Anticoagulants
These are agents which when mixed with blood, prevent coagulation. When used inside the body these are called anticoagulants in vivo. Anticoagulants in vitro are used outside the body, e.g., laboratory, blood bank, etc.
(1) Heparin:
It is a mucoitin polysulphuric acid (Sulphated muco-polysaccharide) and is formed within the body. It is present as a polymer on a protein backbone and a highly negatively charged molecule. It is found in basophils and mast cells. (Heparin was discovered by a second year medical student of johns Hopkins University, USA). It can be used as anticoagulant both in vivo and in vitro. It is measured by unit. One unit is the amount of heparin which will prevent coagulation of 1 ml of cat's blood for 24 hours. One milligram of heparin may be equivalent up to 100 units. Heparin acts by inhibiting IXa. But the most important action is, it facilitates binding of antithrombin-III with plat thrombin; this inhibits thrombin and thus coagulation. This The antithrombin-III and heparin complex also inhibits factors IXa, Xa, Xla, XIla. It also activates the lipoprotein lipase and is called plasma clearing factor as it helps in deposition of lipids from plasma to the fat depots. It is used in the treatment of different thrombotic disorders. A small dose (0.5 to 1 mg/kg) is quite effective. If overdose occurs, treatment is done with protamine sulphate which neutralises heparin. Now a days a low molecular weight heparin is used with some advantage but is costly.
(2) Vitamin K antagonists:
These are also called oral anticoagulants and include warfarin, dicoumaro, diphenadione etc. These substances interfere in the formation of vitamin K dependent factors ike II, VII, IX and X. These are synthesised in liver where vitamin K acts as a cofactor. In presence of these vitamin K antagonists vitamin K cannot act and synthesis of the above factors is hampered. If these factors are decreased, coagulation is hampered, hence these agents are used to prevent coagulation, i.e., in the treatment of thrombosis. These are given orally and take some time to affect coagulation. It is clear from the mode of action that these will not act in vitro. Overdose of these anticoagulants is treated by vitamin K.
(3) Ca+ chelating agents:
These agents form
insoluble salts with Ca+ in plasma, so the concentration of Ca++ is decreased
and the process of coagulation stops. These are:
(a) Potassium-ammonium oxalate: It is made of two parts of K-oxalate with three parts of NH,-Oxalate by mixing 0.4 ml and 0.6 ml of 1% solution of potassium oxalate and ammonium oxalate respectively. The solution is put in a vial and then dried up. This is sufficient for 5 ml of blood. It is of value for determination of the haematocrit as it is thought to keep the cell volume unchanged. Now a days it is replaced by EDTA (see below) which is superior in this respect.
(b) Sodium
or potassium oxalate may be used so also the sodium fluoride. Oxalates act by
chelating Ca+ but floride acts as enzyme inhibitor.
(c) Sodium citrate as
trisodium citrate: It forms sodium calcium citrate, i.e., Ca2+ is complexed. It
is used in vitro as a 3.8% solution in the ratio of citrate: blood = 1:4. It is
used in ESR estimation and in blood banking as citrate, phosphate, dextrose
(CPD). (p. 64)
(d) EDTA (ethylene diamine tetra-acetic acid) : Its sodium salt is used. It acts by chelating Ca++. Out of the whole group only the citrate can be used n vivo because others are highly toxic.
(4) Blood can be kept in fluid state also by
(a) Keeping at low temperature.
(b) Keeping in siliconised vessels.