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1985 (1)
1Author    H. Stieve, M. Pflaum, J. Klomfaß, H. G. AubeRequires cookie*
 Title    Calcium/Sodium Binding Competition in the Gating of Light-Activated Membrane Conductance Studied by Voltage Clamp Technique in Limulus Ventral Nerve Photoreceptor  
 Abstract    The mem brane current vs. voltage dependence was measured in Limulus ventral nerve photo­ receptors at various external C a2+ and N a+ concentrations, using the voltage clam p technique. Lowering the external concentration of the divalent cations Ca~+ and Mg+ to < 1 |im ol/l by adding EDTA causes 1) the light-induced transient conductance increase to disappear and 2) Irev J d ! the reversal potential of the mem brane current in the dark to shift to a positive value between + 10 and + 2 0 mV. This value is about the same as the (V^J-l), reversal potential of the total light current under normal ionic conditions. If the external N a+ is lowered to 50 mmol/1 (i.e. 10% of the normal concentration) simultaneously with the lowering o f the divalent cation concentration described above, the light response is not abolished and V TevJ p is shifted less. The extent of this antagonism depends on the sodium sub­ stitute; it is stronger if choline is used instead of lithium. Lowering of sodium alone to 50 mmol/1, in a saline containing normal C a2+ and Mg2+ concen­ trations, does not change the mem brane dark current vs. voltage curve and so is not altered; VT ^ A J L, the reversal potential of the light-induced current, however, is reduced by 10 mV (from + 2 0 to + 1 0 mV). This reduction in %evA JL can be accounted for by the reduction of the sodium gradient across the cell membrane. Raising the external C a2+ concentration to 40 or 100 mmol/1 has no conspicuous effect on the mem brane current vs. voltage dependence and the gating o f the light-induced conductance in­ crease. The results are consistent with our working hypothesis that the gating o f the light-activated ion channels in Limulus photoreceptor is controlled by negative binding sites for which calcium-and sodium ions compete with antagonistic actions. Abbreviations: ReP, receptor potential (m embrane voltage response to the light stimulus); HMAX [mV], peak am pli­ tude of the ReP; PMP [mV], pre-stimulus mem brane po­ tential (difference between the extracellularly recorded zero line and the intracellularly recorded base line; initial and final zero were averaged to compensate for drift); VM [mV], mem brane voltage; J M [nA], mem brane current; J d [nA]» dark current, m em brane current during voltage clamp, measured 1 s after clam p onset; J L [nA], am plitude o f total light current; am plitude of total m em brane current following the light flash (J D + A JL)\ AJL, am plitude of light-induced current; ^ ev7D [mV], reversal potential of dark current; ^ ev7 L[mV], reversal potential of light cur­ rent; I(evj y L[mV], reversal potential of the light-induced current evoked by the light stimulus; g G [|iS], membrane (slope) conductance in the dark; ^lD 1^], total membrane (slope) conductance during illum ination; J g L[|iS], light induced mem brane (slope) conductance; PS, physiological saline (see Table I); [N a+]ex[Ca2+]ex, sodium, or calcium ion concentration of the external saline; EDTA, ethylene dimethyl tetra acetic acid; EGTA, ethylene glycol-bis (2-aminoethylether) N ,N '-tetra acetic acid. 
  Reference    Z. Naturforsch. 40c, 278—291 (1985); received N ovember 19 1984 
  Published    1985 
  Keywords    Limulus Ventral Nerve Photoreceptor, Current-Voltage Relation, Varied External Calcium and Sodium Concentration, Calcium /Sodium Binding Competition, Lithium 
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 TEI-XML for    default:Reihe_C/40/ZNC-1985-40c-0278.pdf 
 Identifier    ZNC-1985-40c-0278 
 Volume    40