Depolarization induced voltage and conductance changes were studied in frog motoneurones in isolated, perfused spinal cord slices. Two types of afterhyperpolarization are observed following action potentials in normal Ringer, a fast afterhypolarization lasting 5-10 msec and a slow afterhyperpolarization lasting 60-200 msec. Both afterhyperpolarizations are mediated by an increased K+ conductance. The slow afterhyperpolarization and the conductance increase underlying it are selectively and reversibly inhibited by perfusion with solutions containing low [Ca2+] (≤ 0.2 mM) or the Ca2+ antagonists Mn2+ (1 mM) or Co2+ (5 mM), and are enhanced by perfusion with high [Ca2+]. Addition of 2-5 mM tetraethylammonium ion (TEA+) to the perfusing solution prolongs the falling phase of the action potential and abolishes the fast afterhyperpolarization, but does not inhibit the slow afterhyperpolarization. When the voltage dependent Na+ current is blocked by perfusion with TTX (10-5 M), intracellularly applied depolarizing current steps evoke fast and slow hyperpolarizations with kinetics and pharmacological sensitivities similar to those of the fast and slow afterhyperpolarizations, respectively. The fast hyperpolarization is maximally activated by brief, intense depolarizations, the slow hyperpolarization by prolonged, less intense depolarizations. These pharmacological and kinetic data demonstrate that in frog motoneurones the repolarization fast afterhyperpolarization sequence and the slow afterhyperpolarization are produced by different K+ conductance systems. The fast K+ conductance activates rapidly on depolarization, decays rapidly on repolarization, and is TEA+ sensitive, while the slow K+ conductance activates and decays more slowly and is Ca2+ dependent. Motoneurones perfused with TEA+ and TEA often show a slow, regenerative depolarizing response to applied depolarizing currents. These regenerative depolarizations are probably produced by an influx of Ca2+ because they persist in isotonic CaCl2 and are blocked by Mn2+ or low [Ca2+]. The Ca2+ dependence of the slow afterhyperpolarization and the increase in slow afterhyperpolarization magnitude observed following the slow Ca2+ potentials suggest that a depolarization evoked Ca2+ influx activates the K+ conductance underlying the slow afterhyperpolarization. Motoneurones in which the slow Ca2+ and K+ conductance systems have been enhanced by high [Ca2+] or blocked by Mn2+ show altered discharge patterns in response to intracellularly applied depolarizing current steps. Perfusion with twice normal [Ca2+] (4 mM) causes motoneurones to discharge more slowly at all current intensities, and reduces the slope of the 'steady state' frequency current relationship. Mn2+ perfused motoneurones exhibit fairly normal high frequency discharge at the onset of the current step, but unlike normal motoneurones, do not discharge at frequencies below 60/sec. These data indicate that the slow Ca2+ and K+ conductance systems are necessary for, and help regulate, low frequency discharge.
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