Pharmacological characterization of noradrenaline-induced contractions of the porcine isolated palmar lateral vein and palmar common digital artery

Interaction of prostanoid EP₃ and TP receptors in guinea-pig isolated aorta: contractile self-synergism of 11-deoxy-16,16-dimethyl PGE₂

BACKGROUND AND PURPOSE

Surprisingly high contractile activity was reported for 11-deoxy-16,16-dimethyl prostaglandin E₂ (DX-DM PGE₂) on pig cerebral artery when used as a selective EP₃ receptor agonist. This study investigated the selectivity profile of DX-DM PGE₂, focusing on the interaction between its EP₃ and TP (thromboxane A₂-like) agonist activities.

EXPERIMENTAL APPROACH

Contraction of guinea-pig trachea (EP₁ system) and aorta (EP₃ and TP systems) was measured in conventional organ baths.

KEY RESULTS

Strong contraction of guinea-pig aorta to sulprostone and 17-phenyl PGE₂ (EP₃ agonists) was only seen under priming with a second contractile agent such as phenylephrine, histamine or U-46619 (TP agonist). In contrast, DX-DM PGE₂ induced strong contraction, which on the basis of treatment with (DG)-3ap (EP₃ antagonist) and/or BMS-180291 (TP antagonist) was attributed to self-synergism arising from co-activation of EP₃ and TP receptors. EP₃/TP self-synergism also accounted for contraction induced by PGF(2α) and its analogues (+)-cloprostenol and latanoprost-FA. DX-DM PGE₂ also showed significant EP₁ agonism on guinea-pig trachea as defined by the EP₁ antagonists SC-51322, (ONO)-5-methyl-1 and AH-6809, although AH-6809 exhibited poor specificity at concentrations ≥3 µM.

CONCLUSIONS AND IMPLICATIONS

EP₃/TP self-synergism, as seen with PGE/PGF analogues in this study, may confound EP₃ agonist potency comparisons and the characterization of prostanoid receptor systems. The competitive profile of a TP antagonist may be distorted by variation in the silent/overt contraction profile of the EP₃ system in different studies. The relevance of self-synergism to in vivo actions of natural prostanoid receptor agonists is discussed.

The effect of cooling on the contractility of equine digital small lamellar arteries: modulating role of the endothelium

The equine hoof displays thermoregulatory functions, and the blood vessels lying under the hoof wall are temperature sensitive. The aim of this study was to investigate the effect of cooling on the contractile responses to α-adrenoceptor and 5-HT receptor stimulation in equine small lamellar arteries using wire myography. The role of the endothelium in the response mediated by 5-HT was also evaluated. Moderate cooling caused a reduction of the contraction induced by depolarizing Krebs solution (DKS, containing 118 mm KCl) and the maximal contraction caused by UK-14304 (α(2)-adrenoceptor agonist). The potency of methoxamine (α(1)-adrenoceptor agonist) was reduced by cooling [pD(2) (-log EC(50)) at 22°C, 5.7 (5.5-6.0) versus 30°C, 5.9 (5.7-6.1)]; however, the efficacy was maintained. The contractions evoked by sumatripan and α-methyl 5-HT (5-HT receptor agonists) were not modified by moderate cooling. In contrast, a cooling-enhanced contraction was observed in response to 5-HT [maximum response (E(max)) at 22°C, 213.2 ± 13% DKS versus 30°C, 179.9 ± 11% DKS]. Furthermore, this effect was more evident in endothelium-denuded lamellar arteries (E(max) at 22°C, 270.2 ± 26% DKS versus 30°C, 172.2 ± 20% DKS), suggesting a potential modulating role of the endothelium. The L-NAME/ibuprofen-resistant relaxation in response to carbachol was reduced by cooling in small lamellar arteries precontracted with 5-HT but not phenylephrine. Therefore, a moderate reduction of temperature modulates the reactivity of small lamellar arteries by enhancing the 5-HT-mediated contraction, but inhibits the α-adrenoceptor-mediated response. Furthermore, the endothelium of these blood vessels may play an important role in preventing excessive vasoconstriction in response to 5-HT and maintaining digital blood flow in cool environmental temperatures.

A61603-induced contractions of the porcine meningeal artery are mediated by alpha1- and alpha2-adrenoceptors

It has recently been shown that A61603 (N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydro-naphthalen-1-yl]methane sulphonamide), a potent alpha(1A)-adrenoceptor agonist, decreased carotid artery conductance in anaesthetized pigs by a novel non-adrenergic mechanism. In this study, we set out to pharmacologically characterize A61603-induced contractions of the porcine isolated meningeal artery. While the maximum contractile responses of the artery were similar, A61603 (E(max): 183 +/- 23% of 100 mM KCl; pEC(50): 7.25 +/- 0.18) was more potent than noradrenaline (E(max): 156 +/- 16%; pEC(50): 5.75 +/- 0.17) or phenylephrine (E(max): 163 +/- 20%; pEC(50): 5.63 +/- 0.02). Prazosin (pA(2): 9.36 +/- 0.23) and, to a lesser extent, rauwolscine (pK(b): 6.36 +/- 0.38) and yohimbine (pK(b): 7.30 +/- 0.15) antagonised the contractions to A61603. The 5-HT(1B) (GR127935; N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide) and 5-HT(2) (ritanserin) receptor antagonists failed to affect the responses to A61603, but methiothepin, which, in addition, has a high affinity for alpha-adrenoceptors, proved an effective antagonist. The A61603-induced responses were suppressed by the cAMP stimulator forskolin, but not by the protein kinase C inhibitor chelerythrine. Our results suggest that the contraction of porcine isolated meningeal artery by A61603 is mediated via mainly alpha(1)-(probably alpha(1A)) and, to a lesser extent, alpha(2)-adrenoceptors, involving the adenylyl cyclase, but not the diacylglycerol pathway.

Characterization of the postjunctional alpha 2C-adrenoceptor mediating vasoconstriction to UK14304 in porcine pulmonary veins

BACKGROUND AND PURPOSE

In terms of postjunctional alpha(2)-adrenoceptors in the pulmonary circulation, no evidence is available with regard to the receptor subtypes mediating vasoconstriction. Therefore, we characterized the alpha(2)-adrenoceptor subtypes mediating contraction in isolated porcine pulmonary veins.

EXPERIMENTAL APPROACH

alpha-adrenoceptor-mediated vasoconstriction was studied using a tissue bath protocol. mRNA profile and relative quantification of alpha(2)-adrenoceptor subtypes were determined in porcine pulmonary veins using reverse-transcriptase polymerase chain reaction (RT-PCR) and real-time PCR.

KEY RESULTS

In porcine pulmonary veins, noradrenaline, phenylephrine (alpha(1)-adrenoceptor agonist), UK14304 and clonidine (alpha(2)-adrenoceptor agonists) caused concentration-dependent contractions. The rank order of agonist potency was: NA approximately UK14304 approximately clonidine > phenylephrine. UK14304 responses were antagonised by MK912 (noncompetitive antagonist parameter pD'(2): 10.1), rauwolscine (pK(B): 9.5), yohimbine (pK(B): 9.1), WB4101 (pK(B): 8.7), ARC239 (pK(B): 7.5), prazosin (pK(B): 7.1) and BRL44408 (pK(B): 7.0). Antagonist potencies fitted best with radioligand binding data (pK(i)) at the human recombinant alpha(2C)-adrenoceptor (r(2)=0.96, P=0.0001). Correlation with alpha(2B)-adrenoceptors was lower (r(2)=0.74, P>0.01) and no correlation was obtained with alpha(2A)-adrenoceptors. Moreover, RT-PCR studies in porcine pulmonary veins showed mRNA signals for alpha(2A)- and alpha(2C)-adrenoceptors, but not for alpha(2B)-adrenoceptors, whilst real-time PCR studies indicated a prominent expression of alpha(2C)-adrenoceptor mRNA.

CONCLUSIONS AND IMPLICATIONS

Postjunctional alpha(2C)-adrenoceptors mediated contraction in porcine pulmonary veins. alpha(1)-Adrenoceptors also seem to be present in this tissue. Since alpha(2)-adrenoceptor responsiveness is increased when pulmonary vascular tone is elevated, alpha(2C)-adrenoceptor antagonists may be beneficial in diseases such as pulmonary hypertension or congestive heart failure.

Characterisation of the relaxant response to raloxifene in porcine coronary arteries

The present study characterises the vasorelaxant response to raloxifene in isolated rings of porcine coronary artery. Tissues precontracted either with KCl (30 mM) or prostaglandin F(2alpha) (PGF(2alpha); 3 microM) were concentration-dependently relaxed by raloxifene (0.1-10 microM). Relaxation was not inhibited by the estrogen receptor antagonist 7alpha-[9-[(4,4,5,5,5-pentafluoropentyl)sulfinyl]nonyl]-estra-1,3,5(10)-triene-3,17beta-diol (ICI 182,780; 1 microM). Preincubation with raloxifene (1-3 microM) caused an inhibition of the KCl or PGF(2alpha)-induced contraction. The effects of raloxifene were independent of the endothelium. The relaxant response to raloxifene was slow in the onset and could not be reversed after repeated washings. Raloxifene did not affect Ca(2+) release from intracellular stores since it failed to inhibit a transient contraction induced by caffeine (10 mM). Raloxifene-induced relaxation was not influenced by the intracellular calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA-AM; 10-20 microM). Calcium-induced contractions in Ca(2+)-free high K(+) (60 mM) depolarising medium were concentration-dependently inhibited by raloxifene (0.3-3 microM). If arterial rings were incubated with the L-type Ca(2+) channel activator (S)-(-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-3-pyridine carboxylic acid methyl ester ((S)-(-)-Bay K 8644; 0.1 microM), cumulative concentration-response curves to Ca(2+) were shifted to the left. Raloxifene (0.3-3 microM) inhibited the effect of (S)-(-)-Bay K 8644 in a concentration-dependent manner. 4-(4-Fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole (SB 203580; 10 microM), an inhibitor of p38 mitogen-activated protein kinase (MAPK), diminished raloxifene-induced relaxation in endothelium-denuded arterial rings. Western blot analysis demonstrated that raloxifene stimulated p38 MAPK. It is concluded that raloxifene has an inhibitory effect on voltage-gated and receptor-operated L-type Ca(2+) channels in porcine coronary arteries, thus inducing vascular relaxation independent of the endothelium. p38 MAPK is, at least in part, involved in the relaxant response to raloxifene.

The Role of Rho Kinase and Extracellular Regulated Kinase-Mitogen-Activated Protein Kinase in α2-Adrenoceptor-Mediated Vasoconstriction in the Porcine Palmar Lateral Vein

Alpha2-adrenoceptor-mediated vasoconstriction in the porcine palmar lateral vein is dependent upon activation of the extracellular signal-regulated kinase-mitogen-activated protein (ERK-MAP) kinase signal transduction pathway. Recent studies have shown that alpha2-adrenoceptor-mediated vasoconstriction in the rat aorta is also dependent upon activation of Rho kinase. The aim of this study was to determine whether Rho kinase and ERK-MAP kinase are part of the same signaling pathway. The Rho kinase inhibitor Y27632 (trans-4-[(1R)-1-aminoethyl]-N-4-pyridinylcyclohexanecarboxamide dihydrochloride) (10 microM) almost completely inhibited the contractile response to the alpha2-adrenoceptor agonist UK14304 (5-bromo-6-[2-imidazolin-2-ylamine]-quinoxaline bitartrate) in segments of porcine palmar lateral vein [maximum response 2.9 +/- 2.3% of 60 mM KCl response (mean +/- S.E.M.) in the presence of Y27632, compared with 64.9 +/- 7.1% in control tissues, n = 4]. However, Y27632 had no effect on alpha2-adrenoceptor-mediated ERK activation, as measured by Western blotting. Alpha2-adrenoceptor-mediated vasoconstriction was associated with an increase in phosphorylation of the myosin phosphatase-targeting subunit (MYPT) at Thr696 (the Rho kinase phosphorylation site). This phosphorylation was inhibited by 10 microM Y27632. In contrast, inhibition of ERK activation with the MAP kinase kinase inhibitor PD98059 (2-amino-3-methoxyflavone) (50 microM) had no effect on MYPT phosphorylation. Both Y27632 and PD98059 inhibited myosin light chain phosphorylation. These data indicate that alpha2-adrenoceptor-mediated vasoconstriction in the porcine palmar lateral vein is dependent upon both Rho kinase and ERK activation, although these are separate pathways. Rho kinase causes vasoconstriction through inhibition of myosin phosphatase and an increase in myosin light chain phosphorylation, whereas ERK causes vasoconstriction through a myosin phosphatase-independent pathway.

The role of several alpha(1)- and alpha(2)-adrenoceptor subtypes mediating vasoconstriction in the canine external carotid circulation

1. It has recently been shown that both alpha(1)- and alpha(2)-adrenoceptors mediate vasoconstriction in the canine external carotid circulation. The present study set out to identify the specific subtypes (alpha(1A), alpha(1B) and alpha(1D) as well as alpha(2A), alpha(2B) and alpha(2C)) mediating the above response. 2. Consecutive 1 min intracarotid infusions of phenylephrine (alpha(1)-adrenoceptor agonist) and BHT933 (alpha(2)-adrenoceptor agonist) produced dose-dependent decreases in external carotid blood flow, without affecting mean arterial blood pressure or heart rate. 3. The responses to phenylephrine were selectively antagonized by the antagonists, 5-methylurapidil (alpha(1A)) or BMY7378 (alpha(1D)), but not by L-765,314 (alpha(1B)), BRL44408 (alpha(2A)), imiloxan (alpha(2B)) or MK912 (alpha(2C)). In contrast, only BRL44408 or MK912 affected the responses to BHT933. 4. The above results support our contention that mainly the alpha(1A), alpha(1D), alpha(2A) and alpha(2C)-adrenoceptor subtypes mediate vasoconstriction in the canine external carotid circulation.

Subtypes of functional alpha1- and alpha2-adrenoceptors

In this review, subtypes of functional alpha1- and alpha2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. Alpha1-adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th alpha1-adrenoceptor, the alpha1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. Alpha2-adrenoceptors can be divided into alpha2A-, alpha2B- and alpha2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha2-adrenoceptors are predominantly of the alpha2A-adrenoceptor subtype (the alpha2D-adrenoceptor is a species orthologue), although alpha2C-adrenoceptors may also occur prejunctionally. Although alpha2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of alpha-adrenoceptor subtypes.

Alpha2-adrenoceptor-mediated contractions of the porcine isolated ear artery: evidence for a cyclic AMP-dependent and a cyclic AMP-independent mechanism

1. The aim of this study was to determine the conditions under which the alpha2-adrenoceptor agonist UK14304 produces vasoconstriction in the porcine isolated ear artery. 2. UK14304 (0.3 microM) produced a small contraction of porcine isolated ear arteries which was 7.8+/-3.3% of the response to 60 mM KCl. Similar sized contractions were obtained after precontraction with either 30 nM angiotensin II, or 0.1 microM U46619 (8.2+/-1.8% and 10.2+/-2.6% of 60 mM KCl response, respectively). However, an enhanced alpha2-adrenoceptor response was uncovered if the tissue was precontracted with U46619, and relaxed back to baseline with 1-2 microM forskolin before the addition of UK14304 (46.9+/-9.6% of 60 mM KCl response). 3. The enhanced responses to UK14304 in the presence of U46619 and forskolin were not inhibited by the alpha1-adrenoceptor antagonist prazosin (0.1 microM), but were inhibited by the alpha2-adrenoceptor antagonist rauwolscine (1 microM), indicating that the enhanced responses were mediated via postjunctional alpha2-adrenoceptors. 4. In the presence of 0.1 microM U46619 and 1 mM isobutylmethylxanthine (IBMX), 1 microM forskolin produced an increase in [3H]-cyclic AMP levels in porcine isolated ear arteries. Addition of 0.3 microM UK14304 prevented this increase. 5. The enhanced UK14304 response was dependent upon the agent used to relax the tissue. After relaxation of ear arteries precontracted with 10 nM U46619 and relaxed with forskolin the UK14304 response was 46.9+/-9.6% of the 60 mM KCl response, and after relaxation with sodium nitroprusside (SNP) the response was 24.8+3.3%. However, after relaxation of the tissue with levcromakalim the UK14304 response was only 8.2+/-1.7%, which was not different from the control response in the same tissues (12.2+/-5.6%). An enhanced contraction was also obtained after relaxation of the tissue with the cyclic AMP analogue dibutyryl cyclic AMP (23.2+/-1.3%) indicating that at least part of the enhanced response to UK14304 is independent of the ability of the agonist to inhibit cyclic AMP production. 6. Relaxation of U46619 contracted ear arteries with SNP could be inhibited by the NO-sensitive guanylyl-cyclase inhibitor 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) indicating that production of cyclic GMP is necessary for the relaxant effect of SNP. However, ODQ had no effect on the relaxation of tissue by forskolin, suggesting that this compound does not act via production of cyclic GMP. Biochemical studies showed that while forskolin increases the levels of cyclic AMP in the tissues, SNP had no effect on the levels of this cyclic nucleotide. 7. In conclusion, enhanced contractions to the alpha2-adrenoceptor agonist UK14304 can be uncovered in porcine isolated ear arteries by precontracting the tissue with U46619, followed by relaxation back to baseline with forskolin, SNP or dibutyryl cyclic AMP before addition of UK14304. There was a greater contractile response to UK14304 after relaxation with forskolin than with SNP or dibutyryl cyclic AMP, suggesting that cyclic AMP-dependent and- independent mechanisms are involved in the enhancement of the UK14304 response.

Characterization of alpha 2-adrenoceptors mediating contraction of dog saphenous vein: identity with the human alpha 2A subtype

1. In the dog saphenous vein alpha 1- and alpha 2-adrenoceptors mediate noradrenaline-induced contractions in vitro. In order to study the alpha 2-adrenoceptor in isolation, alpha 1-adrenoceptors were inactivated by treatment of tissues with the alkylating agent phenoxybenzamine (3.0 microM for 30 min) in the presence of rauwolscine (1 microM) to protect alpha 2-adrenoceptors. 2. Noradrenaline-induced contractions of tissues treated with phenoxybenzamine were antagonized competitively by the selective alpha 2-adrenoceptor antagonist rauwolscine, pKB = 8.63 +/- 0.07 (means +/- s.e. mean; n = 3), consistent with an interaction at alpha 2-adrenoceptors. 3. Noradrenaline was a full agonist at alpha 2-adrenoceptors in dog saphenous vein. By use of the method of partial receptor alkylation and analysis of concentration-effect curve data by direct, operational model fitting methods, the affinity (pKA) and efficacy (tau) were 5.74 +/- 0.07 and 7.50 +/- 1.05, respectively (n = 6). Nine other agonists which were examined each had affinities higher than noradrenaline, but with the exception of the imidazoline, A-54741 (5,6-dihydroxy-1,2,3,4-tetrahydro-1-naphthyl-imidazoline) had relatively lower efficacies. 4. To compare the alpha 2-adrenoceptor in dog saphenous vein to the human recombinant subtypes, the affinities of twenty-one compounds were estimated in functional studies in the dog saphenous vein and in radioligand binding studies for the human alpha 2A, alpha 2B and alpha 2C receptor subtypes expressed in Chinese hamster lung (CHL) cells. 5. Of twenty-one compounds examined in ligand binding studies, only nine had greater than ten fold selectivity for one human receptor subtype over either of the other two. These compounds were A-54741, oxymetazoline, guanfacine, guanabenz, prazosin, spiroxatrine, tolazoline, WB 4101 and idazoxan. In dog saphenous vein, their affinities (pKA and pKB for agonists and antagonists respectively) were: A-54741 (pKA = 8.03 +/- 0.05), oxymetazoline (pKA = 7.67 +/- 0.09), guanfacine (pKA = 6.79 +/- 0.03); guanabenz (pKA = 7.02 +/- 0.13); prazosin (pKB = 5.19 +/- 0.08), spiroxatrine (pKB = 6.59 +/- 0.04), tolazoline (pKB = 6.21 +/- 0.07), WB 4101 (pKB = 7.42 +/- 0.09) and idazoxan (pKB = 7.11 +/- 0.08). 6. Comparisons of affinity estimates for these nine compounds at the receptor in dog saphenous vein and at the human recombinant subtypes suggest that the vascular receptor is most similar to the h alpha 2A subtype; correlation coefficients (r) were 0.82 (h alpha 2A), 0.24 (h alpha 2B) and 0.04 (h alpha 2C).

Investigation of the subtype of alpha 2-adrenoceptor mediating pressor responses in the pithed rat

We have investigated the subtype of alpha 2-adrenoceptor mediating postjunctional pressor responses in the pithed rat in comparison with alpha 2-adrenoceptor ligand binding sites. In pithed rats, postjunctional alpha 2-adrenoceptors were investigated in terms of the ability of alpha 2-adrenoceptor antagonists to shift the pressor potency of the alpha 2-adrenoceptor agonist xylazine. Antagonist potency at postjunctional alpha 2-adrenoceptors in the pithed rat was correlated with antagonist affinity at alpha 2-adrenoceptor ligand binding sites in membranes of rat kidney (alpha 2B), Sf9 cells expressing human recombinant receptors (alpha 2C) and rat submandibular gland (alpha 2D) labelled with [3H]yohimbine. The correlation with the postjunctional alpha 2-adrenoceptor mediating pressor responses in the pithed rat was better for the alpha 2D-adrenoceptor ligand binding site of rat submandibular gland (r = 0.95, n = 9, P < 0.0001) and the alpha 2B-adrenoceptor ligand binding site of rat kidney (r = 0.90, n = 9, P < 0.001) than with the human recombinant alpha 2C-adrenoceptor ligand binding site (r = 0.81, n = 9, P < 0.01). When the pressor potencies of three additional antagonists were included in the correlations for alpha 2B- and alpha 2D-sites only, the correlation with alpha 2D-adrenoceptor ligand binding site of rat submandibular gland (r = 0.91, n = 12, P < 0.0001) was much better than with the alpha 2B-adrenoceptor ligand binding site of rat kidney (r = 0.77, n = 12, P < 0.01). It is concluded that the functional postjunctional alpha 2-adrenoceptors mediating pressor responses in the pithed rat most closely resemble the alpha 2D-adrenoceptors subtype.

Alpha 2-adrenoceptor mediated inhibition of forskolin-stimulated cyclic AMP accumulation in isolated porcine palmar lateral veins

The aim of this study was to use a 3H-adenine pre-labelling technique to characterise the effect of alpha 2-adrenoceptor activation on forskolin-stimulated cyclic AMP accumulation in the isolated porcine palmar lateral vein. Forskolin (10(-7)-10(-4) M) stimulated 3H-cyclic AMP accumulation in the isolated porcine palmar lateral vein in a biphasic and concentration-dependent manner. In the absence of the cyclic AMP-selective phosphodiesterase inhibitor rolipram, forskolin stimulated 3H-cyclic AMP accumulation approximately 7-8 fold. The response reached a peak after 5 min. In the presence of rolipram (10(-5) M), basal 3H-cyclic AMP levels were approximately 70% higher than in its absence (basal: 1823 +/- 57 dpm; rolipram: 3088 +/- 229, n = 3) and forskolin (3 x 10(-5) M) stimulated 3H-cyclic AMP accumulation approximately 8 fold. The latter response reached a plateau 10 min after the addition of forskolin. In all subsequent studies, the tissues were incubated with forskolin (3 x 10(-5) M) for 5 min in the absence of rolipram. Noradrenaline (NA; 10(-9)-10(-4) M) and UK14304 (10(-9)-10(-4) M) inhibited forskolin-stimulated 3H-cyclic AMP accumulation in a concentration-dependent manner with mean pIC50 values of 7.61 +/- 0.37 (n = 4) and 7.76 +/- 0.23 (n = 5), respectively. With either NA or UK14304, the maximal inhibition of the forskolin response obtained was approximately 75%. Neither NA (10(-4) M) nor UK14304 (10(-4) M) altered basal 3H-cyclic AMP levels.(ABSTRACT TRUNCATED AT 250 WORDS)

The relationship between density of alpha-adrenoceptor binding sites and contractile responses in several porcine isolated blood vessels

1. The aim of this study was to investigate constrictor alpha-adrenoceptors in three isolated blood vessels of the pig, the thoracic aorta (TA), the splenic artery (SA) and marginal ear vein (MEV) and then compare the functional response with the densities of alpha 1- and alpha 2-adrenoceptor binding sites in these and several other porcine vascular tissues, palmar common digital artery (PCDA), palmar lateral vein (PLV) and ear artery (EA). 2. Noradrenaline (NA), phenylephrine (PE) and UK14304 (all at 0.03-10 microM) elicited concentration-dependent contractions in the TA and MEV, with a rank order of potency of UK14304 > NA > PE. UK14304 produced maximal responses which were 58% (TA) and 65% (MEV) of that of NA. In the SA, UK14304 and PE produced maximal responses which were less than 10% and 50% of the NA-induced maximal response respectively, with an order of potency of NA > PE. In the SA, NA-induced contractions were competitively antagonized by prazosin (pA2 = 8.60 +/- 0.15). Further, rauwolscine (1-10 microM) antagonized NA-induced contractions with an apparent pKB of 6.09 +/- 0.11 (n = 6), indicating an action at alpha 1-adrenoceptors. The combination of the two antagonists at concentrations selective for alpha 1- (0.1 microM) and alpha 2-adrenoceptors (1 microM) had no greater effect than either antagonist alone. This suggests that the SA expresses only post-junctional alpha 1-adrenoceptors. 3. In the TA, prazosin produced non-parallel shifts in the NA-induced CRC and this was also observed with rauwolscine, where reductions in the maximal responses were also observed. In the MEV, prazosin was largely inactive in antagonizing NA-induced contractions. In both these vessels a combination of these two antagonists had a greater effect than either alone, indicating the presence of functional alpha 1- and alpha 2-adrenoceptors. The post-junctional alpha 2-adrenoceptors in all of these vessels were resistant to prazosin, suggesting the alpha 2-adrenoceptor to be of the alpha 2A/2D subtype. The expression of functional alpha 2-adrenoceptors was MEV > TA > PLV > PCDA > SA. 4. In radioligand binding studies using TA P2 pellet membranes, [3H]-prazosin and [3H]-RX821002 ([1,4-[6,7(n)-3H] benzodioxan-2-methoxy-2-yl)-2-imidazole) labelled different high affinity sites, and in competition studies using identical membranes corynanthine displaced [3H]-prazosin with 10 fold higher affinity than rauwolscine, indicating that [3H]-prazosin was selectively binding to alpha 1-adrenoceptor sites. Further, rauwolscine displaced [3H]-RX821002 with approximately 100 fold greater affinity compared to corynanthine, which is indicative of selective alpha2-adrenoceptor binding.5. Separation of the P2 pellet into plasma membrane and mitochondrial fractions was carried out using a differential sucrose density gradient. [3H]-prazosin and [3H]-RX821002 binding sites were found in both the plasma membrane and mitochondrial fractions.6. In saturation studies all tissues produced single site saturation curves with no difference in the Kd(range 0.13-0.20nM) of the alpha1-adrenoceptor sites for [3H]-prazosin. However, there was considerable variation in Bmax of alpha 1-adrenoceptor sites; the highest density was found in the TA (397.9 =/- 52.7 fmol mg-1, n = 4), followed by the PCDA (256.7 +/- 22.7 fmol mg-1, n = 4), the PLV and SA having approximately equal density (143.6 +/- 3.9 and 159.1 +/- 7.0 fmol mg-1 respectively, n = 4 for both), followed bythe EA (91.3 +/- 10.5 fmol mg-1, n = 3) and the MEV had the lowest density (48.9 +/- 11.4 fmol mg-1,n = 3).7. In saturation studies using [3H]-RX821002, all tissues produced single site saturation curves with no differences in the Kd values (range 1.31 +/- 2.16 nM) but the highest densities were found in the TA and MEV (545.3 +/- 36.2 and 531.0 +/- 40.9 fmol mg-1 respectively), followed by the PLV (418.4 +/- 39.4 fmol mg-1), then the EA (266.3 +/- 40.0 fmol mg-1), and low densities of [3H]-RX821002 binding being found in the PCDA and SA (155.9 +/- 18.1 and 117.5 +/- 19.3 fmol mg-1 respectively).8. The pattern of binding site distribution for alpha l- and alpha 2-adrenoceptors is in reasonable agreement with functional studies carried out in these porcine vascular tissues; the TA has the highest densities of alpha 1-and alpha2-adrenoceptors; in the SA and PCDA there is a predominance (although small) of alpha l-adrenoceptor binding sites, the reverse of which is observed both in the PLV and MEV (i.e. greater density of alpha2-adrenoceptor sites). Thus, it would appear that alpha 1- and alpha2-adrenoceptor densities play a role in the expression of functional responses via these receptor subtypes; although it is interesting to note that the SA did have a small density of alpha 2-adrenoceptor binding sites, no functional response was observed after alpha2-adrenoceptor activation.