Extracellular zinc stimulates a calcium-activated chloride conductance through mobilisation of intracellular calcium in renal inner medullary collecting duct cells

Acute lead (Pb2+) exposure increases calcium oxalate crystallization in the inner medullary collecting duct, and is ameliorated by Ca2+/Mg2+-ATPase inhibition, as well as Capa receptor and SPoCk C knockdown in a Drosophila melanogaster model of nephrolithiasis

Calcium oxalate (CaOx) kidney stones accumulate within the renal tubule due to high concentrations of insoluble deposits in the urine. Pb2+-induced Ca2+ mobilization along with Pb2+-induced nephrotoxic effects within the proximal tubule have been well established; however, Pb2+ mediated effects within the collecting duct remains insufficiently studied. Thus in vitro and ex vivo model systems were treated with increasing concentrations of lead (II) acetate (PbAc) ± sodium oxalate (Na2C2O4) for 1 h, both individually and in combination. Pb2+-mediated solution turbidity increased 2 to 5 times greater post-exposure to 75, 100 and 200 μM Pb2+ with the additional co-treatment of 10 mM oxalate in mouse inner medullary collecting duct (mIMCD-3) cells. Additionally, 100 μM and 200 μM Pb2+ alone induced significant levels of intracellular Ca2+ release. To validate Pb2+-mediated effects on the formation of CaOx crystals, alizarin red staining confirmed the presence of CaOx crystallization. Pb2+-induced intracellular Ca2+ was also observed ex vivo in fly Malpighian tubules with significant increases in CaOx crystal formation via Pb2+-induced intracellular Ca2+ release significantly increasing the average crystal number, size, and total area of crystal formation, which was ameliorated by tissue-specific SPoCk C transporter and Capa receptor knockdown. These studies demonstrate Pb2+-induced Ca2+ release likely increases the formation of CaOx crystals, which is modulated by a Gq-linked mechanism with concurrent Ca2+ extracellular mobilization.

A voltage-dependent Ca2+ influx pathway regulates the Ca2+-dependent Cl(-) conductance of renal IMCD-3 cells

We have previously shown that the membrane conductance of mIMCD-3 cells at a holding potential of 0 mV is dominated by a Ca2+-dependent Cl(-) current (I(CLCA)). Here we report that I(CLCA) activity is also voltage dependent and that this dependence on voltage is linked to the opening of a novel Al3+-sensitive, voltage-dependent, Ca2+ influx pathway. Using whole-cell patch-clamp recordings at a physiological holding potential (-60 mV), ICLCA was found to be inactive and resting currents were predominantly K+ selective. However, membrane depolarization to 0 mV resulted in a slow, sigmoidal, activation of ICLCA (T(0.5) approximately 500 s), while repolarization in turn resulted in a monoexponential decay in I(CLCA) (T (0.5) approximately 100 s). The activation of I(CLCA) by depolarization was reduced by lowering extracellular Ca2+ and completely inhibited by buffering cytosolic Ca2+ with EGTA, suggesting a role for Ca2+ influx in the activation of I(CLCA). However, raising bulk cytosolic Ca2+ at -60 mV did not produce sustained I(CLCA) activity. Therefore I(CLCA) is dependent on both an increase in intracellular Ca2+ and depolarization to be active. We further show that membrane depolarization is coupled to opening of a Ca2+ influx pathway that displays equal permeability to Ca2+ and Ba2+ ions and that is blocked by extracellular Al3+ and La3+. Furthermore, Al3+ completely and reversibly inhibited depolarization-induced activation of ICLCA, thereby directly linking Ca2+ influx to activation of I(CLCA). We speculate that during sustained membrane depolarization, calcium influx activates ICLCA which functions to modulate NaCl transport across the apical membrane of IMCD cells.