Na⁺ −K⁺ −Cl⁻ co-transport in the intestine of a marine teleost

Abstract

A key element in the absorption of salt and water by various epithelia is the presence, in the apical or brush border membrane, of a mechanism for directly coupling the inward movements of Na⁺ and Cl⁻ (ref. 1). In this way the downhill gradient for Na⁺ from lumen to cell, which is maintained by the cell's Na⁺/K⁺ pump, can provide the energy for cellular accumulation and transepithelial transport of Cl⁻. A requirement for K⁺ at the serosal surface of the epithelium has long been accepted as essential to the operation of the Na⁺/K⁺ pump and therefore to salt absorption. Until very recently, however, the effect of luminal K⁺ on NaCl absorption has not been explored. We show here that luminal K⁺ stimulates NaCl absorption in the intestine of a marine teleost, the winter flounder, Pseudopleuronectes americanus. Furthermore, K⁺ uptake across the brush border membrane depends on the presence of both Na⁺ and Cl⁻ in the bathing medium and is inhibited by furosemide, which also inhibits the coupled uptake of Na⁺ and Cl⁻ (ref. 2). Thus, flounder intestine seems to have a Na⁺ −K⁺ −Cl⁻ co-transport system similar to that demonstrated in erythrocytes^3–5, cultured tumour cells⁶, cultured kidney cells⁷ and squid giant axon⁸. Electrophysiological studies of the thick ascending limb of Henlé's loop in mammalian kidney⁹ and the diluting segment in amphibian kidney¹⁰ also indicate that luminal K⁺ is required for salt absorption. Co-transport of Na⁺, K⁺ and Cl⁻ may therefore be characteristic of a variety of cells and tissues, including salt-absorbing epithelia.