Effects of hyperosmotic solutions on endolymphatic pressure

Hydrostatic pressure changes in endolymph as well as perilymph and cerebrospinal fluid (CSF) following intravenous injection of glycerol, urea, and mannitol were studied in the guinea pig using a servo-controlled micropipet system. After the initial rise in pressure observed during and immediately after injection of each hyperosmotic agent, all pressures fell below baseline levels for about 20 minutes. The largest reductions in pressure with each of the hyperosmotics showed no significant difference between CSF and labyrinth. In the glycerol and urea studies, labyrinthine pressures stabilized at their minimum levels 20 to 40 minutes after injection, while CSF pressure recovered slowly. However, alteration of labyrinthine pressures induced by mannitol paralleled that of CSF. These results may suggest a possible osmotic dehydration effect in the labyrinth for glycerol and urea, but an ambiguous effect for mannitol.