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 porcin e GH/kg body weight (BW) or its vehicle twice daily for 2 months. Anim als 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 h istomorphometric data (using double tetracycline labeling) were measur ed 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 w eight (p < 0.02) in GH-treated pigs than in controls. The similar valu es of apparent bone density (weight/volume) and ash/bone volume or ash /dry matter in the two groups suggest adequate coupling between bone g rowth and mineralization in GH-treated pigs. Histomorphometric data fo r the distal metacarpal metaphysis indicated greater trabecular bone v olume (p < 0.01), osteoblastic surface (p < 0.01), and mineral apposit ion 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 th roughout the experiment. These data and the elevated plasma alkaline p hosphatase activity (p < 0.05) in GH-treated pigs suggest that GH spec ifically 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 ca lcitriol may help adapt bone mineralization to accelerated bone format ion during growth hormone treatment. Plasma phosphate (p < 0.05) and i nsulin-like growth factor (IGF-I, p < 0.01) were elevated, whereas pla sma 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 re sult from directly or indirectly, via IGF-I stimulation of phosphate t ubular reabsorption by GH. Thus exogenous GH increases bone mass via b one formation in growing pigs. GH is also involved in vitamin D and ph osphorous metabolism.