Discrimination between alpha 1- and alpha 2-adrenergic receptors in the isolated perfused ileum

Adrenergic control over intestinal homeostasis has been associated with changes in intestinal vascular resistance, motility, and transport. With the use of selective alpha-adrenergic agents, this study was designed to discriminate between the vascular and transport effects. Rabbit 20 cm ileal segments (n = 31) were vascularly perfused at a rate of 1.5 ml/min by means of a modified Krebs solution containing 15% to 20% red cells. The intestinal lumen was perfused with an isotonic solution containing carbon 14-polyethylene glycol as a nonabsorbable marker. Net fluxes of water and electrolytes were calculated during 20-minute basal, experimental, and recovery periods. Norepinephrine (mixed alpha 1- and alpha 2-agonist) significantly increased intestinal absorption and vascular resistance. Phenylephrine (alpha 1-agonist) significantly increased vascular resistance without altering transport. Clonidine (alpha 2-agonist) stimulated intestinal absorption without changing vascular perfusion pressure. Yohimbine (alpha 2-antagonist) prevented norepinephrine-induced absorption but had no effect on norepinephrine-induced increases in perfusion pressure. In this isolated perfused whole gut model, alpha 1-adrenergic stimulation was responsible for increases in vascular resistance, and alpha 2-adrenergic stimulation was responsible for increases in the absorption of water and electrolytes. The ability to discriminate between alpha 1- and alpha 2-effects has potential therapeutic implications in patients with malabsorption and diarrhea.     

Muscarinic cholinergic receptors in human gastric mucosa

Muscarinic cholinergic receptor sites in human gastric mucosa were analyzed directly by using radioligand binding techniques with the specific muscarinic antagonist³H-quinuclidinyl benzilate (QNB) as ligand. Specific binding of³H-QNB to membrane preparations from human gastric mucosa was saturable, of high affinity (Kd=4.17±1.94 nM, Bmax=0.37±0.04 pmol/mg protein) and selectively inhibited by muscarinic antagonists (atropine, scopolamine) and agonists (acetylcholine, pilocarpine). These findings provide direct evidence for the existence of muscarinic cholinergic receptors in human gastric mucosa. The specific³H-QNB binding to its receptor was blocked by atropine but not by histamine, cimetidine, pentagastrin, or synthetic human gastrin. The muscarine and histamine H₂-receptor, or muscarine and gastrin receptor, probably do not share the same locus.     

Comparison of muscarinic cholinergic and alpha adrenergic receptors in canine ileum, colon, intestinal urinary reservoirs and bladder

The muscarinic cholinergic (MCh) and alpha 2 adrenergic receptor densities in canine ileum, colon, ileal and colonic urinary reservoirs and bladder were determined using radioligand receptor binding methods in order to provide a rational basis for pharmacologic management of urinary incontinence following bladder replacement with intestinal segments. Muscarinic cholinergic and alpha 2 adrenergic receptor binding sites were studied in these tissues using saturation experiments with 3H-NMS and 3H-rauwolscine, respectively. The mean equilibrium dissociation constants for 3H-NMS binding (0.13 to 0.17 nM) in these tissues were similar (p greater than 0.05) indicating homogeneity of muscarinic cholinergic binding sites. The mean equilibrium dissociation constants for 3H-rauwolscine binding (1.27 to 1.98 nM) in these tissues were also similar (p greater than 0.05). A substantial density of MCh (1.06 to 1.22 fmol/mg. wet wt.) and alpha 2 adrenergic (0.47 to 1.11 fmol/mg. wet wt.) binding sites was identified in the intestinal tissues assayed. The density of ileal and colonic MCh and alpha 2 adrenergic binding sites was not altered following construction of urinary intestinal reservoirs. The presence of a substantial density of MCh and alpha 2 adrenergic binding sites in the intestinal tissues suggests that MCh and alpha 2 adrenergic analogs may be utilized for the management of urinary incontinence following bladder replacement with intestinal urinary reservoirs.     

Muscarinic cholinergic receptors in bladder exstrophy: insights into surgical management

The surgical management of classical bladder exstrophy (functional bladder closure or urinary diversion) should be influenced by the inherent detrusor function of the exstrophied bladder. Cystometrograms performed previously on individuals with successful exstrophy closures demonstrate normal bladder function. The biochemical and neurophysiological properties of the exstrophied bladder have otherwise not been investigated. In this study radioligand receptor binding techniques were used to compare the density and equilibrium dissociation constant of muscarinic cholinergic receptors in control and exstrophy bladders. The density of muscarinic cholinergic receptors in the control and exstrophy groups was 1.97 plus or minus 0.29 and 1.44 plus or minus 0.21 fmol. per microgram deoxyribonucleic acid (mean plus or minus standard error of mean), respectively. The dissociation constant of the control and exstrophy groups was 0.15 plus or minus 0.02 and 0.14 plus or minus 0.02 nM. (mean plus or minus standard error of mean), respectively. These data show that the muscarinic receptor density and binding affinity in control and exstrophy bladders are similar. Therefore, the neurophysiological composition of the exstrophied bladder is not grossly altered during the anomalous development.     

Dexmedetomidine produces dual alpha2-adrenergic agonist and alpha1-adrenergic antagonist actions on human isolated internal mammary artery

OBJECTIVE: To investigate the direct effects of dexmedetomidine (DEX) on isolated human internal mammary artery (IMA). DESIGN: In vitro experimental study. SETTING: Cardiovascular Pharmacology Laboratory, Department of Pharmacology, Gulhane School of Medicine, Ankara, Turkey. PARTICIPANTS: IMA segments were obtained from 18 patients undergoing coronary artery bypass surgery. INTERVENTIONS: The response in IMA was recorded isometrically by a force displacement transducer in isolated organ baths. DEX-induced contractions were tested in the presence of the alpha2-adrenoceptor antagonist yohimbine (10(-7) mol/L) and the alpha1-adrenoceptor antagonist prazosin (10(-8) M). The effect of DEX (10(-7), 10(-6), and 10(-5) mol/L) on phenylephrine (10(-9)-3 x 10(-4) mol/L)-induced contactions was also tested. MEASUREMENT AND MAIN RESULTS: DEX (10(-9) mol/L-3 x 10(-5) mol/L) caused contraction in IMA segments. The contraction at lower concentrations of DEX (10(-9) mol/L-3 x 10(-7) mol/L) was attenuated by yohimbine (10(-7) mol/L), whereas prazosin (10(-8) mol/L) attenuated the contractions at higher concentrations of DEX (10(-6) mol/L-3 x 10(-5) mol/L). Incubation of IMA segments with high concentrations of DEX (10(-6) mol/L and 10(-5) mol/L) caused an inhibition of phenylephrine (10(-9) mol/L-3 x 10(-4) mol/L)-induced contraction. CONCLUSION: These data suggest that DEX causes contraction by activating alpha2-adrenoceptors at lower concentrations, but it may also activate alpha1-adrenoceptors at higher concentrations in IMA. The action of DEX on phenylephrine-induced contraction may be related to an alpha1-adrenoceptor antagonistic effect produced via partial alpha1-adrenoceptor agonistic action.     

The density of cholinergic and alpha and beta adrenergic receptors in the normal and hyper-reflexic human detrusor

The density of cholinergic and of alpha and beta adrenergic receptors was measured in samples of normal and hyper-reflexic human detrusor muscle. There was a significant reduction in the density of cholinergic receptors and a significant increase in the density of alpha adrenergic receptors in the hyper-reflexic samples as compared with normal. There was no difference between the two groups in the density of beta adrenergic receptors.     

Neuropeptide Y-induced intestinal absorption: mediation by alpha 2-adrenergic receptors.

In the intestine neuropeptide Y (NPY) is contained in sympathetic nerves, in neuroendocrine cells of the mucosa, and in neurons of the enteric plexuses. After a meal is ingested the concentration of NPY in the blood rises, and intestinal absorption of water and ions increases. We have recently demonstrated a proabsorptive effect of NPY on water and ion transport in the small intestine. The current experiments tested the hypothesis that the alpha 2-adrenergic receptor mediates NPY-induced intestinal absorption. Rabbit ileal segments (n = 35) were harvested and arterially perfused ex vivo. The intestinal lumen was perfused with an isotonic solution containing carbon 14-labeled polyethylene glycol. Net fluxes of H2O, Na+, and Cl- were calculated for three 20-minute periods: basal, drug infusion, and recovery. Five groups were randomly studied: (1) NPY (500 pmol/min); (2) terazosin (1 microgram/min, alpha 1-adrenergic receptor antagonist); (3) NPY + terazosin; (4) yohimbine (1 microgram/min, alpha 2-adrenergic receptor antagonist); and (5) NPY + yohimbine. The infusion of NPY alone caused a significant (p less than 0.05) proabsorptive response for H2O, Na+, and Cl-. Neither terazosin nor yohimbine alone had a significant effect on the transport state of the intestine. Yohimbine, but not terazosin, completely prevented the NPY-induced proabsorptive response. These data support the hypothesis that the proabsorptive effect of NPY is mediated by the alpha 2-adrenergic receptor system.     

Dual alpha(2)-adrenergic agonist and alpha(1)-adrenergic antagonist actions of dexmedetomidine on human isolated endothelium-denuded gastroepiploic arteries

The actions of dexmedetomidine (DEX) on human vascular smooth muscle are unclear. We investigated its effects on isolated, endothelium-denuded human gastroepiploic arteries in vitro and compared them with clonidine (CLO). DEX had little direct effect on resting tension, whereas CLO produced small contractile responses, an effect which is blocked by the alpha(1)-adrenergic antagonist prazosin. DEX markedly enhanced the high K(+) (40 mmol/L)-induced contraction, and this effect was reversed by the alpha(2)-adrenergic antagonists yohimbine and rauwolscine but unaffected by prazosin. However, CLO had little effect on the K(+) contractions. Interestingly, larger concentrations (>10(-7) mol/L) of both alpha(2)-adrenergic stimulants significantly inhibited the contractions elicited by the alpha(1)-adrenergic agonist phenylephrine (10(-6) mol/L) and, to a lesser extent, those elicited by the alpha(1)/alpha(2)-agonist norepinephrine (10(-6) mol/L). These results suggest the possibility that DEX and CLO each have a high affinity for alpha(1)-adrenoceptors in human isolated gastroepiploic arteries, resulting in a reduced efficacy of alpha(1)-adrenergic activation by alpha-agonists. The differing affinities of the drugs for alpha(1)- and alpha(2)-adrenoceptors may help explain their additional actions: 1) DEX enhances the high K(+)-induced contraction presumably through alpha(2)-adrenoceptor activation, and 2) CLO acts on alpha(1)-adrenoceptors as a partial agonist when present alone. IMPLICATIONS: Dexmedetomidine may not directly affect smooth muscle in human peripheral resistance vessels within the usual range of plasma concentrations (<10(-7) mol/L) achieved in clinical practice. However, in large doses, it could enhance the response to nonadrenergic vasoconstrictor agonists while antagonizing the vasoconstrictor response to alpha(1)-adrenoceptor agonists.     

Functional characterization of alpha1-adrenergic receptors in experimental vein grafts.

BACKGROUND: Studies on the pharmacology of the smooth muscle cells in vein bypass grafts suggest that the function of G-proteins and adrenergic receptors is altered. This study examines the alpha-adrenergic responsiveness of smooth muscle cells in vein bypass grafts as compared with those in the common carotid arteries and external jugular veins. METHODS: New Zealand White rabbits received jugular vein interposition bypass grafts of the common carotid. Vessel segments of the vein bypass grafts harvested after 28 days, common carotid arteries, and external jugular veins were sectioned into 5-mm rings (four per vessel) for studies of isometric tension in response to phenylephrine (10(-10) to 10(-4) M) alone and in the presence of prazosin, an alpha1-adrenergic antagonist; WB4101 and 5-methylurapidil (5-MU), alpha1A antagonists; chloroethylclonidine (CEC); an alpha1B antagonist; or the Gi/o G-protein inhibitor pertussis toxin (PTx). RESULTS: All vessels had prazosin-sensitive responses. The jugular veins appear to have functional alpha1A receptors (WB4101 and 5-MU sensitive, CEC insensitive) which are associated with pertussis toxin-sensitive G-proteins. Carotid arteries appear to have atypical alpha1 receptors (WB4101 and 5-MU insensitive, CEC insensitive) associated with pertussis toxin-insensitive G-proteins. Vein grafts appear to have functional alpha1B receptors (WB4101 and 5-MU insensitive, CEC sensitive) which are associated with pertussis toxin-insensitive G-proteins. CONCLUSIONS: These results show that placement of a vein into the arterial circulation induces a change in alpha1-adrenergic receptor subtypes (alpha1A to alpha1B) and in the G-protein coupling of the receptors (PTx sensitive to PTx insensitive), reflecting a signficant phenotypic change in smooth muscle cell signal transduction.     

Conduction block by clonidine is not mediated by alpha2-adrenergic receptors in rat sciatic nerve fibers

BACKGROUND AND OBJECTIVES: Clonidine, an alpha(2)-adrenergic agonist, has been shown to prolong local anesthesia. It appears that clonidine by itself produces conduction block by acting on peripheral nerves. However, whether clonidine-induced conduction block is mediated through alpha(2)-adrenergic receptors remains unclear. The purpose of this study was to see if clonidine's nerve-blocking action was through alpha(2)-adrenergic receptors by examining clonidine's action in the presence of alpha(2)-adrenergic antagonists. METHODS: The compound action potentials (CAPs) evoked by electrical stimuli were recorded from the isolated rat sciatic nerve in a recording chamber. Conduction block was examined by analyzing CAPs with regard to peak amplitude and time-to-peak in the presence of clonidine alone or clonidine plus alpha(2)-adrenergic antagonist yohimbine or idazoxan. RESULTS: Both clonidine and yohimbine produced concentration-dependent, reversible, conduction block. Based on concentration-response relationships, the 50% of effective concentration (EC(50)) were estimated to be 1.61 +/- 0.51 mmol/L (mean +/- SD) for clonidine and 51.4 +/- 27.2 micromol/L for yohimbine. A mixture of equal volumes of 2.07 mmol/L clonidine and 55.6 micromol/L yohimbine produced conduction block to a level close to the mean value between conduction blocks induced by 2.07 mmol/L clonidine alone and 55.6 micromol/L yohimbine alone. Addition of idazoxan, a more specific alpha(2)-adrenergic antagonist than yohimbine, to clonidine was without effect on clonidine-induced conduction block. CONCLUSIONS: The results indicated that the mixture of clonidine and yohimbine, in which either drug inhibited impulse conduction, produced conduction block in an additive manner, and that clonidine-induced conduction block was not reversed by coapplication with a specific alpha(2)-adrenergic antagonist idazoxan. These data suggest that clonidine's effects likely depend on mechanisms not mediated by alpha(2)-adrenergic receptors.     

Expression of IL-2alpha and IL-2beta receptors on the membrane surface of human sperm

Cytokines are secreted proteins that act as local immunological mediators. Increased seminal cytokine concentrations are associated with fertility problems. The purpose of the present study was to investigate the presence of IL-2alpha, and IL-2beta receptors on fresh and isolated sperm by flow cytometry and transmission electron microscopy. Twenty sperm samples from oligospermic men were incubated with CD25, a mouse monoclonal antibody specific for IL-2alpha-chain receptor, and CD122, a mouse monoclonal antibody specific for IL-2beta-chain receptor. The strong initial fluorescence intensity and, subsequently, a labeling index yielded by CD25 and CD122 decreased in sperm centrifuged on a Percoll gradient (p < .05). The expression of CD25 and CD122 correlated negatively with fresh sperm concentration, but in sperm centrifuged on a Percoll gradient there was no correlation. Labeling with CD25 and CD122 antibody was evident on the head and the middle piece in fresh sperm, while in sperm centrifuged on a Percoll gradient a weak labeling was observed only on the principal piece. The authors have identified and localized cytokine receptors on human sperm for the first time. Cytokine receptors may be involved in the regulation of pathophysiological events in sperm cell functions and male infertility. The exact pathway involved in modulation of these receptors requires further investigation. These results contribute to the understanding of cytokine-sperm relationships.     

Synergistic interaction between alpha 2-adrenergic agonists and benzodiazepines in rats.

Both alpha 2-adrenergic agonists and benzodiazepines exert anxiolytic and sedative effects when administered as preoperative medications. Clinical effects achieved with a combination of drugs, representative of these classes of compounds, is greater than that which could be expected from a simple additive response. Therefore, we investigated the nature of the interaction between dexmedetomidine, the highly-selective alpha 2-adrenergic agonist, and midazolam in a series of in vivo and in vitro studies in rats. Rats were administered midazolam, dexmedetomidine, or a combination of midazolam and dexmedetomidine intravenously to derive three dose-response curves for loss of righting reflex (LRR). LRR was determined in rats in a rotating cage (4 rotations/min) by observing whether the rat failed to maintain its upright posture for greater than or equal to 15 s exactly 2.5 min after drug administration. The effect of either flumazenil (benzodiazepine receptor antagonist) or atipamezole (the alpha 2-adrenergic antagonist) on the LRR was also determined. A probit analysis was performed and an isobologram for the ED50 was derived to assess the nature of the interaction. Rat brain membranes were prepared for receptor binding assays using [3H]-flumazenil and [3H]-rauwolscine to characterize the benzodiazepine and alpha 2-adrenergic receptors, respectively. The ability of either midazolam or dexmedetomidine to displace the radiolabeled ligand from the alternative receptor was assessed. To detect a possible kinetic interaction between the two drugs, separate cohorts of rats were administered the two drugs individually or in combination at the combination ED50 doses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adrenergic and cholinergic receptors in the human prostate, prostatic capsule and bladder neck.

The adrenergic and cholinergic receptors of the human prostatic capsule, prostatic "adenoma", and bladder neck, were investigated by the in-vitro isometric technique. The prostatic capsule was found to be very rich in both alpha-adrenergic receptors and cholinergic receptors. The prostatic adenoma was moderately rich in alpha-adrenergic receptors, but cholinergic receptors were absent. Beta-adrenergic receptors were absent in the prostatic adenoma, and there was an equivocal response in less than half the specimens of the prostatic capsule. An attempt was made to distinguish between the trigonal component at the posterior bladder neck, and the true bladder neck muscle both posteriorly and antero-laterally. The results indicat that the "posterior bladder neck" seen at operation is predominantly trigonal muscle, and is poor in cholinergic receptors. The adrenergic response is variable in the true bladder neck muscle, but is present and strong in the trigonal muscle. This response is characteristically gradual in its development. In view of the findings in this investigation, it is suggested that certain instances of acute retention of urine in prostatic patients are due to over-stimulation of the alpha-adrenergic receptors, particularly those in the prostatic capsule. Similarly, the accepted clinical contraindication to the use of cholinergic drugs for retention in the prostatic patient is supported by the distribution of the cholinergic receptors in the tissues examined.     

Nuclear androgen receptors in the epithelium and stroma of human benign prostatic hypertrophic glands

Androgen receptors have been characterized and quantified in nuclear extracts of separated epithelium and stroma from human benign prostatic hypertrophic (BPH) glands. Tritiated dihydrotestosterone was used as the ligand and incubation was carried out at 15 degrees C for 18-20 hr before separation of bound and free ligand using dextran-coated charcoal. The results were analysed by Scatchard-type analysis. The concentration of receptor was found to be significantly (p = 0.022) greater in stromal than in epithelial nuclei: 1765 +/- 152 vs 1030 +/- 227 fmol/mg DNA (SEM, n = 6). Fourteen competitors were tested and the results indicated the presence of specific androgen receptors rather than contaminating sex-hormone-binding globulin. This was also borne out by the results of agar gel electrophoresis and sucrose gradient ultracentrifugation studies. The results are in line with current opinion that prostatic stroma is an important androgen-sensitive tissue, particularly in human BPH.