Methods Homozygous adult male rats of the Brattleboro strain, obtained from the Yale Medical School colony and from Blue Spruce Farms (Altamont, New York), were used in all experi-ments. The water diuretic status of each animal used was confirmed by overnight observation in a metabolic cage immediately prior to experimental study; data for urine volumes and osmolalities, as well as sodium and potassium excretion during this period, are shown in Table 1. Free access to water and standard rat chow (# 5010, Ralston Purina, St. Louis, Missouri) were allowed up to the time of experiment. Animals were anesthetized with mactin, 100 mg/kg body wt, ip, and placed on a thermoregulated table designed to hold body temperature at 37 C. The right carotid artery was catheterized to allow BP monitoring and periodic blood sampling for measurement of hematocrit, inulin radioactivity, plasma osmolality and sodium and potassium concentrations. The left jugular vein was cannulated with a length of PE tubing connected via an adaptor to three separate infusion pumps to enable independent adjustment of fluid delivery from each (Fig. 1). The left kidney was prepared for micropuncture as previously described from this laboratory [14].

Two series of experiments were performed, involving either free-flow distal micropuncture (series 1) or distal tubular micro-perfusion (series 2). In all studies, an identical protocol of infusions and clearance collections was followed (Fig. 1) to define the overall effect of ADH on renal water and electrolyte excretion, and GFR. Immediately after insertion of the venous cannula, an infusion of hypotonic (0.45%) sodium chloride was commenced at 10 mllhr, following a priming injection of this fluid equal to 5% of body weight to reverse pre-existing dehydration [121, maintain BP, and replace surgical fluid losses. Simultaneously, two infusions of 0.9% NaC1 at 1 mL/hr were begun: one to act as" vehicle" for the later addition of ADH, the other to deliver {methoxy-3H] inulin (New England Nuclear Corporation, Boston, Massachusetts) at 90 pCi/hr following a priming injection of 90 pCi. After completion of surgery (60 to 75 mm), a further 1-hr period was allowed for equilibration; two 30-mm clearance periods were then observed during water diuresis (" control" phase). Following this, ADH was added to its saline vehicle as arginine vasopressin (Grade VIII synthetic AVP, Sigma Chemical Co., St. Louis, Missouri) so as to deliver 6 mU AVP/hr. This resulted in a dramatic fall in urine flow rate