Harvey and Nedergaard ('64 Proc. Nat. Acad. Sci.,
have shown that midguts isolated from mature larvae of the Cecropia silkworm, when perfused i n aerated, agitated physiological solution, exhibit a large electrical potential .with the lumen-side positive to the blood-side. Isotope studies show that potassium carries 83% of the current generated by the midgut when the potential is shortcircuited. These and other data demonstrate that potassium is actively transported from blood-side to lumen-side of the midgut epithelium. Neither the potential nor the current requires sodium.
The effects of various chemicals on this sodium-independent active transport of potassium were examined. The short-circuit current was rapidly and reversibly inhibited by anoxia and 2,4-dinitrophenol. An irreversible inhibition was effected by iodoacetate. No observable change was produced by cholinesterase inhibitors, adrenalin, pituitary hormones or small changes i n pH. Ouabain, a cardiac glycoside which is thought to be a specific inhibitor of sodium transport, was without effect at concentrations as high as 10-4M.
Barely affected by 5% COZ, the current was strongly and reversibly depressed by 25% C 0 2 . The carbonic anhydrase inhibitor hygroton at 10-3M was without effect, but the related sulfonamide cardrase caused 36% inhibition a t this concentration. The sulfonamides are barely soluble in water and perhaps penetrate the midgut cells with difficulty. Another type of carbonic anhydrase inhibitor, sodium sulfide, caused reversible inhibitions of 31% at lo-* M and 87% at M respectively. Clearly the potassium transporting system of Hyalophora cecropia has important differences from sodium systems, and possibly employs a potassium, hydrogen ion-linked pump.