Morphologic Effects of Glycerol and Urea on Cochlear Tissues of the Chinchilla

Abstract

Glycerol and urea are used as test agents in confirming the diagnosis of endolymphatic hydrops. Although both substances act as osmotic diuretics, recent evidence suggests that they may have differing physiologic effects on the inner ear. This study was designed to compare the morphologic effects of urea and glycerol on cochlear tissues, using the chinchilla as an experimental model. Animals were given subcutaneous injections of glycerol (2 g/kg) or urea (1.2 g/kg) over periods of 3 hours, 4 days, or 1 week. Both agents were found to produce ultrastructural changes, including spiral ligament vacuolization, intracellular alterations of the stria vascularis, and increased numbers of Hensen's bodies in outer hair cells. These alterations appeared indicative of metabolic stress, but not toxicity. The morphologic findings provided no evidence that glycerol and urea affect the inner ear by fundamentally different mechanisms of action.

Physiological functions of urea transporter B

“…The contents of urea and glycerol are commonly used for diagnosis of Meniere’s disease. Urea can increase the permeability gradient between blood and inner ear fluid and reduce the volume of endolymph hydrops [7, 30, 87]. The steady state of the volume, pressure, and chemical composition in the endolymph is crucial for the electromechanical conduction of sound in the ear.…”

Electrocochleographic Evaluation of the Guinea Pig Model of Endolymphatic Hydrops

“…5 Likewise, a similarity was found in the effect of the two dehydrating agents on cochlear blood flow and on the vasodilatory response threshold. 6 In a study of patients with Meniere's disease, Van de Water et al 7 noted that glycerol produced greater shifts in serum osmolarity than did urea.…”

Glycerol Administration Modulates the Pattern of the 3-Fucosyl-N-Acetyl-Lactosamine (CD15) Epitope in the Adult Guinea Pig Inner Ear

“…Glycerol has usually been regarded to act as an osmotic diuretic withdrawing fluid from the expanded endolymphatic space [1], However, recent observations favour the view that glycerol inter feres with local regulatory mechanism(s) of fluid compart ments inducing short-lasting structural changes of compo nents of the transduction apparatus [2][3][4], In addition, glycerol affects the morphology of the endolymphatic sac (ES) [5,6], Structural changes of the ES found after glycer ol administration were paralleled by increased intra-epithelial synthesis of carbohydrate residues and their secre tion into the sac lumen and degradation [7,8], It has been suggested that such residues may help to prevent the col lapse of the lumen of the ES and to regulate transepithelial fluid transport [8], Several carbohydrate complexes -glycoconjugates (GCs) -have been localized in the inner ear of adult and developing animals. They have been identified at differ ent locations in the inner ear, such as the cupula, the oto lithic membrane, the tectorial membrane (TM) as well as in the glycocalyx of the sensory organs and in the ES and its lumen [reviewed in ref.…”