Different Dietary Calcium and Phosphorus Inclusion Levels Alter Satellite Cell Activity in Neonatal Pigs

Postnatal skeletal muscle growth requires satellite cell activation and fusion with existing myofibers. Delay or absence of satellite cell proliferation and differentiation leads to a reduction in postnatal muscle hypertrophy and reduced growth potential. Calcium and phosphate are necessary for normal muscle development, and deficiencies cause altered satellite cell function and reduced muscle accumulation. Dietary PO₄ deficiencies were shown to cause decreased satellite cell proliferation in vitro while excess PO₄ caused increased proliferation. The objective of this study was to determine proliferation and differentiation of satellite cells in response to combined Ca-PO₄ deficient, adequate, and excess diets in neonatal pigs. Twenty-seven newborn piglets (24h±6h age) were assigned to dietary treatments, based on NRC requirements, containing either an adequate Ca-PO₄ available diet, a 25% deficient Ca-PO₄ available diet, or a 25% excess Ca-PO₄ available diet over a 17-day trial. Feed intake and body weight were recorded daily, and blood samples collected at 8d and 16d. Oral bromodeoxyuridine (BRDU) was administered 18hrs prior to sacrifice. Following sacrifice, longissimus dorsi muscle (LD) was collected for immunohistochemistry and satellite cell (SC) isolation. Isolated SC were cultured for in vitro proliferation and differentiation assays. Dietary treatments altered serum phosphate where adequate treatment increased serum phosphate by 8% compared to the deficient diet at 8d, but was 12% less in the excess diet compared to the deficient at 16d (P<0.05). There were no differences in average daily gain between treatments (P=0.16). Dietary deficiency reduced proliferating satellite cells (BRDU/MyoD+) in vivo (P=0.02) compared to adequate and excess diets. However, in vitro SC from excess diets tended to proliferate 13.7% (P= 0.08) less than SC from adequate diets with deficient diets being intermediate and not different from the two (P≥0.49). Similarly, myoblast fusion rates were greatest in adequate diets with deficient diets having 9.2% lower fusion rates (P= 0.05), and excess diets exhibiting 20.7% lower than those fed adequate diets (P<0.001). These data indicate that diets deficient in Ca-PO₄ decrease satellite cell activity which may lead to a reduction in postnatal muscle growth. Additionally, excess dietary Ca-PO₄ appears to reduce satellite cell proliferation and differentiation in vitro. Diets adequate in Ca-PO₄ have the greatest proliferation and differentiation rates which may improve the growth potential of the animal.