Permeability and phase-boundary potentials determined from conductance in a transmitter-activated potassium channel in Aplysia californica in the absence of a constant field.

A potassium-selective, chemically excitable channel, whose characteristics cannot be accurately described by "constant field" theory, is studied using a new approach based on diffusion theory but having 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 chord conductance, 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 8.77 (10)-6 cm/sec 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.