Alpha-adrenoceptors in equine digital veins Evidence for the presence of both alpha1 and alpha2-receptors mediating vasoconstriction من كتب طب بيطرى

vasoconstriction من كتب طب بيطرى 

J. vet. Pharmacol. Therap. 20, 308±317, 1997. SYSTEMIC PHARMACOLOGY
Alpha-adrenoceptors in equine digital veins: Evidence for the presence of
both alpha1 and alpha2-receptors mediating vasoconstriction
Elliott, J. Alpha-adrenoceptors in equine digital veins: Evidence for the presence
of both alpha1 and alpha2-receptors mediating vasoconstriction. J. vet.
Pharmacol. Therap. 20, 308±317.
Rings of equine digital vein examined under conditions of isometric tension
recording constricted to alpha-adrenoceptor agonists with an order of potency
of 5-bromo-6-[2-imidazolin-2-yl-amino]-quinoxaline bitartrate (UK 14304) =
noradrenaline 4 6-Allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-(4,5-d) azepine
(BHT-920) 4 phenylephrine 4 dopamine 4 methoxamine. The maximum
force generated was greatest for the non-selective agonist noradrenaline and
lowest for the alpha2-selective agonist BHT-920 with the other agonists
between these two extremes. Selective inactivation of alpha1-adrenoceptors
(achieved by treating yohimbine-protected tissues with phenoxybenzamine)
reduced the maximum responses of all agonists, the EC50 values of UK 14304,
BHT-920 and noradrenaline and increased the EC50 values of phenylephrine
and methoxamine. Prazosin (30 nM) had no inhibitory effect on responses to
low concentrations of BHT-920 and UK 14304 and caused competitive
inhibition of responses to phenylephrine and noradrenaline giving pKb values of
8.49+0.18 and 8.23+0.14, respectively. Yohimbine (0.1 mM) caused
significant competitive inhibition of responses to BHT-920 and noradrenaline
with calculated pKb values of 8.43+0.11 for BHT-920 and 7.43+0.31 for
noradrenaline and non-competitive inhibition of responses to UK 14304. 2-[2-
methoxy-1,4-benzodioxan-2-yl]-2-imidazoline (RX 821002; 10 nM) caused
competitive inhibition of responses to BHT-920 (pKb 9.04+0.27) and
dopamine (pKb 8.2+0.2). These data indicate that equine digital veins possess
both post-synaptic alpha1 and alpha2-adrenoceptors.
(Paper received 4 November 1996; accepted for publication 11 April 1997)
Jonathan Elliott, Department of Veterinary Basic Sciences, Royal Veterinary College,
Royal College Street, London NW1 0TU, UK.

Abstract
Rings of equine digital vein examined under conditions of isometric tension recording constricted to alpha-adrenoceptor agonists with an order of potency of 5-bromo-6-[2-imidazolin-2-yl-amino]-quinoxaline bitartrate (UK 14304) = noradrenaline > 6-Allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-(4,5-d) azepine (BHT-920) > phenylephrine > dopamine > methoxamine. The maximum force generated was greatest for the non-selective agonist noradrenaline and lowest for the alpha2-selective agonist BHT-920 with the other agonists between these two extremes. Selective inactivation of alpha1-adrenoceptors (achieved by treating yohimbine-protected tissues with phenoxybenzamine) reduced the maximum responses of all agonists, the EC50 values of UK 14304, BHT-920 and noradrenaline and increased the EC50 values of phenylephrine and methoxamine. Prazosin (30 nM) had no inhibitory effect on responses to low concentrations of BHT-920 and UK 14304 and caused competitive inhibition of responses to phenylephrine and noradrenaline giving pKb values of 8.49 +/- 0.18 and 8.23 +/- 0.14, respectively. Yohimbine (0.1 microM) caused significant competitive inhibition of responses to BHT-920 and noradrenaline with calculated pKb values of 8.43 +/- 0.11 for BHT-920 and 7.43 +/- 0.31 for noradrenaline and non-competitive inhibition of responses to UK 14304. 2-[2-methoxy-1,4-benzodioxan-2-yl]-2-imidazoline (RX 821002; 10 nM) caused competitive inhibition of responses to BHT-920 (pKb 9.04 +/- 0.27) and dopamine (pKb 8.2 +/- 0.2). These data indicate that equine digital veins possess both post-synaptic alpha1 and alpha2-adrenoceptors.

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