Renal physiology of nocturia.

Renal function, diurnal fluctuations in arginine vasopressin (AVP) secretion, sex, and advanced age affect urine formation and may contribute to nocturia. Renal effects of AVP are mediated by AVP V2 receptors in the kidney collecting duct. Changes in AVP concentration have the greatest relative effects on urine volume when AVP levels are low; therefore small changes can have a large effect on renal water excretion. AVP is the major regulator of water excretion by the kidneys, and AVP levels have been shown to affect nocturnal voiding. Results of several studies show that patients with nocturia had no significant variation in plasma AVP, whereas patients without nocturia had significant diurnal variation in plasma AVP. The V2 receptor gene is located on the X chromosome, which has important sex-specific consequences. For example, mutations in the V2 gene can cause nephrogenic diabetes insipidus, predominantly in men. Age-related changes in water metabolism are associated with overall body composition, kidney, and brain. Older people generally experience decreased extracellular fluid and plasma volume, which leads to increased adverse consequences from net body water gain or loss. Renal function declines with age, and the ability to concentrate urine and conserve sodium is reduced in the elderly. Thirst perception is also decreased in the elderly, who, compared with younger people, tend to hypersecrete AVP in response to higher plasma osmolality, possibly resulting in hyponatremia. These aspects of renal physiology should be considered when antidiuretic drugs are prescribed for the treatment of nocturia.