Calcium and calcimimetics regulate paracellular Na⁺ transport in the thin ascending limb of Henle's loop in mouse kidney

The effect of Ca²⁺ and calcimimetics on NaCl transport was investigated in the in vitro isolated microperfused mouse thin ascending limb of Henle's loop. In the presence of a transmural NaCl gradient, the transepithelial diffusional potential was 13.7±0.4 mV (n=17). When the Ca²⁺ in the bath was increased from 1.5 to 4.5 mM at 37°C, the relative permeability of Na⁺ to Cl^- (P _Na /P _Cl) estimated from the diffusional voltage deflection due to the transepithelial NaCl gradient (V _d) changed from 0.371±0.017 to 0.341±0.015 (n=10, P<0.0001). When the Ca²⁺ in the lumen was increased from 1.5 to 4.5 mM, the P _Na /P _Cl decreased from 0.349±0.013 to 0.330±0.013 (n=5, P<0.002). The addition of 0.1 mM neomycin and 0.2 mM gentamicin to the bath or lumen also decreased the P _Na /P _Cl. The same effect on P _Na /P _Cl of Ca²⁺ and calcimimetics occurred in ClC-K1 (kidney-specific chloride channel) knockout mice. The addition of 300 μg/ml protamine to the bath strongly inhibited changes to P _Na /P _Cl induced by basolateral Ca²⁺. These data indicate that ambient Ca²⁺ and calcimimetics inhibit Na⁺ transport in the thin ascending limb, which is known to occur via the paracellular shunt pathway. Our observations strongly suggest that Ca²⁺ is involved in the regulation of paracellular Na⁺ permeability in the thin ascending limbs.