Myosin content and myosin isoenzyme distribution in the heterotopic rat heart allograft.

Heterotopic cardiac transplants are vascularly perfused organs that can be used to study the regulation of myocardial protein content. Prior studies have demonstrated that cardiac isografts undergo marked atrophy with a decrease in weight and myosin content. In the present studies we have investigated the changes in size, myosin content and myosin isoenzyme distribution in the heterotopic cardiac allografts. Six days after transplantation allograft hearts were not spontaneously beating and histologically showed evidence of necrosis and cellular infiltration. Total heart weight (816 +/- 16 mg) and protein content (117 +/- 7 mg) were significantly greater in the allografts compared to in situ hearts (471 +/- 11 and 90 +/- 5 mg respectively, (P less than 0.01). In contrast to the increase in weight there was a simultaneous decrease in myosin ATPase (26%), the V1 isoform of the myosin isoenzyme (43%), and myosin content (53%) in the allograft heart. These studies demonstrate that similar to cardiac isografts, allograft hearts undergo a decrease in myosin content and a shift in myosin isoenzymes. In contrast to the marked atrophy of the cardiac isograft, the allograft heart weight is increased most likely due to rejection with cellular infiltration and an increased water content.