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Vol. 282, Issue 1, 248-255, 1997
Department of Physiology and Pharmacology, Auburn University,
College of Veterinary Medicine, Auburn University, Alabama
Whole-cell electrophysiological studies suggest that sympathetic nerve
alpha-2 adrenergic receptors are coupled to
voltage-dependent N-type calcium channels through the Gi
family of proteins to inhibit neurotransmitter release. Because most
nerve terminals are too small for direct electrophysiological
recordings, the aim of this study was to examine the relationship
between alpha-2 adrenergic receptor-mediated inhibition
of norepinephrine release and the rise in cytosolic calcium in neurites
from cultured sympathetic neurons. In cultured rat superior cervical
ganglion neurons, the alpha-2 adrenergic receptor
agonists, UK-14304 (0.01-10 µM) and oxymetazoline (0.1-10 µM),
and the N-type calcium channel blocker, 
-conotoxin GVIA (0.1-10
nM), inhibited the release of tritiated norepinephrine in response to
electrical stimulation (1 Hz, 30 pulses, 0.1 ms, 70 V). The inhibitory
effect of the alpha-2 adrenergic receptor agonists was
not altered by pretreatment with pertussis toxin (200 ng/ml, 18 h), although pertussis toxin blocked the inhibition of
forskolin-stimulated cAMP accumulation by UK-14304. In fura-2 loaded
cells, electrical stimulation (1 Hz, 30 pulses, 0.1 ms, 70 V) increased
cytosolic calcium in sympathetic neuronal processes. Blockade of N-type
calcium channels with -conotoxin (1 and 10 nM) reduced the rise in
cytosolic calcium by 25 ± 3% and 52 ± 6%, respectively,
whereas UK-14304 and oxymetazoline did not alter the electrically
stimulated rise in cytosolic calcium. These data suggest that blockade
of N-type calcium channels with -conotoxin GVIA inhibits stimulated
norepinephrine release and cytosolic calcium measured with fura-2 at
similar concentrations, whereas activation of alpha-2
adrenergic receptor inhibits norepinephrine release by a pathway that
is insensitive to pertussis toxin and changes in cytosolic calcium in
neurites from cultured rat superior cervical ganglion cells.
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