Diet manipulation, muscle glycogen metabolism and work performance in the equine

Typescript (photocopy).Two longitudinally designed experiments to study the influence of diet and exercise manipulation on muscle glycogen and performance of aerobic work on a treadmill and anaerobic work pulling a weighted sled were conducted with 5 and 4 mature geldings respectively. In the aerobic experiment horses were treated using a graded exercise challenge to fatigue administered before the trial began, after a 28-day conditioning period, at the end of a 5-day glycogen depletion period and at the end of a 3-day glycogen repletion period. Muscle biopsies for glycogen analyses were taken before and after each test. In addition, heart and respiration rates and blood samples for analyses of lactic acid and glucose were taken at rest, 10 and 30 minutes of exercise (EX), at fatigue (FTG) and 2, 5, 10, 30 and 60 minutes of recovery (REC). Time to fatigue (FTGT) was also recorded.Muscle glycogen was significantly reduced after depletion and significantly increased after a 3-day repletion period when compared to either pre-trial or post-conditioning measurements. While increasing muscle glycogen stores had no effect (P > .05) on FTGT, when muscle glycogen stores were depressed FTGT was significantly shortened and horses were unable to achieve maximum heart rates at FTG. Further blood glucose and lactic acid were depressed (P < .05) at FTG and throughout recovery. Increasing muscle glycogen stores elevated heart and respiration rates (P < .05) throughout recovery, an indication of a larger oxygen debt supported by the finding of elevated lactic acid levels (P < .05) at FTG and throughout recovery. Blood glucose followed a similar pattern. In the anaerobic experiment horses were tested by pulling increasing amounts of weight to FTG with tests conducted following a 28-day training period, after depletion and repletion periods as previously described and after a 2-week recovery period. Muscle biopsies for glycogen analyses were taken before and after each test and relative work performance (RWP) for each test was calculated as mass times distance. Heart and respiration rates and blood samples for analyses of lactic acid and glucose were taken at rest, after a warm up pull (WU), at FTG and 2, 5, 10, 30 and 60 REC. Muscle glycogen was again significantly depressed after depletion and higher (P < .05) after repletion compared to post-conditioning levels. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UM