[Influence of preload on myocardial relaxation in the rat].

Preload, which determines the initial muscle length, has proved to be a basic determinant of the muscle relaxation phase. The mechanical properties of the papillary muscle of Wistar rats (n = 20) were studied at different initial lengths (L): Lmax and 98 p. 100, 94 p. 100, 90 p. 100, 86 p. 100 and 82 p. 100 Lmax. In isometry, the relaxation phase was proportionally less sensitive to a reduction of preload than the contraction phase. When L decreased the peaks of maximal force derivative of the contraction phase (+dF.dt-1 max) and relaxation phase (-dF.dt-1 max) were linearly and significantly lowered at L = 94 p. 100 Lmax and L = 90 p. 100 Lmax respectively. Conversely, in isotonia the relaxation phase was much more sensitive to a reduction of preload than the contraction phase. On a muscle preloaded from 82 p. 100 Lmax to Lmax, the highest max Vr value (i.e. maximal speed of isotonic relaxation) was measured at Lmax, which is the apex of the Starling's isometric curve. When L decreased max Vr decreased linearly and significantly at 94 p. 100 Lmax. In contrast, the maximal speed of contraction (max Vc) increased between Lmax and 94 p. 100 Lmax, became maximal, then significantly decreased beyond 82 p. 100 Lmax. Thus, in the physiological range of the myocardium (Lmax - 85 p. 100 Lmax) the rat papillary muscle tended to maintain its maximal isotonic speed of contraction, or even to increase that speed, whereas the isometric contraction, isometric relaxation and isotonic relaxation phases were early and significantly depressed.(ABSTRACT TRUNCATED AT 250 WORDS)