Pressure overload changes cardiac skinned-fiber mechanics in rats, not in guinea pigs

The mechanical properties of detergent-treated skinned fibers from pressure-overloaded rat and guinea pig hearts have been compared with those of sham-operated animals. Overload was obtained 4 wk after abdominal aortic stenosis, an intervention that increases ventricular weight by 56% in rats and 57% in guinea pigs. The time constant (T, in ms) for tension recovery after a quick stretch was significantly lower in normal guinea pig than in rat. It was lengthened by the process of overload in both species, but this was much more pronounced in rats where T increases by 84% than in guinea pig where it was only slightly augmented by 14% for a doubling of the heart weight. By contrast the maximum tension obtained at pCa 4.5, the stiffness, and the sensitivity to calcium of the fibers were unmodified by chronic overload. In rat, not in guinea pig, a slight decrease in MgATP sensitivity was also observed, whereas no change in creatine kinase efficiency was seen. These results are interpreted as indicating that the slowing of the turnover rate of cross-bridge cycling explains the drop in shortening velocity observed on papillary muscles in rat but not in guinea pig; a species in which membrane modifications must be predominant in the process of adaptation.