A comparison of the ultrastructure and metabolic response of the skeletal muscle of horses performing intense treadmill exercise at 20 and 35 degreesC

The aim of this study was to determine whether the metabolic response and u ltrastructure of muscle differed when horses performed intense exercise at different ambient temperatures. Four Standardbred geldings performed treadm ill exercise, including an intensive trot of 2600 m on two different occasi ons, either at a high ambient temperature of 35 degrees C or at a temperatu re of 20 degrees C. The horses had a warm-up period of 23.5 min of submaxim al exercise, followed by 2 h of box rest before the intensive exercise. Mus cle biopsy data of adenine nucleotides, creatine phosphate, lactate and gly cogen concentrations measured before the warm-up period were similar to tho se measured before the period of intensive exercise. Muscle lactate concent rations did not differ between the two temperatures, but increased signific antly after intense exercise to levels of 34.7 +/- 8.3 mmol/kg d.w. at 20 d egrees C, and to 41.7 +/- 12.5 mmol/kg d.w. at 35 degrees C. Muscle glycoge n and creatine phosphate concentrations did not differ between the two ambi ent temperatures, but decreased significantly by 122 +/- 82 mmol/kg d.w. an d 25.2 +/- 17.4 mmol/kg d.w., respectively, after the intensive exercise. N o changes were seen in adenosine triphosphate, adenosine diphosphate and ad enosine monophosphate concentrations. The muscle biopsies were investigated by electron microscopy, and showed no marked changes in the ultrastructure of the muscle due to exercise at the two different ambient temperatures. I n conclusion, no marked changes were seen in the muscle metabolic response or in the ultrastructure of the muscle when the horses performed intense ex ercise at 35 degrees C compared to 20 degrees C.