Abstract
beta-Adrenergic receptors are often studied as prototypes of the large
family of G-protein-coupled receptors, which includes many other
well-known members such as the muscarinic acetylcholine receptors,
but also the receptors for light, taste and olfaction. These receptors
are regulated by multiple mechanisms which can affect either their
function or their expression to a rapidly changing environment. The
most obvious changes are effected by receptor agonists, and this
process is called receptor desensitization. On the functional level,
the most intriguing and important mechanism of desensitization involves
the phosphorylation of beta-adrenergic and homologous receptors by
specific receptor kinases, termed the G-protein-coupled receptor
kinases (GRKs). This phosphorylation is followed by binding of arrestins
to the receptors, which causes uncoupling of receptors and G-proteins
and thus results in a loss of receptor function. On the expression
level, there appear to be two major pathways leading to a reduction
of the receptor number: degradation of the receptors themselves,
or reduced receptor synthesis brought about by reduced receptor mRNA
levels. Heart failure is accompanied by a markedly reduced responsiveness
of the beta-adrenergic receptor system, which in many ways resembles
the phenomena seen in agonist-induced receptor desensitization. The
levels of beta 1-adrenergic receptors are reduced, and this reduction
is paralleled by similar decreases in the levels of the corresponding
mRNA. At the same time, the activity and the mRNA levels of one of
the GRK-isoforms, GRK2 (which is identical to the beta-adrenergic
receptor kinase 1) are increased. These alterations may contribute
to the loss of beta-adrenergic receptor responsiveness in heart failure
and result in further impairment of cardiac function.
Users
Please
log in to take part in the discussion (add own reviews or comments).