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Smooth Muscle Group, Department of Physiology, The Queen's University of Belfast, BT9 7BL Belfast, United Kingdom
Freshly dispersed
interstitial cells from the rabbit urethra were studied by using the
perforated-patch technique. When cells were voltage clamped at
60 mV
and exposed to 10 µM norepinephrine (NE) at 80-s intervals, either
large single inward currents or a series of oscillatory inward currents
of diminishing amplitude were evoked. These currents were blocked by
either phentolamine (1 µM) or prazosin (1 µM), suggesting that the
effects of NE were mediated via
1-adrenoceptors.
NE-evoked currents were depressed by the blockers of
Ca2+-activated Cl
currents, niflumic acid (10 µM), and 9-anthracenecarboxylic acid (9-AC, 1 mM). The reversal
potential of the above currents changed in a predictable manner when
the Cl
equilibrium potential was altered, again
suggesting that they were due to activation of a Cl
conductance. NE-evoked currents were decreased by 10 µM cyclopiazonic acid, suggesting that they were dependent on store-released
Ca2+. Inhibition of NE-evoked currents by the phospholipase
C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate
(100 µM) suggested that NE releases Ca2+ via an inositol
1,4,5-trisphosphate (IP3)-dependent mechanism. These
results support the idea that stimulation of
1-adrenoceptors releases Ca2+ from an
IP3-sensitive store, which in turn activates
Ca2+-activated Cl
current in freshly
dispersed interstitial cells of the rabbit urethra. This elevates slow
wave frequency in these cells and may underlie the mechanism
responsible for increased urethral tone during nerve stimulation.
chloride; pacemaker cell; neurotransmission
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