Differential Effects of Hypoxic and Hyperoxic Stress-Induced Hypertrophy in Cultured Chick Fetal Cardiac Myocytes

The adult heart responds to contraction demands by hypertrophy, or enlargement, of cardiac myocytes. Adaptive hypertrophy can occur in response to hyperoxic conditions such as exercise, while pathological factors that result in hypoxia ultimately result in heart failure. The difference in the outcomes produced by pathologically versus physiologically induced hypertrophy suggests that the cellular signaling pathways or conditions of myocytes may be different at the cellular level. The structural and functional changes in myocytes resulting from hyperoxia (simulated using hydrogen peroxide) and hypoxia (using oxygen deprivation) were tested on fetal chick cardiac myocytes grown in vitro. Structural changes were measured using immunostaining for alpha-sarcomeric actin or MyoD, while functional changes were assessed using immunostaining for calcium/calmodulin-dependent kinase (CaMKII) and by measuring intracellular calcium fluxes using live cell fluorescence imaging. Both hypoxic and hyperoxic stress resulted in an upregulation of actin and MyoD expression. Similarly, voltage-gated channels governing myocyte depolarization and the regulation of CaMK were unchanged by hyperoxic or hypoxic conditions. However, the dynamic features of calcium fluxes elicited by caffeine or epinephrine were different in cells subjected to hypoxia versus hyperoxia, suggesting that these different conditions differentially affect components of ligand-activated signaling pathways that regulate calcium. Our results suggest that changes in signaling pathways, rather than structural organization, may mediate the different outcomes associated with hyperoxia-induced versus hypoxia-induced hypertrophy, and these changes are likely initiated at the cellular level.