ROLE OF VOLUME STATUS IN VASOPRESSIN-INDUCED NATRIURESIS - STUDIES INBRATTLEBORO RATS

The influence of volume status on the effect of physiological doses of vasopressin on sodium excretion was assessed in anaesthetized Brattle boro rats. Following a 1 h control period, animals were divided into f our groups. Group 1 (control) rats were kept in water balance througho ut (by adjustment of the rate of i.v. glucose infusion) and received n o vasopressin. In group 2 rats, vasopressin (20 mu U/min) was infused i.v. for 2 h, then withdrawn during the following 2 h; the vasopressin -induced antidiuresis and subsequent return to water diuresis were mat ched by appropriate changes in the i.v. infusion, thus maintaining wat er balance. In this group, vasopressin had no effect on sodium excreti on. Group 3 rats received the same dose of vasopressin, but the infusi on rate of the glucose solution was not reduced; consequently these ra ts became water-loaded. In this group, sodium excretion increased sign ificantly during vasopressin infusion, and rapidly returned to baselin e values when the vasopressin was discontinued. Group 4 rats were trea ted in the same way as group 3 animals except that the vasopressin inf usion was maintained (but without additional water loading) for a furt her 2 h; this did not prevent the fall in sodium excretion during the final 2 h of the experiment. We conclude that the natriuretic effect o f physiological levels of vasopressin reported elsewhere may be depend ent on an accompanying acute volume expansion during infusion of the h ormone.