COMPENSATORY MECHANISMS ASSOCIATED WITH THE HYPERDYNAMIC FUNCTION OF PHOSPHOLAMBAN-DEFICIENT MOUSE HEARTS

Phospholamban ablation is associated with significant increases in the sarcoplasmic reticulum Ca2+-ATPase activity and the basal cardiac con tractile parameters. To determine whether the observed phenotype is du e to loss of phospholamban alone or to accompanying compensatory mecha nisms, hearts from phospholamban-deficient and age-matched wild-type m ice were characterized in parallel. There were no morphological altera tions detected at the light microscope level. Assessment of the protei n levels of the cardiac sarcoplasmic reticulum Ca2+-ATPase, calsequest rin, myosin, actin, troponin I, and troponin T revealed no significant differences between phospholamban-deficient and wild-type hearts. How ever, the ryanodine receptor protein levels were significantly decreas ed (25%) upon ablation of phospholamban, probably in an attempt to reg ulate the release of Ca2+ from the sarcoplasmic reticulum, which had a significantly higher diastolic Ca2+ content in phospholamban-deficien t compared with wild-type hearts (16.0+/-2.2 versus 8.6+/-1.0 mmol Ca2 +/kg dry wt, respectively). The increases in Ca2+ content were specifi c to junctional sarcoplasmic reticulum stores, as there were no altera tions in the Ca2+ content of the mitochondria or A band. Assessment of ATP levels revealed no alterations, although oxygen consumption incre ased (1.6-fold) to meet the increased ATP utilization in the hyperdyna mic phospholamban-deficient hearts. The increases in oxygen consumptio n were associated with increases (2.2-fold) in the active fraction of the mitochondrial pyruvate dehydrogenase, suggesting increased tricarb oxylic acid cycle turnover and ATP synthesis. P-31 nuclear magnetic re sonance studies demonstrated decreases in phosphocreatine levels and i ncreases in ADP and AMP levels in phospholamban-deficient compared wit h wild-type hearts. However, the creatine kinase activity and the crea tine kinase reaction velocity were not different between phospholamban -deficient and wild-type hearts. These findings indicate that ablation of phospholamban is associated with downregulation of the ryanodine r eceptor to compensate for the increased Ca2+ content in the sarcoplasm ic reticulum store and metabolic adaptations to establish a new energe tic steady state to meet the increased ATP demand in the hyperdynamic phospholamban-deficient hearts.