Myocardial membrane cholesterol: effects of ischaemia

Evidence has recently been presented that myocardial ischaemia is associated with a significant increased mitochondrial cholesterol content, suggesting a redistribution of cholesterol within the ischaemic cell (Rouslin et al. 1980, 1982). The aim of this study was therefore to determine the effects of different periods of ischaemia and reperfusion on the cholesterol content of myocardial mitochondria, sarcoplasmic reticulum and sarcolemma. Using the isolated perfused rat heart as experimental model, it was demonstrated that increasing periods of ischaemia (15-60 min) caused a progressive loss of cholesterol from the tissue as well as from the sarcolemma and sarcoplasmic reticulum, concomitant with a significant increase in mitochondrial cholesterol content. These compositional changes were associated with a marked increase in sarcolemmal and mitochondrial microviscosity, while that of the sarcoplasmic reticulum was reduced. To gain more insight into the mechanisms controlling intracellular cholesterol distribution, control and ischaemic hearts were perfused with either exogenous cholesterol or its precursor [U-14C]acetate as an indicator of endogenous cholesterol synthesis. Perfusion with exogenous cholesterol resulted in significant increases in the membrane cholesterol content of control hearts. However, hypoxic, low flow perfusion prevented cholesterol enrichment of the sarcolemmal and sarcoplasmic reticulum membranes, while the cholesterol content of the mitochondria was increased from 99.48 +/- 12.75 to 127.61 +/- 1.84 nmols/mg protein, indicating specific incorporation into this membrane system. Incorporation of [U-14C]acetate into cholesterol in the sarcoplasmic reticulum was increased by 120% in ischaemic conditions. However, a marked redistribution of newly synthesized cholesterol was observed within the ischaemic cell: under control conditions most of the labelled cholesterol was transferred to the sarcolemma and least to the mitochondria, while this distribution pattern was reversed in ischaemia. In view of the fact that exchange of cholesterol between membranes is affected by both phospholipid polar head-group composition and acyl chain length and saturation, it is suggested that prior ischaemia-induced membrane compositional changes might lead to intracellular cholesterol redistribution. Finally, to determine whether cholesterol loss affects sarcolemmal permeability, hearts enriched in sarcolemmal cholesterol were subjected to 15 or 30 min global ischaemia followed by reperfusion and the rate of enzyme release determined. However, enzyme release was similar in treated and untreated hearts, indicating that sarcolemmal cholesterol loss probably does not affect its permeability.