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 alpha (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 alpha. 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 alpha. The best fit for the data was obtained for alpha = 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 alpha = 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.

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