Comparison of the effects of isoprenaline, orciprenaline, salbutamol and isoetharine on the cardiovascular system of anaesthetized dogs

Understanding the effects on constitutive activation and drug binding of a D130N mutation in the β2 adrenergic receptor via molecular dynamics simulation

G-protein-coupled receptors (GPCRs) are currently one of the largest families of drug targets. The constitutive activation induced by mutation of key GPCR residues is associated closely with various diseases. However, the structural basis underlying such activation and its role in drug binding has remained unclear. Herein, we used all-atom molecular dynamics simulations and free energy calculations to study the effects of a D130N mutation on the structure of β2 adrenergic receptor (β2AR) and its binding of the agonist salbutamol. The results indicate that the mutation caused significant changes in some key helices. In particular, the mutation leads to the departure of transmembrane 3 (TM3) from transmembrane 6 (TM6) and marked changes in the NPxxY region as well as the complete disruption of a key ionic lock, all of which contribute to the observed constitutive activation. In addition, the D130N mutation weakens some important H-bonds, leading to structural changes in these regions. Binding free energy calculations indicate that van der Waals and electrostatic interactions are the main driving forces in binding salbutamol; however, binding strength in the mutant β2AR is significantly enhanced mainly through modifying electrostatic interactions. Further analysis revealed that the increase in binding energy upon mutation stems mainly from the H-bonds formed between the hydroxyl group of salbutamol and the serine residues of TM5. This observation suggests that modifications of the H-bond groups of this drug could significantly influence drug efficacy in the treatment of diseases associated with this mutation.

Involvement of alpha 2-adrenoceptors in mechanism of intragastric nicotine protection against ethanol injury in rat stomach

To elucidate the role of alpha- and beta-adrenoceptors in the mechanism of intragastric nicotine protection against ethanol-induced gastric mucosal injury, the following studies were performed. At 0.5-hr prior to the injury study, rats were pretreated with: subcutaneous control, prazosin (0.5 mg/kg) or yohimbine (5 mg/kg) to block alpha 1- or alpha 2-adrenoceptors; or intraperitoneal control, metoprolol (2 mg/kg) or butoxamine (4 mg/kg) to block beta 1- or beta 2-adrenoceptors, respectively. At 1-hr intervals, rats received intragastric vehicle or nicotine (4 mg/kg) and 40% ethanol (10 ml/kg). Total lengths of the linear gastric corpus mucosal lesions were measured by an unbiased observer using a caliper. In a separate study, 0.5-hr after subcutaneous control or yohimbine (5 mg/kg), rats were treated with intragastric vehicle or nicotine (4 mg/kg). One hour later, gastric mucus volume, gastric juice volume and pH, and titratable acid in the gastric juice were measured. In the rat stomach, the intragastric nicotine protection against 40% ethanol-induced mucosal injury was not blocked by selective alpha 1-(prazosin), beta 1-(metoprolol), or beta 2-(butoxamine) adrenoceptor antagonists. The protection was significantly reduced although not completely abolished by selective alpha 2-(yohimbine) adrenoceptor antagonist. Yohimbine also significantly reduced basal and nicotine-stimulated increase in gastric mucus volume. These data suggest that alpha 2-adrenoceptors are involved in the protective effect of intragastric nicotine against 40% ethanol-induced gastric mucosal injury possibly by a mucus-dependent mechanism.

Intragastric nicotine protects against 40% ethanol-induced gastric mucosal injury despite pretreatment with propranolol or N-ethylmaleimide in rats

We tested the hypotheses that the protective effect of intragastric nicotine against ethanol-induced gastric mucosal injury is dependent on propranolol- or N-ethylmaleimide-sensitive mechanisms. Propranolol was administered in doses (2 and 20 mg/kg) that provided dose-related blockade of beta-adrenoceptors (significant decreases in heart rate). N-Ethylmaleimide was administered in doses that previously had been shown to increase gastric vascular permeability (10 mg/kg) or inhibit gastric mucosal sulfhydryl compounds (50 mg/kg). At 0.5 hr after these or control subcutaneous pretreatments, the rats received intragastric nicotine (4 mg/kg) or vehicle. One hour later 40% ethanol was given intragastrically. The gastric corpus mucosal lesions were recorded by polaroid photographs after another hour, and their areas measured unbiasedly by computerized image analysis. The results showed that N-ethylmaleimide, but not propranolol, aggravated ethanol-induced gastric mucosal injury. The protective effect of intragastric nicotine was not modified by either pretreatment. We conclude that the mechanism mediating intragastric nicotine protection against 40% ethanol-induced gastric mucosal injury is independent of propranolol- or N-ethylmaleimide-sensitive mechanisms.

In vivo inhibition of polymyxin B-induced hypotension: evidence of beta-adrenergic inhibitory activity on rat mast cells

In vitro, rat mast cells show a dose-inhibition profile when stimulated with compound 48/80 in the presence of isoprenaline. Isoprenaline-induced inhibition is stereoselective at low concentrations, being greater on pleural mast cells. In vitro, polymyxin B (0.02-0.05 mg/kg) induces a marked hypotension and slow heart rate, but not tachyphylaxis. Both effects were not suppressed in the presence of isoprenaline (with short half-life in vivo), but orciprenaline does induce a clear inhibition of Polymyxin B-induced hypotension, which correlates well with heart rate.

Enhanced arrhythmogenic activity of beta-adrenoceptor stimulants in desoxycorticosterone-pretreated rats

The arrhythmogenic activity of four beta-adrenoceptor stimulants, metaproterenol, salbutamol, isoetharine, and dobutamine, was determined in adult, Wistar rats pretreated for 3-4 weeks with desoxycorticosterone (DOCA) and 1% saline as drinking fluid. Doses (SC) of these stimulants which were well tolerated in untreated animals elicited severe ventricular dysrhythmias, often leading to death in ventricular fibrillation. The LD50 in mg/kg was as follows: metaproterenol, 0.28; salbutamol, 0.60; isoetharine, 2.3; and dobutamine, 4.8. The relative potency of metaproterenol, salbutamol, and isoetharine correlates well with their ability to stimulate beta 1-adrenoceptors. It is suggested that DOCA-saline pretreatment in rats may be used as a model for the rapid screening of drugs affecting beta-adrenoceptors.

The role of beta1- and beta2-adrenoceptors in the inhibition of gastric acid secretion in the dog

1. Characterization of the beta-adrenoceptors mediating inhibition of gastric acid secretion in the conscious Heidenhain pouch dog has been investigated by determination of the effects of propranolol, (+)-propranolol, practolol and H35/25 on salbutamol and isoprenaline-induced inhibition of gastric acid secretion. 2. The gastric antisecretory effect of salbutamol was significantly blocked by propranolol and H35/25 but not by practolol or (+)-propranolol. The effect of isoprenaline was significantly blocked by propranolol and practolol but not by H35/25 or (+)-propranolol. 3. It is concluded that both beta1- and beta2-adrenoceptors can mediate inhibition of pentagastrin-induced gastric secretion in conscious dogs with a Heidenhain pouch. Salbutamol exerts its antisecretory effect through beta2-adrenoceptors, whereas isoprenaline mediates its effects primarily through beta1-adrenoceptors. 4. The results are discussed with regard to the sub-classification of beta-adrenoceptors and to the possible role of adrenoceptors in the physiological control of gastric secretion. 5. In this study it is concluded that the tachycardia induced by isoprenaline or salbutamol is mediated primarily through reflexes activated by beta2-adrenoceptor mediated vasodilatation.

Lack of cardiac or bronchodilator tachyphylaxis to isoprenaline in the dog

1. Tachyphylaxis to heart rate and bronchodilator effects of (plus or minus)-isoprenaline was studied in anaesthetized open-chest dogs by 5 procedures. 2. Heart rate responses to a series of intravenous injections of isoprenaline were essentially unchanged before and after a 15 min infusion of isoprenaline (at 3 doses). 3. Heart rate and bronchodilator responses in the same animal to an intravenous injection of isoprenaline were not significantly different before and after a 30 min infusion of isoprenaline (at 2 doses). 4. Heart rate responses were relatively constant to an isoprenaline injection given every 30 min during a 4 h infusion of isoprenaline (at 3 doses), but the magnitude of the response was smallest for the largest infusion dose (highest background heart rate) and greatest for the smallest infusion dose (lowest background heart rate). 5. Heart rate and bronchodilator responses in the same animal to isoprenaline were relatively constant during a 5 h infusion of isoprenaline (at 2 doses). 6. Bronchodilator responses to intratracheally administered isoprenaline aerosol were essentially unchanged during a 4-5 h period using various doses and procedures. 7. Tachyphylaxis to the heart rate of bronchodilator effects of isoprenaline was not observed. The present data give no support to the hypothesis that tachyphylaxis to isoprenaline aerosols is an important mechanism in asthma mortality.

Metabolic and cardiovascular effects of isoprenaline and salbutamol in the dog

1 The intravenous infusion of isoprenaline and salbutamol into the greyhound increased heart rate and levels of free fatty acids, lactic acid and glucose. 2 On terminating the infusions of isoprenaline the changes produced declined rapidly but the effects produced by salbutamol were more persistent. When high doses of salbutamol had been infused, glucose and lactic acid levels in fact increased during the 20 min following the infusions. 3 These results support suggestions that, in the dog, lipolysis is mediated by beta1-adrenoceptors and liver glycogenolysis by beta2-adrenoceptors. The beta-adrenoceptors mediating muscle glycogenolysis could not be assigned inequivocally to either subtype. 4 The differences in the behaviour of isoprenaline and salbutamol in the period following the infusions are considered to be due partly to slower removal of salbutamol. Increases in lactic acid levels after infusion of large amounts of salbutamol may be secondary to the persistence of high glucose levels.

Effects of adrenaline, noradrenaline, isoprenaline and salbutamol on the production and release of renin by isolated renal cortical cells of the cat

1 Isolated renal cortical cells of the cat have been demonstrated to produce renin on incubation in vitro. After 2 h of incubation, without added agonist, the total amount of renin in the flask increased by a mean of 27.2 percent. The increase in renin content of the incubation flask was found to be present in the medium. 2 Noradrenaline (1.18 times 10-minus 4M) and adrenaline (1.09 times 10-minus 4M) added to the incubation medium stimulated renin production by 45 and 34% respectively, compared with the incubated controls. Most of the increase in renin production was present in the incubation medium. 3 Isoprenaline did not stimulate renin production. However, when added to the incubation medium at a concentration of 0.72 times 10-minus 4M there was a significant decrease in the cellular content and a significant increase in the medium content of renin. This increase was at least as great as that observed with adrenaline and noradrenaline. 4 Salbutamol had an effect similar to isoprenaline, i.e. it induced the release of renin into the medium without affecting production. In this respect it was about a third as potent as isoprenaline.