Lack of effect of the alpha2C-adrenoceptor Del322-325 polymorphism on inhibition of cyclic AMP production in HEK293 cells.

BACKGROUND AND PURPOSE: The alpha(2C)-adrenoceptor has multiple functions, including inhibiting release of noradrenaline from presynaptic nerve terminals. A human alpha(2C) polymorphism, Del322-325, a potential risk factor for heart failure, has been reported to exhibit reduced signalling in CHO cells. To further understand the role of the Del322-325 polymorphism on receptor signalling, we attempted to replicate and further study the reduced signalling in HEK293 cells. EXPERIMENTAL APPROACH: Human alpha(2C) wild-type (WT) and Del322-325 adrenoceptors were stably transfected into HEK293 cells. Radioligand binding was performed to determine affinities for both receptors. In intact cells, inhibition of forskolin-stimulated cyclic AMP production by WT and Del322-325 clones with a range of receptor densities (200-2320 fmol.mg(-1) protein) was measured following agonist treatment. KEY RESULTS: Noradrenaline, brimonidine and clonidine exhibited similar binding affinities for WT and Del322-325. Brimonidine and clonidine also had similar efficacies and potencies for both receptors for the inhibition of cyclic AMP production at all receptor densities tested. A linear regression analysis comparing efficacy and potency with receptor expression levels showed no differences in slopes between WT and Del322-325. CONCLUSIONS AND IMPLICATIONS: The alpha(2C) WT and Del322-325 adrenoceptors exhibited similar binding properties. Additionally, inhibition of cyclic AMP production by Del322-325 was similar to that of WT over a range of receptor densities. Therefore, in intact HEK293 cells, the alpha(2C)-Del322-325 polymorphism does not exhibit reduced signalling to adenylyl cyclase and may not represent a clinically important phenotype.

Differential effects of neonatal norepinephrine lesions on immediate early gene expression in developing and adult rat brain.

Activity regulated cytoskeletal protein (Arc), c-fos and zif268 are immediate early genes (IEGs) important for adult brain plasticity. We examined developmental expression of these IEGs and the effect of neonatal noradrenergic lesion on their expression in developing and mature brain. N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4), a specific noradrenergic neurotoxin, was administered to rats on postnatal day (PND) 3 and in situ hybridization was used to assay Arc, c-fos and zif268 mRNA on PND 13, 25 and 60. In contrast to decreases in Arc, c-fos and zif268 expression produced by noradrenergic lesions of mature brain, lesions on PND 3 yield a strikingly different effect. Neonatal lesions produce increases in c-fos and zif268 expression in specific frontal cortical layers on PND 13, while Arc shows no change. These lesions lead to increases in zif268 expression in frontal cortical layers on PND 25, with no changes in c-fos or Arc expression, and on PND 60 they produce a significant increase in c-fos expression in hippocampus with no significant changes in Arc or zif268 expression. 2-[2-(2-Methoxy-1,4-benzodioxanyl)]imidazoline hydrochloride (RX821002), an alpha-2 adrenergic receptor (A2AR) antagonist, administered to control PND 60 animals produces elevations of Arc, zif268 and c-fos mRNAs. This response was eliminated in animals lesioned with DSP-4 on PND 3. These data indicate that norepinephrine regulation of IEG expression differs in developing and mature brain and that loss of developmental norepinephrine leads to abnormally high postnatal IEG expression. Previous studies have shown an important role for norepinephrine in brain development. Our data support the idea that norepinephrine plays an important role during CNS development and that changes in noradrenergic signaling during development may have long lasting effects, potentially on learning and memory.

Agonist-stimulated [35S]GTPgammaS autoradiography: optimization for high sensitivity.

The receptor-stimulated accumulation of [35S]GTPgammaS provides a measure of functional coupling of G proteins with receptors. Sensitivity for autoradiographic visualization of [35S]GTPgammaS binding was improved two- to threefold in rat brain sections by optimizing assay conditions. Non-specific (NSB), basal and agonist-stimulated [35S]GTPgammaS binding were measured, using methadone, 5-carboxamidotryptamine and epinephrine for mu-opiate receptors, 5-HT(1A) receptors and alpha(2)-adrenoceptors. Basal and NSB were low in glycylglycine buffer compared to Tris or HEPES buffers, and agonist-stimulated [35S]GTPgammaS binding was more easily observed. Further optimization using glycylglycine buffer found increased signal-to-noise ratio with inclusion of dithiothrietol, increased [35S]GTPgammaS incubation time (2-4 h) and guanosine 5'-diphosphate (GDP) preincubation (20-30 min), and use of [35S]GTPgammaS at 0.1 nM. Improved sensitivity was due to decreased NSB and basal [35S]GTPgammaS binding and agonist-stimulated binding were similarly affected for each receptor system. The assay conditions described should extend the use of agonist-stimulated [35S]GTPgammaS autoradiography to receptors, which produce low levels of [35S]GTPgammaS binding and to the measurement of changes in receptor-G protein coupling.

Cloning and expression of the alpha 2C-adrenergic receptor from the OK cell line.

The alpha 2-adrenergic receptors have been divided into four pharmacological subtypes, alpha 2A, alpha 2B, alpha 2C, and alpha 2D. The OK cell line, a cell line derived from an opossum kidney, expresses the alpha 2C-adrenergic receptor and is the prototypical cell line for the alpha 2C receptor subtype. The cloned human alpha 2C-C4 and rat RG10 receptors have been shown to express alpha 2C pharmacology. Here we report the cloning and expression of the OK alpha 2C-adrenergic receptor, OKc2. The receptor has 64% deduced amino acid identity and 21% similarity to the alpha 2-C4 receptor, giving an overall similarity of 85%. The clone, expressed in Chinese hamster ovary cells, has a pharmacology that correlates very well (r = 0.97) with that of the native OK cell alpha 2C-adrenergic receptor, and it is negatively coupled to adenylyl cyclase.

Molecular determinants of the alpha-2D adrenergic receptor subtype.

The alpha-2 adrenergic receptor in the bovine pineal gland and the rodent homologues of the human alpha-2-C10 receptor express alpha-2D subtype pharmacological characteristics. The alpha-2 adrenergic receptor in the chicken pineal expresses characteristics similar to the alpha-2A subtype found in human and pig. The rodent receptors (alpha-2D) contain a serine residue at position 201 whereas the human and porcine receptors (alpha-2A) have a cysteine at this position. Our results indicate that the bovine pineal receptor has a serine at position 201, supporting the alpha-2D classification. However, the chicken pineal receptor also contains a serine at position 201 suggesting that other amino acids may be responsible for the differences in pharmacological characteristics.

Pharmacological characteristics of alpha 2-adrenergic receptors: comparison of pharmacologically defined subtypes with subtypes identified by molecular cloning.

On the basis of extensive radioligand data and more limited functional data, three pharmacological subtypes of alpha 2-adrenergic receptors have been identified. More recently, three human genes or cDNAs for alpha 2-adrenergic receptors have been identified by molecular cloning. The relationship, however, among the pharmacologically defined subtypes and those identified by molecular cloning has not been clear. In order to resolve this issue, we have compared the pharmacological characteristics of the receptors identified by molecular cloning and expressed in COS-7 cells with the characteristics of the pharmacologically defined receptors in their respective prototypic tissue or cell line. The affinities (Ki values) of 12 subtype-selective alpha 2-adrenergic antagonists were determined for the alpha 2 receptor in the six preparations, by radioligand binding. Correlation analyses of the pKi values indicate that the alpha 2A subtype, as defined in the HT29 cell line, the alpha 2B receptor of the neonatal rat lung, and the alpha 2C subtype, as defined in an oppossum kidney cell line, correspond to the cloned human alpha 2-C10, alpha 2-C2, and alpha 2-C4 receptor subtypes, respectively.

Desensitization and down-regulation of the 5-hydroxytryptamine1B receptor in the opossum kidney cell line.

In the opossum kidney cell line the 5-hydroxytryptamine (serotonin; 5-HT)1B receptor is negatively coupled to adenylyl cyclase via a Gi protein. Preincubation of opossum kidney cell line cell monolayers with 5-HT resulted in 5-HT1B receptor-mediated desensitization expressed as a 4-fold rightward shift of the dose-response curve and a 10 to 29% decrease of maximal inhibition of forskolin-stimulated cyclic AMP production. These moderate decreases in potency and efficacy were concentration- and time-dependent. Maximal desensitization occurred with 3 hr of 5-HT preincubation. Preincubation with 5-HT caused no change in the potency or efficacy of alpha-2 adrenergic agonist-mediated inhibition of forskolin-stimulated cyclic AMP production. Therefore, the desensitization caused by 5-HT preincubation appears to be homologous. Down-regulation of the 5-HT1B receptor, assessed with the high affinity radioligand [125I]iodocyanopindolol, also occurred, and was concentration- and time-dependent. Maximum down-regulation of 40% occurred after 20 hr of exposure to 10 microM 5-HT. These results demonstrate that, like other receptors coupled to the inhibition of adenylyl cyclase, exposure of 5-HT1B receptors to an agonist causes desensitization of the functional response followed by down-regulation of the receptor.

Identification and characterization of alpha 2D-adrenergic receptors in bovine pineal gland.

The vertebrate mechanisms for generating circadian rhythms, regulating the synthesis of melatonin, and modulating the activities of pineal indole-synthesizing enzymes differ significantly among species. The synthesis of melatonin in mammalian pineal glands is stimulated by beta 1-adrenergic receptor agonists and is potentiated by alpha 1-adrenergic receptor agonists, whereas in the avian pineal gland alpha 2-adrenergic receptor agonists inhibit its synthesis. By using [3H]rauwolscine, a selective alpha 2-adrenergic receptor antagonist, we have identified for the first time alpha 2-adrenergic receptor sites in a mammalian pineal gland. [3H] Rauwolscine bound in a saturable manner to a single class of receptors, with an equilibrium dissociation constant (KD) of 1.4 nM and a density (Bmax) of 71 fmol/mg of protein, in crude synaptic membrane preparations from bovine pineal gland. Competition studies carried out with various adrenergic antagonists supported the conclusion that [3H]rauwolscine binding sites were alpha 2-adrenergic receptors. The bovine pineal alpha 2-adrenergic receptor appears to represent a pharmacological subtype distinct from the three currently proposed subtypes, i.e., alpha 2A found in a human colonic adenocarcinoma cell line (HT29 cell), alpha 2B found in rat lung, and alpha 2C found in an opossum kidney cell line (OK cell). However, the pharmacologic profile of the pineal alpha 2 receptor resembles that found in the rat submaxillary gland. We suggest that the bovine pineal receptor may represent a fourth pharmacological subtype, which would be designated as alpha 2D.

Alpha 2-adrenergic receptor stimulation of phospholipase A2 and of adenylate cyclase in transfected Chinese hamster ovary cells is mediated by different mechanisms.

The effect of alpha 2-adrenergic receptor activation on adenylate cyclase activity in Chinese hamster ovary cells stably transfected with the alpha 2A-adrenergic receptor gene is biphasic. At lower concentrations of epinephrine forskolin-stimulated cyclic AMP production is inhibited, but at higher concentrations the inhibition is reversed. Both of these effects are blocked by the alpha 2 antagonist yohimbine but not by the alpha 1 antagonist prazosin. Pretreatment with pertussis toxin attenuates inhibition at lower concentrations of epinephrine and greatly potentiates forskolin-stimulated cyclic AMP production at higher concentrations of epinephrine. alpha 2-Adrenergic receptor stimulation also causes arachidonic acid mobilization, presumably via phospholipase A2. This effect is blocked by yohimbine, quinacrine, removal of extracellular Ca2+, and pretreatment with pertussis toxin. Quinacrine and removal of extracellular Ca2+, in contrast, have no effect on the enhanced forskolin-stimulated cyclic AMP production. Thus, it appears that the alpha 2-adrenergic receptor in these cells can simultaneously activate distinct signal transduction systems; inhibition of adenylate cyclase and stimulation of phospholipase A2, both via G1, and potentiation of cyclic AMP production by a different (pertussis toxin-insensitive) mechanism.

Effects of alpha 2-adrenergic agonist preincubation on subsequent forskolin-stimulated adenylate cyclase activity and [3H]forskolin binding in membranes from HT29 cells.

alpha 2-Adrenergic agonist preincubation resulted in a leftward shift in the subsequent concentration-response curve to forskolin-stimulated adenylate cyclase activity in membranes from HT29 cells, a human colon adenocarcinoma cell line. This effect was much less pronounced than the effect seen in the intact cell cyclic AMP production assays. Removal of GTP from the assay caused a further slight leftward shift in the concentration-response curve. In [3H]forskolin binding experiments, alpha 2-adrenergic agonist preincubation caused a doubling of the maximal number of binding sites (80 vs 31 fmol/mg protein) compared to control. The addition of MgCl2 and NaF to the assay buffer increased control binding 5-fold. With agonist preincubation, there was a further increase in binding in the presence of MgCl2 and NaF which was not significantly different from the appropriate control. Pertussis toxin pretreatment blocked both the leftward shift in the forskolin concentration-response curve and the increase in maximal number of binding sites, indicating that a pertussis toxin sensitive protein is involved in these changes. Activation of cyclic AMP production in the intact cell by cholera toxin followed by norepinephrine preincubation and then stimulation by forskolin resulted in a degree of sensitization similar to that seen in the membrane adenylate cyclase and binding assays. Pertussis toxin also blocked this sensitization. It appears that if the cyclase system is highly activated, then the degree of sensitization is similar in the membrane and intact cell assay.

Desensitization of the alpha-2 adrenergic receptor in HT29 and opossum kidney cell lines.

Preincubation of HT29 cells with an alpha-2 adrenergic agonist resulted in a parallel rightward shift in the subsequent dose-response curve to 5-bromo-6-[2-imidazoline-2-yl-amino] quinoxaline (an alpha-2 adrenergic agonist) in inhibiting vasoactive intestinal peptide-stimulated cyclic AMP production. This rightward shift in the dose-response curve, which was concentration and time dependent, was interpreted as desensitization of the alpha-2 adrenergic receptor-mediated inhibition of cyclic AMP production. The fact that no decrease in efficacy was observed appears to result from a receptor reserve. Agonist preincubation effects on subsequent p-aminoclonidine (an alpha-2 adrenergic partial agonist) inhibition and partial irreversible inactivation of receptors confirmed the presence of an alpha-2 adrenergic receptor reserve in HT29 cells. Desensitization appeared to have a heterologous component since inhibition of vasoactive intestinal peptide-stimulated cyclic AMP production by somatostatin was also attenuated. We also assessed the effect of alpha-2 agonist preincubation on subsequent 5-bromo-6-[2-imidazoline-2-yl-amino] quinoxaline inhibition of parathyroid hormone-stimulated cyclic AMP production in OK cells. As with HT29 cells, agonist preincubation resulted in a concentration- and time-dependent shift in the dose-response curve. In both cell lines, long-term preincubation with an alpha-2 adrenergic agonist resulted in a 40% decrease in subsequent [3H]yohimbine binding, indicating down-regulation of the alpha-2 adrenergic receptor.

Alpha-2A and alpha-2B adrenergic receptor subtypes: attenuation of cyclic AMP production in cell lines containing only one receptor subtype.

At least two subtypes of alpha-2 adrenergic receptors have been identified on the basis of antagonist affinities as determined mainly by radioligand binding assays. The human platelet and the HT29 human colonic adenocarcinoma cell contain alpha-2A adrenergic receptors, whereas the neonatal rat lung and the NG108 neuroblastoma X glioma hybrid cell contain the alpha-2B adrenergic receptor. Using the attenuation of the cyclic AMP accumulation as a functional assay, the affinities of various antagonists for the alpha-2 adrenergic receptor were determined in HT29 and NG108 cell lines. Dose-response curves to 5-bromo-6-(2-imidazoline-2-yl-amino)quinoxaline (UK 14,304) an alpha-2 adrenergic agonist, were generated in the absence and presence of three concentrations of various antagonists. Schild regressions were used to determine pA2 values and then dissociation constants (KB value) were calculated. Whereas phentolamine and yohimbine were equipotent at the receptor in the two cell lines, 2-(2,4-(O-methoxyphenyl)-piper-azin-1-yl)ethyl-4,4-dimethyl-1,3-(2 H,4H)- isoquinolindione (ARC-239) and prazosin were 100- and 30-fold more potent in the NG108 cell line than in the HT29 cell. These potency ratios determined from functional experiments are the same as those obtained from radioligand binding experiments. These functional data are consistent with the previous and more extensive binding data, and thus support the existence and definition of alpha-2A and alpha-2B adrenergic receptor subtypes.

Characterization of serotonin-1B receptors negatively coupled to adenylate cyclase in OK cells, a renal epithelial cell line from the opossum.

We have previously reported the presence of a 5-HT1 (serotonin)-like receptor coupled in an inhibitory manner to adenylate cyclase in the opossum kidney cell line, which is derived from the kidney of a North American opossum. Pharmacological data from binding and cyclic AMP production studies indicate that this receptor does not have characteristics of a 5-HT1A, 5-HT1C or 5-HT1D receptor, but is similar to 5-HT1B receptors found in rodent tissues. Many serotonergic drugs, including 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyrindinyl)1H-indol, 5-HT and methysergide, but not (+/-)-8-hydroxy-2-(di-N-propylamino)tetralin hydrobromide or buspirone, were full agonists at this receptor as defined by the inhibition of bovine parathyroid hormone peptide fragment 1-34-stimulated cyclic AMP production in an intact cell assay. Several classical beta adrenergic antagonists including propranolol and cyanopindolol were also full agonists at this receptor. Radioligand binding studies using [125I](-)-iodocyanopindolol gave a Bmax of 88 fmol/mg of protein and a KD of 47 pM for saturation experiments carried out in the presence of GTP. In the absence of GTP, the binding data were significantly better fit by a two-site model with KD values of 10 and 345 pM. Inhibition binding experiments were consistent with the results of the cyclic AMP experiments. The identification of 5-HT1B receptors in a tissue derived from the opossum kidney suggests that these receptors may be distributed more widely than previously thought, inasmuch as other studies have found them only in neuronal tissues of rodents.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and possible mechanisms of alpha 2-adrenergic receptor-mediated sensitization of forskolin-stimulated cyclic AMP production in HT29 cells.

Preincubation with an alpha 2-adrenergic agonist sensitized subsequent forskolin- and vasoactive intestinal peptide-stimulated cyclic AMP production in HT29 cells, a human colonic adenocarcinoma cell line. Preincubation with somatostatin, another agonist negatively coupled to adenylate cyclase, sensitized forskolin-stimulated cyclic AMP production to a lesser extent. alpha 2-Adrenergic agonist preincubation also resulted in desensitization as indicated by a shift to the right in the dose-response curve of a subsequent challenge by an alpha 2-adrenergic agonist. In an effort to elucidate the mechanism for sensitization, we examined protein kinase C and the Na+/H+ antiporter. Whereas these components had marked effects on forskolin stimulation, there was no effect on sensitization. Changes in the concentration of extra-cellular Ca2+ or Mg2+ had no effect on either forskolin stimulation or sensitization. Pertussis toxin pretreatment caused a time-dependent decrease in sensitization, an attenuation of inhibition of cyclic AMP production, and a decrease in subsequent [32P]ADP-ribosylation by pertussis toxin. The time course for these three events was similar, implicating the inhibitory guanine nucleotide regulatory protein in the mechanism for alpha 2-adrenergic receptor-mediated sensitization of forskolin-stimulated cyclic AMP production. In addition, pertussis toxin dramatically decreased forskolin-stimulated cyclic AMP production, although with a different time course. These results suggest that the mechanism of sensitization is via an as yet undefined sequence of biochemical events that includes the inhibitory guanine nucleotide regulatory protein, but does not include inhibition of adenylate cyclase nor activation of the Na+/H+ antiporter.

Oxymetazoline inhibits adenylate cyclase by activation of serotonin-1 receptors in the OK cell, an established renal epithelial cell line.

The nonselective alpha-adrenergic agonist oxymetazoline inhibits parathyroid hormone (PTH)-stimulated cAMP production in intact OK cells, an epithelial cell line derived from an American opossum kidney. This inhibition, however, is not blocked by alpha 2-adrenergic receptor antagonists. After excluding several alternate hypotheses to explain this anomalous activity of oxymetazoline, we hypothesized that oxymetazoline activates a receptor in OK cells that is negatively coupled to adenylate cyclase but distinct from the alpha 2-adrenergic receptor. Prior exposure of OK cells to pertussis toxin blocks the inhibitory response to oxymetazoline, suggesting involvement of a guanine nucleotide-binding regulatory protein. Screening various compounds for attenuation of PTH-stimulated adenylate cyclase showed that serotonin (5HT) is a potent and fully efficacious agonist. Desensitization of alpha 2-receptor-mediated inhibition of cAMP production by epinephrine did not alter the response to either 5HT or oxymetazoline, indicating that these compounds do not produce their effect by activating alpha 2-adrenergic receptors. The 5HT1 receptor-selective antagonist methiothepin, but not ketanserin (5HT2-selective) or ICS-205,930 (5HT3-selective), blocked the response to both 5HT and oxymetazoline. The potency of methiothepin for antagonizing oxymetazoline-induced inhibition of PTH-stimulated cAMP production was not significantly different from its potency for the 5HT-induced effect. These data indicate that OK cells express a 5HT1 receptor that is negatively coupled to adenylate cyclase and that oxymetazoline is an agonist at these receptors.

Characterization of alpha-2 adrenergic receptors in the OK cell, an opossum kidney cell line.

We have characterized alpha-2 adrenergic receptors in OK cells, an opossum kidney-derived cell line. In membrane saturation binding experiments, [3H]rauwolscine (Kd = 74 pM) was 3-fold more potent than [3H]yohimbine (Kd = 230 pM). Each labeled a single class of binding sites with densities of 135 and 124 fmol/mg of protein for [3H]rauwolscine and [3H]yohimbine, respectively. Inhibition of [3H]rauwolscine and [3H]yohimbine binding by several alpha adrenergic agonists and antagonists demonstrated the radioligands labeled an alpha-2 type adrenergic receptor with a pharmacological profile similar to the alpha-2B receptor subtype. The rank order of potency for antagonist inhibition of binding was yohimbine greater than prazosin = phentolamine greater than chlorpromazine = corynanthine, whereas the rank order of agonist potency was oxymetazoline = clonidine greater than or equal to UK-14,304 greater than or equal to (-)-epinephrine greater than (-)-norepinephrine. The oxymetazoline, clonidine and antagonist inhibition curves were routinely monophasic and modeled best as a single class of binding sites. For the other agonists, inhibition binding curves were biphasic with approximately 35% of the binding sites existing in a high affinity state. These curves were shifted to the right in the presence of 0.1 mM GTP, and in general modeled as a single class of binding sites. UK-14,304, (-)-epinephrine, (-)-norepinephrine and oxymetazoline attenuated parathyroid hormone-stimulated cyclic AMP production by up to 70% in whole cell monolayers in a dose-dependent manner via a pertussis toxin-sensitive mechanism. With the exception of oxymetazoline, this inhibition could be reversed with alpha adrenergic antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the vasoactive intestinal peptide receptor in rat submandibular gland: radioligand binding assay in membrane preparations.

The vasoactive intestinal peptide (VIP) receptor in membranes from rat submandibular gland was studied using radioligand binding assays with 125I-VIP and various unlabeled competing ligands. In addition to the necessity of working within the parameters under which all radioligand binding assays should be performed, binding studies with 125I-VIP, as with other peptide hormones and neurotransmitters, are subject to additional technical difficulties. Specific problems that were addressed included radioligand proteolysis, the identification of an effective protease inhibitor (leupeptin) and the deleterious effects of a commonly used inhibitor (bacitracin); avid radioligand absorption to incubation tubes that was eliminated by precoating of the tubes with a combination of polyethylenimine and an organosilane; and a disproportionate effect of increasing membrane protein concentration on affinity estimates. Under optimized conditions, the affinity (Kd) and density Bmax values for 125I-VIP obtained from saturation assays (76 pM, 2.0 pmol/mg) were in excellent agreement. Membrane protein (or receptor) levels beyond the linear portion of the receptor concentration curve are often used in radioligand binding assays. Results from 125I-VIP binding studies at elevated receptor concentrations revealed the predicted marked decrease in receptor affinity. In addition, the rank order potency of unlabeled ligands in inhibition binding assays was changed. The optimization of the assay for measuring VIP receptors in submandibular gland membrane provides a reliable method for studying the role of receptor regulation in stimulus-secretion coupling for this neuropeptide.

Alpha-1 adrenergic receptor binding in aortas from rat and dog: comparison of [3H]prazosin and beta-iodo-[125I]-4-hydroxyphenyl-ethyl-aminomethyl-tetralone.

We have identified and characterized pharmacologically alpha-1 adrenergic receptors in aorta from dog and rat utilizing two alpha-1 antagonists, [3H]prazosin and beta-3-iodo[125I]-4-hydroxyphenyl-ethyl-aminoethyl-tetralone [125I]HEAT, to determine receptor number and affinity. The rat and dog alpha-1 receptors were found to be very similar. In the dog, KD values were 27 and 11 pM for [3H]prazosin and [125I]HEAT, respectively, whereas maximum binding values were 89 and 65 fmol/mg of protein. KD values in rat aorta were 22 and 15 pM for [3H]prazosin and [125I]HEAT, respectively, whereas maximum binding values were 62 and 47 fmol/mg of protein. Both tissues demonstrated a rank order potency of antagonist inhibition of the two radioligands consistent with an alpha-1 adrenergic receptor with prazosin greater than phentolamine greater than yohimbine. A comparison between IC50 values for both the ligands and the tissues shows strong correlations suggesting the two radioligands are identifying a similar receptor population and that the receptor labeled by the ligands has similar pharmacologic characteristics in the two tissues.

Alpha 2-adrenergic receptor-mediated sensitization of forskolin-stimulated cyclic AMP production.

Preincubation of HT29 human colonic adenocarcinoma cells with alpha 2-adrenergic agonists resulted in a 10- to 20-fold increase in forskolin-stimulated cyclic AMP production as compared to cells preincubated without agonist. Similar results were obtained using either a [3H]adenine prelabeling assay or a cyclic AMP radioimmunoassay to measure cyclic AMP levels. This phenomenon, which is termed sensitization, is alpha 2-adrenergic receptor-mediated and rapid in onset and reversal. Yohimbine, an alpha 2-adrenergic receptor-selective antagonist, blocked norepinephrine-induced sensitization, whereas prazosin (alpha 1-adrenergic) and sotalol (beta-adrenergic) did not. The time for half-maximal sensitization was 5 min and the half-time for reversal was 10 min. Only a 2-fold sensitization of cyclic AMP production stimulated by vasoactive intestinal peptide was observed, indicating that sensitization is relatively selective for forskolin. Sensitization reflects an increased production of cyclic AMP and not a decreased degradation of cyclic AMP, since incubation with a phosphodiesterase inhibitor and forskolin did not mimic sensitization. Increasing the levels of cyclic AMP during the preincubation (using a phosphodiesterase inhibitor) had no effect on sensitization, indicating that sensitization is not caused by decreased cyclic AMP levels during the preincubation. This rapid and dramatic sensitization of forskolin-stimulated cyclic AMP production is a previously unreported effect that can be added to the growing list of alpha 2-adrenergic responses that are not mediated by a decrease in cyclic AMP.