Cyclic AMP-receptor responses to hypergravity.

Background: Altered gravity (G) encountered during spaceflight causes physiologic changes in humans and in experimental animals. In addition to weightlessness (0G) in space, sharply increased G forces are exerted on the spacecraft during the lift-off and reentry phases. Previous studies showed major changes in cAMP-associated activity of rat heart muscle after spaceflight, indicating that (hormone) signaling pathways may have been affected.

Hypothesis: The present study was designed to test the hypothesis that cAMP-related cellular responses of exocrine glands after simulated hypergravity (centrifugation at 1.7G) differ from the effects of 0G.

Methods: A portion of the parotid and lachrymal gland tissue was fixed for morphologic and immunocytochemical study, and another was used for biochemical determinations. A short-term tissue culture was established from each gland to determine the effects of stimulation by norepinephrine. Heart muscle (ventricle) was also studied. Soluble and particulate fraction extracts of tissue homogenates were prepared, photoaffinity labeled with the [32P]8-N3-analog of cAMP, proteins separated by electrophoresis and the cAMP-reactive proteins (cARP) identified by autoradiography.

Results: Differences were seen in protein banding patterns of the gland extracts and in altered cARP distribution in the 1.7G samples of heart ventricle and exocrine gland tissues, when compared with 1G controls. In the heart, cARP increased in the soluble fraction, while the particulate fraction extract showed no change. In acinar cells of the parotid, labeled cARP had accumulated, but decreased after stimulation to the level of the 1G controls. Immunogold labeling showed an increased content of amylase in the secretory granules of the 1.7G animals, while morphologic observation revealed few changes in the structure of parotid acinar cells. The response in the lachrymal gland was translocation of an isoform of cARP from the particulate to the cytoplasmic compartment.

Conclusions: Changes distinct from those due to 0G, but specific for hyper-G were found in cARP activity, protein synthesis, as well as in an apparent inhibition of regulated secretion.