Active transport by the cecropia midgut. IV. Specificity of the transport mechanism for potassium.

Abstract

ABSTRACT Although the addition of 32 mM/l. choline chloride to the standard bathing solutions depresses the short-circuit current of the isolated midgut, the replacement of the added choline chloride by either sodium or lithium chloride has no further affect on the current. No net flux of either sodium or lithium is measureable by atomic absorption spectrometry. Therefore neither sodium nor lithium is actively transported across the midgut. Rubidium is able to substitute fully for potassium in maintaining a short-circuit current. The net fluxes of rubidium and potassium are approximately in the same ratio as their concentrations in the solutions bathing the blood-side of the isolated midgut. Therefore the midgut is unable to distinguish between potassium and rubidium. Ammonium is unable to replace potassium in sustaining a current. In double-label experiments using 42K to measure one unidirectional flux and 86Rb to measure the other, the net potassium-rubidium flux accounts for about 87 % of the short-circuit current. The short-circuit current increases with increasing potassium concentration, at least in the range from 2 to 64 mM/l., although the exact function depends on the conditions under which the measurements are made. The midgut isolated from the Cecropia silkworm appears to possess a ‘potassium pump’ with the simple ionic requirement that potassium be present in the solution bathing the blood-side and presumably that potassium be present in the cells.