Postextrasystolic potentiation in isolated rat myocardium: dependence on resting muscle length.

The degree and decay of postextrasystolic potentiation were studied in isolated rat papillary muscle at Lmax, the length at which developed force was maximum, and at shorter lengths. Potentiation curves were constructed relating the degree of potentiation with the time interval between any given extrasystole and the last steady-state beat. At all stimulation rates tested (ranging from 3 to 60/min), potentiation curves were found to be dependent on muscle length: at any interval the degree of potentiation increased progressively with decreasing initial muscle length. At all lengths and stimulation frequencies tested, the decay of potentiation was a function of the number of beats following the extrasystole. This relationship could be fitted by a single exponential equation. The exponential constant of the decay was significantly smaller at Lmax than at shorter lengths. These results suggest that muscle length affects more than one step of the E-C coupling processes.