Perilymphatic and endolymphatic PO2. Variations during anoxia, hyperoxia, and hypercapnia

Effects of CO2/HCO3− in Perilymph on the Endocochlear Potential in Guinea Pigs

The effect of CO(2)/HCO(3)(-) on the endocochlear potential (EP) was examined by using both ion-selective and conventional microelectrodes and the endolymphatic or perilymphatic perfusion technique. The main findings were as follows: (i) A decrease in the EP from approximately +75 to approximately +35 mV was produced by perilymphatic perfusion with CO(2)/HCO(3)(-)-free solution, which decrease was accompanied by an increase in the endolymphatic pH (DeltapH(e), approximately 0.4). (ii) Perilymphatic perfusion with a solution containing 20 mM NH(4)Cl produced a decrease in the EP (DeltaEP, approximately 20 mV) with an increase in the pH(e) (DeltapH(e), approximately 0.2), whereas switching the perfusion solution from the NH(4)Cl solution to a 5% CO(2)/25 mM HCO(3)(-) solution produced a gradual increase in the EP to the control level with the concomitant recovery of the pH(e). (iii) The perfusion with a solution of high or low HCO(3)(-) with a constant CO(2) level within 10 min produced no significant changes in the EP. (iv) Perfusion of the perilymph with 10 microg/ml nifedipine suppressed the transient asphyxia-induced decrease in EP slightly, but not significantly. (v) By contrast, the administration of 1 microg/ml nifedipine via the endolymph inhibited significantly the reduction in the EP induced by transient asphyxia or perilymphatic perfusion with CO(2)/HCO(3)(-)-free or 20 mM NH(4)Cl solution. These findings suggest that the effect of CO(2) removal from perilymphatic perfusion solution on the EP may be mediated by an increase in cytosolic Ca(2+) concentration induced by an elevation of cytosolic pH in endolymphatic surface cells.

Biophysical basis for inner ear decompression sickness

Isolated inner ear decompression sickness (DCS) is recognized in deep diving involving breathing of helium-oxygen mixtures, particularly when breathing gas is switched to a nitrogen-rich mixture during decompression. The biophysical basis for this selective vulnerability of the inner ear to DCS has not been established. A compartmental model of inert gas kinetics in the human inner ear was constructed from anatomical and physiological parameters described in the literature and used to simulate inert gas tensions in the inner ear during deep dives and breathing-gas substitutions that have been reported to cause inner ear DCS. The model predicts considerable supersaturation, and therefore possible bubble formation, during the initial phase of a conventional decompression. Counterdiffusion of helium and nitrogen from the perilymph may produce supersaturation in the membranous labyrinth and endolymph after switching to a nitrogen-rich breathing mixture even without decompression. Conventional decompression algorithms may result in inadequate decompression for the inner ear for deep dives. Breathing-gas switches should be scheduled deep or shallow to avoid the period of maximum supersaturation resulting from decompression.

Sensitivity of the endocochlear potential level to cochlear blood flow during hypoventilation

To study the relationship between endocochlear DC potential (EP) and cochlear blood flow (CoBF) under hypoxic conditions, we recorded the EP and CoBF from the basal turn of the cochlea in 21 guinea pigs. Hypoventilation for 10 minutes was induced by reducing the respiratory rate and volume. During hypoventilation, the EP declined in most of the cases to an intermediate level of the positive range in a few minutes. At the midpoint of the 10-minute hypoventilation, angiotensin II (5 microg/kg or 1 mL/kg) was infused for 60 seconds to raise the systemic blood pressure. In this experimental manipulation of systemic blood pressure, the CoBF and EP generally rose transiently. We determined the sensitivity of the EP to a CoBF change (delta) by calculating the deltaEP/deltaCoBF. More specifically, we analyzed the relationship between the deltaEP/deltaCoBF and the EPi (EP level just before angiotensin II infusion). The deltaEP was equal to the maximum EP level after angiotensin II infusion minus the EPi. The deltaCoBF was equal to the maximum CoBF value after angiotensin II infusion minus the CoBF value just before infusion. The deltaEP/deltaCoBF increased most in the range near 70% of the EPi. That is, the deltaEP/deltaCoBF was greater and the EPi was lower in the range above 70% of the EPi. To elucidate this linear correlation in the range above 70% of the EPi, we must consider several factors. In the supplementary experiments for blood gas analysis using 11 guinea pigs, most of the data of the EPi in the range above 70% were found to be obtained under conditions of a PaO2 of more than 12 mm Hg. As to the sensitivity increase of the EP to the deltaCoBF above mentioned, we propose that among several factors in the stria vascularis during hypoxemia, the activation of glycolysis in aerobic metabolism may be involved. As another possible factor, we postulate the increase in the reactive rate of the enzymatic activities that are linked with EP production and respond to the elevated cyclic adenosine monophosphate activity induced by the sympathicotonic state due to hypercapnia.

Effects of experimental cochlear thrombosis on oxygenation and auditory function of the inner ear

To elucidate the etiology and pathogenesis of sudden hearing loss, the effect of experimental cochlear thrombosis on oxygenation and the auditory function of the inner ear was investigated in anesthetized guinea pigs. Impairment of cochlear blood flow (CBF) was induced by ferromagnetic obstruction of cochlear blood vessels at lowered body temperature. Perilymphatic oxygen partial pressure (PO2) in the basal scala tympani (about 200 microm below the round window membrane) was measured polarographically using micro-coaxial needle electrodes. Auditory function was examined by recording cochlear microphonic (CM) frequency responses, compound action potentials (CAP) and auditory evoked brainstem responses (ABR). Findings demonstrated a considerable decrease in the mean perilymphatic PO2 of 40%, 2 h after the start of the experiment. Mean CM and N1 CAP amplitudes were reduced by about 25% each and ABR by 18%. No significant changes were observed in the latencies of either CAP or ABR. Mean basal CBF was found to decrease by 35%, as measured by laser Doppler flowmetry in a parallel study. The present findings demonstrate that vascular impairment in the inner ear results in a considerable drop in intracochlear oxygenation, causing a significant loss in the auditory response.

The effect of CO2- and O2-gas mixtures on laser Doppler measured cochlear and skin blood flow in guinea pigs

The effects of carbogen (5% CO2: 95% O2) 10% CO2-in-air and 100% O2 on cochlear blood flow (CBF), skin blood flow (SBP), blood pressure (BP) and arterial blood gases were investigated in the anesthetized, respired or self-respiring guinea pig. In respired animals, CBF and SBF were increased with carbogen and 10% CO2-in-air and decreased with O2. BP was elevated with each gas. In freely breathing animals, only 10% CO2-in-air caused a small increase in CBF; both carbogen and O2 caused CBF to decrease. SPF changes were similar in form, but larger than those seen in respirated subjects. No consistent change in BP was seen during breathing of these mixtures. Arterial PO2 was increased by carbogen and 10% CO2-in-air for both groups. PCO2 increased for both CO2 gas mixtures during forced respiration; but in free-breathing animals PCO2 only increased for 10% CO2-in-air (normal PCO2 values were maintained with carbogen thorough increased breathing rate). The observed changes in CBF were consistent with a balance between a combined vasoconstrictive effect of PO2 and vasodilation effect of PCO2 on cochlear vessels. Analysis of cochlear vascular conductivity (CBF/BP) indicated that vasodilation was significant only with 10% CO2-in-air in respirated animals. In all other conditions the increased CBF apparently reflects the increase profusion pressure associated with respiration of each gas. For clinical purposes, while carbogen does not appear to directly cause vasodilation of cochlear vessels it does lead to an increased oxygenation of the cochlea blood and would appear to avoid the cochlear vasoconstriction caused by 100% O2.

Influence of topically applied adrenergic agents on cochlear blood flow

This study was designed to assess the role of adrenergic receptors in the control of cochlear blood flow. Laser Doppler flowmetry was used to determine the effects of adrenergic drugs topically applied to the round window membrane of the cochlea. The relative influence of the various receptor types (alpha 1, alpha 2, beta 1, and beta 2) was examined by a selection of agonists and antagonists. The agonists norepinephrine and epinephrine, which have mixed alpha- and beta-receptor effects, and phenylephrine, a strong alpha 1-agonist, all induced a dose-dependent reduction in cochlear blood flow. The agonists isoproterenol (beta-active), salbutamol (alpha 2-active) had no effect on cochlear blood flow. Of the antagonists, when tested alone, only the selective alpha 1-antagonist prazosin had a direct effect on cochlear blood flow, demonstrating an increase in cochlear blood flow. The selective alpha 2-antagonist idazoxan, the beta-antagonist propranolol, and the unselective alpha-antagonist phentolamine had no effect on cochlear blood flow. Interaction studies of agonists and antagonists were performed to specifically define the receptor subclasses responsible for the cochlear blood flow increases with norepinephrine and epinephrine. The results are consistent with the presence of an alpha 1-adrenergic sympathetic control of cochlear blood flow.

Feasibility of pulse oxymetry to measure arterial O2 saturation in studies on cochlear blood circulation

To understand the characteristics of oxygen transport to the inner ear, the relationship between arterial O2 saturation and cochlear microcirculation was investigated under different respiratory condition in guinea pigs. To monitor arterial O2 saturation a pulse oxymeter instead of an arterial blood gas analyzer was used. When the arterial O2 saturation was measured in the foot pad by a pulse oxymeter under different respiratory conditions, the data showed a close correlation with the results of blood gas analysis. For the measurement of cochlear microcirculation, a pulse oxymeter was found to be a feasible respiratory monitor for animal experiments. With this apparatus our study demonstrated a slower reaction in the decrease of perilymphatic oxygen tension than of cochlear blood flow during stepwise induction of hypoventilation monitored by a pulse oxymeter. Under certain conditions of hyperventilation in which arterial O2 saturation and perilymphatic oxygen tension increased gradually, cochlear blood flow was found to decrease. This decrease of cochlear blood flow could be attributed to chemical controls which are regulated, as in the cerebral blood circulation, by the content of CO2 and H+ in the vascular bed in the cochlea.

Autoregulation of inner ear blood flow in normal and hydropic guinea pigs

Inner ear and brainstem blood circulation was measured by three different techniques: the laser Doppler (LD), hydrogen clearance (HC) and oxygen tension (PO2) measurements, while systemic blood pressure (BP) was modulated by norepinephrine infusion or removal of whole blood. Results were as follows: (i) The blood flow (BF) change determined by LD correlated well with that measured by HC and PO2 techniques; (ii) BF in the brainstem was maintained constant in the BP range of 35 to 80 mmHg; however, inner ear BF showed a poor autoregulatory function relative to the change of systemic BP; (iii) although the change of BF was similar for cochlea and semicircular canal the amount of PO2 decrease for lowered BP was significantly less in the cochlea than in the canal; (iv) in guinea pigs with unilaterally obliterated endolymphatic sac and duct, the decrease in cochlear BF was larger on the operated side than on the intact side. This suggests that the autoregulatory function for BF is impaired in the hydropic ear.

Relationship between cochlear blood flow and perilymphatic oxygen tension

To clarify the characteristics of the blood circulation in the cochlea, we correlated cochlear blood flow and perilymphatic oxygen tension at various blood pressures. Cochlear blood flow was measured in guinea pigs by laser Doppler flowmetry, and perilymphatic oxygen tension by polarography. Blood pressure changes were induced by angiotensin II injection, trimetaphan camsylate injection and blood withdrawal. Cochlear blood flow generally paralleled systemic blood pressure, indicating a close correlation. In contrast, perilymphatic oxygen tension was slower to increase and decrease. However, when systemic blood pressure was lowered more gradually, perilymphatic oxygen tension did not show the same lag. These findings indicate that perilymphatic oxygen tension parallels systemic blood pressure when changes induced are slower and in a physiological range.

Measurement of cochlear blood flow: intravital fluorescence microscopy

A technique is described for directly observing in vivo cochlear microvasculature in the gerbil for physiologic and experimentally induced changes in vessel diameter and blood flow velocity. Measurements are made from computer processed video images of surgically exposed microvessels. These images are obtained using intravital fluorescence microscopy (IFM) with epi-illumination. The Mongolian gerbil is an ideal animal model for circulatory studies of the inner ear. It has a stable heart rate and blood pressure under urethane/alpha-chloralose anesthesia and its cochlea is surgically accessible. A window is created over the feeding artery (anterior inferior cerebellar artery) and over the stria vascularis of the second turn of the cochlea, atraumatically exposing radiating arterioles and strial capillaries. Our system of IFM provides images that are videorecorded, digitally analyzed with a computer image processor, and enhanced according to the type of measurement desired. Velocity measurements are obtained by tracking plasma gaps or single fluorescent labeled red blood cells through successive frames of the videorecorded images. This experimental technique allows us to analyze circulatory responsiveness to a variety of vasoactive drugs administered regionally to the cochlea in concentrations not affecting systemic circulation.

Cochlear blood flow: measurement techniques

The measurement of inner ear blood flow and other microvascular variables is subject to unique technical problems which are compounded by methodological limitations. As a result, the interpretation of experimental results is often difficult. This report discusses the most important methods currently available for cochlear blood circulation measurements and the technical problems associated with their use. The use of a combination of measurements to resolve problems of interpretation is stressed. An extensive review of the pertinent literature is provided in relation to each method.

In vivo capillary diameters in the stria vascularis and spiral ligament of the guinea pig cochlea

Blood microvessels in the membraneous lateral wall of the cochlea were examined using intravital microscopic techniques. A video analysis system made serial diameter measurements at 1 micron intervals along the length of selected vessel segments during four experimental conditions. For each vessel segment, the serial measurements were statistically converted into a single diameter estimate, such that the flow resistance in a uniform vessel of this diameter would equal the resistance of the real non-uniform vessel. Nominal vessel diameters found (spiral ligament: 9-12 micron; stria vascularis: 12-16 micron) were nearly double those reported earlier in histological observations (Axelsson, 1968). During stimulation the largest diameter change seen was a 3.7% dilation (about 0.5 micron) in response to breathing 5% CO2 in oxygen. Theoretically, this change could reduce vascular fluid resistance by 16%, nearly enough to explain the observed flow increase of 20%. No diameter changes occurred for 5% CO2 in air despite a 50% flow increase, nor for air pressure pulses applied at the tympanic membrane. Round window electrical stimulation of 50 microA also produced dilation (less than 2.5%), but higher current levels were ineffective. In general, blood flow increases seen in this study could not adequately be attributed to the small lateral wall vessel diameter increases nor systemic causes, suggesting that lateral wall blood flow in these instances is dependent on control within the modiolus.

Effects of carbon monoxide on cochlear electrophysiology and blood flow

The belief that the cochlea is particularly vulnerable to a reduction in oxygen availability comes predominantly from studies reporting the disruption of electrophysiological measures, such as the compound action potential, endocochlear potential, inner hair cell intracellular potentials or afferent nerve fiber responses by asphyxiation. Because hypoxia has frequently been suggested as an underlying mechanism by which many ototoxic agents produce injury, and because such agents are not likely to completely disrupt oxygen delivery, we investigated the effects of graded hypoxia (using doses of carbon monoxide) on cochlear blood flow, the compound action potential (CAP) and the cochlear microphonic (CM). High doses of carbon monoxide injected intra-peritoneally yielded reversible loss of the CAP sensitivity for high frequency tone bursts, the extent of which was dose dependent. The loss was observed first at the highest frequency tested (50 kHz) and as carboxyhemoglobin levels increased, contiguous lower frequencies were influenced. Recovery progressed from low to high frequencies as carboxyhemoglobin levels declined. Carbon monoxide administration also produced a dose dependent elevation in the cochlear blood flow measured by a laser Doppler flow monitor. The data suggest that carbon monoxide administration disrupts cochlear function only under extremely severe exposure conditions. An elevation in cochlear blood flow may well serve as a protective mechanism which maintains cochlear function in the face of declining blood oxygen carrying capacity and delivery. While the site of action of carbon monoxide in the cochlea is uncertain, the data clearly indicate that elements involved in the generation of the CAP for high frequency tones are particularly vulnerable.(ABSTRACT TRUNCATED AT 250 WORDS)