AJP - Cell Physiology, Vol 257, Issue 2 C385-C396, Copyright © 1989 by American Physiological Society
Computer analysis reveals changes in renal Na+-glucose cotransporter in diabetic rats
M. E. Blank, F. Bode, K. Baumann and D. F. Diedrich
Department of Cell Physiology, University of Hamburg, Federal Republic of Germany.
A novel, computer-assisted program was developed to analyze the time course
of Na+-glucose cotransport by rat renal cortical brush-border membrane
vesicles (BBMV). Transporter characteristics can be measured, which routine
kinetic analyses fail to distinguish: cotransporter membrane density is
derived from the picomoles of D-glucose bound per milligram of protein.
Binding is stereospecific, blocked by phlorizin, and supported equally well
by Na+ or K+ (but not Cs+). Quasi-first-order influx and efflux rate
constants for the composite Na+-driven influx and the (presumed)
Na+-independent efflux processes were highly dependent on glucose
concentration. Either two Na+-glucose transporters exist in proximal
tubules or a single mechanism abruptly changes rate when glucose falls to
low levels. The major operation mode is slow, has a high capacity but low
affinity, and may have a 2 Na+:2 glucose stoichiometry (Hill coefficient is
unity). The minor system is a fast, smaller-capacity, higher-affinity
operation with a 2 Na+:1 glucose stoichiometry that was not distinguishable
when the same data were analyzed in conventional kinetic plots. Results
with streptozocin-induced diabetic rats illustrate the method's utility.
Low-glucose-affinity cotransporters were upregulated in hyperglycemic, but
not in cachectic, ketoacidotic animals. Rate constants, especially for
efflux, were decreased in diabetes.
