We studied the effect of an isolated increase in preload on isovolumetric relaxation in the intact dog heart and isometric relaxation in isolated cardiac muscle (dog and rat) preparations. In eight anesthetized dogs, 8 to 12 ml of blood was infused into the left ventricle during a single diastole. The exponential time constant (T) of isovolumetric relaxation was measured in single-beat experiments in which the left ventricular systolic pressure increased (112 ± 2 to 128 ± 3 mm Hg; p < .05, n =

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... 2). In a second series of experiments, left ventricular systolic pressure was held constant (109 ± 2 to 107 + 2 mm Hg; p = NS, n = 23) by simultaneous ventricular infusion and aortic unloading. In the first protocol, T increased from 28.0 ± 0.4 to 30.7 ± 0.4 msec (p < .05), whereas in the second protocol (constant systolic pressure) there was no change in T. The time course of isometric relaxation was also studied in six rat left ventricular papillary muscles and four dog right ventricular trabecular muscles. Preload was varied from 30% to 100% of the peak of the isometric length-tension curve in each muscle. Over this wide range of preload, the isometric force decline recordings were superimposable as long as the comparisons were made at equal levels of total load. Thus an isolated increase in preload does not influence the time course of isovolumetric relaxation. Circulation 73, No. 5, 103773, No. 5, -104173, No. 5, , 1986. THE TIME COURSE of left ventricular isovolumetric pressure decline is determined by a series of interacting factors, including loading conditions, the inactivation rate of individual fibers, and the degree of fiber inhomogeneity within the wall of the ventricle. 1 These factors are continuously modulated by autonomic tone and metabolic events, and for these reasons it is difficult to interpret some of the reported changes in left ventricular isovolumetric relaxation rate. For example, CIRCULATION by guest on July 22, 2018 http://circ.ahajournals.org/ Downloaded from 8. Wiegner AW, Bing OHL: Isometric relaxation of rat myocardium at end-systolic fiber length. Circ Res 43: 865, 1978 9. Mirsky I: Assessment of diastolic function: suggested methods and future considerations.