Ectopic heart transplant: A model of whole heart atrophy

The ectopically transplanted heart, because it does not have the role of support of the animal's circulation, is mechanically unloaded and as such should undergo atrophy. Characteristics of myocardial atrophy in unloaded papillary muscles include an increase in the passive stiffness of the muscle and a shift to the left of the force-velocity and active length-tension relationships (G. Cooper & R.J. Tomanek, Circ. Res. 50: 788-798, 1982). We tested whether similar changes occur in the whole heart when mechanically unloaded. Donor rabbit hearts were arrested and cooled with cardioplegia and transplanted into the abdomens of recipients (donor aorta to recipient abdominal aorta, donor pulmonary artery to recipient inferior vena cava). In this model the only volume pumped by the left ventricle is that due to thebesian flow. After 96 hours, the function of the left ventricle was evaluated using Langendorff techniques and compared to controls (not transplanted) and to hearts that were transplanted but evaluated immediately after transplantation. Peak systolic pressure and associated volume and diastolic pressure were recorded. The transplant procedure resulted in an 18% loss of systolic function with no change in diastolic function. After 96 hours, systolic function was further depressed to 75% of control. Diastolic function, indicated by the ventricular volume at which peak systolic function occurred, was also depressed to 75% of control. These data indicate that the mechanically unloaded heart has increased stiffness and decreased active length-tension characteristics, indicators of atrophy.