Laboratory Investigation

Kidney International (1993) 44, 337–343; doi:10.1038/ki.1993.249

Simulation of lithium transport along the thin segments of Henle's loop

Junichi Taniguchi, David G Shirley, Stephen J Walter and Masashi Imai

Department of Pharmacology, Jichi Medical School, Tochigi, Japan, and Department of Physiology, Charing Cross and Westminster Medical School, London, England, United Kingdom

Correspondence: Masashi Imai MD, Department of Pharmacology, Jichi Medical School, Minamikawachi, Kawachi, Tochigi 329-04, Japan.

Received 4 November 1992; Revised 8 March 1993; Accepted 8 March 1993.

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Abstract

Simulation of lithium transport along the thin segments of Henle's loop. Although the renal clearance of lithium is widely used as an index of sodium and water delivery to the end of the proximal tubule, micropuncture studies of superficial nephrons suggest that lithium may be reabsorbed additionally in the loop of Henle. In order to examine the possibility of lithium transport in the thin loop segments of deep nephrons, we conducted a computer simulation study using a modification of the mathematical model reported previously [1]. The data for the model were obtained from measurements of osmolality and electrolyte concentrations in systemic plasma and renal papillary interstitial fluid of control and furosemide-treated rats. Papillary interstitium/ plasma Li+ concentration ratios were 3.23 and 1.48 in the control and the furosemide group, respectively. Assuming that solute concentration in the renal medulla increases as an exponential function, and applying phenomenological coefficients obtained from hamsters, the transport profiles of Li+ along the thin loop segments were calculated to be very similar to those of Na+. In the control group, about 82% of delivered Li+ was reabsorbed along the entire thin loop segments, with all segments contributing equally. This value may represent the highest possible capacity of the thin loop segments. In the furosemide group, Li+ reabsorption in the thin loop was reduced to 31% of delivered Li+. From these analyses, we conclude that an appreciable amount of lithium may be reabsorbed in the thin loop segments of long-looped nephrons by passive mechanisms. Furosemide inhibits Li+ reabsorption by dissipating the osmotic concentration gradient in the renal medulla. The findings suggest that results from lithium clearance may underestimate end-proximal fluid delivery in deep nephrons and that lithium clearance studies should therefore be interpreted with caution, particularly in situations where the medullary osmotic gradient is likely to be disturbed.

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