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 sell voltage clamped to the K+ equilibrium potential (E-K) or C l- equilibrium potential (E-Cl) under standard conditions, the choline rgic agonist, carbachol, induced a large oscillating K+ current but on ly 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) (GTP gamma S), to pipettes stimu lated large oscillating K+ and inward currents. This occurred even whe n Ca2+ was absent from the bath but not when the Ca2+ chelator, ethyle ne glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid, was a dded to pipettes. When all pipette and bath K+ was replaced with Na+ a nd cells were voltage clamped between E-Na and E-Cl, GTP gamma S activ ated oscillating Na+ and Cl- currents. Finally, addition of inositol 1 ,4,5-trisphosphate [Ins(1,4,5)P-3] to pipettes activated large oscilla ting 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)P-3 si gnaling pathway. In addition, this or another G protein activates Cl- current by directly gating Cl- channels to increase their sensitivity to Ca2+.