Swelling of rat mesangial cells induces a Ca²⁺-dependent Cl⁻ conductance

Abstract

Membrane voltage (V _m) and ion currents of rat mesangial cells in primary culture were measured with the patch-clamp technique in the fast whole-cell configuration.V _m was −44 ± 1 mV (_n = 138). A reduction of the osmolality from 290 to 190 mosmol/kg depolarizedV _m from −44 ± 1 to −29 ± 1 mV (_n = 118) and increased the inward and outward conductances (Gm) from 14±2 to 39 ± 4 nS and 13±2 to 37 ± 4 nS (_n = 84), respectively. During the hypotonicity-induced depolarization the cell capacitance increased significantly from 33 ± 3 to 42 ± 4 pF (_n = 40). The effect of hypotonic cell swelling onV _m was increased in a bath with a reduced extracellular Cl⁻ of 32 mmol/l (by 71 ± 4%,_n = 23), indicating that a Cl⁻ conductance was activated. The permselectivity of this conductance was I⁻ ≥ Br⁻ > Cl⁻. TheV _m response was not affected in the presence of a reduced extracellular Na⁺ of 5 mmol/l (n = 13) and was inhibited in a solution with reduced extracellular Ca²⁺ concentration (by 63 ± 9%,n = 14). In microfluorescence measurements with the Ca²⁺-sensitive dye fura-2 hypotonic cell swelling induced a sustained increase of the intracellular Ca²⁺ activity, [Ca²⁺]_i (_n = 19). The increase of [Ca²⁺]_i was completely inhibited when the extracellular solution was free of Ca²⁺. TheV _m response to hypotonic cell swelling was not attenuated in the presence of the L-type Ca²⁺ channel blockers nicardipine (n = 5), nifedipine (n = 5) and verapamil (n = 5) (all at 1 μmol/l). The data indicate that in rat mesangial cells, osmotic swelling induces a Ca²⁺ influx from extracellular space. This Ca²⁺ influx activates a Cl⁻ conductance resulting in a depolarization ofV _m. The enhanced Cl⁻ conductance may lead to KCl extrusion and hence regulatory volume decrease.