EVIDENCE FOR RECEPTOR-MEDIATED MINERALOCORTICOID ACTION IN RAT OSTEOBLASTIC CELLS

Aldosterone significantly enhanced the proliferation of osteoblastic c ells from rat calvaria, and this effect was inhibited by RU 26752 and ZK 91587, two antagonists specific to the mineralocorticoid receptor ( MCR). In addition, aldosterone inhibited the activity of alkaline phos phatase, a marker of the osteoblastic phenotype, and this effect was a lso reversed by RU 26752. Cytoplasmic staining for MCR was observed in rat calvaria osteoblasts incubated with a specific polyclonal antiser um raised against rat kidney MCR. This anti-MCR immunoglobulin G immun oprecipitated and macroaggregated the MCR-[H-3]RU 26752 complex in ost eoblastic cytosol. A single 98-kDa band was observed when osteoblastic cytosol was analyzed by Western blotting with anti-MCR serum. The 98- kDa band was also obtained after autoradiography of irradiated osteobl astic cytosol-[H-3]R 5020 complex, and this was abolished in the prese nce of RU 26752. A p26MR probe, specific to COOH-terminal end of MCR, hybridized with the predicted product after amplification of total cel l RNA by polymerase chain reaction technique. Furthermore, hybridizati on of poly(A)(+) mRNA from rat calvaria osteoblastic cells with the p2 6MR probe revealed a major band of similar to 4.2 kb. Collectively, ou r studies demonstrate the existence of a functional MCR in rat calvari a osteoblasts.