Effects of lifelong dietary restriction on somatotropes: immunohistochemical and functional aspects.

We investigated the effects of lifelong dietary restriction (DR) on growth hormone (GH)-immunoreactivity and secretory function of somatotropes using a computerized image analysis in tissue sections and a reverse hemolytic plaque assay (RHPA) in dispersed pituitary cells of male F344 rats. DR did not prevent an aging-related reduction in GH immunoreactivity in somatotropes. However, it did augment the mean optical density of GH-immunoreactive area in somatotropes, although it did not alter the immunoreactive area in somatotropes, suggestive of a greater amount of immunoreactive GH in somatotropes of DR rats. DR also increased the percentage of aggregated GH-immunoreactive area in the anterior lobe, indicating a higher cell density of immunoreactive somatotropes. RHPA also confirmed the presence of larger proportions of GH-secreting cells in dispersed cells of DR rats in response to GH-releasing hormone (GHRH) at 24 months, although no change by DR was observed in the estimated amount of GH secreted from individual cells. Our results suggest that lifelong dietary restriction may influence the cytoplasmic GH-content at the steady state, while not modulating the responsiveness of individual somatotropes to GHRH for GH release, and that the major action of DR on somatotropes is an increment in the cell number.