Impairment in the phosphorylation of canine cardiac sarcoplasmic reticulum following endotoxin administration

Abstract

Effects of endotoxin administration on the phosphorylation and dephosphorylation of phospholamban in canine cardiac sarcoplasmic reticulum (SR) were studied. Results obtained 4 h after endotoxin administration show that the Ca 2+-calmodulin dependent phosphorylation of phospholamban was reduced by 17–25% ( P<0.05). Kinetic analysis reveals that the V max values for Ca 2+, for calmodulin, and for ATP for the Ca 2+-calmodulin dependent phosphorylation were significantly decreased, while the S 0.5 values (for Ca 2+ and calmodulin) and the K m (for ATP) and Hill coefficients (for Ca 2+ and calmodulin) remained unaffected during endotoxic shock. The cAMP-dependent phosphorylation of phospholamban measured in the presence of the exogenously added catalytic subunit of the cAMP-dependent protein kinase remained unaffected. The basal/endogenous (cAMP- and Ca 2+-independent) phosphorylation of phospholamban was significantly decreased after endotoxin administration. The half-time for the dephosphorylation of phospholamban prephosphorylated in the presence of Ca 2+ and calmodulin was shortened by 58% ( P<0.01) in endotoxin shock. These data indicate that the phosphorylation of phospholamban was inhibited while the dephosphorylation was stimulated in canine cardiac SR during endotoxin shock. Since the phosphorylation and dephosphorylation of phospholamban in cardiac SR plays an important role in the control of myocardial contractility, these findings may have a pathophysiological significance in contributing to the understanding of myocardial dysfunction in endotoxin shock.