Effect of glycerol on inner ear fluid electrolytes and osmolalities in guinea pigs

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.

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

Glycerol (2 g/kg body weight), or 0.15 M NaCl for control animals, was administered to rats by i.v. injection. The dose was chosen in order to obtain an osmolarity increase in plasma of about 15 mosm/l 1 h after the glycerol administration, an increase which is similar to that observed in the human glycerol dehydration test. Endolymph and perilymph were sampled from the basal turn of the cochlea; cerebrospinal fluid (CSF) was sampled from cisterna magna. Plasma osmolarity, endocochlear potential, Na and K concentrations in endolymph, perilymph and CSF were determined 1 and 2 h after the glycerol injection. Compared with control animals, glycerol induced an increase in Na and K concentration in perilymph and endolymph, respectively, 1 and 2 h after the glycerol injection. No modification of the endocochlear potential was observed. These results are compatible with an increase in inner ear fluids osmolarity induced by glycerol.

The effect of glycerol on the guinea-pig hydropic ear

The aim of this paper was to investigate the effect of glycerol on experimental endolymphatic hydrops in guinea-pigs. The right endolymphatic sac and duct were obliterated through an extradural posterior fossa approach. Some animals received a 3 g/kg dose of glycerol for a period of 7 days, whereas others received the same dose for 30 days. The activity of glycerol was studied by investigating the volumetric changes in the scala media determined with a computerized planimeter. Glycerol induced a significant reduction of the hydrops showing its effectiveness and suggesting a strial metabolic response.

Effect of glycerol on pressure difference between perilymph and endolymph

The pressure difference between the perilymph and the endolymph following administration of 50% glycerol (12 ml/kg) was studied in guinea pigs. The perilymphatic and endolymphatic pressures were measured simultaneously with two sets of a servo-nulling system. Glycerol was administered for about one minute via a gastrocatheter. Both the perilymphatic and endolymphatic pressures began to decrease about 5 min after the administration of glycerol, and thereafter the decrease continued for about one hour with no significant difference between the two pressures. We concluded from the results that the glycerol-induced pressure difference between the perilymph and the endolymph, if present, is only a very small one, although a collapse of the scala media after glycerol intake was reported in guinea pigs and chinchillas.

Electrochemical composition of the cochlear fluids in the early experimental hydrops. Preliminary results

The composition of endolymph and perilymph was studied in the guinea pig cochlea after 2 and 6 weeks of blockage of the vestibular aqueduct in an experimental model of hydrops. Compound action potential was monitored several times in the observation period. The endocochlear potential was measured and the endolymph was sampled at the first and third turns of the scala media. The Na, K, and Cl concentrations were determined in nanolitre aliquots of endolymph and of perilymph, the latter sampled from the basal scala vestibuli. After 2 weeks, no change in endolymphatic electrochemical composition was observed. After 6 weeks, endocochlear potential was decreased by 25% at both cochlear turns; K concentration was decreased in endolymph of the basal turn and Cl concentration was decreased in both turns; the calculated osmolality (Na + K + Cl) was decreased in both turns. These results indicate that the blockage of the vestibular aqueduct induced early auditory dysfunction whereas alterations of the electrochemical composition of endolymph occurred later after a time lag of more than 2 and less than 6 weeks.