Two types of rat pituitary somatotrophs secrete growth hormone with different biological and immunological profiles

Two stable subpopulations of somatotrophs reside in the rat pituitary gland. We tested the hypothesis that one produced growth hormone (GH) with greater activity when tested in the tibial line bioassay (BGH) than the other, while differences in the activities between the two groups would be less dramatic when measured by immunoassay (IGH).

A series of studies using hypophysectomized rats, hollow fibers, treatments and culture models were used to differentiate differences in Type I and Type II anterior pituitary somatotrophs in both function and production of immunoactive and bioactive growth hormone.

We found that dense, Type II somatotrophs (>1.070g·cm^-3) differed markedly in their secretion patterns of IGH vs BGH in different In vitro and in vivo tests. In culture, Type II cells secreted five times as much BGH, and three fourths as much IGH as the less dense Type I cells. Production (storage and secretion) of BGH was 7-fold greater by Type II cells whereas IGH production was identical for the two cell types. Implantation of Type II cells into hypophysectomized rats significantly increased body weight, epiphyseal cartilage thickness, and muscle weight of the recipients; in contrast, Type I cells elicited only a small increase in body weight. Type I somatotrophs isolated from rats which had been previously fasted or insulin-treated subsequently showed only small, inconsistent changes in release relative to that from cells in the unfractionated cell population. However, release of BGH from the Type II cells was markedly decreased.

Both IGH and BGH should be considered in the elucidation of GH physiology.