Alkaline phosphatases in endochondral ossification of rats low in calcium and vitamin D deficient

Young rats fed a low calcium and vitamin D deficient diet for 2 weeks developed hypocalcemia and increased alkaline phosphatase activity in serum. The serum alkaline phosphatase activity (pNPPase) was found to be of skeletal origin. In accordance, the total non-specific alkaline phosphatase (pNNPase) activity in the microsomal fraction of tibial epiphyseal cartilage and metaphysis increased in the deficiently fed group when compared to the normal group. An increased activity in the microsomal fraction of tibial epiphyseal cartilage and metaphysis was shown both for inorganic pyrophosphatase and total ATP-degrading enzyme activity in the deficient group. This was also found in the presence of R 8231, indicating an increased activity of Ca2+-ATPase, shown to be present in both the epiphyseal plate and the metaphysis. These increased enzyme activities were consistent with the known effects of hypocalcemia and/or parathyroid hormone (PTH) on bone alkaline phosphatase activity. The increase in Ca2+-ATPase might, however, be a direct response to the hypocalcemia present in the deficient animals. Furthermore, the findings in the present study support the view that the same alkaline phosphatase iso-enzyme is present at different calcification loci.