Changes in serum osmolarity influence the function of outer hair cells

Fast motility of outer hair cells (OHC) is thought to be based on a hydrome chanic principle. In vitro, the function of OHCs can be disturbed by a chan ge in the osmolarity of the culture medium. Whether changes in the serum os molarity in vivo can also interfere dth OHC motility has not been investiga ted as yet. Serum osmolarity of New Zealand White rabbits (n = 18) was elev ated by a continuous infusion of glucose 40%, decreased by an infusion of a qua dest, or kept constant by an infusion of saline. OHC function was monit ored using distortion products of otoacoustic emissions (DPOAE). Input:outp ut curves were established between 2 and 5 kHz (geometric mean of f2) with primaries of levels between 35 and 55 dB SPL. Cochlear perfusion was measur ed using a fluorescence microsphere method. Elevation of the serum osmolari ty from 306 +/- 17 mosm/l to 365 +/- 23 induced a decrease of DPOAE between 3 and 12 dB SPL. Cochlear blood flow increased from 0.11 +/- 0.09 to 0.15 +/- 0.10 ml/min/g. When decreasing the serum osmolarity from 303 +/- 9 to 2 81 +/- 8 mosm/l, only slight changes of the DPOAE could be verified. As in the control group, cochlear perfusion was almost unchanged. In the control group, neither serum osmolarity nor DPOAE changed. Comparable to findings i n vitro, increasing the serum osmolarity can lead to a disturbance of OHC f unction. In patients suffering from sudden hearing loss, dehydration due to physical or mental stress is often observed. This new and promising pathop hysiological concept needs further clinical evaluation.