The metabolic changes in the homografted canine heart were studied in order to define the biochemical alterations accompanying homograft rejection. In several experiments, homograft rejection was accelerated by prior sensitization of the host animal.

The homografted heart released pyruvate and lactate as well as malic dehydrogenase and aldolase. Extraction of glucose by the graft usually remained positive. During the accelerated rejection, the release of pyruvate and lactate was more pronounced, and even glucose appeared in increased concentrations in coronary vein blood. In many experiments the respiratory quotient of the transplanted heart as well as its glucose-oxygen extraction ratio were elevated.

It seemed likely that the elevated respiratory quotients were the result of conversion of carbohydrates to fat, since the injection of thiamine hydrochloride resulted in further elevation of the respiratory quotient and in an increased myocardial pyruvate extraction. Apparently, thiamine corrected a metabolic block at the level of the cocarboxylase.

The metabolic block or blocks present in the transplanted heart are likely to be the result of diminution in intracellular enzymes and coenzymes resulting from increased cellular permeability. The redox potential across the transplanted heart was positive, indicating the absence of anoxia. The results illustrate that glycolysis proceeds in the transplanted heart in the presence of oxygen.

Histopathologic and histochemical studies show the earliest lesion to be an accumulation of lymphocytes around vessels at 3 hours. Swelling of vascular endothelium occurs. By 5 hours a polar perivascular cellular infiltrate of lymphocytes, plasma cells, macrophages, and histiocytes exists. Changes following at 19 hours show the appearance of Aschoff- and Anitschkow-like cells. Granulomatous myocarditis which was first perivascular became interstitial with lymphocytic and histiocytic invasion of the myocardium.

After 8 days acceleration of swelling of vascular endothelium and granulomatous lesions were observed and necrosis of the myocardium was prominent.

Endothelial hyperplasia occurred at 14 days. In the accelerated reaction these changes were intensified and necrosis began as early as 4 hours after grafting.

Histochemical changes of DPNH diaphorase, lactic, malic, and succinic dehydrogenase showed only significant diminution of malic dehydrogenase in the cardiac muscle which was concurrent with the increase of this enzyme in the serum.

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