ION PERMEATION AND GLUTAMATE RESIDUES LINKED BY POISSON-NERNST-PLANCKTHEORY IN L-TYPE CALCIUM CHANNELS

L-type Ca channels contain a cluster of four charged glutamate residue s (EEEE locus), which seem essential for high Ca specificity. To under stand how this highly charged structure might produce the currents and selectivity observed in this channel, a theory is needed that relates charge to current. We use an extended Poisson-Nernst-Planck (PNP2) th eory to compute (mean) Coulombic interactions and thus to examine the role of the mean field electrostatic interactions in producing current and selectivity. The pore was modeled as a central cylinder with tape red atria; the cylinder (i.e., ''pore proper'') contained a uniform vo lume density of fixed charge equivalent to that of one to four carboxy l groups. The pore proper was assigned ion-specific, but spatially uni form, diffusion coefficients and excess chemical potentials. Thus elec trostatic selection by valency was computed self-consistently, and sel ection by other features was also allowed. The five external parameter s needed for a system of four ionic species (Na, Ca, CI, and H) were d etermined analytically from published measurements of three limiting c onductances and two critical ion concentrations, while treating the po re as a macroscopic ion-exchange system in equilibrium with a uniform bath solution. The extended PNP equations were solved with these param eters, and the predictions were compared to currents measured in a var iety of solutions over a range of transmembrane voltages. The extended PNP theory accurately predicted current-voltage relations, anomalous mole fraction effects in the observed current, saturation effects of v aried Ca and Na concentrations, and block by protons. Pore geometry, d ielectric permittivity, and the number of carboxyl groups had only wea k effects. The successful prediction of Ca fluxes in this paper demons trates that ad hoc electrostatic parameters, multiple discrete binding sites, and logistic assumptions of single-file movement are all unnec essary for the prediction of permeation in Ca channels over a wide ran ge of conditions. Further work is needed, however, to understand the a tomic origin of the fixed charge, excess chemical potentials, and diff usion coefficients of the channel. The Appendix uses PNP2 theory to pr edict ionic currents for published ''barrier-and-well'' energy profile s of this channel.