Tissues were post-fixed and processed for electron microscopy. Adenylate cyclase activities were monitored by following the conversion of 3H-ATP to 3H-cAMP, and cAMP was removed by column chromatography. Low Km and high Km cAMP-PDE activities were determined, and the reaction product (3H-5 AMP) was elutriated from a polyacrylamide boronate affinity gel chromatographic column. cAMP-dependent protein kinase activity was measured as the incorporation of P-ATP into protein. Assays were carried out with protamine as the exogenous phosphoacceptor protein and in either the absence or presence of exogenous (10-6M) cAMP, as was phosphorylation of endogenous substrate proteins without the addition of protamine. Compartmental subunits distribution of regulatory cA-PK subunits was determined using a photoaffinity labeling method with [32P]-8- azido cyclic AMP, and type I and type II cA-PK were separated on SDS-gradient gels and identified by autoradiography of Western blots.

Results:

Changes in ultrastructure and biochemistry were found in the heart tissue of flown rats. An accumulation of lipid droplets and changes in glycogen deposits and microtubules were observed at the electron microscope level as a result of space flight. Adenylate cyclase and low Km PDE activities were not altered, but a decrease in high Km phosphodiesterase was seen in flight homogenates. Protein kinase activity decreased, while activity ratios increased, indicating that the holoenzyme was more extensively dissociated and inactivated during space flight. Together with other studies of responses to catecholamines, results show that ß-adrenergic responses were affected during space flight. While events at the cell surface level were within the range of homeostatic control, intracellular signal processing of the receptor interactions was modified. Results suggest that heart muscle energy metabolism may be altered in microgravity.