Contribution of delayed rectifier potassium currents to the electrical activity of murine colonic smooth muscle

1. We used intracellular microelectrodes to record the membrane potential ( V-m) of intact murine colonic smooth muscle. Electrical activity consisted of spike complexes separated by quiescent periods (V-m approximate to -60 m V). The spike complexes consisted of about it dozen action potentials of ap proximately 30 mV amplitude. Tetraethylammonium (TEA, 1-10 mM) had little e ffect on the quiescent periods but increased the amplitude of the action po tential spikes. 4-Aminopyridine (4-AP, greater than or equal to 5 mM) cause d continuous spiking. 2. Voltage clamp of isolated myocytes identified delayed rectifier K+ curre nts that activated rapidly (time to half-maximum current, 11.5 ms at 0 mV) and inactivated in two phases (tau(f) = 96 ms, tau(s) = 1.5 s at 0 mV). The half-activation voltage of the permeability was -27 mV, with significant a ctivation at -50 mV. 3. TEA (10 mM) reduced the outward current at potentials positive to 0 mV. 4-AP (5 mM) reduced the early current but increased outward current at late r times (100-500 ms) consistent with block of resting channels relieved by depolarization. 4-AP inhibited outward current at potentials negative to -2 0 mV, potentials where TEA had no effect. 4. Qualitative PCR amplification of mRNA identified transcripts encoding de layed rectifier K+ channel subunits Kv1.6, Kv4.1, Kv4.2, Kv4.3 and the Kv b eta 1.1 subunit in murine colon myocytes. mRNA encoding Kv 1.4 was not dete cted. 5. We find that TEA-sensitive delayed rectifier currents are important dete rminants of action potential amplitude but not rhythmicity. Delayed rectifi er currents sensitive to 4-AP are important determinants of rhythmicity but not action potential amplitude.