AJP - Cell Physiology, Vol 258, Issue 2 C234-C242, Copyright © 1990 by American Physiological Society
Whole cell recording of sugar-induced currents in LLC-PK1 cells
C. Smith-Maxwell, E. Bennett, J. Randles and G. A. Kimmich
Department of Biophysics, School of Medicine and Dentistry, University of Rochester, New York 14642.
Gigaohm-seal whole cell recording techniques were used to monitor function
of the Na(+)-coupled sugar transport system in LLC-PK1 cells. The currents
coupled to sugar transport were identified as those that are induced by the
presence of 10 mM alpha-methylglucoside (AMG) in either the extracellular
or intracellular compartment and were inhibited by addition of 320-800
microM phlorizin to the extracellular bathing medium. The sugar-induced
currents are small, 15-20 pA, but of the expected magnitude as determined
from the known kinetic parameters for Na(+)-coupled sugar transport in
LLC-PK1 cells. The phlorizin-sensitive currents are Na+ dependent and can
be studied under conditions in which the net Na+ and sugar flux (and
consequently the Na+ electrical current) is in either the inward or outward
direction. The reversal potential of the sugar-induced currents measured
under conditions with high Na+ and AMG concentrations inside the cell is
close to values predicted from thermodynamic principles, assuming a
coupling stoichiometry of 2 Na+: 1 sugar for the transport system. The
reversal potential of the sugar-induced currents with high extracellular
Na+ and AMG is not equal to the predicted value, but it is of the polarity
expected for inward-imposed solute gradients. Reasons for the observed
discrepancy between observed and calculated values are discussed.
