Absorptive and Post-absorptive states Flashcards
define Absorptive state?
a fed state, when food is in the GIT and nutrients are being actively absorbed into the blood or lymph
define Post-absorptive state?
fasting state, when nutrients are not being absorbed
what is the first thing to happen in absorptive state?
the hormone-sensitive tissues of energy metabolism are:
Liver
Muscle
Adipose tissue
Absorbed nutrients (monosaccharides, amino acids and lipids) enter the portal blood and therefore first go to the liver
describe the pathway of glucose
Glucose first passes into the liver via the enterohepatic circulation.
In the liver it can be converted into glycogen or when it is in excess it can be converted into Triglycerides.
Glucose can also be taken up by muscle tissue and converted into glycogen.
Glucose can be taken up by adipose tissue and converted into triglycerides.
Glucose can be taken up by other tissues and be used to produce energy via the Krebs cycle.
describe the pathway of AA
Amino acids can be taken up by the liver to generate energy and keto acids.
Amino acids can also be taken by the muscle tissue to produce proteins
describe the pathway of Triglycerides
Triglycerides are predominantly going to be deposited by adipose tissue.
Triglycerides cannot pass through the cell membrane freely.
They first undergo lipolysis to produce free fatty acids and glycerol.
The FFA and glycerol can now be taken up by the adipose tissue and are then esterified back to triglycerides to be stored in the adipose tissue.
what is the switch from absorptive to post absorptive state regulated by?
regulated by hormones
The Switch from the Post-absorptive state to the Absorptive state is due to an increase in glucose & insulin in the blood.
You have just had a meal
You enter the absorptive state:
Food enters the GIT and nutrients (e.g. glucose) are actively absorbed into the blood.
This results in an increase blood glucose concentration.
Increase in blood glucose is sensed by beta cells –> insulin exocytosis
Insulin causes increased GLUT-4 expression on the surface on cells, in particular muscle and adipocytes.
This allows these cells to take glucose in from circulation.
Therefore, blood glucose concentration falls.
This means stimulus for insulin secretion is removed –> blood insulin concentration falls –> you are now back in post absorptive state.
- this induces insulin exocytosis
what is the mechanism of secretion of insulin?
Insulin is secreted by β-cells of the pancreas when there is high concentration of glucose or amino acids in the blood
Insulin secretion mechanism:
GLUT-2 is the transporter responsible for β-cell glucose uptake
Glucose metabolism increases intracellular ATP concentration - inhibits the activity of ATP sensitive K+ channels (KATP)
This results in depolarisation of the cell as K+ efflux is reduced
Depolarisation causes voltage gated Ca2+ channels to open
Increased intracellular calcium causes exocytosis of insulin
how does Insulin regulate the transport of glucose into muscle and adipose tissue?
Muscle and adipose tissue both need insulin in order for their cells to take up glucose.
GLUT-4 is a glucose transporter stored within the muscle cells and adipocytes.
Insulin binds to receptors on the cell surface and ultimately results in translocation of GLUT-4 to the plasma membrane
This allows for enhanced glucose uptake.
how does matching of Insulin and Glucose happen?
Insulin and glucose levels match each other:
Increase in glucose concentration in blood triggers insulin release.
As insulin is released it causes decreases in blood glucose concentration
This decrease in blood glucose concentration in turn causes a decrease in blood insulin concentration.
Note characteristic lag
What are the effects of insulin on the liver, adipose tissue and muscle tissue during the absorptive state?
During the absorptive state nutrients (e.g. glucose) are absorbed into the blood.
Glucose concentration in the blood increases.
Therefore in the fasted state insulin levels are high in the fed state
Insulin has the following effects on the tissues:
liver
muscle
adipocyte
what effect does insulin have on the liver?
The uptake of glucose by the liver is not insulin sensitive:
Glucose enters hepatocytes through GLUT-2 transported (NOT insulin sensitive) down its concentration gradient.
Once glucose enters hepatocytes it is converted into glucose-6-phosphate (enhanced by insulin)
Glycogenesis (Glycogen synthesis)
Glucose 6 phosphate is converted into glycogen (enhanced by insulin)
Glycogenolysis:
Insulin also inhibits glycogenolysis (glycogen breakdown)
Glycolysis
Insulin also enhances glycolysis
Lipogenesis
If Glucose is in excess it is converted into Acetyl CoA 🡪 converted into triglycerides.
This is known as lipogenesis and is also enhanced by insulin.
Therefore, in total Glycogen synthesis, Glycolysis and Fat synthesis (in times of excess) will all be
NB: In diagram:
Green arrows represent events activated by insulin
Red arrow represent events inhibited by insulin.
what effect does insulin have on the muscle?
The uptake of glucose by muscles is insulin sensitive:
Insulin binds to the insulin receptors on the plasma membrane
This triggers translocation of GLUT-4 to the plasma membrane 🡪 more glucose can enter the cell
The insulin then drives Glycogen synthesis, Glycolysis and Protein synthesis.
Therefore, glucose uptake, glycogen synthesis, glycolysis and protein synthesis are all enhanced by insulin.
what effect does insulin have on the adipocyte?
The uptake of glucose by adipocytes is insulin sensitive:
Insulin binds to the insulin receptors on the plasma membrane
This triggers translocation of GLUT-4 to the plasma membrane 🡪 more glucose can enter the cell
Insulin then enhances glycolysis and lipogenesis (triglyceride synthesis)
It also inhibiting lipolysis (breakdown of triglycerides)
Mechanism:
Insulin inhibits hormone sensitive lipase (HSL).
Therefore, TG cannot be broken down into fatty acids
Therefore, FA cannot be transported out of the adipocyte, so stay stored.
Insulin also stimulates synthesis of lipoprotein lipase, which then moves to the surface of endothelial cells:
This enzyme allows chylomicrons to release free fatty acids.
Free fatty acids then enter adipocytes and are re-esterified.
Therefore, insulin drives glucose uptake, glycolysis and TG synthesis, it also inhibits lipolysis.
what happens in post absorptive state?
2 types of reactions are…?
In the post-absorptive state, you need to utilise your body’s energy stores because glucose is not being absorbed from the GIT
There are 2 types of reaction in which this can occur:
Glucose-supplying reactions
Glucose-sparing reactions