On the mechanism of plasma membrane turnover in the salt gland of ducklings. Implications from DNA content, rates of DNA synthesis, and sites of DNA synthesis during the osmotic stressing and destressing cycle.

This study provides information on the rates of DNA synthesis, sites of DNA synthesis, and DNA content of the avian salt gland during the osmotic stressing (plasma membrane synthesis) and destressing (plasma membrane turnover) cycle, in an effort to better understand the relationship of cell turnover to the initial events in plasma membrane amplification, differentiation, and turnover. The rate of DNA synthesis increases 12--24 h after the onset of osmotic stress, is maximal at about 24 h of osmotic stress, and decreases thereafter in fully stressed and destressed glands. The maximum DNA and protein content, and the maximum protein/DNA ratio are obtained after about 3 days of stress. Autoradiograms show that at 24 hr of stress 70--80% of DNA synthesis occurs in connective tissue cells and 20--30% in parenchymal cells, but by 6 days of stress, synthesis occurs about equally in these cell groups. Because destressing is characterized by a large decrease in plasma membrane and in glandular protein, but by little DNA turnover or loss, the loss of plasma membrane is likely due to some type of cell dedifferentiation rather than cell turnover.