title: 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.
mesh_terms: D000818:Animals; D002217:Carbachol / Q000494:pharmacology*; D002463:Cell Membrane Permeability / Q000187:drug effects; D004058:Diffusion; D004553:Electric Conductivity; D005724:Ganglia / Q000378:metabolism*; D008564:Membrane Potentials; D008974:Mollusca; D011188:Potassium / Q000378:metabolism*
chemical_list: D002217:Carbachol; D011188:Potassium