Shoichi Sawada

The effects of V2 antagonist (OPC-31260) on endolymphatic hydrops.

In the present study, two experiments were performed to investigate the influence of OPC-31260 on experimentally induced endolymphatic hydrops in guinea pigs and the regulation of aquaporin-2 (AQP2) mRNA expression in the rat inner ear. In morphological studies, the increases in the ratios of the length of Reissners membrane (IR-L) and the cross-sectional area of the scala media (IR-S) were quantitatively assessed among normal guinea pigs (normal ears) and three groups with hydropic ears: hydropic ears with no infusion (non-infusion hydropic ears), hydropic ears with an infusion of physiological saline into the scala tympani (saline-infused hydropic ears) and hydropic ears with infusion of 0.3% OPC-31260 into the scala tympani (OPC-infused hydropic ears). IR-Ls in the experimental groups were markedly larger than in the normal ear group, but there was no significant difference among the groups of non-infusion hydropic ears, saline-infused hydropic ears and OPC-infused hydropic ears. The IR-Ss of non-infusion hydropic ears and saline-infused hydropic ears (48.8-49.3%) were statistically different from that of normal ears (6.5%) (Dunnet multiple comparison test, P<0.01). However, IR-S of the OPC-infused hydropic ears (-14.8%) was significantly smaller than those of non-infusion hydropic ears and saline-infused hydropic ears (one-way ANOVA, P<0.01). In the quantitative polymerase chain reaction study, a comparison of the ratio of AQP2 and beta-actin mRNA (MAQP2/Mbeta-actin) was made between water-injected and OPC-31260-injected rats. An intravenous injection of OPC-31260 resulted in a significant decrease in MAQP2/Mbeta-actin both in the cochlea and in the endolymphatic sac (t-test, P<0.001). These results indicate that water homeostasis in the inner ear is regulated via the vasopressin-AQP2 system, and that the vasopressin type-2 antagonist OPC-31260 is a promising drug in the treatment of Menieres disease.

Aquaporin-2 regulation by vasopressin in the rat inner ear

Our previous studies have suggested a close relationship between vasopressin and endolymphatic hydrops, or the increased volume of endolymph in the inner ear. Endolymphatic hydrops is also thought to occur in Ménières disease patients. In the kidney collecting duct, vasopressin induces the expression of aquaporin-2 (AQP2), resulting in increased water reabsorption. We explored the possibility, using a quantitative PCR method, that vasopressin regulates the expression of AQP2 mRNA in the rat inner ear, as it does in the kidney. The levels of AQP2 mRNA in the cochlea and endolymphatic sac were significantly higher in rats treated with vasopressin than the levels in control animals. We speculate that over-expression of AQP2 may be involved in the formation of endolymphatic hydrops.

Aquaporin-1 (AQP1) is expressed in the stria vascularis of rat cochlea.

Cochlea endolymph, produced by the stria vascularis, is essential for normal inner ear function. Abnormal endolymphatic volumes correlate closely with pathological conditions such as Ménières disease. The critical roles played by aquaporins, which facilitate osmotic movement of water molecules, are known in a variety of tissues. We investigated the expression of aquaporin-1 (AQP1) in the rat inner ear using reverse transcription polymerase chain reaction and immunohistochemical methods. We obtained novel data showing that not just AQP1 mRNA but also AQP1 protein is expressed in the stria vascularis, in addition to other data confirming previous reports. AQP1 immunoreactivity localized to the intermediate cells in the stria vascularis. The above finding suggests that AQP1 may play a role in the water distribution associated with vigorous ion transport in the stria vascularis since the intermediate part of the stria vascularis contains both intermediate cells and the basolateral parts of marginal cells, both of which express ion transporters abundantly.

Bumetanide-induced enlargement of the intercellular space in the stria vascularis critically depends on Na+ transport

The intercellular space in the stria vascularis (intrastrial space) is a closed space and isolated from both the endolymph and the perilymph in normal tissue. Loop diuretics such as bumetanide and furosemide cause an acute enlargement of the intrastrial space in association with a decline in the endocochlear potential. It is known that bumetanide inhibits the Na+-K+-2Cl- cotransporter, which is expressed abundantly in the basolateral membrane of marginal cells. We studied ionic mechanisms underlying the bumetanide-induced enlargement of the intrastrial space using perilymphatic perfusion in guinea pigs. Perilymphatic perfusion with artificial perilymph containing 100 microM bumetanide caused marked enlargement of the intrastrial space, as reported previously. Removal of K+ from the perilymph did not affect the bumetanide-induced enlargement, whereas removal of Na+ from the perilymph inhibited it almost completely. Perilymph containing 1 mM amiloride also inhibited the enlargement of the intrastrial space almost completely. These results indicate that perilymphatic Na+, but not K+, and amiloride-sensitive pathways are essential to the bumetanide-induced enlargement of the intrastrial space. Two possible pathways could yield these results. Na+ in the perilymph could enter the endolymph via Reissners membrane or the basilar membrane; Na+ in the endolymph would then be taken up by marginal cells via the apical membrane and secreted into the intrastrial space by Na+-K+-ATPase in the basolateral membrane of them. Another, less likely possibility is that Na+ in the perilymph is transported into basal cells or fibrocytes in the spiral ligament, then into intermediate cells via gap junctions, and finally secreted into the intrastrial space via Na+-K+-ATPase of intermediate cells.

Effects of lithium on endolymph homeostasis and experimentally induced endolymphatic hydrops

There is evidence to suggest that water homeostasis in the inner ear is regulated via the vasopressin (VP)-aquaporin 2 (AQP2) system in the same fashion as in the kidney. The VP-AQP2 system in the kidney is well known to be inhibited by lithium, resulting in polyuria due to a decrease in reabsorption of water in the collecting duct of the kidney. Therefore, lithium is also likely to inhibit the VP-AQP2 system in the inner ear, and consequently exert some influence on inner ear fluid homeostasis. In this study, we investigated the effects of lithium on AQP2 expression in the rat inner ear, and on the cochlear fluid volume in hydropic ears of guinea pigs. A quantitative PCR study revealed that lithium reduced AQP2 mRNA expression in the cochlea and endolymphatic sac. Lithium application also decreased the immunoreactivity of AQP2 in the cochlea and endolymphatic sac. In a morphological study, lithium intake significantly reduced endolymphatic hydrops dose-dependently. These results indicate that lithium acts on the VP-AQP2 system in the inner ear, consequently producing a dehydratic effect on the endolymphatic compartment.

Time course of dehydrating effects of isosorbide on experimentally induced endolymphatic hydrops in guinea pigs.

Osmotic diuretics are therapeutic agents used to reduce endolymphatic hydrops. However, glycerol-induced change in endolymph volume is followed by a rebound phenomenon. In this study, we investigated the rebound phenomenon occurring with isosorbide, an osmotic diuretic used as a therapeutic agent for Ménière’s disease in Japan. Forty guinea pigs underwent surgical obliteration of the endolymphatic sac. Thirty received isosorbide orally 1 month after surgery. These animals were sacrificed 3, 6, or 12 h after isosorbide intake. The remaining 10 animals served as controls. Quantitative assessment of changes in the endolymphatic space was performed light-microscopically. Isosorbide reduced cochlear endolymph volume, with a peak reduction 6 h after intake. Thereafter, no prominent rebound phenomenon was noted. Clinically, since isosorbide is orally administered every 8 h, rebound phenomenon need not be considered in the treatment with isosorbide.

Diagnostic value of plasma antidiuretic hormone, electrocochleography, and glycerol test in patients with endolymphatic hydrops.

Objective: To investigate the relationship between the plasma antidiuretic hormone (p-ADH) level, electrocochleogram (ECoG), and the glycerol test in patients with endolymphatic hydrops (ELH). Patients and Methods: The subjects were 60 patients, including 51 with Ménière’s disease (except for cochlear Ménière’s disease), 7 with delayed ELH, and 2 with syphilitic ELH. The time period for measurements of the p-ADH level, ECoG and the glycerol test was within 4 weeks. Results: 13 patients showed positive results for all tests. 58 patients showed positive results for at least one of three tests. Only 2 patients showed negative results for all tests. Conclusion: The p-ADH level, ECoG and the glycerol test show different selectivity of ELH detection. It is useful to perform all three tests to diagnose ELH.

The effects of basic fibroblast growth factor on postoperative mastoid cavity problems.

Objectives: To present the effects of basic fibroblast growth factor on intractable cavity problems and discuss the mechanisms of its effects. Methods: We treated three ears with postoperative open cavities. All three cases had suffered from chronic discharge of the ear for 7, 10, and 30 years, respectively; 100 μg/ml of trafermin (genetic recombination) solution, as basic fibroblast growth factor, was dropped into the open cavity once daily. If bacterial and/or fungal infection was observed, antibiotics and/or antifungal agents were administered locally twice daily. Results: The cavities epithelialized and were cured within 2 months using this treatment. Conclusion: Our results suggest that basic fibroblast growth factor stimulates the proliferation and differentiation of keratinocytes, fibroblasts, and endothelial cells, leading to accelerated wound healing. The basic fibroblast growth factor agent appears to be highly effective in treating intractable cavity problems.

Immunohistochemical Localization of Histamine Receptors in Rat Cochlea

Objective: Histamine may have physiologic functions in the inner ear. The locations of histamine receptors, however, have not yet been identified in the mammalian cochlea. The aim of this study was to investigate the localization of histamine receptor subtypes (H1, H2, and H3 receptors) in rat cochlea.

Cerebrospinal fluid otorrhea in a patient with congenital inner ear dysplasia

A case of a 10-month-old boy with a cerebrospinal fluid (CSF) fistula in his right ear is reported. In the same ear, the patient also showed congenital inner ear dysplasia. The CSF fistula was obstructed surgically. Surgical exploration showed a fistula superior to the oval window and a normally shaped stapes. The stapes was removed and the fistula was closed by obliteration of the vestibulum with the temporal fascia and fat tissue. The location of the fistula was very rare; to our knowledge, this is the first reported case of CSF fistula superior to the oval window. The relationship between perilymphatic fistula and the microfissure revealed by temporal bone study is discussed.