H. G. Hempling

Analysis of ion channels by modeling the osmotic effects of weak acids and bases

SummaryThis paper describes computer programs which may be used to identify and analyze cation and anion channels. Weak acids are used to increase intracellular proton concentrations and by so doing to promote the exchange of osmotically active cations with protons. The time course of cation exchange is readily identified from the changes in cell volume which accompany the net changes in osmotically active cations. Weak bases are used to identify and analyze hydroxyl/anion exchange by a comparable strategy. The model was able to produce data that agreed with experimental data in the literatur with an accuracy equal to experimental error. One program, called PROPIONATE, uses the weak acid, propionic acid, to identify cation channels such as the sodium-proton exchanger or the calcium-dependent, potassium channel. A second program, called BASE, is more general because either a weak acid such as propionic acid or a weak base such as ammonia may be used individually or together. When experimental data ara available, the programs may be used to calculate permeability coefficients for ion channels and the capacity of intracellular buffers. The programs may be used also in the design of experiments. Initial values may be assigned to intracellular and extracellular electrolyte and proton concentrations. Values for intracellular buffer capacity and channel permeabilities may be chosen. The program will then generate changes in ions, cell volume, and intracellular pH when either a weak acid, a weak base of combination of the two is added to the external medium.

Mass Transfer of Liquids Across Biological Barriers

I believe that I was asked to address my colleagues in Cryobiology because of my interest in water and how it moves across biological barriers. Apparently freezing produces differential changes in the osmolarity of cell and medium and having water around which can crystallize is not a happy event. Therefore a major portion of this talk will be devoted to the transfer of water between compartments as a consequence of osmotic gradients.

Analyzing an electrogenic cotransporter

A model for the sodium-dependent accumulation of glutamate by synaptosomes has been presented which fits the data of Wheeler and his coworkers and supports their hypothesis of an electrogenic cotransporter. Since their hypothesis was based on experimental data on the operation of the cotransporter on the outer membrane, the model was expanded to predict events when the cotransporter was operating on both sides of the membrane. The model predicts that the accumulation of glutamate is sensitive to the synaptosomal sodium and emphasizes the importance of the sodium/potassium pump to maintain this value. A model which uses only an electrogenic form of the cotransporter on the external membrane and a neutral form on the inside of the membrane predicts too much or too little accumulation of glutamate at different membrane potentials. A model which uses an electrogenic cotransporter on the external membrane and a concentration-dependent sodium glutamate leak would require a significant increase in the permeability of sodium glutamate when the membrane depolarizes. Only the operation of all four mentioned mechanisms will fit experimental data at two different external sodium concentrations and over the range of membrane potentials measured experimentally.