Page 97 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Catecholamine, Adrenergic α 2, ! 1 and ! 2) can be distinguished according
Transmission and Adrenoceptors to their affinity to E and NE and to numerous
agonists and antagonists. All adrenoceptors re-
Certain neurons can enzymatically produce L- spond to E, but NE has little effect on ! 2-
dopa (L-dihydroxyphenylalanine) from the adrenoceptors. Isoproterenol (isoprenaline)
amino acid L-tyrosine. L-dopa is the parent activates only !-adrenoceptors, and phen-
substance of dopamine, norepinephrine, tolamine only blocks α-adrenoceptors. The ac-
and epinephrine—the three natural cate- tivities of all adrenoceptors are mediated by G
proteins (! p. 55).
cholamines, which are enzymatically synthe-
Autonomic Nervous System (ANS) matic L-amino acid decarboxylase). Dopamine Their location and function are as follows: CNS
sized in this order. Dopamine (DA) is the final
Different subtypes (α 1 A, α 1 B, α 1 D) of α 1-
adrenoceptors can be distinguished (! B1).
step of synthesis in neurons containing only
the enzyme required for the first step (the aro-
(sympathetic activity"), salivary glands, liver
is used as a transmitter by the dopaminergic
(glycogenolysis"), kidneys (alters threshold
for renin release; ! p. 184), and smooth
neurons in the CNS and by autonomic neurons
muscles (trigger contractions in the arterioles,
that innervate the kidney.
uterus, deferent duct, bronchioles, urinary
Norepinephrine (NE) is produced when a
second enzyme (dopamine-!-hydroxylase) is
bladder, gastrointestinal sphincters, and di-
also present. In most sympathetic postgan-
lator pupillae).
Activation of α 1-adrenoceptors (! B1), me-
neurons, NE serves as the neurotransmitter
along with the co-transmitters adenosine tri-
(PLC!), leads to formation of the second mes-
3 glionic nerve endings and noradrenergic central diated by G q proteins and phospholipase C!
phosphate (ATP), somatostatin (SIH), or neu-
sengers inositol tris-phosphate (IP 3), which in-
ropeptide Y (NPY). creases the cytosolic Ca 2+ concentration, and
Within the adrenal medulla (see below) diacylglycerol (DAG), which activates protein
and adrenergic neurons of the medulla ob- kinase C (PKC; see also p. 276). G q protein-me-
longata, phenylethanolamine N-methyltrans- diated α 1-adrenoceptor activity also activates
2+
+
ferase transforms norepinephrine (NE) into Ca -dependent K channels. The resulting K +
epinephrine (E). outflow hyperpolarizes and relaxes target
The endings of unmyelinated sympathetic smooth muscles, e.g., in the gastrointestinal
postganglionic neurons are knobby or varicose tract.
(! A). These knobs establish synaptic contact, Three subtypes (α 2 A, α 2 B, α 2 C) of α 2-adreno-
albeit not always very close, with the effector ceptors (! B2) can be distinguished. Their lo-
organ. They also serve as sites of NE synthesis cation and action are as follows: CNS (sympa-
and storage. L-tyrosine (! A1) is actively thetic activity#, e.g., use of the α 2 agonist
taken up by the nerve endings and trans- clonidine to lower blood pressure), salivary
formed into dopamine. In adrenergic stimula- glands (salivation#), pancreatic islets (insulin
tion, this step is accelerated by protein kinase secretion#), lipocytes (lipolysis#), platelets
A-mediated (PKA; ! A2) phosphorylation of (aggregation"), and neurons (presynaptic au-
the responsible enzyme. This yields a larger toreceptors, see below). Activated α 2-adreno-
dopamine supply. Dopamine is transferred to ceptors (! B2) link with G i protein and inhibit
chromaffin vesicles, where it is transformed (via α i subunit of G i) adenylate cyclase (cAMP
into NE (! A3). Norepinephrine, the end prod- synthesis#, ! p. 274) and, at the same time, in-
uct, inhibits further dopamine synthesis crease (via the !γ subunit of G i) the open-
+
(negative feedback). probability of voltage-gated K channels (hy-
NE release. NE is exocytosed into the synap- perpolarization). When coupled with G 0 pro-
tic cleft after the arrival of action potentials at teins, activated α 2-adrenoceptors also inhibit
2+
2+
the nerve terminal and the initiation of Ca in- voltage-gated Ca 2+ channels ([Ca ] i#).
flux (! A4 and p. 50). All "-adrenoceptors are coupled with a G S
Adrenergic receptors or adrenoceptors protein, and its α S subunit releases cAMP as a
84 (! B). Four main types of adrenoceptors (α 1, second messenger. cAMP then activates pro-
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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