Quantitative Analysis of Resting Membrane Electrogenesis in Insect (diptera) Skeletal Muscle H. Testing of a Model Involving Contributions from Potassium and Sodium Ions, and the Anomalous Effect of Reducing Extracellular Sodium

A model of resting membrane electrogenesis in skeletal muscles of prepupal Calliphora erythrocephala was formulated. From experiments in which reversible effects of changing extracellular K and Na activities on the membrane potential (EM) were measured, three different values of a (the ratio of the partial permeabilities of the membrane to Na and K) were derived, each from a different range of extracellular Na and K activities. Two independent tests were carried out to determine the most realistic value of a. Intracellular K and Na activities and EM values were measured in a population of cells, and the EM values predicted using the Goldman-Hodgkin-Katz equation for different values of a. The best fit for the data was obtained for a-= 0-036. In ionic substitution experiments, in which passive movements of Cl~ were prevented or minimized, the changes in EM around the resting level could be explained with a high degree of accuracy by assuming again that a = 0-036. However, tests of the model by investigation of direct effects of reducing extracellular Na concentration over a wide range of EM values gave an anomalous result. In low-Na + Ringer, EM values became more positive than the respective resting levels. The anomalous effect of low-Na Ringer on EM did not involve a change in the K + equilibrium potential. Instead, it was probably due to a reduction in the K permeability of the membrane. Possible mechanisms underlying this effect are discussed.