Effects of clamp rise-time on rat brain IIA sodium channels in Xenopus oocytes.
The kinetic properties of wild-type rat brain IIa sodium channels in excised macropatches were studied using step depolarizations and ramp depolarizations to imitate the slow settling-time of voltage in two-electrode voltage clamp. Ramp depolarizations longer than 1 ms produce an increasing suppression of peak sodium current (I[Na]). Two rates of inactivation can be seen in macroscopic sodium current records from excised patches following both step and ramp depolarizations. During slow ramp depolarizations, reduction in peak I[Na] is associated with selective loss of the fastest rate of test-pulse inactivation. This change can be interpreted as resulting from inactivation of a separate sub-population of 'fast mode' channels. The slow rate of test-pulse inactivation is relatively unaffected by changing ramp durations. These results are sufficient to explain the typically slow inactivation kinetics seen in two-electrode voltage clamp recordings of sodium channels in Xenopus oocytes. Thus, the kinetics of sodium channels expressed in Xenopus oocytes are not readily characterizable by two-electrode clamp because of the large membrane capacitance and resulting slow clamp settling time which artifactually selects for slow mode channels.
Ruben
P C
PC
Department of Biology, Utah State University, Logan 84322-5305, USA. pruben@cc.usu.edu
Fleig
A
A
Featherstone
D
D
Starkus
J G
JG
Rayner
M D
MD
D000818:Animals; D001921:Brain / Q000378:metabolism*; D004566:Electrodes; D005260:Female; D008959:Models, Neurological; D009865:Oocytes / Q000378:metabolism*; D018408:Patch-Clamp Techniques / Q000295:instrumentation / Q000379:methods; D051381:Rats; D015222:Sodium Channels / Q000378:metabolism / Q000502:physiology*; D014982:Xenopus laevis
D015222:Sodium Channels