Biophysical Journal 18: 323-349 (1977)
© 1977 the Biophysical Society

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Schwartz, T L Articles by Kado, R T Search for Related Content PubMed PubMed Citation Articles by Schwartz, T L Articles by Kado, R T

Permeability, phase-boundary potential and conductance in a cholinergic channel without constant field.

ABSTRACT

A potassium-selective, chemically excitable channel, whose characteristics cannot be accurately described by constant-field theory, is studied by a new approach based on diffusion theory but with no need for the classical assumptions of constant field, homogeneous membrane, and equal phase-boundary potentials at both interfaces. Permeability is defined, free of these constraints, and the Goldman coefficient is demonstrated to be a special case useful only when the constraints apply. Permeability can be evaluated directly from current-voltage data, and it is found not to be a parameter in this channel, but rather a function of both the voltage and the concentration of the permeant ion. However, it becomes concentration-independent when the membrane voltage is equal to the sum of the phase-boundary potentials. That sum can therefore be determined from these data, and it is -65 mV in this channel. The permeability at that potential is a channel parameter, and equal to 8.66 X 10(-6) cm/s for this channel. A constant field is shown not to exist in this channel and the Goldman coefficient not to be a parameter but a function of potential and concentration. Although errors introduced into this coefficient by nonconstant field and unequal surface potentials partially cancel each other, the coefficient is nevertheless not a correct measure of permeability.