Presentation on theme: "TUBULAR REABSORPTION OF GLUCOSE, AMINO ACIDS, UREA & OTHER ELECTROLYTES LECTURE 6."— Presentation transcript:
1 TUBULAR REABSORPTION OF GLUCOSE, AMINO ACIDS, UREA & OTHER ELECTROLYTES LECTURE 6
2 Glucose reabsorption general consideration Glucose reabsorption is calculated as the difference between the amount of glucose filtered by the kidney and the amount excreted.When plasma glucose (PG) is increased to near 200 mg/dl, glucose begins to appear in the urine – this is called the “glucose renal threshold”
3 As glucose is further increased, more glucose appears in the urine. At very high filtered glucose, reabsorption remains constant, this is called “tubular transport maximum” for glucose (TmG)At this maximum transport, all the glucose carriers are saturated and no more glucose can be transported
4 Glucose reabsorption Mechanism of glucose reabsorption Secondary active transportLuminal membraneCotransport with NaBasolateral membraneGLUT2
5 Cellular Mechanism for Glucose Reabsorption The luminal membrane of the epithelial cells faces the tubular fluid (lumen) and contains the Na+-glucose co-transporter. The peritubular membrane or basolateral membrane of the cells faces the peritubular capillary blood and contains the Na+-K+ ATPase and the facilitated glucose transporter.
6 Cellular Mechanism for Glucose Reabsorption LUMENBLOODCell of the proximal tubuleNa+Na+K+GlucoseGlucoseCellular Mechanism for Glucose Reabsorption
7 Steps involved in reabsorbing glucose from tubular fluid into peritubular capillary blood Glucose move from tubular fluid cell by binding with Na+ to the cotransport protein (GLUT1) which rotates in the membrane Na+ and glucose released to ICF. Glucose is transported against an electrochemical gradient.Na+ gradient is maintained by the Na-K ATPase in the peritubular membrane. Because ATP is used directly to energize the Na-K ATPase and indirectly to maintain the Na gradient, Na+-glucose cotransport called secondary active transport.Glucose transported from cell peritubular capillary blood by facilitated diffusion (GLUT2). Glucose move down electrochemical gradient, no energy required.
8 Glucose Reabsorption GLUT 2 SGLT 2 Na+ K+ Glucose One Na+ Early proximaltubule cellINTERSTITIALFLUIDTUBULARLUMENK+Na+GlucoseGlucoseGLUT 1SGLT 1Two Na+Late proximaltubule cell
9 Glucose Titration Curve and Tm A glucose titration curve depicts the relationship between plasma glucose concentration and glucose reabsorption. It is best understood by examing each relationship separately and then by considering all three relationships together.
12 Tubular maximum (Tmg) Renal threshold Maximum absorptive capacity for glucose by renal tubular cells375 mg/min (female 300mg/min)Renal thresholdPlasma glucose level at which glucose first appear in urine200mg/dl in arterial; 180 mg/dl in venous
13 Glucose absorption is inhibited by Phlorhizin competes for binding to the carrier blood glucose level ( renal threshold) exceed Tm glucose in urine glucosuria Diabetes Mellitus
14 Glucose filtered, reabsorbed, 800600400200FilteredGFR x PGExcretedUG x VGlucose filtered, reabsorbed,or excreted (mg/min)TmG (375)ReabsorbedSplayThreshold (200)Plasma glucose (mg/dl)
15 The threshold for glucose is affected by the following: GFR – a low GFR causes an increased threshold because the filtered glucose is decreased and the kidney can reabsorb the filtered glucose even though the plasma glucose is increased (more time for reabsorption)TmG – a decreased TmG lowers the threshold because the tubules have a reduced capacity to reabsorb glucose.Splay – “rounded” as it approaches its maximum which is caused by different nephrons having different reabsorption and filtering capacities.
16 Amino acid reabsorption All filtered AAs are reabsorbed in PCTLuminal membraneCotransport with NaBasolateral membranediffusion
17 Bicarbonate reabsorption 90% of filtered is reabsorbed in PCTFiltered HCO3 + H2O H2CO3H2CO3 H2O + CO2 in the presence of carbonic anhydraseCO2 diffuses into the cell + H2OH2CO3H2CO3 CA H + HCO3HCO3 is reabsorpedH+ is secreted in exchange for Na +
19 Phosphate reabsorption Bones, teeth & skeleton (80%)Intracellular P (20%)Plasma P 1mmol/l freely filtered1/3 of filtered is excreted in urineCotransported with NaRate of absorption is under the control of PTH & VD (rate of absorption)Compete with glucose: blocking glucose P reabsorption
20 Urea reabsorption Plasma urea concentraion 15-40mg/100ml End product of protein metabolism40-50% of filtered urea reabsorbedPassive diffusionReabsorbed in consequent of Na reabsorption50-60% excretedGFRConcentration in blood
21 Urea reabsorption cont. GFR (renal disease; low renal blood flow) urea concentraion in plasmaGFR urea filteredGFR slow flow rate of filterate more urea is absorbed to blood