Water excretion in the elderly.

Osmoreceptor sensitivity is enhanced in healthy elderly subjects and AVP secretion is increased, relative to that of younger subjects, when plasma osmolality rises. Increased AVP secretion/unit increase in plasma tonicity reflects a decrease in collecting tubule sensitivity to AVP by an as yet unknown mechanism in the aged kidney. This change in sensitivity is not completely offset by increased ADH release, so that maximum Uosm achievable under hydropenic conditions (concentrating ability) is reduced in the elderly. CH2O in older subjects decreases in proportion to the fall in GFR; thus, CH2O is intact in older subjects with preserved GFR. In subjects with age-related reductions in GFR, minimal Uosm achievable is usually less than 100 mOsm per kg H2O and thus usually sufficient to meet the demands of solute-free water intake so that plasma hypo-osmolarity does not result. Increasing exposure of the elderly to pharmacologic agents that reduce CH2O is primarily responsible for the impression that aged patients are at increased risk for hyponatremia.