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

We determined the effects of food supply and low-intensity training on grow th, serum thyroid hormone levels and the Na+,K+-pump concentration in equin e skeletal muscle. Twenty-two Shetland ponies were subjected to two differe nt feeding regimes for 2(1)/(2) years (11 ponies per group): food restricti on (body condition score kept at 2) or an libitum fed (body condition score kept at 8). Five ponies in each group underwent low-intensity training. Gl uteus medius muscle and serum samples were obtained in April 1998. Subseque ntly, 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 (T-4) level was increased, the free T-4 remaine d at the control level. The serum total tri-iodothyronine (T-3) and free T- 3 were reduced by 30% and 49% respectively. After 6 months of refeeding the re 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 m edius 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 th e 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 T-3. The reduction in serum T-3 only slightly influ enced the Na+,K+-ATPase concentration in skeletal muscle, indicating that m uscle tissue of different species may respond differently to changes in cir culating thyroid hormones.