A micropuncture study of collecting tubule function in rats with hereditary diabetes insipidus

The reabsorption of water and solute by the papillary collecting duct was studied during water diuresis and vasopressin-induced antidiuresis in young rats with hereditary hypothalamic diabetes insipidus. The tip of the left renal papilla was exposed and fluid was obtained by micropuncture from loops of Henle and from collecting ducts at the papillary tip, and at an average of 1 mm proximal to the tip. In water diuresis the ratio of tubule fluid to plasma (TF/P) osmolality (osm) of loop fluid was 1.73 +/-0.058 (SE); of fluid from the proximal collecting duct, 0.63 +/-0.027; and from the tip, 0.55 +/-0.024; indicating a substantial osmotic pressure difference across the collecting duct epithelium. The fraction of filtered water reabsorbed (x 100) by the terminal collecting duct was 1.58% +/-0.32. In antidiuresis the TF/P osm of loop fluid was 2.65 +/-0.109; of fluid from the proximal collecting duct, 2.20 +/-0.093; and from the tip, 2.71 +/-0.111; indicating a marked decrease in the driving force for water reabsorption. The fraction of filtered water reabsorbed (x 100) by the terminal collecting duct was reduced to 0.58% +/-0.08, while the delivery of solute to the same segment was unchanged from that in water diuresis. The glomerular filtration rate (GFR) of the right kidney declined from 327 +/-24.4 mul/min in water diuresis to 274 +/-24.4 mul/min in antidiuresis (P < 0.005); similar results were obtained in a study comparing right and left GFRs in five additional rats. Thus, fractional reabsorption (and very likely the absolute volume) of water reabsorbed by the terminal collecting duct was less in antidiuresis than in water diuresis (mean difference, 1.01% +/-0.29, P < 0.005).