Reversible changes in osmoregulation of vasopressin release due to impaired water excretion

Studies of renal water handling and the effects of altered hydration and posture on the osmoregulation of vasopressin release were performed on a chronically hyponatremic patient with complete cervical spinal cord transection at the C-5 level. Acute oral water loading studies showed marked reduction in free water clearance and urine diluting ability, despite appropriate suppression of plasma vasopressin concentrations. Orthostatic reductions in arterial blood pressure during head-up tilting and following the assumption of sitting posture were also demonstrable, and may have contributed to, but could not fully account for, the defect in renal water excretion, which persisted in supine posture. Hypertonic sodium chloride infusion studies performed before fluid restriction showed that low preinfusion plasma osmolality was associated with a reduced osmotic threshold for vasopressin release, which was subsequently corrected by a period of fluid restriction that restored the patient's plasma osmolality to a normal level. This shift in osmotic threshold can be inferred from both linear regression and log-linear regression analysis of the data. These studies show that marked impairment of renal water excretion coupled with unrestricted water intake can result in altered osmoregulation of vasopressin release in association with persistent plasma hypo-osmolality, which can be corrected by fluid restriction.