Changes in immunoreactivity for cathepsin H in rat type II alveolar epithelial cells and its proteolytic activity in bronchoalveolar lavage fluid over 24 hours

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Abstract
Variations in immunoreactivity for cathepsin H in rat type II alveolar epithelial (type II) cells and in its proteolytic activity in bronchoalveolar lavage fluid (BALF) were examined at six evenly spaced times over 24 hr (light period: 0600–1800 hr). Ring-shaped immunodeposits for cathepsin H were detected in type II cells (lamellar bodies), their sizes varying over 24 hr. The large ring-shaped immunodeposits increased during the light period, decreasing rapidly from 2000 to 0000 hr. Small intense immunodeposits abundantly appeared in the cells at 0000 hr. The area densities of immunodeposits in type II cells and their optical densities also varied with the time of day; both densities were high during the dark period, peaking at the mid dark period, whereas they were decreased during the light period. Proteolytic activities of cathepsin H in BALF (6 ml/rat) examined at each time revealed a distinct variation over 24 hr, corresponding to the variation in the immunoreactivity in type II cells. The activities in BALF were high from 1600 to 0400 hr and low at 0800 hr. These results suggest that the variation in immunoreactivity for cathepsin H in type II cells over 24 hr reflects the intracellular growth of lamellar bodies and secretory activity of the cells. Similar variations in the immunoreactivity and proteolytic activity of cathepsin H in the cells and BALF indicate its cosecretion with surfactants.