Relationship between inotropy and relaxation in rat myocardium.

Myocardial mechanics of 56 rat papillary muscles were investigated in various inotropic states, i.e., early postpartum (1st day, 1st mo) and, in adult, by altering external calcium concentration, [Ca2+]0, stimulation frequency (F), and initial muscle length (L). In all these protocols, the decrease of the inotropic level paralleled a progressive diminution in load sensitivity of relaxation. In such protocols modifying cardiac contractility, peak shortening velocity of the twitch with preload only at Lmax (Vcmax) was always less depressed than peak lengthening velocity (Vrmax), and the ratio R1 = Vcmax/Vrmax always increased. This increase of R1 was linearly related with the decrease of the maximum unloaded shortening velocity (Vmax). The ratio R2 (positive over negative peak force derivatives of the isometric twich at Lmax) was significantly lowered in newborn and was lowered in adult for [Ca2+]0 less than or equal to 1 mM and L less than or equal to 94% Lmax. For a given change in Vmax, each of the experimental protocols gave rise to specific changes in contraction-relaxation coupling tested at low load (R1) and at heavy load (R2).