EVIDENCE FOR CALCIUM-PHOSPHATE SUPERSATURATION IN THE LOOP OF HENLE

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 supersaturatio n ratios in all nephron segments. The collecting ducts and urine showe d expected supersaturation with stone-forming salts. Fluid in the thin segment of the loop of Henle may be supersaturated with calcium carbo nate and calcium phosphate under certain conditions. Because calculati ons cannot predict the actual course of crystallization, we made solut ions to mimic, in vitro, presumed conditions in the loop of Henle. The solid phases that formed were analyzed by X-ray powder diffraction, e lectron microprobe, and infrared spectroscopy. All samples were identi fied as poorly crystallized or immature apatite. The descending limb o f Henle's loop creates a unique condition as it extracts water but not sodium, bicarbonate, calcium, or phosphate, giving a calcium concentr ation at the bend of 3 mM, pH 7.4, and a phosphate concentration that varies from 0.8 to 48 mM, depending on parathyroid hormone and dietary phosphate. We conclude that conditions in the thin segment potentiall y could create a solid calcium phosphate phase, which may initiate nuc leation of calcium oxalate salts in the collecting ducts, potentiating nephrolithiasis and nephrocalcinosis.