EVIDENCE FOR CALCIUM-BINDING PROTEINS AND CALCIUM-DEPENDENT REGULATORY PROTEINS IN SENSORY CELLS OF THE ORGAN OF CORTI

Calcium is thought to play a major signaling role in outer hair cells to control metabolism, cytoskeletal integrity, cell shape and cell exc itability. For this to happen, in resting cells the concentration of f ree calcium ions must be maintained at low levels so that focal increa ses can trigger specific events. In this paper, the localization of ca lcium, calcium-binding and calcium-dependent regulatory proteins in se nsory cells from the guinea pig inner ear was demonstrated using immun ocytochemical and histochemical techniques. We found the calcium buffe r and/or calcium sensor proteins calmodulin, calbindin and calsequestr in predominantly in sensory cells and that when present, these protein s can be enriched in the outer hair cells. Calmodulin is found in the stereocilia, in the cuticular plate and in the cytoplasm and calbindin is found only in the cuticular plate and cytoplasm of both the inner and outer hair cells. The staining for these proteins in the outer hai r cells is homogeneous, with no apparent compartmentalization along th e lateral wall. Calsequestrin, thought to store and release calcium fr om membrane bound intracellular storage sites is found only in the cyt oplasm of outer hair cells. There, it has a more punctuate staining pa ttern than does calmodulin or calbindin suggesting that it may be pres ent in calciosomes rather than soluble in the cytoplasm. We did not de tect caldesmon and S-100. Using the potassium pyroantimonate technique , we found precipitates containing calcium ions distributed throughout the cytoplasm of outer hair cells, with no evidence that the subsurfa ce cisterns along the lateral wall act as calcium storage sites. Thus, calcium in resting cells is found in the cytoplasm along with calbind in and calmodulin and appears to have a punctate distribution consiste nt with a co-localization with calsequestrin. The implications of this distribution with respect to the slow shortening and elongation seen in outer hair cells are discussed.