Altered ryanodine receptor of canine cardiac sarcoplasmic reticulum and its underlying mechanism in endotoxin shock.

Effects of endotoxin administration on ryanodine receptor in canine cardiac junctional sarcoplasmic reticulum (SR) were studied. The results show that the Bmax for [3H]ryanodine binding to cardiac junctional SR was decreased by 25% (8 +/- 0.38 vs 6 +/- 0.41 pmole/mg protein for control and endotoxic, respectively; (P less than 0.01) while the kd (13.7 +/- 1 nM for control vs 13.2 +/- 2 nM for endotoxic) was unaffected 4 hr following endotoxin administration. Ca2+ activated [3H]ryanodine binding in both groups sigmoidally but the Vmax for Ca2+ activation was decreased by 24% (P less than 0.05) while the S0.5 and the Hill coefficient values remained unchanged after endotoxin injection. Caffeine, ATP, and AMP-PCP activated while calmodulin, SKF-525A, ruthenium red, and Mg2+ inhibited [3H]ryanodine binding in both groups but the A0.5 (concentration requires for half-maximum activation) and the I50 (concentration requires for half-maximum inhibition) for the above-mentioned activators and inhibitors, respectively, were unaffected during endotoxin shock. Digestion of cardiac SR isolated from control dogs with phospholipase A2 inhibited [3H]ryanodine binding and the inhibition was reversed completely by the addition of phosphatidylserine. Addition of phosphatidylserine to cardiac SR isolated from endotoxic dogs stimulated [3H]ryanodine binding and the stimulation represents a complete reversal of the inhibition caused by endotoxin administration. Based on these findings together with previous observation that phospholipase A2 activity is activated during endotoxin shock, it is concluded that endotoxin administration decreases the number of ryanodine receptor in canine cardiac junctional SR and the decrease in ryanodine receptor is associated with a mechanism involving a modification of membrane lipid microenvironment in response to phospholipase A2 activation.