Contact sites between the inner and outer mitochondrial membranes in stunned versus hibernating myocardium

This study characterizes the energy state and the influence of calcium on myocardial stunning and chronic hibernation via the quantification of a calcium-sensitive phenomenon known as mitochondrial contact sites. For stunning, the left anterior descending artery of mongrel dogs was occluded for 15 minutes, followed by a 150-minute reperfusion; for chronic hibernation, we used human biopsies obtained during coronary artery bypass graft (CABG) from viable (positron emission tomography-controlled) asynergic areas. Both sample groups were processed for electron microscopy, and the ratio of surface densities of contact sites to mitochondrial membranes was quantified with morphometry. Therefore a cycloidal pattern was superimposed on the electron micrographs, and the ratio between the total number of intersections between cycloids and contact sites and the total number of intersections between cycloids and mitochondrial membranes is the ratio of surface densities (Ss). In stunned cells, Ss = 0.46 ± 0.06, which is significantly higher than the ratio in the normokinetic cells, Ss = 0.355 ± 0.003, although the general ultrastructure of the subcellular compartments is practically identical to those in the normoxic area. The distinction between cells affected by chronic hibernation and normal cells was based on structural criteria. The ratio of surface densities, expressed Ss = 0.27 ± 0.05, was significantly lower than the ratio in the normoxic area (Ss = 0.356 ± 0.005). The high ratio of surface densities in the stunned cells lends credibility to the notion that stunning implies an increased intracellular calcium content and energy demand, whereas hibernation might be a kind of low demand-low supply situation with a low intracellular calcium level.