Impaired calcium uptake by cardiac sarcoplasmic reticulum and its underlying mechanism in endotoxin shock

Effects of endotoxin administration on the ATP-dependent Ca²⁺ uptake by canine cardiac sarcoplasmic reticulum (SR) were investigated. Results obtained 4 h after endotoxin administration show that ATP-dependent Ca²⁺ uptake by cardiac SR was decreased by 27–43% (p < 0.05). Kinetic analysis indicates that the Vmax values for Ca²⁺ and for ATP were significantly decreased while the S_0.5 and the Hill coefficient values were not affected during endotoxin shock. Magnesium (1–5 mM) stimulated while vanadate (25–50 M) inhibited the ATP-dependent Ca²⁺ uptake, but the Mg²⁺-stimulated and the vanadate-inhibited activities remained significantly lower in the endotoxin-treated animals. Phosphorylation of SR by the exogenously added catalytic subunit of the cAMP-dependent protein kinase or by the addition of calmodulin stimulated the ATP-dependent Ca²⁺ uptake activities both in the control and endotoxin-injected dogs. However, the phosphorylation-stimulated activities remained significantly lower in the endotoxin-injected dogs. Dephosphorylation of SR decreased the ATP-dependent Ca²⁺ uptake, but the half-time required for the maximal dephosphorylation was reduced by 31% (p < 0.05) 4 h post-endotoxin. These data indicate that endotoxin administration impairs the ATP-dependent Ca²⁺ uptake in canine cardiac SR and the endotoxininduced impairment in the SR calcium transport is associated with a mechanism involving a defective phosphorylation and an accelerated dephosphorylation of SR membrane protein. Since ATP-dependent Ca²⁺ uptake by cardiac SR plays an important role in the regulation of the homeostatic levels of the contractile calcium, our findings may provide a biochemical explanation for myocardial dysfunction that occurs during endotoxin shock.