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Journal of Neuroscience, Vol 14, 740-750, Copyright © 1994 by Society for Neuroscience
ATP-activated current and its interaction with acetylcholine-activated current in rat sympathetic neurons
K Nakazawa
Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115.
Ionic current activated by extracellular ATP was characterized using
whole-cell voltage clamp of rat sympathetic neurons isolated from superior
cervical ganglia. ATP (10-1000 microM) activated an inward current (IATP)
at negative holding potentials in almost all the cells (> 97%). The
current was reversed near 0 mV in a quasi-physiological external solution,
and the concentration dependence could be fitted by a curve with an EC50
value of 60 microM and a Hill coefficient of 1.8. The relationship between
IATP and the current activated by ACh (IACh) was examined. ACh (100 microM)
activated an inward current one- to fivefold larger than the current
activated by 100 microM ATP. IATP and IACh were not additive; the current
activated by simultaneous application of ATP and ACh was only as large as
the current activated by ACh alone. During current activation by ATP or
ACh, the current activated by the other agonist became smaller, suggesting
that these agonists reciprocally inhibit their excitatory responses. The
reciprocal inhibition appeared to depend on the extent of channel opening
because the reduction of the current elicited by each agonist was relieved
when the current elicited by the other agonist had been desensitized.
Suramin (100 microM), a purinoceptor antagonist, selectively inhibited IATP
whereas two open-channel blockers of nicotinic receptor channels,
hexamethonium (10-100 microM) and d- tubocurarine (1-10 microM), inhibited
IACh without affecting IATP. When 140 mM glucosamine was used as an
external cation, only ATP but not ACh activated a considerable inward
current at -150 mV. The ATP-induced glucosamine influx was reduced by
simultaneous application of ACh. These results suggest that channel
activation by ATP and that by ACh are not independent, and these two
excitatory neurotransmitters negatively interact with each other at
postsynaptic level in rat sympathetic neurons. The interaction between the
ATP- and the ACh- induced conductances was hypothetically explained based
upon our previous proposal of "channel overlap"; that is, ATP activates a
subpopulation of the nicotinic receptor channels.
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