Defects in Osmoregulation of Vasopressin Secretion

Four patients are presented, all having pathologic responses in plasma vasopressin concentration to changes in plasma osmolality. Clinically, three of them had the syndrome of inappropriate antidiuretic hormone secretion (SIADH). The fourth had diabetes insipidus associated with an increased threshold for plasma osmolality. Two of the patients with SIADH had lowered thresholds for ADH secretion and one of these also had increased sensitivity to increments in osmolality. The latter patient, like another earlier reported similar case, had a defective blood-cerebrospinal fluid barrier. The third SIADH patient had no response at all to changes in osmolality from 230 to 305 mOsm/kg and had a small cell carcinoma of the lung. These examples illustrate that both the setting and the sensitivity of osmoreceptors and AVP-secreting neurons probably are under complex control from the CNS, and that CNS diseases can affect one or several of these pathways thereby affecting the setting of the osmoreceptor-vasopressin system.