Drugs that lower lipid and lipoproteins levels in the blood are known as hypolipidemic drugs.
The first measure in controlling hyperlipidemias is dietary restriction. However, if it proves unsatisfactory, hypolipidemic drugs are prescribed.
Hypolipidemic drugs have attracted considerable attention since they prevent cardiovascular diseases by retarding atherosclerosis in hyperlipidemic individuals.
- Carried in plasma in form of lipoproteins, after getting associated with several apolipoproteins.
- Plasma lipid concentrations are dependent upon the concentration of lipoproteins.
- Lipoprotein globules consist of core of triglyceride and cholesterol esters. The outer polar layer has phospholipids, apoproteins and free cholesterol.
- Divided into 6 classes depending upon particle size and density. These are:
b. Chylomicron remnants
a) Chylomicrons are the largest lipoproteins containing about 90% triglycerides and about 5% cholesterol.
b) Chylomicron remnants contain more cholesterol than triglycerides.
c) VLDL contains 50-60% triglycerides and 10% cholesterol.
d) IDL, just like chylomicron remnants, contain more cholesterol than triglycerides.
e) LDL contain 50% cholesterol and 5% triglycerides.
f) HDL contain 20% cholesterol and 10% triglycerides.
- Dietary lipids are absorbed in the intestine with help of bole acids and chylomicrons are formed.
- They are passed into lacteals and reach blood stream.
- During passage via capillaries, the endothelial lipases hydrolyze TGs and convert them into fatty acids, which are carried to muscles to be utilized as energy source or are taken up by fat cells, where they are reconverted into fat cells.
- Remaining part left is chylomicron remnant, which is engulfed by liver, which has receptors for it.
- Free cholesterol liberated is either stored in liver cells or excreted in bile in form of bile acids.
- Liver secretes VLDL which contain more TGs than cholesterol esters.
- Lipases act and convert it into IDL, which contains more cholesterol than TGs
- Half IDL is taken up by liver cells, while remaining half loses TGs and becomes LDL, which contains cholesterol.
- LDL circulates in plasma for a long time and uptake into cells is dependent upon the needs of cells for cholesterol.
- Cholesterol ester present in LDL is de-esterified and mainly used in cell membrane formation.
- The cholesterol released from degradation of membrane is incorporated into HDL. It is then esterified with help of enzyme and transferred back to VLDL or IDL. Thus completing the cycle.
- Excess lipoproteins in plasma are phagocytosed by macrophages for disposal.
- If too much lipoprotein is to be degraded in this manner, excess of cholesterol is in atheromas i.e. arterial walls and xanthomas i.e. in skin and tendons.
- Raised levels of VLDL, IDL, LDL and rarely those of chylomicrons and chylomicron remnants all are atherogenic.
- Raised levels of HDL, however, may be protective since HDL facilitates the removal of cholesterol from tissues.
Causes of Hyperlipidemias:
- Nephrotic syndrome
Types of Hyperlipidemias
- Type 1 (Familial Hyperchylomicronemias)(mainly increased VLDL levels)
- Type 11A (Familial Hypercholesterolemia) (mainly increased LDL levels)
- Type 11B (Familial Combined Hyperlipidemias) (mainly increased LDL and VLDL levels)
- Type 111 (Familial Dysbetalipoproteinemias) (mainly increased VLDL levels)
- Type IV (Familial Hypertriglyceridemia) (mainly increased VLDL levels)
- Type V(Familial Mixed Hypertriglyceridemia) (mainly increased LDL and VLDL levels)