Cell volume density alterations within the stria vascularis after administration of a hyperosmotic agent.

The purpose of this study was to determine the effects of an intravenous injection of a hyperosmotic agent (mannitol) on the volume density (Vv) of the primary components of the stria vascularis (SV). Chinchillas received either a 2.0 g/kg injection of mannitol or an equal volume of saline as a control. At 1, 10 and 60 min after the injection, the right cochleas were fixed with osmium tetroxide and prepared for transmission electron microscopy. At a distance of 70% from the cochlear apex, the complete radial area of the SV was photographed and stereologically analyzed. Additional animals received mannitol or bumetanide for the purpose of measuring serum osmolality and the endocochlear potential (EP). The present results showed elevation of serum osmolality after mannitol but not after bumetanide and depression of the +EP after bumetanide but not after mannitol. Vv alterations of SV components after mannitol were similar to those Vv changes observed in a previous study, after bumetanide. After treatment with either diuretic, the Vv of the marginal cells decreased and the Vv of the intermediate cells and intercellular spaces increased. We conclude that since the Vv alterations of the SV components are so similar after both diuretics, none of these alterations is a morphological correlate of a depressed +EP which was observed after bumetanide. A model of the action of mannitol on the SV is proposed.