Energy dispersive X-ray analysis of intracochlear ion shifts produced by anoxia.

Abstract

AbstractContemporary understanding of cochlear physiology has changed significantly in the last twenty years, due in large measure to experimental advances based on technological innovations. Since the early 1950's, it has been known that the cochlea is the site of several unique electrical events, among them the endocochlear potential. This potential is known to be biphasic, consisting of both positive and negative components. That the positive component of this potential is oxygen dependent, employs as part of its mechanism an ATP‐driven potassium pump, and is sensitive to several metabolic poisons is well established. It is the second or negative portion of the endocochlear potential about which significant controversy still exists.The importance of the endocochlear potential in contemporary theories of auditory function requires that more be known about its subcomponents in order that deafness be better understood. Measurements of the unique cationic concentrations within the endolymph have been made in the past in order to correlate concentration changes that may occur over time with already known changes in the negative endocochlear potential per se. Practical as well as theoretical shortcomings of studies employing micropipette, resistance measurement and ion‐specific electrode techniques have led to this investigation in which ion concentration changes during prolonged anoxia were investigated by first freeze‐drying the cochleas to be studied to prevent contamination. Second, material recovered and isolated after freeze drying was examined by energy dispersive X‐ray analysis, a highly sensitive and precise method of determining elemental concentration within biological fluids.Relative concentrations of Na+, K+ and Cl− were recorded within the first hour and at subsequent hourly intervals after decapitation of chinchillas. During the time of emergence and subsequent decay of the negative endocochlear potential, no significant change in the relative concentration of K+ or Na+ occurred. At much longer intervals (6 to 8 hours) large changes were measurable by the methods employed.K+ flux or diffusion has, in the past, been thought by many to be the likely source of the negative endocochlear potential. However, observed measurements of the relative concentration of K+ do not correlate with the magnitude of change in the negative endocochlear potential, rendering endolymphatic K+ concentration or flux an unlikely source of the negative endolymphatic potential. Other experimental data suggesting alternate regions within the cochlea such as Reissner's membrane or the organ of Corti, specifically the hair cells or supporting cells as more likely sources of the negative endocochlear potential, are reviewed.