Length-dependent changes in myocardial contractile state.

PARMLEY, WILLIAM W., AND LEONARD CHUCK. Length-dependent changes in myocardial contractile state. Am. J. Physiol. 224(5) : 11951199. 1973.-The present study describes an alteration of myocardial contractile state which is intimately associated with changes in muscle length. Seventy-one right ventricular cat papillary muscles were studied isometrically or isotonically in vitro. When muscle length was increased from 10 y0 less than L TnaX up to L mIlx, there was an’ immediate rise in isometric force due to the Starling mechanism. This was followed by a gradual secondary increase in force (average 25 %), which occurred over the subsequent 10 min. T’his effect was reversible, since a reduction in muscle length to lOY, less than L mBX produced a fall in force due to the Starling mechanism, followed by a gradual secondary reduction in force over the next 10 min to the original base line. This slow secondary change in contractile state was unrelated to the cross-sectional area of the muscles or catecholamine depletion by reserpine and persisted during hypoxia. The same effect was seen during isotonic contractions with a preload only, suggesting that it was primarily a length-dependent, rather than force-dependent, phenomenon. This study indicates that one cannot simply relate muscle performance to length without knowledge of the previous history.