fetuses at 20, 21, and 22 d gestation, and also from newborn rats at 3-5 h, 1 and 3 d, were rapidly frozen, sectioned, freeze-dried, and examined cold (-100 degrees C) in a transmission electron microscope equipped with a fully quantitative energy-dispersive x-ray detector and analyzer. X-ray spectra were collected from the lamellar bodies and cytoplasm of type II cells at each time point. Lamellar body calcium concentration in the fetus was approximately twice that of the adult (70 +/- 4 vs. 37 +/- 2 mmol/kg dry wt +/- SEM, P less than 0.01), and it decreased rapidly after birth to adult levels. Apically located lamellar bodies in the fetus have a significantly higher calcium concentration than those in a perinuclear position (76 +/- 4 vs. 52 +/- 3, P less than 0.01). There is significant correlation of calcium and chloride concentrations in lamellar bodies, suggesting that factors responsible for the distribution of chloride, i.e., pH, may also be responsible for the accumulation of calcium by these organelles. These results show that mature calcium transport in lamellar bodies is achieved prenatally in the rat, and suggest that the calcium required for normal surfactant function at birth is secreted with the lamellar body.
R G Eckenhoff