ACTIVE TRANSPORT BY THE CECROPIA MIDGUT. I. INHIBITORS, STIMULANTS, AND POTASSIUM-TRANSPORT.

Abstract

AbstractHarvey and Nedergaard ('64 Proc. Nat. Acad. Sci., 51: 757–765) have shown that midguts isolated from mature larvae of the Cecropia silkworm, when perfused in 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 in pH. Ouabain, a cardiac glycoside which is thought to be a specific inhibitor of sodium transport, was without effect at concentrations as high as 10−4 M.Barely affected by 5% CO2, the current was strongly and reversibly depressed by 25% CO2. The carbonic anhydrase inhibitor hygroton at 10−3 M was without effect, but the related sulfonamide cardrase caused 36% inhibition at 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 10−4M and 87% at 10−3 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.