Doctoral Dissertations

Date of Award

5-1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Human Ecology

Major Professor

Roy E. Beauchene

Committee Members

Dileep Sachan, James Bailey, Edward Howley

Abstract

The impact of long term food restriction and exercise on the pituitary-thyroid axis and growth was examined in aging male Wistar rats. Four groups were evaluated: ad libitum-fed, sedentary controls (A); ad libitum-fed, exercised (AE); restricted-fed, sedentary (R); and restricted-fed, exercised (RE). Restriction and exercise regimens were implemented at weaning and continued for the life of the animals. Pituitary-thyroid axis functioning was assessed by serum concentration measurements of thyroid stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) at 12, 16, 20, 24, and 28 months of age. Renal cortical NaK ATPase activity was evaluated at concurrent intervals. Aging resulted in a decline in circulatory levels of T4 and T3, and in the T4 to T3 ratio. TSH values increased with age; this increase was delayed by restriction. Restriction also slowed the age-associated decrease in serum T4 and T3 levels and lowered absolute T3 and TSH levels. Exercise resulted in lower absolute serum T3 concentration, but had little effect on serum T4 levels. Exercise did not slow the age-associated decline in T4 or T3 serum concentration in either ad libitum-fed or restricted-fed groups. Renal cortex NaK ATPase activity decreased with age. This effect seems, in part, attributable to thyroid hormone status. T3 serum concentration was positively correlated with body weight; T4 was not. Food restriction significantly lowered the predicted mature body weight and terminal body weight. Exercise had no effect upon terminal body weight in AE or RE but increased predicted mature body weight in RE. Food restriction decreased the relative rate of maturation; exercise further decreased the relative rate of maturation in restricted animals. The results of this study indicate that the pituitary remains capable of mounting an appropriate response with age and suggests that the thyroid gland itself is central to the pituitary-thyroid axis decline with aging. The pituitary-thyroid axis is responsive to alterations in food intake and energy expenditure. Its age-associated decline may be slowed by food restriction.

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