Premium
Isotonic transport by the Na + ‐glucose cotransporter SGLT1 from humans and rabbit
Author(s) -
Zeuthen T.,
Meinild A.K.,
Loo D. D. F.,
Wright E. M.,
Klaerke D. A.
Publication year - 2001
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2001.0631h.x
Subject(s) - cotransporter , water transport , voltage clamp , osmosis , tonicity , biophysics , chemistry , intracellular , membrane potential , membrane transport , reversal potential , osmotic pressure , patch clamp , biochemistry , membrane , sodium , biology , water flow , environmental engineering , receptor , organic chemistry , engineering
1 In order to study its role in steady state water transport, the Na + ‐glucose cotransporter (SGLT1) was expressed in Xenopus laevis oocytes; both the human and the rabbit clones were tested. The transport activity was monitored as a clamp current and the flux of water followed optically as the change in oocyte volume. 2 SGLT1 has two modes of water transport. First, it acts as a molecular water pump: for each 2 Na + and 1 sugar molecule 264 water molecules were cotransported in the human SGLT1 (hSGLT1), 424 for the rabbit SGLT1 (rSGLT1). Second, it acts as a water channel. 3 The cotransport of water was tightly coupled to the sugar‐induced clamp current. Instantaneous changes in clamp current induced by changes in clamp voltage were accompanied by instantaneous changes in the rate of water transport. 4 The cotransported solution was predicted to be hypertonic, and an osmotic gradient built up across the oocyte membrane with continued transport; this resulted in an additional osmotic influx of water. After 5‐10 min a steady state was achieved in which the total influx was predicted to be isotonic with the intracellular solution. 5 With the given expression levels, the steady state water transport was divided about equally between cotransport, osmosis across the SGLT1 and osmosis across the native oocyte membrane. 6 Coexpression of AQP1 with the SGLT1 increased the water permeability more than 10‐fold and steady state isotonic transport was achieved after less than 2 s of sugar activation. One‐third of the water was cotransported, and the remainder was osmotically driven through the AQP1. 7 The data suggest that SGLT1 has three roles in isotonic water transport: it cotransports water directly, it supplies a passive pathway for osmotic water transport, and it generates an osmotic driving force that can be employed by other pathways, for example aquaporins.