Biomedical subjects
D J Marlin
Publications and source records attributed to D J Marlin.
Plasma aspartate aminotransferase and creatine kinase activities in thoroughbred racehorses in relation to age, sex, exercise and training.
In order to investigate the effect of age, sex and month on the response of plasma aspartate aminotransferase (AST) and creatine kinase (CK) to exercise, blood samples were collected once a month between March and September from a group of 40 2- and 3-year-old (2yo and 3yo) thoroughbred racehorses (kept under the same managemental regimen) at rest before exercise (PRE) and at 2 (2H) and 24 h (24H) post-exercise. The absolute change in activities between the 2H and PRE samples (2H delta) and the 24H and PRE samples (24H delta) was also calculated. Age had a significant effect on all measured and calculated parameters for colts (C), apart from 24H delta CK but showed no effect in the fillies (F). Sex only had a significant effect in the 3yo in the 2H delta CK. In the 2yo, significant effects of sex were found for both CK and AST in the PRE, 2H and 24H samples. The effect of month varied according to the classification group with only the 2yoC not showing any significant effect on any parameter. Fillies were, in general, more likely than colts to show greater than a twofold increase in CK activity at 2H post-exercise and the number of animals showing such an increase decreased as the season progressed. Very little change in AST activities occurred with exercise.
Effects of administration of water versus an isotonic oral rehydration solution (ORS) at rest and changes during exercise and recovery.
The administration of 41 of an isotonic, plasma-like oral rehydration solution (ORS) with an osmotic skeleton and 41 of water (water; no osmotic skeleton), were evaluated in five thoroughbred horses. Solutions were administered by nasogastric tube 4 h after feeding. Uptake of deuterium, concentrations of plasma sodium, potassium, chloride, glucose, total protein and packed cell volume, pH, PCO2, HCO3-, total CO2, actual base excess, standard base excess, plasma volume and weight loss were assessed both at rest, and during and after exercise on a treadmill. Each horse underwent four experimental sessions (water-resting; ORS-resting; water-exercise; ORS-exercise). There was an indication of uptake of both water and ORS by 10 min post-administration. Based on the appearance of deuterium in plasma, there was no significant difference in the rate of uptake of water or ORS at rest, although there was a trend for the uptake of ORS to be slower than water during the exercise session. The mean decrease in total protein (TP, 3.0 g l-1) and the increase in plasma volume (PV, 4.6 ml kg-1) after administration of ORS at rest was greater (P < 0.05) than that of water (TP, 1.3 g l-1 and PV, -1.2 ml kg-1). There was no significant difference in TP or PV following administration of water or ORS during the exercise treatment. Exercise had little effect on plasma sodium concentration. The results confirmed that administration of 41 of isotonic, plasma-like ORS provided a much more distinct and durable contribution to the maintenance of plasma volume and circulation than administration of an equal amount of water at rest. Subsequent exercise may, however, mask some of the effects observed at rest.
Rehydration following exercise: effects of administration of water versus an isotonic oral rehydration solution (ORS).
The effects of administering (1) 6L isotonic oral rehydration solution (ORS), similar in composition to plasma (except for an elevated potassium concentration) and with an osmotic skeleton and (2) 6L water (no osmotic skeleton), were evaluated in five thoroughbred horses following exercise-induced dehydration. The horses were exercised on a treadmill for 10 min at walk (1.7 m.s-1; approximately 15% VO2max), 40 min at trot (3.7 m.s-1; approximately 25% VO2max) and 10 min at walk (1.7 m.s-1; approximately 15% VO2max). Exercise was undertaken on a 3 degrees incline at 30 degrees C/80% RH. Solutions of water or ORS at 20 degrees C were administered by nasogastric tube over 60s 5 min following exercise. Mean weight loss following exercise was 9.2 +/- 1.7 kg (2.0 +/- 0.4% body weight; mean +/- SEM) with water and 9.2 +/- 1.1 kg (2.0 +/- 0.2% body weight) with ORS and was not different between treatments (P > 0.05). Water treatment resulted in a fall in plasma [Na+] (approximately 3 mmol.L-1) and C1- (1-2 mmol.L-1) concentrations by 30 min after administration and the effect persisted until the end of the study (300 min post fluids). There was little change in plasma total protein (TP) from that at the end of exercise, suggesting a failure of water to restore or maintain PV. In contrast, ORS administration resulted in a small increase in plasma [Na+] (1-2 mmol.L-1) and [Cl-] (2-3 mmol.L-1) with a corresponding decrease in plasma TP. By 120 min post ORS, plasma TP and PV were no longer significantly different from rest or pre-exercise (P > 0.05), whilst with water, TP was elevated (approximately 3-4 g.L-1) and PV reduced (approximately 4-5 mL.kg-1). Total urine output was not significantly different between water (1096 +/- 135 mL) and ORS (750 +/- 215 mL, P > 0.05). Estimates of expected plasma volume and electrolyte concentration changes as a result of either treatment compared well with measured changes of TP and PV. On the basis of calculated or measured changes, it was estimated that only 1L of water contributed to rehydration following exercise compared to 4L isotonic, plasma-like ORS. The administration of 6L ORS restored the PV deficit induced by exercise with minimal or no disturbance of plasma electrolyte concentrations. In contrast, water alone resulted in minimal improvement in PV. When fluid intake after periods of fluid loss, such as induced by exercise or transport, is not accompanied by food intake, the present study has clearly demonstrated that water alone is ineffective in promoting rehydration compared with an isotonic, plasma-like ORS.
Acclimation and acclimatisation of the equine athlete.
A number of studies have clearly shown that the ability of both man and horses to perform a standardised exercise test in hot humid conditions is reduced compared to that in hot dry or cool conditions (see reviews by Maughan and Lindinger, 1995; Lindinger and Marlin, 1995). In man it is clear that a period of acclimation or acclimatisation is beneficial. A search of the literature prior to 1995 indicates no published information relating to the acclimation or acclimatisation of the horse to hot or hot humid environmental conditions. Concerns for the appropriate preparation for and welfare of horses at the 1996 Atlanta Olympic Games prompted a notable increase in studies relating to equine thermoregulation, and four studies describing acclimation or acclimatisation responses.
Post exercise changes in compartmental body temperature accompanying intermittent cold water cooling in the hyperthermic horse.
Whereas the efficacy of cold water cooling of horses has been demonstrated by several studies, the dynamics of temperature changes within and between compartments (primarily muscle, blood [core], skin and deep core [rectal]) have not been investigated. Changes in body temperature associated with cold water cooling were investigated in the hyperthermic horse. Muscle (TMU), pulmonary artery (TPA), rectal (TREC), tail-skin (TTSK) and coat surface (TCOAT) temperatures, were monitored continuously in 5 Thoroughbred horses during and after exercise in hot humid (30 degrees C and 80% RH) conditions on a treadmill. Horses were cooled in the hot humid environment with cold water (approximately 6 degrees C) for 6 30 s periods. Between each 30 s cooling period the horses stood for 30 s. A total of 180 l of cold water was applied. Horses were monitored for a further 4 min following the final cooling period. From the end of exercise to the end of the final cooling (6.5 min), mean (+/- s.e.) rates of decrease for TTSK and TPA were similar (0.8 +/- 0.1 and 0.8 +/- 0.1 degrees C/min, respectively). The effects on TMU and TREC were less marked, with average rates of 0.2 +/- 0.1 and 0.0 +/- 0.1 degrees C/min, respectively. During the first 4 min of cooling, TPA fell during the 30 s period of water application and rose during each 30 s period of standing. When TPA fell below approximately 36.5 degrees C, these variations were suppressed and TPA rose steadily, despite continued applications; TREC and TMU continued to fall, although less rapidly than before. These observations are consistent with the onset of skin vasoconstriction at low TPA. The mechanism is mediated through a cooling of circulating blood volume providing a greater capacity for heat transfer between muscle and circulation. Intermittent application of cold water (approximately 6 degrees C) improves heat removal without apparent deleterious effects and is well tolerated. Even when hypothermia develops (based on TPA), muscle and rectal temperatures continue to fall.
Exercise-induced pulmonary haemorrhage (EIPH) in horses results from locomotory impact induced trauma--a novel, unifying concept.
Exercise-induced pulmonary haemorrhage (EIPH) in horses, although of major welfare and economic importance worldwide, is of uncertain cause. It is accepted that the dorsocaudal region of the lung is particularly prone to the condition, but present theories of causation cannot satisfactorily explain the mechanism or pattern of occurrence. We propose that EIPH results from locomotory impact induced trauma; the mechanism being similar to that producing lung tissue damage following thoracic impact injury. In impact injury, the localised impulsive load on the chest wall is transmitted by pressure waves through the lung at a slower speed than in the chest wall. The waves are subsequently reflected from the distal chest wall and other structures, producing a complex pattern of wave motion; waves travelling from regions of large cross-section to narrower ones are amplified in magnitude, consequently these regions can experience very high local stresses. Compression/dilation and shear waves are produced within the parenchyma and the latter particularly have been implicated as the cause of parenchymal damage and rupture with oedema and haemorrhage. This form of soft tissue damage has been shown to occur at remarkably low loads with an impact velocity greater than about 11 m/s and pressure exceeding approximately 14 kPa. In the horse, the lung is subjected to comparable levels of locomotory derived impulsive force during moderate to high speed exercise and this is the basis of the mechanism causing EIPH. During locomotion, the force following ground-strike of the front legs is transmitted, with some attenuation, through the forelimbs to the scapulae. The anatomical arrangement of the scapula, coupled with the direction of the force at the shoulder (scapulo humeral joint) produces an impulsive force on the rib cage, approximately just below mid height of the frontal aspect of the chest approximately over the fourth rib. As a result, pressure waves are transmitted through the lung parenchyma towards the dorsal and caudal regions; these waves are subsequently reflected at the distal chest wall, spine and diaphragm causing a complex pattern of wave interaction. The observed locations of EIPH are at the sites where wave intensity is expected to be greatest due to changes in cross section and reflection. Based on available information, it is estimated that impulsive forces of more than 100 kPa, lasting approximately 10 ms, would be applied to the chest wall by each scapula in a 500 kg horse when galloping; this level of force would be sufficient to cause oedema and haemorrhage as observed in impact induced injury.
Effects of nitric oxide inhibition on thermoregulation during exercise in the horse.
We investigated the role of NO in the control of thermoregulation. We measured sweating rate and body temperatures (core, rectal and skin) in five thoroughbred horses during exercise of variable intensity on a high-speed treadmill. A standard exercise test (SET) consisting of three canters (8 m s-1), with walking and trotting between each canter, was performed twice, in random order, by each horse and N omega-nitro-L-arginine methyl ester (L-NAME; 20 mg ml-1), a competitive inhibitor of nitric oxide synthase (NOS), was infused into the central circulation after the first canter in the test SET only. L-Arginine (200 mg ml-1), a substrate of NOS, was injected after the second canter in both control and test SETs. L-NAME significantly (p < 0.05) reduced the sweating rate measured on the neck (31.6 +/- 6.4 versus 9.7 +/- 4.2 g/min/m2) and rump (14.7 +/- 5.2 versus 4.8 +/- 1.6 g/min/m2) while raising the core temperature (39.7 +/- 0.2 versus 40.6 +/- 0.7 degrees C, p < 0.05) during the second canter. In the third canter, sweating rate had increased after giving L-arginine during the test SET, but had not returned to levels measured at similar times during the control SET. Core, rectal and skin temperatures continued to rise and were significantly higher than control levels, despite giving L-arginine. The results show that inhibition of NO production reduces sweating rate in the horse during exercise thereby inducing a rise in body temperatures.
Metabolic adaptation to fat-supplemented diet by the thoroughbred horse.
Following 10 weeks of fat supplementation a group of aerobically trained thoroughbred horses exhibited a significant decrease in postprandial plasma triacylglycerol concentration. This decrease was associated with a mean 50% increase in plasma total lipase activity following pentosan polysulfate administration and an increase in postprandial plasma cholesterol concentration. A significant increase in the activity of muscle citrate synthase (EC 4.1.3.7), expressed as a ratio to the total fractional area occupied by type I and type IIa muscle fibres, was also observed. No significant change in the concentration of resting muscle glycogen or triacylglycerol occurred as a result of fat supplementation. These results suggest that there was improved management of the fat load and that the triacylglycerol-clearing capacity of the horses was increased as a result of fat supplementation. It is suggested that the increase in plasma total lipase activity following pentosan polysulfate administration may have reflected an increase in muscle lipoprotein lipase (EC 3.1.1.34) activity, which would increase the capacity of muscle for free fatty acid uptake from circulating triacylglycerol-rich plasma lipoproteins. Fat supplementation may also enhance the oxidative capacity of muscle, as suggested by the significant increase in muscle citrate synthase and the trend towards an increase in beta-hydroxyacyl CoA dehydrogenase (EC 1.1.1.35) following 10 weeks of fat supplementation.
Nitric oxide and thermoregulation during exercise in the horse.
The effect of inhibition of nitric oxide production on sweating rate (SR) and on core, rectal, and tail skin temperatures was measured in five Thoroughbred horses during exercise of variable intensity on a high-speed treadmill. A standard exercise test consisting of three canters [approximately 55% maximum O2 uptake (VO2max)], with walking (approximately 9% VO2max) and trotting (approximately 22% VO2max) between each canter, was performed twice (control or test), in random order, by each horse. N(G)-nitro-L-arginine methyl ester (L-NAME; 20 mg/kg), a competitive inhibitor of nitric oxide synthase, was infused into the central circulation and induced a significant reduction in the SR measured on the neck (31.6 +/- 6.4 vs. 9.7 +/- 4.2 g x min(1) x m(-2); 69%) and rump (14.7 +/- 5.2 vs. 4.8 +/- 1.6 g x min(-1) x m(-2); 67%) of the horses during canter (P < 0.05). Significant increases in core, rectal, and tail skin temperatures were also measured (P < 0.05). L-Arginine (200 mg/kg iv) partially reversed the inhibitory effects of L-NAME on SR, but core, rectal, and tail skin temperatures continued to increase (P < 0.05), suggesting a cumulation of body heat. The results support the contention that nitric oxide synthase inhibition diminishes SR, resulting in elevated core and peripheral temperatures leading to deranged thermoregulation during exercise. The inhibition of sweating by L-NAME may be related to peripheral vasoconstriction but may also involve the neurogenic control of sweating.
Nitric oxide and exercise in the horse.
1. The effects of exercise on the production rate of nitric oxide (NO) in exhaled air (VNO) and the effects of inhaled NO (80 p.p.m.) on cardiovascular and respiratory parameters were investigated in five Throughbred horses. 2. The concentration of NO ([NO]) in exhaled air collected from within the nasal opening was lower when collected at a high flow rate of 80 l min-1 than at a low flow rate of 20 l min-1: when trotting at 3.7 m s-1 the values were 0.78 +/- 0.15 and 1.23 +/- 9.14 p.p.b., respectively, and when cantering at 9 m s-1 the values were 1.69 +/- 0.31 and 2.25 +/- 0.32 p.p.b., respectively. 3. Nebulized methoxamine (40 mg ml-1 for 60 s), an alpha 1-adrenergic agonist, further reduced [NO] during the 9 m s-1 canter to 1.05 +/- 0.14 and 1.99 +/- 0.41 p.p.b. when collected at 80 and 20 l min-1, respectively, and induced cyclical changes in the breathing pattern. 4. Exercise induced a linear increase in VNO with work intensity to a maximum (428.1 +/- 31.6 pmol min-1 kg-1) which coincided with the maximal oxygen uptake for the horses (138.3 +/- 11.7 ml min-1 kg-1), although a further increase in VNO (779.3 +/- 38.4 pmol min-1 kg-1) occurred immediately after exercise. The changes in VNO correlated well with the tidal volume (r = 0.968; P < 0.01) and the haematocrit (r = 0.855; P < 0.01). 5. In the first 2 min of high intensity exercise, inhaled NO (80 p.p.m.) significantly (P < 0.05) reduced the pulmonary artery pressure: during the first minute, pulmonary artery pressure was 83.1 +/- 7.6 mmHg compared with a control value of 94.4 +/- 6.3 mmHg, and during the second minute, 84.2 +/- 7.1 mmHg compared with a control value of 98.4 +/- 4.7 mmHg. There were no other significant changes in cardiovascular or respiratory indices, including cardiac output, measured during exercise between control and inhaled NO tests. 6. The results show that exhaled NO is released from the airways of the horse and may contribute to the regulation of pulmonary vascular tone during exercise.
Plasma iron in elite horses at rest and after transport.
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Effects of transporting horses facing either forwards or backwards on their behaviour and heart rate.
The effects of transporting horses facing either forwards or backwards were compared by transporting six thoroughbred horses in pairs in a lorry on one journey facing in the direction of travel, and on another journey facing away from the direction of travel, over a standard one-hour route. Heart rate monitors were used to record their heart rate before, during and after the journey and the horses' behaviour was recorded by scan sampling each horse every other minute. The average heart rate was significantly lower (P < 0.05) when the horses were transported facing backwards, and they also tended to rest on their rumps more (P = 0.059). In the forward-facing position, the horses moved more frequently (P < 0.05) and tended to hold their necks in a higher than normal position and to vocalise more frequently (P = 0.059). During loading the average peak heart rate was 38 bpm lower (P < 0.05) when the horses were backed into the horse box for rear-facing transport than when they were loaded facing forwards. However, there was no difference between transport facing forwards or backwards in terms of the peak unloading heart rate, or the average heart rate during loading or unloading. The horses seemed to find being transported less physically stressful when they were facing backwards than when they were facing forwards.
Effects of exercise intensity and environmental stress on indices of oxidative stress and iron homeostasis during exercise in the horse.
The effects of prolonged variable-intensity and short-term high-intensity exercise on indices of oxidative stress and iron homeostasis were compared in six fit horses under cool [20 degrees C, 40% relative humidity (RH)] or hot/humid (30 degrees C, 80% RH) environmental conditions. The exercise protocols were designed to simulate equine competition, including racing (intense exercise) or the speed and endurance phase of a 3-day event (prolonged exercise). Increased plasma concentrations of lipid hydroperoxides and haemolysate concentrations of oxidised glutathione (GSSG) were measured within 30 min of the completion of exercise, indicating production of reactive oxygen species (ROS) and lipid membrane peroxidation. The horses were unable to complete the prolonged exercise protocol at high temperature and humidity. This coincided with higher maximal values of lipid hydroperoxides [138.2 (17.7) microM and GSSG [110.6 (18.2) microM], compared to high-intensity [105.2 (14.9) microM and 63.6 (8.6) microM, respectively] or prolonged [100.7 (18.7) microM and 86.2 (9.1) microM, respectively] exercise performed under cooler environmental conditions. Significant correlations were found between the duration of the final stage of exercise during hot/humid environmental conditions and increased levels of lipid hydroperoxides (r = 0.85), GSSG (r = 0.94), xanthine (r = 0.92) and uric acid (r = 0.96). Exercise also decreased the iron (Fe)-binding antioxidant activity of the plasma and increased the total plasma Fe levels, although this was only significant for prolonged exercise in ambient conditions. There was no detectable free Fe in the plasma at any stage of exercise. Other changes in biochemical parameters had returned to pre-exercise levels within 24 h after exercise. The results show that exercise can induce changes in biochemical parameters that are indicative of oxidative stress in the fit horse and that this was, exacerbated during exercise at high temperature and humidity.
Pulmonary artery pressure during exercise in the horse after inhibition of nitric oxide synthase.
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Use of the Wet Bulb Globe Temperature (WBGT) index to quantify environmental heat loads during three-day-event competitions.
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Gait, estimated net cost of transport and heat production at different speeds in three-day-event horses.
Heart rate and gait characteristics (stride length and frequency) were studied in 6 horses subjected to a standardised incremental exercise test, involving moving at the trot and increasing speeds up to a fast gallop and subsequently during the steeplechase phase of a 3-day-event. The studies were performed in hot conditions. Appropriate scaling, based on hindleg length (hh), stride length (L), stride frequency (f) and speed (Sp) for nondimensional stride length (lambda = (L/hh), nondimensional stride frequency (phi = f(hh/g)1/2) and nondimensional velocity (û = Sp/(ghh)1/2), where g is the gravitational acceleration, demonstrated that there were no major differences in characteristics over the full range studied lambda = 2.3 û0.68. However, there were subtle differences in some horses that could endow a benefit in locomotory efficiency when compared to others exercising at the same absolute speed. There were clear changes in the relationship between nondimensional stride length and frequency with increasing speed, from trot to canter (at û approximately 1.4) and to full gallop (at û approximately 2.3); when trotting, lambda was less than 2.2 and the transition from canter to gallop took place at lambda approximately 3.2. The cost of transport/kg/m, estimated from the heart rates measured continuously during each study, decreased with increasing speed and bodyweight. In some animals, there appeared to be a weak minimum around the canter-gallop transition speed. When interpreted as oxygen cost, using published values for the oxygen consumption-heart rate relationship, the cost fell from an average of 0.201 ml/kg/m at the trot to 0.161 mlO2/kg/m when galloping during the incremental exercise tests. During the steeplechase, the cost was approximately 7.5% higher than at the same speed in the exercise tests; this was probably due to jumping effort. Estimated power consumption increased linearly with speed. In the steeplechase, power consumption was also 7.5% higher than during the exercise tests at the same absolute speed; this was equivalent to an average rate of heat production of 346 kcal/min (24 kW) or 59.5 kcal/min/m2 of the measured body surface area.
Modified ventilated capsule for the measurement of sweating rate in the exercising horse.
Modification of a ventilated capsule technique for the measurement of sweating rate (SR) is described for application in an exercising horse. The plastic capsule was sealed against the coat of the horse. Dry air was blown through the capsule over the skin and the change in absolute humidity (g/m3) between air entering and leaving the capsule was used to calculate SR. The effect of flow rate on the response characteristics of the system was investigated and measured over a range of sweating rates in a group of horses completing a short exercise test. Repeatability of SR measurements made at 2 sites, one 10 cm distal to the tuber sacrale (SRS) and the other approximately overlying C4 in the neck (SRN), were compared. To encourage the rapid and complete evaporation of moisture from the area of coat enclosed by the SR capsule, it was demonstrated that the degree of saturation of the air leaving the capsule should be maintained below 15% relative humidity (RH). This was best achieved at flow rates exceeding 35 l/min. No significant difference was found between the 2 repeated tests separated by 3 days, with respect to right atrial temperature (TRA) and SR. The modified system for the measurement of SR in the exercising horse used in this study was shown to be fast responding and capable of measuring a wide range of sweating rates. A good seal was maintained throughout and the capsule was removed without causing damage to the coat.