Separate Cl^- Conductances Activated by cAMP and Ca2+ in Cl^--Secreting Epithelial Cells

We studied the cAMP- and Ca²⁺-activated secretory Cl^- conductances in the Cl^--secreting colonic epithelial cell line T84 using the whole-cell patch-clamp technique. Cl^- and K^+ currents were measured under voltage clamp. Forskolin or cAMP increased Cl^- current 2-15 times with no change in K^+ current. The current-voltage relation for cAMP-activated Cl^- current was linear from -100 to +100 mV and showed no time-dependent changes in current during voltage pulses. Ca²⁺ ionophores or increased pipette Ca²⁺ increased both Cl^- and K^+ currents 2-30 times. The Ca²⁺-activated Cl^- current was outwardly rectified, activated during depolarizing voltage pulses, and inactivated during hyperpolarizing voltage pulses. Addition of ionophore after forskolin further increased Cl^- conductance 1.5-5 times, and the current took on the time-dependent characteristics of that stimulated by Ca²⁺. Thus, cAMP and Ca²⁺ activate Cl^- conductances with different properties, implying that these second messengers activate different Cl^- channels or that they induce different conductive and kinetic states in the same Cl^- channel.