Day, TRJ; Potter, GD; Morris, EL; Greene, LW; Simmons, JB; EQUINE NUTR & PHYSIOL SOC; EQUINE NUTR & PHYSIOL SOC
It may be possible to alter the rate of bone demineralization and remineralization by exogenous administration of growth hormone, which increases IGF-I, a local factor in osteoblastic activity in the blood This study was conducted to determine the physiologic and skeletal response in two-year old horses in race training treated with exogenous equine somatotropin (eST). Sixteen Quarter Horse geldings were paired by age (ave. age = 26 months), and one horse from each pair was assigned at random to either the eST treatment group or the control. The experiment was conducted over 112 d dining which the horses were broke to ride and trained on a dirt track in a regimen typical for race horses in training. Radiographs were taken of the left third metacarpal on d 0, 28, 42, 56, 70, 84 and 112. Jugular blood samples were collected every 14 d and analyzed for calcium (Ca), phosphorous (P), osteocalcin, 25-dihydroxyvitamin D (Vit D), parathyroid hormone (PTH), IGF-I and GH.
Daily injections of eST were well tolerated and resulted in no observed side effects. There was a trend (P=.14) for increased plasma GH concentrations in the treated horses as the trial progressed Plasma IGF-I concentrations were increased (P<.01) in the treated horses by d 14 and remained higher than the control horses for the duration of the study. As seen in other studies, there was a pattern of decreasing and subsequent increasing of both medial (P<.02) and lateral (P<.01) cortical bone density during the training period, but treatment differences were not significant. Serum osteocalcin was affected by d (P<.01) and was higher (P<.04) in the eST treated horses on d 42. Serum Vitamin D, calcium, and phosphorous were affected by d (P<.01) but, not by treatment (P>.05). In this study, administration of eST was effective in elevating plasma IGF-I, which is stimulatory to osteoblasts, and serum osteocalcin, which is a product of osteoblastic activity.