Academic literature on the topic 'B2-adrenoceptors'

Dexmedetomidine (Dex) is an alpha-2 agonist used for sedation during various procedures. Dex activates 2-adrenoceptors, and causes the decrease of sympathetic tone, with attenuation of the neuroendocrine and hemodynamic responses to anesthesia and surgery; it reduces anesthetic and opioid requirements; and causes sedation and analgesia. Objective: it was to compare the perioperative effects of different doses of Dex at different timing in patients using Dex during the perioperative period adopting a medical data classification algorithm based on optimized semi-supervised collaborative training (Tri-training). Methods: 495 patients requiring surgical treatment in Xingtai People’s Hospital were randomly selected as the study subjects. The patients were divided into group A (used before induction), group B (used during induction), and group C (used after induction) according to different induction timing, with 165 cases in each group. Then, groups A, B, and C were divided into groups A1, B1, and C1 (0.4 μg/(kg·h) rate), groups A2, B2, and C2 (0.6 μg/(kg·h) rate), and groups A3, B3, and C3 (0.8 μg/(kg·h) rate) according to the dose used, with 55 cases in each group. Intraoperative anesthesia and postoperative adverse reactions were compared among the 9 groups. Results: the similarity between the Tri-training algorithm optimized by Naive Bayes (NB) classification algorithm and the actual classification (93.49%) was clearly higher than that by decision tree (DT) and K-nearest neighbor (kNN) classification algorithm (76.21%, 74.31%); heart rate (HR), systolic blood pressure (SBP), and diastolic blood pressure (DBP) values decreased obviously after Dex used in groups B1, C1, B2, C2, B3, and C3 (P <0.05), but did not change significantly in groups A1, A2, and A3 (P >0.05); the proportion of patients with satisfactory Ramsay score in group A3 was distinctly superior than that in groups A1, A2, B1, B2, B3, C1, C2, and C3 (P <0.05); the incidence of adverse reactions in group A3 was significantly inferior than that in groups A1, A2, B1, B2, B3, C1, C2, and C3 (P <0.05). Conclusion: the optimization effect of NB classification algorithm was the best, and the injection of Dex at the injection rate of 0.8 μg/(kg·h) before induction of anesthesia could apparently improve the fluctuation of HR, SBP, and DBP during perioperative period, and effectively reduce the occurrence of adverse reactions in patients, with better sedative effect on patients.

Bradykinins have only recently been identified in fish, and a detailed analysis of their cardiovascular actions is lacking. The present study examines the cardiovascular effects of trout bradykinin ([Arg0,Trp5,Leu8]bradykinin; tBK) in conscious trout, Oncorhynchus mykiss. tBK (1-10 nmol/kg body wt bolus) produced triphasic pressor-depressor-pressor responses. In phase 1, cardiac output (CO), ventral aortic (P(VA)), dorsal aortic (P(DA)), and central venous pressure increased, whereas systemic (R(S)) and gill resistance (R(G)) were unchanged. In phase 2, R(G) increased, whereas R(S), CO, and heart rate decreased, reducing P(VA) and P(DA). Plasma prostaglandin E2 and the prostacyclin metabolite, 6-ketoprostaglandin F1alpha, were significantly elevated during phase 2, whereas leukotrienes C4 and B4 and thromboxane B2 were unaffected. Phase 3 was produced by an increased CO and R(S) and the return of R(G) to control. Phase 1 pressor response was not blocked by inhibitors of cyclooxygenase, angiotensin-converting enzyme (ACE) or alpha-adrenoceptors (alpha-AD), whereas phase 2 depressor and plasma prostaglandin responses were prevented by cyclooxygenase inhibition. Phase 3 was partially blocked by ACE and alpha-AD inhibitors and is a response to the preceding hypotension. In vitro, tBK only decreased vascular resistance in the perfused splanchnic or skeletal muscle-kidney preparations. These results show that although tBK has multiple effects on the trout cardiovascular system, none of the effects are due to direct tBK stimulation of vascular smooth muscle. Phase 2 vasodilation has features consistent with release of vasodilator prostaglandins while the mechanism of phase 1 constriction is unknown.

There were two main objectives for this study. The first of these was to develop a research methodology with which both the affinity and efficacy of B-adrenoceptor agonists on protein degradation in skeletal muscle could be studied.

B2-Adrenoceptors were isolated from toad skeletal muscle and were characterised in radioligand binding experiments. The resultant affinity constants for 14 drugs were compared with values obtained from bovine Bradrenoceptors, and paired t-test analysis indicated that no significant difference existed between these two populations of B2-adrenoceptors. Thus, toad skeletal muscle B2-adrenoceptors provide a suitable model for screening B-ligand affinities, and values may be extrapolated to the corresponding receptors in cattle (target species).

Recombinant Escherichia coli expressing human B1,-and human B2-adrenoceptors were also characterised in radioligand binding experiments. The data obtained verified that each clone provided a source of homogeneous receptors, which displayed the ligand binding affinities appropriate for the respective receptor subtype. Thus, recombinant bacteria are a useful tool for screening the subtype-selectivity of ligands.

The bacterial clone expressing human B2-adrenoceptors displayed ligand binding affinities that were not significantly different to drug affinity determinations performed with bovine B2-adrenoceptors. Thus, the B2-strain is a useful model of the Bradrenoceptor expressed in the skeletal muscle of cattle. Additionally, recombinant B2-adrenoceptors exhibited differential binding behaviour for agonists and antagonists: all antagonists tested displayed monophasic displacement curves, whereas agonist molecules having molecular weight of less than 250 g/mol and an octanol/water partition coefficient (log P) less than zero, displayed biphasic binding behaviour. From these experiments, the cloned Escherichia coli cell lines were selected as the preferred source of B2-adrenoceptors for radioligand binding screens.

The B-adrenoceptor agonist efficacy assay selected for this study was the isolated perfused rat hemicorpus. In the first experiment, the rate of 3-methylhistidine (3-MH) release from perfused hemicorpora was examined: the 3 MH efflux rate of 0.227 ± 0.009 Amo1/30 g/3 hours corresponded to a protein degradation rate of 8.7 ± 0.3 %/day. While this value was comparable to other estimates reported in the literature, the HPLC analytical technique used to quantify 3-MH was at the lower limit of detection sensitivity. This author was not confident that the small changes in 3-MH release that may result from B-agonist treatment would be detected using this method. Hence, an alternative approach of measuring proteolysis as a function of other essential amino acids was examined. With this second method, reincorporation of amino acid markers into protein was prevented by the protein synthesis inhibitor, cycloheximide. In the presence of radiolabelled phenylalanine and leucine, the addition of 40 Amo1/1 cycloheximide to the perfusion medium was found to inhibit protein synthesis by 78 to 90%, without influencing the composition of the free amino acid pools within muscle. Furthermore, this radiolabel incorporation data (for rat hindlimbs perfused in the absence of cycloheximide) enabled the protein synthetic rate of the muscle to be estimated at between 2.0 and 3.06 %/day.

When proteolysis is measured as a function of the release of essential amino acids, the rate of myofibrillar protein breakdown is indistinguishable from non-myofibrillar breakdown. Thus, specific increases in myofibrillar degradation may potentially be undetected. However, the probability of detecting significant changes in degradation may be improved by co-analysing a total of eight essential and semi-essential amino acids. This approach was endorsed when the amino acid efflux rate from rat hemicorpora perfused in the presence and absence of cycloheximide were examined: logically, the blockade of protein synthesis would be expected to result in elevated rates of amino acid efflux. Indeed, the amino acid efflux rates from cycloheximide treated tissues were consistently, though not necessarily significantly, higher than the rates determined in cycloheximide-free tissues. However, when a paired t-test analysis was performed with all eight amino acids, the level of statistical significance improved dramatically.

The results from perfusion experiments indicated that all subsequent studies of protein degradation should be performed in the presence of 40/Imola cycloheximide; and the essential amino acids, valine, methionine, tryptophan, phenylalanine, isoleucine, lysine, leucine, and the semi-essential amino acid tyrosine should be used as co-indices of the rate of protein degradation.

When rat hemicorpora were perfused in the presence of 40 Amo1/1 cycloheximide, the mean ± SEM rate of potassium released from 30 grams of skeletal muscle was 740 ± 60 Amo1/3 hours. Likewise, the mean ± SEM rate of glucose consumed by 30 grams of skeletal muscle was 330 ± 20 Amo1/3 hours. HPLC analysis of muscle amino acids derived from both intact rats, and from hemicorpora which had been perfused for three hours, revealed that the intracellular free amino acid content of muscle was not significantly altered by the perfusion process. Thus, the rate of protein degradation in perfused muscle tissue was estimated directly from the rate of amino acid efflux for eight essential/semi-essential amino acids.

The second major objective of this study was to use the rat hemicorpus preparation to determine whether the B2-adrenoceptor agonist, clenbuterol, mediates its growth promoting effects on skeletal muscle through a reduction in the rate of myofibrillar protein degradation.

Clenbuterol treatment (4 mg/kg diet, 265 μg/kg BWT/day) resulted in net protein accretion, and significantly enhanced the growth rate of treated animals. In mature male rats, clenbuterol treatment in the diet increased the rate of weight gain to 260% of the untreated level. However, the magnitude of the growth response declined after six days of treatment, which is consistent with B-adrenoceptor desensitisation. Clenbuterol-induced changes in blood flow to different tissues was evident from the change in the colour of adipose tissue in abdominal fat depots.

Clenbuterol treatment had no significant effect on either the perfusate pressure range or the rate of glucose uptake in perfused rat hemicorpora. In contrast, potassium efflux rates in chronically treated hindlimb preparations were elevated by 95%. The increase in potassium efflux may have been an artefact of the cycloheximide-induced inhibition of protein synthesis. However, more recent evidence of elevated plasma potassium levels in cows chronically treated with clenbuterol indicate that this may be a real effect of long-term B-agonist treatment.

When the rate of protein degradation was evaluated in terms of the sum of essential amino acids effluxed from perfused rat hemicorpora, acute (three hours) clenbuterol treatment caused a significant, but transient increase in proteolysis (11%). In contrast, longer-term clenbuterol treatment (6 and 12 days) effected a non-significant decrease in the rate of protein degradation (11.8 and 7.6%, respectively), and this effect was diminished over time, in a manner consistent with B-adrenoceptor desensitisation. Furthermore, the withdrawal of clenbuterol from chronically-treated tissues resulted in an immediate and significant increase (25.1%) in the rate of protein degradation, which indicates that the continued presence of clenbuterol is required to maintain the effect of reduced proteolysis.

Clearly, the reduction in the rate of protein degradation observed with chronic clenbuterol treatment is insufficient to account for the 160% increase seen in the growth rate of treated rats. Consequently, it can be concluded that protein synthetic rates must have also been elevated. Indeed, it has been estimated that the 160% increase in the growth rate of clenbuterol-treated rats could be accounted for by a net 16.4% change in the component processes of protein turnover (ie an 11.8% reduction in protein breakdown, and a 4.6% increase in protein synthesis). This is equivalent to a 10.25% change in protein turnover for every 100% increase in growth rate, and compares exceptionally well with the 10 to 13% change estimated by Kim and Sainz (1992).

From these studies, it was concluded that the mechanism of clenbuterol-induced muscle accretion includes components of both increased protein synthesis and decreased protein degradation. In tropical cattle, this saving in metabolic energy resulting from decreased proteolysis may offer an advantage to the animal during the dry winter months where seasonal weight losses are often observed.

The aim of this project was to investigate physiological methods for increasing growth rate in beef cattle. The effect of glucocorticoids with or without clenbuterol on B₂- adrenoceptors (B₂-AR) in skeletal muscle and the effect of fasting on serum leptin were studied.

Muscle growth can be promoted in meat producing animals, humans, and experimental animals by the use of B-agonists. However, these anabolic effects rapidly attenuate due to a decrease in B₂-AR density in skeletal muscle following treatment. Glucocorticoids were demonstrated to increase B₂-AR density in rat lung. Thus, the first section of the project was designed to determine whether glucocorticoids could prevent clenbuterol-induced downregulation of B₂-ARs.

Corticosterone, dexamethasone and clenbuterol were tested for effects on muscle growth and B₂-AR density in male rats. Corticosterone caused a dose-related decrease in total weight gain and gain in gastrocnemius/plantaris bundle (GP muscle), but failed to increase B-AR density in the muscle at any of doses tested (6.25-50mg/kg, sc 5d). GP muscle and the ratio of gain/food intake were increased in the group treated with clenbuterol and decreased in the group treated with dexamethasone when administrated daily for 10d. B-AR density in GP muscle was decreased by both clenbuterol and dexamethasone. In contrast, B-AR density in rat lung was increased by dexamethasone and decreased by clenbuterol. hi concurrent treatment group, dexamethasone prevented clenbuterol-induced downregulation of B₂-ARs in rat lung, but not in GP muscle. Thus, dexamethasone does not enhance the anabolic effect of clenbuterol, but causes a marked catabolic effect instead. The results demonstrate that a B-agonist/glucocorticoid combination would not be useful in animal production, and the discovery that B₂-drenoceptors are regulated differently in different tissues warrants further investigation.

The leptin system is postulated to be a key regulatory hormonal pathway associated with the regulation of body weight through effects on metabolism and appetite both in humans and in rodents. Thus, in the second section of this study, an immunological method for detecting bovine leptin and the effects of fasting on circulating leptin, cortisol, glucose and FFA in beef cattle were investigated.

Polyclonal antibodies against bovine recombinant leptin were produced by immunizations of mice, rats, sheep and cattle. Immunization resulted in a positive response with maximum titres assayed by ELISA of 800 in 6/16 of the C57 BL/6 mice and 4/8 of cattle, 3000 in 7/16 rat, and 102400 in 3/3 sheep. No immune response was observed in BALB/c mice. Spleen lymphocytes derived from immunized rats were fused with the murine myeloma cells. Eight hundred hybridomas were cultured. Unfortunately, no leptin specific monoclonal antibody was found on screening the supernatants from these clones.

Cattle and sheep antibodies were affinity purified and conjugated with horseradish peroxidase or biotin. Two immunoassays, avidin-biotin ELISA and competitive ELISA, were developed for determining bovine leptin. The detection limit for recombinant bovine leptin was 500ng/m1 in the avidin-biotin ELISA and 5ng/m1 in the competitive ELISA. The application of the competitive ELISA developed in this study was tested by measuring leptin concentrations of bovine serum and serum plus standards. The results showed that the leptin concentration of different diluted serum was 45.4 ± 3.31 ng/ml.

However, the concentrations of leptin of 1:2 diluted serum plus each standard were all higher than expected, possibly due to the presence of reserve leptin binding proteins in the serum. Thus, after the problem of putative interference of binding protein in serum is resolved, determination of large numbers of sample from beef cattle, rapidly and economically using the competitive assay, will be possible.

The effect of 24 h fasting on serum leptin levels was studied in beef cattle. Serum leptin 4.17 ± 0.11 ng human equivalent /ml was detected in these cattle using a human leptin RIA kit. The leptin levels of fasting animals decreased as compared to the animals before fasting and re -feeding. Fasting caused a 72.33 ± 28.18 % increase in serum cortisol and 203.01 ± 30.94 % increase in plasma FFA. Plasma glucose did not change. A positive correlation between serum leptin concentrations and body weight were observed in seven cattle (before fasting: r2 = 0.663, P = 0.026; after re-feeding: r2 = 0.902, P = 0.0011), but one other animal was outlying. These results indicated that depriving cattle of food caused a decrease of serum leptin and an increase of stress steroids and plasma FFA, while glucose level maintained normal, which demonstrated that leptin levels are readily influenced by physiological changes and are consistent with a homeostatic signaling function for this hormone.

According to the findings of the fasting study, further work would be needed: (1) to investigate whether there are some relationships between leptin and FFA, glucose, insulin and other hormones in circulation in beef cattle as in rats and mice; and (2) to establish whether immunization of cattle with leptin will decrease serum leptin and whether lower leptin level will stimulate food intake, increase growth and enhance the productivity of beef cattle.