Bidirectional forces generated along the cortex of mammalian outer auditory hair cells (OHC) lengthen and shorten the cell in response to changes of membrane potential. This electromechanical transduction process can cycle in phase with the stimulus at acoustic frequencies. We have now established that the entire mechanism of voltage dependent force generation is exclusively located in the plasma membrane, more specifically in the lateral domain between the apical tight junction belt and a region just above the basal synaptic end. The mechanism is incorporated int he plasma membrane as many locally activated elements that can be driven independently in isolated patches inside a patch electrode. The distribution of motor activity coincides with a dense array of large transmembrane proteins. Voltage dependent changes in the arrangement of proteins within this array could generate forces in the plane of the membrane and thus form the basis of electromechanical transduction. We have also found that drugs that inhibit water transport markedly affect the electrokinetic response. In another experimental approach using the calcium dye Fluo-3 we were able to determine that the cortical endoplasmic reticulum in OHC is a calcium storage compartment. We also found that calcium effects on electromechanical responses are mediated by changes in membrane conductance Coincident with the distribution of electrokinetic activity the lateral plasma membrane of OHC also contains stretch activated potassium channels with 130 pS conductance. These channels are not affected by 1 uM Gd (3+) but can be blocked by 1.5mM quinine which eliminates stretch sensitivity. This stretch sensitivity of the lateral wall may provide a feedback mechanism to the electromechanical transduction process.
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