Heterogeneity of growth hormone (GH)-producing cells in aging male rats: in vitro GH releasing activity of somatotrope subpopulations.

Studies on the age-related decline of growth hormone (GH) release have ignored that the population of GH-producing cells (somatotropes) is heterogeneous. In aging male rats, centrifugation of dispersed pituitary cells in a density gradient yields two somatotrope subpopulations, i.e. low- (LD) and high-density (HD) cells. A previous analysis of ultrastructure and GH mRNA levels has shown that storage and biosynthetic features were inversely related in both subsets. Furthermore, ultrastructural and molecular differences between LD- and HD-cells were retained throughout the rat lifespan, suggesting that the heterogeneity of somatotropes may have a biological meaning. Accordingly, the main objective of the present study was to analyze the functional heterogeneity of the somatotrope population during the aging process in male rats. For this purpose, the response of LD- and HD-somatotropes from 5-, 19-, and 26-month-old male rats was analyzed with an optimized cell immunoblot assay both under basal conditions, and after GH-releasing factor (GRF) and/or somatostatin (SS) treatments. Simultaneous measurements of hormonal release, intracellular GH content, and cell size were performed at the single-somatotrope level. Average values for those parameters were significantly higher in HD- than in corresponding LD-cells, such differences being irrespective of age or treatment. Releasing activity and GH content were significantly reduced with age in both subpopulations. GRF stimulated GH release from LD- and HD-somatotropes, and the GRF responsiveness was similar in both subpopulations and in all ages. On the other hand, SS prevented GRF-stimulated GH release in most cases. At the level of single cells, both releasing activity and cell size showed a significant, linear dependence on intracellular GH content, correlations being irrespective of age, subpopulation, or treatment. Taken together, our results demonstrate that LD- and HD-somatotrope subpopulations display quantitative differences in releasing activity that are essentially retained through aging. This functional heterogeneity is more dependent on the basal GH release of these somatotrope subsets than in their responsiveness to GRF and SS. The present findings suggest that the reduction in secretory activity at the single somatotrope level observed in both subpopulations underlies the age-related decline of pituitary GH release. Finally, a theoretical model of secretory cycle is proposed which might contribute to the understanding of the biological meaning of the somatotrope subpopulations in aging male rats.