PGs, TX and prostacyclin act on their own specific receptors located on cell membrane. Five major types of prostanoid receptors have been designated, each after the natural PG for which it has the greatest affinity.
PROSTANOID RECEPTORS
PGs,
TX and prostacyclin act on their own specific receptors located on cell
membrane. Five major types of prostanoid receptors have been designated, each
after the natural PG for which it has the greatest affinity. This has been
supported by receptor cloning. All prostanoid receptors are Gprotein coupled
receptors which utilize the IP3/ DAG or cAMP transducer mechanisms.
Some selective antagonists of prostanoid receptors have been produced. The
prostanoid receptors are:
DP Has greatest affinity for PGD2, but
PGE2 also acts on it; activation increases cAMP which inhibits
platelet aggregation.
EP Has greatest affinity for PGE2; enprostil is a selective agonist. It has been
subdivided into EP1 which causes smooth muscle contraction through
IP3/DAG pathway and EP2 which mediates smooth muscle
relaxation by increasing cAMP. Cloning studies have identified two more
subtypes EP3 and EP4. PGE2 enhances Cl¯ and
water secretion in intestinal mucosa also by increasing cAMP. However, in some
tissues (adipocytes) PGE2 inhibits cAMP formation—responsible for
its antilipolytic action. EP1 receptors are activated by PGF2α also.
FP Has greatest affinity for PGF2α; fluprostenol is a selective agonist. The
most prominent effect of activation of this receptor is smooth muscle
contraction mediated through IP3/DAG formation.
IP Has greatest affinity for PGI2; PGE
also acts on it and cicaprost
is a selective agonist. It functions by activating adenylyl cyclase in platelets
(inhibiting aggregation) and smooth muscles (relaxation).
TP Has greatest affinity for TXA2; PGH2 also acts on it. It utilizes IP3/DAG
as second messengers which mediate platelet aggregation and smooth muscle
contraction.
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