Zinc is present in normal urine and may retard calcium phosphate stone formation (Meyer & Angino, 1977). Our previous studies using renal inner medullary collecting duct cells (IMCD-3 cell-line) have identified the presence of a spontaneously active apical Cl^– conductance (Stewart et al. 2001) that is regulated by kinins, and external ATP via increased intracellular calcium ([Ca²⁺]_i) (Stewart et al. 2001). Here we report stimulation of IMCD Cl^– conductance by extracellular Zn²⁺ via release of intracellular Ca²⁺.

Cl^– currents were measured using the perforated (amphotericin 240 mg ml^-1) patch clamp whole cell recording configuration with a standard NaCl-rich bath solution and a KCl-rich pipette solution (Stewart et al. 2001). Initial recordings with a holding potential of 0 mV with excursions to ± 60 mV gave outwardly rectifying time-independent currents (current densities ± 60 mV, 131 ± 14 pA pF^-1, -74 ± 8 pA pF^-1 (means ± S.E.M.), n = 6 ) with an E_rev of -14.1 ± 2.8 mV. In the presence of 400 µM ZnCl₂ there was a progressive stimulation of current magnitude to a sustained plateau at 4-5 min, of 253 ± 31 pA pF^-1 and -139 ± 18 pA pF^-1 at ± 60 mV (n = 6), whilst E_rev shifted close to the Cl^– equilibrium potential (-6.5 ± 0.9 mV). The stimulated current was partially reversed on Zn²⁺ removal.

In order to investigate whether extracellular Zn²⁺ stimulation of current density was a direct action, measurements of [Ca²⁺]_i were made using fura-2 loaded IMCD-3 cells. Superfusion of the standard Na⁺-rich Krebs solution plus 400 µM ZnCl₂ resulted in a transient increase in [Ca²⁺]_i, and/or a progressive increase in [Ca²⁺]_i that was partially reversible (n = 9). Superfusion of 400 µM ZnCl₂ in Ca²⁺-free medium failed to abolish Zn²⁺-induced increments in [Ca²⁺]_i (n = 5).

Since our previous studies have excluded the presence of the extracellular Ca²⁺-sensing receptor in IMCD-3 cells (Stewart et al. 2001), these data suggest that a zinc-sensing receptor activates intracellular [Ca²⁺]_i so increasing Cl^– conductance in IMCD cells.

This work was supported by an NKRF studentship award to J.L.