Ca2+ entry in squid axons during voltage-clamp pulses is mainly Na+/Ca2+ exchange.
Intact squid axons were injected with aequorin and bathed in 3 mM Ca seawater (a concentration close to that of squid blood). Sodium and potassium currents were pharmacologically blocked and repetitive voltage-clamp pulses of a duration of 1.5 ms were applied (to simulate the duration of an action potential) at amplitudes of +30 to +90 mV and at frequencies of 100/s. In a very fresh axon (low internal Na concentration) no detectable change in aequorin glow resulted from this treatment, whether the axons were in Na-containing or in Na-free seawater. In axons subjected to modest Na loading, repetitive voltage-clamp pulsing did not result in an increased aequorin glow when the pulses were delivered in Na seawater, whereas in Na-free seawater there was an easily measurable increase in aequorin light emission during repetitive pulsing. The increase in aequorin photons emitted per voltage-clamp pulse was e-fold for 22 mV of depolarization, and the process showed no signs of saturating at pulse amplitudes of +180 mV (i.e., at a membrane potential close to ECa). The aequorin light emission per voltage-clamp pulse increased linearly with pulse duration (at constant amplitude).
Mullins
L J
LJ
Requena
J
J
Whittembury
J
J
D000200:Action Potentials; D000331:Aequorin; D000818:Animals; D001369:Axons / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D049832:Decapodiformes / Q000378:metabolism; D066298:In Vitro Techniques; D007473:Ion Channels / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*
D007473:Ion Channels; D000331:Aequorin; D012964:Sodium; D002118:Calcium