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Calcium current and calcium‐activated inward current in the oocyte of the starfish Leptasterias hexactis.
Author(s) -
Lansman J B
Publication year - 1987
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1987.sp016708
Subject(s) - chemistry , voltage clamp , divalent , reversal potential , calcium , depolarization , biophysics , current (fluid) , analytical chemistry (journal) , membrane potential , patch clamp , biochemistry , thermodynamics , physics , biology , chromatography , receptor , organic chemistry
1. Inward currents in the immature oocyte of the starfish Leptasterias hexactis were studied with a two‐micro‐electrode voltage clamp. Experiments investigated the role of Ca2+ in the Na+‐dependent plateau of the action potential. 2. Voltage steps more positive than ‐55 mV produced inward currents in normal sea water that activated and then decayed to a non‐zero level with a double‐exponential time course. Returning the voltage to the resting potential produced an inward tail current that relaxed slowly to zero with a time course of seconds. 3. Replacing Na+ with choline abolished the slowly decaying component as well as the slow tail current which followed the end of the voltage pulse. This suggested that inward current in Na+‐containing sea water consisted of a rapidly decaying component that flowed through Ca2+ channels and a more slowly decaying component carried by Na+. 4. Ca2+ current was isolated in Na+‐free sea water. Activation followed a sigmoidal time course that could be described with m2 kinetics. Inactivation during a maintained depolarization followed first‐order kinetics and was voltage dependent. 5. When Ba2+ was substituted for Ca2+ as the divalent ion charge carrier, inward currents in Na+‐containing sea water decayed along a single‐exponential time course. The absence of a slowly decaying Na+ current in Ba2+‐containing sea water suggested that Na+ current depended on Ca2+ influx. 6. The effects of altering Ca2+ influx on the time course of Na+ current were investigated. Na+ current decayed more rapidly as the test pulse potential was made more positive, while raising [Ca2+]o slowed the decaying phase without altering its dependence on membrane potential. 7. Tail currents measured after rapidly stepping the membrane potential back to the resting level consisted of a fast component associated with the closing of Ca2+ channels and a slow component that was abolished by removing Na+. 8. The variation of the amplitude of the slow component of tail current with the duration of the voltage‐clamp pulse indicated that Na+ current is associated with a time‐dependent component of membrane conductance. 9. Possible mechanisms for the slowly decaying Na+ current are considered. The results are discussed in relation to the idea that the conductance change to Na+ follows the time course of Ca2+ accumulation and removal from the cytoplasm.