MECHANICAL RESTITUTION AND RECIRCULATION FRACTION IN CARDIAC MYOCYTESAND LEFT-VENTRICULAR MUSCLE OF ADULT-RATS

Unloaded cell shortening was measured in electrically stimulated myocy tes from adult rat hearts to compare the contractile response to stimu lation with that in isometrically contracting left ventricular papilla ry muscles under similar experimental conditions, but preloaded to pro duce maximum twitch tension. Mechanical restitution in cells followed a biexponential function with time constants of 0.19+/-0.03 s and 36.4 +/-10.2 s (7 cells from 5 hearts, n = 7/5). The time constants for pap illary muscles were 0.58+/-0.05 s and 14.6+/-1.0 s(n = 6/6). in myocyt es, maximum post-rest potentiation occurred after 30 to 60 s of rest. The potentiation after 60 s of rest was 2.48+/-0.31 times the steady-s tate in cells and 2.63+/-0.16 in papillary muscles. Recirculation frac tion of C2+ as calculated from the decay of post-rest potentiation was 0.84+/-0.04 in single cells and 0.59+/-0.02 in papillary muscles (p < 0.005). Caffeine (3mM) abolished post-rest potentiation in both types of preparations. The numerical values for the time constants of mecha nical restitution, potentiation factor and recirculation fraction in p apillary muscles did not depend on preload. It is concluded that inter val-dependent changes of contractility are preserved in single cardiac cells but the kinetics of decay of potentiation appear to have change d quantitatively.