Sugar Transport Heterogeneity in the Kidney: Two Independent Transporters or Different Transport Modes through an Oligomeric Protein? 1. Glucose Transport Studies |
| |
Authors: | N Oulianova A Berteloot |
| |
Institution: | (1) Membrane Transport Research Group, Department of Physiology, Faculty of Medicine, Université de Montreal, CP 6128, succursale ``Centre-Ville,' Montreal (Québec) Canada H3C 3J7, CA |
| |
Abstract: | The kinetics of Na+/d-glucose cotransport (SGLT) were reevaluated in rabbit renal brush border membrane vesicles isolated from the whole kidney
cortex using a fast-sampling, rapid-filtration apparatus (FSRFA, US patent #5,330,717) for uptake measurements. Our results
confirm SGLT heterogeneity in this preparation, and both high (HAG) and low (LAG) affinity glucose transport pathways can
be separated over the 15–30°C range of temperatures. It is further shown that: (i) Na+ is an essential activator of both HAG and LAG; (ii) similar energies of activation can be estimated from the linear Arrhenius
plots constructed from the V
max data of HAG and LAG, thus suggesting that the lipid composition and/or the physical state of the membrane do not affect much
the functioning of SGLT; (iii) similar V
max values are observed for glucose and galactose transport through HAG and LAG, thus demonstrating that the two substrates share
the same carrier agencies; and (iv) phlorizin inhibits both HAG and LAG competitively and with equal potency (K
i
= 15 μm). Individually, these data do not allow us to resolve conclusively whether the kinetic heterogeneity of SGLT results from
the expression in the proximal tubule of either two independent transporters (rSGLT1 and rSGLT2) or from a unique transporter
(rSGLT1) showing allosteric kinetics. Altogether and compared to the kinetic characteristics of the cloned SGLT1 and SGLT2
systems, they do point to a number of inconsistencies that lead us to conclude the latter possibility, although it is recognized
that the two alternatives are not mutually exclusive. It is further suggested, from the differences in the K
m
values of HAG transport in the kidney as compared to the small intestine and SGLT1 cRNA-injected oocytes, that renal SGLT1
activity is somehow modulated, maybe through heteroassociation with (a) regulatory subunit(s) that might also contribute quite
significantly to sugar transport heterogeneity in the kidney proximal tubule.
Received: 25 October 1995/Revised: 10 June 1996 |
| |
Keywords: | : Na+-dependent sugar transport — Substrate specificity — Phlorizin inhibition — Temperature dependence — Membrane vesicles (rabbit kidney) |
本文献已被 SpringerLink 等数据库收录! |
|