Contribution of Kv4 channels toward the A-type potassium current in murine colonic myocytes

Abstract

A rapidly inactivating K⁺ current (A-type current; I_A) present in murine colonic myocytes is important in maintaining physiological patterns of slow wave electrical activity. The kinetic profile of colonic I_A resembles that of Kv4-derived currents. We examined the contribution of Kv4 α-subunits to I_A in the murine colon using pharmacological, molecular and immunohistochemical approaches. The divalent cation Cd²⁺ decreased peak I_A and shifted the voltage dependence of activation and inactivation to more depolarized potentials. Similar results were observed with La³⁺. Colonic I_A was sensitive to low micromolar concentrations of flecainide (IC₅₀ = 11 μM). Quantitative PCR indicated that in colonic and jejunal tissue, Kv4.3 transcripts demonstrate greater relative abundance than transcripts encoding Kv4.1 or Kv4.2. Antibodies revealed greater Kv4.3-like immunoreactivity than Kv4.2-like immunoreactivity in colonic myocytes. Kv4-like immunoreactivity was less evident in jejunal myocytes. To address this finding, we examined the expression of K⁺ channel-interacting proteins (KChIPs), which act as positive modulators of Kv4-mediated currents. Qualitative PCR identified transcripts encoding the four known members of the KChIP family in isolated colonic and jejunal myocytes. However, the relative abundance of KChIP transcript was 2.6-fold greater in colon tissue than in jejunum, as assessed by quantitative PCR, with KChIP1 showing predominance. This observation is in accordance with the amplitude of the A-type current present in these two tissues, where colonic myocytes possess densities twice that of jejunal myocytes. From this we conclude that Kv4.3, in association with KChIP1, is the major molecular determinant of I_A in murine colonic myocytes.