Muscle ATP loss and lactate accumulation at different work intensities in the exercising Thoroughbred horse

The effect of 2 min treadmill exercise, at speeds of 6-12 m.s-1 on an incline of 5 degrees, upon muscle adenine nucleotide loss and lactate accumulation was studied in six Thoroughbred horses. Minimal change occurred in the adenosine triphosphate (ATP) content of the middle gluteal muscle at speeds of 10 m.s-1 or less, but significant loss (up to 47%) had occurred in all horses by 12 m.s-1. The decline in ATP significantly correlated with the accumulation of muscle lactate, beginning shortly after the accumulation of 40 mmol.kg-1 dry muscle lactate. Decline in muscle ATP was mirrored closely by the appearance of ammonia, and to a lesser extent, hypoxanthine and uric acid in plasma. The results suggest that peak accumulation of any of these, or simply the concentration at a specified recovery time, may be used as a measure of ATP loss in the musculature as a whole. This was not so in the case of xanthine, which may also be formed from the degradation of guanidine nucleotides. An In-In plot of plasma ammonia against treadmill speed indicated a break point in accumulation between 8 and 9 m.s-1. The kinetics of ammonia accumulation with speed differed from those of lactate.