An increase in [Ca2+]i activates basolateral chloride channels and inhibits apical sodium channels in frog skin epithelium

 The aim of this study was to investigate the mechanisms by which increases in free cytosolic calcium (Ca²⁺]_i) cause a decrease in macroscopic sodium absorption across principal cells of the frog skin epithelium. Ca²⁺]_i was measured with fura-2 in an epifluorescence microscope set-up, sodium absorption was measured by the voltage-clamp technique and cellular potential was measured using microelectrodes. The endoplasmic reticulum calcium-ATPase inhibitor thapsigargin (0.4 μM) increased Ca²⁺]_i from 66 ± 9 to 137 ± 19 nM (n = 13, P = 0.002). Thapsigargin caused the amiloride-sensitive short circuit current (I _sc) to drop from 26.4 to 10.6 μA cm^–2 (n = 19, P<0.001) concomitant with a depolarization of the cells from –79 ± 1 to –31 ± 2 mV (n = 18, P<0.001). Apical sodium permeability (P ^a _Na) was estimated from the current/voltage (I/V) relationship between amiloride-sensitive current and the potential across the apical membrane. P ^a _Na decreased from 8.01·10^–7 to 3.74·10^–7 cm s^–1 (n = 7, P = 0.04) following an increase in Ca²⁺]_i. A decrease in apical sodium permeability per se would tend to decrease I _sc and result in a hyperpolarization of the cell potential and not, as observed, a depolarization. Serosal addition of the chloride channel inhibitors 4,4′-diisothiocyanatostilbene-2,2′-disulphonic acid (DIDS), diphenylamine-2-carboxylate (DPC), indanyloxyacetic acid 94 (IAA-94) and furosemide reversed the depolarization induced by thapsigargin, indicating that chloride channels were activated by the increase in Ca²⁺]_i. This was confirmed in wash-out experiments with ³⁶Cl where it was shown that thapsigargin increased the efflux of chloride from 32.49 ± 5.01 to 62.63 ± 13.3 nmol·min^–1 cm^–2 (n = 5, P = 0.04). We conclude that a small increase in Ca²⁺]_i activates a chloride permeability and inhibits the apical sodium permeability. The activation of chloride channels and the closure of apical sodium channels will tend to lower the macroscopic sodium absorption. Received: 25 June 1996 / Received after revision: 28 August 1996 / Accepted: 2 September 1996