Caroline M. Scott

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.

Quantification of the response of equine apocrine sweat glands to β2-adrenergic stimulation

The aim of the present study was to characterise the quantitative sweating response of the horse to beta2-adrenergic stimulation. The sweating responses of 6 horses to the randomised infusion of 8 different adrenaline concentrations (0.025, 0.05, 0.075, 0.1, 0.2, 0.4, 1.0 or 2.0 microg/kg bwt/min), was investigated. Sweating rate (SR) and skin temperature (TSK) on the neck (N) and gluteal region (G), and plasma adrenaline and noradrenaline concentrations were measured. Peak SR was approximately 15 (N) and approximately 9 g/m2/min (G) during infusion of both 1.0 and 2.0 microg/kg bwt/min adrenaline. Sweat produced per nmol/l plasma adrenaline peaked during the infusion of 0.075 microg/kg bwt/min adrenaline. Higher adrenaline infusion concentrations resulted in a progressive decrease in the amount of sweat produced per nmol/l plasma adrenaline and a plateau of 6 g/m2/(nmol/l) plasma adrenaline was reached for infusions between 1.0 and 2.0 microg/kg bwt/min. Peak SR were far lower than we have previously reported during exercise. There was no evidence of sweat gland fatigue or vasoconstriction during infusion, suggesting saturation of sweat gland beta2 receptors. We conclude that sweating in the horse is under dual control from a combination of hormonal and neural mechanisms.

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.

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.

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.