LARGE-CONDUCTANCE CA2-ACTIVATED K+ CHANNELS IN THE SOMA OF RAT MOTONEURONS()

Properties of large conductance Ca2+-activated K+ channels were studie d in the soma of motoneurones visually identified in thin slices of ne onatal rat spinal cord. The channels had a conductance of 82 +/- 5 pS in external Ringer solution (5.6 mM K(o)(+)parallel to 155 mM K-i(+)) and 231 +/- 4 pS in external high-K-o solution (155 mM K(o)(+)parallel to 155 mM K+). The channels were activated by depolarization and by a n increase in internal Ca2+ concentration. Potentials of half-maximum channel activation (E-50) were -13, -34, -64 and -85 mV in the presenc e of 10(-6), 10(-5), 10(-4) and 10(-3) M internal Ca2+, respectively. Using an internal solution containing 10(-4) M Ca2+, averaged K-Ca cur rents showed fast activation within 2-3 msec after a voltage step to 50 mV. Averaged K-Ca currents did not inactivate during 400 msec volta ge pulses. External TEA reduced the apparent single-channel amplitude with a 50% blocking concentration (IC50) of 0.17 +/- 0.02 mM. K-Ca cha nnels were completely suppressed by externally applied 100 mM charybdo toxin. It is concluded that K-Ca channels activated by Ca2+ entry duri ng the action potential play an important role in the excitability of motoneurones.