Effects of DDAVP and AVP on sodium and water balance in conscious rat.

Many aspects of the physiological adaptation to chronic vasopressin and hypotonic fluid administration remain unclear. We therefore infused vasopressin [either 1-desamino-8-D-arginine vasopressin (DDAVP) at 0.112 ng/h or arginine vasopressin (AVP) at 2.4 mU/h] and hypotonic fluid (0.22% NaCl at 3.1 ml/h) into conscious unrestrained rats for 4-6 days. To determine if a decrease in the hydrosmotic effect of vasopressin occurred, urinary osmolality, flow rate, and free water reabsorption were measured sequentially in vasopressin-treated and control animals (receiving 0.22% NaCl alone). Progressive increases in urine flow and decreases in urine osmolality and free water reabsorption occurred in vasopressin-treated animals. This decreased hydrosmotic effect was noted with both DDAVP and AVP and could be dissociated from hormonal degradation and urinary prostaglandin E2 excretion. Sodium and water balance were measured to assess the determinants of the hypotonic state following chronic vasopressin and hypotonic fluid. In DDAVP-treated animals, sodium balance remained constant and hyponatremia resulted from water retention alone. In AVP-treated animals, a greater degree of hyponatremia was observed and resulted from combined positive water balance and negative sodium balance. The difference in sodium balance observed when DDAVP- and AVP-treated animals were compared could not be attributed to differences in either magnitude of water retention or filtered load of sodium.