Large conductance Ca(2+)-activated K+ channels are involved in both spike shaping and firing regulation in Helix neurones.
1. The role of BK-type calcium-dependent K+ channels (K+Ca) in cell firing regulation was evaluated by performing whole-cell voltage clamp and patch clamp experiments on the U cell neurones in the snail Helix pomatia. These cells were selected because most of the repolarizing K+ current flowed through K+Ca channels. 2. U cells generated overshooting Ca(2+)-dependent spikes in Na(+)-free saline. In response to prolonged depolarizing current, they fired a limited number of spikes of decreasing amplitude, and behaved like fast-adapting or phasic neurones. 3. Under voltage clamp conditions, the K+Ca current had a slow onset at voltages that induced small Ca2+ entries. By manipulating the Ca2+ entry (either with appropriate voltage programmes or by changing the Ca2+ content of the bath), the K+Ca channel opening was found to be rate limited by the Ca2+ binding step and not by the voltage-dependent conformational change to the open state. 4. Despite the slow activation rate observed in voltage-clamped cells, 25-30% of the available K+Ca current was found to be active during isolated spikes. These data were based on patch clamp, spike-like voltage clamp and hybrid current clamp-voltage clamp experiments. 5. The fact that spikes led the slowly rising K+Ca current to shift into a fast activating mode was accounted for by the large surge of Ca2+ current concomitant with spike upstroke. The early calcium surge resulted in local increases in cytosolic calcium, which speeded up the binding of calcium ions to the closed K+Ca channels. From changes in the null Ca2+ current voltage, it was calculated that the submembrane [Ca2+]i increase to 50-80 microM during the spike. 6. Due to their fast voltage dependence, K+Ca channels appeared to play no role in shaping the interspike trajectory. 7. Even in the fast activating mode, the K+Ca current had a finite rate of rise and was not involved in repolarizing short duration Na(+-dependent action potentials. The current became more and more active, however, when voltage-gated K+ channels were progressively inactivated during firing. 8. The fast adaptation exhibited by U cells upon sustained depolarization was not paralleled by a recruitment of K+Ca channels because of the cumulative Ca2+ entries. During a spike burst, the K+Ca current progressively overlapped the depolarizing Ca2+ current, which ultimately stopped the firing. The early opening of K+Ca channels was ascribed to residual Ca2+ accumulation that kept part of the channels in the Ca(2+)-bound state ready to be opened quickly by cell depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)
Crest
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Laboratoire de Neurobiologie, CNRS, Marseille, France.
Gola
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D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D003600:Cytosol / Q000378:metabolism; D004594:Electrophysiology; D017952:Ganglia, Invertebrate / Q000166:cytology / Q000502:physiology; D006372:Helix, Snails / Q000502:physiology*; D066298:In Vitro Techniques; D008839:Microelectrodes; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*
D002121:Calcium Channel Blockers; D015221:Potassium Channels; D002118:Calcium