Influx theory and size of potassium and rubidium pools in the midgut of Hyalophora cecropia.

1. The midgut contains a 'pool' which must mix with tracer added to the blood-side, before the transported potassium reaches a steady specific activity. The size and mixing time of the pool can be deduced from the time-course of rate of appearance of tracer on the lumen-side, the tracer influx. The theory has been extended to cover cases where the pool-size and the transport rate vary with time. 2. By using 42K and 86Rb together, it is shown that the midgut treats potassium and rubidium differently, rubidium being transported nine-tenths as fast as potassium, when both are present in equal concentration. The influx pool for potassium is one-third larger than that for rubidium, but mixes four-fifths as fast with blood-side tracer. 3. The size of the potassium pool decreases with time, but that for rubidium does not, and the ratios of mixing times and transport rates are constant. 4. The implications of the results are discussed, both in terms of the accuracy of previous investigations and with respect to the probable intracellular location of part of the influx pool.