Evidence for calcium phosphate supersaturation in the loop of
henle.
Asplin, John R., Neil S. Mandel, and F. L. Coe.
Nephrology Section, University of Chicago, Chicago, Illinois,
Departments of Medicine, Biochemistry, and Orthopedic Surgery,
Medical College of Wisconsin, and the Zablocki VA Medical Center,
Milwaukee, Wisconsin
APStracts 2:0184F, 1995.
We have used published rat micropuncture data to construct a matrix of
ion concentrations along the rat nephron. With an iterative computer
model of known ion interactions, we calculated relative
supersaturation ratios in all nephron segments. The collecting ducts
and urine showed expected supersaturation with stone forming salts.
Fluid in the thin segment of the loop of Henle, may be supersaturated
with calcium carbonate and calcium phosphate under certain
conditions. Since calculations cannot predict the actual course of
crystallization, we made solutions to mimic, in vitro, presumed
conditions in the loop of Henle. The solid phases that formed were
analyzed by x-ray powder diffraction, electron microprobe, and
infrared spectroscopy. All samples were identified as poorly
crystallized or immature apatite. The descending limb of Henle's loop
creates a unique condition as it extracts water but not sodium,
bicarbonate, calcium, or phosphate, giving a calcium concentration at
the bend of 3 mM, pH of 7.4 and a phosphate concentration which
varies from 0.8 mM to 48 mM depending on parathyroid hormone and
dietary phosphate. We conclude conditions in the thin segment
potentially could create a solid calcium phosphate phase which may
initiate nucleation of calcium oxalate salts in the collecting ducts,
potentiating nephrolithiasis and nephrocalcinosis.
Received 21 June 1995; accepted in final form 13 October 1995.
APS Manuscript Number F200-5.
Article publication pending Am. J. Physiol. (Renal Fluid Electrolyte
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95