Role of ionic adsorption in the excitable membrane

Ion flow through biomembranes. Physical theory explains its high sensitivity

Numerous biomembranes exhibit a sensitivity to changes in electrical potential greater than predicted as possible from the classical application of the Boltzmann relation, a phenomenon which has long defied explanation, the actual sensitivity of some Na+ channels being many times greater than the classical limit. This paper explains, using a minimum of mathematics, how the very rapid gating effect of adsorbed Ca2+ (or other impermeable divalent cations) can directly affect the conductance of channels, and thus interact with the electric field within the channel to produce a change in the potential across the channel's gate much greater than the change in the membrane potential, with a corresponding change in the fraction of open conformational gates and change in conductance. These results are not in conflict with the Boltzmann relation, the necessary energy being made available from the total potential difference across the membrane by a long unrecognized stochastic process; the full mathematical theory is given in cited references.

Ion flow through membranes and the resting potential of cells

A knowledge of the relationship between ion flow, both passive and active, ionic concentration, and membrane potential is essential to the understanding of cellular function. The problem has been analyzed on the basis of elementary physical and biophysical principles, providing a theoretical model of current flow and resting potential of cells, including those in epithelia. The model assumes that the permeability of the ion channels is not voltage dependent, but applies to gated channels when the gates are open. Two sources of nonlinearity of the current-voltage relationship are included in the analysis: ionic depletion and accumulation at the channels' mouths, and channel saturation at higher concentrations. The predictions of the model have been quantitative, validated by comparison with experiment, which has been limited to the only two cases in which adequate data was found. Application of the theory to the scala media of the mammalian cochlea has explained the source of its high positive potential and provided estimates of the Na+ and K+ permeabilities of the membranes of its marginal cells. This analysis provides a theoretically sound alternative to the widely used Goldman equation, the limited validity of which was emphasized by Goldman (D.E. Goldman, 1943, J. Gen. Physiol, 27:37-60), as well as its derivatives, including the Goldman-Hodgkin-Katz equation for resting potentials.

Positive endocochlear potential: mechanism of production by marginal cells of stria vascularis

The positive endocochlear potential (EP+) and high K+ concentration of the endolymph in the scala media of the mammalian cochlea are unusual. They have long been assumed to be due to a putative K-pump in the luminal membrane of the marginal cells of the stria vascularis, which were believed to have a negative internal potential. We show that the cell potential is more positive than the EP+, and that the ion pump is conventional Na,K-ATPase, probably in the basolateral membrane. The latter was determined from experiments in which the ionic environment of the strial cells was controlled by perfusion of the perilymphatic space of the cochlea, in the absence of vascular circulation. While the usual EP+ was maintained by normal perfusate, replacement of Na+ by choline resulted in a negative EP, showing that Na,K-ATPase is necessary for the production of EP+. Elimination of K+ as well as Na+ from the perfusate did not change the value of the negative EP, showing that no K-ATPase is involved.

Sensitivity of membranes to their environment. Role of stochastic processes

Ionic flow through biomembranes often exhibits a sensitivity to the environment, which is difficult to explain by classical theory, that usually assumes that the free energy available to change the membrane permeability results from the environmental change acting directly on the permeability control mechanism. This implies, for example, that a change delta V in the trans-membrane potential can produce a maximum free energy change, delta V X q, on a gate (control mechanism) carrying a charge q. The analysis presented here shows that when stochastic fluctuations are considered, under suitable conditions (gate cycle times rapid compared with the field relaxation time within a channel), the change in free energy is limited, not by the magnitude of the stimulus, but by the electrochemical potential difference across the membrane, which may be very much greater. Conformational channel gates probably relax more slowly than the field within the channel; this would preclude appreciable direct amplification of the stimulus. It is shown, however, that the effect of impermeable cations such as Ca++ is to restore the amplification of the stimulus through its interaction with the electric field. The analysis predicts that the effect of Ca++ should be primarily to affect the number of channels that are open, while only slightly affecting the conductivity of an open channel.