a-61603 and 5-methylurapidil

It has been recently shown that the supersensitivity of distal segments of the rat tail artery to phenylephrine after chemical sympathectomy with reserpine results from the appearance of alpha(1D)-adrenoceptors. It is known that both alpha(1A)- and alpha(1D)-adrenoceptors are involved in the contractions of proximal portions of the rat tail artery. Therefore, this study investigated whether sympathectomy with reserpine would induce supersensitivity in proximal segments of the rat tail artery, a tissue in which alpha(1D)-adrenoceptors are already functional. Proximal segments of tail arteries from reserpinised rats were three- to sixfold more sensitive to phenylephrine and methoxamine than were arteries from control rats (n = 6-2; p < 0.05). The imidazolines N-[5-(4,5-Dihydro-1H-imidazol-2-yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl]methanesulfonamide hydrobromide (A-61603) and oxymetazoline, which activate selectively alpha(1A)-adrenoceptors, were equipotent in tail arteries from control and reserpinised rats (n = 4-2; p < 0.05), whereas buspirone, which activates selectively alpha(1D)-adrenoceptor, was approximately 4-fold more potent in tail arteries from reserpinised rats (n = 4-6; p < 0.05). Prazosin (nonselective) and 5-methylurapidil (alpha(1A)-selective), were competitive antagonists of contractions induced by phenylephrine and were equipotent in tail arteries from control and reserpinised rats (n = 4-6). The selective alpha(1D)-adrenoceptor antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione dihydrochloride (BMY-7378) presented similar complex antagonism in tail arteries from control and reserpinised rats, with Schild slopes much lower than 1.0 (p < 0.05, n = 4-6). Semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) revealed that mRNA encoding alpha(1A)-and alpha(1B)-adrenoceptors are similarly distributed in tail arteries from control and reserpinised rats, whereas mRNA for alpha(1D)-adrenoceptors is twice more abundant in the tail artery from reserpinised rats. In conclusion, the supersensitivity induced by reserpine is related only to alpha(1D)-adrenoceptors, even in tissues where this receptor subtype is already present and functional. Only the use of subtype-selective alpha(1)-adrenoceptor agonists detected the increased alpha(1D)-adrenoceptor component after reserpinisation, as the antagonists behaved similarly in tail arteries from control and reserpinised rats.

Topics: Adrenergic alpha-1 Receptor Agonists; Adrenergic alpha-1 Receptor Antagonists; Animals; Arteries; Buspirone; Gene Expression; Imidazoles; In Vitro Techniques; Male; Methoxamine; Muscle Contraction; Muscle, Smooth, Vascular; Oxymetazoline; Phenylephrine; Piperazines; Prazosin; Rats; Rats, Wistar; Receptors, Adrenergic, alpha-1; Reserpine; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; Sympathectomy; Tail; Tetrahydronaphthalenes

Ioudina, M. V., Dyer, D. C. Pharmacological characterization of alpha1-adrenoceptor subtypes in the bovine tail artery. J. vet. Pharmacol. Therap. 25, 363-369. The purpose of this study was to identify the alpha1-adrenoreceptor subtypes present in the bovine tail artery which mediate contractions to adrenergic agonists. A61603, an alpha1A-selective agonist, was more potent compared with norepinephrine and phenylephrine. The pKA value of A61603 was 6.93 +/- 0.19 microM (n=6). Antagonists, BMY 7378, WB 4101 and 5-methylurapidil, caused a parallel shift to the right of the concentration-response curve to A61603 with pA2 values of 6.62, 9.27 and 8.86, respectively. Prazosin, BMY 7378 and WB 4101 inhibited phenylephrine induced contraction with pA2 values of 9.47, 7.17 and 9.73, respectively. The pA2 values obtained for 5-methylurapidil, WB 4101, BMY 7378 and prazosin against alpha1-adrenoceptor agonists were significantly correlated with pKi values (Zhu, Zhang & Han, 1997, Eur. J. Pharmacol.329, 55-61) for the cloned alpha1a-adrenoceptor but not with the cloned alpha1b- or alpha1d-adrenoceptor. BMY 7378, a selective alpha1D-adrenoceptor antagonist, was significantly more potent against the nonsubtype selective agonist phenylephrine than to A61603. Chloroethylclonidine (50 microM for 10 min) did not affect contractile responses to A61603, but caused a significant inhibition of contractile responses to phenylephrine. In conclusion, it appears that alpha1A- and alpha1D-adrenoceptors play a role in adrenergic mediated contraction in the bovine tail artery.

Topics: Adrenergic alpha-Agonists; Adrenergic alpha-Antagonists; Animals; Arteries; Cattle; Dioxanes; Dose-Response Relationship, Drug; Imidazoles; Norepinephrine; Phenylephrine; Piperazines; Prazosin; Receptors, Adrenergic, alpha-1; Tail; Tetrahydronaphthalenes; Vasoconstriction

alpha(1)-adrenoceptor subtypes in human skeletal muscle resistance arteries were characterized using agonists noradrenaline (non-selective) and A61603 (alpha(1A)-selective), the antagonists prazosin (non-selective), 5-methyl-urapidil (alpha(1A)-selective) and BMY7378 (alpha(1D)-selective) and the alkylating agent chloroethylclonidine (preferential for alpha(1B)). Small arteries were obtained from the non-ischaemic skeletal muscle of limbs amputated for critical limb ischaemia and isometric tension recorded using wire myography. Prazosin antagonized responses to noradrenaline with a pA(2) value of 9.18, consistent with the presence of alpha(1)-adrenoceptors, although the Schild slope (1.32) was significantly different from unity. 5-Methyl-urapidil competitively antagonized responses to noradrenaline with a pK(B) value of 8.48 and a Schild slope of 0.99, consistent with the presence of alpha(1A)-adrenoceptors. In the presence of 300 nM 5-methyl-urapidil, noradrenaline exhibited biphasic concentration response curves, indicating the presence of a minor population of a 5-methyl-urapidil-resistant subtype. Contractile responses to noradrenaline were not affected by 1 microM chloroethylclonidine suggesting the absence of alpha(1B)-adrenoceptors. Maximum responses to noradrenaline and A61603 were reduced to a similar extent by 10 microM chloroethylclonidine, suggesting an effect of chloroethylclonidine at alpha(1A)-adrenoceptors at the higher concentration. BMY7378 (10 and 100 nM) had no effect on responses to noradrenaline. BMY7378 (1 microM) poorly shifted the potency of noradrenaline giving a pA(2) of 6.52. These results rule out the presence of the alpha(1D)-subtype. These results show that contractile responses to noradrenaline in human skeletal muscle resistance arteries are predominantly mediated by the alpha(1A)-adrenoceptor subtype with a minor population of an unknown alpha(1)-adrenoceptor subtype.

Topics: Adrenergic alpha-Agonists; Adrenergic alpha-Antagonists; Arteries; Clonidine; Dose-Response Relationship, Drug; Humans; Imidazoles; In Vitro Techniques; Muscle, Skeletal; Norepinephrine; Piperazines; Prazosin; Receptors, Adrenergic, alpha-1; Tetrahydronaphthalenes; Vasoconstriction

In this study, alpha(1)-adrenoceptor subtypes were characterised in rat femoral resistance arteries mounted on a small vessel myograph. A-61603 was found to be more potent than noradrenaline and phenylephrine in these arteries. Brimonidine (UK 14304) could not evoke any contractile responses and the sensitivity to noradrenaline and phenylephrine was not affected by (8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13a-decahydro-3-methoxy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]-naphthyridine (RS 79948), ruling out the presence of alpha(2)-adrenoceptors. Prazosin, 5-methyl-urapidil and 2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4-benzodioxane (WB 4101) produced rightward shifts in the sensitivity to noradrenaline, giving pA(2) values of 9.6, 9.4 and 10.4, respectively, in agreement with the presence of alpha(1A)-adrenoceptors. (8-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione (BMY 7378; 1 microM) produced a small shift in the sensitivity of noradrenaline giving a pK(B) of 7.2. In the presence of 300 nM 5-methyl-urapidil, sensitivity to noradrenaline was not further shifted by 1 microM BMY 7378. Responses to noradrenaline were unaffected by the alpha(1B)-adrenoceptor alkylating agent chloroethylclonidine (1 microM). These results suggest alpha(1A)-adrenoceptors mediate contractile responses to noradrenaline in rat femoral resistance arteries.

Topics: Adrenergic alpha-1 Receptor Agonists; Adrenergic alpha-1 Receptor Antagonists; Adrenergic alpha-2 Receptor Antagonists; Adrenergic alpha-Agonists; Adrenergic alpha-Antagonists; Animals; Brimonidine Tartrate; Clonidine; Dioxanes; Dose-Response Relationship, Drug; Femoral Artery; Imidazoles; In Vitro Techniques; Isoquinolines; Male; Naphthyridines; Norepinephrine; Phenylephrine; Piperazines; Prazosin; Quinoxalines; Rats; Rats, Wistar; Receptors, Adrenergic, alpha-1; Tetrahydronaphthalenes; Vascular Resistance; Vasoconstriction

Alpha1-adrenoceptor (alpha1-AR) stimulation increases sarcolemmal Na+-H+ exchanger (NHE) activity. The present study was designed to determine the role(s) of alpha1-AR subtype(s) in mediating this response. As an index of NHE activity, acid efflux rates (JHs) were determined in single rat ventricular myocytes loaded with the pH-sensitive fluoroprobe carboxy-seminaphthorhodafluor-1 after 2 consecutive intracellular acid pulses in bicarbonate-free medium. JH at pHi 6.90 did not change significantly during the second pulse relative to the first in control cells but increased in a dose-dependent manner when the second pulse occurred in the presence of phenylephrine (nonselective alpha1-AR agonist) or A61603 (alpha1A-AR-selective agonist), with EC50 values of 1.24 micromol/L and 3.6 nmol/L, respectively (both agonists given together with 1 micromol/L atenolol). Stimulation of NHE activity by 10 micromol/L phenylephrine was inhibited in a dose-dependent manner by the competitive antagonists prazosin, WB4101, and 5-methylurapidil, with IC50 values of 12, 32, and 149 nmol/L, respectively. Analyses of the relative EC50 and IC50 values obtained (and Ki values estimated from the antagonist IC50s) in relation to the relative potencies of these agents at native rat alpha1-AR subtypes and their relative affinities for recombinant rat alpha1-ARs suggest that alpha1-adrenergic stimulation of sarcolemmal NHE activity is likely to be mediated selectively by the alpha1A-AR.

Topics: Adrenergic alpha-Agonists; Adrenergic alpha-Antagonists; Animals; Cattle; Clonidine; Dioxanes; Heart Ventricles; Humans; Imidazoles; Kinetics; Myocardium; Phenylephrine; Piperazines; Prazosin; Rats; Receptors, Adrenergic, alpha-1; Sarcolemma; Sodium-Hydrogen Exchangers; Tetrahydronaphthalenes

In rats, the pressor response to intravenous (i.v.) phenylephrine is mediated by vascular alpha 1A- and alpha 1B-adrenoceptors, but the role of alpha 1D-adrenoceptors is uncertain. These studies evaluated the effect of a selective alpha 1D-adrenoceptor antagonist, BMY 7378 (8-[2-[4-(2- methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol[4,5]decane-7,9-dio ne 2HCl), on the pressor effect to i.v. phenylephrine (alpha 1A/B/D-adrenoceptor agonist) and (R)A-61603 (alpha 1A-adrenoceptor agonist; N-[5-(4,5-dihydro-1 H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-l -yl] methanesulfonamide HBr) in pithed rats. Pretreatment with BMY 7378 (0.1-1 mg/kg) competitively antagonized the phenylephrine pressor response, but not the (R)A-61603 pressor curve. At 10 mg/kg, BMY 7378 antagonized the (R)A-61603 response, indicating the non-selective blockade of alpha 1A-adrenoceptors. These findings demonstrate that i.v. phenylephrine can activate vascular alpha 1D-adrenoceptors in the pithed rat.

Topics: Adrenergic alpha-1 Receptor Antagonists; Adrenergic alpha-Agonists; Adrenergic alpha-Antagonists; Animals; Blood Pressure; Dose-Response Relationship, Drug; Imidazoles; Male; Phenylephrine; Piperazines; Rats; Rats, Inbred Strains; Receptors, Adrenergic, alpha-1; Tetrahydronaphthalenes