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.