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) ≃ -60 mV). The spike complexes consisted of about a dozen action potentials of approximately 30 mV amplitude. Tetraethylammonium (TEA, 1-10 mM) had little effect on the quiescent periods but increased the amplitude of the action potential spikes. 4-Aminopyridine (4-AP, ≥ 5 mM) caused continuous spiking. 2. Voltage clamp of isolated myocytes identified delayed rectifier K⁺ currents that activated rapidly (time to half-maximum current, 11.5 ms at 0 mV) and inactivated in two phases (τ(f) = 96 ms, τ(s) = 1.5 s at 0 mV). The half-activation voltage of the permeability was -27 mV, with significant activation 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 later times (100-500 ms) consistent with block of resting channels relieved by depolarization. 4-AP inhibited outward current at potentials negative to -20 mV, potentials where TEA had no effect. 4. Qualitative PCR amplification of mRNA identified transcripts encoding delayed rectifier K⁺ channel subunits Kv1.6, Kv4.1, Kv4.2, Kv4.3 and the Kvβ1.1 subunit in murine colon myocytes. mRNA encoding Kv1.4 was not detected. 5. We find that TEA-sensitive delayed rectifier currents are important determinants of action potential amplitude but no rhythmicity. Delayed rectifier currents sensitive to 4-AP are important determinants of rhythmicity but not action potential amplitude.