Evidence that phosphoinositides mediate motility in cochlear outer hair cells

Cochlear outer hair cells are postulated to modulate auditory transduction by their mechanical properties which presumably are controlled by efferent neurons and regulated by the levels of intracellular calcium. In a number of biological systems these calcium levels are controlled by inositol trisphosphate (InsP3), the second messenger of the phosphoinositide cascade. We have investigated whether the phosphoinositides function in the signal transfer in mammalian auditory sensory cells. Live isolated outer hair cells synthesize the lipids of the phosphoinositide cycle, phosphatidic acid (Ptd), phosphatidylinositol (PtdIns), phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). After one hour of incubation with [32P]orthophosphate, Ptd constituted 21 +/- 7% of labeled lipids, PtdIns 19 +/- 5%, PtdInsP 28 +/- 7% and PtdInsP2 33 +/- 7%. Contractions were elicited in isolated cells after their membrane was permeabilized with Triton X-100. Addition of calcium and ATP resulted in contractions (average, 7.2% of initial cell length) in 81% of the cells studied; the omission of ATP or calcium (i.e. the presence of the calcium-chelator EGTA) reduced the incidence of contractions to 0% and 17%, respectively. In the absence of calcium, the putative second messenger, InsP3, caused contractions in 63% of the observed cells while the physiologically inactive agents inositol and inositol bisphosphate failed to elicit responses. The results are compatible with the suggestion that InsP3 mediates contractility in outer hair cells.