Contributions of secondary active transport processes to membrane potentials.

Equations are developed to examine the effects of secondary active transport processes on the steady-state membrane potential of symmetrical cells. It is shown that, with suitable modifications, equations of the type developed by Goldman, Hodgkin and Katz may be derived to accommodate the contributions to the membrane potential of both electroneutral and electrogenic transporters. Where the membrane potential is a function of the dominant medium ions (Na, K, and Cl), other contributions can come only from an electrogenic Na pump and from neutral co- and counter-transporters if, and only if, these involve the dominant ions. Experimental approaches to measure the parameters necessary to solve the equations developed here are discussed.