Functional characterization of alpha 1-adrenoceptor subtypes in vascular tissues using different experimental approaches: a comparative study

Inhibition of the norepinephrine transporter rescues vascular hyporeactivity to catecholamine in obstructive jaundice

In patients with obstructive jaundice, the cardiovascular system exhibits hypotension and vascular hyporeactivity. Most norepinephrine is taken up through the neuronal norepinephrine transporter (NET), which is implicated in cardiovascular diseases. A previous study demonstrated that pharmacological NET inhibition could increase resting blood pressure. However, the role of NETs in vascular hyporeactivity induced by obstructive jaundice is poorly understood. This study used the NET inhibitor nisoxetine and a rat model of bile duct ligation (BDL) to investigate whether NET is associated with BDL-induced vascular hyporeactivity. Rats were injected with nisoxetine via the tail vein for 7 consecutive days after BDL. Samples of the superior cervical sympathetic ganglion (SCG) and thoracic aortic rings were processed for investigations. Our results showed that NET expression in the SCG was significantly increased after BDL. Nisoxetine prevented the augmentation of NET expression, increased α₁-adrenoceptor activation, and enhanced the weakened contractile responses of thoracic aortic rings after BDL. Our study demonstrates that nisoxetine plays a protective role in BDL-induced vascular hyporeactivity through increased α₁-adrenoceptor activation in rats.

Activation of α1A -adrenoceptors desensitizes the rat aorta response to phenylephrine through a neuronal NOS pathway, a mechanism lost with ageing

BACKGROUND AND PURPOSE

A NO-mediated desensitization of vasoconstrictor responses evoked by stimulation of α₁ -adrenoceptors has been reported in different vessels. We investigated the involvement of each α₁ -adrenoceptor subtype and constitutive NOS isoforms and the influence of ageing and hypertension on this process.

EXPERIMENTAL APPROACH

Wistar and spontaneously hypertensive rats (SHR), 16, 32, 52 and 72 weeks-old, were used to evaluate the desensitization process. Expression of α₁ -adrenoceptor subtypes, endothelial NOS (eNOS) and neuronal NOS (nNOS) were determined in rat aorta and left ventricle (LV). Expression levels were also evaluated in LV of a group of heart failure patients with a wide age range.

KEY RESULTS

Repeated application of phenylephrine decreased subsequent α₁ -adrenoceptor-mediated vasoconstriction by increasing nNOS protein expression in aorta, but not in tail or mesenteric resistance arteries, where mRNA levels of nNOS were undetectable. This desensitization process disappeared in the absence of endothelium or in the presence of L-NAME (100 μM), nNOS inhibitors, SMTC (1 μM) and TRIM (100 μM), and 5-methylurapidil (100 nM, α_1A -antagonist), but not BMY7378 (10 nM, α_1D -antagonist). The α_1A /nNOS-mediated desensitization was absent in aged SHR and Wistar animals, where the expression of α_1A -adrenoceptors was reduced in aorta and LV. In human LV, a negative correlation was found between age and α_1A -adrenoceptor expression.

CONCLUSIONS AND IMPLICATIONS

The α_1A -adrenoceptor subtype, through endothelial nNOS-derived NO, may act as a physiological 'brake' against the detrimental effects of excessive α₁ -adrenoceptor-mediated vasoconstriction. Reduced α_1A -adrenoceptor- and nNOS-mediated desensitization in aged patients could be involved in the age-dependent elevation of adrenergic activity.

Action of an extract from the seeds of Fraxinus excelsior L. on metabolic disorders in hypertensive and obese animal models

Nuzhenide and GI3, the principal secoiridoids of an extract obtained from the seeds of Fraxinus excelsior L. (FXE), are believed to be the active compounds responsible for the previously reported hypoglycemic effects of this extract. In this study, the effects of FXE were studied in two animal models which are representative of metabolic disorders: spontaneously hypertensive rats (SHR) and obese Zucker rats. SHR were acutely treated (oral gavage) with different doses of FXE. In addition, SHR and Zucker rats were chronically fed (20 or 5 weeks, respectively) with standard chow supplemented with FXE. Acute treatment with FXE (200 mg per kg body weight) decreased systolic blood pressure as in the case with captopril (50 mg per kg body weight). Chronic treatment with FXE at 100 mg per kg body weight per day, a dose equivalent to that showing hypoglycemic activity in humans, resulted in a significant decrease in glycemia (-16.3%), triglyceridemia (-33.4%) and body weight (-8.1%) in Zucker rats as well as a significant decrease in SBP in SHR (-6.7%), with a concomitant improvement in endothelial function in both strains. The broad-ranging effects of FXE may be due to a unique compositional profile that could be useful to prevent the metabolic syndrome, characterized by obesity, insulin resistance, glucose intolerance, hypertriglyceridemia and elevated blood pressure.

Changes in adrenoceptors and G-protein-coupled receptor kinase 2 in L-NAME-induced hypertension compared to spontaneous hypertension in rats

This work compares the expression of adrenoceptors (ARs) and G-protein-coupled receptor kinase (GRK) 2 (RT-PCR and immunoblotting) and functional responses in conductance (aorta) and resistance vessels (mesenteric resistance arteries; MRA) in two different models of rat hypertension: hypertension induced by chronic treatment with L-NAME (N(G)-nitro-L-arginine methyl-ester) (L-NAME-treated rats; LNHR), and genetically induced hypertension (spontaneously hypertensive rats; SHR). Changes found in the aorta, but not in the MRA, were: (1) a loss of contractile capacity, more evidently in α1-AR-mediated contraction, and an impairment of endothelium-dependent vasorelaxation, with both changes occurring independently of the hypertensive model; (2) a diminished sensitivity to α1-AR-induced vasoconstriction along with increased β2-AR-mediated vasodilation in LNHR, and (3) a lower expression of ARs and GRK2 in LNHR. The two latter changes are the opposite of those previously found in aortas of SHR. In the MRA of LNHR, a diminished sensitivity to isoprenaline, in parallel with a reduced expression of β1-AR, was observed without changes in GRK2 expression. In the MRA of SHR, the increased GRK2 expression was not accompanied by significant changes in either β-AR expression or the vasorelaxant potency of isoprenaline. The present results highlight that changes in AR function differ not only between vessels but also between hypertensive models. Moreover, they suggest that changes in GRK2 expression could contribute to regulating β2-AR function in conductance vessels but not β1-AR function in resistance vessels.

α1D-Adrenoceptors are responsible for the high sensitivity and the slow time-course of noradrenaline-mediated contraction in conductance arteries

The objective of this study was to determine whether the different time-course characteristics of α₁-adrenoceptor-mediated contraction in arteries can be related to the subtypes involved. Contractile responses to noradrenaline (NA) were compared with inositol phosphate accumulation and extracellular signal-regulated kinase (ERK)1/2 phosphorylation after α₁-agonist stimuli in the same vessels in the presence or absence of α₁-antagonists in rat or in α₁-subtype knockout (KO) mice. Aorta, where α_1D-AR is the main functional subtype, had higher sensitivity to NA (in respect of inositol phosphate [IP], pERK1/2, and contractile response) than tail artery, where the α_1A-adrenoceptor subtype is predominant. Furthermore, the contraction in aorta exhibited a slower decay after agonist removal and this was consistent in all strains harboring α_1D-adrenoceptors (from rat, α_1B-KO, and wild-type [WT] mice) but was not observed in the absence of the α_1D-adrenoceptor signal (α_1D-adrenoceptor blocked rat aorta or aorta from α_1D-KO). IP formation paralleled α₁-adrenoceptor-mediated contraction (agonist present or postagonist) in aorta and tail artery. High sensitivity to agonist and persistence of response after agonist removal is a property of α_1D-adrenoceptors. Therefore, the preponderance of this subtype in noninnervated conductance arteries such as aorta allows responsiveness to circulating catecholamines and prevents abrupt changes in vessel caliber when the stimulus fluctuates. Conversely, in innervated distributing arteries, high local concentrations of NA are required to activate α_1A-adrenoceptors for a response that is rapid but short lived allowing fine adjustment of the contractile tone by perivascular sympathetic nerves.

The three α1-adrenoceptor subtypes show different spatio-temporal mechanisms of internalization and ERK1/2 phosphorylation

We analyzed the kinetic and spatial patterns characterizing activation of the MAP kinases ERK 1 and 2 (ERK1/2) by the three α1-adrenoceptor (α1-AR) subtypes in HEK293 cells and the contribution of two different pathways to ERK1/2 phosphorylation: protein kinase C (PKC)-dependent ERK1/2 activation and internalization-dependent ERK1/2 activation. The different pathways of phenylephrine induced ERK phosphorylation were determined by western blot, using the PKC inhibitor Ro 31-8425, the receptor internalization inhibitor concanavalin A and the siRNA targeting β-arrestin 2. Receptor internalization properties were studied using CypHer5 technology and VSV-G epitope-tagged receptors. Activation of α1A- and α1B-ARs by phenylephrine elicited rapid ERK1/2 phosphorylation that was directed to the nucleus and inhibited by Ro 31-8425. Concomitant with phenylephrine induced receptor internalization α1A-AR, but not α1B-AR, produced a maintained and PKC-independent ERK phosphorylation, which was restricted to the cytosol and inhibited by β-arrestin 2 knockdown or concanavalin A treatment. α1D-AR displayed constitutive ERK phosphorylation, which was reduced by incubation with prazosin or the selective α1D antagonist BMY7378. Following activation by phenylephrine, α1D-AR elicited rapid, transient ERK1/2 phosphorylation that was restricted to the cytosol and not inhibited by Ro 31-8425. Internalization of the α1D-AR subtype was not observed via CypHer5 technology. The three α1-AR subtypes present different spatio-temporal patterns of receptor internalization, and only α1A-AR stimulation translates to a late, sustained ERK1/2 phosphorylation that is restricted to the cytosol and dependent on β-arrestin 2 mediated internalization.

The impact of alpha1-adrenoceptors up-regulation accompanied by the impairment of beta-adrenergic vasodilatation in hypertension

In human and animal hypertension models, increased activity of G-protein-coupled receptor kinase (GRK) 2 determines a generalized decrease of beta-adrenergic vasodilatation. We analyzed the possibility of differential changes in the expression and functionality of alpha(1A), alpha(1B), alpha(1D), beta(1), beta(2), and beta(3)-ARs also being involved in the process. We combined the quantification of mRNA levels with immunoblotting and functional studies in aortas of young and adult spontaneously hypertensive rats (SHRs) and their controls (Wistar Kyoto). We found the expression and function of beta(1)-adrenoceptors in young prehypertensive SHRs to be higher, whereas a generalized increase in the expression of the six adrenoceptors and GRK2 was observed in aortas of adult hypertensive SHRs. alpha(1D)- and beta(3)-adrenoceptors, the subtypes that are more resistant to GRK2-mediated internalization and mostly expressed in rat aorta, exhibited an increased functional role in hypertensive animals, showing two hemodynamic consequences: 1) an increased sensitivity to the vasoconstrictor stimulus accompanied by a decreased sensitivity to the vasodilator stimulus (alpha(1D)-ARs are the most sensitive to agonists, and beta(3)-ARs are the least sensitive to agonists); and 2) a slower recovery of the basal tone after adrenergic stimulus removal because of the kinetic characteristic of the alpha(1D) subtype. These functional changes might be involved in the greater sympathetic vasoconstrictor tone observed in hypertension.

Smooth muscle alpha1D-adrenoceptors mediate phenylephrine-induced vasoconstriction and increases in endothelial cell Ca2+ in hamster cremaster arterioles

BACKGROUND AND PURPOSE

alpha(1)-Adrenoceptor agonists induce Ca(2+)-transients in endothelial cells (ECs) of arterioles. However, the presence of alpha(1)-adrenoceptors on arteriolar ECs has not been excluded, and the identity of alpha(1)-adrenoceptor subtypes in arterioles only has been inferred from pharmacology. Therefore, we determined which subtypes were expressed by vascular smooth muscle cells (VSMCs) and ECs, and which subtype mediated alpha(1)-adrenoceptor-induced constriction.

EXPERIMENTAL APPROACH

EC Ca(2+)-transients in isolated, cannulated hamster cremasteric arterioles or freshly isolated ECs were studied using Fura 2. Arteriolar diameter was measured by video microscopy. alpha(1)-Adrenoceptor expression was assessed by western blot of whole-arteriolar homogenates and real-time RT-PCR on enzymatically isolated VSMCs and ECs.

KEY RESULTS

Phenylephrine-induced constriction and EC Ca(2+)-transients were abolished by the alpha(1)-adrenoceptor antagonist prazosin (30 nM) in arterioles. Phenylephrine-induced constriction was inhibited by the alpha(1D)-adrenoceptor antagonist BMY 7378 (K(B)=2.96 nM) and the alpha(1A)-adrenoceptor antagonist 5-methylurapidil (K(B)=4.08 nM), suggesting a significant role for alpha(1D)-adrenoceptors. Western blots confirmed alpha(1D)-adrenoceptor expression, but did not detect alpha(1A)-adrenoceptors. VSMCs expressed alpha(1D)- and alpha(1A)-, but not alpha(1B)-, adrenoceptor transcripts. No alpha(1)-adrenoceptor transcripts were detected in ECs. Neither phenylephrine (10 microM) nor noradrenaline (0.1-1 microM) elicited Ca(2+)-transients in freshly isolated ECs, whereas the endothelium-dependent vasodilators methacholine (1 microM) and substance P (100 nM) consistently increased Ca(2+).

CONCLUSIONS AND IMPLICATIONS

We reject the hypothesis that hamster cremasteric arteriolar ECs express alpha(1)-adrenoceptors and conclude that alpha(1)-adrenoceptor agonists predominantly act on VSMC alpha(1D)-adrenoceptors to cause vasoconstriction and a subsequent rise in EC Ca(2+).

Orthostatic hypotensive effect of antipsychotic drugs in Wistar rats by in vivo and in vitro studies of alpha1-adrenoceptor function

RATIONALE

Many antipsychotics cause orthostatic hypotension possibly due to antagonist action on resistance vessel alpha1A-adrenoceptors (alpha1A-AR).

OBJECTIVE

We have tested this possibility by determining in Wistar rats how the orthostatic hypotensive effect of several antipsychotic drugs compares with their affinity for adrenoceptors in mesenteric small arteries (MSA with mainly alpha1A-AR) and aorta (mainly alpha1D-AR).

MATERIALS AND METHODS

Using a tilt setup, orthostatic hypotension was measured in anaesthetized rats for prazosin and the antipsychotics haloperidol, sertindole, risperidone, clozapine, ziprasidone, domperidone, olanzapine, and aripiprazole. For in vitro studies, segments of MSA and aorta were mounted on a wire myograph for isometric tension recording. Cumulative concentration-response curves were constructed to phenylephrine (PE) in the absence and presence of the drugs. Apparent affinity (pA2) was calculated by Schild analysis.

RESULTS

Prazosin antagonized tilt-induced and PE responses in both studies (threshold 4 ng/ml, pA2 9.52 MSA, 10.1 aorta). The rank order of the potency of the antipsychotics in the tilt experiments correlated (r2 = 0.69, P = 0.01) with the pA2-values in MSA: Risperidone and sertindole had the highest potency in the tilt test (threshold 159 and 97 ng/ml) and the highest apparent affinity in MSA (pA2 8.92 and 8.78), in contrast with aripiprazole and domperidone, which had the lowest in each case (threshold 4.1 and 3.0 microg/ml, pA2 7.17 and 6.99). In aorta, the pA2 values did not correlate with the in vivo potencies; in particular, sertindole had no functional affinity in aorta.

CONCLUSION

We conclude that the orthostatic hypotensive effect in rats of the antipsychotic drugs investigated is mediated through alpha1A-ARs.

Impact of alpha1-adrenoceptor expression on contractile properties of vascular smooth muscle cells

Low-frequency blood pressure oscillations (Mayer waves) are discussed as a marker for sympathetic modulation of vascular tone. However, the factors that determine the frequency response of the vasculature to sympathetic stimuli are not fully understood. Possible mechanisms include functions related to alpha(1)-adrenergic receptors (alpha(1)-AR) and postreceptor processes involved in the vascular contractile response. The purpose of the present study was to examine the hypothesis that expression levels of alpha(1)-AR and their subtype distribution determine velocity and magnitude of alpha(1)-AR-mediated vascular smooth muscle cell (VSMC) contraction. alpha(1A)-, alpha(1B)-, and alpha(1D)-AR subtypes were transfected into VSMCs from rat aorta and characterized immunocytochemically via confocal microscopy. Functional studies in isolated cells were performed using video microscopy. The alpha(1)-AR agonist phenylephrine produced dose-dependent contractions of VSMCs. All transfected groups were more sensitive to phenylephrine compared with controls. Maximal contraction velocity almost doubled in transfected cells. However, no differences in observed parameters were found between the three transfected groups. Contractile properties in response to membrane depolarization with KCl were similar in all groups. In conclusion, alpha(1)-AR density determines velocity and sensitivity of alpha(1)-AR-mediated contraction in VSMCs. alpha(1)-AR subtype distribution does not appear to influence vasoconstriction to sympathetic stimuli.

Correlation between mRNA levels and functional role of alpha1-adrenoceptor subtypes in arteries: evidence of alpha1L as a functional isoform of the alpha1A-adrenoceptor

The mRNA levels for the three alpha1-adrenoceptor subtypes, alpha1A, alpha1B, and alpha1D, were quantified by real-time RT-PCR in arteries from Wistar rats. The alpha1D-adrenoceptor was prominent in both aorta (79.0%) and mesenteric artery (68.7%), alpha1A predominated in tail (61.7%) and small mesenteric artery (73.3%), and both alpha1A- and alpha1D-subtypes were expressed at similar levels in iliac artery. The mRNA levels of the alpha1B-subtype were a minority in all vessels (1.7-11.1%). Concentration-response curves of contraction in response to phenylephrine or relaxation in response to alpha1-adrenoceptor antagonists on maximal sustained contraction induced by phenylephrine were constructed from control vessels and vessels pretreated with 100 micromol/l chloroethylclonidine (CEC) for 30 min. The significant decrease in the phenylephrine potency observed after CEC treatment together with the inhibitory potency displayed by 8-{2-[4-(2-methoxyphenyl)-1-piperazinyl]-8-azaspiro (4,5) decane-7-dionedihydrochloride} (BMY-7378, an alpha1D-adrenoceptor antagonist) confirm the relevant role of alpha1D-adrenoceptors in aorta and iliac and proximal mesenteric arteries. The potency of 5-methylurapidil (an alpha1A-adrenoceptor antagonist) and the changes in the potency of both BMY-7378 and 5-methylurapidil after CEC treatment provided evidence of a mixed population of alpha1A- and alpha1D-adrenoceptors in iliac and distal mesenteric arteries. The low potency of prazosin (pIC50 < 9) as well as the high 5-methylurapidil potency in tail and small mesenteric arteries suggest the main role of alpha1A/alpha1L-adrenoceptors with minor participation of the alpha1D-subtype. The mRNA levels and CEC treatment corroborated this pattern and confirmed that the alpha1L-adrenoceptor could be a functional isoform of the alpha1A-subtype.

Insights into the functional roles of alpha(1)-adrenoceptor subtypes in mouse carotid arteries using knockout mice

1. alpha(1)-Adrenoceptor (AR) subtypes in mouse carotid arteries were characterised using a combination of agonist/antagonist pharmacology and knockout (KO) mice. 2. Phenylephrine (PE) was most potent in the alpha(1B)-KO (pEC(50)=6.9+/-0.2) followed by control (pEC(50)=6.3+/-0.06) and alpha(1D)-KO (pEC(50)=5.5+/-0.07). Both N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl] methanesulphonamide hydrobromide (A-61603) and 5-hydroxytryptamine (5-HT) were more potent in the alpha(1D)-KO (pEC(50)=7.4+/-0.27 and 7.4+/-0.05, respectively) than the control (pEC(50)=6.9+/-0.09 and 6.9+/-0.08, respectively) and equipotent with the control in the alpha(1B)-KO (pEC(50)=6.7+/-0.07 and 6.8+/-0.04). Maximum responses to PE and A-61603 were reduced in the alpha(1D)-KO compared to control; there was no difference in maximum responses to 5-HT. 3. In control arteries, prazosin and 5-methylurapidil acted competitively with pA(2) of 9.6 and 7.5, respectively. BMY7378 produced antagonism only at the highest concentration used (100 nM; pK(B) 8.3). 4. Prazosin, 5-methylurapidil and BMY7378 acted competitively in alpha(1B)-KO carotid arteries with pA(2) of 10.3, 7.6 and 9.6, respectively. 5. In the alpha(1D)-KO, against PE, 5-methylurapidil produced a pA(2) of 8.1. pK(B) values were calculated for prazosin (10.6) and BMY7378 (7.0). Against A-61603, 5-methylurapidil had a pA(2) of 8.5, prazosin 8.6, while BMY7378 had no effect. 6. In conclusion, the alpha(1B)-KO mediates contraction solely through alpha(1D)-ARs and the alpha(1D)-KO through alpha(1A)-ARs. Extrapolating back to the control from the knockout data suggests that all three subtypes could be involved in the responses, but we propose that the alpha(1D)-AR causes the contractile response and that the role of the alpha(1B)-AR is mainly regulatory.

Differential Distribution of Functional α1-Adrenergic Receptor Subtypes along the Rat Tail Artery

The rat tail artery has been used for the study of vasoconstriction mediated by alpha(1A)-adrenoceptors (ARs). However, rings from proximal segments of the tail artery (within the initial 4 cm, PRTA) were at least 3-fold more sensitive to methoxamine and phenylephrine (n = 6-12; p < 0.05) than rings from distal parts (between the sixth and 10th cm, DRTA). Interestingly, 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)-ARs, were equipotent in PRTA and DRTA (n = 4-12), whereas buspirone, which activates selectively alpha(1D)-AR, was approximately 70-fold more potent in PRTA than in DRTA (n = 8; p < 0.05). The selective alpha(1D)-AR antagonist 8-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione dihydrochloride (BMY-7378) was approximately 70-fold more potent against the contractions induced by phenylephrine in PRTA (pK(B) of approximately 8.45; n = 6) than in DRTA (pK(B) of approximately 6.58; n = 6), although the antagonism was complex in PRTA. 5-Methylurapidil, a selective alpha(1A)-antagonist, was equipotent in PRTA and DRTA (pK(B) of approximately 8.4), but the Schild slope in DRTA was 0.73 +/- 0.05 (n = 5). The noncompetitive alpha(1B)-antagonist conotoxin rho-TIA reduced the maximal contraction induced by phenylephrine in DRTA, but not in PRTA. These results indicate a predominant role for alpha(1A)-ARs in the contractions of both PRTA and DRTA but with significant coparticipations of alpha(1D)-ARs in PRTA and alpha(1B)-ARs in DRTA. Semiquantitative reverse transcription-polymerase chain reaction revealed that mRNA encoding alpha(1A)- and alpha(1B)-ARs are similarly distributed in PRTA and DRTA, whereas mRNA for alpha(1D)-ARs is twice more abundant in PRTA. Therefore, alpha(1)-ARs subtypes are differentially distributed along the tail artery. It is important to consider the segment from which the tissue preparation is taken to avoid misinterpretations on receptor mechanisms and drug selectivities.

Differential participation of protein kinase C and Rho kinase in α1-adrenoceptor mediated contraction in rat arteries

The major functional α1-adrenoceptor in the rat aorta is of the α1Dsubtype and that in the caudal artery is of the α1Asubtype. In the present study, the participation of protein kinase C (PKC) and Rho kinase (RhoK) in contractile responses to stimulation of the α1-adrenoceptors in these two arteries was investigated. Both the PKC inhibitor Ro-318220 and the RhoK inhibitor Y-27632 significantly blocked contractile responses of the aorta to phenylephrine (PE) and the selective α1A-adrenoceptor agonist A61603. When used in combination, the inhibitors had an additive blocking effect. In the caudal artery, Y-27632 but not Ro-318220 inhibited contractile responses to PE and A61603, and, in combination, the antagonism produced was no greater than that by Y-27632 alone. Contractile responses to direct activation of PKC with phorbol 12,13-dibutyrate were much smaller and levels of CPI-17 (PKC-activated protein phosphatase inhibitor of 17 kDa) were much lower in the caudal artery than the aorta. The results suggest that both PKC and RhoK contribute independently to contractile responses to stimulation of α1D-adrenoceptors in the aorta. However, RhoK, but not PKC, participates in contractile responses to stimulation of α1A-adrenoceptors in the caudal artery. This difference may largely be due to differences between the two arteries in the extent to which PKC participates in contraction.Key words: vascular smooth muscle, α1-adrenoceptors, protein kinase C, rho kinase, phenylephrine.

Alpha-1D adrenoceptors are involved in reserpine-induced supersensitivity of rat tail artery

1. We examined reserpine-induced chemical denervation supersensitivity with special reference to alpha-1 adrenoceptor (AR) subtypes. 2. Chronic treatment with reserpine for 2 weeks depleted noradrenaline in the tail artery and spleen of rats. Noradrenaline in the thoracic aorta was negligible before and after reserpine treatment. 3. The treatment with reserpine produced supersensitivity in the contractile responses of the rat tail artery to phenylephrine, 5-HT and KCl, resulting in leftward shift of concentration-response curves (11.6-, 2.5- and 1.1-fold at EC(50) value, respectively). These results suggest a predominant sensitization of the alpha-1 AR-mediated response by reserpine treatment. 4. BMY 7378 at a concentration (30 nm) specific for blocking the alpha-1D AR subtype, but not KMD-3213 at a concentration (10 nm) selective for blocking the alpha-1A AR subtype, inhibited the supersensitivity of the phenylephrine-induced response in the reserpine-treated artery. On the other hand, the response to phenylephrine in reserpine-untreated artery was selectively inhibited by the same concentration of KMD-3213, but not by BMY 7378. Prazosin, a subtype-nonselective antagonist, blocked the responses to phenylephrine with the same potency, regardless of reserpine treatment. 5. In the thoracic aorta and spleen, no supersensitivity was produced in the responses to phenylephrine by reserpine treatment. 6. In a tissue segment-binding study using [(3)H]-prazosin, the total density and affinity of alpha-1 ARs in the rat tail artery were not changed by treatment with reserpine. However, alpha-1D AR with high affinity for BMY 7378 was significantly detected in reserpine-treated tail artery, in contrast to untreated artery. Decreases in alpha-1A AR with high affinity for KMD-3213 and alpha-1B AR with low affinities for KMD-3213 and BMY 7378 were also estimated in reserpine-treated tail artery. 7. Alpha-1D AR mRNA in rat tail artery increased to three-folds by reserpine treatment, whereas the levels of alpha-1A and 1B mRNAs were not significantly changed. 8. The present results suggest that chronic treatment with reserpine affects the expression of alpha-1 AR subtypes of rat tail artery and that the induction of alpha-1D ARs with high affinity for catecholamines is in part associated with reserpine-induced supersensitivity.

Alpha-1 adrenoceptors: evaluation of receptor subtype-binding kinetics in intact arterial tissues and comparison with membrane binding

The binding kinetics of [3H]-prazosin were measured using intact segments of rat tail artery (RTA) and thoracic aorta (RAO), and the data were compared with those obtained using a conventional membrane ligand-binding method. In intact RTA and RAO segments, [3H]-prazosin bound reversibly in a time-dependent and receptor-specific manner at 4 degrees C to alpha-1 adrenoceptors (ARs) of the plasma membrane, with affinities (pKD): 9.5 in RTA; 9.9 in RAO) that were in agreement with values estimated by a conventional membrane ligand-binding method. However, nonspecific binding was considerably higher in RAO than RTA, failing to detect clearly the specific binding at high concentrations (>300 pm) of [3H]-prazosin in binding experiments with RAO segments and membranes. The abundance of receptor in the RTA and RAO (Bmax mg-1) of total tissue protein), estimated using the tissue segment-binding approach (527+/-14 fmol mg-1 for RTA; 138+/-4 fmol mg-1 for RAO), was about 25-fold higher than values estimated using a conventional membrane-binding method (22+/-5 fmol mg-1) for RTA; 5+/-1 fmol mg-1 for RAO). Binding competition experiments using intact tissue segments or membranes derived from RTA tissue yielded comparable data, indicating a coexistence of alpha-1A AR (high affinity for prazosin, KMD-3213 and WB4101 and low affinity for BMY 7378) and alpha-1B AR (high affinity for prazosin but low affinity for KMD-3213, WB4101 and BMY 7378). In RAO tissue, careful evaluation of the tissue segment-binding assay revealed the coexpression of alpha-1B AR (high affinity for prazosin, but low affinity for KMD-3213 and BMY 7378) and alpha-1D AR (high affinity for prazosin and BMY 7378, but low affinity for KMD-3213), whereas the membrane-binding approach failed to detect these receptor subtypes with certainty. The present study indicates that previous estimates of alpha-1 AR density and alpha-1 AR subtypes obtained by a conventional membrane-binding approach, as opposed to our improved tissue segment-binding assay, may have substantially underestimated the abundance of receptors present in arterial tissues, and may have failed to identify accurately the presence of receptor subtypes. Advantages and disadvantages of the tissue segment-binding approach are discussed.British Journal of Pharmacology (2004) 141, 468-476. doi:10.1038/sj.bjp.0705627