Effects of dietary calcium depletion and repletion on dynamic determinants of tibial bone volume in two inbred strains of mice

Abstract

As an adjunct to our efforts to identify the genes that determine peak bone density, we examined phenotypic differences between two inbred strains of mice, C3H/HeJ (C3H) and C57BL/6J (B6), which are of similar size but differ with respect to peak bone density (e.g., C3H mice have 53% higher femoral bone density than B6 mice). The current studies were intended to compare the skeletal responses of C3H and B6 mice to 2 weeks of dietary calcium (Ca) depletion, followed by 2 weeks of Ca repletion. Initial studies showed that: (a) femur dry weight decreased during Ca depletion in both C3H and B6 mice (by 25% and 19%, respectively, p < 0.001) and most of this loss was recovered during Ca repletion; and (b) serum alkaline phosphatase (ALP) activity increased during Ca depletion, in both strains of mice ( p < 0.001), and returned to normal after Ca repletion. Histological analyses of ground cross sections prepared at the tibiofibular junction showed that Ca-depletion increased medullary area in both C3H and B6 mice (indicating endosteal bone loss, p < 0.01), with reversal during Ca repletion. There were no effects of Ca depletion or repletion on periosteal bone growth. Endosteal bone forming surface and endosteal mineral apposition decreased during Ca depletion and increased during repletion in both C3H and B6 mice ( p < 0.05). Net bone formation decreased during Ca depletion in C3H mice, but not B6 mice ( p < 0.01), and was normal during Ca repletion in both strains. Endosteal bone resorbing surface and net bone resorption increased during Ca depletion and decreased during repletion in both strains ( p < 0.01). A supplemental study (of Ca depletion without repletion) confirmed the effects of Ca depletion on femoral dry weight and serum ALP activity ( p < 0.001 for each). This supplemental study also showed that Ca deficiency increased serum parathyroid hormone (PTH) ( p < 0.05) and decreased (tibial) cortical bone area and cortical mineral content ( p < 0.05 to p < 0.001) in both strains of mice. Together, these data demonstrate that the skeletal responses to Ca depletion and repletion are, qualitatively, similar in C3H and B6 mice.