Larvae of Manduca sexta were fed on tobacco leaves or synthetic diets with differing potassium content. For all diets tested, midgut lumen K+ activity coefficients were low, and midgut lumen K+ activities, K+ concentrations and pH values were as closely regulated as in blood. The transmidgut electrochemical potential difference for potassium based on activities was lower than had previously been estimated from concentrations, but was dominated by the electrical gradient established by the electrogenic K+ ATPase. Midgut K+ activities and pH profiles were similar, as would be suggested by a model in which the generation of high pH was linked to electrogenic K+ transport.

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