To determine whether the mechanical and structural characteristics of the myocardium from the left and right ventricles are comparable in the adult rat and whether aging affects the two ventricles in a similar manner, the left and right posterior papillary muscles of rats at 10-11 and 19-20 mo of age were studied in terms of function and quantitative morphology. At 10-11 mo of age, the duration of isometric contraction was shorter and resting tension higher in the right muscle than in the left muscle. Isotonic velocity of shortening and relengthening were quicker in the left, but time-to-peak shortening was faster in the right. With aging (19-20 mo), velocity of shortening fell in the left ventricle while duration of contraction became prolonged. No such age-related effects were found in the right papillary muscle. On a structural basis, myocardial damage in the form of replacement scarring was present in the left and right muscles at 10-11 mo of age. However, the left muscle was more severely injured, and this difference persisted with age because a comparable increase in replacement fibrosis occurred in both muscles. It was concluded that cardiac performance is distinctly different in the left and right papillary muscles and aging exerts its detrimental impact on the left more than in the right myocardium, an observation that is further supported by the magnitude of tissue injury.

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To determine whether digoxin protects the myocardium during the initial phases of hypertension and diabetes combined, adult male Wistar rats with two-kidney, one-clip renal hypertension and streptozotocin-induced diabetes mellitus were treated with digoxin (500 micrograms.kg-1.day-1) by gavage for 10 wk immediately after the onset of hypertension and diabetes. Systemic arterial blood pressures, ventricular pressures, the first time derivative of left ventricular pressure, diastolic wall stress, and the quantitative analysis of the number and distribution of myocardial lesions and capillary density of the myocardium were measured. In comparison to untreated hypertensive-diabetic animals, digoxin-treated rats showed a lesser elevation in left ventricular end-diastolic pressure and diastolic and systolic wall stress despite comparable degrees of hypertension and blood glucose levels. In addition, chamber diameter was smaller and the diffusion distance for oxygen was within normal values in animals treated with this glycoside. However, the numerical density of the foci of replacement fibrosis was similar to that found in untreated hypertensive-diabetic animals. In conclusion, digoxin reduces the magnitude of ventricular remodeling and diastolic wall stress in this model of hypertension and diabetes.

To determine whether the variation in the magnitude of work load sustained by the left and right ventricles during adulthood and senescence affects the load-dependent aspect of relaxation, posterior papillary muscles from the left and right ventricles of rats at 4, 10, and 20 months of age were studied under variably loaded conditions in vitro. Because of differences between the life spans of Fischer and Sprague-Dawley rats, the functional characteristics of relaxation were investigated to evaluate the possibility of a differential age-associated response in these two strains of animals. The kinetic performance of the diastolic phase of myocardial contraction was measured by assessing the relative time during which load bearing occurred in a series of afterloaded isotonic twitches. This measurement was expressed as the ratio of the duration of afterloaded isotonic shortening and relengthening to the time required for isometric force to decline to the same level during isometric relaxation. A ratio of less than unity identified a load-dependent state whereas a value greater than one reflected a load-independent condition. Results showed that the right myocardium was completely load independent whereas the left myocardium was fully load dependent at all physiological afterloads. Aging reduced the load independence of the right ventricle and the load dependence of the left ventricle in Fischer rats. In contrast, no aging effect on the properties of afterloaded isotonic relaxation was seen in Sprague-Dawley rats. In conclusion, distinct differences exist in the mechanical dynamics of inactivation between the left and right ventricular myocardium. Aging reduced these variations in Fischer rats but had no apparent influence in Sprague-Dawley animals up to 20 months after birth.

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