B. Brodin et al., AN INCREASE IN [CA2-CHANNELS IN FROG-SKIN EPITHELIUM(](I) ACTIVATES BASOLATERAL CHLORIDE CHANNELS AND INHIBITS APICAL SODIUM), Pflugers Archiv, 433(1-2), 1996, pp. 16-25
The aim of this study was to investigate the mechanisms by which incre
ases in free cytosolic calcium ([Ca2+)](i)) cause a decrease in macros
copic sodium absorption across principal cells of the frog skin epithe
lium. [Ca2+](i) was measured with fura-2 in an epifluorescence microsc
ope set-up, sodium absorption was measured by the voltage-clamp techni
que and cellular potential was measured using microelectrodes. The end
oplasmic reticulum calcium-ATPase inhibitor thapsigargin (0.4 mu M) in
creased [Ca2+](i) from 66 +/- 9 to 137 +/- 19 nM (n = 13, P = 0.002).
Thapsigargin caused the amiloride sensitive short circuit current (I-s
c) 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 fr
om the current/voltage (I/V) relationship between amiloride-sensitive
current and the potential across the apical membrane. P-Na(a) decrease
d from 8.01 . 10(-7) to 3.74 . 10(-7) cm s(-1) (n = 7, P = 0.04) follo
wing an increase in [Ca2+](i). A decrease in apical sodium permeabilit
y 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'-diisothiocyanatostil
bene-2,2'-disulphonic acid (DIDS), diphenylamine-2-carboxylate (DPC),
indanyloxyacetic acid 94 (IAA-94) and furosemide reversed the depolari
zation induced by thapsigargin. indicating that chloride channels were
activated by the increase in [Ca2+](i). This was confirmed in wash-ou
t experiments with Cl-36 where it was shown that thapsigargin increase
d 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 s
odium permeability. The activation of chloride channels and the closur
e of apical sodium channels will tend to lower the macroscopic sodium
absorption.