Voltage-clamp studies of the Na+/glucose cotransporter cloned from rabbit small intestine.
Inward Na+ currents associated with the cloned intestinal Na+/glucose cotransporter expressed in Xenopus oocytes have been studied using the two-microelectrode voltage-clamp method. The steady-state current/voltage relations showed voltage-dependent (Vm from +20 to -75 mV) and relatively voltage-independent (Vm from -75 to -150 mV) regions. The apparent Imax for Na+ and glucose increased with negative membrane potentials, and the apparent K0.5 for glucose (K(Glc)0.5) depended on Vm and [Na]o. Increasing [Na]o from 7 to 110 mmol/l had the same effect in decreasing K(Glc)0.5 from 0.44 to 0.03 mmol/l as increasing the Vm from -40 to -150 mV. The I/V curves under saturating conditions (20 mmol/l external sugars and 110 mmol/l [Na]o) were identical for D-glucose, D-galactose, alpha-methyl D-glucopyranoside and 3-O-methyl D-glucoside. The specificity of the cotransporter for sugars was: D-glucose, D-galactose, alpha-methyl D-glucopyranoside greater than 3-O-methyl D-glucoside much greater than D-xylose greater than D-allose much greater than D-mannose. Ki for phlorizin (approximately 10 mumol/l) was independent of Vm at saturating [Na]o. We conclude that a variety of sugars are transported by the cloned Na+/glucose cotransporter at the same maximal rate and that membrane potential affects both the maximal current and the apparent K0.5 of the cotransporter for Na+ and glucose.
Birnir
B
B
Department of Physiology, UCLA School of Medicine 90024-1751.
Loo
D D
DD
Wright
E M
EM
D000818:Animals; D001667:Binding, Competitive; D001693:Biological Transport, Active; D003001:Cloning, Molecular; D005947:Glucose / Q000378:metabolism; D007421:Intestine, Small / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials; D008839:Microelectrodes; D009004:Monosaccharide Transport Proteins / Q000235:genetics / Q000378:metabolism*; D009865:Oocytes / Q000378:metabolism; D010695:Phlorhizin / Q000494:pharmacology; D011817:Rabbits; D012964:Sodium / Q000378:metabolism; D014982:Xenopus laevis
D009004:Monosaccharide Transport Proteins; D012964:Sodium; D010695:Phlorhizin; D005947:Glucose