The electron transport system in the cochlear hair cell: The ultrastructural cytochemistry of respiratory enzymes in hair cell mitochondria of the Guinea pig.

A systematic analysis of the electron transport system in hair cell mitochondria of the guinea pig is presented. This is the first such attempt to explain the underlying oxido‐reduction mechanisms for the aerobic metabolism of the hair cell. The study is based on recent developments in ultrastructural cytochemical techniques. These techniques have been modified, expanded, and applied to the study of the inner ear. The use of distyryl ditetrazolium salt (DS‐NBT) for dehydrogenase experiments and 3,3 1 diaminobenzidine (DAB) for cytochrome experiments allows precise localization of enzyme activities due to the amorphous electron opaque reaction products which do not obliterate ultrastructural detail. Rigorous morphologic and cytochemical controls are employed to confirm the specificity of the reactions studied. Enzyme systems from the three major classes of oxido‐reduction respiratory chain enzymes are studied: the pyridine‐linked dehydrogenases — lactic acid dehydrogenase, flavin‐linked dehydrogenases — succinic dehydrogenase and NADH 2 ‐diaphorase, and cytochromes — cytochrome oxidase. Our results indicate that the respiratory oxido‐reduction reactions, from NADH to cytochrome oxidase are located on the inner mitochondrial membrane, in the outer compartment and the intracristate space. This electron transport system has direction (vectorial metabolism) and anisotropy (spatial orientation). Its properties are best explained by the chemiosmotic hypothesis. The hair cell mitochondria are cytochemically indistinguishable (with regard to these four reactions) from free liver mitochondria. A cursory review of mitochondria of the stria vascularis, supporting cells of the organ of Corti, and the vestibular system reveals a similar, ultrastructural cytochemical picture. The results of these experiments are in close agreement with the studies on rat heart, liver, and muscle mitochondria, and with the accepted biochemical fractionation data. The question of whether the mitochondria can perform mechanical work, in terms of conformational changes, is open to debate. Our results demonstrate that it is hazardous to interpret the morphologic mitochondrial alterations in terms of functional activity. This is due to the inherent artifacts in all morphologic experiments which are dependent on prefixation and osmolarity alterations.