Prolonged food restriction and mild exercise in Shetland ponies: Effects on weight gain, thyroid hormone concentrations and muscle Na+,K+-ATPase

Abstract

We determined the effects of food supply and low-intensity training on growth, serum thyroid hormone levels and the Na+,K+-pump concentration in equine skeletal muscle. Twenty-two Shetland ponies were subjected to two different feeding regimes for 2 1/2 years (11 ponies per group): food restriction (body condition score kept at 2) or ad libitum fed (body condition score kept at 8). Five ponies in each group underwent low-intensity training. Gluteus medius muscle and serum samples were obtained in April 1998. Subsequently, all ponies were fed ad libitum and the training programme was stopped. Muscle biopsies and serum samples were collected again in November 1998. Food restriction was associated with a 30-50% reduction of body weight gain. While the total thyroxine (T4) level was increased, the free T4 remained at the control level. The serum total tri-iodothyronine (T3) and free T3 were reduced by 30% and 49% respectively. After 6 months of refeeding there were no differences in any of the hormone levels between the ad libitum fed and the food-restricted groups. Food restriction produced a minor, but not significant, decrease in the Na+,K+-pump concentration in the gluteus medius muscle of the Shetland ponies. Low-intensity, training reduced weight gain of the ad libitum fed group by 25%, but had no detectable effect on the concentration of the Na+,K+-pumps. We conclude that prolonged food restriction in Shetland ponies results in a weight gain reduction of 30-50%, and is associated with similar decreases in serum total and free T3. The reduction in serum T3 only slightly influenced the Na+,K+-ATPase concentration in skeletal muscle, indicating that muscle tissue of different species may respond differently to changes in circulating thyroid hormones.