Biomedical Engineering Reference
In-Depth Information
Tabl e 8. 7.
-Adrenoceptors in the lung and their function (Source: [
725
]). The tone of airway
smooth myocytes is mainly regulated by members of the Gq/11 and Gs subclasses of G-protein
subunits that cause bronchoconstriction and bronchodilation, respectively.
α
Site
Predominant
Effect
subtype
α
Airway smooth myocyte
1
Contraction
Vascular smooth myocyte
α
1
Contraction
Autonomic nerves
α
2
Neurotransmitter release decrease
Secretory glands
α
1
Secretion increase
Mastocyte
Mediator release increase
Adrenergic Receptors
In
human
lungs,
the
amount
of
β
-adrenoceptors
is
greater
than
that
of
α
-adrenoceptors [
725
].
α
-Adrenoceptors
Airway
smooth
myocytes
synthesize
both
α
1-
and
α
2-adrenoceptors [
725
].
α
1a-Adrenoceptors pri-
marily act on calcium channels, thereby fostering entry of extracellular Ca
2
+
ions.
α
-Adrenoceptors cause bronchoconstriction (Table
8.7
).
α
1b-Adrenoceptors activates phospholipase-C that processes phosphatidylinositol
(4,5)-bisphosphate in the plasma membrane to form inositol (1,4,5)-trisphosphate
and diacylglycerol, which triggers Ca
2
+
release from intracellular stores and
activates Ca
2
+
- and phospholipid-dependent protein kinase-C. The latter may
contribute to smooth muscle contraction by increasing the Ca
2
+
sensitivity of
contractile
proteins.
α
2-Adrenoceptors inhibit
adenylate
cyclase
via
guanine
nucleotide inhibitory protein (Gi).
Both noradrenaline and adrenaline increase very weakly the bronchial tone
after pretreatment with
-adrenoceptor antagonists in bronchi isolated from healthy
human lungs; the response is not enhanced in the presence of histamine [
725
].
β
β
-Adrenoceptors
β
β
∼
Both
20 and 80%,
respectively) in human lungs [
725
]; hence, in human bronchial smooth myocytes,
β
1- and
2-adrenoceptors lodge on airway smooth myocytes (
β
β
-Adrenoceptors can also be
detected in respiratory epithelial as well as mucous and serous gland cells of the
trachea; however, most of the
-adrenoceptors belong mainly to the
2-subtype.
β
-adrenoceptor pool of the human lung localizes in
alveolar cells.
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