CALMODULIN-DEPENDENT PROTEIN-KINASES MEDIATE CALCIUM-INDUCED SLOW MOTILITY OF MAMMALIAN OUTER HAIR-CELLS

Cochlear outer hair cells in vitro respond to elevation of intracellul ar calcium with slow shape changes over seconds to minutes ('slow moti lity'). This process is blocked by general calmodulin antagonists sugg esting the participation of calcium/calmodulin-dependent enzymatic rea ctions. The present study proposes a mechanism for these reactions. Le ngth changes of outer hair cells isolated from the guinea pig cochlea were induced by exposure to the calcium ionophore ionomycin. ATP level s remained unaffected by this treatment ruling out depletion of ATP (b y activation of calcium-dependent ATPases) as a, cause of the observed shape changes. Involvement of protein kinases was suggested by the in hibition of shape changes by K252a, a broad-spectrum inhibitor of prot ein kinase activity. Furthermore, the inhibitors ML-7 and ML-9 blocked the shape changes at concentrations compatible with inhibition of myo sin light chain kinase (MLCK). KN-62, an inhibitor of Ca2+/calmodulin- dependent protein kinase II (CaMKII), also attenuated the length chang es. Inhibitors with selectivity for cyclic nucleotide-dependent protei n kinases (H-89, staurosporine) were tested to assess potential additi onal contributions by such enzymes. The dose dependence of their actio n supported the notion that the most likely mechanism of slow motility involves phosphorylation reactions catalyzed by MLCK or CaMKII or bot h.