CALCIOTROPIC HORMONES AND THE AGING BONE

The aging of bone is conditioned by intrinsic and extrinsic factors. The decrease of bone density, of cortical and of trabecular bone volume with age goes along with an increase of interstitial trabecular bone (Courpron, 1981), in addition to other intrinsic structural changes. Interstitial bone has a lower cellularity and a lower metabolic activity than the bone tissue which is closer to the trabecular surface. This increase of interstitial bone serves as an example for the supposed decrease of bone cellularity, cellular activity, and hormonal receptors with age in bone. Therefore, the "playground" for calciotropic hormones is reduced in advanced age, independently from the hormonal changes which will be discussed thereafter. The relative role of this decrease of the target organ with age is not yet fully understood. The process of aging can interfere with each step of calcium metabolism from calcium intake and absorption, vitamin D intake and absorption to vitamin D production, hydrolysation and action, as well as to secretion and action of calciotropic hormones such as PTH, calcitonine, and gonadal steroids. © 1990.