Heparin B

A clotting factor is activated when the factor that precedes it in the clotting cascade splits off a protein fragment and thereby exposes an enzymatic center. The latter can again be inactivated physiologically by complexing with anti-thrombin III (AT III), a circulating glycoprotein. Heparin acts to inhibit clotting by accelerating formation of this complex more than 1000-fold. Heparin is present (together with histamine) in the vesicles of mast cells; its physiological role is unclear. Therapeutically used heparin is obtained from porcine gut or bovine lung. Heparin molecules are chains of amino sugars bearing -COO-and -SO4 groups; they contain approx. 10 to 20 of the units depicted in (B); mean molecular weight, 20,000. Anticoagulant efficacy varies with chain length. The potency of a preparation is standardized in international units of activity (IU) by bioassay and comparison with a reference preparation.

The numerous negative charges are significant in several respects: (1) they contribute to the poor membrane pe-netrability—heparin is ineffective when applied by the oral route or topically onto the skin and must be injected; (2) attraction to positively charged lysine residues is involved in complex formation with ATIII; (3) they permit binding of heparin to its antidote, protamine (polycationic protein from salmon sperm).

If protamine is given in heparin-in-duced bleeding, the effect of heparin is immediately reversed.

For effective thromboprophylaxis, a low dose of 5000 IU is injected s.c. two to three times daily. With low dosage of heparin, the risk of bleeding is sufficiently small to allow the first injection to be given as early as 2 h prior to surgery. Higher daily i.v. doses are required to prevent growth of clots. Besides bleeding, other potential adverse effects are: allergic reactions (e.g., thrombocy-topenia) and with chronic administration, reversible hair loss and osteoporosis.

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Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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