Relationship between Plasma Oamolality and Plasma Concentration of Antidiuretic Hormone in Normal Subjects, Patients with Chronic Renal Diseases, and Patients with Central Diabetes Insipidus

Abstract

Using improved bioassay techniques, plasma concentration of the anti-diuretic hormone (ADH) was determined in thirteen normal subjects, seven patients with chronic renal diseases, and ten patients with central diabetes insipidus. Three different experimental conditions were examined, at two hr after lunch or supper at rest (standard conditions); after overnight restriction of water intake (dehydrated conditions) and after loading excess water (over-hydrated conditions). Osmolality of plasma and urine was also determined. The mean values for ADH concentration in plasma of normal subjects under the standard, dehydrated and over-hydrated conditions were 3.2 ± 0.5, 4.0 ± 0.8, and 0.8 ± 0.2 μU/ml, respectively. Collected data obtained from normal subjects revealed that plasma ADH concentration sharply rose when plasma osmolality exceeded 270 mOsm/kg, but apparent plasma threshold osmolality which caused U/P osmolar ratio of more than unity was around 275-285 mOsm/kg. When compared the ADH levels between standard and dehydrated conditions for individual subjects there were significant increases after the dehydration despite unchanged plasma osmolality, suggesting that some factor or factors other than plasma osmolality may modify the relationship beteen plasma ADH concentration and osmolality. In patients with chronic renal diseases, plasma ADH concentration for the standard and dehydrated states were 3.3 ± 0.9 and 4.9 ± 1.3 μU/ml, respectively. These patients responded to dehydration with an increase in ADH concentration but with much less increase in U/P osmolar ratio. The findings are in accord with the pathophysiological conditions of these patients with impaired renal concentrating capacity without impaired ADH releasing mechanism. In patients with central diabetes insipidus, plasma ADH concentra-tion under the standard and dehydrated states (after 3% decrease in body weight) were 0.4±0.1 and 0.9 ± 0.2 μU/ml, respectively. These patients responded to the dehydration test with a remarkable increase in plasma osmolality (288.6 ± 5.3 to 300 ± 7.3 mOsm/kg) without significant increase in plasma ADH concentration.