ABSTRACT
The isolated midgut of lepidopteran larvae actively transports K from haemolymph to lumen, providing a model for K transport in insect secretory and excretory tissues (reviewed by Harvey, 1980). The dependence of this transport on extracellular K has been studied by several investigators. Recently, Zerahn (1982) estimated a value of 10 mM for Km in Hyalophora cecropia from his data and also from previous observations (Harvey & Zerahn, 1972), and obtained a value of 40 mM from data for Manduca sexta (Moffett, 1979). In the studies upon H. cecropia, the only cation present in the bathing solution was K; in the study upon M. sexta, 5 mM-Ca and 5 mM-Mg were also present as well as sufficient NaCl to maintain constant osmolarity. Although there may be slight innate differences in the transport kinetics of the two species, we show here that the reported differences can be accounted for by the presence or absence of divalent cations.
