Subtype-specific dimerization of alpha 1-adrenoceptors: effects on receptor expression and pharmacological properties

The potential role of dimerization in controlling the expression and pharmacological properties of alpha1-adrenoceptor subtypes was examined using coimmunoprecipitation of epitope-tagged receptors. Human alpha1-adrenoceptor subtypes (alpha1A, alpha1B, alpha1D) were tagged at their amino-termini with Flag or hemagglutinin epitopes and transfected into human embryonic kidney 293 cells. Homodimerization of all three subtypes was observed by coimmunoprecipitation of receptors with different tags and was not altered by norepinephrine treatment. Heterodimer formation between hemagglutinin-tagged alpha1B-adrenoceptors and Flag-tagged alpha1A- or alpha1D-adrenoceptors was also observed. However, no alpha1A/alpha1D-adrenoceptor heterodimers were observed, suggesting that dimerization is subtype-specific. The extent of heterodimerization was also unaltered by norepinephrine treatment. alpha1-Adrenoceptor truncation mutants lacking carboxyl or amino-terminal sequences formed homo- and heterodimers similarly to full-length receptors, suggesting that these domains play little or no role in dimerization. Biotinylation with a membrane-impermeable agent showed that monomers and homo- and hetero-oligomers of all three subtypes are expressed on the cell surface. Radioligand binding studies showed that heterodimerization did not alter the affinity of alpha1-adrenoceptors for norepinephrine, prazosin, or subtype-selective antagonists, suggesting that dimerization does not result in pharmacologically distinct subtypes. However, coexpression of alpha1B-adrenoceptors significantly increased both binding site density and protein expression of alpha1A- and alpha1D-adrenoceptors, and increased cell surface expression of alpha1D-adrenoceptors, suggesting a functional role for heterodimerization. Conversely, coexpression of alpha1A-with alpha1D-adrenoceptors, which did not heterodimerize, had no effect on receptor density or protein. These studies demonstrate subtype-selective heterodimerization of alpha1-adrenoceptors, which does not change their pharmacological properties but seems to have functional consequences in regulating receptor expression and trafficking.     

Gender difference in levels of alpha2-adrenoceptor mRNA in the rat tail artery

To investigate the hypothesis that differing mRNA levels underlie gender differences in the contractile response of the rat tail artery, alpha2-adrenoceptor mRNA was measured using in situ hybridization. Messenger RNA for the alpha2A- and alpha2C-adrenoceptor subtypes was found localized to the smooth muscle layer. There was no detectable mRNA present for the alpha2B-adrenoceptor subtype. Levels of alpha2C-adrenoceptor mRNA were greater in female compared to male tail arteries (417 +/- 35 vs. 263 +/- 38 dpm/mg, P = 0.01), while levels of alpha2A-adrenoceptor mRNA were the same in both sexes. Levels of alpha2-adrenoceptor mRNA may parallel levels of functioning protein present in the rat tail artery.     

High-affinity interactions between human alpha1A-adrenoceptor C-terminal splice variants produce homo- and heterodimers but do not generate the alpha1L-adrenoceptor

Using combinations of bioluminescence resonance energy transfer, time-resolved fluorescence resonance energy transfer and the functional complementation of pairs of inactive receptor-G protein fusion proteins, the human alpha(1A-1)-adrenoceptor was shown to form homodimeric/oligomeric complexes when expressed in human embryonic kidney (HEK) 293 cells. Saturation bioluminescence resonance energy transfer studies indicated the alpha(1A-1)-adrenoceptor homodimer interactions to be high affinity and some 75 times greater than interactions between the alpha(1A-1)-adrenoceptor and the delta opioid peptide receptor. Only a fraction of the alpha(1A-1)-adrenoceptors was at the plasma membrane of HEK293 cells at steady state. However, dimers of alpha(1A-1)-adrenoceptors were also present in intracellular membranes, and the dimer status of those delivered to the cell surface was unaffected by the presence of agonist. Splice variation can generate at least three forms of the human alpha(1A-1)-adrenoceptor with differences limited to the C-terminal tail. Each of the alpha(1A-1), alpha(1A-2a), and alpha(1A-3a)-adrenoceptor splice variants formed homodimers/oligomers, and all combinations of these splice variants were able to generate heterodimeric/oligomeric interactions. Despite the coexpression of these splice variants in human tissues that possess the pharmacologically defined alpha(1L)-adrenoceptor binding site, coexpression of any pair in HEK293 cells failed to generate ligand binding characteristic of the alpha(1L)-adrenoceptor.     

[3H]-MK 912 binding delineates two alpha 2-adrenoceptor subtypes in rat CNS one of which is identical with the cloned pA2d alpha 2-adrenoceptor.

1. Simultaneous computer modelling of control and guanfacine-masked [3H]-MK 912 saturation curves as well as guanfacine competition curves revealed that the drugs bound to two alpha 2-adrenoceptor subtypes in the rat cerebral cortex with very different selectivities. These alpha 2-adrenoceptor subtypes were designated alpha 2A and alpha 2C. The Kd value of [3H]-MK 912 for the alpha 2A-subtype was 1.77 nM and for the alpha 2C-subtype 0.075 nM; the receptor sites showing capacities 296 and 33 fmol mg-1 protein, respectively. The Kds of guanfacine were 19.9 and 344 nM, respectively. 2. Binding constants of 26 compounds for the two rat cerebral cortex alpha 2-adrenoceptor subtypes were determined by simultaneous computer modelling of control and guanfacine-masked drug competition curves as well as plain guanfacine competition curves using [3H]-MK912 as labelled ligand (i.e. a '3-curve assay'). Of the tested drugs WB4101, corynanthine, rauwolscine, yohimbine, ARC 239 and prazosin were found to be clearly alpha 2C-selective with selectivities ranging from 16 to 30 fold whereas guanfacine, oxymetazoline, BRL 44408 and BRL 41992 were found to be alpha 2A-selective with selectivities ranging from 9 to 22 fold. 3. The Kds of compounds obtained for the cerebral cortex alpha 2C-adrenoceptors showed an almost 1:1 correlation with the corresponding Kds for alpha 2-adrenoceptors expressed by the pA2d-gene (the rat 'alpha 2-C4' adrenoceptor) in CHO-cells. The cerebral cortex alpha 2A-adrenoceptors did not correlate well with the pA2d alpha 2-adrenoceptor Kds. 4. In the rat spinal cord [3H]-MK 912 bound to alpha 2A- and alpha 2C-adrenoceptor sites with similar affinities as in the cerebral cortex and with densities 172 and 7.4 fmol mg-1 protein, respectively. Drug affinities for some compounds showing major selectivity for alpha 2A- and alpha 2C-adrenoceptors were fully compatible with the notion that the spinal cord sites were alpha 2A- and alpha 2C-adrenoceptors.     

Contractile 5-HT1B receptors in human cerebral arteries: pharmacological characterization and localization with immunocytochemistry

1 The cerebrovascular receptor(s) that mediates 5-hydroxytryptamine (5-HT)-induced vasoconstriction in human cerebral arteries (HCA)has proven difficult to characterize, yet these are essential in migraine. We have examined 5-HT receptor subtype distribution in cerebral blood vessels by immunocytochemistry with antibodies selective for human 5-HT1B and human 5-HT1D receptors and also studied the contractile effects of a range of 5-HT receptor agonists and antagonists in HCA. 2 Immunocytochemistry of cerebral arteries showed dense 5-HT1B receptor immunoreactivity (but no 5-HT1D receptor immunoreactivity) within the smooth muscle wall of the HCA. The endothelial cell layer was well preserved and weak 5-HT1B receptor immunoreactivity was present. 3 Pharmacological experiments on HCA with intact endothelium showed that 5-carboxamidotryptamine was significantly more potent than alpha-methyl-5-HT, 2-methyl-5-HT and 5-HT in causing vasoconstriction. The 5-HT1B/1D receptor agonists naratriptan, sumatriptan, zolmitriptan and 181C91 (N-desmethyl zolmitriptan), all induced equally strong contractions and with similar potency as 5-HT. The maximum contractile response was significantly less for avitriptan and dihydroergotamine. There was a significant correlation between vasoconstrictor potency and 5-HT1B- and 5-HT1D-receptor affinity, but not with 5-HT1A-, 5-ht1F or 5-HT2- receptor affinity. 4 The 5-HT1B/1D-receptor antagonist GR 55562 (10-7 - 10-6 M) inhibited the contractile responses to sumatriptan and 5-CT in a competitive manner with a pKB value for GR 55562 of 7.4. Furthermore, ketanserin (10-7 M), prazosin (10-7 M), and sulpiride (10-7 M) were devoid of significant antagonistic activity of 5-HT-induced contraction in the HCA. 5 The results are compatible with the hypothesis that the 5-HT1B receptors play a major role in 5-HT-induced vasoconstriction in HCA.     

Alpha1-adrenoceptor subtypes.

Alpha1-adrenoceptors are one of three subfamilies of receptors (alpha1, alpha2, beta) mediating responses to adrenaline and noradrenaline. Three alpha1-adrenoceptor subtypes are known (alpha1A, alpha1B, alpha1D) which are all members of the G protein coupled receptor family, and splice variants have been reported in the C-terminus of the alpha1A. They are expressed in many tissues, particularly smooth muscle where they mediate contraction. Certain subtype-selective agonists and antagonists are now available, and alpha1A-adrenoceptor selective antagonists are used to treat benign prostatic hypertrophy. All subtypes activate phospholipase C through the G(q/11) family of G proteins, release stored Ca2+, and activate protein kinase C, although with significant differences in coupling efficiency (alpha1A > alpha1B > alpha1D). Other second messenger pathways are also activated by these receptors, including Ca2+ influx, arachidonic acid release, and phospholipase D. Alpha1-adrenoceptors also activate mitogen-activated protein kinase pathways in many cells, and some of these responses are independent of Ca2+ and protein kinase C but involve small G proteins and tyrosine kinases. Direct interactions of alpha1-adrenoceptors with proteins other than G proteins have not yet been reported, however there is a consensus binding motif for the immediate early gene Homer in the C-terminal tail of the alpha1D subtype. Current research is focused on discovering new subtype-selective drugs, identifying non-traditional signaling pathways activated by these receptors, clarifying how multiple signals are integrated, and identifying proteins interacting directly with the receptors to influence their functions.     

Characterization of rat white fat cell alpha 1B-adrenoceptors.

In isolated rat white adipocytes, epinephrine (in the presence of 10 microM propranolol) increased the uptake of [32P]Pi into phosphatidylinositol in a dose-dependent fashion. When the cells were pretreated with the irreversible antagonist chlorethylclonidine, this alpha 1-adrenergic effect was markedly diminished. The effect of epinephrine was dose-dependently antagonized by selective alpha 1-adrenergic antagonists, with the potency order prazosin greater than 5-methylurapidil greater than or equal to WB4101. Binding studies using crude membrane preparations were performed with the ligands [3H]bunazosin and 125I-HEAT. Both ligands bound to membrane sites with high affinity (Kd values of 0.75 +/- 0.20 nM for [3H]bunazosin and 125 +/- 20 pM for 125I-HEAT), in a rapid, reversible, and saturable (Bmax, 9-12 fmol/mg of protein) fashion, and with the expected pharmacological characteristics for alpha 1-adrenoceptors. Binding displacement studies with these ligands indicated a potency order of prazosin greater than 5-methylurapidil greater than or equal to WB4101. Northern blot analysis using receptor subtype-specific gene probes showed that adipocyte mRNA hybridized with the alpha 1B-adrenergic probe. All these data suggest that the alpha 1-adrenoceptors of rat white adipocytes belong to the alpha 1B subtype.     

Selective agonists reveal alpha(1A)- and alpha(1B)-adrenoceptor subtypes in caudal artery of the young rat

Multiple alpha(1)-adrenoceptors were evaluated in caudal artery of the young Wistar rat using selective agonists and antagonists. Arteries were exposed to the selective alpha(1A)-adrenoceptor agonist, A-61603 (N-[5-(4,5-dihydro-1H-imidazol-2-yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl] methanesulfonamide) or to phenylephrine and to prazosin (alpha(1)-adrenoceptor antagonist), or the selective alpha(1A)-adrenoceptor antagonists 5-methylurapidil, RS 100329 (5-methyl-3-[3-[4-[2-(2,2,2,-trifluoroethoxy)phenyl]-1-piperazinyl]propyl]-2,4-(1H)-pyrimidinedione), RS 17053 (N-[2(2-cyclopropylmethoxy) ethyl]-5-chloro-alpha, alpha-dimethyl-1H-indole-3-ethanamide), and the selective alpha(1D)-adrenoceptor antagonist BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5] decane-7,9-dione). Results showed a 100-fold higher potency of A-61603 for the alpha(1)-adrenoceptor present in the artery, compared with phenylephrine. Prazosin displaced both agonists with high affinity, whereas 5-methylurapidil, RS 100329 and RS 17053 displaced A-61603 with high affinity, indicating the presence of alpha(1A)-adrenoceptors. The selective alpha(1A)-adrenoceptor antagonists blocked phenylephrine responses with low affinity, suggesting that phenylephrine activated a second receptor population in caudal artery. BMY 7378 antagonized with low affinity both A-61603 and phenylephrine-induced contractions, indicating absence of alpha(1D)-adrenoceptors in the vessel. The results suggest that functional alpha(1B)-adrenoceptors are present in caudal arteries of the young Wistar rat.     

Different affinities of native alpha1B-adrenoceptors for ketanserin between intact tissue segments and membrane preparations

The pharmacological profiles of alpha1-adrenoceptors for ketanserin, prazosin, silodosin, and BMY 7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]decane-7,9-dione dihydrochloride) were examined under different assay conditions. Among the tested antagonists and alpha1-adrenoceptors subtypes, ketanserin showed significantly lower affinity for the alpha1B-adrenoceptor subtype in intact tissue sampled from the rat tail artery, thoracic aorta, and cerebral cortex (functional pKB and binding pKi were approximately 6), than in cerebral cortex membrane preparations or whole cell and membrane preparations of alpha1B-adrenoceptor transfected human embryonic kidney 293T (HEK 293T) cells (pKi was approximately 8). In these tissues and cells, however, ketanserin showed a similar affinity (pKi = approximately 8) for alpha1A- and alpha1D-adrenoceptors even though the assays were conducted under different conditions. In contrast, the affinities of alpha1A-, alpha1B-, and alpha1D-adrenoceptors for prazosin, silodosin, and BMY 7378 did not significantly change under different assay conditions and in different tissues. The present study reveals that the pharmacological profiles of native alpha 1B-adrenoceptors for ketanserin is strongly influenced by the assay conditions and suggest that antagonist affinity is not necessarily constant.     

Dopamine D2-like receptors on human peripheral blood lymphocytes: a radioligand binding assay and immunocytochemical study

1 Peripheral blood lymphocytes express dopamine D₁-like and D₂-like receptors which were investigated using radioligand binding assay and molecular biology techniques. Analysis of dopamine D₂-like receptors expressed by human peripheral blood lymphocytes with radioligand binding assay may offer a rapid technique for assessing receptor changes in disorders characterized by involvement of the dopaminergic system. However, the suitability of radioligand binding assay techniques to measure dopamine D₂-like receptors is questioned. 2 In view of the discrepancy between data of dopamine D₂-like receptor determination with molecular biology and radioligand binding assay techniques, we have assayed dopamine D₂-like receptors expressed by human peripheral blood lymphocytes using as radioligands the dopamine receptor agonist 7-[³H]-hydroxy-N,N-di-n-propyl-2-aminotetraline ([³H]-7-OH-DPAT) and two antagonists ([³H]-spiperone and [³H]-nemonapride). 3 Analysis of saturation curves revealed a concentration-dependent binding of all compounds to human peripheral blood lymphocytes. Dissociation constant (K_d) values averaged between 0.15 and 0.40 n m for different radioligands. The maximum density of binding sites (B_max) was low, ranging from 4.15 ± 0.05 fmol/10⁶ cells with [³H]-spiperone and 8.66 ± 0.04 fmol/10⁶ cells with [³H]-7-OH-DPAT. 4 Displacement curves of [³H]-7-OH-DPAT, [³H]-spiperone and [³H]-nemonapride binding to human peripheral blood lymphocytes revealed, using radioligand concentrations giving the highest specific:non-specific binding ratio, a pharmacological profile consistent with the labelling of dopamine D₂-like receptors. The use of higher radioligand concentrations resulted in a poorly displaceable and characterizable binding. 5 Detection of dopamine D₂, D₃ and D₄ receptor immunoreactivity in cytospin centrifuged peripheral blood lymphocytes revealed dopamine D₃ and D₄ but not D₂ receptor immunostaining. 6 The above findings indicate in agreement with molecular biology studies, that dopamine D₂-like receptors expressed by human peripheral blood lymphocytes belong to the D₃ and D₄ receptor subtypes. These receptors are detectable using either dopamine D₂-like receptor agonists and antagonists as radioligands if controlled experimental conditions are followed. The standardisation of immunocytochemical techniques for detecting human peripheral blood lymphocyte dopamine receptors may contribute to clarify their role in lymphocyte function or as a peripheral marker of the status of the dopaminergic system.     

Comparison of guinea-pig,bovine and rat alpha 1-adrenoceptor subtypes.

1. To elucidate a possible role of species differences in the classification of alpha 1-adrenoceptor subtypes, we have characterized the alpha 1-adrenoceptors in guinea-pig spleen, kidney and cerebral cortex and in bovine cerebral cortex using concentration-dependent alkylation by chloroethylclonidine and competitive binding with 5-methlurapidil, methoxamine, (+)-niguldipine, noradrenaline, oxymetazoline, phentolamine, SDZ NVI-085, tamsulosin and (+)-tamsulosin. Rat liver alpha 1B-adrenoceptors were studied for comparison. Chloroethylclonidine-sensitivity and (+)-niguldipine affinity were also compared at cloned rat and bovine alpha 1a-adrenoceptors. 2. Chloroethylclonidine concentration-dependently inactivated alpha 1-adrenoceptors in all five tissues. While chloroethylclonidine inactivated almost all alpha 1-adrenoceptors in rat liver and guinea-pig kidney and brain, 20-30% of alpha 1-adrenoceptors in guinea-pig spleen and bovine brain were resistant to alkylation by 10 microM chloroethylclonidine. With regard to concentration-dependency guinea-pig kidney and brain were approximately 10 fold less sensitive than guinea-pig spleen or rat liver. 3. In rat liver, all drugs tested competed for [3H]-prazosin binding with steep and monophasic curves. Drug affinities were relatively low and resembled most closely those of cloned rat alpha 1b-adrenoceptors. 4. In guinea-pig spleen, all drugs tested competed for [3H]-prazosin binding with steep and monophasic curves. Drug affinities were relatively low and resembled most closely those of cloned rat alpha 1b-adrenoceptors. 5. In guinea-pig kidney most drugs tested competed for [3H]-prazosin binding with steep and monophasic curves and had relatively low drug affinities close to those of cloned rat alpha 1b- and alpha 1d-adrenoceptors. However, noradrenaline and tamsulosin had consistently biphasic competition curves recognizing 36-39% high and 61-64% low affinity sites. 6. In guinea-pig cerebral cortex, all drugs tested competed for [3H]-prazosin binding with shallow and biphasic curves. While most drugs recognized approximately 25% high affinity sites, tamsulosin and noradrenaline recognized approximately 50% high affinity sites. Drug affinities at the high and low affinity sites except those for tamsulosin and noradrenaline resembled those at cloned alpha 1a- and alpha 1b-adrenoceptors, respectively. 7. In bovine cerebral cortex all drugs tested except for noradrenaline competed for [3H]-prazosin binding with shallow and biphasic curves. All drugs recognized approximately 70% high affinity sites. Drug affinities at the high and low affinity sites resembled those at cloned alpha 1a- and alpha 1b-adrenoceptors, respectively. Noradrenaline competition curves in bovine cerebral cortex were steep and monophasic. 8. When cloned rat and bovine alpha 1a-adrenoceptors transiently expressed in COS cells were studied in a direct side-by-side comparison, both species homologues had similar chloroethylclonidine-sensitivity and (+)-niguldipine affinity. 9. We conclude that properties of bovine alpha 1A- and alpha 1B-adrenoceptors are very similar to those of other species such as rat. alpha 1-Adrenoceptor subtypes in guinea-pigs resemble alpha 1A- and alpha 1B-adrenoceptors in other species but chloroethylclonidine sensitivity and competition binding profiles of noradrenaline and tamsulosin are not compatible with previously established alpha 1-adrenoceptor subtype classification.     

Studies on the location of catecholamine receptors in canine sympathetic ganglia.

Receptors mediating catecholamine-induced inhibition were studied in cardiac ganglia of pentobarbital-anesthetized dogs. Using selective agonists and antagonists the presence of three receptor subtypes was verified: alpha 1- and alpha 2-adrenoceptors and dopamine D2 receptors. Activation of alpha 1-adrenoceptors or dopamine D2 receptors reduced the response to preganglionic nerve stimulation but not to direct stimulation of the nicotinic acetylcholine receptors of the principal ganglion cells: response to both types of stimulation were reduced by activation of ganglionic alpha 2-adrenoceptors. These results suggested that two inhibitory systems were present in canine sympathetic ganglia and mediated the effects of exogenous catecholamines. One system involved alpha 1-adrenoceptors and dopamine D2 receptors located proximal to the synapse of the pre- and postganglionic neurons and the other involved alpha 2-adrenoceptors located distal to the intraganglionic synapse.     

Different roles of alpha1-adrenoceptor subtypes in mediating cardiomyocyte protein synthesis in neonatal rats

1. Three different alpha1-adrenoceptor subtypes, designated alpha1A, alpha1B and alpha1D, have been cloned and identified pharmacologically in cardiomyocytes. In vitro studies have suggested that alpha1-adrenoceptors play an important role in facilitating cardiac hypertrophy. However, it remains controversial as to which subtype of alpha1-adrenoceptors is involved in this response. In the present study, we investigated the different role of each alpha1-adrenoceptor subtype in mediating cardiomyocyte protein synthesis, which is a most important characteristic of cardiac hypertrophy in cultured neonatal rat cardiomyocytes. 2. Cardiomyocyte hypertrophy was monitored by the following characteristic phenotypic changes: (i) an increase in protein synthesis; (ii) an increase in total protein content; and (iii) an increase in cardiomyocyte size. 3. The role of each alpha1-adrenoceptor subtype in mediating cardiomyocyte protein synthesis was investigated by the effect of specific alpha1-adrenoceptor subtype-selective antagonists on noradrenaline-induced [3H]-leucine incorporation. In addition, pKB values for alpha1-adrenoceptor subtype-selective antagonists were calculated and compared with the corresponding pKi values to further identify their effects. 4. Activation of alpha1-adrenoceptors by phenylephrine or noradrenaline in the presence of propranolol significantly increased [3H]-leucine incorporation, protein content and cell size. 5. Pre-incubating cardiomyocytes with 5-methyl-urapidil, RS 17053 or WB 4101 significantly inhibited noradrenaline-induced [3H]-leucine incorporation. However, there was no effect when cardiomyocytes were pre-incubated with BMY 7378. The correlation coefficients between pKB values for alpha1-adrenoceptor subtype-selective antagonists and pKi values obtained from cloned alpha1A-, alpha1B- or alpha1D-adrenoceptors were 0.92 (P <0.01), 0.66 (P >0.05) and 0.24 (P >0.05), respectively. 6. Our results suggest that the alpha1-adrenoceptor is dominantly responsible for adrenergic hypertrophy of cultured cardiomyocytes in neonatal rats. The efficiency in mediating cardiomyocyte protein synthesis is alpha1A > alpha1B > alpha1D.     

Modification of classical drug receptor mechanisms]

It is generally accepted that full and partial agonists interact with the same receptors according to the classical receptor mechanisms. We have modified the drug receptor mechanisms of M3-, alpha 1- and beta-receptors. Among the muscarinic receptors, there are two subtypes of M3-cholinoceptors, propylbenzilylcholine mustard (PrBCM)-sensitive receptors and (PrBCM)-resistant ones. Full agonists contract the guinea pig ileum through both types of cholinoceptors, while the partial agonists produce contractions through only the PrBCM-sensitive receptors. Two subtypes of alpha 1-adrenoceptors, alpha 1A and alpha 1B, were demonstrated in some arteries. Full agonists contracted the rabbit aorta through both the alpha 1A- and alpha 1B-adrenoceptors, while the partial agonists mediated contraction through only the alpha 1A-adrenoceptors. beta-Chloroethylamines (PrBCM and chloroethylclonidine) can discriminate the subtype of M3- or alpha 1-receptors in the presence of GTP. beta-Adrenoceptors have two different types of binding sites, high and low affinity sites. The competitive antagonistic effect of the partial agonist is due to their ability to compete with the full agonists for the high affinity site, while the partial agonists interact with the low affinity site to induce the beta-adrenergic effect. A regional difference in alpha 1-adrenoceptor mechanisms was discussed. The potency of norepinephrine in veins is related to alpha 1-adrenoceptor densities. In contrast, the potency of norepinephrine is linearly related to the agonist dissociation constant. This discrepancy suggests a qualitative difference between alpha 1-adrenoceptor mechanisms in the veins and arteries.     

Subtype-specific alpha1- and beta-adrenoceptor signaling in the heart

Recent studies of adrenoceptors have revealed subtype-specific signaling, promiscuous G-protein coupling, time-dependent switching of intracellular signaling pathways, intermolecular interactions within or between adrenoceptor subfamilies, and G-protein-independent signaling pathways. These findings have extended the classical linear paradigm of G-protein-coupled receptor signaling to a complex "signalome" in which an individual adrenoceptor initiates multiple signaling pathways in a temporally and spatially regulated manner. In particular, persistent stimulation of beta-adrenoceptor subtypes causes a time-dependent switch of signaling pathways and elicits different, even opposing, functional roles of these receptors in regulating cardiac structure and function. Recent progress in the understanding of subtype-specific functions and signaling mechanisms of cardiac adrenoceptor subtypes, particularly beta(1)-adrenoceptors, beta(2)-adrenoceptors, alpha(1A)-adrenoceptors and alpha(1B)-adrenoceptors, might have important pathogenic and therapeutic implications for heart disease.