title: Loss of adaptive mechanisms during aging.
abstract: Factors contributing to modifications in the capability for enzyme adaptation as an expression of aging are reviewed. Specific examples of altered enzyme adaptations during aging include the responses of hepatic glucokinase activity to glucose and hepatic tyrosine aminotransferase activity to starvation in Sprague-Dawley rats. These impaired enzyme adaptations apparently are not the consequence of alterations in hepatic function during aging. Instead, they reflect disturbances in extrahepatic hormonal regulatory mechanisms. Specific examples include modifications in the control of circulating levels of insulin glucagon, corticosteroids, and thyroid hormones. Age-dependent changes in the regulation of circulating levels of insulin probably originate within the impaired ability of pancreatic islets of Langerhans to secrete the hormone in response to glucose. The rationale for exploiting this experimental approach as a means to understand biological aging is discussed.
mesh_terms: D000222:Adaptation, Physiological; D000375:Aging; D000818:Animals; D003345:Corticosterone / Q000502:physiology; D005934:Glucagon / Q000502:physiology; D005941:Glucokinase / Q000502:physiology; D007328:Insulin / Q000502:physiology; D008099:Liver / Q000201:enzymology*; D008136:Longevity; D008297:Male; D051381:Rats; D013217:Starvation / Q000201:enzymology; D013963:Thyroid Hormones / Q000502:physiology; D014444:Tyrosine Transaminase / Q000502:physiology
chemical_list: D007328:Insulin; D013963:Thyroid Hormones; D005934:Glucagon; D014444:Tyrosine Transaminase; D005941:Glucokinase; D003345:Corticosterone