Abstract
In fourth instar larvae of Camptochironomus tentans, net sodium uptake from 2 mM-NaCl has an electrogenic component. During net uptake the transepithelial potential (TEP) alters from a value of approximately - 40 mV (sign refers to haemolymph), in depleted animals, to approximately o mV. The TEP in depleted larvae is dependent upon external sodium concentration above about I mM-Na+, becoming increasingly electropositive (haemolymph relative to medium) at high sodium concentrations. This effect is exaggerated in Na2SO4 compared with NaCl. At an external concentration of 2mM-NaCl, chloride is carried by an electroneutral mechanism, probably a closely coupled Cl-/anion exchange. However, it is possible that chloride transport could become somewhat electrogenic at higher concentrations. Lithium competes with sodium for the electrogenic pump. Observed TEPs differ greatly from those required to maintain passively the haemolymph concentrations of sodium and chloride.
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