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Cardiorespiratory effects of Clenbuterol in fit Thoroughbred horses during a maximal exercise test
... Acute and short-term dosing studies in horses have failed to show significant alterations in any indices of aerobic performance (Rose et al., 1983;Rose and Evans, 1984;Kiely, 1985;Kiely and Jenkins, 1985;Kallings et al., 1991;Slocombe et al., 1992). However, longitudinal studies have shown that long-term administration of a mid-level approved dose rate of the drug (2.4 lg/kg) resulted in deleterious aerobic performance Beekley et al. 2003). ...
... Several investigators have also looked at the effects of clenbuterol on exercise performance. Rose and Evans (1984) used a crossover design to study the effect of a single clenbuterol pretreatment (0.8 lg/kg) on maximal exercise performance in Thoroughbred geldings (n = 5). Prior to the maximal exercise test, each horse was administered either a saline or clenbuterol treatment. ...
... Prior to the second test, each horse received 0.8 lg/kg of clenbuterol IV. As was the case with the study of Rose and Evans (1984), clenbuterol failed to show any significant effects in the tested variables, but its effects on RAO during exercise are still to be determined. Kallings et al. (1991) examined the effects of short-term (5.5 days) pre-treatment with clenbuterol during a standard exercise test. ...
Clenbuterol is a beta(2)-agonist and potent selective bronchodilator that is used to treat bronchospasm in the horse. The drug is normally administered to horses orally as a syrup formulation. Once absorbed into the systemic circulation, clenbuterol has the potential to cause many side effects, including a repartitioning effect and major alterations in cardiac and skeletal muscle function. Recent studies have also reported that clenbuterol can affect bone and the immune, endocrine and reproductive systems. A great deal of information has been published on the beneficial effects of short term therapeutic doses of clenbuterol on the equine respiratory system, although there is limited information about chronic administration, particularly since this has been associated with adverse physiological effects on other systems. This review summarizes the relevant understanding of clenbuterol for clinicians and horse owners who may administer this drug to pleasure and performance horses.
... Figura 3.Mudanças na freqüência cardíaca (A) e na lactacidemia (B) de eqüinos da raça Puro Sangue Árabe (PSA) submetidos à exercício de intensidade crescente em esteira rolante. Médias seguidas de letras diferentes diferem estatisticamente pelo teste Tukey (p≤0,05).As respostas das variáveis hematológicas ao esforço de intensidade crescente em esteira rolante foram similares àquelas encontradas porROSE et al. (1983) eTYLER-MCGOWAN et al. (1999), que estudaram cavalos da raça Puro Sangue Inglês (PSI) submetidos a testes físicos com incrementos de velocidade. Todas as variáveis hematológicas revelaram aumento nos seus valores médios (Figura 1), relacionado com o aumento gradual do esforço. ...
... Todas as variáveis hematológicas revelaram aumento nos seus valores médios (Figura 1), relacionado com o aumento gradual do esforço. Este fato é normalmente imputado à contração esplênica esforço-dependente que é proporcional ao exercício e especialmente importante nos eqüinos(ROSE et al., 1983). Segundo PERSSON(1967), nos eqüinos, a mobilização da reserva esplênica é ativada pelo estímulo simpático que sensibiliza receptores adrenérgicos α 1 , contraindo o baço e causando hemoconcentração pelo aumento de hemácias circulantes e da hemoglobinemia (Figura 1). ...
Currently, equine exercise physiology is in a considerable development in Brazil. The use of scientific methods for the evaluation of the exercise and training protocols is fundamental for the maximization of equine performance. It also can be applied to evaluate the effects of legal (ergogenics) and/or illegal substances during competitions on athletic horses. Some metabolic, endocrine, cardiac and hematologic responses of trained Arab horses submitted to intense exercise and to the administration of caffeine, aminophylline and clenbuterol, a single dose given at 30 minutes before the effort. Doses were, respectively, 5 mg.kg-1, 10 mg.kg-1 and 0,8 g.kg-1. A treadmill was employed for the evaluation of exercise and drug effects, with 10% slope and speed increments. Blood samples were withdrawn 15 seconds before the end of each exercise bout. Hematocrit, erythrocytes and leucocytes count, hemoglobin, glucose, lactate, insulin and cortisol concentration, were determined. Heart rate was also monitored at the same time points. A minimum criterion of P≤0,05 was adopted for statistical significance. Our results revealed significant variations in all physiologic parameters studied. Caffeine impaired the aerobic capacity but improved the aerobic potency. Aminophylline interfered in glycemic curve during intense exercise, but improved anaerobic performance. Finally, clenbuterol did not improve the parameters association with aerobic metabolism, but, markedly, impaired cardiac response, and increased insulin concentration compromising glycemic control during intense exercise.
... Due to the widespread use of inhalers to combat bronchospasm, concern has been raised as to the potential ergogenic properties of these 2-agonists, even when prescribed at therapeutic doses (1). Data regarding the ergogenic effect of short-term use of 2-agonist in humans have been equivocal (1,4,7,35,36,46) whereas data in the healthy horses have failed to show any significant increase in performance (20,23,24,42,43,48). However, there are no data concerning the relationship between exercise performance and long-term administration of therapeutic levels of clenbuterol. ...
... The reductions in aerobic performance seen in the present study are consistent with findings from previous studies in other species that used much higher doses (~mg·kg Ϫ1 body weight) of clenbuterol (13,14,19,29,51,52). Previous studies of the horse (20,23,24,42,43,48) have not demonstrated any alterations in performance; however, those studies used either acute (1 d) or short-term (Ͻ5 d) therapeutic doses (0.8 -3.2 g·kg Ϫ1 body weight) of clenbuterol. This is the first study in the horse to demonstrate any adverse effects on performance by using doses in a prescribed therapeutic range recommended by the manufacturer. ...
The purpose of this 8-wk study was to examine the effect of therapeutic levels of clenbuterol on aerobic performance and hemodynamics associated with exercise.
Twenty-three unfit Standardbred mares were divided into four experimental groups, clenbuterol (2.4 microg x kg(-1) body weight twice daily) plus exercise (20 min at 50% O2max; CLENEX; N = 6), clenbuterol only (CLEN; N = 6), exercise only (EX; N = 5), and control (CON; N = 6). All horses performed an incremental exercise test (GXT) to measure maximal oxygen consumption (O2max), blood lactate concentration, total plasma protein concentration, and hematocrit. Plasma volume, heart rate, right ventricular pressure (RVP), and pulmonary artery pressure (PAP) were measured before and after the treatment/training. Each horse also performed an exercise capacity test (ECT) in which they ran at their pretreatment O2max speed until exhausted.
There were no significant changes in blood lactate, total protein, or hematocrit for any group during either the GXT or ECT. CLENEX decreased (P < 0.05) O2max (-6.2%) and velocity to O2max (-10.0%), whereas both CLENEX and CLEN decreased (P < 0.05) in time to exhaustion (-20.5+/-4.7 and -20.9 +/- 5.6%). EX alone increased (P < 0.05) O2max (+6.5%), velocity to O2max (+10.0%), velocity to produces lactate concentration of 4 mmol (+13.5%), and time to exhaustion (+32.3 +/- 15.0%). Plasma volume was altered (P < 0.05) in CLENEX (-10%) and EX (+27%) but not in CLEN. Posttest recovery HR was higher (P < 0.05) at 2 min post-GXT in the CLENEX, CLEN, and CON compared with their pretest values; RVP remained elevated at 2 min of recovery in the CLEN and CON groups; however, in the EX, recovery HR and RVP had returned to pre-GXT levels by 2 min of recovery.
These data suggest that the combined effect of therapeutic levels of clenbuterol and training decrease aerobic performance and that the resultant reduction in plasma volume may affect improvements in cardiovascular function during recovery normally seen with exercise training.
... In SB, clenbuterol treatment increased fat free mass (Kearns et al., 2001) but impaired aerobic performance (Kearns and Mckeever, 2002). Similarly, clenbuterol failed to improve aerobic performance in TB (Rose and Evans, 1987). The current study demonstrates a greater capacity for aerobic metabolism from a young age in SB and TB when compared with QH. ...
Responses of equine skeletal muscle characteristics to growth and training have been shown to differ between breeds. These differential responses may arise in part because muscle fiber type and mitochondrial density differ between breeds, even in untrained racing-bred horses. However, it is not known when these breed-specific differences manifest. To test the hypothesis that weanling Standardbreds (SB) and Thoroughbreds (TB) would have higher mitochondrial measures than Quarter Horses (QH), gluteus medius samples were collected from SB (mean ± SD; 6.2 ± 1.0 mo; n = 10), TB (6.1 ± 0.5 mo; n = 12), and QH (7.4 ± 0.6 mo; n = 10). Citrate synthase (CS) and cytochrome c oxidase (CCO) activities were assessed as markers of mitochondrial density and function, respectively. Mitochondrial oxidative (P) and electron transport system (E) capacities were assessed by high-resolution respirometry (HRR). Data for CCO and HRR are expressed as integrated (per mg protein and per mg tissue wet weight, respectively) and intrinsic (per unit CS). Data were analyzed using PROC MIXED in SAS v 9.4 with breed as a fixed effect. Mitochondrial density (CS) was higher for SB and TB than QH (P ≤ 0.0007). Mitochondrial function (integrated and intrinsic CCO) was higher in TB and QH than SB (P ≤ 0.01). Integrated CCO was also higher in TB than QH (P < 0.0001). However, SB had higher integrated maximum P (PCI+II) and E (ECI+II) than QH (P ≤ 0.02) and greater integrated and intrinsic complex II-supported E (ECII) than both QH and TB (P ≤ 0.02), while TB exhibited higher integrated P with complex I substrates (PCI) than SB and QH (P ≤ 0.003) and higher integrated PCI+II and ECI+II than QH (P ≤ 0.02). In agreement, TB and QH had higher contribution of complex I (CI) to max E than SB (P ≤ 0.001), while SB had higher contribution of CII than QH and TB (P ≤ 0.002). Despite having higher mitochondrial density than QH and TB, SB showed lower CCO activity and differences in contribution of complexes to oxidative and electron transport system capacities. Breed differences in mitochondrial parameters are present early in life and should be considered when developing feeding, training, medication, and management practices.
... In SB, clenbuterol treatment increased fat free mass (Kearns et al., 2001) but impaired aerobic performance (Kearns and Mckeever, 2002). Similarly, clenbuterol failed to improve aerobic performance in TB (Rose and Evans, 1987). The current study demonstrates a greater capacity for aerobic metabolism from a young age in SB and TB when compared with QH. ...
... This progressive increase in VE at successive levels of work load was achieved mainly by an increase of/ resp and, to a lesser extent, also of VT. This trend in respiratory pattern is a common observation in horses (Rose and Evans, 19876;Woakes etal. 1987;Pelletier etal. ...
The respiratory mechanics together with the rate of work of breathing were studied in five healthy adult Standardbred horses at rest and during different levels of exercise on a treadmill (0.8–2.3 ms−1, 7% slope). In three of the horses the stride frequency was also determined.
The ventilatory response to exercise increased linearly with treadmill speed up to 9.6 times the resting values and was sustained more by an increase in respiratory frequency than by an increase in tidal volume. At the most elevated work load, respiratory frequency and tidal volume averaged 5.2 and 1.4 times the resting values, respectively. Lung resistance decreased during the most intense level of exercise to about 30% of the average value observed at rest, while lung dynamic compliance remained unchanged. The rate of work of breathing (Ẇresp, in W) increased exponentially with ventilation (, in 1min−1) according to the equation: . In the three animals investigated a definite locomotor-respiratory coupling was observed at a trot and in two animals also during walking. This observation, together with other considerations based on available data on the energetics of respiration, suggests that the efficiency of the equine respiratory system is relatively high. In addition, when compared with other smaller mammals (man and dog), horses are, in mechanical terms, more economical breathers.
... There have been limited investigations into the effect of R adrenergic receptor agonists on conditioned performance horses. Administration of clenbuterol to exercising horses resulted in higher Pao, and lower P a c q post exercise (Rose et al. 1983), but was not found to enhance cardiorespiratory function (Rose and Evans 1987) or exercise performance (Slocombe et al. 1992). This study was designed to test the hypothesis that a specific I32 adrenergic receptor agonist, albuterol sulphate, when delivered by metered-dose inhaler (MDI) would affect the performance of fit Thoroughbred horses during incremental exercise testing. ...
This study was designed to test the hypothesis that a specific ß2 adrenergic receptor agonist, albuterol sulphate, when delivered by metered-dose inhaler (MDI) would affect the performance of Thoroughbred horses during incremental exercise testing. Six conditioned Thoroughbred horses were randomly assigned to one of 2 treatment groups: Group 1 horses received placebo and Group 2 received albuterol. Each horse received both treatments in a crossover design with a 3 week interval between trials. Horses inspired albuterol (900 μg) or placebo over a 5 min period followed by an incremental, peak oxygen consumption (V̇O2peak) exercise test. Facial arterial blood gases (Pao2, Paco2), pHa, temperature, haemoximetry (O2Hb, COHb, MetHb, RHb) and electrolytes (Na+, K+, iCa2+, Cl−) were measured. Heart rate (HR), V̇O2peak, CO2 production (V̇CO2), plasma lactate, speed at failure, exercise duration, and urine levels of albuterol and metabolites (Alburine) were monitored. The Paratrend7 intravascular sensor provided continuous arterial blood gas data. Mixed effects linear modelling using generalised least-squares estimation was used to assess treatment and speed main effects interaction. Significant differences (P<.01) were found between placebo and albuterol-treated horses for total run time (mean ± s.e. 405.8 ± 8.8 vs. 430.5 ± 6.7 s) and run time from completion of 8 m/s until fatigue (169.3 ± 8.6 vs. 187.3 ± 6.7 s), as well as V̇O2peak (121.7 ± 3.7 vs. 130.3 ± 3.8 ml/kg/min), respectively. In conclusion, inhaled albuterol sulphate has a small but significant effect on the performance of fit Thoroughbred horses during incremental, V̇O2peak exercise testing.
... Those studies focused on chronic administration of clenbuterol. Unfortunately, most investigations of the effects of clenbuterol on cardiorespiratory function have only studied short-term (either one acute dose or following 5.5 d of dosing) administration of the drug, and none has looked at immune function (Rose et al., 1983;Rose and Evans, 1987;Slocombe et al., 1992). ...
Effects of longitudinal exercise training and acute intensive exercise (simulated race test) on immune function have not been reported in horses. Clenbuterol, a beta2-adrenergic agonist, is used to manage inflammatory airway disease in horses. This study investigated the interaction of 8 wk of exercise training with or without 12 wk of clenbuterol administration in horses. Twenty-three untrained standardbred mares (10 +/- 3 yr, Mean +/- SE) were used and divided into four experimental groups. Horses given clenbuterol plus exercise (CLENEX; n = 6) and clenbuterol alone (CLEN; n = 6) received 2.4 microg/kg BW of clenbuterol twice daily (in an average volume of 20 mL) on a schedule of 5 d on and 2 d off for 12 wk. The CLENEX group was also aerobically trained 3 d/wk. Mares given exercise alone (EX; n = 5) were aerobically trained for 3 d/wk, and the control group (CON; n = 6) remained sedentary. Both EX and CON horses were administered similar volumes (approximately 20 mL) of molasses twice daily. A simulated race test (SRT) resulted in an elevation in lymphocyte number postexercise (P < 0.05). There was no significant difference after acute exercise in either monocyte or granulocyte number. Acute exercise resulted in a decrease (P < 0.05) in the percentage of CD4+ and an increase (P < 0.05) in the percentage of CD8+ cells. The SRT resulted in a decreased lymphoproliferative response to pokeweed mitogen (P < 0.05). A SRT had no effect on antibody production in response to equine influenza vaccine. The EX group demonstrated greater cortisol concentrations at rest and at all other time points postexercise after completing the training regimen compared with CLENEX horses (P < 0.05). Preexercise (SRT) peripheral blood monocyte number was lower in CLENEX horses than in other treatment groups (P < 0.05). Clenbuterol and exercise training did not significantly affect post-SRT changes in leukocyte numbers. Exercise training resulted in a decrease (P < 0.05) in the percentage of CD8+ cells post-SRT compared with other groups, but the percentage of CD4+ cells was not altered by either clenbuterol or exercise conditioning. Lymphocyte proliferative response was not affected by clenbuterol or exercise treatment. Horses demonstrated responses to bouts of acute exercise as noted with other species, namely humans and rodents.
... A reduced FO2 by Padrenergic treatment has previously not been observed. In horses, too, VE and HR were moderately increased by clenbuterol treatment (Rose and Evans, 1986;Rose et al., 19881. A decrease of NEFA levels (after a n initial increase1 several hours after P-A treatment has been previously noted in calves (Blum and Flueckiger, 1988). ...
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.
... ultrasound; equine; 2-agonist CLENBUTEROL, A 2-AGONIST, is a potent selective bronchodilator that initially was used as a drug to treat bronchospasm and to alleviate the symptoms of chronic obstructive pulmonary disease (COPD) in the horse (38). Several investigators have studied the effect of short-term (either acute or 5.5 days) clenbuterol treatment on various cardiorespiratory functions in a variety of horse breeds (15,36,37,40). Clenbuterol has also been shown to improve clinical signs of bronchitis and pneumonia (38) and to increase mucociliary transport rate in both normal horses and horses with COPD (18,19). ...
The purpose of this study was to examine the effect of therapeutic levels of clenbuterol, with and without exercise training, on body composition. Twenty-three unfit Standardbred mares were divided into four experimental groups: clenbuterol (2.4 microg/kg body wt twice daily) plus exercise (ClenEx; 20 min at 50% maximal oxygen consumption 3 days/wk; n = 6), clenbuterol only (Clen; n = 6), exercise only (Ex; n = 5), and control (Con; n = 6). Rump fat thickness was measured at 2-wk intervals by using B-mode ultrasound, and percent body fat (%fat) was calculated by using previously published methods. For Ex, body fat decreased (P < 0.05) at week 4 (-9.3%), %fat at week 6 (-6.9%), and fat-free mass (FFM) increased (P < 0.05) at week 8 (+3.2%). On the other hand, Clen had significant changes in %fat (-15.4%), fat mass (-14.7%), and FFM (+4.3%) at week 2. ClenEx had significant decreases in %fat (-17.6%) and fat mass (-19.5%) at week 2, which was similar to Clen; however, this group had a different FFM response, which significantly increased (+4.4%) at week 6. Con showed no changes (P > 0.05) in any variable at any time. These results suggest that exercise training and clenbuterol have additive effects with respect to %fat and fat mass but antagonistic effects in terms of FFM. Furthermore, chronic clenbuterol administration causes significant repartitioning in the horse, even when administered in therapeutic doses.
The aim of the study was to determine the effect of clen-buterol on the anaerobic threshold of horses on a tread-mill with increasing physical stress, measuring heart rate (HR) and blood levels of lactate, glucose, and insulin. Twelve Arabian horses were submitted to two physical tests separated by a 10-day interval. Clenbuterol (CL) at 0.8 mg/kg or saline (control—C) was administered intravenously 30 minutes before the test. The treadmill exercise test consisted of an initial warmup followed by a gradually increasing effort. There was no statistical difference in either V 2 or V 4 (velocity at which plasma lactate concentration reached 4 and 2 mmol/L, respec-tively) between the two experimental groups. For the CL group, V 200 , V 180 , V 160 , and V 140 (velocity at which the rate heart is 140, 160, 180, and 200 beats/minute, respectively) decreased significantly. At rest as well as times 4, 6, and 10 minutes, insulin levels were higher in the group that received clenbuterol (P < .05). Con-trary to what was expected, apparently, there was no im-provement in aerobic metabolism in animals when given a therapeutic dose of the bronchodilator. The elevated heart rate observed could have been attributable to the stimulation of cardiac b 1 adrenoceptors and the in-creased insulin levels to the stimulation of pancreatic b 2 receptors.
A comparison between the relaxant effects of clenbuterol and theophylline on horse tracheal smooth muscle has been made in vitro. Rat tracheal smooth muscle was also investigated as a reference. The tracheal preparations were initially contracted with carbachol since the smooth muscle did not spontaneously develop tone. The response of the carbachol-contracted preparations to theophylline was the same in the two species. The response to clenbuterol varied. In only five out of eleven horses were the tracheal smooth muscles sensitive to clenbuterol (mean pD2 = 7.92 M). In the remaining six horses the tracheal smooth muscles were insensitive to clenbuterol (mean pD2 = 3.59 M), yet the preparations responded well to theophylline with complete relaxation. All rat tracheal preparations were insensitive to clenbuterol.
To evaluate the effects of clenbuterol on cardio-respiratory parameters and blood lactate relation to exercise tolerance, experimental horses performed standardized exercise tests on a high-speed treadmill before and after administration of the drug. Clenbuterol was administered in feed to six healthy Standardbreds at a dose rate of 0.8 micrograms/kg b.wt twice daily for 5.5 days. Each horse was tested twice, without and with a respiratory mask, during two consecutive days. One week elapsed between the baseline tests without drug and the tests with clenbuterol treatment (each horse served as its own control). The results show an unchanged heart rate response to exercise 2 h after the last clenbuterol administration. The blood lactate response and the arterial oxygen tension during exercise did not differ before and after drug treatment. The oxygen uptake as well as pulmonary ventilation relative to the work load performed was essentially unaffected. The arterial pH during exercise was significantly increased (P less than 0.05) following clenbuterol treatment. Plasma levels of clenbuterol were maximal 2 h post-administration with values between 0.45 and 0.75 ng/ml. The plasma half-life of elimination was 10.4 h (+/- 2.25 SD). In conclusion, clenbuterol did not cause any major effects on the cardio-respiratory and blood lactate parameters studied in healthy horses performing submaximal exercise tolerance tests.
This article has presented information on the importance of the sympathetic nervous system in the response to exercise. The authors have reviewed the very limited information on the effects of sympathomimetic and sympatholytic drugs on exercise performance in the horse. Most of these drugs are specifically prohibited under the rules of racing, and they have significant side effects that either decrease performance or make their use dangerous to both the horse and horse-man. Additionally, all of these drugs or their metabolites are readily detected by current drug testing protocols. Further information is needed to expand our knowledge of how each of these substances affects the health of the equine athlete.
Six ponies performed a standardised exercise test on a motorised treadmill at each of three randomly assigned treadmill elevations (1, 4, or 7 degrees). The exercise test consisted of four, 4 min increments of increasing treadmill speed from 1.0 to 3.4 m/sec. Heart rate, blood lactate concentration, and packed cell volume (PCV) were determined, during the last min of each exercise level, and at 4 and 12 mins post exercise. Regardless of treadmill elevation, no differences were observed in pre-exercise heart rate (49 +/- 2) beats/min), lactate (1.2 +/- 0.1 mM), and PCV (0.32 +/- 0.01 litres. During exercise, heart rate and PCV were highly correlated to treadmill speed and elevation. Peak exercise heart rates (determined at 3.4 m/sec) were 159 +/- 10, 182 +/- 5, and 216 +/- 6 beats/min at 1, 4, and 7 degrees, respectively, while peak PCVs were 0.37 +/- 0.01, 0.40 +/- 0.01 and 0.42 +/- 0.02 lit/litre at 1, 4, and 7 degrees, respectively. Blood lactate did not change significantly from pre-exercise levels during the exercise test at a treadmill elevation equal to 1 degree, but increased markedly at 4 and 7 degrees. Peak lactates were 1.9 +/- 0.7, 5.3 +/- 1.0, and 18.1 +/- 1.5 mM at 1, 4, and 7 degrees, respectively. There was a highly significant correlation between heart rate and lactate at all treadmill speeds and elevations. Therefore, during graded, submaximal exercise increasing treadmill elevation up to 7 degrees results in increases in heart rate, blood lactate concentration and PCV comparable to those seen with increasing treadmill speed alone.
The present study was carried out to ascertain whether beta2-adrenergic receptor stimulation with clenbuterol would attenuate the pulmonary arterial, capillary and venous hypertension in horses performing high-intensity exercise and, in turn, modify the occurrence of exercise-induced pulmonary haemorrhage (EIPH). Experiments were carried out on 6 healthy, sound, exercise-trained Thoroughbred horses. All horses were studied in the control (no medications) and the clenbuterol (0.8 pg/kg bwt, i.v.) treatments. The sequence of these treatments was randomised for every horse, and 7 days were allowed between them. Using catheter-tip-transducers whose in-vivo signals were referenced at the point of the left shoulder, right heart/pulmonary vascular pressures were determined at rest, sub-maximal exercise and during galloping at 14.2 m/s on a 3.5% uphill grade--a workload that elicited maximal heart rate and induced EIPH in all horses. In the control experiments, incremental exercise resulted in progressive significant increments in right atrial as well as pulmonary arterial, capillary and venous (wedge) pressures and all horses experienced EIPH. Clenbuterol administration to standing horses caused tachycardia, but significant changes in mean right atrial or pulmonary vascular pressures were not observed. During exercise performed after clenbuterol administration, heart rate as well as right atrial and pulmonary arterial, capillary and wedge pressures also increased progressively with increasing work intensity. However, these values were not found to be statistically significantly different from corresponding data in the control study and the incidence of EIPH remained unaffected. Since clenbuterol administration also does not affect the transpulmonary pressure during exercise, it is unlikely that the transmural force exerted onto the blood-gas barrier of exercising horses is altered following i.v. clenbuterol administration at the recommended dosage.
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