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 equ ally fed (within blocks) fortified corn-soybean meal diets (1.30% lysine) c ontaining 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 (pS T)/d. Following a 7-d adjustment period, total collection of feces and urin e was performed during two periods (d 1 to 10 and d 20 to 30) for the deter mination of Ca and P apparent digestibility (absorption) and retention. Pig s were bled after each period(d 10, 20, and 30) for the determination of se rum metabolites associated with Ca, P, and bone metabolism. Feed intake for the 30-d period averaged 2,020 g/d. There were no treatment x period inter actions, so the absorption and retention data were pooled across periods. T he 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 < .0 1) 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 re tention on an absolute basis (g/d) were more pronounced in pST-treated pigs consuming the higher Ca and P diet (interaction, P < .10). Serum concentra tions of 1,25-dihydroxyvitamin D-3, osteocalcin, and IGF-I on d 10 and 30 w ere increased (P < .07) with PST administration. However, the increases in 1,25-dihydroxyvitamin D-3 and osteocalcin in pST-treated pigs were more pro nounced when the lower dietary Ca and P levels were fed (interaction, P < . 08). Urinary excretion of hydroxyproline increased (P < .01) with pST admin istration, 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 C a and P level. However, serum measures associated with Ca, P, and bone meta bolism in pST-treated pigs were dependent on the Ca and P content of the di et, suggesting an effect of pST on the homeostatic control of Ca, P, and bo ne metabolism.