SYNDROME OF INAPPROPRIATE ANTIDIURESIS WITHOUT INVOLVING INAPPROPRIATE SECRETION OF VASOPRESSIN IN AN ELDERLY WOMAN - EFFECT OF INTRAVENOUSADMINISTRATION OF THE NONPEPTIDE VASOPRESSIN V2 RECEPTOR ANTAGONIST OPC-31260

We describe a 78-year-old female patient with severe hyponatremia owin g to inappropriate antidiuresis. Despite hyponatremia, the urinary sod ium excretion persisted with urine osmolality exceeding plasma osmolal ity. Although a water load decreased plasma sodium concentration and o smolality, the patient excreted only 40% of the water load after 4 h w ithout decreased urine sodium concentrations and osmolality. The plasm a vasopressin levels relative to plasma osmolality were not inappropri ately elevated. Intravenous administration of the selective nonpeptide vasopressin V2 antagonist OPC-31260 decreased sodium concentration an d osmolality in urine to lower values than in plasma. Concomitantly, t he urine volume excretion increased markedly. In addition, restriction of water or administration of demeclocycline improved plasma sodium a nd plasma vasopressin levels relative to plasma osmolality to be norma l. The findings indicate that the inappropriate antidiuresis in this p atient was related to hyperfunction of the arginine vasopressin V2 rec eptor in the kidney which is not due to inappropriately secreted vasop ressin.