Active transport of potassium by the Cecropia midgut; tracer kinetic theory and transport pool size

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Abstract
Tracer influx kinetics have been analysed theoretically to determine the size of the transport pool with no assumptions regarding the transport pathway. For a calculation of the size of the transport pool to be made, the following six conditions are required by the theory: tracer steady state attained, tissue steady state attained, Isc measures net flux, small magnitude and constant time-course of efflux, and correction for decay in pumping rate. The size of the pool, S1 is given by the steady state influx, J∞, divided by the mixing-time constant, α. Some experimental results are analysed by three different graphical methods, and it is shown that these three methods are equivalent. Specifically, a is equal to the reciprocal of the 75 % mixing time, divided by In 4 and is equal to the reciprocal of the lag time, X. The tracer kinetic theory is applied to active potassium transport across the isolated short-circuited midgut: the transport meets the six conditions required by the theory. The size of the transport pool of potassium in one midgut is calculated to be 80·5 μequiv./g wet weight under high-K steady-state conditions. A value as high as this suggests that the pool is intracellular.