Purification and characterization of the human platelet alpha 2-adrenergic receptor

Human platelet alpha 2-adrenergic receptors have been purified approximately 80,000-fold to apparent homogeneity by a five-step chromatographic procedure. The overall yield starting from the membranes is approximately 2%. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of radioiodinated protein from purified receptor preparations shows a single major band of Mr 64,000. The specific binding activity of the alpha 2-adrenergic receptor after four chromatographic steps is 14.5 nmol/mg protein. This value is consistent with the expected theoretical specific activity (15.6 nmol/mg) for a protein with a molecular mass of 64,000 daltons if it is assumed that there is one ligand-binding site/receptor molecule. The purified protein can be covalently labeled with the alkylating alpha-adrenergic ligand, [3H]phenoxybenzamine. This labeling is specific, and it shows that the Mr 64,000 protein contains the ligand binding site of the alpha 2-adrenergic receptor. In addition, the competitive binding of ligands to the purified receptor protein shows the proper alpha 2-adrenergic specificity. The alpha 2-adrenergic receptor contains an essential sulfhydryl residue. Thus, exposure of the purified receptor to the sulfhydryl-specific reagent, phenylmercuric chloride, resulted in an 80% loss of binding activity. This loss of binding activity was prevented when exposure to phenylmercuric chloride was done in the presence of alpha 2-adrenergic ligands, and it was reversed by subsequent exposure to dithiothreitol. Partial proteolysis of purified alpha 2-adrenergic receptors was obtained with Staphylococcus aureus V-8 protease, alpha-chymotrypsin, and papain. In a comparison with purified beta 2-adrenergic receptors, no common partial proteolytic products were found.     

Identification of alpha1-adrenergic receptors and their involvement in phosphoinositide hydrolysis in the frog heart

The aim of this study was to characterize alpha(1)-adrenergic receptors in frog heart and to examine their related signal transduction pathway. alpha(1)-Adrenergic binding sites were studied in purified heart membranes using the specific alpha(1)-adrenergic antagonist [(3)H]prazosin. Analysis of the binding data indicated one class of binding sites displaying a K(d) of 4.19 +/- 0.56 nM and a B(max) of 14.66 +/- 1.61 fmol/mg original wet weight. Adrenaline, noradrenaline, or phenylephrine, in the presence of propranolol, competed with [(3)H]prazosin binding with a similar potency and a K(i) value of about 10 microM. The kinetics of adrenaline binding was closely related to its biological effect. Adrenaline concentration dependently increased the production of inositol phosphates in the heart in the presence or absence of propranolol. Maximal stimulation was about 8.5-fold, and the half-maximum effective concentration was 30 and 21 microM in the absence and presence of propranolol, respectively. These data clearly show that alpha(1)-adrenergic receptors are coupled to the phosphoinositide hydrolysis in frog heart. To our knowledge, this is the first direct evidence supporting the presence of functional alpha(1)-adrenergic receptors in the frog heart.     

Molecular size of the human platelet alpha 2-adrenergic receptor as determined by radiation inactivation

The size of the alpha 2-adrenergic receptor in human platelet membranes has been determined by radiation inactivation/target size analysis to be 160,000 daltons. The size of the platelet alpha 2-receptors is essentially identical to that of the alpha 1-adrenergic receptor of the rat liver membrane.     

The effects of lithium on platelet phosphoinositide metabolism.

The effects on phosphoinositide metabolism of preincubation of platelets for 90 min with 10 mM-Li+ were studied. Measurements were made of [32P]phosphate-labelled phosphoinositides and of [3H]inositol-labelled inositol mono-, bis- and tris-phosphate (InsP, InsP2 and InsP3). Li+ had no effect on the basal radioactivity in the phosphoinositides or in InsP2 or InsP3, but it caused a 1.8-fold increase in the basal radioactivity in InsP. Li+ caused a 4-, 3- and 2-fold enhanced thrombin-induced accumulation of label in InsP, InsP2 and InsP3 respectively. Although the elevated labelling of InsP2 and InsP3 returned to near-basal values within 30-60 min, the high labelling of InsP did not decline over a period of 60 min after addition of thrombin to Li+-treated platelets, consistent with inhibition of InsP phosphatase by Li+. The effect of Li+ was not due to a shift in the thrombin dose-response relationship; increasing concentrations of thrombin enhanced the initial rate of production of radiolabelled inositol phosphates, whereas Li+ affected either a secondary production or the rate of their removal. The only observed effect of Li+ on phosphoinositide metabolism was a thrombin-induced decrease (P less than 0.05) in labelled phosphatidylinositol 4-phosphate in Li+-treated platelets; this suggests an effect on phospholipase C. Li+ enhanced (P less than 0.05) the thrombin-induced increase in labelled lysophosphatidylinositol, suggesting an effect on phospholipase A2. It is concluded that Li+ inhibits InsP phosphatase and has other effects on phosphoinositide metabolism in activated platelets. The observed effects occur too slowly to be the mechanism by which Li+ potentiates agonist-induced platelet activation.     

Cloning of the human serotonin 5-HT2 and 5-HT1C receptor subtypes.

We report the cloning and the deduced amino acid sequence of cDNAs encoding both the human serotonin 5-HT2 and 5-HT1C receptors. The human 5-HT2 and 5-HT1C receptors shared 87% and 90% amino acid homology, respectively, with their rat counterparts. The most divergent regions of the 5-HT2 receptor between human and rat were the N-terminal extracellular domain (75% homology) and the C-terminal intracellular domain (67% homology between amino acids 426-474). The greatest variability between the human and rat 5-HT1C receptors were at the N-terminal extracellular domain (78% homology) and the third cytoplasmic loop (71% homology). The availability of the cloned human 5-HT2 and 5-HT1C receptors will help facilitate the further understanding of the molecular pharmacology and physiology of these receptors.     

QSAR study on the affinity of some arylpiperazines towards the 5-HT1A/alpha1-adrenergic receptor using the E-state index

QSAR models represent the relationship of biological activity with either physicochemical parameters or structural indices. QSAR study was performed on some arylpiperazines as 5-HT(1A)/alpha(1)-adrenergic receptor antagonists using E-state indices to identify the pharmacophoric requirements. It was found that some of the atoms played important roles to both activities and some played important role in selectivity of compound to the 5-HT(1A) antagonistic activity. The presence of COONHPr group at the ortho-position of the phenyl ring might be disadvantageous and Br at meta-position might be conducive to the activity. COOPr at the ortho-position might be disfavored the adrenergic alpha(1)-antagonistic activity, thus increase the selectivity.     

Methodological considerations for the human platelet 5-HT2A receptor binding kinetic assay

Analysis of an extensive database of human platelet 5-HT2A receptor binding assays has been conducted in order to identify factors that may affect the assay results. Despite anecdotal reports that storage of frozen platelet pellets may affect 5-HT2A binding affinity and capacity, no quantitative study has been reported in the literature. Analysis of binding data for 373 frozen samples with a storage time up to three years is presented in this paper. It is shown that prolonged storage significantly decreases binding. The loss of binding capacity begins in the first six month of storage. Bmax declines by half after 17 month. The impact of storage time on the binding affinity is much smaller. There is only about 20% increase in the value of affinity K(D) during the half-life of Bmax. Differences in sample storage time may partly explain discrepancies in results between different research groups. Nonspecific binding due to binding to filter material diminishes accuracy and reliability of the binding assays as a result of a decrease in the ratio of specific to nonspecific ratio. A data analysis based on our suggested mathematical model shows that this effect depends on tissue concentration in test tube and becomes pronounced when the concentration is below 0.1 mg protein/ml (at 0.2 nM of ligand). Above 0.1 mg protein/ml, percentage of specific to total binding exceeds 65%, which is an acceptable level for the ratio. The majority of the binding studies reported in the literature employed a tissue concentration more than 0.5 mg/ml, well above the minimal limit sufficient for a reliable assay. However, development of microassays to conserve precious tissue must take the limit into consideration.     

Muscarinic receptor activation of phosphatidylcholine hydrolysis. Relationship to phosphoinositide hydrolysis and diacylglycerol metabolism

We examined the relationship between phosphatidylcholine (PC) hydrolysis, phosphoinositide hydrolysis, and diacylglycerol (DAG) formation in response to muscarinic acetylcholine receptor (mAChR) stimulation in 1321N1 astrocytoma cells. Carbachol increases the release of [3H]choline and [3H]phosphorylcholine ([3H]Pchol) from cells containing [3H]choline-labeled PC. The production of Pchol is rapid and transient, while choline production continues for at least 30 min. mAChR-stimulated release of Pchol is reduced in cells that have been depleted of intracellular Ca2+ stores by ionomycin pretreatment, whereas choline release is unaffected by this pretreatment. Phorbol 12-myristate 13-acetate (PMA) increases the release of choline, but not Pchol, from 1321N1 cells, and down-regulation of protein kinase C blocks the ability of carbachol to stimulate choline production. Taken together, these results suggest that Ca2+ mobilization is involved in mAChR-mediated hydrolysis of PC by a phospholipase C, whereas protein kinase C activation is required for mAChR-stimulated hydrolysis of PC by a phospholipase D. Both carbachol and PMA rapidly increase the formation of [3H]phosphatidic acid ([3H]PA) in cells containing [3H]myristate-labeled PC. [3H]Diacylglycerol ([3H]DAG) levels increase more slowly, suggesting that the predominant pathway for PC hydrolysis is via phospholipase D. When cells are labeled with [3H]myristate and [14C]arachidonate such that there is a much greater 3H/14C ratio in PC compared with the phosphoinositides, the 3H/14C ratio in DAG and PA increases with PMA treatment but decreases in response to carbachol. By analyzing the increase in 3H versus 14C in DAG, we estimate that the DAG that is formed in response to PMA arises largely from PC. Muscarinic receptor activation also causes formation of DAG from PC, but approximately 20% of carbachol-stimulated DAG appears to arise from hydrolysis of the phosphoinositides.     

The serotonin 5-HT2 receptor system, but not the alpha 1-adrenergic receptor system, is involved in the estrogen-induced afternoon prolactin surge in the rat

Ketanserin (Ket), a new serotonergic (5-HT2) antagonist, has recently been shown to block the estrogen-induced afternoon PRL surge (Endocrinology 120: 2070-2077, 1987). It is not certain, however, whether the effect of Ket was due to its serotonergic or adrenergic receptor antagonistic property. Another 5-HT2 receptor antagonist, LY53857, which possesses no alpha 1-adrenergic receptor affinity, as well as an alpha 1-adrenergic receptor antagonist, prazosin, were used in this study to further clarify the mechanism of 5-HT in the control of PRL secretion. Adult female Sprague-Dawley rats ovariectomized for 2-3 weeks and given a single injection of polyestradiol phosphate were studied 6 days later. Ket, LY53857 and prazosin were examined singly or in combination and animals were injected twice on the sampling day at 1200 and 1300h, respectively. The dosages were as follows: Ket and LY53857, 3 mg/kg BW, ip and 2 mg/kg BW, sc; prazosin, 1 mg/kg BW, ip and 0.7 mg/kg BW, sc. Blood samples were drawn from indwelling intraatrial catheters throughout the afternoon PRL surge.     

Agonist-dependent phosphorylation of the alpha 2-adrenergic receptor by the beta-adrenergic receptor kinase

Desensitization of the beta-adrenergic receptor, a receptor which is coupled to the stimulation of adenylate cyclase, may be regulated via phosphorylation by a unique protein kinase. This recently discovered enzyme, known as the beta-adrenergic receptor kinase, only phosphorylates the agonist-occupied form of the beta-adrenergic receptor. To assess whether receptors coupled to the inhibition of adenylate cyclase might also be substrates, we examined the effects of beta-adrenergic receptor kinase on the partially purified human platelet alpha 2-adrenergic receptor. Phosphorylation of the reconstituted alpha 2-adrenergic receptor was dependent on agonist occupancy and was completely blocked by coincubation with alpha 2-antagonists. The time course of phosphorylation of the alpha 2-adrenergic receptor was virtually identical to that observed with the beta-adrenergic receptor with maximum stoichiometries of 7-8 mol of phosphate/mol of receptor in each case. In contrast, the alpha 1-adrenergic receptor, which is coupled to stimulation of phosphatidylinositol hydrolysis, is not a substrate for the beta-adrenergic receptor kinase. These results suggest that receptors coupled to either stimulation or inhibition of adenylate cyclase may be regulated by an agonist-dependent phosphorylation mediated by the beta-adrenergic receptor kinase.     

Phorbol esters inhibit alpha 1-adrenergic receptor-stimulated phosphoinositide hydrolysis and contraction in rat aorta: evidence for a link between vascular contraction and phosphoinositide turnover

We investigated the actions of two biologically active phorbol esters, phorbol dibutyrate (PDB) and phorbol myristate acetate (PMA), on receptor-stimulated phosphoinositide hydrolysis in rat aorta. We found both PDB and PMA potently inhibited norepinephrine (NE) stimulated PI hydrolysis in rat aortic rings. The biologically inactive phorbol, 4-alpha-phorbol was ineffective. In the presence of the calcium channel antagonist nitrendipine, PDB potently inhibited both the phasic and tonic components of NE-induced contraction. These results suggest a functional coupling between receptor-stimulated PI turnover and vascular contraction. They also suggest a mode of feed-back regulation in vascular tissue involving phorbol esters in receptor-stimulated PI hydrolysis.     

Discrete amino acid sequences of the alpha 1-adrenergic receptor determine the selectivity of coupling to phosphatidylinositol hydrolysis.

We have constructed a variety of chimeric beta 2/alpha 1 adrenergic receptors (AR) in which selected portions of the third intracellular loop of the alpha (1B)AR were substituted into the corresponding regions of the beta 2AR. The mutant receptors were both transiently and permanently expressed in COS-7 or L-cells, respectively, and tested for their ability to mediate epinephrine-induced activation of polyphosphoinositide (PI) hydrolysis and adenylylcyclase. We have determined that 27 amino acids of the alpha (1B)AR (residues 233-259) derived from the N-terminal portion of the third intracellular loop represent the structural determinant conferring to the beta 2AR the ability to activate PI hydrolysis. This finding suggests that in the alpha (1B)AR the N-terminal portion of the third intracellular loop plays a major role in determining the selectivity of receptor-G protein coupling. However, replacement of alpha 1B sequences in the third intracellular loop of the beta 2AR did not abolish the latter receptor's coupling to activation of adenylylcyclase, thus resulting in chimeric adrenergic receptors which activated both PI hydrolysis and adenylylcyclase. These results indicate that, even if the N-terminal portion of the third intracellular loop is a major determinant of the selectivity of receptor-G protein coupling, other structural domains of the receptors also modulate this property. The comparison of the amino acid sequences which determine the selectivity of G protein coupling in functionally similar receptors may help to elucidate the structural basis for activation of specific G protein-effector systems.     

Effects of ethanol on phosphoinositide hydrolysis and muscarinic acetylcholine receptor number in SH-SY5Y cells

Previous studies suggest that the effects of ethanol on carbachol-stimulated I(1,4,5)P3 formation and on the number of mAChRs may be independent of each other. The aim of this work was to further study this hypothesis. Human neuroblastoma SH-SY5Y cells were used as a model system. Acute exposure of the cells to 100 mM ethanol induced a decrease in [3H]N-methylscopolamine ([3H]NMS) binding at 30 seconds which was of lower magnitude and of shorter duration than the previously described ethanol-induced inhibition of the peak of carbachol-stimulated I(1,4,5)P3 formation. Long-term ethanol treatment of the cells induced a time- and concentration-dependent increase in [3H]NMS binding. Three hours of 100 mM ethanol treatment were sufficient to increase the number of mAChRs at the cell surface but these receptors were not immediately functionally active, suggesting that they may be newly synthesized. Furthermore, the ethanol-induced potentiation of carbachol-stimulated I(1,4,5)P3 formation, after two days, was, for all ethanol concentrations tested, of higher magnitude than the ethanol-induced increase in mAChR number. Together, these data indicate that both acute and chronic ethanol-induced changes in carbachol-stimulated I(1,4,5)P3 formation may not only be explained by changes in mAChR density at the cell surface but may rather be the consequence of actions of ethanol down-stream of the receptor.     

Synthesis of new piperazine-pyridazinone derivatives and their binding affinity toward alpha1-, alpha2-adrenergic and 5-HT1A serotoninergic receptors

We report the design and synthesis of a new class of piperazine-pyridazinone analogues. The arylpiperazine moiety, the length of the spacer, and the terminal molecular fragment were varied to evaluate their influence in determining the affinity of the new compounds toward the alpha1-adrenergic receptor (alpha1-AR), alpha2-adrenergic receptor (alpha2-AR), and the 5-HT1A serotoninergic receptor (5-HT1AR). Biological data showed that most of the compounds have an alpha1-AR affinity in the nanomolar or subnanomolar range, while affinity toward the other two receptors was lower in most cases. However, several of the tested compounds also showed very good (in the nanomolar range) or moderate affinity toward the 5-HT1AR subtype.     

Localization of the fourth membrane spanning domain as a ligand binding site in the human platelet alpha 2-adrenergic receptor

The human platelet alpha 2-adrenergic receptor is an integral membrane protein which binds epinephrine. The gene for this receptor has been cloned, and the primary structure is thus known [Kobilka et al. (1987) Science 238, 650-656]. A model of its secondary structure predicts that the receptor has seven transmembrane spanning domains. By covalent labeling and peptide mapping, we have identified a region of the receptor that is directly involved with ligand binding. Partially purified preparations of the receptor were covalently radiolabeled with either of two specific photoaffinity ligands: [3H]SKF 102229 (an antagonist) or p-azido[3H]clonidine (an agonist). The radiolabeled receptors were then digested with specific endopeptidases, and peptides containing the covalently bound radioligands were identified. Lysylendopeptidase treatment of [3H]SKF 102229 labeled receptor yielded one peptide of Mr 2400 as the product of a complete digest. Endopeptidase Arg-C gave a labeled peptide of Mr 4000, which was further digested to the Mr 2400 peptide by additional treatment with lysylendopeptidase. Using p-azido[3H]clonidine-labeled receptor, a similar Mr 2400 peptide was obtained by lysylendopeptidase cleavage. This Mr 2400 peptide corresponds to the fourth transmembrane spanning domain of the receptor. These data suggest that this region forms part of the ligand binding domain of the human platelet alpha 2-adrenergic receptor.     

Simultaneous coupling of alpha 2-adrenergic receptors to two G-proteins with opposing effects. Subtype-selective coupling of alpha 2C10, alpha 2C4, and alpha 2C2 adrenergic receptors to Gi and Gs.

Coupling of the three alpha 2-adrenergic receptor (alpha 2AR) subtypes to Gi and Gs was studied in membranes from transfected CHO cells. We observed that in the presence of low concentrations of the alpha 2AR agonist UK-14304, alpha 2C10 mediated inhibition of adenylyl cyclase activity, whereas at high concentrations of agonist, alpha 2C10 mediated stimulation of adenylyl cyclase activity. We considered that this biphasic response was due to the coupling of alpha 2C10 to both Gi and Gs. To isolate functional Gs and Gi coupling, cells were treated with pertussis toxin or cholera toxin in doses sufficient to fully ADP-ribosylate the respective G-proteins. Following treatment with cholera toxin, agonists elicited only alpha 2C10-mediated inhibition (approximately 50%) of adenylyl cyclase while after pertussis toxin treatment, agonists elicited only alpha 2C10-mediated stimulation (approximately 60%) of adenylyl cyclase. Incubation of membranes with antisera directed against the carboxyl-terminal portion of Gs alpha blocked this functional alpha 2AR.Gs coupling to the same extent as that found for beta 2AR.Gs coupling. In addition to functional Gs coupling, we also verified direct, agonist-dependent, physical coupling of alpha 2AR to Gs alpha. In agonist-treated membranes, an agonist-receptor-Gs alpha complex was immunoprecipitated with a specific alpha 2C10 antibody, and the Gs component identified by both western blots using Gs alpha antibody, and cholera toxin mediated ADP-ribosylation. Due to the differences in primary amino acid structure in a number of regions of the alpha 2AR subtypes, we investigated whether G-protein coupling was subtype-selective, using UK-14304 and cells with the same alpha 2AR expression levels (approximately 5 pmol/mg). Coupling to Gi was equivalent for alpha 2C10, alpha 2C4, and alpha 2C2: 53.4 +/- 8.8% versus 54.9 +/- 1.0% versus 47.6 +/- 3.5% inhibition of adenylyl cyclase, respectively. In marked contrast, distinct differences in coupling to Gs were found between the three alpha 2AR subtypes: stimulation of adenylyl cyclase was 57.9 +/- 6.3% versus 30.7 +/- 1.1% versus 21.8 +/- 1.7% for alpha 2C10, alpha 2C4, and alpha 2C2, respectively. Thus, alpha 2AR have the potential to couple physically and functionally to both Gi and Gs; for Gi coupling we found a rank order of alpha 2C10 = alpha 2C4 = alpha 2C2, while for Gs coupling, alpha 2C10 greater than alpha 2C4 greater than alpha 2C2.     

Subtype-specific clinically important effects of alpha 2-adrenergic receptors

A- B- and C-subtypes of alpha 2-adrenoreceptors present in all mammals are involved in responses to currently existing subtype-nonselective ligands of these receptors widely used in medicine. Each of the subtypes has its own specific distribution in tissue and cells, onthogenetic pattern, specific regulation of activity and expression, and, as result, specific physiological functions. The latter suggests opportunities of using the subtype-specific for correction of the functions depending on this receptor. The article reviews the role of individual subtypes of alpha 2-adrenoreceptors in regulation of neurochemical transmission of cardiovascular system, psychoemotional state and development of psychic disorders, and also male sexual behaviour.     

Interaction of clonidine and clonidine analogs with human platelet alpha 2-adrenergic receptors

Several new clonidine analogs were synthesized and their ability to inhibit [3H]phentolamine binding to human platelet alpha 2-adrenergic receptors was tested. The order of potency and calculated dissociation constants for clonidine and its analogs were as follows: clonidine (0.020 +/- 0.005 microM) greater than p-aminoclonidine (0.100 +/- 0.010 microM) greater than hydroxy-phenacetyl-aminoclonidine (0.20 +/- 0.03 microM) greater than p-dansyl clonidine (1.00 +/- 0.20 microM) greater than t-boc-tyrosine clonidine (1.80 +/- 0.60 microM). Thus, p-amino substitution reduces alpha 2-adrenergic affinity in the platelet system. The effects of clonidine and its p-amino analogs on platelet adenylate cyclase were also evaluated. This enzyme is inhibited by epinephrine acting via alpha 2-adrenergic receptors. Both clonidine and p-aminoclonidine cause slight inhibition of basal adenylate cyclase and reverse the inhibition induced by epinephrine. These observations indicate that clonidine is a partial agonist for platelet alpha 2-adrenergic receptors.     

Cross-regulation between G-protein-coupled receptors. Activation of beta 2-adrenergic receptors increases alpha 1-adrenergic receptor mRNA levels.

Stimulation of DDT1 MF-2 vas deferens cells with epinephrine resulted in a time- and dose-dependent loss of alpha 1-adrenergic receptor-specific ligand binding. Regulation of alpha 1-adrenergic receptor mRNA was characterized. In monolayer culture, cells displayed 0.7 +/- 0.05 amol of alpha 1-adrenergic receptor mRNA/microgram of total cellular RNA. Epinephrine, which acts at both alpha 1- and beta 2-adrenergic receptors of DDT1 MF-2 cells, induced a short term (2-8 h) increase (50-70%) in the abundance of alpha 1-adrenergic receptor mRNA. Propranolol, a beta 2-adrenergic receptor antagonist, attenuated the epinephrine-mediated increase in alpha 1-adrenergic receptor mRNA but did not affect the decrease in alpha 1-adrenergic receptor-specific ligand binding. Phentolamine, an alpha 1-adrenergic receptor antagonist, did not attenuate the epinephrine-mediated increase in alpha 1-adrenergic receptor mRNA at 4 h but did block the decrease in alpha 1-adrenergic receptor-specific ligand binding. The half-life of the alpha 1-adrenergic receptor mRNA was approximately 7 h in untreated cells as well as in cells challenged with epinephrine. The epinephrine-promoted increase in alpha 1-adrenergic receptor mRNA was found to result from cross-regulation via beta 2-adrenergic receptors. Cholera toxin, forskolin, as well as the cyclic AMP analog CPT cAMP (8-(4-chlorophenylthio)adenosine 3':5'-cyclic monophosphate) increased the alpha 1-adrenergic receptor mRNA at 4 h, as did epinephrine in the presence of alpha 1-antagonists but not in the presence of a beta-adrenergic antagonist. This is the first report of heterologous up-regulation of mRNA levels of adrenergic receptors. Cross-regulation between alpha 1- and beta 2-adrenergic receptor-mediated pathways at 4 h occurs at the level of mRNA whereas later down-regulation of alpha 1-receptor mRNA and binding proceed via agonist activation of alpha 1-adrenergic receptors.