ISSN:
1432-1017
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Physics
Notes:
Abstract Using a very low noise voltage clamp technique it has been possible to record from the squid giant axon a slow component of gating current (I g ) during the inactivation phase of the macroscopic sodium current (I Na ) which was hitherto buried in the baseline noise. In order to examine whether this slowI g contains gating charge that originates from transitions between the open (O) and the inactivated (I) states, which would indicate a true voltage dependence of inactivation, or whether other transitions contribute charge to slowI g , a new model independent analysis termed isochronic plot analysis has been developed. From a direct correlation ofI g and the time derivative of the sodium conductance dg Na/d the condition when only O-I transitions occur is detected. Then the ratio of the two signals is constant and a straight line appears in an isochronic plot ofI g vs. dg Na/d . Its slope does not depend on voltage or time and corresponds to the quantal gating charge of the O-I transition (q h ) divided by the single channel ionic conductance (γ). This condition was found at voltages above − 10 mV up to + 40 mV and a figure of 1.21e − was obtained forq h at temperatures of 5 and 15°C. At lower voltages additional charge from other transitions, e.g. closed to open, is displaced during macroscopic inactivation. This means that conventional Eyring rate analysis of the inactivation time constant τ h is only valid above − 10 mV and here the figure forq h was confirmed also from this analysis. It is further shown that most of the present controversies surrounding the voltage dependence of inactivation can be clarified. The validity of the isochronic plot analysis has been confirmed using simulated gating and ionic currents.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01561139
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