Loop of Henle during the water-to-antidiuresis transition in Brattleboro rats

A mathematical model of the renal medulla predicted patterns of NaCl and urea concentration and flow rate when a rat undergoing water diuresis receives antidiuretic hormone (ADH), We tested these predictions with micropuncture and clearance experiments in Brattleboro rats lacking endogenous ADH. The model successfully predicted urine flow rate, urea excretion rate, and ascending vasa recta (AVR) solute concentration patterns. A novel result the model correctly foresaw was that AVR urea concentration lagged behind NaCl, because ADH temporarily interrupted urea recycling. The model suggested that this lag would be carried over to descending limb of Henle's loop (DLH) fluid only if DLH permeabilities allowed solutes as well as water to move across the DLH wall, but not if solute movement was negligible. Experimental results showed the lag, therby suggesting significant solute entry into DLH fluid during the transition, and, by extension, in other conditions as well, As shown in a companion paper (Am. J. Physiol. 239 (Renal Fluid Electrolyte Physiol. 8): F57-F71, 1980), solute permeation across DLH limits hypertonic urine formation in the inner medulla unless thin ascending limbs perform active transport of NaCl.