Effects of Porcine Somatotropin on Calcium and Phosphorus Balance and Markers of Bone Metabolism in Finishing Pigs

Six sets of four littermate barrows initially averaging 75.5 kg BW were equally fed (within blocks) fortified corn-soybean meal diets (1.30% lysine) containing two concentrations of Ca (.50 and 1.00%) and P (.45% and .90%) in a 34-d test. One-half of the pigs were injected with 4 mg of porcine ST (pST)/d. Following a 7-d adjustment period, total collection of feces and urine was performed during two periods (d 1 to 10 and d 20 to 30) for the determination of Ca and P apparent digestibility (absorption) and retention. Pigs were bled after each period (d 10, 20, and 30) for the determination of serum metabolites associated with Ca, P, and bone metabolism. Feed intake for the 30-d period averaged 2,020 g/d. There were no treatment × period interactions, so the absorption and retention data were pooled across periods. The absorption and retention of Ca and P were greater (P < .01) in pigs fed the higher Ca and P levels. Within each Ca and P level, pST reduced (P < .01) fecal Ca and P excretion. Administration of pST did not affect urinary P excretion, but it increased (P < .03) urinary Ca excretion in pigs fed the low-Ca diet. The absorption and retention of Ca and P were increased (P < .01) by pST; however, the increases in Ca retention and P absorption and retention on an absolute basis (g/d) were more pronounced in pST-treated pigs consuming the higher Ca and P diet (interaction, P < .10). Serum concentrations of 1,25-dihydroxyvitamin D₃, osteocalcin, and IGF-I on d 10 and 30 were increased (P < .07) with pST administration. However, the increases in 1,25-dihydroxyvitamin D₃ and osteocalcin in pST-treated pigs were more pronounced when the lower dietary Ca and P levels were fed (interaction, P < .08). Urinary excretion of hydroxyproline increased (P < .01) with pST administration, but this effect was more pronounced in pST-treated pigs fed the lower Ca and P diet (interaction, P < .09). These results suggest that pST increases the absorption and retention of Ca and P independent of dietary Ca and P level. However, serum measures associated with Ca, P, and bone metabolism in pST-treated pigs were dependent on the Ca and P content of the diet, suggesting an effect of pST on the homeostatic control of Ca, P, and bone metabolism.