A ryanodine receptor-like molecule expressed in the osteoclast plasma membrane functions in extracellular Ca²⁺ sensing

Ryanodine receptors (RyRs) reside in microsomal membranes where they gate Ca²⁺ release in response to changes in the cytosolic Ca²⁺ concentration. In the osteoclast, a divalent cation sensor, the Ca²⁺ receptor (CaR), located within the cell's plasma membrane, monitors changes in the extracellular Ca²⁺ concentration. Here we show that a RyR-like molecule is a functional component of this receptor. We have demonstrated that [³H] ryanodine specifically binds to freshly isolated rat osteoclasts. The binding was displaced by ryanodine itself, the CaR agonist Ni²⁺ and the RyR antagonist ruthenium red. The latter also inhibited cytosolic Ca²⁺ elevations induced by Ni²⁺. In contrast, the responses to Ni²⁺ were strongly potentiated by an antiserum Ab¹²⁹ raised to an epitope located within the channel-forming domain of the type II RyR. The antiserum also stained the surface of intact, unfixed, trypan blue-negative osteoclasts. Serial confocal sections and immunogold scanning electron microscopy confirmed a plasma membrane localization of this staining. Antiserum Ab³⁴ directed to a putatively intracellular RyR epitope expectedly did not stain live osteoclasts nor did it potentiate CaR activation. It did, however, stain fixed, permeabilized cells in a distinctive cytoplasmic pattern. We conclude that an RyR-like molecule resides within the osteoclast plasma membrane and plays an important role in extracellular Ca²⁺ sensing.