EFFECT OF EXERCISE ON ERYTHROCYTE BETA-ADRENERGIC RECEPTORS AND PLASMA-CONCENTRATIONS OF CATECHOLAMINES AND THYROID-HORMONES IN THOROUGHBRED HORSES

The effects of exercise stress on erythrocyte beta-adrenergic receptor characteristics and plasma concentrations of adrenaline, noradrenalin e and thyroid hormones were studied in Thoroughbred racehorses during rest and after exercise.Five minutes after a maximal speed race of 120 0 +/- 200 m (mean +/- s.d.), both plasma adrenaline and noradrenaline concentrations increased with respect to basal values (from 2.48 +/- 0 .15 to 3.83 +/- 0.27 and from 2.13 +/- 0.11 to 3.53 +/- 0.27 nmol/l re spectively). The increment of adrenaline was greater in high performan ce (HP) as compared to low performance (LP) horses (76.9 vs. 43.5%), i n accordance with the contribution of the adrenal medulla in the sympa thoadrenal response to exercise. Triiodothyronine (T-3), but not thyro xine (T-4) levels increased 5 min after exercise (from 55.6 +/- 2.9 to 81 +/- 3.7 ng/dl and from 0.67 +/- 0.04 to 0.70 +/- 0.05 mu g/dl resp ectively). No differences were observed in basal values of thyroid hor mones or in the percentage of T-3 increment, when comparing HP vs. LP horses. Erythrocyte membranes obtained 5 min after racing showed decre ased concentrations of beta-adrenergic receptors (beta-AR) and dissoci ation constant as compared to basal values (50.1 +/- 7.0 vs. 95.7 +/- 12.0 fmol/mg protein and 0.97 +/- 0.24 vs. 2.04 +/- 0.3 nmol/l respect ively). This temporal pattern suggest that the observed changes in bet a-AR characteristics could be mediated by catecholamines, but not by t hyroid hormones, in this model. This down regulation of beta-AR may ac t as a protecting mechanism preventing the erythrocytes from the decre ase in membrane fluidity known to be provoked by adrenergic agonists. The accomplished study showed that, in the Thoroughbred horse, there i s a homeostatic response to race stress, characterised by a sudden inc rease in plasma catecholamines and T-3 and a parallel decrease in beta -AR concentration on the erythrocyte membrane. In this way the racing horse could rapidly adjust its metabolism to the exercise stress, but at the same time override one possible undesirable side-effect caused by these hormonal changes. further studies will be required to establi sh performance-related differences occurring in endocrine changes.