Pituitary glands were dissociated into single cell suspensions with viabilities greater than 95%. GH cell function in somatotrophs prepared from pituitary glands of rats exposed to microgravity were compared with those from corresponding controls. Somatotroph numbers were determined by flow cytometric immunofluorescence. Western blot analysis of alkaline (pH 8) cell extracts or culture media was done by electrophoresis followed by staining for GH variants by enzyme-linked immunoassay. For culture, 2.5x103 cells/well were maintained for six days with an intervening medium change on the third day. GH was assayed by RIA or 3T3 cell bioassay. For determination of GH cell function in vivo, cells were implanted into the lateral ventricles of 100 g rats using the hollow fiber encapsulation procedure. After sixteen days, tibial plates of recipients were measured to provide an index of long bone growth resulting from the implantation.

Results:

Pituitary growth hormone cells from glands of rats flown on SL-3 contained two to three times more intracellular hormone than controls, but released significantly less GH in subsequent in vitro and in vivo tests. After implantation into hypophysectomized rats, cells from both sizes of flight rats released ~50% as much GH relative to those from the control group. These diminished GH secretory patterns cannot be attributed simply to changes in variant forms of the GH molecule(s), though, since Western blot profiles in both intracellular and secreted GH revealed only minor differences in concentration and number of GH variants. Since the balance between somatostatin and GH releasing hormone presumably regulates hormone secretion in vivo, it is possible that microgravity alters their ratio to bring about high intracellular GH levels and lower release rates. As encapsulated cells were in an environment where they were exposed to hypothalamic regulatory peptides, one could also suggest that experimental cells were less sensitive to these peptides as a result of microgravity.