Yohimbine Binding Research Articles

Noradrenergic alterations in Parkinson's disease: a combined 11C-yohimbine PET/neuromelanin MRI study.

Degeneration of the noradrenergic system is now considered a pathological hallmark of Parkinson's disease, but little is known about its consequences in terms of parkinsonian manifestations. Here, we evaluated two aspects of the noradrenergic system using multimodal in vivo imaging in patients with Parkinson's disease and healthy controls: the pigmented cell bodies of the locus coeruleus with neuromelanin sensitive MRI; and the density of α2-adrenergic receptors (ARs) with PET using 11C-yohimbine. Thirty patients with Parkinson's disease and 30 age- and sex-matched healthy control subjects were included. The characteristics of the patients' symptoms were assessed using the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS). Patients showed reduced neuromelanin signal intensity in the locus coeruleus compared with controls and diminished 11C-yohimbine binding in widespread cortical regions, including the motor cortex, as well as in the insula, thalamus and putamen. Clinically, locus coeruleus neuronal loss was correlated with motor (bradykinesia, motor fluctuations, tremor) and non-motor (fatigue, apathy, constipation) symptoms. A reduction of α2-AR availability in the thalamus was associated with tremor, while a reduction in the putamen, the insula and the superior temporal gyrus was associated with anxiety. These results highlight a multifaceted alteration of the noradrenergic system in Parkinson's disease since locus coeruleus and α2-AR degeneration were found to be partly uncoupled. These findings raise important issues about noradrenergic dysfunction that may encourage the search for new drugs targeting this system, including α2-ARs, for the treatment of Parkinson's disease.

Structural Insight into Groove Binding of Yohimbine with Calf Thymus DNA: A Spectroscopic, Calorimetric, and Computational Approach.

A variety of anticancer and antibacterial drugs target DNA as one of their primary intracellular targets. Understanding ligand-DNA interactions and developing new, promising bioactive molecules for clinical use are greatly aided by elucidating the interaction between small molecules and natural polymeric DNAs. Small molecules' ability to attach to and inhibit DNA replication and transcription provides more information on how drugs impact the expression of genes. Yohimbine has been broadly studied in pharmacological properties, while its binding mode to DNA has not been explicated so far. In this study, an attempt was made to explore the interaction between Yohimbine (YH) and calf thymus (CT-DNA) by using varying thermodynamics and in silico approaches. Minor hypochromic and bathochromic shifts of fluorescence intensity were observed, suggesting the binding of YH to CT-DNA. The Scatchard plot analysis using the McGhee-von Hipple method revealed noncooperative binding and affinities in the range of 105 M-1. The binding stoichiometry value is 2:1 (2 molecules of YH were span by 1 base pair) and was determined by Job's plot. The thermodynamic parameters suggested exothermic binding, which was favored by negative enthalpy and positive entropy changes from both isothermal titration calorimetry and temperature-dependent fluorescence experiment. Salt-dependent fluorescence suggested that the interaction between the ligand and DNA was governed by nonpolyelectrolytic forces. Kinetics experiment confirmed the static type of quenching. The results of iodide quenching, urea denaturation assay, dye displacement, DNA melting, and in silico molecular docking (MD) suggested groove binding of YH to CT-DNA. Circular dichroism spectra confirmed minimal perturbation of CT-DNA with YH binding via groove region. Therefore, the groove binding mechanism of interaction was validated by biophysical techniques and in silico, MD approaches. The findings supported here may contribute to the development of new YH therapeutics possessing better efficacy and lesser side effects.

Distribution of α2-Adrenergic Receptors in the Living Human Brain Using [11C]yohimbine PET.

The neurofunctional basis of the noradrenergic (NA) system and its associated disorders is still very incomplete because in vivo imaging tools in humans have been missing up to now. Here, for the first time, we use [11C]yohimbine in a large sample of subjects (46 healthy volunteers, 23 females, 23 males; aged 20-50) to perform direct quantification of regional alpha 2 adrenergic receptors' (α2-ARs) availability in the living human brain. The global map shows the highest [11C]yohimbine binding in the hippocampus, the occipital lobe, the cingulate gyrus, and the frontal lobe. Moderate binding was found in the parietal lobe, thalamus, parahippocampus, insula, and temporal lobe. Low levels of binding were found in the basal ganglia, the amygdala, the cerebellum, and the raphe nucleus. Parcellation of the brain into anatomical subregions revealed important variations in [11C]yohimbine binding within most structures. Strong heterogeneity was found in the occipital lobe, the frontal lobe, and the basal ganglia, with substantial gender effects. Mapping the distribution of α2-ARs in the living human brain may prove useful not only for understanding the role of the NA system in many brain functions, but also for understanding neurodegenerative diseases in which altered NA transmission with specific loss of α2-ARs is suspected.

Combined In Vivo Microdialysis and PET Studies to Validate [11C]Yohimbine Binding as a Marker of Noradrenaline Release.

The noradrenaline system attracts attention for its role in mood disorders and neurodegenerative diseases but the lack of well-validated methods impairs our understanding when assessing its function and release in vivo. This study combines simultaneous positron emission tomography (PET) and microdialysis to explore if [11C]yohimbine, a selective antagonist radioligand of the α2 adrenoceptors, may be used to assess in vivo changes in synaptic noradrenaline during acute pharmacological challenges. Anesthetised Göttingen minipigs were positioned in a head holder in a PET/CT device. Microdialysis probes were placed in the thalamus, striatum and cortex and dialysis samples were collected every 10 min. Three 90 min [11C]yohimbine scans were acquired: at baseline and at two timepoints after the administration of amphetamine (1-10 mg/kg), a non-specific releaser of dopamine and noradrenaline, or nisoxetine (1 mg/kg), a specific noradrenaline transporter inhibitor. [11C]yohimbine volumes of distribution (VT) were obtained using the Logan kinetic model. Both challenges induced a significant decrease in yohimbine VT, with time courses reflecting their different mechanisms of action. Dialysis samples revealed a significant increase in noradrenaline extracellular concentrations after challenge and an inverse correlation with changes in yohimbine VT. These data suggest that [11C]yohimbine can be used to evaluate acute variations in synaptic noradrenaline concentrations after pharmacological challenges.

Prioritization of bioactive compounds envisaging yohimbine as a multi targeted anticancer agent: insight from molecular docking and molecular dynamics simulation

Recently, multi-targeted drugs have attracted much attention in cancer therapy where several therapeutic proteins are targeted by a single agent. Using the published scientific literature, we selected sixteen well-known anticancer targets and seven potential phytobioactive chemicals to find a multitargeted compound by screening through molecular docking. The feasible protein–ligand interaction was further predicted by protein–ligand interaction analysis and molecular dynamic simulation. The phytochemical yohimbine exhibited the lowest docking score in the range of −8.3 to −10.0 kcal/mol over other ligands with all the studied protein targets. Molecular interaction data also revealed the feasible binding of yohimbine with all targets. Moreover, the molecular simulation data also confirmed the stability of protein-ligand complexes with three most scored targets viz. ERK2, PARP1 and PIK3α. Based on our results, yohimbine seems to be the most potent compound out of those selected compounds and can be considered as effective lead molecule against the studied target proteins. Communicated by Ramaswamy H. Sarma

Radioligand binding analysis of \u03b12 adrenoceptors with [11C]yohimbine in brain in vivo: Extended Inhibition Plot correction for plasma protein binding

We describe a novel method of kinetic analysis of radioligand binding to neuroreceptors in brain in vivo, here applied to noradrenaline receptors in rat brain. The method uses positron emission tomography (PET) of [11C]yohimbine binding in brain to quantify the density and affinity of α2 adrenoceptors under condition of changing radioligand binding to plasma proteins. We obtained dynamic PET recordings from brain of Spraque Dawley rats at baseline, followed by pharmacological challenge with unlabeled yohimbine (0.3 mg/kg). The challenge with unlabeled ligand failed to diminish radioligand accumulation in brain tissue, due to the blocking of radioligand binding to plasma proteins that elevated the free fractions of the radioligand in plasma. We devised a method that graphically resolved the masking of unlabeled ligand binding by the increase of radioligand free fractions in plasma. The Extended Inhibition Plot introduced here yielded an estimate of the volume of distribution of non-displaceable ligand in brain tissue that increased with the increase of the free fraction of the radioligand in plasma. The resulting binding potentials of the radioligand declined by 50–60% in the presence of unlabeled ligand. The kinetic unmasking of inhibited binding reflected in the increase of the reference volume of distribution yielded estimates of receptor saturation consistent with the binding of unlabeled ligand.

Neonatal domoic acid alters in vivo binding of [11C]yohimbine to α2-adrenoceptors in adult rat brain.

Epilepsy is a debilitating seizure disorder that affects approximately 50 million people. Noradrenaline reduces neuronal excitability, has anticonvulsant effects and is protective against seizure onset. We investigated the role of α2-adrenoceptors in vivo in a neonatal domoic acid (DOM) rat model of epilepsy. We injected male Sprague-Dawley rats daily from postnatal day 8-14 with saline or one of two sub-convulsive doses, 20μg/kg (DOM20) or 60μg/kg (DOM60) DOM, an AMPA/kainate receptor agonist. The rats were observed in open field, social interaction and forced swim tests at day 50, 75 and 98, respectively. At ~120days of age, four rats per group were injected and scanned with [11C]yohimbine, an α2-adrenoceptor antagonist, and scanned in a Mediso micro positron emission tomography (PET) scanner to measure α2-adrenoceptor binding. DOM60-treated rats spent more time in the periphery during the open field test and had a significant 26-33% reduction in [11C]yohimbine binding in the hypothalamus, hippocampus and orbital prefrontal cortex compared to saline-treated rats. On the other hand, DOM20 rats had a significant 34-40% increase in [11C]yohimbine binding in the hypothalamus, amygdala and entorhinal cortex compared to saline-treated rats, with no obvious behavioural differences. The current data clearly indicate that low concentrations of DOM given to rats in their second week of life induces long-term changes in α2-adrenoceptor binding in rat brain that may have relevance to the progression of an epilepsy phenotype.

Impaired responsiveness of platelets to epinephrine due to α2A adrenoreceptor deficiency in Male Chinese

Epinephrine is known as a weak, but important, agonist for platelet activation. It has been reported that the responsiveness of platelets to epinephrine was markedly impaired in 6% of Caucasians and in 16% of Japanese. The purpose of this study was to screen and characterize this abnormality in healthy Taiwanese Chinese volunteers. We used aggregometry, flow cytometry and platelet function analyzer (PFA)-100 system to assess in 50 healthy male volunteers the responsiveness of platelets to epinephrine stimulation. Using α2A adrenoceptor antagonist BRL44408 maleate competition and a [3H]yohimbin binding assay, we evaluated α2A adrenoceptors on platelets. The aggregation of platelets after stimulation with 10 μM of epinephrine indicated two distinct groups of study participants: 24 (48.0%) good- and 26 (52.0%) impaired-responders to epinephrine. Flow cytometric analysis of platelets after stimulated with 1 μM epinephrine showed that glycoprotein (GP) IIb/IIIa and P-selectin expression of epinephrine good- and impaired-responders were 27.1 ± 11.0% vs. 9.9 ± 5.4% (p = 0.003) and 12.2 ± 6.2% vs. 3.6 ± 3.5% (p < 0.001), respectively. The PFA-100 system showed that epinephrine-impaired-responders had a longer collagen-epinephrine induced closure time. Good-responder platelets incubated with BRL44408 maleate had an impaired response to epinephrine stimulation. [3H]yohimbine binding studies showed fewer α2A adrenoreceptors on the platelets of epinephrine-impaired-responders than on those of good-responders. The prevalence of impaired responsiveness to epinephrine was high and probably due to α2A adrenoreceptor deficiency in male Taiwanese Chinese.

Quantification of [(11)C]yohimbine binding to α2 adrenoceptors in rat brain in vivo.

We quantified the binding potentials (BPND) of [11C]yohimbine binding in rat brain to alpha-2 adrenoceptors to evaluate [11C]yohimbine as an in vivo marker of noradrenergic neurotransmission and to examine its sensitivity to the level of noradrenaline. Dual [11C]yohimbine dynamic positron emission tomography (PET) recordings were applied to five Sprague Dawley rats at baseline, followed by acute amphetamine administration (2 mg/kg) to induce elevation of the endogenous level of noradrenaline. The volume of distribution (VT) of [11C]yohimbine was obtained using Logan plot with arterial plasma input. Because alpha-2 adrenoceptors are distributed throughout the brain, the estimation of the BPND is complicated by the absence of an anatomic region of no displaceable binding. We used the Inhibition plot to acquire the reference volume, VND, from which we calculated the BPND. Acute pharmacological challenge with amphetamine induced a significant decline of [11C]yohimbine BPND of ~38% in all volumes of interest. The BPND was greatest in the thalamus and striatum, followed in descending order by, frontal cortex, pons, and cerebellum. The experimental data demonstrate that [11C]yohimbine binding is sensitive to a challenge known to increase the extracellular level of noradrenaline, which can benefit future PET investigations of pathologic conditions related to disrupted noradrenergic neurotransmission.

Kinetics of yohimbine binding and displacement in porcine brain in vivo

Kinetics of yohimbine binding and displacement in porcine brain in vivo

The second extracellular loop of α2A‐adrenoceptors contributes to the binding of yohimbine analogues

Rodent alpha(2A)-adrenoceptors bind the classical alpha(2)-antagonists yohimbine and rauwolscine with lower affinity than the human alpha(2A)-adrenoceptor. A serine-cysteine difference in the fifth transmembrane helix (TM; position 5.43) partially explains this, but all determinants of the interspecies binding selectivity are not known. Molecular models of alpha(2A)-adrenoceptors suggest that the second extracellular loop (XL2) folds above the binding cavity and may participate in antagonist binding. Amino acids facing the binding cavity were identified using molecular models: side chains of residues 5.43 in TM5 and xl2.49 and xl2.51 in XL2 differ between the mouse and human receptors. Reciprocal mutations were made in mouse and human alpha(2A)-adrenoceptors at positions 5.43, xl2.49 and xl2.51, and tested with a set of thirteen chemically diverse ligands in competition binding assays. Reciprocal effects on the binding of yohimbine and rauwolscine in human and mouse alpha(2A)-adrenoceptors were observed for mutations at 5.43, xl2.49 and xl2.51. The binding profile of RS-79948-197 was reversed only by the XL2 substitutions. Positions 5.43, xl2.49 and xl2.51 are major determinants of the species preference for yohimbine and rauwolscine of the human versus mouse alpha(2A)-adrenoceptors. Residues at positions xl2.49 and xl2.51 determine the binding preference of RS-79948-197 for the human alpha(2A)-adrenoceptor. Thus, XL2 is involved in determining the species preferences of alpha(2A)-adrenoceptors of human and mouse for some antagonists.

In vitro effect of lithium on alpha2D‐adrenoceptors binding in rat cerebral cortex

The aim of this study was to examine the effect of lithium on the density and the affinity of rat α2D-adrenoceptors (α2D-AR) in our experimental model. Sodium, which has been reported to modulate certain adrenoceptors, was also used. Rat cerebral cortex slices were incubated with increasing concentrations of LiCl (0.5 − 100 x 10−3 M) or NaCl (1 − 100 x 10−3 M) at 37°C for 10 min and [3H]yohimbine saturation experiments were then carried out. In untreated tissue the density and the affinity of [3H]yohimbine binding sites were 197.8 ± 16.9 fmol/mg protein and 8.55 ± 1.0 nM, respectively. ANOVA analysis revealed that the [3H]yohimbine dissociation constant in lithium- and sodium-incubated tissues was similar to that in untreated samples (F6, 23 = 0.700; p= 0,654 for lithium; F3, 15 = 0.334; p=0.801 for sodium). Moreover, neither lithium nor sodium treatment affected the density of [3H]yohimbine binding sites (F6, 23 = 2.411; p=0.072 for lithium: F3, 15=1.204; p=0.350 for sodium). Although it has been reported an allosteric modulation of α2-AR binding by monovalent cations, our data show that the affinity and density values of α2D-AR for [3H]yohimbine in the presence of LiCl or NaCl were not modified in our experiemental model. Density of α2D-AR (fmol/mg prot) ± SEM (n = 5–9).

The Effects of the α2‐Adrenergic Receptor Agonists Clonidine and Rilmenidine, and Antagonists Yohimbine and Efaroxan, on the Spinal Cholinergic Receptor System in the Rat

Cholinergic agonists produce spinal antinociception via mechanisms involving an increased release of intraspinal acetylcholine. The cholinergic receptor system interacts with several other receptor types, such as alpha2-adrenergic receptors. To fully understand these interactions, the effects of various receptor ligands on the cholinergic system must be investigated in detail. This study was initiated to investigate the effects of the alpha2-adrenergic receptor agonists clonidine and rilmenidine and the alpha2-adrenergic receptor antagonists yohimbine and efaroxan on spinal cholinergic receptors in the rat. Spinal microdialysis was used to measure in vivo changes of acetylcholine after administration of the ligands, with or without nicotinic receptor blockade. In addition, in vitro binding properties of the ligands on muscarinic and nicotinic receptors were investigated. It was found that clonidine and rilmenidine increased, while yohimbine decreased spinal acetylcholine release. Efaroxan affected acetylcholine release differently depending on concentration. Nicotinic receptor blockade attenuated the effect of all ligands. All ligands showed poor binding affinity for muscarinic receptors. On the other hand, all ligands possessed affinity for nicotinic receptors. Clonidine and yohimbine binding was best fit to a one site binding curve and rilmenidine and efaroxan to a two site binding curve. The present study demonstrates that the tested alpha2-adrenergic receptor ligands affect intraspinal acetylcholine release in the rat evoked by nicotinic receptor mechanisms in vivo, and that they possess binding affinity to nicotinic receptors in vitro. The binding of alpha2-adrenergic receptor ligands to nicotinic receptors might affect the intraspinal release of acetylcholine.

Effect of α 2-adrenoceptor blockade on lithium action in the rat brain

The inhibitory effect of different concentrations of lithium (0.15–10×10 −3 M) on cAMP production induced by isoprenaline (1×10 −4 M) after the blockade of α 2-adrenoceptors in the rat cerebral cortex was investigated. Low lithium concentrations (0.3–0.6×10 −3 M) exerted a significant inhibitory effect after yohimbine (1×10 −5 M) addition, but had no effect when isoprenaline alone or prazosin (1×10 −7 M) was added. The recovery of [ 3H]yohimbine binding after irreversible inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was evaluated in cortical membranes to study how α 2-adrenoceptor repopulation affects the action of lithium on the adenylyl cyclase–cAMP system. When the density of α 2-adrenoceptors was lower than 21%, lithium showed a significant inhibitory effect at all concentrations tested. However, at higher densities, increased concentrations of lithium were required to inhibit cAMP production. Our results suggest that the inhibitory effect of lithium on cAMP levels in the rat brain is conditioned by α 2D-adrenoceptors.

Site-directed mutagenesis studies on the Drosophila octopamine/tyramine receptor

The cloned Drosophila octopamine/tyramine receptor can be coupled to second messenger pathways in an agonist-specific fashion by the endogenously occurring biogenic amines, octopamine and tyramine, when expressed in Chinese hamster ovary cells. We have mutated to alanine a range of receptor amino acids that could potentially form hydrogen bonds with the β-hydroxyl group of octopamine based on homologies with α- and β-adrenergic receptor subtypes. After stable expression of the mutant receptors in CHO cells we have compared the ability of octopamine and tyramine to displace [ 3H]yohimbine binding to membrane fractions from the mutant cell lines with their ability to modulate adenylyl cyclase activity in intact cells. The results suggest that none of the mutated amino acids residues, at least in isolation, are likely to be involved in interactions with the β-hydroxyl group of the octopamine side chain. It is possible that amino acids not mutated in the present study are somehow involved in this interaction. Alternatively, it is also possible that the β-hydroxyl group of the octopamine side chain is capable of interacting with more than one of the amino acids mutated in the present study.

Characteristics of norepinephrine and clonidine displacement of [ 3H]yohimbine binding to platelet alpha 2-adrenoreceptors in healthy volunteers

Clonidine's estimates of platelet α 2-adrenoreceptor (α 2AR) density are substantially lower than yohimbine's. This discrepancy could have contributed to inconsistent results from studies on the role of α 2AR in depression. Furthermore, few studies have investigated the relative distribution of α 2AR between the high- and low-affinity states or their G i protein coupling. [ 3H]Yohimbine saturable binding to platelet α 2AR, its displacement by norepinephrine and clonidine, and the effects of Gpp(NH)p on agonist displacement curves were investigated in 11 healthy volunteers. Clonidine estimates of α 2AR density were close to norepinephrine estimates, and both were strongly correlated. Clonidine's K L/ K H ratio was lower than norepinephrine's, consistent with its partial agonist nature. Norepinephrine and clonidine displacement curves revealed two affinity states. Gpp(NH)p induced a significant rightward shift to a single low-affinity state. When used in combination with a specific antagonist, clonidine's estimates of α 2AR density were similar to those of norepinephrine's, and both were higher than previously reported, when clonidine was used alone. Re-evaluation of previous studies on α 2AR in depression using clonidine is needed. The combined use of antagonist-saturation and agonist-displacement experiments to examine possible dysregulation in α 2AR coupling to G i protein in psychiatric disorders is recommended.

Platelet and brain alpha 2-adrenoceptors and cardiovascular sensitivity to agonists in dogs suffering from endotoxic shock.

We examined the changes in alpha 2-adrenoceptor binding on platelet and brain membranes of dogs treated with a non-lethal dose of endotoxin (0.1 mg/kg intravenously), and the alpha 2-adrenoceptor mediated cardiovascular effects during endotoxin shock. At 2 h, 24 h, and 7 days after endotoxin administration, the number of binding sites (Bmax) of [3H]yohimbine binding decreased and equilibrium dissociation constants (Kd) increased in platelets, whereas both Bmax and Kd decreased in either cerebral cortex or medulla oblongata. After 30 days of endotoxin administration, there were no significant differences in Bmax or Kd between the treated and untreated animals in both platelets and brain tissues. Significant positive correlations were observed for Bmax values between platelets and brain tissues, although negative correlations for Kd values between platelets and brain were not significant. Significant negative correlations were also observed between plasma catecholamine concentrations and platelet alpha 2-adrenoceptor number, and between plasma noradrenaline and medulla alpha 2-adrenoceptor number. Pretreatment with E coli endotoxin diminished cardiovascular effects such as bradycardia, hypotension, and increase in systemic vascular resistance induced by either i.v. clonidine or xylazine. This suggests that alpha 2-adrenoceptor activity is impaired in the central nervous system as well as in the peripheral vascular system during endotoxin shock. Therefore, platelets may in part represent a good model which reflects the alpha 2-adrenoceptor changes in the central nervous system and peripheral vascular system during and after endotoxin shock.

Vascular alpha-2 adrenoceptor function is decreased in rats with congestive heart failure

Vascular alpha-2 adrenoceptor function of rats with congestive heart failure (CHF) was characterized in both in vivo and in vitro experiments. CHF was induced in Sprague-Dawley rats by coronary artery ligation. Sham-operated rats served as normal controls. Postjunctional alpha-2 adrenergic responsiveness was assessed in vivo using the pithed rat model and in vitro in organ bath. Vascular alpha-2 adrenoceptor density was studied by receptor binding assay. Four to 6 weeks after this surgical procedure, plasma catecholamines were markedly increased in CHF rats. In vivo vascular responses to alpha-2 adrenoceptor agonists BHT933 and clonidine were significantly decreased in CHF rats (P < 0.001). Clonidine elicited dose-dependent responses in endothelium intact mesenteric arteries in both CHF and sham-operated rats. The dose-response curve in CHF was shifted to the right with a pD2 value of 5.5 +/- 0.2 compared with control rats 6.2 +/- 0.2 (P < 0.05). The response to clonidine was selectively blocked by an alpha-2 adrenergic antagonist rauwolscine in both groups. Endothelium denuded arteries showed an enhanced response to clonidine in both CHF and control rats. However, the response to clonidine is still decreased in CHF compared to sham-operated rats (P < 0.05). Alpha-2 adrenoreceptor density, as determined by [3H]yohimbine binding in membrane preparations from mesenteric arteries was decreased in CHF compared to sham-operated rats (Bmax 43 +/- 6 vs. 104 +/- 20 fmol/mg protein, P < 0.05). Vascular alpha-2 adrenoceptor function is decreased in rats with CHF.

Effect of left ventricular pressure and volume overload on α-adrenoceptor activity in patients with rheumatic heart valvular disease

We have investigated the possibility that the various left ventricular load conditions may exert different effects on the sympathetic function by comparing the influence of volume (VOL) and pressure (POL) overload on platelet α-adrenoceptor activity, plasma catecholamines and cAMP in 44 patients with rheumatic heart valvular disease. Receptor activity was determined by radioligand binding methods, catecholamines by HPLC using an electrochemical detector, and cAMP by radioimmunoassay. The mean a-adrenoceptor density (B max) of the control group ( n = 29) was 4.71±0.41 fmol per 10 7 platelets and the corresponding dissociation constant ( K d ) was 2.47±0.15 nM. In VOL patients, the density was elevated by 70% ( P<0.001), but it remained unchanged in the POL patients. In contrast to the B max the K d of the VOL group was not changed, and it increased by 34% ( P<0.01) in the POL group. Norepinephrine was elevated by 91% ( P<0.05) in POL, and epinephrine increased by 65% ( P<0.05) in POL and 71% ( P<0.05) in VOL. These results suggest that the sympathetic nervous system responds to left ventricular volume overload by increasing α-adrenoceptor density with no apparent change in receptor affinity toward [ 3H]-yohimbine binding, and to left ventricular pressure overload by decreasing their binding affinity without a parallel decrease in receptor density. The increase in receptor density in VOL is accompanied by an increase in plasma epinephrine, and the decrease in binding affinity in POL is associated with increased plasma norepinephrine and epinephrine levels

The significance of platelet alpha 2-adrenoceptor and sympathetic nerve activity in the hypertensive mechanism under essential hypertension--with special reference to pressor response to norepinephrine

Although many studies have examined the metabolism of catecholamines and cardiovascular responsiveness to norepinephrine in essential and various experimental hypertension, the role of sympathetic nervous system in the pathogenesis of hypertension has not been elucidated. In this study, therefore,the role of sympathetic nerve activity related to platelet alpha 2-adrenoceptor was investigated to clarify the mechanism in which sympathetic nervous system augments the blood pressure elevation in patients with essential hypertension (EHT). Tritiated yohimbine binding was used to estimate platelet membrane alpha 2-adrenoceptor characteristics in 27 hospitalized patients with mild to moderate EHT and 27 normotensive subjects (NT) receiving a regular diet containing 120mEq/day of sodium and 75mEq/day of potassium. In this study, mean arterial pressure (MAP) and plasma norepinephrine concentration (pNE) was significantly higher in EHT than those in NT. Total binding sites (Bmax) and dissociation constant (Kd) for 3H-yoshimbine in EHT was also significantly higher than those in NT. There was a significant positive correlation between Bmax and age in NT, but not in EHT. A significant positive correlation was observed between the pressor response to infused norepinephrine (NE-R:increments in MAP induced by i.v. infused 0.2 micrograms/kg/min of NE) and Bmax both in NT and EHT. On the other hand, no significant correlation was found between NE-R and Kd in NT and EHT. In addition, Bmax was correlated inversely with PNE in both NT and EHT. These findings suggest that down-regulation mechanism exists in platelet alpha 2-adrenoceptor number responded to PNE levels. Moreover, the increased density of alpha-adrenoceptor might have something to do with the augmented NE-R in EHT, indicating an important pathophysiological role of this receptor in the hypertensive mechanisms in EHT.