Beta receptors
So far three subtypes of b receptors have been identified and cloned. They differ in their distribution, the b1 type being found in the heart, the b2 in lung, smooth muscle, skeletal muscle and liver, while the b3 type occurs in adipose tissue. There is evidence that b2 adrenoceptors occur on the lymphocyte membrane also but the precise function there is unknown. The antihypertensive drugmetoprolol is a clinically effective example of a b1 antagonist. All the b receptor subtypes are linked to adenylate cyclase as the second messenger system. It seems that both b1 and b2 receptor types occur in the brain and that their activation leads to excitatory effects. Of particular interest to the psychopharmacologist is the finding that chronic antidepressant treatment leads to a decrease in the functional responsiveness of the b receptors in the brain, and in the density of these receptors on lymphocytes, which coincides with the time necessary for the therapeutic effects of the drugs to be manifest. Such changes have been ascribed to the drugs affecting the activity of the G-proteins that couple the receptor to the cyclase sub-unit. The adrenergic receptors have been purified and their genes cloned. They have seven membrane-spanning units, which are involved in binding the selective agonists and antagonists.
So far three subtypes of b receptors have been identified and cloned. They differ in their distribution, the b1 type being found in the heart, the b2 in lung, smooth muscle, skeletal muscle and liver, while the b3 type occurs in adipose tissue. There is evidence that b2 adrenoceptors occur on the lymphocyte membrane also but the precise function there is unknown. The antihypertensive drugmetoprolol is a clinically effective example of a b1 antagonist. All the b receptor subtypes are linked to adenylate cyclase as the second messenger system. It seems that both b1 and b2 receptor types occur in the brain and that their activation leads to excitatory effects. Of particular interest to the psychopharmacologist is the finding that chronic antidepressant treatment leads to a decrease in the functional responsiveness of the b receptors in the brain, and in the density of these receptors on lymphocytes, which coincides with the time necessary for the therapeutic effects of the drugs to be manifest. Such changes have been ascribed to the drugs affecting the activity of the G-proteins that couple the receptor to the cyclase sub-unit. The adrenergic receptors have been purified and their genes cloned. They have seven membrane-spanning units, which are involved in binding the selective agonists and antagonists.
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