Lecture 24: Traffic Across Cells: Glucose Absorption Flashcards
How many steps in glucose absorption?
2
What protein contransports glucose and sodium across the apical membrane?
SGLT transport membrane (sodium glucose symporter)
How does glucose exit the Basolateral membrane?
By facilitative diffusion mediated by GLUT (transporter protein)
What provides the energy for glucose entry?
The sodium gradient, maintained by the sodium pump
What happens to water during glucose absorption?
The movement of glucose across the epithelium creates osmotic imbalance, which drives the absorption of water (but this is paracellular)
6 features of the glucose absorption cell?
- Tight junctions=apical/Basolateral domains
- Na/K pump sets up ion gradients
- Sodium glucose symporter uses energy of Na gradient to actively accumulate glucose above its transportation gradient
- Facilitative glucose transporter mediates glucose exit across Basolateral membrane via passive diffusion down gradient
- Na taken up by symporter, exits via pump
- Transport of Na and glucose across epithelium induces paracellular Cl and water fluxes
How does the glucose gradient work?
When eating, glucose levels accumulate in the cell (above blood level) and passively diffuse into blood. When fasting, glucose levels in cell are low so must actively cross to blood, or blood glucose can be transport to cell vice versa. But depends on gradient
What is oral rehydration therapy?
Giving a simple sugar/water solution, instead of straight water, to improve hydration
How does oral rehydration work?
By including sugar, the glucose restores electrolytes in your body (such as sodium, chloride, potassium and water), all of which are enhanced by the uptake of glucose
Who is oral rehydration most effect for?
Dehydrated babies suffering from diarrhoea- this saves millions of lives per year
What is glucose-galactose malabsorption syndrome?
A mutation where it knocks out gene 1 (one type of sodium-glucose co-transporter protein) (SGLT)
Where is gene 1 of the glucose-sodium transporter primarily found?
In the small intestine
What does the glucose-galactose malabsorption syndrome result in?
No uptake of glucose in cells, so sugar is retained in the intestine lumen
What is galactose?
Sugar from milk
What does the resulting non-breakdown of starch result in?
A change in osmolarity (do water flows into the small intestine)
What does the increase in lumen osmolarity result in?
A water efflux, where the osmotic imbalance creates increased water flow produces produced diarrhoea (watery chyme)
How is glucose-galactose malabsorption syndrome treated?
By removing glucose/galactose from the diet and using a different transporter to uptake fructose (GLUT5)
What transporter is used to treat glucose-galactose malabsorption syndrome?
The facilitative transporter specifically for fructose (a 5 carbon sugar) (GLUT5)
What is glucose reabsorption in the kidneys?
Glucose in the plasma needs to be reabsorbed or it will appear in the urine
There needs to be a high uptake of glucose as glucose is constantly coming
What happens if glucose system in kidneys can’t cope?
It gets saturated (not enough binding pockets)
What is glucosuria?
Glucose in the urine (When transport maximum of SGLT protein is reached)
What is the most common cause of glucosuria?
Diabetes mellitis, because insulin activity is deficient and blood sugar is too high (over 200mg/ml)
What happens to the glucose symporter in diabetes?
It cannot absorb glucose fast enough, hence glucose appears in urine
Do kidneys and the small intestines use the same transporter for glucose?
No, kidneys use SGLUT2 which has a higher capacity (but if there’s too much glucose, even SGLUT2 can’t cope)
What happens if glucose absorption is impaired in kidney, or transporter is saturated?
Glucose appears in urine
When does filtered glucose stop being reabsorbed?
When the renal threshold is reached
What happens when the renal threshold is reached?
Glucose appears in urine and the renal threshold reflects the transport maximum of SGLT2
What is the transport maximum and renal threshold?
Transport maximum= 375 mg/min
Renal threshold= 200mg/ml
