Life extension in heart disease: an animal model

Chronic circadian desynchronization decreases the survival of animals with cardiomyopathic heart disease

Shift work is associated with increased cardiovascular morbidity and mortality. Whereas it has been suggested that continuous shifting of the circadian clock/sleep-wake cycle may have negative effects on health, there is very little experimental evidence to support such a hypothesis. Cardiomyopathic Syrian hamsters were either maintained on a fixed light-dark (LD) cycle (n = 31) or were subjected to a 12-h phase shift in the LD cycle on a weekly basis (n = 32). The duration of the life span was recorded for each animal. Chronic reversal of the external LD cycle at weekly intervals resulted in a significant decrease in the survival time in cardiomyopathic hamsters with the median life span being reduced by 11%. Disrupting normal circadian rhythmicity in an animal susceptible to early mortality due to cardiac disease results in a further decrease in longevity. The deleterious effects of the chronic phase shifts in the LD cycle in cardiomyopathic hamsters may be related to reports of increased cardiovascular morbidity and mortality in humans engaged in shift work.

Phototherapeutic effects in hamsters with heart disease

In previous experiments we have shown that hamsters with inherited heart disease--cardiomyopathic hamsters (CMHs)--live longer if they spend their lives in an environment devoid of time cues. The purpose of this experiment was to test the several hypotheses by which life in constant light could extend life in CMHs. To do this, CMHs were allowed to spend their lives in one of six different lighting conditions: constant light, LD 12:12, LD 23:1, LD 1:23, LD 1:23.2, and LD 1:23.6. The only schedule to produce a significant extension of life was LD 1:23.6; in contrast to LD 1:23.2, this schedule is photostimulatory. Of the hypotheses tested to evaluate the life-enhancing effects of constant light, support was found for only the one stating that non-24-h LD regimens are health enhancing. Although some evidence was found relating testicular size to life span, dissociations between these variables indicate that testicular function does not play an overriding role in modulating the phototherapeutic effects.

Effect of life in a constant light environment on the course of hypertension in Dahl rats

An important goal for biobehavioral scientists is to evaluate treatments that might extend life in the presence and absence of disease. The prototypic example of such a treatment is food restriction. Importantly, we have shown that exposure to a life-long environment of constant light extends life in hamsters with severe, life-threatening heart disease. The purpose of this experiment was to determine whether constant light would also extend the life of rats with an inherited form of hypertension. Constant light neither delayed the progression of hypertension nor extended life in this model. These data suggest that constant light may have a more limited use as an experimental therapeutic modality to extend life as compared to food restriction.

Cardiovascular aging in Syrian hamsters: similarities between normal aging and disease

The functional status of the cardiovascular system in health and cardiomyopathic (CM) hamsters was assessed throughout their lives by measuring heart weight and fluid in the pleural and peritoneal cavities. An index of cardiovascular age was developed from a multiple regression model of changes in these variables with chronological age. This index showed parallel changes in healthy and CM hamsters with increasing age, but CM hamsters with shorter lifespans underwent the changes at earlier ages. It was also a better predictor of viability than chronological age. The cardiovascular age index correctly predicted the early death of those hamsters which died before the average death age, whereas they should have been alive according to their chronological age. Conversely, hamsters which lived beyond the average death age had cardiovascular ages younger than the average death age, whereas their chronological ages erroneously indicated they should have been dead. This index may have been able to assess viability because it was correlated with histopathological signs of congestive heart failure in both strains of hamsters, as well as with the total amount of pathology found in them. Hamsters which die naturally at a particular age should have older cardiovascular indices than those sacrificed at the same age, and CM hamsters which died at 11-13 months of age did have older cardiovascular ages than hamsters that were sacrificed at these ages. Three experiment examined the effects of various treatments on the index of cardiovascular age. Life in constant light decreased the cardiovascular age of CM hamsters by 30% and extended life by 25%. Chronic digitalis treatment will improve cardiovascular performance, and it prevented increases in cardiovascular age during the end stages of heart failure. Finally, life-threatening chronic stress increased cardiovascular age in CM hamsters, which suggested that severe stress brought CM hamsters nearer to death. However, it was not possible to determine whether the effects of these treatments were on aging or health. Thus, the data suggested that aging, at least in hamsters, may be closely related to the natural development of heart failure which represents a common end stage of life in both cardiomyopathic hamsters and very old, supposedly healthy hamsters.

Gonadal function during prolongation of life produced by constant light in hamsters with heart failure

In a prior experiment, we showed that living in constant light prolonged the lives of cardiomyopathic (CM) hamsters dying of heart failure. One possible explanation for this therapeutic effect related to the physiological effects of living in short and long days. The control hamsters for our constant light experiment lived in light/dark 12:12, a short day regimen which produces inhibitory effects on the gonadal function of healthy hamsters. To see whether the CM hamster responded to short days like healthy hamsters, we measured gonadal and seminal vesicle mass and plasma testosterone at 1 year of age in CM hamsters raised in the 2 light conditions and assessed testicular size repeatedly over the lives in a second group of hamsters raised in the 2 light conditions. As in healthy hamsters, we found gonadal function in CM hamsters to be greatly inhibited by LD 12:12. Importantly, we replicated our finding that living in constant light prolongs the life of CM hamsters in heart failure. We also found that the stress and trauma inherent in our repeatedly using surgery to assess testicular size acted as an additional risk factor controlling the lifespan of these animals. Our findings suggest that a photoperiodic effect of day length should be considered as one possible explanation of the mechanism of prolongation of life produced in hamsters with heart failure by constant light.