An energy-barrier model for the permeation of monovalent and divalent cations through the maxi cation channel in the plasma membrane of rye roots

The depolarization-activated, high-conductance "maxi" cation channel in the plasma membrane of rye (Secale cereale L.) roots is permeable to a wide va riety of monovalent and divalent cations. The permeation of K+, Na+, Ca2+ a nd Ba2+ through the pore could be simulated using a model composed of three energy barriers and two ion binding sites (a 3B2S model), which assumed si ngle-file permeation and the possibility of double cation occupancy. The mo del had an asymmetrical free energy profile. Differences in permeation betw een cations were attributed primarily to differences in their free energy p rofiles in the regions of the pore adjacent to the extracellular solution. In particular, the height of the central free energy peak differed between cations, and cations differed in their affinities for ion binding sites. Si gnificant ion repulsion occurred within the pore, and the mouths of the por e had considerable surface charge. The model adequately described the diver se current vs. voltage (IN) relationships obtained over a wide variety of e xperimental conditions. It described the phenomena of non-Michaelian unitar y conductance vs. activity relationships for K+, Na+ and Ca2+, differences in selectivity sequences obtained from measurements of conductance and perm eability ratios, changes in relative cation permeabilities with solution co mposition, and the complex effects of Ba2+ and Ca2+ on K+ currents through the channel. The model enabled the prediction of unitary currents and ion f luxes through the maxi cation channel under physiological conditions. It co uld be used, in combination with data on the kinetics of the channel, as in put to electrocoupling models allowing the relationships between membrane v oltage, Ca2+ influx and Ca2+ signaling to be studied theoretically.