TAURODEOXYCHOLATE ACTIVATES POTASSIUM AND CHLORIDE CONDUCTANCES VIA AN IP3-MEDIATED RELEASE OF CALCIUM FROM INTRACELLULAR STORES IN A COLONIC CELL-LINE (T84)
Dc. Devor et al., TAURODEOXYCHOLATE ACTIVATES POTASSIUM AND CHLORIDE CONDUCTANCES VIA AN IP3-MEDIATED RELEASE OF CALCIUM FROM INTRACELLULAR STORES IN A COLONIC CELL-LINE (T84), The Journal of clinical investigation, 92(5), 1993, pp. 2173-2181
Whole-cell patch-clamp techniques and fluorescence measurements of int
racellular Ca2+ concentration, (Ca2+)i, were used to investigate the m
echanism of taurodeoxycholate (TDC) stimulation of Cl- secretion in th
e T84 colonic cell line. During perforated whole-cell recordings, the
cell membrane voltage was alternately clamped to E(K) and E(Cl). Initi
ally, TDC (0.75 mM) stimulated inward nonselective cation currents tha
t were composed of discrete large conductance single-channel events. T
his initial response was followed by activation of K+ and Cl- currents
with peak values of 385+/-41 pA and 98+/-28 pA, respectively (n = 12)
. The K+ and Cl- currents oscillated while TDC was present and returne
d to baseline levels upon its removal. The threshold for activation of
the oscillatory currents was 0.1 mM TDC. Taurocholate, a bile acid th
at does not stimulate colonic Cl- secretion, induced no current respon
se. The TDC-induced currents could be activated in Ca2+-free bathing s
olutions. Preincubation of cells with the Ca2+ chelator, bis-(o-aminop
henoxy)-ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethy)ester (2
0 muM), (BAPTA-AM), eliminated the K+ and Cl- current responses, altho
ugh the nonselective cation channel events were still present. Replace
ment of bath Na+ with NMDG+ inhibited the TDC-induced nonselective cat
ion current but did not affect the K+ or Cl- currents. TDC induced a t
ransient (Ca2+)i rise of 575+/-70 nM from a baseline of 71+/-5 nM (n =
15); thereafter, (Ca2+)i either plateaued or oscillated. TDC-induced
(Ca2+), oscillations were observed in the absence of bath Ca2+; howeve
r, removal of bath Ca2+ during the TDC response caused (Ca2+)i to retu
rn to near baseline values. Simultaneous K+ current and (Ca2+)i measur
ements confirmed that the initial nonselective cation current was inde
pendent of (Ca2+)i, while K+ current oscillations were in phase with t
he (Ca2+)i oscillations. TDC induced inositol monophosphate (IP) accum
ulation, reflecting production of inositol 1,4,5-trisphosphate (IP3) d
uring TDC stimulation. The response to TDC during standard whole-cell
patch-clamp was similar to that observed with perforated whole-cell re
cordings, except the nonselective cation current was prolonged. When h
eparin (1 mg/ml) was added to the pipette under these conditions, the
Ca2+-activated currents were inhibited, but the nonselective cation cu
rrents were unaffected. These data suggest that TDC induces a Ca2+-ind
ependent nonselective cation conductance, perhaps by directly permeabi
lizing the plasma membrane. TDC stimulates Cl- secretion by activating
K+ and Cl- conductances via an IP3-mediated release of Ca2+ from intr
acellular stores.