Catecholamine-induced subsensitivity of adenylate cyclase associated with loss of beta-adrenergic receptor binding sites

The metabolic, stress axis, and hematology response of zilpaterol hydrochloride supplemented beef heifers when exposed to a dual corticotropin-releasing hormone and vasopressin challenge

The objective of this study was to determine the metabolic, stress, and hematology response of beef heifers supplemented with zilpaterol hydrochloride (ZH) when exposed to an endocrine stress challenge. Heifers ( = 20; 556 ± 7 kg BW) were randomized into 2 treatment groups: 1) control (CON), no ZH supplementation, and 2) zilpaterol (ZIL), supplemented with ZH at 8.33 mg/kg (DM basis). The ZIL group was supplemented ZH for 20 d, with a 3-d withdrawal period. On d 24, heifers received an intravenous bolus of corticotropin-releasing hormone (CRH; 0.3 µg/kg BW) and arginine vasopressin (VP; 1.0 µg/kg BW) to activate the stress axis. Blood samples were collected at 30-min intervals for serum and 60-min intervals for plasma and whole blood, from -2 to 8 h relative to the challenge at 0 h (1000 h). Samples were analyzed for glucose, insulin, NEFA, blood urea nitrogen (BUN), cortisol, epinephrine, norepinephrine, and complete blood cell counts. Following the challenge, cattle were harvested over a 3-d period. Liver, LM, and biceps femoris (BF) samples were collected and analyzed for glucose, lactate, and glycolytic potential (GP). There was a treatment ( ≤ 0.001) effect for vaginal temperature (VT), with ZIL having a 0.1°C decrease in VT when compared with CON. A treatment × time effect ( = 0.002) was observed for NEFA. A treatment effect was observed for BUN; ZIL had decreased BUN concentrations compared with CON ( < 0.001) prior to the challenge; however, no treatment × time effect was observed. There was also a treatment effect for cortisol ( ≤ 0.01) and epinephrine ( = 0.003); ZIL had decreased cortisol and epinephrine during the CRH/VP challenge when compared with CON. There was a time effect for total white blood cells, lymphocytes, and monocytes; each variable increased ( ≤ 0.01) 2 h postchallenge. Additionally, neutrophil counts decreased ( ≤ 0.01) in response to CRH/VP challenge in both treatment groups. Glucose concentrations within the LM were greater ( = 0.03) in CON when compared with ZIL. Lactate concentrations and GP within the BF were greater in CON ( = 0.05) when compared with ZIL. These data suggest there are some variations observed between treatments in terms of response to the CRH/VP challenge; however, in the environmental conditions of this trial, none of the variations observed suggest that the supplementation of ZH detrimentally alters the ability of cattle to effectively respond to stressful stimuli.

Adrenergic receptors. Methods of determination and mechanisms of regulation

The radioligand binding technique has been proven useful in the study of altered responsiveness after exposure to adrenergic agents. A reduction in receptor number has generally been reported after interventions serving to increase the stimulatory input to the receptors prior to assay. Conversely a decrease in receptor stimulation has been demonstrated to induce an elevation in receptor density. These phenomena have been described for alpha- as well as for beta-adrenoceptors in various tissues under a variety of experimental conditions such as prolonged agonist exposure, chronic receptor blockade, denervation and interference with catecholamine turnover. A number of clinically relevant cardiovascular experimental models have been characterized by altered adrenoceptor densities, possibly reflecting a compensatory resetting of sympathetic tone in response to an aberrant haemodynamic pattern. The mechanisms underlying receptor density changes as those discussed have been suggested to involve an internalization process which may involve phospholipase and transglutaminase activation in the cell membrane.

New Insights in the Pathogenesis of Atopic Disease

A causal link between the increasing environmental pollution and the fast spreading of allergic diseases is currently discussed.

The exogenic and endogenic noxious agents contributing to the total environmental load are primarily acting through immunotoxic, sensitizing and neurotoxic mechanisms in animal experiments and in humans. Beside classic allergic–triggering factors (allergen potency, intermittent exposure to different allergen concentrations, presence of microbial bodies and sensitizing phenols), the adjuvant role of environmental pollutants gains increasing importance in allergy induction.

Our therapy experience with more than 18.000 atopic eczema patients shows that beside allergic reactions pseudoallergic mechanisms through toxic environmental agents (formaldehyde, industrial and traffic smog, wood preservatives, microbial toxins, additive–rich food, nicotine, alcohol, pesticides, solvents, amalgam–heavy metals) are increasingly incriminated as causal factors for the complex symptomatology.

The avoidance and elimination of such triggering factors before and during pregnancy and in early childhood may result in a significant decrease of the incidence of atopic diseases.

New insights in the pathogenesis of atopic disease

A causal link between the increasing environmental pollution and the fast spreading of allergic diseases is currently discussed. The exogenic and endogenic noxious agents contributing to the total environmental load are primarily acting through immunotoxic, sensitizing and neurotoxic mechanisms in animal experiments and in humans. Beside classic allergic-triggering factors (allergen potency, intermittent exposure to different allergen concentrations, presence of microbial bodies and sensitizing phenols), the adjuvant role of environmental pollutants gains increasing importance in allergy induction. Our therapy experience with more than 18.000 atopic eczema patients shows that beside allergic reactions pseudoallergic mechanisms through toxic environmental agents (formaldehyde, industrial and traffic smog, wood preservatives, microbial toxins, additive-rich food, nicotine, alcohol, pesticides, solvents, amalgam-heavy metals) are increasingly incriminated as causal factors for the complex symptomatology. The avoidance and elimination of such triggering factors before and during pregnancy and in early childhood may result in a significant decrease of the incidence of atopic diseases.

The dual effect of isoproterenol on insulin release is suppressed in pancreatic islets from hypothalamic obese rats

Hyperinsulinemia in obesity has been attributed to insulin oversecretion by pancreatic beta-cells. Beta-cells are equipped with cholinergic and adrenergic receptors; whereas overall acetylcholine action is to potentiate, catecholamines' effect is to inhibit glucose-induced insulin release (GIIR) via alpha2-adrenoceptor. However, it has been shown that beta-adrenergic agonists potentiate glucose response. GIIR was studied in pancreatic islets from hyperinsulinemic adult obese rats, obtained by L-glutamate monosodium (MSG) neonatal treatment. Islets from MSG-rats were more glucose responsive than control ones. Isoproterenol, a beta-adrenergic agonist, inhibited the GIIR in islets from MSG-obese rats. Results indicate that MSG treatment causes alteration on function of beta-cell adrenoceptors.

Uncoupling and endocytosis of 5-hydroxytryptamine 4 receptors. Distinct molecular events with different GRK2 requirements

The 5-hydroxytryptamine type 4 receptors (5-HT4Rs) are involved in memory, cognition, feeding, respiratory control, and gastrointestinal motility through activation of a G(s)/cAMP pathway. We have shown that 5-HT4R undergoes rapid and profound homologous uncoupling in neurons. However, no significant uncoupling was observed in COS-7 or HEK293 cells, which expressed either no or a weak concentration of GRK2, respectively. High expression of GRK2 in neurons is likely to be the reason for this difference because overexpression of GRK2 in COS-7 and HEK293 cells reproduced rapid and profound uncoupling of 5-HT4R. We have also shown, for the first time, that GRK2 requirements for uncoupling and endocytosis were very different. Indeed, beta-arrestin/dynamin-dependent endocytosis was observed in HEK293 cells without any need of GRK2 overexpression. In addition to this difference, uncoupling and beta-arrestin/dynamin-dependent endocytosis were mediated through distinct mechanisms. Neither uncoupling nor beta-arrestin/dynamin-dependent endocytosis required the serine and threonine residues localized within the specific C-terminal domains of the 5-HT4R splice variants. In contrast, a cluster of serines and threonines, common to all variants, was an absolute requirement for beta-arrestin/dynamin-dependent receptor endocytosis, but not for receptor uncoupling. Furthermore, beta-arrestin/dynamin-dependent endocytosis and uncoupling were dependent on and independent of GRK2 kinase activity, respectively. These results clearly demonstrate that the uncoupling and endocytosis of 5-HT4R require different GRK2 concentrations and involve distinct molecular events.

Altered inotropic reactivity in diabetic rabbit right ventricular myocardium

Alloxan monohydrate was used to induce diabetes in rabbits, which were maintained for a 3-month period with or without daily insulin replacement along with age-matched controls. Isolated right ventricular myocardial strips were used to generate dose–response curves to isoproterenol, forskolin, and Bay K 8644. Basal developed force was significantly elevated in diabetic ventricular strips. While isoproterenol acted as a full inotropic agonist, diabetic preparations revealed a consistent but insignificant decrease in the maximum developed force. While both sensitivity to isoproterenol and β-adrenoceptor density were decreased in preparations from diabetic rabbits, there was no associated increase in circulating plasma catecholamines. In contrast, forskolin and Bay K 8644 were partial agonists in control preparations but full inotropic agonists in diabetic preparations, demonstrating significant increases in maximum developed force. This hyperresponsiveness was not associated with altered calcium channel density. Finally, insulin replacement reduced or prevented all diabetic-related changes. These data indicate that the hyperresponsiveness to forskolin and Bay K 8644 represents an altered utilization of intracellular calcium in the diabetic rabbit, converting them into full agonists similar to isoproterenol. The decrease in sensitivity to isoproterenol correlated with a decrease in β-adrenoceptor density but not elevated circulating catecholamines as previously observed in diabetic rats.Key words: experimental diabetes, β-adrenoceptor density, inotropic state, hyperresponsiveness, subsensitivity, calcium utilization.

Immunoanalogue of vertebrate beta-adrenergic receptor in the unicellular eukaryote Paramecium

Cell fractionation, SDS-PAGE, quantitative Western blot, confocal immunolocalization and immunogold labelling were performed to find an interpretation of the physiological response of the unicellular eukaryote Paramecium to beta-adrenergic ligands. The 69 kDa polypeptide separated by SDS-PAGE in S2 and P2 Paramecium subcellular fractions cross-reacted with antibody directed against human beta2-adrenergic receptor. This was detected by Western blotting followed by chemiluminescent detection. Quantitative image analysis showed that beta-selective adrenergic agonist (-)-isoproterenol--previously shown to enhance phagocytic activity--evoked redistribution of the adrenergic receptor analogue from membraneous (P2) to cytosolic (S2) fraction. The relative increase in immunoreactive band intensity in S2 reached 80% and was paralleled by a 59% decrease in P2 fraction. Confocal immunofluorescence revealed beta2-adrenergic receptor sites on the cell surface and at the ridge of the cytopharynx--where nascent phagosomes are formed. This localization was confirmed by immunoelectron microscopy. These results indicate that the 69 kDa Paramecium polypeptide immunorelated to vertebrate beta2-adrenergic receptor appeared in this evolutionary ancient cell as a nutrient receptor.

Adrenergic receptors and secretory responses of the rat submandibular salivary gland after periodic incisor reduction

The lower and upper incisors of female rats were repeatedly reduced every 48 hr for 21 days. A marked enlargement of the submandibular glands was observed at the end of this period. One day after the final reduction, dose dependent curves to phenylephrine and isoproterenol were obtained in relation to salivary flow rates. Secretory responses, expressed as mg/gland, showed that the dose response curve to the alpha1-adrenomimetic drug was not modified by treatment while that for isoproterenol was shifted to the right of the control. When the responses were expressed as microg of saliva/mg of wet tissue, the dose-response curve to both agonists was shifted to the right in the incisor-reduced group. (Activation of alpha2-adrenergic receptors by clonidine did not inhibit the responses to phenylephrine in the incisor-reduced rats.) Radioligand binding assays of alpha1-, beta- and alpha2-receptors did not show differences between control and experimental glands in terms of densities (Bmax) or affinities (Kd). The lack of correlation between the decrease in alpha2- and beta-mediated responses and the radioligand bindings suggests that postreceptor mechanisms are involved in the diminished secretory responses of the rat submandibular gland after periodic reduction or amputation of incisors.

Modification of catecholamine-induced changes in heart function by food restriction in rats

In view of the common practice of dieting for weight reduction, the influence of severe food restriction (about 25% of ad libitum intake) on adrenergic mechanisms was studied. Cardiac norepinephrine and epinephrine concentrations as well as plasma norepinephrine levels, were increased upon feeding a restricted diet to rats for 14 days in comparison with control rats that ingested about 30 g food/ day. Bradycardia as well as characteristic electrocardiographic abnormalities, including prolongation of the QRS and QT intervals, were observed in food-restricted rats. Diet-restricted rats did not develop ventricular arrhythmias in response to epinephrine injections as readily as control rats. Depression in both + dP/dt and -dP/dt of the heart in situ as well as reductions in the inotropic responses to epinephrine were evident in diet-restricted rats. Beta-adrenergic binding studies revealed a significant decrease in receptor density, but the dissociation constant for binding was also depressed in the food-restricted rat heart. Downregulation of the beta-adrenergic receptors in the heart may explain the lack of an epinephrine-induced increase in contractile force development as well as arrhythmias in food-restricted rats. These data demonstrate that severe food restriction has marked effects on adrenergic mechanisms and heart function, and thus some caution should be exercised at early periods of this therapy for weight reduction.

Catecholamines, lymphocyte subsets, and cyclic adenosine monophosphate production in mononuclear cells and CD4+ cells in response to submaximal resistance exercise

We examined the effect of 30 min of submaximal resistance exercise on free and sulphoconjugated plasma catecholamine concentrations determined by high performance (-pressure) liquid chromatography separation, the distribution of circulating lymphocytes quantified by flow cytometry, and isoproterenol induced cyclic adenosine monophosphate (cAMP) production in mononuclear cells (MNL) and CD4+ cells. Venous blood samples were taken before, immediately after and 45 min after exercise. Resistance exercise increased free plasma adrenaline (A) and noradrenaline (NA) concentrations, whereas sulphoconjugated catecholamine concentrations remained unchanged. Exercise induced leucocytosis and lymphocytosis was predominantly manifested by an increase in the number of total lymphocytes, monocytes, CD3+, CD8+ cells and CD3- CD16/CD56+ cells. Redistribution resulted in a decrease in the CD4+:CD8+ ratio. The total number and distribution of lymphocytes returned to baseline after 45-min rest. An exercise-induced increase in the number of CD3- CD16/CD56+ cells was significantly correlated with the increase in plasma NA (r = 0.66; P = 0.035), indicating a NA dependent process of redistribution. The cAMP-production in MNL was significantly elevated after resistance exercise, when cells were stimulated with 1 mumol.1(-1) isoproterenol [pre-exercise 16.5 (SD 3.3); postexercise 21.6 (SD 9.8); 45 min postexercise 10.7 (SD 2.8)]. The cAMP production in CD4+ cells was not affected by exercise. Therefore, it is discussed whether redistribution is responsible for the exercise induced increase in cAMP production in MNL.

Pharmacokinetics and pharmacodynamics in the critically ill patient

1. Until recently, when drugs were used in critically ill patients they were expected to behave in the same way as in less seriously ill patients. Now the unpredictability of even the most reliable drugs has been recognized. With this there is an awareness of the adverse effects drugs may have on organs other than the ones the drug was intended to act on. In patients with multiorgan dysfunction, poly-pharmacy is usually needed. The drugs may not only interfere with the action of each other at the receptor and enzyme level, but may also change protein binding and elimination. All these effects may be unimportant in less seriously ill patients, but may affect outcome in the critically ill. A high degree of awareness and suspicion of unknown drug-induced adverse reaction is needed by clinicians and pharmacologists alike.

Catecholamines in critical care. The commonly used catecholamines: receptor and clinical profile, indications and dosages

The pharmacology, pattern of receptor activation and resulting clinical impact of the currently most widely used intravenous catecholamines are reviewed. A brief physiological description of the alpha, beta and dopaminergic receptors is used in order to explain the clinical effects of norepinephrine, epinephrine, isoproterenol, dopamine, dobutamine and dopexamine. Each drug is discussed separately according to receptor profile, indications, dosages and current application in critical care. Tables are provided for comparison of relative strengths of these drugs regarding receptor activation, haemodynamic effects, organ perfusion and recommended dosages. The use of combinations of catecholamines to meet a variety of circulatory demands is commented upon.

Increased airway reactivity in the guinea-pig follows exposure to intravenous isoprenaline

1. Intravenous infusion of (+/-) isoprenaline (1-100 micrograms kg-1 h-1) enhanced airway responses (resistance, RL; and compliance, Cdyn) to histamine (1.0-1.8 micrograms kg-1) and bombesin (100-240 ng kg-1), whereas airway responses to vagal stimulation remained unchanged. 2. Bilateral vagotomy before intravenous infusion of (+/-)isoprenaline (100 micrograms kg-1 h-1) prevented development of airway hyperreactivity to histamine or bombesin, yet vagotomy after infusion of isoprenaline was without effect. 3. Prior treatment with atropine (1 mg kg-1) did not influence the capacity of (+/-)isoprenaline (100 micrograms kg-1 h-1) to increase airway reactivity to bombesin. 4. Despite a 500-fold difference in spasmolytic potency in vivo, infusion of (+)isoprenaline (100 micrograms kg-1 h-1) or (-)isoprenaline (100 micrograms kg-1 h-1) increased reactivity of the airways to histamine or bombesin to a comparable extent. 5. Neither adrenaline (100 micrograms kg-1 h-1) nor forskolin (600 micrograms kg-1 h-1) increased reactivity of the airways to histamine or bombesin. 6. Intravenous infusion of dopamine (100 micrograms kg-1 h-1) or noradrenaline (100 micrograms kg-1 h-1) increased reactivity of the airways to histamine or bombesin. 7. Intravenous infusion of (+/-) propranolol (100 micrograms kg-1 h-1) increased reactivity of the airways to histamine or bombesin which was partially inhibited by bilateral vagal section. 8. Depletion of circulating platelets by lytic anti-platelet serum or concomitant infusion of an antagonist of platelet-activating factor (PAF), ginkgolide B (1 mg kg-1 h-1) did not diminish the capacity of (+/-)isoprenaline (100 micrograms kg-1 h-1) to induce hyperreactivity of the airways to histamine or bombesin. 9. These observations indicate that (+/-)isoprenaline can induce airway hyper-reactivity by a mechanism unrelated to beta-adrenoceptor activation, but which is dependent upon intact vagus nerves.

Hepatic encephalopathy in thioacetamide-induced acute liver failure in rats: characterization of an improved model and study of amino acid-ergic neurotransmission

An imbalance of excitatory and inhibitory amino acid-ergic neurotransmission has been suggested to play a role in the pathogenesis of hepatic encephalopathy. For further evaluation of this hypothesis, several parameters of amino acid-ergic neurotransmission were studied in rats with acute liver failure induced by the administration of 300 mg per kg thioacetamide by gavage on two consecutive days. By appropriate supportive care, hypoglycemia, renal failure and hypothermia were avoided. Rats were monitored clinically and neurologically. Hepatic encephalopathy evolved in four distinct, easily recognizable stages. Light and electron microscopic examination of brains of rats with hepatic encephalopathy revealed only a slight swelling of nuclei of neurons and astrocytes without signs of neuronal degeneration or brain edema. In rats with hepatic encephalopathy, the concentrations of GABA, glutamate and taurine were decreased in the cerebral cortex, the hippocampus and the striatum, whereas those of aspartate and glycine were unchanged or increased. GABAA and benzodiazepine receptors were studied as parameters for the postsynaptic GABAA-benzodiazepine receptor complex, glutamic acid decarboxylase as parameter for presynaptic GABA-ergic neurons and stimulation of benzodiazepine binding by GABA as a parameter for a GABA-mediated postsynaptic event. None of these parameters was different in hepatic encephalopathy as compared to controls. Similarly, Ca++/Cl(-)-dependent and -independent glutamate receptors as parameters for glutamatergic neurons were unchanged in rats with hepatic encephalopathy. Thus, in rats with thioacetamide-induced liver failure and hepatic encephalopathy, changes of the concentrations of neurotransmitter amino acids occur in the brain. Other neurochemical parameters, however, failed to identify alterations of GABA-ergic or glutamatergic neurotransmission in hepatic encephalopathy.

The pharmacology of xamoterol: a basis for modulation of the autonomic control of the heart

1. Xamoterol (Corwin, Carwin, Corwil, Xamtol, ICI 118,587) is a beta-adrenoceptor partial agonist which is of benefit in the chronic treatment of heart failure (e.g. The German and Austrian Xamoterol Study Group, 1988). These results contrast with those obtained with other beta-adrenoceptor drugs, prenalterol and pirbuterol which were unsuccessful on chronic dosing (Currie et al., 1984; Glover et al., 1985). 2. Unlike prenalterol and pirbuterol, xamoterol has no significant agonist activity at the beta 2-adrenoceptor and has shown no tachyphylaxis in animals or man. 3. The overall action of xamoterol is to modulate sympathetic control of the heart such that at rest and at low levels of exercise the heart receives inotropic support whilst during more severe exercise, heart rate is reduced. In patients with left ventricular dysfunction these effects lead to an improvement in the relation between filling pressure and cardiac output at all levels of activity such that a given cardiac output is achieved with a lower filling pressure. It is suggested that this alteration in the pattern of cardiac activity over a long period of time results in a beneficial adaptation of the myocardium and is a possible explanation of the observed clinical improvement.

Supersensitivity of beta-adrenoceptor coupled adenylate cyclase in pulmonary tissue of the spontaneously hypertensive rat

Basal adenylate cyclase activity was similar in plasma membranes prepared from the lungs of 12 week old spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY). However, sensitivity to Gpp[NH]p, isoproterenol plus GTP or Gpp[NH]p was significantly greater in the SHR. Beta-receptor density measured by [3H]DHA binding was unaltered. The dissociation constant, Kd, revealed a significantly greater binding affinity of the radioligand in the SHR (6.23 +/- 0.45 nM) compared with the WKY (8.53 +/- 0.82 nM). Activity of Gs was assessed by complementing S49 cyc- acceptor membranes with lung cholate extract. Basal activity of the reconstituted system was decreased 43% in the SHR. However, sensitivity to NaF, Gpp[NH]p, and isoproterenol plus Gpp[NH]p was significantly elevated. These data suggest that desensitization of the adenylate cyclase complex is not a generalized response to chronic hypertension. A tissue specific increase in sympathetic drive appears to be responsible for the lowered concentration of cardiac beta-adrenoceptors in the SHR. In contrast, both indirect and direct evidence indicate an enhanced functional sensitivity of pulmonary Gs in the hypertensive rats.

Effects of xamoterol on resting and exercise haemodynamics in patients with chronic heart failure

1. A bicycle exercise test was used to investigate functional capability and haemodynamics in 30 patients with heart failure (13 NYHA Class II, 17 Class III), before and after i.v. xamoterol (Corwin, Carwin, Corwil, Xamtol, ICI 118,587) 0.2 mg kg-1. 2. Resting heart rate fell from 78 to 74 beats min-1 (P less than 0.05) and cardiac index rose from 2.5 to 2.8 l min-1 m-2 (P less than 0.001) after xamoterol. Blood pressure fell slightly, and systemic vascular resistance was reduced. Stroke work index improved and double product decreased. There were no changes in pulmonary artery wedge pressure ejection fraction or plasma noradrenaline concentrations. 3. On exercise, xamoterol produced a considerable reduction in heart rate increase, improved stroke volume and left ventricular stroke index and lowered double product. Exercise duration increased by 10%, but this did not quite achieve statistical significance. 4. These results are consistent with the concept that a beta 1-partial adrenoceptor agonist with the level of intrinsic sympathomimetic activity (43%) of xamoterol provides moderate inotropic support at rest, and protects the heart against overstimulation on exercise, when sympathetic drive is high. 5. Reduction of double product on exercise implies a lowered oxygen demand, which could be of considerable importance in patients with ischaemic heart disease.

Plasma catecholamine levels in severe atopic eczema

Free plasma catecholamines were measured by means of a standardized HPLC method in 41 adult patients with severe atopic eczema and in 18 healthy volunteers. The circulating norepinephrine levels were significantly higher in the atopic group (P less than 0.005), by contrast only slight differences were found in the epinephrine and dopamine concentrations. The possible mechanisms leading to these changes at concomitant normal DBH activities are discussed.

Regulation of adrenergic receptor number following chronic noradrenaline infusion in the rabbit

In order to study noradrenaline-induced regulation of alpha- and beta-adrenoceptors, groups of male New Zealand White rabbits (n = 8) were treated with intravenous noradrenaline (0.09 mumol/kg x h) or ascorbate (0.1%) for 10 days via osmotic minipumps implanted in the femoral vein, and the number of cardiac, lung and lymphocyte beta-adrenoceptors as well as renal and platelet alpha 2-adrenoceptors were determined. 1. The mean arterial blood pressure, heart rate and catecholamine levels were measured before commencing, and after 24 h and 10 days infusion. Circulating noradrenaline concentrations were elevated approximately 6-fold at 24 h and were sustained at these levels after 10 days administration of noradrenaline. There were no significant alterations in the blood pressure while a significant decrease in the heart rate was observed at 24 h. 2. Alpha 2-adrenoceptor density was assessed using [3H]-yohimbine. A significant decrease in the number of alpha 2-adrenoceptors in the kidney was observed following the 10 days infusion with noradrenaline. This down-regulation was in marked contrast to the lack of alteration in platelet alpha 2-adrenoceptor number and the platelet alpha 2-adrenoceptor mediated aggregatory response. 3. The density of beta-adrenoceptors in lymphocytes, heart and lung were quantified using (-)[125I]iodocyanopindolol (ICYP). The noradrenaline infusions caused significant reductions in beta-adrenoceptor number in the heart and lung (containing predominantly beta 1-adrenoceptors) but not in lymphocytes (possessing mainly beta 2-adrenoceptors). The KD-values (pM) for ICYP binding to heart and lung were also significantly decreased in the present studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of fasting and refeeding on somatostatin concentration and binding to cytosol from rabbit gastric mucosa

Somatostatin like immunoreactivity and the density of somatostatin binding sites were measured in stomach (fundus and antrum) from either fed, 12 to 96 hours fasted, or 96 hours fasted plus 48 hours refed rabbits. The somatostatin concentration increased in fundic and antral mucosa after 24 h and reached its highest value after 96 h of fasting. The number of specific somatostatin binding sites with high and low affinity decreased with the duration of fasting. Refeeding of fasted animals resulted in a normalisation to control values of gastric mucosal somatostatin and somatostatin binding.

Beta-adrenergic receptor-coupled adenylate cyclase. Biochemical mechanisms of regulation

Beta-adrenergic receptor-coupled adenylate cyclase is regulated by both amplification and desensitization processes. Desensitization of adenylate cyclase is divided into two major categories. Homologous desensitization is initiated by phosphorylation of the receptors by a beta-adrenergic receptor kinase. This reaction serves to functionally uncouple the receptors and trigger their sequestration away from the cell surface. These sequestered receptors can rapidly recycle to the cell surface or, with time, become down regulated, being destroyed within the cell. Dephosphorylation of the receptors is accomplished in the sequestered compartment of the cell, which may functionally regenerate the receptors and allow their return to the cell surface. In heterologous desensitization, receptor function is also regulated by phosphorylation, but in the absence of receptor sequestration or down regulation. In this case, phosphorylation serves only to functionally uncouple the receptors, that is, to impair their interactions with the guanine nucleotide regulatory protein Ns. Several protein kinases are capable of promoting this phosphorylation, including the cAMP-dependent kinase and protein kinase C. In addition to the receptor phosphorylation, heterologous desensitization is associated with modifications at the level of the nucleotide regulatory protein Ns and perhaps Ni. Adenylate cyclase systems are also subject to amplification that involves a protein kinase C-mediated phosphorylation of the catalytic unit of the enzyme. Phosphorylation of the catalytic unit enhances its catalytic activity and results in amplified stimulation by the regulatory protein Ns. Other receptor/effector systems exhibit qualitatively similar regulatory phenomena, suggesting that covalent modification (phosphorylation) may represent a general mechanism for regulating receptor function.

Molecular mechanisms of beta-adrenergic receptor desensitization

Multiple mechanisms seem to be involved in regulating the responsiveness of hormone receptor-coupled adenylate cyclase systems. These mechanisms at least involve the receptors and nucleotide regulatory proteins. With the recent development of methods for purifying the catalytic unit of the enzyme it will be possible to assess whether it is also a locus for such regulatory phenomena. At least two major pathways of receptor regulation have been uncovered. Homologous desensitization (Fig. 9) involves the uncoupling and translocation of the receptors out of their normal plasma membrane environment. This process sequesters the receptors away from their effector, the regulatory and catalytic components of adenylate cyclase. The site of receptor sequestration is unclear and might lie within the plasma membrane or within the cell. The sequestered receptors can recycle to the cell surface or become down-regulated, perhaps being destroyed within the cell. Phosphorylation of the receptors through a beta-adrenergic receptor kinase appears to be associated with homologous desensitization. This phosphorylation event may serve either to uncouple functionally the receptors or to trigger their sequestration from the cell surface or both. In heterologous desensitization (Fig. 10), receptor function is regulated by phosphorylation in the absence of receptor sequestration or down-regulation. This covalent modification serves to functionally uncouple the receptors, that is, to impair their interactions with the guanine nucleotide regulatory proteins. Several protein kinases seem to be capable of promoting phosphorylation of the receptors including the cAMP-dependent kinase and protein kinase C. In addition to the receptor modification, heterologous desensitization seems to be associated with functional modifications (phosphorylation?) at the level of nucleotide regulatory proteins (Ns and Ni), (Fig. 10). Further studies of the mechanisms of desensitization of adenylate cyclase-coupled receptors are thus likely to help elucidate modes of regulation of a wide variety of receptor-coupled functions in diverse types of cells.

Effect of chronic administration of denopamine (TA-064), a new positive inotropic agent, on cardiac response of rats to denopamine

Effects of chronic administration of denopamine on acute cardiovascular response to denopamine were studied in anesthetized rats. Effects of repeated treatment with isoproterenol was also investigated. The dose of denopamine to increase LV dp/dtmax by 50% of the control (ED50) was 0.77 mg/kg, p.o. Following chronic administration of denopamine once daily at 10 or 20 mg/kg, p.o., for 14 days, the effect of denopamine (i.v.) on LV dp/dtmax was similar to that in the control group. In the 40 mg/kg-group, however, the positive inotropic effect of denopamine (i.v.) was attenuated significantly at lower doses without a decrease in the maximal response and the ED50 was increased 1.8-fold. Chronic treatment with denopamine in the diet at 20 or 40 mg/kg/day for 14 days did not influence the response to the drug. By subcutaneous administration of 50 micrograms/kg isoproterenol, thrice daily for 3 days, the ED50 of isoproterenol (i.v.) for positive inotropy were increased 6.8-fold. In addition, the maximal response to isoproterenol was depressed to about 70% of that obtained in the control. In the preparation desensitized by isoproterenol (50 micrograms/kg), the inotropic response to denopamine was attenuated at lower doses, but the maximal response was not altered. In the groups desensitized by the two drugs, the positive chronotropic effect of the drugs (i.v.) tended to decrease and the effects on blood pressure was not changed. By Scatchard analysis, the specific 3H-dihydroalprenolol binding to the cardiac membranes (Bmax) was reduced in the 40 mg/kg denopamine (p.o.) group as well as in the isoproterenol-treated groups. In the 10 mg/kg denopamine and 20 mg/kg denopamine groups, however, Bmax was not changed. These results suggest that chronic administration of denopamine hardly results in desensitization of its positive inotropy at the effective doses.

Kinetic analysis of cardiac beta-receptors in perfused working rabbit hearts

Myocardial adrenergic beta-receptors were isolated and partially purified from the nonischemic perfused working rabbit hearts. Using highly radioactive beta-receptor antagonist, dihydroalprenolol (DHA), properties of beta-receptors were investigated by kinetic equilibrium analysis when the physiological function of the heart appeared to be normal. At the concentration of 10 nM DHA dissociation constant (Kd) was 14.9 nM and there were at least two distinctly different DHA binding sites, based on the analysis of the dissociation rate of DHA-receptor complex. Identification of the two distinctly different DHA binding sites was not obvious from the analysis of Scatchard plot.

Molecular mechanisms of receptor desensitization using the beta-adrenergic receptor-coupled adenylate cyclase system as a model

Desensitization, the tendency of biological responses to wane over time despite the continuous presence of a stimulus of constant intensity, is observed in organisms as diverse as bacteria and mammals. Recently, new insights into the molecular mechanisms underlying these phenomena have emerged from the study of the receptors coupled to the ubiquitous second messenger-generating system adenylate cyclase. These mechanisms involve sequestration or down-regulation of the receptors from the cell surface as well as functionally significant covalent modifications of the receptors and/or guanine nucleotide regulatory proteins.

EGF induces receptor down-regulation with no receptor recycling in KB cells

Several ligands, including epidermal growth factor (EGF), have been found to negatively modulate or down-regulate their specific plasma membrane receptors. Using both 125I-EGF and a monoclonal antibody against the EGF-receptor (EGF-R1), we studied the down-regulation of the EGF-receptor in the human adenocarcinoma cell line KB. The results presented here demonstrate that incubating KB cells at 37 degrees C with EGF rapidly decreases the number of plasma membrane EGF-receptors. In addition, there is a concomitant rise of equal magnitude in the number of EGF molecules taken up. The latter result argues strongly that there is negligible recycling of the EGF-receptor in KB cells and that the major portion of internalized EGF-receptor complexes are transported to lysosomes and subsequently degraded. The fate of the EGF-receptor is markedly different from that of receptors not subject to down-regulation. The biochemical signals that operate to regulate such diverse receptor traffic in cells remains to be elucidated.

Enhanced renin secretion in adrenalectomized rats with glucocorticoid-induced hypertension

The role of circulating epinephrine in the regulation of renin release was studied in unanesthetized rats with glucocorticoid-induced hypertension. Biadrenalectomized Wistar rats were made hypertensive with methylprednisolone (20 mg/kg s.c. weekly) for 2 weeks and supplemented with deoxycorticosterone pivalate (10 mg/kg s.c. weekly). Sham-operated controls received the same treatment. Baseline weight, mean intra-arterial blood pressure and heart rate of the groups were the same. In both adrenalectomized and sham-operated rats plasma renin activity was determined after a 30 min infusion of the beta-adrenoceptor stimulant isoproterenol (40 ng/min) or its vehicle. Isoproterenol had no blood pressure effect and accelerated heart rate to a similar extent in rats with and without adrenals. Plasma renin activity was significantly higher in epinephrine-deficient than in sham-operated rats. Renin secretion was significantly enhanced by isoproterenol in both groups of rats. These data therefore indicate that in rats with glucocorticoid-induced hypertension the renin-angiotensin system is activated by adrenalectomy, despite the fact that adrenal insufficiency cannot develop. It also appears that rats lacking of circulating epinephrine for a prolonged period do not exhibit an abnormal responsiveness of renin secretion to the stimulation of renal beta-adrenoceptors.

The effects of chronic inhalation of salbutamol on the acute airway responsiveness to salbutamol and ipratropium bromide in the conscious and the anaesthetized guinea-pig

The effects of chronic exposure to a nebulized mist of salbutamol on the capacity of systemic salbutamol to prolong the time taken for inhaled histamine to produce bronchospasm in guinea-pigs have been examined. Initially a reproducible cough time to inhalation of histamine acid phosphate (0.5 ml ml-1) in 100% O2 was established. Antagonism of this response by intraperitoneal salbutamol or ipratropium Br was assessed to establish submaximal responses to these drugs. A fresh group of animals was then exposed to a persistent mist of nebulized water for 16 days, before and during which each animal was tested by exposure to histamine mist either alone or shortly after salbutamol (10 micrograms kg-1 i.p.) or ipratropium Br (5 micrograms kg-1 i.p.). The nebulized water had no effect on the response to the drugs. The same animals were rested for 7 days and then exposed to nebulized salbutamol solution (5 mg ml-1) for 15 days, during which time tachyphylaxis developed to salbutamol (i.p.) but not to ipratropium Br. At the end of the 15 days the animals were anaesthetized and total lung resistance (RL) measured. At this time, the protective effect of intravenous salbutamol was also diminished by comparison with untreated guinea-pigs while the response to ipratropium Br was unaffected. A separate group exposed to 1 mg ml-1 of nebulized salbutamol for 20 days developed selective tachyphylaxis to intraperitoneal salbutamol. The animals were then allowed to breathe room air and the response to intraperitoneal salbutamol after 13 days returned to normal as did the effects of intravenous salbutamol on the RL response to histamine.

Sympathetic activity and cardiac adrenergic receptors in one-kidney, one clip hypertension in rats

The activity of the sympathetic nervous system, as measured by levels of plasma and cardiac catecholamines and catecholamine metabolites and the function of cardiac alpha- and beta-adrenergic receptors, was evaluated at 3 days and 4 weeks after induction of one-kidney, one clip hypertension (1K1C) in the rat. At 3 days, the plasma level of norepinephrine (NE) was lower in the 1K1C group than the control group (p less than 0.01), whereas epinephrine (E) and the metabolites dihydroxymandelic acid (DOMA), dihydroxyphenylglycol (DOPEG), and normetanephrine (NMN) were similar in both groups. In addition, cardiac content of catecholamines, their metabolites, and adrenergic receptors were similar in both groups. At 4 weeks, plasma levels of NE and DOPEG were lower (p less than 0.01), whereas levels of DOMA and NMN were higher (p less than 0.02 and p less than 0.001, respectively) in the 1K1C group than the control group. Cardiac content of NE (p less than 0.01), and DOPEG (p less than 0.05) was significantly lower, whereas DOMA and NMN were significantly higher (p less than 0.01) in the 1K1C group as compared to controls. In addition, cardiac density of both alpha- and beta-adrenergic receptors was reduced in the 1K1C group, whereas receptor affinities were unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of in vivo treatment with isoprenaline or prenalterol on beta-adrenoceptor mechanisms in the heart and soleus muscle of the cat

The full agonist isoprenaline (5.3-6.6 nmol/kg . min) and the partial beta-adrenoceptor agonist prenalterol (10.6-13.3 nmol/kg . min) were administered to cats continuously via osmotic minipumps (i.p.). After seven days the functional and adenylate cyclase responsiveness to the agonists, as well as the beta-adrenoceptor-binding characteristics, were studied in cardiac and soleus muscle preparations in vitro. After isoprenaline pretreatment, the papillary muscles and soleus muscle strips wer 15-18 times less sensitive to isoprenaline compared with muscles from control cats. The stimulatory potency (pD2) of prenalterol in the papillary muscle was not changed significantly. The affinity of the agonists to the beta-adrenoceptors was unaffected in both tissues by the pretreatment, but the densities of beta-adrenoceptors were significantly reduced, by 36% (myocardium) and 47% (soleus) respectively. In the cat papillary muscle the intrinsic sympathomimetic activity (ISA) of prenalterol on contractile parameters was reduced from 84 (Tmax), 69 (dT/dtmax) and 71% (dT/ dtmin ) in control animals, to 33, 22 and 28%, respectively in the animals pretreated with isoprenaline. Prenalterol pretreatment did not induce any marked changes, either in the stimulatory potency or affinity of the agonists in the two tissues or in the maximal response (ISA) of prenalterol in the papillary muscle. The marked reduction in the stimulatory potency of isoprenaline and the reduced ISA of prenalterol in the myocardium after isoprenaline pretreatment can not be explained by the reduction in beta- adrenoceptor density alone. Since the affinity to the beta- adrenoceptors is unaffected, a reduced efficiency in the signal transmission must be the main cause.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of variation in dietary sodium intake on biochemical indices of sympathetic activity in normal man

The effects of variation in dietary sodium intake on biochemical indices of sympathetic activity were investigated in normal subjects. Sodium restriction for 7 days (mean urinary sodium excretion 21 mmol/day, s.e.m. = 4) increased supine plasma noradrenaline concentration significantly (1.78 nmol/l, s.e.m. = 0.23) compared to levels during medium sodium intake (1.31 nmol/l, s.e.m. = 0.14) and high sodium intake (1.35 nmol/l, s.e.m. = 0.25), when urinary sodium excretion averaged 186 (s.e.m. = 24) and 310 (s.e.m. = 43) mmol/l respectively. Plasma adrenaline concentrations did not vary significantly. During sodium restriction, the apparent release rate of noradrenaline increased significantly; noradrenaline clearance remained unchanged. No changes were observed in the heart rate response after beta-adrenoceptor stimulation with isoprenaline, suggesting that increased sympathetic activity during sodium restriction did not necessarily alter cardiac beta-receptor sensitivity. The responses to cardiac autonomic blockade with propranolol and atropine did not vary with sodium intake. These observations suggest that the sympathetic response to sodium restriction may not be generalized, excluding, in particular, the sympathetic nerves to the heart.