3.carrier mediated transport
-the presence of specialised transport mechanisms without which many essential water-soluble nutrients like monosacharrides ,aminoacids and vitamins will be poorly absorbed.
--the mechanism is thought to involve a component of the membrane called as the carrier.
--the carrier,that binds reversible or non covalent with the solute molecules to be transported.
--this carrier-solute complex transverse across the membrane to the other side where it dissociated & discharges the solute molecule--the carrier then returns to its original site to complete the cycle by accepting a fresh molecule of solute .
--the carrier may be an enzyme or some other component of the membrane.
--the important characteristics of carrier mediated transport are;
1.the transport process is structure specific(I.e. the carriers have affinity for and transfer a drug of specific chemical structure only ;generally the carriers have special affinity for essential nutrients).
2.sonce the system is structure specific, drugs having structure similar to essential nutrients called as false nutrients, are absorbed by the same carrier system.
(This mechanism is important in the absorption of several anti neoplastic agents like 5-fluorouracul&5-bromouracil which serve as false nutrients).
3.as the no.of carriers are limited,the transport system is subjected to competition between agents having similar structure.
4.since the no.of carriers are limited, the system is capacity limited I.e at high drug concentration the system becomes saturated &approaches an asymptole.
--its is important to note that for a drug absorbed by passive diffusion ,the rate of absorption increases linearly with concentration until the carriers become saturated after which it becomes curvilinear and approach a constant value at higher doses.
--such a capacity limited process can be adequately described as mixed order kinetics,also called as michaelis-menten,saturation or non-linear kinetics .the process is called mixed order.because it is first-order ar subsaturation drug concentration. Apparent zero-order at above saturation level.
--adminstration of a large single oral dose of such vitamins Is irrational.
5.specialised absorption/carrier mediated absorption;generally occurs from specific sites of the intestinal tract which are rich in number of carriers.
--such an area In which the carrier system is most dense is called as absorption window.
--drugs absorbed through such absorption windows are poor candidates for controlled release formulations.
2types of carrier mediated transport systems have been identified.they are1.facilitated diffusion.
2.active transport.
3.facilitated diffusion
-carrier mediated transport system .
-concentration gradient operated at faster rate than compared to simple passive diffusion.
-the driving force is concentration gradient.
-the process Is not inhibited by metabolic poisons that interfere with energy production
-facilitated diffusion having a limited importance in the absence of drugs.
*examples:entry of glucose into RBCs,intestinal absorption of vitamin B1&B2,GI absorption of drug..
4.active transport
-it is a carrier mediated transport system
-active transport is more important process than facilitated diffusion in absorption of drugs & nutrients
-it differs in several respects;
1.the drug is transported from. A region of lower to one higher concentration I.e. against the concentration gradient or uphill transport.
2.since,the process is uphill,energy is required
3.as the process requires expenditure of energy,it can be inhibited by metabolic poisons that interfere with energy production like fluoride,cyanide and dinitrophenol&lack of O2 etc.
-endogenous substances that are transported actively include sodium,K,Ca2+,Fe,Glucose..
-drugs having structural similarity are absorbed actively.
Ex:absorption of 5-fluorouracil&5-bromouracil via primitive transport system,absorption of methyldopa&levodopa via L-amino acid transport system,absorption of ACE inhibitor,enalopril via small peptide carrier system.
-active transport is also important I renal&biliary excretion.
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