Nb. Slepecky et M. Ulfendahl, EVIDENCE FOR CALCIUM-BINDING PROTEINS AND CALCIUM-DEPENDENT REGULATORY PROTEINS IN SENSORY CELLS OF THE ORGAN OF CORTI, Hearing research, 70(1), 1993, pp. 73-84
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.