Computer analysis reveals changes in renal Na+-glucose cotransporter in diabetic rats.
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.
Department of Cell Physiology, University of Hamburg, Federal Republic of Germany.
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Diedrich
D000818:Animals; D002458:Cell Fractionation / Q000379:methods; D003196:Computer Graphics; D003921:Diabetes Mellitus, Experimental / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D007672:Kidney Cortex / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008433:Mathematics; D008871:Microvilli / Q000378:metabolism* / Q000648:ultrastructure; D008962:Models, Theoretical; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012016:Reference Values; D013816:Thermodynamics
D009004:Monosaccharide Transport Proteins; D005947:Glucose