EVIDENCE FOR DIFFERENTIAL REGULATION OF CALCIUM BY OUTER VERSUS INNERHAIR-CELLS - PLASMA-MEMBRANE CA-ATPASE GENE-EXPRESSION

The expression of mRNA encoding plasma membrane calcium ATPase (PMCA) subunit isoforms (1-4) and splice variants was examined in the adult a nd developing rat cochlea by PCR and in situ hybridization. High level s of PMCA mRNA expression were observed in the neurons of the spiral g anglion, and in hair cells. Spiral ganglion neurons expressed PMCA 1-3 beginning in embryonic development, reaching high levels shortly afte r birth, and continuing into adulthood. Inner hair cells expressed PMC A 1 at moderate levels from birth to the time of onset of cochlear fun ction on postnatal day 12, and strongly from then until adulthood. Out er hair cells expressed PMCA 2 at high levels from shortly after birth through adulthood. The data suggest that the calcium clearance requir ements of inner and outer hair cells are distinct. PMCA 2 is the isofo rm with the highest affinity for calmodulin, and has also been associa ted with high levels of inositol triphosphate, Its presence in outer h air cells suggests that regulation of the enzyme by calmodulin may be particularly important for this hair cell type. It further suggests th at inositol phosphate may play a unique role in the outer hair cell. ( C) 1998 Elsevier Science B.V. All rights reserved.