Outer hair cells (OHCs) amplify sound by an electromechanical mechanism. Sound-induced vibrations cause OHC membrane potential (E) to change from its resting voltage (Em) to a new value (Em+dE). The induced receptor potential (dE) initiates charge movement (Q) and force production to counteract viscous losses incurred by the traveling wave. Q exhibits a sigmoidal function with E and, because it is most sensitive to dE at the midpoint (E = Vm), Em should equal Vm to ensure maximum amplification.
OHCs were isolated from prepubertal and adult guinea pigs, and Vm measured with whole-cell patch-clamp under constant intracellular pressure in presence of a KCNQ4 blocker to ensure robust voltage clamp. The relationships reflect the OHC transmembrane electric field dependence upon the sexual maturity of the male guinea pigs. For OHCs from prepubertal males (median weight, mw, 355g), Vm is constant (60±20mV) across the cochlea and the charge (0.0015pC/µm2) and peak (0.001pF/µm2) densities are uniform. For OHCs from adult males (mw, 600 g) Vm increases with area of lateral wall, ALW (0.002-0.003mV/µm2) and the charge (1.24/ALW, pC/µm2) and peak (9/ALW) densities vary inversely with area. Similar relationships were observed for OHCs from prepubertal females (mw, 289g) as for the adult males. We estimate the net valence of the charge is positive from the relationship between charge density and Vm suggesting intrinsic charged groups of prestin contribute to Q. After correcting for physiological temperature, the results show our in vitro Vm overlays with Em measured in vitro in gerbil and rat (Johnson 2011) and exhibits a coincidence rate of 40% with Em measured in vivo in the guinea pig (Cody 1987, Kossl 1992, Dallos 1985). More measurements are required to establish whether this coincident rate results from technical shortfalls or is representative of the mammalian cochlea over the measured frequency range (0.1-22 kHz).