α receptors
α receptors have several functions in common, but also individual effects. Common (or still unspecified) effects include:
- Vasoconstriction of arteries to heart (coronary artery).[3]
- Vasoconstriction of veins[4]
- Decrease motility of smooth muscle in gastrointestinal tract[5]
[edit]α1 receptor
Alpha1-adrenergic receptors are members of the G protein-coupled receptor superfamily. Upon activation, a heterotrimeric G protein, Gq, activates phospholipase C (PLC). The PLC cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) which in turn causes an increase in inositol triphosphate (IP3) anddiacylglycerol (DAG). The former interacts with calcium channels of endoplasmic and sarcoplasmic reticulum thus changing the calcium content in a cell. This triggers all other effects.
Specific actions of the α1 receptor mainly involve smooth muscle contraction. It causes vasoconstriction in many blood vessels including those of the skin,gastrointestinal system, kidney (renal artery)[6] and brain[7]. Other areas of smooth muscle contraction are:
- ureter
- vas deferens
- hair (arrector pili muscles)
- uterus (when pregnant)
- urethral sphincter
- bronchioles (although minor to the relaxing effect of β2 receptor on bronchioles)
- blood vessels of ciliary body (stimulation causes mydriasis)
Further effects include glycogenolysis and gluconeogenesis from adipose tissue[8] and liver, as well as secretion from sweat glands[8] and Na+ reabsorption from kidney.[8]
Antagonists may be used in hypertension.
[edit]α2 receptor
- inhibition of insulin release in pancreas.[8]
- induction of glucagon release from pancreas.
- contraction of sphincters of the gastrointestinal tract
- negative feedback in the neuronal synapses
[edit]β receptors
[edit]β1 receptor
Specific actions of the β1 receptor include:
- Increase cardiac output, by raising heart rate (positive chronotropic effect) and increasing impulse conduction and increasing contraction thus increasing the volume expelled with each beat (increased ejection fraction).
- Increase Renin secretion from juxtaglomerular cell of kidney.
- increase ghrelin secretion from the stomach[9]
[edit]β2 receptor
Specific actions of the β2 receptor include the following:
- Smooth muscle relaxation, e.g. in bronchi.[8]
- Lipolysis in adipose tissue.[10]
- Anabolism in skeletal muscle.[11][12]
- Relax non-pregnant uterus
- Relax detrusor urinae muscle of bladder wall
- Dilate arteries to skeletal muscle
- Glycogenolysis and gluconeogenesis
- Contract sphincters of GI tract
- Thickened secretions from salivary glands.[8]
- Inhibit histamine-release from mast cells
- Increase renin secretion from kidney
- Relaxation of Bronchioles (salbutamol, a beta 2 agonist relieves bronchiole constriction)
[edit]β3 receptor
Specific actions of the β3 receptor include:
- Enhancement of lipolysis in adipose tissue. Beta-3 activating drugs could theoretically be used as weight-loss agents, but are limited by the side effect of tremors.
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