Beta-adrenoceptors in equine trachea and heart

Comparison of single-breath continuous positive airway pressure manoeuvre with inhaled salbutamol to improve oxygenation in horses anaesthetized for laparotomy

OBJECTIVE

To compare the efficacy of single-breath continuous positive airway pressure manoeuvre (CPAP-M) with inhaled salbutamol, and a combination of both.

STUDY DESIGN

Randomized, clinical study.

ANIMALS

A total of 62 client-owned horses (American Society of Anesthesiologists status III-V) anaesthetized for laparotomy.

METHODS

Horses were premedicated with intravenous (IV) xylazine (0.4-0.6 mg kg^-1), anaesthesia was induced with midazolam (0.06 mg kg^-1 IV) and ketamine (2.2 mg kg^-1 IV) and maintained with isoflurane in oxygen using volume-controlled ventilation without positive end-expiratory pressure. If PaO₂ was < 100 mmHg (13.3 kPa), either a CPAP-M (50 cmH₂O for 45 seconds) or salbutamol (0.002 mg kg^-1) was administered. The intervention was considered successful if PaO₂ reached 100 mmHg (13.3 kPa). If PaO₂ remained < 100 mmHg (13.3 kPa), treatments were switched. PaO₂/FiO₂ ratio and estimated shunt fraction (F-shunt) were derived from data obtained from arterial blood gas measurements. Dynamic compliance (C_dyn) was calculated from variables recorded at the moment of arterial blood analysis. Fisher's exact tests compared success rates between treatments, and linear models were performed to test whether the treatment modified the values of the measurements; p < 0.05.

RESULTS

Salbutamol was the first intervention in 28 horses and was effective in 22 horses. CPAP-M was the first intervention in 34 horses and was effective in 26 horses. CPAP-M after salbutamol was performed in six horses, with four responders, and salbutamol after CPAP-M was administered to eight horses, with one responder. Salbutamol, but not CPAP-M, significantly decreased F-shunt. Both salbutamol and CPAP-M significantly increased C_dyn.

CONCLUSIONS AND CLINICAL RELEVANCE

Salbutamol and CPAP-M were comparably effective in improving oxygenation and C_dyn in anaesthetized horses with PaO₂ < 100 mmHg (13.3 kPa). Whether combining both treatments might be beneficial needs to be confirmed on a larger number of horses.

The Effect of the Clenbuterol-β2-Adrenergic Receptor Agonist on the Peripheral Blood Mononuclear Cells Proliferation, Phenotype, Functions, and Reactive Oxygen Species Production in Race Horses In Vitro

Clenbuterol, the β2-adrenoceptor agonist, is gaining growing popularity because of its effects on weight loss (i.e., chemical liposuction). It is also popular in bodybuilding and professional sports, due to its effects that are similar to anabolic steroids. However, it is prohibited by anti-doping control. On the other hand, it is suggested that clenbuterol can inhibit the inflammatory process. The cells from 14 untrained and 14 well-trained race horses were collected after acute exercise and cultured with clenbuterol. The expressions of CD4, CD8, FoxP3, CD14, MHCII, and CD5 in PBMC, and reactive oxygen species (ROS) production, as well as cell proliferation, were evaluated by flow cytometry. In addition, IL-1β, IL-4, IL-6, IL-10, IL-17, INF-γ and TNF-α concentrations were evaluated by ELISA. β2-adrenoceptor stimulation leads to enhanced anti-inflammatory properties in well-trained horses, as do low doses in untrained animals. In contrast, higher clenbuterol doses create a pro-inflammatory environment in inexperienced horses. In conclusion, β2-adrenoceptor stimulation leads to a biphasic response. In addition, the immune cells are more sensitive to drug abuse in inexperienced individuals under physical training.

Effects of intravenous terbutaline on heart rate, arterial pressure and blood gases in anesthetized horses breathing air

OBJECTIVE

To investigate the effects of intravenous (IV) administration of terbutaline on PaO₂, PaCO₂, pH, heart rate (HR) and arterial pressures in healthy, laterally recumbent horses breathing ambient air under total intravenous anesthesia (TIVA).

STUDY DESIGN

Prospective experimental study.

ANIMALS

Eight healthy adult horses were enrolled. Six horses, four mares and two geldings weighing 433-624 kg, completed the study.

METHODS

Horses were sedated with xylazine (1.0 mg kg^-1) IV for placement of arterial and venous catheters. Anesthesia was induced with midazolam (0.1 mg kg^-1) and ketamine (2.2 mg kg^-1) IV and maintained with an IV infusion of guaifenesin (50 mg mL^-1), ketamine (2 mg mL^-1) and xylazine (0.5 mg mL^-1) at 1.9 ± 0.3 mL kg^-1 hour^-1. Horses were in left lateral recumbency and breathed air spontaneously. Arterial blood was collected for pH and blood gas analysis during xylazine sedation, 15 minutes after induction of anesthesia, immediately before and 5, 15 and 30 minutes after administration of terbutaline (2 μg kg^-1), and when the horse was standing after recovery from anesthesia. HR, systolic (SAP), mean (MAP) and diastolic (DAP) arterial pressures were recorded at 5 minute intervals during anesthesia. Normal data were analyzed with anova and non-normal data were analyzed with a Friedman test with a p < 0.05 considered significant.

RESULTS

The mean PaO₂ decreased from baseline to <60 mmHg (8.0 kPa) during anesthesia (p < 0.0001) and did not improve after administration of terbutaline. After terbutaline administration, HR increased (p = 0.002), and SAP, MAP and DAP decreased (p < 0.001) with the greatest changes occurring immediately after terbutaline administration.

CONCLUSIONS AND CLINICAL RELEVANCE

Terbutaline (2 μg kg^-1) IV did not improve PaO₂ and was associated with adverse cardiovascular effects during TIVA in healthy, laterally recumbent horses breathing air.

Cardiovascular changes after administration of aerosolized salbutamol in horses: five cases

Prevention and treatment of intraoperative hypoxemia in horses is difficult and both efficacy and safety of therapeutic maneuvers have to be taken into account. Inhaled salbutamol has been suggested as treatment of hypoxia in horses during general anesthesia, due to safety and ease of the technique. The present report describes the occurrence of clinically relevant unwanted cardiovascular effects (i.e. tachycardia and blood pressure modifications) in 5 horses undergoing general anesthesia in dorsal recumbency after salbutamol inhalation. Balanced anesthesia based on inhalation of isoflurane in oxygen or oxygen and air and continuous rate infusion (CRI) of lidocaine, romifidine, or combination of lidocaine and guaifenesine and ketamine was provided. Supportive measures were necessary to restore normal cardiovascular function in all horses but no long-term adverse effects were noticed in any of the cases.

Are so many adrenergic receptor subtypes really present in domestic animal tissues? A pharmacological perspective

Adrenergic receptors (ARs) are the cellular membrane binding sites through which natural catecholamines and sympathomimetic drugs exert their physiological and pharmacological effects. In recent decades, studies to clarify the distribution and function of ARs have been performed mostly on cultured cells, laboratory animals and human target tissues, but little is known about these aspects in domestic animals. This review focuses on AR structure, classification and signalling pathways and on AR subtype distribution in target tissues of some domestic animals, namely dogs, horses and bovines. In these species, different alpha- and beta-AR subtypes have been characterized and the functions controlled by the adrenergic systems have been studied. In the dog, the role played by the adrenergic system in the pathogenesis of cardiovascular disorders and in the modulation of canine aggression has roused particular interest. In dogs affected by dilated cardiomyopathy a significant down-regulation of beta-ARs has been observed both in the heart and circulating lymphocytes. This finding confirms the involvement of the adrenergic system in the pathogenesis and progression of the disorder and suggests new therapeutic strategies. In the horse, AR distribution has been studied in the cardiac, respiratory and gastrointestinal systems as well as in digital veins and arteries. The cardiac beta-ARs in healthy horses seem to be predominantly represented by the beta(1) subtype. In this species, heart failure may increase the expression of the beta(2) subtype, rather than causing AR down-regulation. Different beta- and alpha-AR subtypes have been characterized in the smooth muscle of equine ileum. The sympathetic relaxation of equine ileum smooth muscle seems to depend mainly on beta(3)-AR subtype activation, with minor involvement of the beta(2) subtype. In the respiratory tract, regional differences have been evidenced in the functionality of beta-AR subtype. The beta(2) subtype predominates in all segments but the beta(2) subtype-mediated adenyl cyclase response is tissue-dependent, with higher activity in tracheal membranes than bronchial or pulmonary ones. Both alpha- and beta-AR subtypes are present in the genital tract of cows. Bovine ovarian and myometrial cell membranes express higher concentrations of beta(2)-ARs than the beta(1) subtype, whereas as far as alpha-ARs are concerned, a single class of alpha(1)-ARs and two distinct classes of alpha(2)-AR binding sites have been discriminated. Interestingly, it has been observed that the activation of the sympathetic system could play an important role in the pathogenesis of bovine ovarian cysts as suggested by the modifications in beta-AR levels in the hypophysis and ovary of cows affected by ovarian cysts. In this species, the phenomenon of down-regulation has been well studied in different organs of veal calves treated with clenbuterol as a "partitioning agent". Since differences exist in AR distribution among species, data obtained in laboratory animals or in human beings cannot be extrapolated to domestic animals and further investigation on AR subtypes in domestic animal tissues is necessary.

Muscarinic receptors in equine airways

The distribution of muscarinic receptors in equine airways was investigated using autoradiography. Frozen sections of tissue from six different levels in the bronchial tree, from the trachea to the distal bronchioles, were incubated in vitro with 1.5 nmol/L of the muscarinic receptor antagonist 1-[N-methyl-3H]scopolamine methyl chloride (3H-NMS). In addition, the subtype pattern of muscarinic receptors was investigated in equine tracheal smooth muscle using radioligand binding with methoctramine, tripinamidc, 4-DAMP-methiodide and pirenzipine as competitors against the binding of 1.3 nmol/L 3H-NMS. The autoradiograms showed specific labelling indicating a high density of muscarinic receptors in smooth-muscle tissue in all levels of the airway tree investigated. Besides muscle tissue, subepithelial glands were the only structures specifically labelled. The dominating subtypes in tracheal smooth muscle investigated with radioligand binding studies were found to be M2 and M4, as both methoctramine (pKd = 8.5) and tripinamide (pKd = 8.6 and 6.7 for two different sites) showed high affinity. The density of the M3-muscarinic receptor subtype was low, but this subtype could be detected with statistical significance when methoctramine was used as the competitor against 3H-NMS binding.

Density and binding characteristics of beta-adrenoceptors in the normal and failing equine myocardium

Beta-adrenoceptors are important regulators of cardiac function and their characteristics are known to change in human and canine diseased myocardium. This study aimed to determine the density and subtypes of beta-adrenoceptors in the normal and failing equine ventricular myocardium. Membrane preparations of the left papillary muscles were incubated with increasing concentrations of the nonselective beta-adrenoceptor antagonist [3H]-CGP12177. Saturable and reversible binding of [3H]-CGP12177 to myocardial membranes was demonstrated with Kd values (+/- s.d.) of 0.49 +/- 0.40 and 0.43 +/- 0.22 nmol/l and Bmax values of 93.4 +/- 20.5 and 110.0 +/- 21.2 and fmol/mg protein for normal (n = 19) and heart failure (n = 10) tissues, respectively. Heart failure had no significant effect on the density of ventricular beta-adrenoceptors. The cardiac beta-adrenoceptors were further characterised by studying displacement of [3H]-CGP12177 (0.6 nmol/l) with the beta1-selective antagonists CGP20712A and the beta2-selective antagonist ICI118.551. In normal ventricular muscle, CGP20712A was 26 times more potent than ICI118.551 (Ki values 30.4 +/- 24.8 and 814.1 +/- 485.2 nmol/l, respectively). In heart failure cases, CGP 20712A curves were monophasic with a Ki value of 45.6 +/- 39.7 nmol/l. ICI 118.551 curves were biphasic in 5 horses where 11-31% of the cardiac beta-adrenoceptors had a high affinity for ICI 118.551. These data suggest that the normal equine ventricular myocardium possesses predominately beta1-adrenoceptors, with no evidence for co-existence of a significant population of beta2-adrenoceptors. The density of beta-adrenoceptors did not appear to change in heart failure, but the appearance of receptors with a high affinity for ICI118.551 may suggest that, in some cases, heart failure increases the expression of beta2-adrenoceptors in equine ventricular myocardium. This study provides an insight into the role of the adrenergic system in heart disease in the horse. Further studies in this area are warranted.

Pharmacokinetics and pharmacodynamics of terbutaline in healthy horses

OBJECTIVE

To determine pharmacokinetics of terbutaline in healthy horses and to relate serum terbutaline concentrations with the drug's pharmacodynamic effects.

ANIMALS

6 healthy horses.

PROCEDURE

Horses were given terbutaline i.v. (10 microg/kg of body weight) and, 1 week later, p.o. (100 microg/kg). Responses to drug administration (eg, heart rate and serum lactate concentration) were measured. Serum terbutaline concentration was measured by means of gas chromatography with mass spectrometry. Protein binding was determined in vitro.

RESULTS

Following i.v. administration, median maximum serum terbutaline concentration and mean residence time were 9.3 ng/ml and 30 minutes, respectively. Bioavailability following oral administration was < 1%. All horses developed sweating, trembling, excitement, and tachycardia during i.v. infusion. The 2 horses with the highest serum terbutaline concentrations developed severe tachycardia and CNS stimulation; 30 minutes after the i.v. infusion was completed, they were hyperventilating and lethargic. Heart rate and serum lactate concentration increased as serum terbutaline concentration increased.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that terbutaline is rapidly cleared from the bloodstream following i.v. administration to horses, suggesting that continuous i.v. infusion would be needed to maintain therapeutic serum concentrations. Oral administration of terbutaline to horses is not practical because of the low bioavailability.