Objectives:Physiological changes which are the consequence of altered gravity conditions were studied in order to evaluate their effect on manned travel in space. Cardiac deconditioning has been found in astronauts, in humans after prolonged bed rest, in monkeys exposed to hypokinesia, and rodents exposed to increased gravity or weightlessness. Altered gravity has been shown to induce morphologically observable changes associated with myocardial degeneration (accumulation of lipid droplets, increased glycogen storage, alterations in microtubule distribution). In this experiment, cardiac ultrastructure and cyclic AMP metabolism were studied in tissues of space-flown rats.
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Approach:
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.
Mednieks, M.I. et al.: Cardiac Muscle Ultrastructure and Cyclic AMP Reactions to Altered Gravity Conditions. American Journal of Physiology, vol. 252, 1987, pp.R227-R232.
Philpott DE, et al. Microgravity Changes in Heart Structure and cyclic-AMP metabolism. Abstract S-209; Proceedings of the Seventh Annual Meeting on the IUPS Commission on Gravitational Physiology,1985 October 13-18; Niagara Falls (NY).
Philpott DE, Fine A, Kato K, Egnor R, Cheng L, Mednieks M. Microgravity Changes in Heart Structure and Cyclic-AMP metabolism. Physiologist 1985; 28(6 Suppl): S209-10.
Philpott DE, Fine A, Mednieks M, Kato K. Microgravity Changes in Heart Structure and c-AMP metabolism. Abstract 83.15; 36th Annual Fall Meeting of the American Physiological Society; 1985 Oct 13-18; Buffalo (NY) Physiologist 1985; 28(4):378.
Activity, cAMP-dependent protein kinase, heart, rat, Incorporation of P-ATP into protein
Activity, Cyclases, Heart, Rat, Conversion of 3H-ATP to 3H-cAMP, Chromatography of cAMP,
Activity, Phosphatases, heart, rat, cAMP phosphodiesterase (low & high Km), (3H-5 AMP elutriated from polyacrylamide boronate affinity gel chromatographic column),
Morphology, ultrastructure, Heart (Cardiac), Rat, Electron Microscopy (descriptive)