Heart sarcolemmal Ca2+ transport in endotoxin shock: II. Mechanism of impairment in ATP-dependent Ca2+ transport.

The role of the phosphorylation and dephosphorylation of sarcolemma and that of the alteration of membrane lipids in the endotoxin-induced impairment of the ATP-dependent Ca2+ transport in canine cardiac sarcolemma were investigated. The results indicate that the ATP-dependent Ca2+ transport in canine cardiac sarcolemma was decreased by 30-35% 4 h after endotoxin administration. Phosphorylation of sarcolemma by the catalytic subunit of the cAMP-dependent protein kinase or calmodulin stimulated ATP-dependent Ca2+ transport in both groups, however, the phosphorylation-stimulated activities remained significantly lower in endotoxic animals. Dephosphorylation of sarcolemma decreased ATP-dependent Ca2+ transport in both groups, yet, the time required to reach maximal dephosphorylation was reduced from 120 to 90 min 4 h post-endotoxin. Analysis of sarcolemmal membranes reveals that phosphatidylcholine and phosphatidylethanolamine contents were decreased while their respective lysophosphatide levels were increased significantly after endotoxin injection. Digestion of control heart sarcolemma with phospholipase A2 inhibited Ca2+ transport and the inhibition was reversible by phosphatidylcholine. The inhibition caused by the in vivo administration of endotoxin was completely reversible by the addition of phosphatidylcholine. Based on these data, it is concluded that endotoxin administration impairs ATP-dependent Ca2+ transport in canine cardiac sarcolemma and that the impairment may be due to i) a defective phosphorylation of sarcolemma; ii) a reduced number of Ca2+ pumps; iii) an accelerated dephosphorylation of sarcolemma; and iv) an alteration in membrane phospholipid profile in response to phospholipase A activation.