| pA2
online © Copyright 2003 The British Pharmacological Society |
051P
University of Surrey Summer Meeting June 2003 |
|
Correlation
between ligandsbinding sites and subtypes of
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1-adrenoceptor
mediating contractions of rat vas deferensWe have previously
demonstrated that contractions of rat vas deferens to exogenous noradrenaline
involve predominantly 
1A-adrenoceptors,
but that contractions to endogenous noradrenaline involve predominantly
1D-adrenoceptors
(Honner & Docherty, 1999). In this study, we have examined the subtypes
of 1-adrenoceptor
in rat vas deferens using radioligand binding.
Male Wistar Rats (250 - 300g) were killed by CO2
overdose. Displacement of [3H]-prazosin
binding to 1-adrenoceptors
was measured by incubating membranes obtained from rat whole vas deferens
for 30 min at 25oC with [3H]-prazosin
2nM and increasing concentrations of antagonists. Non specific binding
was determined in the presence of phentolamine 10 µM. Prazosin KD
was 0.33±0.12nM (n=6). Some animals were injected with 6-hydroxy-dopamine
(100mg/kg i.p.) on days 1 and 4 to produce a chemical sympathectomy. Tissues
were investigated on day 5 or 6.
In control tissues, antagonist binding was consistent with a single population
of 1-adrenoceptors,
with Hill slopes not significantly different from unity. Binding affinities
for 10 1-adrenoceptor
antagonists (prazosin, 5-methyl-urapidil, RS 17053, benoxathian, WB 4101,
HV 723, BMY 7378, phentolamine, spiperone, RS 100329) were expressed as
pKi values (e.g. BMY 7378: 7.42±0.21, n=5) and correlated with
known affinities for 1-adrenoceptors.
There were significant correlations with published data for ligand binding
affinities at 1A-
and 1D-adrenoceptors.
However, the pKi values obtained in this study correlated significantly
with the potency of antagonists at inhibiting contractions to exogenous
noradrenaline (an 1A-adrenoceptor
mediated response), but not with the potency of antagonists at inhibiting
the nifedipine-insensitive 1-adrenoceptor
mediated electrically-evoked contraction (1D-adrenoceptor
mediated) (see Honner & Docherty, 1999).
In sympathectomised
tissues, binding affinity for the 1D-adrenoceptor
selective antagonist BMY 7378 fitted best with a 2 site model, with pKi
values of 9.20±0.24 (54±10%) and 7.17±0.14 (46±10%)
for high and low affinity sites respectively (n=8). These affinities closely
fit the published affinities of BMY 7378 for 1D-
and 1A-adrenoceptors,
respectively.
It is concluded that the predominant 1-adrenoceptor
in rat vas deferens is the 1A-adrenoceptor,
both in terms of ligand binding and contractions to exogenous agonists.
The 1D-adrenoceptor,
although involved in nerve-evoked contractions of rat vas deferens, is
only clearly detectable by radioligand binding following chemical sympathectomy.
Sympathectomy increases the proportion of 1D-adrenoceptor
ligand bind sites.
Supported by RCSI.
Honner, V. & Docherty, J.R. (1999). Br. J,. Pharmacol. 128,
1323 1323-1331.