Carbachol induces oscillations of membrane potassium conductance in a colonic cell line, T84

Effects of carbachol on membrane potential and current in T84 cells were determined using whole cell patch-clamp techniques. When the pipettes contained a standard KCl solution and the bath contained a standard NaCl solution, carbachol (100 μM) caused a rapid hyperpolarization to the K⁺ equilibrium potential (E(K⁺)), followed by potential oscillations. When membrane potential was clamped to 0 mV, carbachol induced an outwardly directed K⁺ current in 31 of 37 cells, with a peak value of 618 ± 51 (SE) pA. In 77% of these cells the current oscillated and gradually declined to base line. Atropine (20 μM) blocked this response. In symmetric KCl solutions the carbachol-induced current reversed at 0 mV with no rectification. Ba²⁺ or Cs⁺ did not block the current, but tetraethylammonium ion (TEA) reduced the number of responding cells. Although a Cl^- conductance was found in resting cells, carbachol did not cause an increase in Cl^- current when the cells were voltage-clamped to E(K⁺), or when voltage-clamped to ±60 mV while bathed in symmetric NaCl solutions. When the Ca²⁺-buffering capacity of the pipette solution was increased, 80% of the cells responded to carbachol, but only 10% oscillated; however, no K⁺ current was induced by carbachol when the pipette was made nominally Ca²⁺ free. The current was not affected by removal of Ca²⁺ from the bath. These results show that carbachol induces an oscillating Ca²⁺-activated K⁺ conductance in T84 cells, but no Cl^- conductance. This K⁺ conductance is dependent on the mechanisms that regulate intracellular Ca²⁺.