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The dysregulation of energy homeostasis in obesity involves multihormone resistance. Although leptin and insulin resistance have been well characterized, catecholamine resistance remains largely unexplored. Murine [beta]3-adrenergic receptor expression in adipocytes is orders of magnitude higher compared with that of other isoforms. While resistant to classical desensitization pathways, its mRNA (Adrb3) and protein expression are dramatically downregulated after ligand exposure (homologous desensitization). [beta]3-Adrenergic receptor downregulation also occurs after high-fat diet feeding, concurrent with catecholamine resistance and elevated inflammation. This downregulation is recapitulated in vitro by TNFa treatment (heterologous desensitization). Both homologous and heterologous desensitization of Adrb3 were triggered by induction of the pseudokinase TRIB1 downstream of the EPAC/RAP2A/PI-PLC pathway. TRIB1 in turn degraded the primary transcriptional activator of Adrb3, CEBP[alpha]. EPAC/RAP inhibition enhanced catecholamine-stimulated lipolysis and energy expenditure in obese mice. Moreover, adipose tissue expression of genes in this pathway correlated with body weight extremes in a cohort of genetically diverse mice and with BMI in 2 independent cohorts of humans. These data implicate a signaling axis that may explain reduced hormone-stimulated lipolysis in obesity and resistance to therapeutic interventions with [beta]3-adrenergic receptor agonists., Introduction Body mass is governed by the balance between energy intake and expenditure, which are tightly coupled to ensure homeostasis even during periods of over- or undernutrition (1-4). Disruption of [...]

Byline: J. R. Kaltman (1), P. S. Ro (2), P. Stephens (1), M. G. McBride (1), M. I. Cohen (3), R. E. Tanel (1), V. L. Vetter (1), L. A. Rhodes (1) Keywords: Long QT syndrome; QT dispersion; [beta]-Adrenergic antagonists Abstract: It has been proposed that [beta]-adrenergic antagonist protection against cardiac events in patients with long QT syndrome (LQTS) may be related to a decrease in baseline QTc dispersion. To determine the effects of [beta]-blocker therapy on QT measurements, we evaluated the exercise tests of 25 pediatric patients with LQTS. Measurements were made of the maximum QTc interval and QTc dispersion during the various segments of the exercise test. There was no statistically significant difference between the pre-[beta]-blocker and post-[beta]-blocker maximum QTc interval during the supine (0.473 +- 0.039 vs 0.470 +- 0.038 sec), exercise (0.488 +- 0.044 vs 0.500 +- 0.026 sec), or recovery (0.490 +- 0.031 vs 0.493 +- 0.029 sec) phases of the exercise stress test. There was also no statistically significant difference between the pre-[beta]-blocker and post-[beta]-blocker QTc dispersion during the supine (0.047 +- 0.021 vs 0.058 +- 0.033 sec), exercise (0.063 +- 0.036 vs 0.063 +- 0.028 sec), or recovery (0.045 +- 0.023 vs 0.052 +- 0.026 sec) phases of the exercise stress test. Therefore, the protection that [beta]-blockers offer appears not to be related to a reduction of the baseline QTc interval or a decrease of QTc dispersion. Author Affiliation: (1) Department of Pediatrics, University of Pennsylvania School of Medicine and the Cardiac Center at The Children's Hospital of Philadelphia, 34th and Civic Center Boulevard, Philadelphia, PA 19103, USA (2) Department of Pediatrics, The Ohio State University School of Medicine, Columbus Children's Hospital, 700 Children's Drive, Columbus, OH 43205, USA (3) Pediatric Cardiology Consultants, 333 East Virginia Avenue, Phoenix, AZ 85004, USA Article History: Registration Date: 01/01/2003 Online Date: 04/09/2003 Article note: Paper presented at the Third Annual World Congress of Pediatric Cardiology and Cardiac Surgery, Toronto, Canada, May 2001

The objective of this research was to use recent ractopamine research data to develop an updated mathematical model to describe the daily compositional growth of pigs fed ractopamine. Mean increases of 18.2, 23.1, and 25.0% for daily protein accretion were assumed for 5, 10, and 20 ppm of ractopamine for an overall gain of 40 kg of BW gain during the feeding period. The relative effect of ractopamine described the rapid increase and subsequent decrease in the effect of ractopamine as a function of BW gain or days on test and ractopamine concentration (RC, ppm). The reduction in ME intake produced by ractopamine was described as 0.036 x [(RC/20).sup.0.7] multiplied by the ME intake for the first 20 kg of BW gain, and then increasing to 0.078 x [(RC/20).sup.0.7] at 40 kg of BW gain feeding period. The ratio of fat-free muscle gain to protein accretion increased by 14 to 16% with the feeding of ractopamine, depending on the dietary lysine/essential AA levels. The ratio of carcass fat gain to empty body lipid gain was increased when lysine and essential AA requirements were met. Daily protein accretion and fat-free lean growth were described as functions of dietary lysine/essential AA intakes. The percentage of lysine in protein accretion increased with the feeding of ractopamine from 6.80 to 7.15%, depending on ractopamine concentration. Equations predicting carcass measurements, such as fat and longissimus muscle depths from carcass weight and composition, were modified to incorporate prediction biases produced by ractopamine. For the four concentrations of ractopamine (0, 5, 10, and 20 ppm, respectively) during a 78 to 110 kg of BW feeding period, the model predicted performance levels for ADG (1.03, 1.15, 1.16, and 1.16 kg/d), gain:feed (kg of ADG/kg of ADFI; 0.360, 0.401, 0.412, and 0.425), dressing percentage (75.1, 76.0, 76.3, and 76.4), percentage fat-free lean (48.7, 51.0, 51.5, and 52.2), longissimus muscle area (38.8, 41.8, 42.5, and 43.5 [cm.sup.2]), 10th-rib fat depth (22.1, 19.8, 19.3, and 18.7 mm), and fat-free lean gain (321,446, 467, and 495 g/d), comparable to recent research data. The model allows the effect of ractopamine to be added to farm specific pig growth curves. It can be used to evaluate ways to optimize the use of ractopamine, including duration of ractopamine feeding, concentration of ractopamine, and dietary lysine concentration. Key Words: [beta]-Adrenergic Agonists, Body Composition, Growth, Models, Pigs

Genetic manipulation of the [[alpha].sub.2A]-adrenergic receptor ([[alpha].sub.2A]-AR) in mice has revealed the role of this subtype in numerous responses, including agonist-induced hypotension and sedation. Unexpectedly, [[alpha].sub.2]-agonist treatment of mice heterozygous for the [[alpha].sub.2A]-AR ([[alpha].sub.2A]-A[R.sup.+/-]) lowers blood pressure without sedation, indicating that more than 50% of [[alpha].sub.2A]-AR must be activated to evoke sedation. We postulated that partial activation of [[alpha].sub.2A]-AR in wild-type [[alpha].sub.2A]-A[R.sup.+/+] animals could be achieved with partial agonists, agents with variable ability to couple receptor occupancy to effector activation, and might elicit one versus another pharmacological response. In vitro assays reveal that moxonidine is a partial agonist at [[alpha].sub.2A]-AR. Although moxonidine was developed to preferentially interact with imidazoline binding sites, it requires the [[alpha].sub.2A]-AR to lower blood pressure because we observe no hypotensive response to moxonidine in [[alpha].sub.2A]-AR-null ([[alpha].sub.2A]-A[R.sup.-/-]) mice. Moreover, we observe that moxonidine lowers blood pressure without sedation in wild-type mice, consistent with the above hypothesis regarding partial agonists. Our findings suggest that weak partial agonists can evoke response-selective pathways and might be exploited successfully to achieve [[alpha].sub.2A]-AR pharmacotherapy where concomitant sedation is undesirable, i.e., in treatment of depression or attention deficit hyperactivity disorder, in suppression of epileptogenesis, or enhancement of cognition. Furthermore, rigorous physiological and behavioral assessment of mice heterozygous for particular receptors provides a general strategy for elucidation of pathways that might be selectively activated by partial agonists, thus achieving response-specific therapy.

We find that the adrenergic agonist isoproterenol increases intracellular [Ca.sup.2+] concentration ([[Ca.sup.2+].sub.i]) in cultured rat brown adipocytes. At the concentration used (10 [micro]M), isoproterenol-induced [Ca.sup.2+] responses were sensitive to block by either [[alpha].sub.1] or [beta]-adrenergic antagonists, suggesting an interaction between these receptor subtypes. Despite reliance on [beta]-adrenoceptor activation, the [Ca.sup.2+] response was not due solely to increases in cAMP because, administered alone, the selective [beta]-adrenergic agonist BRL-37344 or forskolin did not increase [[Ca.sup.2+].sub.i]. However, increased cAMP elicited vigorous [[Ca.sup.2+].sub.i] increases in the presence of barely active concentrations of the a-adrenergic agonist phenylephrine or the P2Y receptor agonist UTP. Consistent with isoproterenol recruiting only inositol 1,4,5-trisphosphate (I[P.sub.3])-sensitive [Ca.sup.2+] stores, endoplasmic reticulum store depletion by thapsigargin blocked isoproterenol-induced [Ca.sup.2+] increases, but removal of external [Ca.sup.2+] did not. These results argue that increases in cAMP sensitize the I[P.sub.3]-mediated [Ca.sup.2+] release system in brown adipocytes. adenosine 3',5'-cyclic monophosphate; fura 2 fluorescence imaging; thapsigargin; isoproterenol; caffeine; adipocytes

The effects of ractopamine (RAC) and ractopamine stereoisomers (RR, RS, SR, and SS) on cyclic AMP (cAMP) production, total protein, and DNA concentrations in mouse skeletal muscle cells (C(sub 2)C(sub 12) were evaluated. The RAC (10 (mu)M) caused an approximately 30% increase in cell number, protein, and DNA concentrations in myoblasts after 48 h; no differences were found in myotubes. The RAC-stimulated increase of these variables in myoblasts was blocked by the presence of equimolar concentrations of propranolol. At a later passage, myoblasts failed to exhibit an increase in cell number, protein, or DNA upon exposure to RAC. Both myoblasts and myotubes increased cAMP production in response to 10 (mu)M RAC. The RAC isomers ranked RR >> SR > RS approximately SS in ability to stimulate cAMP production, with essentially no response to SS. The SR produced about 50% of the RR response. Coincubation of propranolol (10 (mu)M) and RAC (10 (mu)M)) prevented RAC-stimulated cAMP production in myotubes but not in myoblasts (approximately 35% of cAMP produced by RAC alone). Turkey satellite cells (derived from biceps femoris of 12-wk-old toms) produced essentially no increased cAMP when exposed to 10 (mu)M RAC stereoisomers. Stability of RAC was evaluated under laboratory storage and culture conditions. The RAC was stable for more than 4 mo when stored in deuterated DMSO (>98% purity) at room temperature or in aqueous solutions at-80 degrees C, as determined from sequential nuclear magnetic resonance studies. Radiolabeled RAC was incubated for 72 h in the presence of serum-containing medium, with or without C(sub 2)C(sub 12) cells. Ninety-eight percent of the parent compound found in the medium at time zero was present in the medium as parent at the end of 72 h. The cellular cAMP response to RAC through (beta)-adrenergic receptors seems to be stereospecific. If the state of myoblasts and myotubes in vitro reflects the in vivo state, then the ractopamine effect in vivo on cellular processes (including cell division and protein and DNA accumulation) may be independent of (beta)-adrenergic receptors in muscle. Key Words: Cells, (beta)-Adrenergic Agonists, Turkey, Ractopamine, Muscles

(Beta)-Adrenergic agonists increase growth rate, but their efficacy is reduced over time as the number of (beta)(sub 2)-adrenoceptors in muscle decreases. Dexamethasone increases (beta)(sub 2)-adrenoceptor density in many tissues, but this effect has not been reported in skeletal muscle. In this study, male rats were treated daily for 10 d with either clenbuterol (4 mg/kg of feed), dexamethasone (.2 mg/kg BW, s.c.), or clenbuterol plus dexamethasone. Untreated rats served as controls. Dexamethasone caused a marked suppression of growth rate, which resulted in decreased (P < .001) body weight (-29%), carcass weight (-30%), hind-limb muscles (-22%), omental fat (-22%), and heart weight (-10%). Feed intake was reduced (-26%), but feed conversion efficiency was also impaired (P < .001). Clenbuterol caused a small increase in growth rate (+6%; P < .05), with an increase in leg muscle (+7%; P < .01) and heart mass (+8%; P < .05). Feed efficiency was improved (P < .001) by clenbuterol. Rats given the combined treatment still showed a reduction in growth rate (-81%). Clenbuterol caused only a mild attenuation of the effects of dexamethasone on feed intake, BW, and carcass weight, but reduced the catabolic effect of dexamethasone on hind-limb muscle to only-8%. Clenbuterol caused a slight increase in the affinity (beta)(sub 2)-adrenoceptors in lung for binding to the radioligand (-)[125I]iodocyanopindolol. Relative to control values, the density of (beta)(sub 2)-adrenoceptors in lung was +31% with dexamethasone treatment,-45% with clenbuterol, and -23% with the combined treatment. Clenbuterol also decreased (beta)(sub 2)-adrenoceptors in skeletal muscle (-35%), but so did dexamethasone (-13%), so the effects of the (beta)-adrenergic agonist were not attenuated through use of the combined treatment (-40%). The results show that the inductive effect of glucocorticoids on (beta)(sub 2)-adrenoceptors is tissue-specific and that glucocorticoid treatment is not a useful adjunct to (beta)-adrenergic agonist treatment in animal production. Key Words: (Beta)-Adrenergic Agonists, Skeletal Muscle, Lungs, Clenbuterol, Dexamethasone, Growth

Research was conducted to examine the alterations in the beta-adrenergic receptor (beta-AR) system of rabbit hearts during the development of heart failure (HF) after myocardial infarction. The objective was to determine if the molecular beta-AR abnormalities associated with human HF are present in the animal model. Results indicate that the myocardial beta-AR system of the rabbit model of HF shares significant biochemical properties with human HF which means that it is an ideal laboratory model to investigate novel therapeutic targets for the treatment of HF.

The objectives of this study were to determine the effects of ractopamine HCl (RAC) stereoisomers (RR, RS, SR, and SS) on performance, carcass composition, and nitrogen retention in growing female rats. Forty-eight rats (eight rats/treatment) were treated with 0 or 320 [[micro]gram]/d of RAC or with 80 [[micro]gram]/d of the RR, RS, SR, or SS stereoisomers of ractopamine. Rats had free access to feed and water before and during the experiment. Ractopamine and stereoisomers were delivered via i.p. implanted osmotic pumps for 14 d, and rats were then slaughtered. Control rats were fitted with osmotic pumps containing saline. Ractopamine increased (P < .05) feed intake (d 1 to 6); body weight; carcass CP; and intake, apparent absorption, retention, and retained:intake ratio of CP on d 1 to 6 of the study. Ractopamine decreased (P < .05) carcass lipid and visceral lipid. Rats dosed with the RR stereoisomer responded similarly to rats dosed with RAC, except for carcass lipid. Carcass lipid was decreased (P < .01) by RAC relative to controls, but it was not different from controls in rats treated with the RR isomer. Compared with controls, BW, carcass CP, and CP retention were increased by the RR stereoisomer, and visceral lipid was decreased. The RS isomer also decreased visceral lipid (P < .10), but variables measured in rats dosed with the RS, SR, and SS isomers generally did not differ from controls. Results of this study indicate that the RR isomer of RAC is responsible for a majority of the leanness-enhancing effects of RAC in rats. Key Words: Ractopamine, Growth, [Beta]-Adrenergic Agonists

Research was conducted to examine the in vitro effects of the short-acting beta-2-adrenoceptor agonist salbutamol and the long-acting beta-2-adrenoceptor agonist salmeterol on hypoxia-induced rat diaphragm force reduction. The impact of propranolol on salbutamol and salmeterol during hypoxia was also tested to check if the effects are mediated through beta-2-adrenoceptor-associated processes. Results indicate that the effects depend on beta-2-adrenoceptor agonist-related processes.

Research was conducted to test whether the beta-adrenergically mediated thermogenesis in men is altered with aging. Intravenous infusion of nonselective beta-agonist isoprenaline was performed on 11 young and nine older men. The older subjects had higher basal plasma norepinephrine and epinephrine concentrations than the younger men. Results indicate that aging is related to a diminished beta-adrenergically mediated thermogenesis which may generate increased obesity and fat storage caused by a positive energy balance.

Research was conducted to examine the effect of Escherichia coli endotoxin, lipopolysaccharide (LPS), on two major fat cell proteins, beta-3-adrenergic receptor (beta-3-AR) and leptin. Dose-dependent elevations of plasma leptin and white fat leptin mRNA in C3/HeOUJ mice resulted were generated by increasing doses of LPS. Results indicate that endotoxemia leads to an induction of leptin mRNA and a decrease in beta-3-AR mRNA in adipose tissue which may have been mediated by alterations in tumor necrosis factor-alpha.

Research was conducted to examine the secretory function of Rap1 by studying the translocation of Rap1 in rat parotid acini in response to isoproterenol (ISO) and its relationship to amylase release. Results suggest that there is a relationship between Rap1 and secretion that was established in the parotid gland. Findings also demonstrate that Rap1 has a substantial role in secretion that is distal to PKA stimulation and proximal to the exocytotic event.

A symposium on asthma therapy development for the 21st century was held at the National Heart and Lung Institute, London, UK. It highlighted current therapy consisting of beta-2-adrenoceptor agonists, theophylline and glucocorticosteroids and efforts to develop new anti-inflammatory therapy for asthma. There is an ongoing development of a second generation theophyllines that inhibit phosphodiesterase to lessen side effects while retaining its beneficial action.

Somatotropin (ST) and synthetic [Beta]-adrenergic agonists ([Beta]-AA) are growth-modifying agents that increase the rate and sometimes, the efficiency of protein deposition in lean tissues of livestock species. The ST-induced increase in muscle protein deposition is effected by a relatively modest increase in protein synthetic rate. This is possibly mediated by the endocrine influence of marked increases in circulating IGF (insulin-like growth factor)-I, and other ST-dependent components of the IGF system; mediation by locally expressed IGF-I may also occur. Increased muscle protein accretion in animals treated with [Beta]-AA seems to be directly mediated by binding of the synthetic agonist to muscle [Beta]-1 or [Beta]-2 receptors, leading to increased muscle protein synthesis, possibly accompanied or followed by decreased protein degradation. This response is transient, due to down-regulation of [Beta]-adrenergic receptors. Maximal responses of muscle protein accretion to both ST and [Beta]-AA are attenuated by feeding inadequate levels of total protein or specific, limiting amino acids. For ST, but not [Beta]-AA, this effect in growing pigs is partially offset by increased efficiency of utilization of absorbed amino acids for protein deposition, with predictable consequences for dietary protein and amino acid requirements. Both ST and [Beta]-AA are less efficacious in promoting muscle protein deposition in very young animals. For ST, this is related to postnatal development of the somatotropic axis; a mechanistic explanation for the similar lack of effect of [Beta]-AA is lacking. In both cases, this phenomenon must be considered against the very high inherent capacity and efficiency of lean tissue protein accretion in the neonate. KEY WORDS: somatotropin; [Beta]-adrenergic agonists; protein nutrition; physiological development

Research indicates that genetic expression of cell components caveolin-1 and caveolin-3 within skeletal muscle is controlled by cardiac beta-adrenergic receptor stimulation. Experiments designed to investigate the physiological impact of chronic beta-adrenergic receptor stimulation in mouse heart tissue are described. Immunoblotting techniques are used to establish caveolin messenger ribonucleic acid expression.

[[Beta].sub.2]-Adrenoceptor agonists provide a mainstay in the treatment of asthma worldwide. However, despite their ability to provide symptomatic relief, chronic or repeated exposure to [[Beta].sub.2]-adrenoceptor agonists does not resolve asthmatic inflammation, because of the rapid development of tolerance by pro-inflammatory and immune cells of the lung. The prevailing belief is that tolerance to the so-called non-bronchodilator actions of [[Beta].sub.2]-adrenoceptor agonists is largely attributable to direct receptor desensitization mediated by G protein receptor-coupled kinases and/or cAMP-dependent protein kinase. Here, Mark Giembycz suggests another, largely ignored, explanation for [[Beta].sub.2]-adrenoceptor desensitization that is based on the accelerated degradation of cAMP by phosphodiesterase.

The four [[Beta].sub.2]-adrenoceptor agonists, clenbuterol, bambuterol, formoterol and salmeterol, are all long-acting bronchodilators, that have distinct mechanisms for their extended duration of action. Various theories have been put forward in an attempt to explain these mechanisms. In this respect, there is strong evidence for the existence of specific additional binding sites (exosites) for salmeterol and related compounds, and that exosites exist on non-ligand recognition regions of the [[Beta].sub.2]-adrenoceptor protein. Here, Robert Coleman and colleagues compare and contrast the profiles of action of these long-acting [[Beta].sub.2]-adrenoceptor agonists, particularly as they relate to the role of exosites.

[Beta]-Adrenergic receptor agonists are growth-promoting drugs with the potential for illegal use in livestock, and human toxicity has resulted from consumption of contaminated meat. On-line liquid chromatography with atmospheric pressure chemical ionization mass spectrometry (LC/APCI-MS) was used for sensitive detection of several [Beta]-agonists in retina, a tissue reported to concentrate and retain such residues for extended periods. Multiresidue extraction, separation, detection, and confirmation procedures were developed for retinal tissue and applied to eyes from cattle treated with clenbuterol (69-201 ppb) and to control eyes spiked with salbutamol (100 ppb) and terbutaline (25-100 ppb). Rapid switching of the potential difference between sampling cone and skimmer in the transport region of the API source was used to optimize acquisition of the protonated molecules and characteristic fragment ions obtained by collision-induced dissociation reactions. The respective selected ions were simultaneously acquired using a single quadrupole mass spectrometer. The accurate and precise agreement observed for diagnostic ion intensity ratios between [Beta]-agonists in retinal samples and authentic standards suggests that LC/APCI-MS can be used for confirmation of analyte structure at trace levels and does not require the use of a triple-stage quadrupole mass analyzer.

Alpha-adrenergic stimulation is equally as effective in preserving the inotropic mechanisms for cardiac contractility that can be diminished after heart failure. Alpha-adrenergic pathways are able to fulfill in place of the predominant beta-adrenergic pathways since they produce similar changes in isometric force and contraction of heart muscles in rats. Stimulation by both pathways are similar even when comparisons were made for normal and failing hearts.

Two independent experiments were carried out to determine whether Clenbuterol is accumulated by pigmented tissues. In the first experiment, unpigmented and pigmented rats were injected two times with 5 [[micro]gram] of Clenbuterol subcutaneously and the eyes were analyzed after 63 h of withdrawal by enzyme immunoassay. Only pigmented rat eyes showed a clear accumulation of Clenbuterol (68.1 to 81.5 ng/g), whereas the eyes of unpigmented treated rats demonstrated levels similar to the negative controls (< .27 ng/g). In the second experiment, two Holstein-Friesian calves were fed with .8 [[micro]gram] of Clenbuterol/kg BW two times a day (therapeutic dose) in milk replacer and black and white hair was collected separately before, during, and after the treatment. The hair was analyzed with an enzyme immunoassay that provided blanks less than 1.0 ng/g and a mean recovery of 67%. This experiment showed an accumulation of Clenbuterol in both sets of hair, but a definitely higher accumulation of Clenbuterol in black (pigmented) hair than in white hair. The black/ white quotient amounted to approximately 50 at 1 wk after treatment. Results of these experiments favor the use of the eye as well as the hair for residue analysis but in case of hair, variation of pigmentation in different animals must be considered. Key Words: Clenbuterol, Pigments, Calves, Rats

A study to determine whether the hyporesponsiveness to beta- adrenergic agonists of the airway smooth muscle of a Basenji-greyhound (BG) dog model of airway hyperresponsiveness is associated with increased cholinergic muscarinic inhibition of adenylyl cyclase suggests that muscarinic m2 receptors may be the cause. Quantitative immunoprecipitation data shows increased numbers of m2 but not m3 muscarinic receptors in the BG airway smooth muscle. The enhanced muscarinic receptor numbers thus antagonizes activation of adenylyl cyclase by beta-adrenergic agonists and lead to impaired relaxation of BG airway smooth muscle.

Both beta1-, beta2-, and beta3-adrenergic receptors (AR) are expressed in the colleting tubule of rats, and are predominant in the cortical collecting duct. These ARs trigger (Ca2+)i variations, indicating their possible coupling to several transduction pathways in the rat. Both subtypes of AR appear to contribute to adenosine 3',5'-cyclic monophosphate accumulation induced by beta-agonists.

The continuous treatment of obese mice with beta 3 adrenergic receptor (AR) agonist causes the an increase of brown adipose tissue with a reregulation of the beta 3 AR. This appears to be related to a decrease in plasma glucose, insulin and nonesterified fatty acid levels. A study in which obese and lean mic were given AR agonists for 10 and 20 day periods in order to determine any resulting metabolic alterations is discussed in detail.

To examine the tissue distribution and residues after withdrawal of various [Beta]-agonists (i.e., clenbuterol, salbutamol, and terbutaline) 160 1-d-old broiler chickens were assigned to four groups. During treatment (16 to 35 d), the birds were fed a control diet or a diet containing 1 ppm of clenbuterol, 10 ppm of salbutamol, or 10 ppm of terbutaline. After d 35 all groups received the control diet. Five birds of each group were then slaughtered and tissues were collected on d 0, 1, 2, 3, 7, 14, and 43 following withdrawal of [Beta]-agonists from the feed. Extraction of [Beta]-agonists from the tissues was carried out by a new method using hetero-bifunctional solid phase extraction. The amount of [Beta]-agonists in the extracts was measured by an enzyme immunoassay (EIA). The highest concentrations of [Beta]-agonists were found in feathers: 224 ng of clenbuterol/g, 1,140 ng of salbutamol/g, and 1,159 ng of terbutaline/g. Clenbuterol accumulated above plasma levels in all tissues that were investigated (liver, kidney, stomach, muscle, fat, feather, eye). Salbutamol was most concentrated in feather, eye, liver, and kidney; terbutaline accumulated only in feather, liver, and kidney. Overall, clenbuterol showed the highest accumulation in the tissues analyzed. A withdrawal period of greater than 2 wk was required for residues in edible tissues to decline below detectable levels.

An experiment was conducted to evaluate the interactive effects among porcine somatotropin (pST), salbutamol, and dietary lysine on growth performance, nitrogen balance, and carcass characteristics of finishing barrows (n = 32; 62.8 kg initially). Two replicate 32-d studies were set up in a split-plot design to evaluate singular and combined use of pST (0 or 4 mg/d) and salbutamol (0 or 2.75 ppm of the diet) as whole-plot treatments and diets containing .8, 1.2, 1.6, or 2.0% lysine as subplot treatments. Dietary lysine levels were administered within subplots in a 4 x 4 Latin square with pigs allowed 4 d of adjustment to diets followed by 4 d of urine and feces collection for determination of N retention and apparent digestibility of DM and N. Interactions between lysine and salbutamol were not found (P [is greater than] .16). A pST x lysine interaction (P [is less than] .05) resulted in ADG being maximized at 1.2% lysine for pST-treated pigs (lysine quadratic, P [is less than] .02) but decreased linearly (P [is less than] .02) with increasing lysine for pigs receiving buffer. Pigs injected with 4 mg/d of pST had improved gain:feed (G:F) up to 1.2% lysine (lysine quadratic, P [is less than] .05), but no further increase to 2% lysine, whereas pigs injected with buffer showed no benefit from increased lysine (pST x lysine, P [is less than] .06). Pigs receiving buffer had greater increases in plasma urea N (PUN) as lysine increased than those receiving pST (pST x lysine, P [is less than] .002), but PUN was reduced by pST (P [is less than] .001) regardless of lysine level. Digestibility of N and DM was similar among pST treatments up to 1.6% lysine, but both were greater for pST-treated pigs at 2% dietary lysine (pST x lysine, P [is less than] .03). Salbutamol increased (P [is less than] .01) ADG, G:F, and digestibility of DM and N independently of lysine or pST treatment. Both pST and salbutamol improved (P [is less than] .001) N retention relative to intake and digestible N, but the magnitude of increase was greater for pST than for salbutamol. Percentage of N retained decreased linearly (P [is less than] .001) as dietary lysine increased independently of pST or salbutamol treatment. Carcass fat depth tended to be increased by salbutamol, whereas pST caused dramatic reductions and negated the effects of salbutamol. Conversely, pST and salbutamol additively increased (P [is less than] .05) longissimus muscle area and percentage of muscle. These data suggest that, regardless of salbutamol treatment, pigs injected with 4 mg/d of pST need approximately 29 g/d of lysine (1.2% of the diet) to maximize G:F and ADG.

The level of protein in the diet both before and during treatment with [Beta]-adrenergic agonist (BAA) was investigated with regard to its effect on growth performance and carcass characteristics of pigs. A total of 76 pigs were divided into two groups and given ad libitum access to diets containing either 12 or 18% crude protein (CP) during growth from 15 to 60 kg. At 60 kg, six pigs from each group were slaughtered and carcass composition was determined. For the test period (growth from 60 to 90 kg) the remainder of the pigs were divided into eight groups of eight pigs each. Half of the pigs from each of the two dietary pretreatment groups were switched to the other diet and half remained on the same diet. Of the four dietary groups, half were fed 0 mg/kg (controls) or 1 mg/kg of BRL 47672. During the test period, all pigs were fed at 90% of their calculated ad libitum DE intake. All pigs were slaughtered at 90 kg and carcass composition was determined. Addition of BRL 47672 to the diet resulted in an overall improvement (P [is less than] .05) in daily gain (.72 vs .64 kg) and gain/feed (.28 vs .26); the greatest improvement occurred with pigs fed the 18% CP diet (24 and 14% for daily gain and gain/feed, respectively). Pigs that had received BRL 47672 also had 12% less backfat, 10% less carcass lipid, 16% larger longissimus muscle area, and 5% more carcass protein (P [is less than] .05 for each), and the greatest changes occurred in pigs fed the 18% CP diet (-26, -16, +21, and +10%, respectively). However, comparing the two groups that had been fed the 18% CP diet, those pigs that had received the 12% CP diet before treatment responded less to BAA treatment than those that had received the 18% CP diet throughout the study. This suggests that pretreatment diet may influence subsequent response to BAA treatment.

Fifty-two gilts were used to investigate the effects of two levels of dietary ractopamine (RAC; 0 and 20 mg/kg) and six levels of dietary protein content (DPC; 8.5, 11.2, 14.0, 16.7, 19.5, and 22.2%) on performance over the live weight range from 60 to 90 kg. Pigs were housed in individual pens and restrictively fed their treatment diets according to a sliding scale related to live weight (average intake approximately 7.0 Mcal of DE/d). The ADG increased with increasing DPC for both control and RAC-treated gilts. The RAC further increased ADG at DPC |is greater than~ 14%; improvements were most apparent during the first 3 to 4 wk. There was a significant linear interaction between DPC and RAC for average daily carcass gain. Although carcass gain was not affected by RAC at the three lowest DPC, it was significantly increased for the three highest DPC. Feed to gain decreased with increasing DPC in a similar manner for both control and RAC-treated gilts. The relationship between empty body protein deposition and DPC could be described by a quadratic and rectilinear model. Regardless of the model employed, protein deposition increased with DPC at the same rate for both the control and RAC-treated gilts over at least the two lowest levels of DPC (|is less than~ 11.2%). At higher DPC maximal rates of protein deposition were at least 21% greater in RAC-treated gilts; these rates were achieved at 3% higher DPC for RAC-treated gilts. Fat deposition decreased with increasing DPC but was unchanged by dietary RAC. Because the biological efficiency of protein use was unchanged, RAC-treated gilts have greater dietary protein requirements than controls do.

The effect of beta-adrenergic agonists mediated contractile function during regulation of vinculin distribution in focal and intracellular junctions of the myofibrillar structure in cultured cardiac myocytes is investigated. Mechanical forces underlining the contractile activity is responsible for vinculin distribution regulation and myofibrillar organization in feline heart cells. Cyclic adenosine monophosphate determines the myofibrillar reorganization.

The influence of genetic capacity for lean tissue (LT) growth on responses of pigs to ractopamine, in terms of rate and efficiency of body growth and the distribution and accretion rate of body tissues, was determined in this study. Two sources of pigs representing low and high LT genotypes were used. Within each source, two littermate barrows from each of eight litters were individually penned and given ad libitum access to a lysine-supplemented, corn-soybean meal diet (17.7% CP, 1.08% lysine) containing 0 or 20 ppm of ractopamine hydrochloride from 63 to 104 kg. Carcasses were physically dissected into muscle, fatty tissue, skin, and bone. Within each source, four additional pigs were killed for determination of initial body composition. Pigs of high LT genotype gained BW and muscle faster (P < .01), required less (P < .01) feed per unit of gain, and produced carcasses that contained more (P < .01) muscle and bone and less (P < .01) fatty tissue. Ractopamine increased (P < .01) weight gain and improved (P < .01) feed:gain ratio in both genotypes. Ractopamine enhanced the accretion rate and the amount of carcass muscle in both genotypes, but the degree of improvement was greater in pigs of the high than in those of the low LT genotype (genotype x ractopamine, P < .02). Ractopamine also reduced the accretion rate and amount of dissectible fat by a greater magnitude in the high LT genotype (genotype x ractopamine, P < .04). Based on these data, ractopamine increases muscle accretion to a greater degree in pigs with a high genetic capacity for LT growth than in those with a low capacity. Key Words: Pigs, Ractopamine, Genotypes, Growth, Muscle, Fat, An experiment was conducted from two sources of pigs possessing low and high lean tissue genotypes to study the influence of genetic capacity for lean tissue growth on responses of pigs to ractopamine. Results of the study show an increase in rate and efficiency of growth as well as improvement of carcass characteristics in pigs with ractopamine added diets. It was further shown that this response to ractopamine is greater in pigs with a high genetic capacity for lean tissue growth compared to those with a low genetic capacity.

Calves perorally administered the [Beta]-adrenergic agonist ([Beta]-A) clenbuterol for 28 d were studied before, during, and after a 12-min treadmill exercise. During exercise on d 1 of clenbuterol administration, respiratory rate, respiratory minute volume, and heart rate and blood glucose, lactate, and insulin concentrations increased more in [Beta]-a-treated calves than in controls. Oxygen extraction rate and growth hormone concentrations were lower in clenbuterol-treated calves, whereas oxygen consumption, carbon dioxide production, and blood cortisol concentration increased similarly in the absence and presence of the [Beta]-A. After 2 wk of daily clenbuterol administration, respiratory rate and respiratory minute volume during exercise were still higher and oxygen extraction was still lower, whereas all other measures were similar to those in controls. The increased heart rate in response to isoproterenol after 3 wk of clenbuterol administration was reduced markedly in resting but only slightly in exercising animals, whereas heart rate reduction by propranolol during exercise was similar to that in controls. Seven days after withdrawal of clenbuterol, newly administered clenbuterol evoked the same effects as on d 1. In conclusion, there were marked reactions to the first clenbuterol treatment that were in part enhanced during treadmill exercise. After 2 wk of [Beta]-A administration, animals responded much less to the [Beta]-A and changes were not different from those in controls. Resensitization to the [Beta]-A was observed 7 d after its withdrawal. Key Words: [Beta]-adrenergic Agonists, Clenbuterol, Exercise, Calves J. Anim. Sci. 1992. 70:2800-2821, Results of a study indicated that significant metabolic, endocrine and respiratory changes were observed in calves at early stages of exposure to Beta-A but prolonged administration resulted in de-sensitization to the drug. The study assessed cardio respiratory, metabolic and hormonal adaptation to treadmill exercise, in calves subjected to Beta-adrenergic agonist (Beta-A) clenbuterol, for short and long duration. Resensitization after withdrawal was also assessed. Calves were administered Beta-A clenbuterol, perorally. Results After withdrawal, re-sensitization was observed within seven days of re-exposure to Beta-A.

The safety of the most widely prescribed antiasthma drugs, [[Beta].sub.2]-adrenoceptor agonists, has recently been questioned. Issues such as their suitability for long-term and regular prophylactic use are addressed in this Comment article by Peter Barnes and Fan Chung, who examine the possibility that the [[Beta].sub.2]-agonists themselves contribute to worsening symptoms in asthma patients, thus setting up a vicious circle with greater use of the drugs. They conclude that it would be prudent to restrict the use of [[Beta].sub.2]-agonists in asthma to on-demand immediate symptom control.

[beta-Arrestin]: A Protein That Regulates [beta-Adrenergic] Receptor Function HOMOLOGOUS DESENSITIZATION is a widespread process that causes specific dampening of cellular responses to stimuli such as hormones, neurotransmitters, or sensory signals [...]

The effects of isoproterenol (ISO, a nonselective [Beta]-agonist), terbutaline (TER, a selective/[Beta]2-agonist), CL316243 (CL, a selective [Beta]3-agonist), and epinephrine (EPI, [Beta]- and [Alpha]2-agonist) on in situ lipolytic response of s.c. adipose tissue were investigated in vivo, using a microdialysis method to measure glycerol release, in 12 adult nonlactating and ovariectomized, underfed Lacaune ewes. All the adrenergic compounds were perfused for 120 min at [10.sup.-6], [10.sup.-5], and [10.sup.-4] M. They had no lipolytic effect at [10.sup.-6] M. Isoproterenol and EPI at [10.sup.-5] and [10.sup.-4] M enhanced, in the same way, maximal response and area under the concentration curve (AUC) of dialysate glycerol, thus suggesting that involvement of [Alpha]2-adrenoceptors in the control of in situ lipolysis is of minor importance in underfed ewes. Terbutaline had only a slight lipolytic effect at [10.sup.-5] M. This low effect could be due to a lower affinity of TER than of ISO for the[Beta]-adrenoceptors. The [Beta]3-agonist, CL, had no lipolytic effect whatever the concentration perfused. Further studies are needed to prove the putative presence of [Beta]3-adrenoceptors and their possible role in the ovine adipose tissue. Key Words: Ewe, Adipose Tissue, Dialysis, Lipolysis, [Beta]-Adrenergic Agonists, Epinephrine

Del Castillo, Jesus R., Julio C. Arevalo, Luis Burguillos, and Maria C. Sulbaran-Carrasco. [Beta]-Adrenergic agonists stimulate [Na.sup.+]-[K.sup.+]-[Cl.sup.-] cotransport by inducing intracellular [Ca.sup.2+] liberation in crypt cells. Am. J. Physiol. 277 (Gastrointest. Liver Physiol. 40): G563-G571, 1999.--Epinephrine and [Beta]-adrenergic agonists ([[Beta].sub.1] and [[Beta].sub.2] for isoproterenol, [[Beta].sub.1] for dobutamine, [[Beta].sub.2] for salbutamol) stimulated [K.sup.+] (or [sup.86]Rb) influx mediated by the [Na.sup.+]-[K.sup.+]-2[Cl.sup.-] cotransporter and the [Na.sup.+]-[K.sup.+] pump in isolated colonic crypt cells. Preincubation with bumetanide abolished the epinephrine effect on the [Na.sup.+]-[K.sup.+] pump, suggesting that the primary effect is on the cotransporter. Maximal effect was obtained with 1 [micro]M epinephrine with an [EC.sub.50] of 91.6 [+ or -] 9.98 nM. Epinephrine-induced [K.sup.+] transport was blocked by propranolol with an [IC.sub.50] of 134 [+ or -] 28.2 nM. [Alpha]-Adrenergic drugs did not modify [K.sup.+] transport mechanisms. Neither [Ba.sup.2+] nor tetraethylammonium nor DIDS modified the adrenergic enhancement on the cotransporter. In addition, epinephrine did not affect [K.sup.+] efflux. Dibutyryl cAMP did not alter [K.sup.+] transport. Reduction of extracellular [Ca.sup.2+] to 30 nM did not influence the response to epinephrine. However, 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid-AM abolished epinephrine-induced [K.sup.+] transport. Ionomycin increased [Na.sup.+]-[K.sup.+]-2[Cl.sup.-] co-transport activity. Moreover, epinephrine increased intracellular [Ca.sup.2+] concentration in a process inhibited by propranolol. In conclusion, epinephrine stimulated the [Na.sup.+]-[K.sup.+]-2[Cl.sup.-] cotransporter in a process mediated by [[Beta].sub.1]- and[[Beta].sub.2]-receptors and modulated by intracellular [Ca.sup.2+] liberation. potassium transport; distal colon; guinea pig

Β-Adrenergic Receptor Kinase: Primary Structure Delineates a Multigene Family REGULATION OF CELLULAR SENSITIVITY TO SENSORY, NEU-ROTRANSMITTER, and hormonal stimuli is observed in diverse signaling systems. Attenuation of cellular responsiveness, generally [...]

Rapid β-Adrenergic Modulation of Cardiac Calcium Channel Currents by a Fast G Protein Pathway STIMULATION OF CARDIAC SYMPATHETIC nerves releases norepinephrine, which increases the rate and force of the heartbeat [...]