A Study on Na⁺ and Water Reabsorption in the Nephron Segment Beyond Proximal Tubule Measured by Lithium Clearance

A Study on Na⁺ and Water Reabsorption in the Nephron Segment Beyond Proximal Tubule Measured by Lithium Clearance

During the past few years it has been proposed that lithium clearance can be used as a reliable measure for the outflow of tubular fluid from the proximal tubule. This study was aimed to characterize the inflow dependent reabsorption of Na in renal tubule beyond the proximal tubule. For this purpose, lithium clearance was used as a measure for the inflow from the proximal tubule and the changes in reabsorption fraction of Na and water were determined in rabbits. Rabbits were pretreated with hypotonic saline solutions for an hour (50 mM/L NaCl, 20 ml/hr/kg). And then a hypertonic solution of 500 mM/L NaCl (20 ml/kg) was administered intraperitoneally in conjunction with a bolus of LiCl solution (2 mM/kg, i.v.) for conditioning the C_Li and urine flow rate. To rule out the effect of Li⁺ on tubular functions, a bolus of NaCl solution (2 mM/kg, i.v.) was administered. Fifteen, thirty, and sixty minutes after injection of hypertonic saline arterial blood and urine samples were taken. Urinary and plasma concentrations as well as urinary output of Li⁺, Na⁺ and K⁺ were measured. From these C_Li, C_Na and the reabsorption fraction of Na and water (Fr_Na & FrH₂O) were calculated. These results were compared with those from control groups in which the same amount of isotonic saline (145 mM/L NaCl) and of 15% dextran solution were administered in the same way as that in experimental group. Followings are the results obtained. 1) The plasma concentration of Na⁺ in rabbits injected with hypertonic saline reached the peak value after 15 min and thereafter no significant change was observed. Hematocrit values did not show any change, while urinary excretion of Na⁺ increased markedly during the first 15 min and decreased thereafter. These results were not affected by an injection of a small amount of LiCl. 2) The clearances of Li⁺, Na⁺ and K⁺ in rabbits injected with hypertonic saline and LiCl solution decreased. 3) In spite of the variation in C_Li, Fr_Na did not show any significant change while FrH₂O increased gradually. 4) C_Li decreased also in rabbits received isotonic saline. Fr_Na tended to be higher than that in hypertonic saline group, while FrH₂O and Fr_Na did not associated with the decrease in C_Li. 5) C_Li of the rabbits received dextran solution fluctuated persistently and Fr_Na and FrH₂O did not change in along with C_Li although Fr_Na had a tendency to be higher than that in hypertonic saline group. 6) From the above results it was concluded that: (a) In rabbits with normal body store of Na⁺, the Fr_Na of renal tubule beyond proximal tubule. calculated from C_Li as a measure of inflow from proximal tubule is constant in spite of variations in C_Li. (b) The FrH₂O calculated from C_Li is dependent largely upon ADH rather than inflow from proximal tubule. (c) When there is a decrease in plasma Na⁺ concentration or ineffective body fluid. Li⁺ reabsorption may occur in the thick segnent of Henle s loop and hence the determination of Fr_Na and FrH₂O will not be easy one, but Fr_Na is constant under the same experimental conditions.