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 lowintensity 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 2½ 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 (T 4 ) level was increased, the free T 4 remained 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 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. Lowintensity 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 T 3 . The reduction in serum T 3 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.

Na+-K+Pump Regulation and Skeletal Muscle Contractility

“…Ouabain-suppressible 42 K uptake showed similar relative changes as [ 3 H]ouabain binding. A more recent study on Shetland ponies showed that food restriction causing 30 -50% reduction in body weight gain produced significant decreases in serum T 3 and free T 3 (30 and 49%, respectively), but only a minor (12-15%) nonsignificant decrease in the content of [ 3 H]ouabain binding sites in the gluteus medius muscle (399). The downregulation of Na ϩ -K ϩ pumps seen during reduced caloric intake might in part reflect reduced physical activity, although it is more rapid than that seen during inactivity.…”

Dynamics of Total and Free Iodothyronines of Jumping Horses on the Responses to Competition and Transport

“…Free T 3 (fT 3 ) is the most potent and active thyroid hormone in circulation, and its measurement can give a clearest picture of true contribution of extrathyroidal tissues, such as the musculoskeletal system, as a consequence of the thyroid stimulation and/or changes in capacity to bind iodothyronines [4][5][6]. Thyroid hormones are considered as markers of stress in horses, showing changes according to training [7], pretraining status and circadian changes [8], exercise [5,6,9], and transport and previous experience [3]. A sudden increase in plasma T 3 levels 5 minutes after exercise was obtained in Thoroughbred horses [9]; in addition, endurance exercise resulted in transient decreases in serum total and free iodothyronines [10].…”

Physiological variables of horses after road transport

“…These include: age (Irvine and Evans, 1975;Chen and Riley, 1981), breed (Malinowski et al, 1996), gender (Fazio et al, 2007), seasonal period (Flisiń ska-Bojanowska et al, 1991), altitude (Greene et al, 2002), changes in fertility (Gutierrez et al, 2002;Meredith and Dobrinski, 2004), daily rhythms (Morris and Garcia, 1983;Duckett et al, 1989), diet (Messer et al, 1995;Powell et al, 2000), training and exercise (Gonzá lez et al, 1998;Suwannachot et al, 2000) and disease, both severity and duration (Peeters et al, 2006).…”