Role of the pituitary and thyroid glands in the decline of minimal O2 consumption with age.

Resting O2 consumption rate (BMR) or minimal O2 consumption rate (MOC) declines with age. Data are presented that suggest that a newly described function of the pituitary may be responsible for a considerable part of the total 75% decline in the MOC with age. The new function appears to decrease the responsiveness of peripheral tissues to thyroid hormones. Response curves to injected thyroxine indicated that immature rats were three times more responsive to thyroxine than adult rats. All the major endocrine ablations were performed in this and earlier work, and only pituitary ablation (a) restored in adults part of the responsiveness to thyroxine found in immature rats and (b) arrested the normal age-associated decrease in responsiveness to thyroxine in immature rats. Bovine pituitary extracts were found that decreased the responsiveness of immature rats to thyroxine. Experiments with the new pituitary function suggested a possible endocrine mechanism to explain why partial starvation doubled the lifespan for rats only when started before puberty.