NITRIC-OXIDE AND EXERCISE IN THE HORSE

1. The effects of exercise on the production rate of nitric oxide (NO) in exhaled air (V over dot(NO)) and the effects of inhaled NO (80 p.p .m.) on cardiovascular and respiratory parameters were investigated in five Thoroughbred horses. 2. The concentration of NO ([NO]) in exhale d 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 m in(-1): when trotting at 3.7 m s(-1) the values mere 0.78 +/- 0.15 and 1.23 +/- 9.14 p.p.b., respectively, and when cantering at 9 m s(-1) t he 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)-adrenergi c 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. 4. Exercise induced a linear increase in V over dot(NO) with work i ntensity to a maximum (428.1 +/- 31.6 pmol min(-1) kg(-1)) which coinc ided with the maximal oxygen uptake for the horses (138.3 +/- 11.7 ml min(-1) kg(-1)), although a further increase in V over dot(NO) (779.3 +/- 38.4 pmol min(-1) kg(-1)) occurred immediately after exercise. The changes in V over dot(NO) 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.) signifi cantly (P < 0.05) reduced the pulmonary artery pressure: during the fi rst minute, pulmonary artery pressure was 83.1 +/- 7.6 mmHg compared w ith 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 respira tory indices, including cardiac output, measured during exercise betwe en control and inhaled NO tests. 6. The results show that exhaled NO i s released from the airways of the horse and may contribute to the reg ulation of pulmonary vascular tone during exercise.