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G proteins activate ionic conductances at multiple sites in T84 cells.

We examined the role of G proteins in activation of ionic conductances in isolated T84 cells during cholinergic stimulation. When cells were whole cell voltage clamped to the K+ equilibrium potential (E(K)) or Cl- equilibrium potential (E(Cl)) under standard conditions, the cholinergic agonist, carbachol, induced a large oscillating K+ current but only a small inward current. Addition of the GDP analogue, guanosine 5'-O-(2-thiodiphosphate), to pipettes blocked the ability of carbachol to activate the K+ current. Addition of the nonhydrolyzable GTP analogue, guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS), to pipettes stimulated large oscillating K+ and inward currents. This occurred even when Ca2+ was absent from the bath but not when the Ca2+ chelator, ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, was added to pipettes. When all pipette and bath K+ was replaced with Na+ and cells were voltage clamped between E(Na) and E(Cl), GTPgammaS activated oscillating Na+ and Cl- currents. Finally, addition of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] to pipettes activated large oscillating K+ currents but only small inward currents. These results suggest that a carbachol-induced release of Ca2+ from intracellular stores is activated by a G protein through the phospholipase C-Ins(1,4,5)P3 signaling pathway. In addition, this or another G protein activates Cl- current by directly gating Cl- channels to increase their sensitivity to Ca2+.