Two types of voltage-dependent inward currents were evoked by depolarization in murine antral smooth muscle cells (SMCs) bathed in Ca2+-containing physiological solution: high-voltage-activated (HVA) and low-voltage-activated (LVA) inward currents. We examined whether the LVA current was due to: 1) T-type Ca2+ channels, 2) Ca2+-activated Cl-channels, 3) nonselective cation channels (NSCC), or 4) voltage-dependent K+ channels. Replacement of external Ca2+ (2 mM) with equimolar Ba2+ increased the amplitude of the HVA current but blocked the LVA current. Nicardipine blocked the HVA current, and in the presence of nicardipine, T-type Ca2+ blockers failed to block LVA current. A Cl- channel antagonist had little effect on LVA current. Cation-free external solution completely abolished both HVA and LVA currents. Addition of Ca2+ to the solution restored only HVA currents. Addition of K+ (5 mM) to otherwise cation-free solution induced LVA current that reversed at -20 mV. These data suggest that LVA current is not due to T-type Ca2+ channels, Ca2+-activated Cl- channels, or NSCC. A-type K+ (KA) currents and delayed rectifying K+ (KDR) currents can be resolved in antral SMCs dialyzed with a solution containing 140 mM K+. When cells were exposed to high K+ external solution and dialyzed with Cs+-rich solution in the presence of nicardipine, LVA current was evoked and reversed at positive potentials. LVA currents were blocked by K+ channel blockers, 4-aminopyridine, and tetraethylammonium. In conclusion, LVA inward currents can be generated by K+ influx via KA channels in murine antral SMCs when cells were dialyzed with Cs+-rich solution.
Keywords: A-type K+ currents; T-type Ca2+ currents; smooth muscle cells.