Age-related decline in protein synthesis in the rat parotid gland

Abstract

This study was undertaken to determine whether protein synthesis declines with age in parorid salivary glands. We have examined the level of incorporation of [3H]-leucine into protein and the metabolism of [U-14C]-D-glucose in 2-, 12-, 18-, 24- and 30-month-old rats. The rate of incorporation of [3H]-leucine into perchloric acid insoluble protein by gland slices decreases progressively with age from 138.43+/-5.43 nmole/mg DNA/h at 2 months to 109.77+/-6.88, 97+/-4.68, 71.62+/-4.13 and 58.08+/-5.61 nmole/mg DNA/h at 12, 18, 24 and 30 months, respectively. The amount of radioactivity in the perchloric acid soluble fraction remains relatively stable at all ages. The ability of gland slices to oxidize [U-14C]-D-glucose to 14CO2 and to incorporate radioactivity into the total lipid does not change with age, whereas the incorporation of radioactivity into protein decreases significantly from 45.06+/-3.55 at 2 months to 21.75+/-1.30 nmoles glucuse/mg DNA/h at 24 months. Histologically, no significant change is detected in the proportion of secretory cells to ductal cells in the different age groups. In radioautographs of the glands incubated with [3H]-leucine, the distribution of cells with grains is similar in all age groups. Lipid droplets occur at the periphery of the lobules and a large number of lipofuscin granules are present in secretory acinar cells of 12-month-old and older rats when viewed in the electron microscope. However, the structural integrity in the cellular organelles is maintained in the acinar cells of all age groups. These results indicate that the ability of parotid gland cells to synthesize protein decreases with age. The decrease reflects specific changes in protein synthesis and not an overall decline in cellular metabolism or a decrease in the number of cells which incorporate leucine.