Muscle mechanics and Ca2+ transport in atrophic heart transplants in rat.

Isometric contractile activity and force-frequency relationship were investigated in heterotopically isotransplanted rat hearts that performed minimal external work for 8 wk and underwent atrophy. No changes were found in the isometric contraction phase of the papillary muscles from the transplants when compared with the hearts of recipient animals that carry full hemodynamic load. However, the relaxation phase of the transplants was impaired at higher frequencies (greater than 24 stimuli/min) of stimulation and at higher levels of calcium (greater than 2.0 mM Ca2+) in the medium. When Ca2+-accumulating activities of cardiac microsomes containing fragments of the sarcoplasmic reticulum (SR) were studied, a significant decrease was seen in the transplanted hearts in comparison with the host hearts. This decrease in Ca2+-uptake activity was observed at different times of incubation as well as different concentrations of free Ca2+ (10(-7) to 10(-5) M Ca2+). Although Ca2+-stimulated ATPase activity did not change, alterations in the phospholipid contents of the SR vesicles from the transplanted hearts were observed. These results provide some evidence for a depression in the SR Ca2+ uptake in transplanted hearts undergoing atrophy and may explain the underlying alteration in the relaxation phase of these hearts.