Question:

The thick ascending limb of the loop of Henle (TAL) drives an important part of the reabsorption of divalent cations. This reabsorption occurs through the paracellular pathway, which in the TAL shows cation selectivity. Claudins (Cldn), a family of tight junction proteins, determine the permeability and selectivity of this pathway. We wanted to study if calcium intake could affect the reabsorptive properties of the TAL and the expression of key claudins in TAL tight junctions.

Methods:

3 groups of young female mice were fed with control, low and high calcium diet for 8 days. We measured metabolic parameters in vivo. TALs were then collected for microperfusion and quantitative PCR for claudin expression analysis. Kidney cryosections were prepared from either fixed (Cldn10, 16, 19) or unfixed deep-frozen kidneys (Cldn14) for immunostaining of the most relevant claudins for cation permeability.

Results:

We observed that low calcium diet did not affect the plasma calcium concentration, but increased phosphate fractional renal excretion. Low calcium diet did not induce any change in the transport, electric properties, or claudin expression in the TAL. In contrast, high calcium diet provoked hypercalcemia, and hypercalciuria with low phosphate fractional excretion. The transcellular transport (short circuit current) remained unchanged, however, transepithelial voltage decreased. In addition, the permeabilities for Ca²⁺ and Mg²⁺ were reduced. The expression of Cldn10b and Cldn14 increased.

Conclusions:

High calcium diet induces a reduction in the tubular (TAL) divalent cation reabsorption, via a diminution in the driving force and less permeability through the paracellular pathway.