Effect of glycerol on electrochemical composition of endolymph and perilymph in the rat

Regulation of the perilymphatic-endolymphatic water shunt in the cochlea by membrane translocation of aquaporin-5

Volume homeostasis of the cochlear endolymph depends on radial and longitudinal endolymph movements (LEMs). LEMs measured in vivo have been exclusively recognized under physiologically challenging conditions, such as experimentally induced alterations of perilymph osmolarity or endolymph volume. The regulatory mechanisms that adjust LEMs to the physiological requirements of endolymph volume homeostasis remain unknown. Here, we describe the formation of an aquaporin (AQP)-based "water shunt" during the postnatal development of the mouse cochlea and its regulation by different triggers. The final complementary expression pattern of AQP5 (apical membrane) and AQP4 (basolateral membrane) in outer sulcus cells (OSCs) of the cochlear apex is acquired at the onset of hearing function (postnatal day (p)8-p12). In vitro, hyperosmolar perfusion of the perilymphatic fluid spaces or the administration of the muscarinic agonist pilocarpine in cochlear explants (p14) induced the translocation of AQP5 channel proteins into the apical membranes of OSCs. AQP5 membrane translocation was blocked by the muscarinic antagonist atropine. The muscarinic M3 acetylcholine (ACh) receptor (M3R) was identified in murine OSCs via mRNA expression, immunolabeling, and in vitro binding studies using an M3R-specific fluorescent ligand. Finally, the water shunt elements AQP4, AQP5, and M3R were also demonstrated in OSCs of the human cochlea. The regulation of the AQP4/AQP5 water shunt in OSCs of the cochlear apex provides a molecular basis for regulated endolymphatic volume homeostasis. Moreover, its dysregulation or disruption may have pathophysiologic implications for clinical conditions related to endolymphatic hydrops, such as Ménière's disease.

Glycerol-induced fluid shifts attenuate the vestibulosympathetic reflex in humans

The glycerol dehydration test (GDT) has been used to test for the presence of Ménière's disease and elicits acute alterations in vestibular reflexes in both normal and pathological states. Activation of the vestibulosympathetic reflex (VSR) increases muscle sympathetic nerve activity (MSNA) and peripheral vascular resistance. We hypothesized that the GDT would attenuate the VSR through fluid shifts of the inner ear. Sixteen male subjects (26 ± 1 yr) were randomly assigned to be administered either glycerol mixed with cranberry juice (97 ± 3 ml glycerol + equal portion of cranberry juice; n = 9) or a placebo control [water + cranberry juice (100 ml each); n = 7]. Subjects in both groups performed head-down rotation (HDR), which engages the VSR, before and after administration of either the glycerol or placebo. MSNA (microneurography), arterial blood pressure, and leg blood flow (venous occlusion plethysmography) were measured during HDR. Before glycerol administration, HDR significantly increased MSNA burst frequency (Δ8 ± 1 bursts/min; P < 0.01) and total activity (Δ77 ± 18%; P < 0.01) and decreased calf vascular conductance (-Δ20 ± 3%; P < 0.01). However, HDR performed postadministration of glycerol resulted in an attenuated MSNA increase (Δ3 ± 1 bursts/min, Δ22 ± 3% total activity) and decrease in calf vascular conductance (-Δ7 ± 4%). HDR significantly increased MSNA burst frequency (Δ5 ± 1 and Δ5 ± 2 bursts/min) and total activity (Δ58 ± 13% and Δ52 ± 18%) in the placebo group before and after placebo, respectively (P < 0.01). Likewise, decreases in calf vascular conductance during HDR before and after placebo were not different (-Δ13 ± 4% and -Δ14 ± 2%, respectively; P < 0.01). These results suggest that fluid shifts of the inner ear via glycerol dehydration attenuate the VSR. These data provide support that inner ear fluid dynamics can have a significant impact on blood pressure regulation via the VSR in humans.

Effects of glycerol intake and body tilt on otoacoustic emissions reflect labyrinthine pressure changes in Menière's disease

It is known that by influencing stapes stiffness thus the ear's impedance, changes in intracranial and intralabyrinthine pressure induce a characteristic phase shift in otoacoustic emissions (OAE) around 1 kHz in human ears. Thus, if the regulation of pressure in intralabyrinthine compartments were abnormal in Menière patients, OAEs might help detect it. Body tilt, which acts on intracranial pressure, and administration of an osmotically active substance provide two simple ways of manipulating intralabyrinthine pressure. Here, 14 patients with typical signs of an attack of unilateral endolymphatic hydrops were submitted to postural changes and a glycerol test. Their OAEs initially collected in upright position served as references, then OAEs were measured in supine position, and back to the upright posture one and 3h after glycerol intake. Twenty control subjects were also tested for body tilt. The main effect of body tilt and glycerol was a phase rotation of OAEs peaking around 1 kHz. Its frequency dependence matched the one due to a pressure-related change in stapes or basilar membrane stiffness predicted by the ear model of Zwislocki (1962). The average glycerol-induced phase shifts were similar in size in Menière vs. asymptomatic ear and audiometric thresholds were stable after glycerol intake in line with the model predicting little change in the magnitude of the transfer function. These data support a simple conductive pressure-related mechanism explaining the action of glycerol on inner ear responses. The fact that the mean postural shift was three times larger in Menière than asymptomatic and control ears suggests an additional effect in allegedly hydropic ears.