Mechanisms of age-related endocrine alterations. Part II.

Aging is associated with a myriad of hormonal changes. The mechanisms underlying these changes are variable. Aging is associated with anatomic changes of the endocrine glands, notably as a result of programmed cell death, autoimmune mediated destruction of the gland, or neoplastic transformation of glandular tissue. Age-related changes in hormonal secretion can also be secondary to physiological changes in circadian and seasonal rhythm, or in frequency or height of hormonal pulses. Some of these changes in hormone secretion are compensatory for the age-related reduction in hormonal clearance, and others are the results of changes in glandular sensitivity to secretagogues or inhibitory stimuli. Age-related changes in hormone action can also be the result of altered bioactivity of hormones, altered transport of hormones to binding receptor sites, altered hormone-receptor interactions or due to postreceptor changes. Within the context of the latter, aging is associated with alterations in plasma membrane properties and intrinsic changes in cellular enzyme activity, and changes in calcium mobilisation and gene expression. Some of these changes are directly related to aging while others are secondary to age-associated diseases and changes in nutritional state. The predominant mechanism in each age-related change varies with the hormone studied. The age-related changes in the mechanisms of endocrine dysfunction are often tissue- and species-specific. Part I of this article discussed the anatomical changes of endocrine glands, changes in hormone secretion, and the changes in the kinetics of hormonal clearance. Part II discusses the changes in hormone action with age.