Metabolic and hormonal changes after exercise in relation to muscle glycogen concentrations.

To test whether, in horses, the concentration of muscle glycogen can be influenced by increasing the uptake of glucose into the muscle cells or by providing a gluconeogenic precursor, 9 trained half-bred riding horses performed on a treadmill a 1.5 h competition exercise test (CET). Each horse performed CET 3 times and 30 min after CET, each was given one of the following solutions: isotonic glucose-electrolyte (GE) solution, GE supplemented with 50 g leucine (GEL) to increase insulin secretion, or GE supplemented with 200 ml propionic acid (GEP), a gluconeogenic precursor. Administration of GE solutions caused no increase in plasma glucose concentration. The highest concentration of insulin was measured after GEL, but also in the GE group the concentration of insulin increased. GEP completely inhibited the increase in insulin concentration. Concentration of glucagon was increased 6 and 22.5 h after CET. None of the post exercise treatments influenced significantly the glycogen content at 22.5 h after CET. This indicates that neither i) elevation of insulin concentration to increase muscle-uptake of glucose, nor ii) increase in the availability of a glucose precursor, propionic acid, was able to increase accumulation of glycogen in the middle gluteal muscle.

Factors affecting accumulation of lactate in red blood cells.

In horses, both the post exercise distribution of lactate between plasma and red blood cells (RBC) and the activity of lactate transporters on the RBC membrane vary widely between individuals. In this study, we investigated the effects of pH, time and temperature on lactate distribution in vitro, and compared the in vitro activity of lactate transporters with the accumulation of lactate into RBC in vivo. To accomplish this, we took venous blood samples at rest and after trotting races. The post exercise accumulation of lactate into RBC was shown to depend on the activity of lactate transporters. The results, in vitro, also indicate that pH, incubation time and temperature influence the activity of lactate transporters and the accumulation of lactate into RBC, underscoring the fact that in practice it is important to standardise the measurement conditions of lactate. These results support the view that whole blood lactate concentrations should be measured in estimating the accumulation of lactate from exercising muscles into the blood, because the effect of blood pH, temperature, time to centrifugation of the sample and also interindividual variation in lactate transport into RBC are therefore minimised.

Effect of a post exercise fat-supplemented diet on muscle glycogen repletion.

Low muscle glycogen at the beginning of exercise may adversely affect performance, increase protein degradation and contribute to the onset of fatigue. As horses are sometimes required to compete on consecutive days both in racing and endurance types of competition, optimal muscle glycogen repletion may improve performance on the day following a race day. The purpose of this experiment was to study the effects of fat supplementation on repletion of muscle glycogen. Twelve Finnhorses performed an exercise test on a treadmill, and 2 and 4 h later they received hay and concentrate (Trial A). Two weeks later these horses performed the same exercise test and were fed the same diet supplemented with either 1000 g of carbohydrate or 400 g of vegetable oil (Trial B). A third trial (Trial C) was 3 weeks later, identical to Trial B, except that the fat group had already been adapted to dietary fat for 3 weeks. Blood samples were analysed for lactate, glucose, glycerol, triglycerides, NEFA, cholesterol, beta-OH-butyrate, insulin and glucagon and muscle samples were analysed for glycogen and triglycerides. The results indicate that in horses not adapted to fat feeding, fat supplementation slows the rate of muscle glycogen repletion, and that after an adaptation period, fat supplementation does not alter the rate of muscle glycogen repletion compared to the rate with a normal diet.