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 mu M) 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 mu A cm(-2) (n = 19, P<0.001) concomitant with a depolarization of the cells from -79 +/- 1 to -31 +/- 2 mV (ii = 18, P<0.001). Apical sodium permeability (P-Na(a)) was estimated from the current/voltage (I/V) relationship between amiloride-sensitive current and the potential across the apical membrane. P-Na(a) 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 Cl-36 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.