CIRCADIAN, ENDOCRINE, AND METABOLIC EFFECTS OF PROLONGED BEDREST: TWO 56-DAY BEDREST STUDIES

摘要

Two bedrest studies of 56 days each, involving a total of 20 male subjects aged 20-26 (one subject aged 40), have been conducted to evaluate the effects of prolonged bedrest on circadian synchrony and endocrine and metabolic function. Measurements included the pituitary-adrenal, thyroid, parathyroid, insulin-glucose-growth hormones, catecholamine excre¬tion, body temperature, and heart rate. The results indicate that a rigorous regimen of isotonic/isometric exercise did not prevent the endocrine and metabolic effects of prolonged bedrest. Changes in circadian, endocrine, and metabolic func¬tions in bedrest appear to be due to changes in hydrostatic pressure and lack of postural cues rather than to inactivity, confinement, or the bleeding schedule. Changes in circulating metabolic and endocrine parameters are unreliable if mea¬sured once per day because their amplitude and time of peak of their diurnal fluctuations are altered during bedrest. Therefore, data should be expressed as units/24 hours. Recovery periods up to 20 days are insufficient for full recovery from 56 days of bedrest. Bedrest beyond 42 days results in periodic hypoglycemia, possibly in response to meals, which may warrant modification of meal composition.nProlonged bedrest, particularly beyond 24 days, results in rhythm desynchronization in spite of well-regulated light/ dark cycles, temperature, humidity, activity, and meal times and meal composition and in increased lability of all endocrine parameters measured.nIt also results in an apparent insensitivity of the glucose response to insulin, of Cortisol secretion to ACTH, and of growth hormone secretion to hypoglycemia. This may be due to an effect of bedrest on the number or sensitivity of target organ receptors;it may reflect a change in radioimmunoassay able levels of the peptide hormones, or it may result from an alteration of the central nervous system's input/feedback integrating mechanisms.