Biochemical aspects of inner ear fluids and possible implications for pharmacological treatment.
Inner ear fluids are in dynamic equilibrium with surrounding fluids, namely blood and cerebrospinal fluid. It is known that substances injected into the blood stream or cerebrospinal fluid are transported into the inner ear fluids. The rate of transport from blood into perilymph is inversely related to the molecular weight or molecular size. There appears to exist a blood-labyrinth barrier. In general, the morphology of capillaries in the spiral ligament and stria vascularis is similar to that of brain capillaries which contribute to formation of the blood-brain barrier. Information on the volume of the inner ear fluids is necessary for estimating the toxicity of the drugs as well as the effective concentration of systemically injected substances. Recently, alteration of the levels of arachidonic acid metabolites, especially the prostaglandins, in perilymph under experimental conditions has been reported. The responses of prostaglandin levels in perilymph to these experimental conditions (aspirin injection, antidiuretic hormone or epinephrine infusion) suggest that prostaglandins may play an important role under physiological conditions. Possible mechanisms of auditory dysfunction due to abnormal prostaglandin metabolism in the auditory system are discussed.