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The Journal of Membrane Biology

, Volume 39, Issue 4, pp 333–367 | Cite as

Current-voltage relationships for the plasma membrane and its principal electrogenic pump inNeurospora crassa: I. Steady-state conditions

  • Dietrich Gradmann
  • Ulf-Peter Hansen
  • W. Scott Long
  • Clifford L. Slayman
  • Jens Warncke
Article

Summary

The nonlinear membrane current-voltage relationship (I–V curve) for intact hyphae ofNeurospora crassa has been determined by means of a 3-electrode voltage-clamp technique, plus “quasi-linear” cable theory. Under normal conditions of growth and respiration, the membraneI–V curve is best described as a parabolic segement convex in the direction of depolarizing current. At the average resting potential of −174 mV, the membrane conductance is ≈190 μmhos/cm2; conductance increases to ≈240 μmhos/cm2 at −300 mV, and decreases to ≈130 μmhos/cm2 at 0 mV. Irreversible membrane breakdown occurs at potentials beyond this range.

Inhibition of the primary electrogenic pump inNeurospora by ATP withdrawal (with 1mm KCN) depolarizes the membrane to the range of −40 to −70 mV and reduces the slope of theI–V curve by a fixed scaling factor of approximately 0.8. For wild-typeNeurospora, compared under control conditions and during steady-state inhibition by cyanide, theI–V difference curve — presumed to define the current-voltage curve for the electrogenic pump — is a saturation function with maximal current of ≈20 μA/cm2, a half-saturation potential near −300 mV, and a projected reversal potential of ca. −400 mV. This value is close to the maximal free energy available to the pump from ATP hydrolysis, so that pump stoichiometry must be close to 1 H+ extruded:1 ATP split.

The time-courses of change in membrane potential and resistance with cyanide are compatible with the steady-stateI–V curves, under the assumption that cyanide has no major effects other than ATP withdrawal. Other inhibitors, uncouplers, and lowered temperature all have more complicated effects.

The detailed temporal analysis of voltage-clamp data showed three time-constants in the clamping currents: one of 10 msec, for charging the membrane capacitance (0.9 μF/cm2) a second of 50–75 msec; and a third of 20–30 sec, perhaps representing changes of intracellular composition.

Keywords

Versus Curve Electrogenic Pump Cable Theory Parabolic Segement Nonlinear Membrane 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer-Verlag New York Inc. 1978

Authors and Affiliations

  • Dietrich Gradmann
    • 1
  • Ulf-Peter Hansen
    • 1
  • W. Scott Long
    • 1
  • Clifford L. Slayman
    • 1
  • Jens Warncke
    • 1
  1. 1.Department of PhysiologyYale School of MedicineNew Haven

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