Effects of training on equine muscle fibres and monocarboxylate transporters in young Coldblooded Trotters

P>Reasons for performing study:Muscular changes caused by training are breed-specific and studies on the Norwegian-Swedish Coldblooded Trotter (NSCT) are limited. Knowledge about lactate-transporters in muscle in this light draught breed used for harness racing is lacking.Objectives:To identify muscular changes associated with training in young NSCTs and investigate muscular distribution of the monocarboxylate transporter 1 (MCT1) and its ancillary protein CD147, which facilitate lactate transport across membranes.Methods:Nine horses were followed from the start of their training period until the end of their 3-year-old season. A biopsy sample of the middle gluteal muscle was collected on 4 occasions. On the last 3 sampling occasions, individual V(La4)-values (the speed corresponding to a blood lactate concentration of 4 mmol/l) were determined in an incremental exercise test on a high-speed treadmill. One horse was excluded due to lameness. Histochemical and immunohistochemical analyses were performed on all muscle samples to determine fibre types (I, IIA, IIAX, IIX), oxidative capacity (NADH) and the expression of MCT1 and CD147. The activity of selected metabolic enzymes in the muscle before and after training was determined.Results:The percentage of type IIX fibres decreased with training while the percentage of type IIAX fibres increased. The activity of citrate synthase and 3-OH-acyl-CoA-dehydrogenase increased with training. The expression of MCT1 was lower in membranes and cytoplasm of type IIX fibres compared to all other fibre types both before and after training. The antibody against CD147 stained membranes and cytoplasm of all fibres. The first V(La4)-value was lower than the last 2 in all horses.Conclusions:Muscular changes with training of NSCTs were similar to those reported in Standardbreds, indicating fibre type transitions and increased oxidative capacity. Expression of MCT1 differed among fibre types and was related to the oxidative capacity of the fibres.