A Quantitative Simulation Model for H-Amino Acid Cotransport To Interpret the Effects of Amino Acids on Membrane Potential and Extracellular pH

The H⁺ cotransport of neutral and acidic amino acids induces transient depolarizations of oat coleoptile (Avena sativa L., var Victory) plasma membranes. The depolarizations, which are completed within 1 or 2 minutes, are followed by repolarizations that are nearly completed within another 2 or 3 minutes. Cysteine induced a two-phased alkalinization of the tissue free space during the electrical changes. The first phase was a rapid, linear increase in pH that coincided with the depolarization; the second phase was a slower, also linear, increase in pH that coincided with the repolarization. Reacidification did not occur until cysteine was withdrawn. Five other acidic, basic, and neutral amino acids also induced persistent alkalinization of the free space.

The notable features of these measurements are that free-space pH was measured more directly than previously, that pH changes corresponded in time to the electrical potential changes, and that reacidification of the free space did not occur. The latter observation indicates that net H⁺ efflux did not occur during repolarization and that the repolarizing current was carried by some other ion. We propose that repolarization could have depended upon depolarization-induced changes in passive K⁺ fluxes combined with an enhanced H⁺ extrusion that increased until it equaled, but did not exceed, the enhanced influx of H⁺.

In support of the feasibility of our hypothesis, we present a quantitative simulation model for cotransport. The simulation model also provides an interpretation of the unique electrical effects of histidine and the basic amino acids. In addition, the model focuses attention upon the difficulties of interpreting H⁺-anion cotransport.