Mechanism of Absorption of DRUG
By four ways the drug can be migrated:
- Simple diffusion (Diffusion directly through lipid)
- Facilitated diffusion (Carrier mediated)
- Active transport (Carrier mediated)
- Pinocytosis (Drinking of cell)
Simple diffusion:
Polar substances dissolve freely in polar solvents and vice versa (like dissolves like). It means non polar substances dissolves freely in lipids( non polar substance), therefore, penetrates cell membrane very freely. It occurs due to conc. gradient, it is moving from high to low conc. no need of energy supply for simple diffusion.
Example: Water- it is moved through the GIT due to gaps between endothelial and along with it .
smaller water soluble substances can be passed such as urea and alcohol etc.
Gases: the gases can be diffused in the lungs by simple diffusion, not due to conc. gradient but due to partial pressure differences of gases i.e., oxygen.
Active diffusion:
Carrier proteins, ion channels are required for this type of diffusion. Here, a carrier protein moves molecules against the conc. gradient.
The drug molecule combine with carrier protein and then it transferred drug to the other side of membrane, there it leaves the molecule and come back to normal situation or form. It is energy mediated. Drugs like L-dopa moves by this way
Through ionized channel, the ions like k+, Ca++, Na+ are transferred.
In simple diffusion, the rate of transport is directly proportional to conc. gradient. Whereas, in carrier mediated transport, the rate of transport is not related with conc. of drug. But it depends on the amount of carriers by which the drug molecules can be bound.
Facilitated diffusion:
In this case also the carrier proteins are required, but transfer is occurred due to absorption of other molecule, which facilitate movement of drug molecule.
The glucose is transported along with sodium from the GIT membrane.
Example: fluorouracil
Pinocytosis:
It is also called as drinking of cell. The drug molecule, when comes in contact with membranes the invagination occurs (pseudopods). They trap the drug molecule and forms vesicles in which the drug molecule is present and taken into the cell. In the cell, some lysozymes are present which acts on the drug molecule and forms active form.
This process occurs rarely.
Example: Barium sulfate. Some molecules like insulin can enter to BBB (blood brain barrier) by this process.
In follicular cells of the thyroid, the colloids are taken by same process and releases T3 and T4 which are useful residues.
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