Renal System and Osmoregulation (3B) Practice Test

A simplified renal physiology study infuses one of two IV solutions for 60 minutes in healthy adults: Solution X (isotonic saline, 0.9% NaCl) or Solution Y (hypertonic saline, 3% NaCl). Subjects are otherwise fasting and supine; GFR is assumed unchanged over the hour. After Solution Y, plasma osmolality rises measurably while extracellular fluid volume also increases. Based on the information, which outcome is most likely as an immediate homeostatic renal response during the infusion of Solution Y?

In a controlled dehydration protocol, healthy volunteers (n=12) were deprived of water for 18 hours while maintaining normal dietary solute intake. At hour 18, mean plasma osmolality increased from 290 to 303 mOsm/kg and mean arterial pressure remained within normal range. Urine output decreased and urine osmolality increased. Investigators then administered a selective vasopressin (V2) receptor antagonist without changing fluid intake. Which renal response would you expect given the scenario to most directly oppose the rise in plasma osmolality under dehydration (before the antagonist is given)?