Na+ binding to the Na(+)-glucose cotransporter is potential dependent

• Published in: The American journal of physiology Vol. 262; no. 2 Pt 1; pp. C510 - C516
• Main Authors: Bennett, E, Kimmich, G A
• Format: Journal Article
• Language: English
• Published: United States 01-02-1992
• Subjects: Cell Line; Electrophysiology; Kinetics; Membrane Potentials; Methylglucosides - metabolism; Models, Biological; Monosaccharide Transport Proteins - metabolism; Osmolar Concentration; Sodium - metabolism
• ISSN: 0002-9513
• DOI: 10.1152/ajpcell.1992.262.2.C510

Activity of the Na(+)-glucose cotransporter in LLC-PK1 epithelial cells was assayed by measuring sugar-induced currents (IAMG) using whole cell recording techniques. IAMG was compared among cells by standardizing the measured currents to cell size using cell capacitance measurements. IAMG at a given membrane potential was measured as a function of alpha-methylglucoside (AMG) concentration and can be fit to Michaelis-Menten kinetics. IAMG at varying Na+ concentrations can be described by the Hill equation with a Hill coefficient of 1.6 at all tested potentials. At high external Na+ levels (155 mM), Na+ is at least 90% saturating at all tested potentials. Maximal currents at a given membrane potential (Im) are calculated from the Michaelis-Menten equation fit to data measuring IAMG vs. AMG concentration at a constant Na+ concentration. Im showed potential dependence under all conditions. Potential-dependent Na+ binding rate(s) cannot alone explain the observed potential dependence of Im under saturating Na+ conditions. Therefore, because Im is potential dependent, at least one step of the transport cycle other than external Na+ binding must be potential dependent. Im was also calculated from data taken at 40 mM external Na+. At all potentials studied, Im at 155 mM Na+ is greater than Im calculated at 40 mM Na+. This implies that the rate of external Na+ binding to the transporter at 40 mM also affects the maximal transport rate. Furthermore, Im at 40 mM external Na+ increases with hyperpolarization faster than Im at 155 mM Na+. Together, these facts indicate that the rate at which Na+ binds to the transporter is also potential dependent.