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

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 Ca2(+)-calmodulin dependent phosphorylation of phospholamban was reduced by 17-25% (P less than 0.05). Kinetic analysis reveals that the Vmax values for Ca2+, for calmodulin, and for ATP for the Ca2(+)-calmodulin dependent phosphorylation were significantly decreased, while the S0.5 values (for Ca2+ and calmodulin) and the Km (for ATP) and Hill coefficients (for Ca2+ 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 Ca2(+)-independent) phosphorylation of phospholamban was significantly decreased after endotoxin administration. The half-time for the dephosphorylation of phospholamban prephosphorylated in the presence of Ca2+ and calmodulin was shortened by 58% (P less than 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.