Minimal concentrations of metabolic substrates capable of supporting cochlear potentials

The objective of this study was to determine the capability of glucose analogues, as well as lactate and pyruvate, to maintain the endolymphatic potential and the cochlear microphonics. In addition, the minimum concentration at which different substrates (including D-glucose) were able to sustain the potentials ('critical' concentration) was determined. Synthetic blood containing various substrates at different concentrations was perfused via the anterior inferior cerebellar artery. The critical concentration for D-glucose was found to be 15 mg% (0.83 mM). l-Glucose, galactose and fructose were not able to support the potentials at concentrations as high as 200 mg%. On the other hand, mannose was capable of supporting the potentials; however, the critical concentration (50 mg% or 2.8 mM) was substantially higher than that of D-glucose. Both lactate and pyruvate could support the potentials, but the critical concentrations (8.5 mM and 6.5 mM, respectively) were markedly higher than in the case of glucose, even when the difference of carbon equivalents was taken into consideration. The data are discussed in the context of the intermediary metabolism and possible carrier systems of the stria vascularis.