Developmental Regulation of a Na+-activated Fast Outward K+ Current in Rat Myoblasts

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Abstract
Myoblasts undergoing differentiation sequentially express multiple K+ channels, and that ion channel expression varies depending on species and state of development. In this report, we reported a developmental regulation of fast activated and fast inactivated outward current in rat myoblasts. The kinetic and pharmacological property of the outward current was investigated by using the patch-clamp technique. The outward current was elicited by a depolarizing step from -100 mV holding potential to +40 mV- +80 mV. The activation properties of this channel changed with days in culture. The outward current was blocked by 4-AP in a concentration dependent manner, with 0.5 mM and 2 mM 4-AP inhibiting the current by 10 +/- 3% and 56 +/- 3%, respectively. When 1 mM tetrodotoxin (TTX) was added to the bath solution or the membrane potential was depolarized to -50 mV, the fast outward current was aborted. Na+ dependent inhibition was observed when Na+ in the bath solution was replaced by Li+. In addition, replacement of K+ in the pipette solution by Cs+ almost completely eliminated the outward current. The developmentally regulated outward current recorded in rat myoblasts is a Na+ influx-dependent outward K+ current, which may contribute to myoblast membrane firing of action potential or myoblast fusion.

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