PHYSIOLOGY OF CARDIAC ADRENOCEPTORS
Keywords:
cholesterol, physiology, cardiovascular physiology, adrenergic-receptorsAbstract
Adrenoceptors (adrenergic receptors, AR.'s) are seven-loop proteins which, docked in the p/asmatic membrane, respond to catecholaminergic stimuli. They are classified in alpha-adrenergic receptors (a.-adrenoceptors ar a.AR. 's) e beta-adrenergic receptors ( -adrenoceptors or AR. 's). There are two main subtypes of a/pha
-adrenergic receptors (a.1anda.) and three subtypes of beta-adrenergic receptors
< v2and ); besides, there's specu/ation about the existence of the receiver 4.
1
The a. - adrenergic receptors activates the intracelular mechanism of
phospholipase C, generating inositol triphosphate (IP3) and diacylglicerol as
intracellular second-messengers, while the a.2- adrenergic receptors is coupled to the adenylate cyclase generating mechanism and, in intracellular milieu, reduce
the formation of adenosine monophosphate cyclic, inhibiting the calcium channel, so that the activation of this subclass receptor promotes inhibitory effects in the ce/1 where it is present. Nevertheless, ai/ -adrenoceptors increase the formation of intracel/ular adenosine monophosphate cyclic. When there are prolonged stimuli, they trigger desensitization of adrenergic receptors, as we/1 as of severa/ other biological signaling systems. The desensitization may happen by the following mechanisms: phosphorylation by specific kinase, phosphorylation by effectory kinase, endocitosis of the receptor; down-regulation, by phosphorylation of the G protein subunity and, final/y, by reduction of the RNA-messenger intracellular transcription. The physio/ogic integrity of adrenergic receptors is dependent on an ideal environment, which may, however, undergo alterations due to diverse factors, such as hypercho/esterolemia and exposure to free radicais.
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