Impairment of the ryanodine-sensitive calcium release channels in the cardiac sarcoplasmic reticulum and its underlying mechanism during the hypodynamic phase of sepsis

Changes in Ca2+-induced Ca2+ release in cardiac sarcoplasmic reticulum (SR) during different phases of sepsis were studied. Sepsis was induced by ceca l ligation and puncture (CLP). The Ca-45(2+) release studies show that the amount of Ca2+ released from the passively and the actively loaded SR vesic les was unaffected during the early sepsis (9 h after CLP), but it was sign ificantly decreased during the late phase (18 h after CLP) of sepsis. The [ H-3]ryanodine binding assays reveal that the B-max for ryanodine binding wa s unaffected during the early phase, but was decreased by 32.1% during the late phase of sepsis. The affinity of ryanodine receptor for Ca2+ remained unchanged during sepsis. ATP, AMP-PCP, and caffeine stimulated binding, whi le MgCl2 and ruthenium red inhibited [H-3]ryanodine binding in control, ear ly sepsis, and late sepsis groups. The EC50 and IC50 values for these regul ators were unaffected during the progression of sepsis. Digestion of contro l SR with phospholipase A(2) decreased [H-3]ryanodine binding and the decre ase was reversible by the addition of phosphatidylcholine (PC), phosphatidy lethanolamine (PE), or phosphatidylserine (PS). Addition of PC, PE, or PS t o the SR isolated from septic rats stimulated [H-3]ryanodine binding. These data demonstrate that Ca2+-induced Ca2+ release from cardiac SR remained r elatively unaffected during the early phase, but was significantly impaired during the late phase of sepsis. The sepsis-induced impairment in SR Ca2release is a result of a quantitative reduction in the number of Ca2+ relea se channels. Furthermore, the reduction is associated with a mechanism invo lving a modification of membrane lipid profile in response to certain stimu li such as activation of phospholipase A(2).