Mechanical restitution of the rat papillary muscle.

The experiments were performed on isometrically contracting rat papillary muscles paced at 1.0 Hz and at a temperature of 37 degrees C. The contractile response of a test contraction continuously rose when the duration of the preceding stimulus interval was gradually increased from 0.2 s. A maximum value was seen at intervals of 60--120 s. This phenomenon was called mechanical restitution of the papillary muscle. The insertion of a priming beat before the test contraction in order to increase the amount of contractile calcium elevated the mechanical restitution curve but it did not change the maximum contractile force seen after 60--120 s intervals. Lowering the extracellular calcium concentration from 2.0 mM to 1.0 mM, however, depressed this maximum contractile force to about 50%. The mechanical restitution is thought to reflect inflow of activator calcium to a cellular store, from which it is later released in response to the action potential. By using two test contractions a simple method is described to estimate the recirculating fraction of activator calcium between beats in this preparation. In 11 preparations the recirculating fraction of activator calcium was 0.72 +/- 0.03 (mean +/- SE). The results are consistent with the view that the same model of metabolism of activator calcium as previously proposed for rabbit papillary muscles is also applicable to the rat heart.