The aim of this study was to identify and characterize the repolarizing currents present in Helix nerve cells that generate long-lasting Ca2+-dependent depolarized plateaus in response to low-frequency stimulation. Two K+ currents were identified: a voltage-gated K(V) current and a Ca2+-activated K+ current or C current. These currents were studied separately in cells injected with either EGTA, tetraethylammonium (TEA+) or Cs+. C current activation was found to be rate-limited by the size of the inward Ca2+ current. Both K(V) and C currents displayed a pronounced relaxation during sustained depolarizations. Inactivation of the K(V) current was voltage-dependent. Inactivation of the C current was induced by either tiny Ca2+ entries or intracellular Ca2+ injections; C current inactivation was found to be more sensitive to intracellular [Ca2+] than the activating process. Similar experiments performed on various nerve cells revealed that the amount and rate of inactivation of both currents, but not their gating properties, varied greatly from cell to cell; plateau-generating cells had the strongest inactivating processes acting on both K+ currents. These properties help to explain how regular firing may turn into long-lasting depolarized plateaus. They point to the existence of cellular processes that might regulate the number of available K+ channels in a manner that is specific to the nerve cell type.
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