KININ RECEPTORS & MECHANISMS
OF ACTION
The
biologic actions of kinins are mediated by specific receptors located on the
membranes of the target tissues. Two types of kinin receptors, termed B1
and B2, have been defined based on the rank orders of agonist
potencies; both are G protein-coupled receptors. (Note that B here stands for
bradykinin, not for β
adrenoceptor.) Bradykinin displays the highest affinity in most B2
receptor sys-tems, followed by lys-bradykinin and then by met-lys-bradykinin.
One exception is the B2 receptor that mediates contraction of venous
smooth muscle; this appears to be most sensitive to lys-bradykinin. Recent
evidence suggests the existence of two B 2-receptor subtypes, which
have been termed B2A and B2B.
B1
receptors appear to have a very limited distribution in mam-malian tissues and
have few known functional roles. Studies with knockout mice that lack
functional B1 receptors suggest that these receptors participate in
the inflammatory response and may also be important in long-lasting kinin
effects such as collagen synthe-sis and cell multiplication. By contrast, B2
receptors have a wide-spread distribution that is consistent with the multitude
of biologic effects that are mediated by this receptor type. Agonist binding to
B2 receptors sets in motion multiple signal transduc-tion events,
including calcium mobilization, chloride transport, formation of nitric oxide,
and activation of phospholipase C, phospholipase A2, and adenylyl
cyclase.
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