LOCALIZATION OF THE CA2-BINDING ALPHA-PARVALBUMIN AND ITS MESSENGER-RNA IN EPIPHYSEAL PLATE CARTILAGE AND BONE OF GROWING RATS()

This study describes the localization of alpha-parvalbumin, in undecal cified tibial epiphyseal cartilage and bone of growing rats by immunoc ytochemistry in the light microscope, and of parvalbumin mRNA by in si tu hybridization. They were compared to the distribution of the calbin din-D9K and its mRNA in rat epiphyseal cartilage. All the chondrocytes of the epiphyseal cartilage were parvalbumin-immunopositive, but ther e was no parvalbumin immunoreactivity in the uncalcified or calcified extracellular cartilage matrix. The intensity of the immunostaining in creased from the resting and proliferative to the mature and hypertrop hic chondrocytes, with the greatest intensity in the terminal hypertro phic chondrocytes in the calcifying zone. The parvalbumin immunostaini ng was located in the cytoplasm, but no immunoreactivity was detected in any chondrocyte processes. The parvalbumin mRNA distribution and le vels, as revealed by in situ hybridization, exactly mirrored those of the parvalbumin protein. In contrast to parvalbumin, calbindin-D9K and its mRNA appeared in mature chondrocytes and decreased in hypertrophi c up to calcifying chondrocytes. Calbindin-D9K was located in the cyto plasm and all along the cell processes. In bone, the osteoblasts and t he osteocytes of trabecular and compact cortical bones were immunoreac tive for parvalbumin and contained parvalbumin mRNA. Parvalbumin lay i n their cytoplasm, but there was no parvalbumin immunostaining in the extracellular uncalcified or mineralized bone matrix. The long process es of osteocytes, in compact bone only, were parvalbumin immunoreactiv e. Osteoclasts contained cytoplasmic parvalbumin immunoreactivity. Thu s, the pattern of immunoreactive parvalbumin distribution indicates th at the protein is not involved in the extracellular mineralization of cartilage and bone matrix. It appears to be associated with specific c alcium-related intracellular functions in chondrocytes and in osteobla sts, osteocytes, and osteoclasts, As the highest cytoplasmic concentra tion of parvalbumin is in the terminal hypertrophic chondrocytes, parv albumin could act as a calcium buffer to delay the death of chondrocyt es. In compact bone, parvalbumin could also have a role throughout the osteocyte processes in regulating the fluxes of calcium ions for mine ral homeostatis.