EFFECTS OF EXOGENOUS GROWTH-HORMONE ON BONE MINERALIZATION AND REMODELING AND ON PLASMA CALCITRIOL IN INTACT PIGS

Growing pigs were given subcutaneous (SC) injections of 40 mu g porcine GH/kg body weight (BW) or its vehicle twice daily for 2 months. Animals were pair-fed with a diet containing 1.1% Ca, 0.6% P, and 1000 IU vitamin D-3/kg. At slaughter, several bone characteristics including histomorphometric data (using double tetracycline labeling) were measured on tibia and metacarpals. GH accelerated growth, with greater (p < 0.01) tibial and metacarpal weights, greater tibial length (p < 0.01) and diameters (outside and inside, p < 0.01), and greater tibial ash weight (p < 0.02) in GH-treated pigs than in controls. The similar values of apparent bone density (weight/volume) and ash/bone volume or ash/dry matter in the two groups suggest adequate coupling between bone growth and mineralization in GH-treated pigs. Histomorphometric data for the distal metacarpal metaphysis indicated greater trabecular bone volume (p < 0.01), osteoblastic surface (p < 0.01), and mineral apposition rate (p < 0.05) in GH-treated pigs. The osteoclast surface, lacuna depth, and osteoid-related parameters in GH-treated and control pigs were similar. The plasma PTH of the two groups of pigs were similar throughout the experiment. These data and the elevated plasma alkaline phosphatase activity (p < 0.05) in GH-treated pigs suggest that GH specifically affects bone formation. GH had no effect on the plasma 25-OH vitamin D-3 but 1,25(OH)(2) vitamin D-3 (calcitriol) was higher (p, < 0.01) in treated pigs throughout the experiment. This suggests that calcitriol may help adapt bone mineralization to accelerated bone formation during growth hormone treatment. Plasma phosphate (p < 0.05) and insulin-like growth factor (IGF-I, p < 0.01) were elevated, whereas plasma calcium did not change in the GH-treated pigs. Because the plasma PTH of the GH-treated pigs was unchanged, the elevated plasma P may result from directly or indirectly, via IGF-I stimulation of phosphate tubular reabsorption by GH. Thus exogenous GH increases bone mass via bone formation in growing pigs. GH is also involved in vitamin D and phosphorous metabolism.