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Feedback control, both negative and positive, is a fundamental feature of biological systems. Some of these systems strive to achieve a state of equilibrium or 'homeostasis'. The major endocrine systems are regulated by negative feedback, a process believed to maintain hormonal levels within a relatively narrow range. Positive feedback is often thought to have a destabilizing effect. Here, we present a 'principle of homeostasis,' which makes use of both positive and negative feedback loops. To test the hypothesis that this homeostatic concept is valid for the regulation of cortisol, we assessed experimental data in humans with different conditions (gender, obesity, endocrine disorders, medication) and analyzed these data by a novel computational approach. We showed that all obtained data sets were in agreement with the presented concept of homeostasis in the hypothalamus-pituitary-adrenal axis. According to this concept, a homeostatic system can stabilize itself with the help of a positive feedback loop. The brain mineralocorticoid and glucocorticoid receptors--with their known characteristics--fulfill the key functions in the homeostatic concept: binding cortisol with high and low affinities, acting in opposing manners, and mediating feedback effects on cortisol. This study supports the interaction between positive and negative feedback loops in the hypothalamus-pituitary-adrenal system and in this way sheds new light on the function of dual receptor regulation. Current knowledge suggests that this principle of homeostasis could also apply to other biological systems. mathematical modeling; mineralocorticoid receptor; glucocorticoid receptor; obesity doi:10.1152/ajpregu.00907.2006

A study conducted among 20 normal controls of two different age groups, under 30 and over 60, and 20 age-matched patients with major depression, reveals that depressed patients of all ages have higher hypercortisolemia and heart rates. Basal hypothalamic-pituitary-adrenal system activity was the same for both young and older healthy controls but older controls had higher stress-induced hormonal secretion during the cognitive challenge. Baseline hypercortisolism leads to a dissociation between the cardiovascular and hormonal responses to a mental challenge in depressed patients but not in normal aged persons.

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Copyright of Oral & Maxillofacial Surgery is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Hypoxic respiratory diseases are frequently accompanied by glucose intolerance. We examined whether hypoxia is a cause of glucose intolerance in healthy subjects. In a double-blind within-subject crossover design, hypoxic versus normoxic conditions were induced in 14 healthy men for 30 minutes by decreasing oxygen saturation to 75% (versus 96% in control subjects) under the conditions of a euglycemic clamp. The rate of dextrose infusion needed to maintain stable blood glucose levels was monitored. Neurohormonal stress response was evaluated by measuring catecholamine and cortisol concentrations as well as cardiovascular parameters, and symptoms of anxiety. To differentiate between the effects of stress hormonal response, and hypoxia itself, on glucose intolerance, we performed hypoglycemic clamps as a nonspecific control. We found a significant decrease in dextrose infusion rate over a period of 150 minutes after the start of hypoxia (p < 0.01). Hypoxia also increased plasma epinephrine concentration (p < 0.01), heart rate (p < 0.01), and symptoms of anxiety (p < 0.05), whereas the other parameters remained unaffected. Glucose intolerance was closely comparable between hypoxic and hypoglycemic conditions (p < 0.9) despite clear differences in stress hormonal responses. Hypoxia acutely causes glucose intolerance. One of the factors mediating this effect could be an elevated release of epinephrine. [ABSTRACT FROM AUTHOR]

OBJECTIVE: The objective of this study was to examine potential clinical predictors of bone density in patients with eating disorders. METHOD: We studied 137 women with anorexia nervosa, bulimia nervosa, or eating disorder not otherwise specified (NOS) after admission to the hospital. Clinical data of patients were collected by clinical interview and standardized questionnaires. Bone mineral density of the lumbar spine was measured by dual energy x-ray absorptiometry. RESULTS: Lumbar bone density was significantly correlated to present and past minimum weight even after correction for height and age. Other factors did not reach significance. CONCLUSION: Normalized present and past weight is the best predictor of lumbar bone density in patients with eating disorders. Factors like reduced caloric intake, binge eating, vomiting, menstrual status, and use of estrogen, laxatives, and nicotine seem to have no independent effect on bone density in this group of patients. [ABSTRACT FROM AUTHOR]