Characterization of beta-adrenergic receptors of human skeletal muscle obtained by needle biopsy

Octopamine drives honeybee thermogenesis

In times of environmental change species have two options to survive: they either relocate to a new habitat or they adapt to the altered environment. Adaptation requires physiological plasticity and provides a selection benefit. In this regard, the Western honeybee (Apis mellifera) protrudes with its thermoregulatory capabilities, which enables a nearly worldwide distribution. Especially in the cold, shivering thermogenesis enables foraging as well as proper brood development and thus survival. In this study, we present octopamine signaling as a neurochemical prerequisite for honeybee thermogenesis: we were able to induce hypothermia by depleting octopamine in the flight muscles. Additionally, we could restore the ability to increase body temperature by administering octopamine. Thus, we conclude that octopamine signaling in the flight muscles is necessary for thermogenesis. Moreover, we show that these effects are mediated by β octopamine receptors. The significance of our results is highlighted by the fact the respective receptor genes underlie enormous selective pressure due to adaptation to cold climates. Finally, octopamine signaling in the service of thermogenesis might be a key strategy to survive in a changing environment.

Alveolar air and oxidative metabolic demand during exercise in healthy adults: the role of single-nucleotide polymorphisms of the β2AR gene

The predominating β‐adrenergic receptor subtype expressed on human alveolar tissue is the β₂AR. The homozygous arginine (Arg16Arg) single‐nucleotide polymorphism (SNP) at codon 16 of the β₂AR gene has been associated with abnormal β₂AR function accompanied by decreased resting alveolar‐capillary membrane gas‐transfer in certain healthy adults. Although not previously studied in the context of the β₂AR gene, pulmonary gas‐transfer is also influenced by alveolar volume (V_A) and with it the availability of alveolar surface area, particularly during exercise. Small V_A implies less alveolar surface area available for O₂ transport. We tested the following hypothesis in healthy adults during exercise: compared with Gly16Gly and Arg16Gly β2AR genotypes, Arg16Arg will demonstrate reduced V_A and ventilation (V̇_A) relative to V̇_E and oxidative metabolic demand. Age‐ BMI‐ and gender‐matched groups of Arg16Arg (N = 16), Gly16Gly (N = 31), and Arg16Gly (N = 17) performed consecutive low (9‐min, 40%‐peak workload) and moderate (9‐min, 75%‐peak workload) intensity exercise. We derived V_A and V̇_A using “ideal” alveolar equations via arterialized gases combined with breath‐by‐breath ventilation and gas‐exchange measurements; whereas steady‐state V̇O₂ was used in metabolic equations to derive exercise economy (EC = workload÷V̇O₂). Variables at rest did not differ across β₂AR genotype. Strongest β₂AR genotype effects occurred during moderate exercise. Accordingly, while V̇_E did not differ across genotype (P > 0.05), decreased in Arg16Arg versus Arg16Gly and Gly16Gly were V̇O₂ (1110 ± 263, 1269 ± 221, 1300 ± 319 mL/(min·m²), respectively, both P < 0.05), V̇_A (59 ± 21, 70 ± 16, 70 ± 21 L/min, respectively, both P < 0.05), and V_A (1.43 ± 0.37, 1.95 ± 0.61, 1.93 ± 0.65 L, respectively, both P < 0.05). Also reduced was EC in Arg16Arg versus Arg16Gly (P < 0.05) and Gly16Gly (P > 0.05) (1.81 ± 0.23, 1.99 ± 0.30, and 1.94 ± 0.26 kcal/(L·m²), respectively). Compared with Gly16Gly and Arg16Gly genotypes, these data suggest the Arg16Arg β₂AR genotype plays a role in the loss of oxidative metabolic efficiency coupled with an inadaptive V_A and, hence, smaller alveolar surface area available for O₂ transport during submaximal exercise in healthy adults.

Acute exposure of primary rat soleus muscle to zilpaterol HCl (β2 adrenergic agonist), TNFα, or IL-6 in culture increases glucose oxidation rates independent of the impact on insulin signaling or glucose uptake

Recent studies show that adrenergic agonists and inflammatory cytokines can stimulate skeletal muscle glucose uptake, but it is unclear if glucose oxidation is similarly increased. Thus, the objective of this study was to determine the effects of ractopamine HCl (β1 agonist), zilpaterol HCl (β2 agonist), TNFα, and IL-6 on glucose uptake and oxidation rates in unstimulated and insulin-stimulated soleus muscle strips from adult Sprague-Dawley rats. Effects on phosphorylation of Akt (phospho-Akt), p38 MAPK (phospho-p38), and p44/42 MAPK (phospho-p44/42) was also determined. Incubation with insulin increased (P<0.05) glucose uptake by ∼47%, glucose oxidation by ∼32%, and phospho-Akt by ∼238%. Insulin also increased (P<0.05) phospho-p38, but only after 2h in incubation. Muscle incubated with β2 agonist alone exhibited ∼20% less (P<0.05) glucose uptake but ∼32% greater (P<0.05) glucose oxidation than unstimulated muscle. Moreover, co-incubation with insulin+β2 agonist increased (P<0.05) glucose oxidation and phospho-Akt compared to insulin alone. Conversely, β1 agonist did not appear to affect basal or insulin-stimulated glucose metabolism, and neither β agonist affected phospho-p44/42. TNFα and IL-6 increased (P<0.05) glucose oxidation by ∼23% and ∼33%, respectively, in the absence of insulin. This coincided with increased (P<0.05) phospho-p38 and phospho-p44/42 but not phospho-Akt. Furthermore, co-incubation of muscle with insulin+either cytokine yielded glucose oxidation rates that were similar to insulin alone, despite lower (P<0.05) phospho-Akt. Importantly, cytokine-mediated increases in glucose oxidation rates were not concomitant with greater glucose uptake. These results show that acute β2 adrenergic stimulation, but not β1 stimulation, directly increases fractional glucose oxidation in the absence of insulin and synergistically increases glucose oxidation when combined with insulin. The cytokines, TNFα and IL-6, likewise directly increased glucose oxidation in the absence of insulin, but were not additive in combination with insulin and in fact appeared to disrupt Akt-mediated insulin signaling. Rather, cytokines appear to be acting through MAPKs to elicit effects on glucose oxidation. Regardless, stimulation of glucose oxidation by these key stress factors did not rely upon greater glucose uptake, which may promote metabolic efficiency during acute stress by increasing fractional glucose oxidation without increasing total glucose consumption by muscle.

Impact of Epinephrine Contained in Local Anesthetic Solution on Serum Lactate Level During Orthognathic Surgery

PURPOSE

There have been many discussions of a relation between endogenous and exogenous epinephrine and hyperlactatemia. This study aimed to identify the impact of epinephrine contained in a local anesthetic solution on serum lactate levels in patients who underwent orthognathic surgery.

MATERIALS AND METHODS

This study was a retrospective record review of cases of maxillary and mandibular osteotomy at the Tokyo University Hospital (Tokyo, Japan) from January 2006 through December 2014. One hundred ninety-three patients were enrolled in this study.

RESULTS

The maximum intraoperative serum lactate level was 22.3 ± 14.7 mg/dL. Of 193 patients, 91 showed an intraoperative serum lactate level that was higher than the normal maximum of 19.8 mg/dL (2.2 mmol/L), and 16 of these had a level that was at least 40 mg/dL (≥4.49 mmol/L). Multiple logistic regression analysis showed 2 factors that could increase the serum lactate level: the amount of epinephrine contained in the local anesthetic solution injected into the oral cavity (odds ratio [OR] = 1.014; 95% confidence interval [CI], 1.006-1.022; P = .0001) and the absence of intraoperative treatment with propranolol (OR, 0.105; 95% CI, 0.019-0.434; P = .0013). Patients with severe serum lactate concentrations (ie, ≥40 mg/dL [≥4.49 mmol/L]) had slight metabolic acidosis. All patients survived 90 days. The number of postoperative hospitalization days for patients with severe serum lactate concentrations was 12.8 ± 2.6 days and that for patients without severe serum lactate concentration was 16.0 ± 8.6 days.

CONCLUSION

Increases in intraoperative serum lactate levels during orthognathic surgery are associated, at least in part, with increased aerobic glycolysis because of β₂-adrenergic signaling. Lactate increase caused by epinephrine contained in a local anesthetic solution does not result in a poor postoperative outcome.

Demonstration of a direct interaction between β2-adrenergic receptor and insulin receptor by BRET and bioinformatics

Glucose metabolism is under the cooperative regulation of both insulin receptor (IR) and β₂-adrenergic receptor (β₂AR), which represent the receptor tyrosine kinases (RTKs) and seven transmembrane receptors (7TMRs), respectively. Studies demonstrating cross-talk between these two receptors and their endogenous coexpression have suggested their possible interactions. To evaluate the effect of IR and prospective heteromerization on β₂AR properties, we showed that IR coexpression had no effect on the ligand binding properties of β₂AR; however, IR reduced β₂AR surface expression and accelerated its internalization. Additionally, both receptors displayed a similar distribution pattern with a high degree of colocalization. To test the possible direct interaction between β₂AR and IR, we employed quantitative BRET² saturation and competition assays. Saturation assay data suggested constitutive β₂AR and IR homo- and heteromerization. Calculated acceptor/donor (AD₅₀) values as a measure of the relative affinity for homo- and heteromer formation differed among the heteromers that could not be explained by a simple dimer model. In heterologous competition assays, a transient increase in the BRET² signal with a subsequent hyperbolical decrease was observed, suggesting higher-order heteromer formation. To complement the BRET² data, we employed the informational spectrum method (ISM), a virtual spectroscopy method to investigate protein-protein interactions. Computational peptide scanning of β₂AR and IR identified intracellular domains encompassing residues at the end of the 7^th TM domain and C-terminal tail of β₂AR and a cytoplasmic part of the IR β chain as prospective interaction domains. ISM further suggested a high probability of heteromer formation and homodimers as basic units engaged in heteromerization. In summary, our data suggest direct interaction and higher-order β₂AR:IR oligomer formation, likely comprising heteromers of homodimers.

cAMP signaling in skeletal muscle adaptation: hypertrophy, metabolism, and regeneration

Among organ systems, skeletal muscle is perhaps the most structurally specialized. The remarkable subcellular architecture of this tissue allows it to empower movement with instructions from motor neurons. Despite this high degree of specialization, skeletal muscle also has intrinsic signaling mechanisms that allow adaptation to long-term changes in demand and regeneration after acute damage. The second messenger adenosine 3',5'-monophosphate (cAMP) not only elicits acute changes within myofibers during exercise but also contributes to myofiber size and metabolic phenotype in the long term. Strikingly, sustained activation of cAMP signaling leads to pronounced hypertrophic responses in skeletal myofibers through largely elusive molecular mechanisms. These pathways can promote hypertrophy and combat atrophy in animal models of disorders including muscular dystrophy, age-related atrophy, denervation injury, disuse atrophy, cancer cachexia, and sepsis. cAMP also participates in muscle development and regeneration mediated by muscle precursor cells; thus, downstream signaling pathways may potentially be harnessed to promote muscle regeneration in patients with acute damage or muscular dystrophy. In this review, we summarize studies implicating cAMP signaling in skeletal muscle adaptation. We also highlight ligands that induce cAMP signaling and downstream effectors that are promising pharmacological targets.

MicroRNA let-7 establishes expression of beta2-adrenergic receptors and dynamically down-regulates agonist-promoted down-regulation

Although β(2)-adrenergic receptors (β(2)AR) are expressed on most cell types, mechanisms that establish expression levels and regulate expression by chronic agonist remain unclear. The 3' UTR of ADRB2 has a conserved 8-nucleotide seed region that we hypothesized is targeted by the let-7 family of miRNAs leading to translational repression. In luciferase assays with transfected cells, luc-β(2)WT3'UTR had decreased expression when cotransfected with let-7f, but a mutated luc-β(2)3'UTR lacking the seed was unaffected by let-7f; a mutated let-7f also had no effect on luc-β(2)WT3'UTR expression. ADRB2 mRNA was in greater abundance in immunoprecipitates of Ago2, a core component of the miRNA-induced silencing complex, when cells were transfected with let-7f, but not with a mutated let-7f, indicating a direct interaction with the silencing mechanism. H292 cells transfected with let-7f caused ∼60% decrease in native β(2)AR expression, but transfection with let-7f-specific locked nucleic acid anti-miRNA increased β(2)AR expression by ∼twofold. We considered that an increase in let-7f leading to greater repression of translation contributes to agonist-promoted down-regulation. Paradoxically, in cells and in lungs from mice treated in vivo, an ∼50% decrease in let-7f occurs during long-term agonist exposure, indicating a counterregulatory event. Consistent with this notion, let-7f locked nucleic acid transfection caused depressed agonist-promoted down-regulation. Thus, let-7f miRNA regulates baseline β(2)AR expression and decreases in let-7f evoked by agonist attenuate down-regulation. This positive feedback loop has not previously been described for a G protein-coupled receptor and its miRNA. Methods to decrease let-7f expression in targeted cells may increase therapeutic responses to β-agonist by increasing β(2)AR expression or minimizing tachyphylaxis.

β2-Adrenergic receptor downregulation and performance decrements during high-intensity resistance exercise overtraining

Previous research on overtraining due to excessive use of maximal resistance exercise loads [100% 1 repetition maximum (1 RM)] indicates that peripheral muscle maladaptation contributes to overtraining-induced performance decrements. This study examined the cellular and molecular responses of skeletal muscle to performance decrements due to high-relative-intensity (%1 RM) resistance exercise overtraining. Weight-trained men were divided into overtrained (OT, n = 8) and control (Con, n = 8) groups. The OT group performed 10 x 1 at 100% 1 RM daily for 2 wk, whereas the Con group performed normal training 2 days/wk. Muscle biopsies from the vastus lateralis muscle, voluntary static and dynamic muscle performances, and nocturnal urinary epinephrine were assessed before (pre) and after (post) overtraining. Overtraining occurred as indicated by a decrease in 1-RM strength for the OT group (mean +/- SE; OT pre = 159.3 +/- 10.1 kg, OT post = 151.4 +/- 9.9 kg, Con pre = 146.0 +/- 12.9 kg, Con post = 144.9 +/- 13.3 kg), as well as a 36.3% decrease in mean power at 100% 1-RM loads. Normal training could be resumed only after 2-8 wk of training cessation. Muscle beta(2)-adrenergic receptor (beta(2)-AR; fmol/mg protein) density significantly decreased by 37.0% for the OT group and was unchanged for the Con group (-1.8%). Nocturnal urinary epinephrine for the OT group increased by 49%, although this was not significant (effect size = 0.42). The ratio of nocturnal urinary epinephrine to beta(2)-AR density suggested a decreased beta(2)-AR sensitivity for the OT group (2.4-fold increase). Overtraining occurred based on decreased muscular force and power. Desensitization of the beta(2)-AR system suggests that this may be an important contributor to performance decrements due to excessive use of maximal resistance exercise loads.

AMP-activated protein kinase activation by adrenoceptors in L6 skeletal muscle cells: mediation by alpha1-adrenoceptors causing glucose uptake

AMP-activated protein kinase (AMPK), which functions as a sensor of cellular energy homeostasis, was phosphorylated after norepinephrine stimulation in L6 skeletal muscle cells. This effect was mediated by alpha1-adrenoceptors, with no stimulatory effects due to interactions at alpha2- or beta-adrenoceptors. Alpha1-adrenoceptors are Gq-coupled receptors, and calcium but not phorbol esters could mimic the effect of alpha1-adrenergic stimulation; and we show that protein kinase C is not involved as an upstream signal to AMPK by alpha1-adrenergic stimulation and that the AMP-to-ATP ratio is unaltered after alpha1-adrenergic stimulation. We further show that glucose uptake mediated by alpha1- but not by beta-adrenoceptors can be inhibited by AMPK inhibition. Acetyl-CoA carboxylase (ACC) is phosphorylated at Ser218 by AMPK, and alpha1- but not beta-adrenoceptor stimulation results in phosphorylation of ACC at this residue. These results suggest a novel pathway where alpha1-adrenoceptor activation, independent of protein kinase C, leads to activation of AMPK in skeletal muscle, which contributes to alpha1-adrenoceptor-mediated increases in glucose uptake.

Major differences in noradrenaline action on lipolysis and blood flow rates in skeletal muscle and adipose tissue in vivo

AIMS/HYPOTHESIS

The regulation of skeletal muscle lipolysis is not fully understood. In the present study, the effects of systemic and local noradrenaline administration on lipolysis and blood flow rates in skeletal muscle and adipose tissue were studied in vivo.

METHODS

First, circulating noradrenaline levels were raised tenfold by a continuous i.v. infusion (n=12). Glycerol levels (an index of lipolysis) were measured in m. gastrocnemius and in abdominal adipose tissue using microdialysis. Local blood flow was determined with the (133)Xe clearance technique and whole-body lipolysis rates assessed with a stable glycerol isotope technique ([(2)H(5)] glycerol). Second, interstitial glycerol levels in m. gastrocnemius, m. vastus and adipose tissue were measured by microdialysis during local perfusion with noradrenaline (10(-8)-10(-6) mol/l) (n=10). Local blood flow was monitored with the ethanol perfusion technique.

RESULTS

With regard to systemic noradrenergic stimulation, no change in fractional release of glycerol (difference between tissue and arterial glycerol) was seen in skeletal muscle. In adipose tissue it transiently increased twofold (p<0.0001), and the rate of appearance of glycerol in plasma showed the same kinetic pattern. Blood flow was reduced by 40% in skeletal muscle (p<0.005) and increased by 50% in adipose tissue (p<0.05). After noradrenaline stimulation in situ, a discrete elevation of skeletal muscle glycerol was registered only at the highest concentration of noradrenaline (10(-6) mol/l) (p<0.05). Adipose tissue glycerol doubled already at the lowest concentration (10(-8) mol/l) (p<0.05). In skeletal muscle a decrease in blood flow was seen at the highest noradrenaline concentrations (p<0.05).

CONCLUSIONS/INTERPRETATION

Lipolysis and blood flow rates are regulated differently in adipose tissue and skeletal muscle. Adipose tissue displays a high, but transient (tachyphylaxia) sensitivity to noradrenaline, leading to stimulation of both lipolysis and blood flow rates. In skeletal muscle, physiological concentrations of noradrenaline decrease blood flow but have no stimulatory effect on lipolysis rates.

Effect of beta1- and beta2-adrenergic stimulation on energy expenditure, substrate oxidation, and UCP3 expression in humans

In humans, beta-adrenergic stimulation increases energy and fat metabolism. In the case of beta1-adrenergic stimulation, it is fueled by an increased lipolysis. We examined the effect of beta2-adrenergic stimulation, with and without a blocker of lipolysis, on thermogenesis and substrate oxidation. Furthermore, the effect of beta1-and beta2-adrenergic stimulation on uncoupling protein 3 (UCP3) mRNA expression was studied. Nine lean males received a 3-h infusion of dobutamine (DOB, beta1) or salbutamol (SAL, beta2). Also, we combined SAL with acipimox to block lipolysis (SAL+ACI). Energy and substrate metabolism were measured continuously, blood was sampled every 30 min, and muscle biopsies were taken before and after infusion. Energy expenditure significantly increased approximately 13% in all conditions. Fat oxidation increased 47 +/- 7% in the DOB group and 19 +/- 7% in the SAL group but remained unchanged in the SAL+ACI condition. Glucose oxidation decreased 40 +/- 9% upon DOB, remained unchanged during SAL, and increased 27 +/- 11% upon SAL+ACI. Plasma free fatty acid (FFA) levels were increased by SAL (57 +/- 11%) and DOB (47 +/- 16%), whereas SAL+ACI caused about fourfold lower FFA levels compared with basal levels. No change in UCP3 was found after DOB or SAL, whereas SAL+ACI downregulated skeletal muscle UCP3 mRNA levels 38 +/- 13%. In conclusion, beta2-adrenergic stimulation directly increased energy expenditure independently of plasma FFA levels. Furthermore, this is the first study to demonstrate a downregulation of skeletal muscle UCP3 mRNA expression after the lowering of plasma FFA concentrations in humans, despite an increase in energy expenditure upon beta2-adrenergic stimulation.

The rainbow trout skeletal muscle beta-adrenergic system: characterization and signaling

The presence and functionality of beta-adrenoceptors (beta-ARs) were examined in red (RM) and white muscle (WM) membranes isolated from the rainbow trout Oncorhynchus mykiss. Specific binding assays revealed the presence of a single class of binding sites with similar affinities in both muscle types (K(d) in nM: 0.14 +/- 0.03 and 0.18 +/- 0.03 for RM and WM, respectively) but with a significantly higher number of binding sites in RM compared with WM (B(max) in fmol/mg protein: 3.22 +/- 0.11 and 2.60 +/- 0.13, respectively). Selective and nonselective beta-adrenergic agonists (beta-AAs) and antagonists indicated an atypical beta-AR pharmacology. This result may represent a nonmammalian beta-AR classification or, more likely, the presence of more than one beta-AR subtype in trout muscles with similar affinities that could not be kinetically resolved. Adenylyl cyclase (ACase) assays showed a dose-dependent increase in cAMP production as concentrations of beta(2)-AAs increased in both muscle membranes with significantly higher basal cAMP production in RM compared with WM (cAMP production in pmol cAMP. mg protein(-1). 10 min(-1): 24.67 +/- 3.06 and 9.64 +/- 3.45, respectively). The agonist-induced increase in cAMP production was blocked by the beta-adrenergic antagonist propranolol, while the ACase activator forskolin increased cAMP production by 7- to 14-fold above basal and approximately 3-fold above all beta-AAs tested. This study demonstrated the presence of atypical beta(2)-ARs on RM and WM membranes of trout, suggesting that beta(2)-AAs may be a tool to enhance protein accretion through this signaling pathway.

Effect of aging on rat skeletal muscle beta-AR function in male Fischer 344 x brown Norway rats

In this study, we tested the hypothesis that, in the male Fischer 344 x Brown Norway (F344xBN) rat, aging would be associated with an increase in sympathetic nervous system activity and a decrease in skeletal muscle beta-adrenergic-receptor (beta-AR) density and function. Radioligand-binding studies using [125I] iodocyanopindolol were done to evaluate beta-AR density (Bmax) and antagonist-binding affinity in gastrocnemius and cardiac muscle from 6-, 18-, and 28-mo-old male F344xBN rats. beta-AR function was measured as adenylyl cyclase (AC) activity stimulated by the beta-AR agonist isoproterenol (Iso, 10(-4) M). Basal arterial plasma norepinephrine (pNE) concentrations were higher in the 28-than in the 6- and 18-mo-old rats. Bmax was greatest and Iso-stimulated AC activity was unchanged in gastrocnemius muscle of the 28-mo-old age group. In contrast, there was an age-associated decrease in Bmax and Iso-stimulated AC activity in cardiac muscle. In conclusion, there was an age-associated increase in pNE concentrations in male F344xBN rats, suggesting an increase in sympathetic nervous system activity. In addition, there was an age-associated increase in skeletal muscle beta-AR density, whereas in skeletal muscle beta-AR-stimulated AC activity remained unchanged with age.

Classical and atypical binding sites for beta-adrenoceptor ligands and activation of adenylyl cyclase in bovine skeletal muscle and adipose tissue membranes

1. The radioligand [125I]-iodocyanopindolol ([125I]-ICYP) was used under standard ligand binding conditions, to detect beta 1- and beta 2-adrenoceptors in membrane preparations from bovine skeletal muscle and adipose tissue. High concentrations of [125I]-ICYP were also used, to identify an 'atypical' binding site in skeletal muscle. Finally, adenosine 3':5'-cyclic monophosphate (cyclic AMP) production was measured in the same membrane preparations, to determine the relationship between the beta-adrenoceptor sub-types present and the production of this second-messenger. 2. According to the results of radioligand binding studies, both skeletal muscle and adipose tissue membranes have beta 2-adrenoceptors, characterized by a high affinity for the beta 2-selective antagonist, ICI 118551 (pK 8.3 and 8.6 respectively); and a low affinity for the beta 1-selective antagonist CGP 20712A (pK 5.2 in both tissues). Antagonism of (-)-isoprenaline-stimulated cyclic AMP production by low concentrations of ICI 118551, yielded pseudo pA2 values in muscle and adipose tissue of 7.6 and 8.7 respectively, confirming that beta 2-adrenoceptors in these tissues are linked to the production of the second-messenger. 3. Although beta 1-adrenoceptors could not be detected in either skeletal muscle or adipose tissue membranes by use of ligand binding techniques, high pseudo pA2 values were obtained (8.0 and 8.2 respectively), when CGP 20712A was used to block the stimulation of cyclic AMP production by (-)-isoprenaline. This finding is consistent with the presence in both tissues of a population of beta 1-adrenoceptors which is small, but efficiently coupled to the second-messenger. 4.In addition to identifying standard beta 1- and beta 2-adrenoceptors, it was also established that skeletal muscle membranes have an 'atypical' binding site which has a relatively low affinity for [125]-ICYP(pK8.84), but which exists in abundance. At high concentrations of radioligand, the 'atypical' site accounted for 89% of the total [125I]-ICYP binding sites present.5. The results of second-messenger studies do not support the hypothesis that skeletal muscle or adipose tissue membranes contain functional beta 3-adrenoceptors: based on the failure of a beta 3-adrenoceptor-selective agonist (BRL 37344) to stimulate cyclic AMP production, the absence of a biphasic response to (-)-isoprenaline, and the observation that cyclic AMP production was not resistant to blockade by either ICI 118551 or CGP20712A.6. It is concluded that data from radioligand binding studies do not accurately reflect the contribution made by beta 1- and beta 2-adrenoceptors to cyclic AMP production in bovine skeletal muscle and adipose tissue membranes. Furthermore, the 'atypical' [125I]-ICYP binding site identified in bovine skeletal muscle does not represent a functional bovine beta 3-adrenoceptor.

Selective beta-adrenoceptor partial agonist effects of pindolol and xamoterol on skeletal muscle assessed by plasma creatine kinase changes in healthy subjects

1. The effects of selective beta-adrenoceptor partial agonist activity on plasma creatine kinase (CK) and skeletal muscle symptoms were studied in normal volunteers. 2. A drug with beta 1-selective partial agonist activity (xamoterol) and one with partial agonist activity acting mainly through beta 2-adrenoceptors (pindolol) were each given for 3 weeks in a randomised double-blind crossover study in 10 subjects. Five additional subjects received only one drug. Plasma CK levels were monitored during a baseline placebo run-in phase, the active treatment period and a placebo washout phase which continued until CK levels returned to baseline. 3. The degree of beta-adrenoceptor antagonism was determined by the inhibition of exercise-induced tachycardia and was similar for the two drug doses used. 4. During pindolol administration plasma CK levels rose compared with pretreatment baseline levels and with levels during xamoterol administration which did not rise. After pindolol was withdrawn CK levels reached higher peaks in some subjects after 1-5 days. 5. Muscle cramps were reported by five subjects during pindolol administration and by one of these subjects but to a lesser extent during xamoterol administration. 6. Pindolol may produce this effect, which was not seen with xamoterol, because of its specific beta 2-adrenoceptor partial agonist activity. Elevations in plasma CK produced by this type of drug or its withdrawal may cause confusion in the diagnosis of muscle disease or myocardial infarction unless the myocardial isoenzyme is measured.

Multiple actions of beta-adrenergic agonists on skeletal muscle and adipose tissue

Images

Increased fat and skeletal muscle beta-adrenergic receptors but unaltered metabolic and hemodynamic sensitivity to epinephrine in vivo in experimental human thyrotoxicosis

Based largely on evidence of increased target tissue beta-adrenergic receptor densities and responsiveness in animal and, to a lesser extent, human tissues, it is often assumed that thyroid hormone excess results in increased sensitivity to catecholamines in vivo, thus explaining several clinical manifestations of thyrotoxicosis. To test the hypothesis that thyrotoxicosis results in increased target tissue beta-adrenergic receptor densities and correspondingly increased metabolic and hemodynamic sensitivity to epinephrine in vivo, we measured these in 10 normal humans before and after administration of triiodothyronine (100 micrograms daily) for 10 d. Thyrotoxicosis increased beta-adrenergic receptor densities in fat (approximately 60%) and skeletal muscle (approximately 30%). Despite increments in beta-adrenergic receptor densities in these and probably other target tissues, metabolic and hemodynamic sensitivity to epinephrine in vivo was unaltered. An apparently adaptive increase in insulin secretion plausibly explains normal glycemic, glycogenolytic/glycolytic, lipolytic, and ketogenic sensitivity to epinephrine in the thyrotoxic state. In view of this striking homeostatic efficiency of the intact individual, the finding of altered adrenergic receptors, even in relevant target tissues, should not be extrapolated to altered sensitivity to catecholamines in vivo in the absence of direct testing of that hypothesis. With respect to the clinical issue, these data suggest that increased sensitivity to catecholamines does not explain clinical manifestations of thyrotoxicosis in humans.