Cadmium block of squid calcium currents. Macroscopic data and a kinetic model.
The mechanism of Cd2+ block of Ca2+ currents (ICa) was explored in squid neurons using whole-cell patch clamp. Control currents activated sigmoidally, more rapidly at more positive potentials, and did not inactivate significantly. External Cd2+ up to 250 microM reduced ICa reversibly. For small depolarizations, the current for a step of 10 ms increased to a maintained value, resembling the control; but for Vm greater than 0 mV, the increase was followed by a decrease, as Cd2+ block became greater. Final block was greater for larger depolarizations. At 0 mV the half-blocking concentration was 125 microM. Tail currents, measured as channels close, had an initial "hook" when recorded in Cd2+: currents increased transiently, then decreased. This suggests that Cd2+ escapes from some channels, which then conduct briefly before closing. Analysis of tail currents shows that Cd2+ does not slow channel closing. The data can be explained if Cd2+ is a permeant blocker of Ca2+ channels and if channels can close when occupied by Cd2+. Cd2+ permeates the channels, but binds transiently to a site in the pore, obstructing the passage of other ions (e.g., Ca2+). Dwell time depends on the transmembrane potential, becoming shorter for more negative internal potentials. A five-state model was used to simulate the steady-state and kinetic features. It combines a Hodgkin-Huxley type m2 gating scheme and a one-site Woodhull ionic blockage model for a permeant blocker and includes a closed blocked state. To fit the data, the binding site for Cd2+ had to be near the outer end of the pore, with a well depth of -12.2 RT, and with a barrier at each end of the pore. The model predicts that the Cd2+ entry rate is nearly voltage independent, but the exit rate is steeply voltage dependent (e-fold/17 mV). Analysis further suggests that the channel closes at a normal rate with Cd2+ in the pore.
Chow
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RH
Department of Physiology, University of Pennsylvania, Philadelphia 19104-6085.
D000818:Animals; D002104:Cadmium / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D049832:Decapodiformes / Q000502:physiology*; D004553:Electric Conductivity / Q000502:physiology; D015640:Ion Channel Gating / Q000502:physiology*; D007700:Kinetics; D008564:Membrane Potentials / Q000502:physiology; D008954:Models, Biological; D008956:Models, Chemical; D009474:Neurons / Q000502:physiology
D015220:Calcium Channels; D002104:Cadmium