AJP - Cell Physiology, Vol 262, Issue 2 C510-C516, Copyright © 1992 by American Physiological Society
Na+ binding to the Na(+)-glucose cotransporter is potential dependent
E. Bennett and G. A. Kimmich
Department of Biophysics, University of Rochester, School of Medicine and Dentistry, New York 14642.
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.(ABSTRACT TRUNCATED AT 250 WORDS)
