Plasma ammonia and lactate responses using incremental and constant speed exercise tests

Plasma NH3, formed during intense exercise, results principally from the deamination of AMP in muscle. Its formation during exercise may be influenced both by the pool of fibres recruited and by changes in the intracellular environment affecting ADP homeostasis. This study compared incremental and constant speed exercise as possible protocols for the investigation of plasma NH3 accumulation with intense exercise. Six trained Thoroughbred horses, one of which had recently been operated on for recurrent laryngeal neuropathy, undertook a step-wise treadmill test with 1 min incremental steps of 6, 8, 10, 11 and 12 m/s (7.5% incline). Two and 4 weeks later horses performed a constant-speed, maximum-exercise tolerance test at 115% VO2max (7.5% incline). Blood samples from the jugular vein were drawn at 20 s intervals in all 3 tests, for plasma NH3 and lactate. There were marked differences between and within horses in their time dependant lactate and NH3 responses to exercise. Three of the 6 horses studied showed a distinct threshold for onset of plasma NH3 accumulation with incremental exercise. Distinct thresholds for the onset of NH3 accumulation were apparent also in 5 of the 6 horses during exercise at constant rate. The present study demonstrates clearly the practicality of measuring NH3 concentration curves, even during a short incremental step test which has the advantage that other measures relating to cardiovascular and respiratory functions can be measured simultaneously.