The b receptor is a glycoprotein within the cell membrane whose stimulation ultimately results in increased cytosolic calcium, which is integral for excitation-contraction coupling. The b receptor is coupled to an intracellular second messenger, cyclic adenosine monophosphate (cAMP), by a regulatory protein within the cell membrane called the G protein. Upon b-receptor stimulation, the G protein undergoes a conformational change that activates adenyl cyclase, increasing intracellular cAMP. Cyclic AMP stimulates protein kinase, which phosphorylates calcium channels, leading to calcium entry into the cell. Calcium entry into the cell triggers additional calcium release from storage organelles. Phosphodiesterase then hydrolyzes cAMP.
Three b-receptor subtypes have been identified: the b 1 subunit in the myocardium, kidney, and eye; the b2 subunit in adipose tissue, pancreas, liver, and muscle; and the b3 subunit in adipose tissue. Stimulation of each subunit serves a unique function. For example, b 1 stimulation increases the force and rate of myocardial contraction. b2 stimulation relaxes vascular smooth muscle. b2 activation also increases metabolic substrate availability needed for stress by stimulating lipolysis and glycogenolysis. The b3 subunit alters lipid metabolism.
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