F. Scheibe

Intensity-related changes in cochlear blood flow in the guinea pig during and following acoustic exposure

SummaryThis study examined the effects of acoustic exposure at different intensities on cochlear blood flow (CBF) using laser Doppler flowmetry. CBF was measured in anesthetized guinea pigs exposed to either a 10 kHz pure tone at 125, 105, or 90 dB SPL, or wide-band noise at 85 dB SPL for 1 h. Mean arterial blood pressure and heart rate were recorded continuously. Arterial acid-base status, cochlear temperature, cochlear microphonics (CM), and compound action potentials (CAP) were measured before and after exposure. There was a small, but significant, steady decline in basal CBF after 40 min loud sound exposure (125 dB SPL), but no change in basal CBF occurred with the lower intensities (85–105 dB SPL). In contrast, there was a significant increase in apical CBF after 1 h exposure to either moderate wideband noise (85 dB SPL) or a 10 kHz tone at 105 dB SPL. These changes persisted during a 20-min post-exposure period. In most cases, the cochlear temperature and cardiorespiratory variables monitored remained unchanged during and after the exposures as compared to the controls. CM and CAP amplitudes showed extensive losses after acoustic overstimulation (125 dB SPL), but no permanent changes were found at the lower intensities used. The present findings confirm the occurrence of intensity-related effects of acoustic exposure on the cochlear microcirculation.

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 μm 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 perilymphaticPO2 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.

Laser light transmission and laser Doppler blood flow measurements on human, rat and guinea pig cochlea

SummaryIn order to test the applicability of laser-Doppler flowmetry in monitoring cochlear blood flow clinically, the thickness and the helium-neon laser light transmission of specimens of human, rat and guinea pig promontory bone and human skin were determined. Furthermore, comparative laser-Doppler measurements were taken from the promontory in patients, rats and guinea pigs. Due to the different thicknesses of the promontory bone in different species, the light transmission was found to be considerably higher for the animal cochlea (rat, 15%; guinea pig, 6.6%) than the human cochlea (1.7%). However, a clearly higher laser-Doppler signal was recorded from both the human and the rat cochleas as compared with the guinea pig. The relative laser light attenuation by the human skin specimens corresponded to that of the human promontory bone. The findings are discussed with regard to the suitability of the laser-Doppler method for blood flow measurements in the human cochlea.

Changes in cochlear oxygenation, microcirculation and auditory function during prolonged general hypoxia

SummaryChanges in cochlear microcirculation and oxygenation and auditory function were examined in anesthetized guinea pigs during prolonged hypoxic ventilation (8% oxygen in nitrogen) for 1 h. Cochlear blood flow and perilymphatic oxygen partial pressure were measured using laser Doppler flowmetry and oxygen-sensitive microelectrodes. Auditory function was examined by recording cochlear microphonics, compound action potentials and auditory evoked brainstem response. Blood pressure and heart rate were monitored. During systemic hypoxia, the perilymphatic PO2 dropped on average to about 70% of the initial value. Cochlear and brainstem potentials showed a mean reduction to 75–82%. Different effects of hypoxia on cochlear blood flow were observed and included decreases as well as increases. Mean arterial blood pressure declined significantly during hypoxia, while the heart rate remained constant. The changed variables returned to normal during reventilation with room air. The findings are discussed with regard to their significance as an animal model for the study of hypoxia-induced cochlear pathophysiology.

On sources of error in the biochemical study of perilymph (guinea-pig).

SummaryContamination of perilymph with other fluids (cerebrospinal fluid, tissue fluid, blood, endolymph) together with sampling, anaesthesia, surgical intervention or food intake of the animals may considerably affect the analytical result. The numerous possible artefacts seem to be the main reason why varying values are given in the literature for the same chemical component of perilymph. This is also partly true of cerebrospinal fluid and blood. The effect of some sources of error on selected chemical components of perilymph, cerebrospinal fluid and blood is briefly summarized.

Cochlear blood flow following temporary occlusion of the cerebellar arteries

SummaryIn order to induce acute transient local blood flow impairment in the guinea pig cochlea, terminal vessels arising from the proximal and middle third of the basilar artery and from the rostral part of the collateral vertebral artery near their junction were occluded by temporary (1–2 min) compression. Clear identification of the anterior inferior cerebellar artery was impeded due to great variation in branching of the cerebellar arteries. To quantify the effect of the vascular occlusion on the cochlear microcirculation, laser Doppler flowmetry was used. Successive compression of the cerebellar arteries resulted in individually differentiated effects on the Doppler signal from the cochlea. In about half of the experiments there was a decrease in the laser signal to 25–55% of the initial level, while in the other cases no clear decrease occurred. Proximal obstruction of the basilar artery was found to reduce the cochlear blood flow up to 10–25% of its initial level, indicating that the Doppler signal primarily reflects microcirculation within the membranous cochlea. The findings are discussed with regard to their significance as an animal model for acute transient cochlear hypoxia.

Laser Doppler measurements of cochlear blood flow during loud sound presentation

SummaryThe laser Doppler flowmeter may give responses to loud sound that reflect the vibration of cochlear structures rather than changes in cochlear blood flow. The present study demonstrates that the positive artifactual response (i.e., increased flow reading) to sound at frequencies above approximately 5 kHz can be eliminated by using flowmeters which have electronic filters at 4 and 12 kHz, limiting the bandwidth of the optical Doppler shifted frequency range to lower frequencies. However, when using the 4 kHz filter a “residual” immediate negative response to loud high-frequency sound (10 kHz tone at 125 dB SPL) is evident at the beginning of the exposure. These findings are discussed with regard to the suitability of the method for investigating the effect of sound/noise on cochlear blood flow.

Vergleichende Untersuchungen der Laktatkonzentration von Perilymphe, Blut und Liquor cerebrospinalis normaler und schallbelasteter Meerschweinchen

SummaryThe paper deals with comparative studies of lactate concentration in the perilymph (PL) of scala tympani and of scala vestibuli, arterial and venous blood, serum and cerebrospinal fluid (CSF) of normal and sound exposed guinea pigs, special consideration having been given to possible sources of error in the methods employed. Lactate was determined enzymatically using a micromodification of the Boehringer UV-test combination adapted to 1 μl PL. The lactate concentrations in the PL of scala tympani and scala vestibuli did not differ significantly. The mean values amounted to 4.5–5.2 mM/l in the case of the opened and of the unopened subarachnoid space (Table 1). The lactate concentrations in the PL of both cochlea scales were significantly higher already ten minutes post-mortem.In the exposure experiments the animals were unilaterally exposed to sound for 1 h in an acoustically isolated system using a wide-band noise at an intensity of 120 dB SPL for one series and 2-kHz pure-tone at intensities of 112 and 122 dB SPL for two other series. We did not detect any changes in the lactate concentrations neither in the PL nor in the blood and in the CSF, following sound exposure (Tables 2 and 3).The lactate concentrations of arterial and venous blood and CSF did not differ significantly. The mean values amounted to 1.4–1.8 mM/l (Table 2). However, if blood was not deproteinized or centrifuged immediately after being taken, the lactate concentration increased markedly.A comparison of the present results has shown that the lactate concentration in the PL is about three times as high as in blood and in CSF. This difference in concentration suggests that the PL lactate is of intracochlear origin and that glycolytic processes take place in the inner ear also under normal conditions. Systematic studies of additional metabolic parameters must be conducted before a definitive physiological interpretation of the present analytical results can be given.The paper deals with comparative studies of lactate concentration in the perilymph (PL) of scala tympani and of scala vestibuli, arterial and venous blood, serum and cerebrospinal fluid (CSF) of normal and sound exposed guinea pigs, special consideration having been given to possible sources of error in the methods employed. Lactate was determined enzymatically using a micromodification of the Boehringer UV-test combination adapted to 1 mul PL. The lactate concentrations in the PL of scala tympani and scala vestibuli did not differ significantly. The mean values amounted to 4.5-5.2 mM/l in the case of the opened and of the unopened subarachnoid space (Table 1). The lactate concentrations in the PL of both cochlea scales were significantly higher already ten minutes post-mortem. In the exposure experiments the animals were unilaterally exposed to sound for 1 h in an acoustically isolated system using a wide-band noise at an intensity of 120 dB SPL for one series and 2-kHz pure-tone at intensities of 112 and 122 dB SPL for two other series. We did not detect any changes in the lactate concentrations neither in the PL nor in the blood and in the CSF, following sound exposure (Table 2 and 3). The lactate concentrations of arterial and venous blood and CSF did not differ significantly. The mean values amounted to 1.4-1.8 mM/l (Table 2). However, if blood was not deproteinized or centrifuged immediately after being taken, the lactate concentration increased markedly. A comparison of the present results has shown that the lactate concentration in the PL is about three times as high as in blood and in CSF. This difference in concentration suggests that the PL lactate is of intracochlear origin and that glycolytic processes take place in the inner ear also under normal conditions. Systematic studies of additional metabolic parameters must be conducted before a definitive physiological interpretation of the present analytical results can be given.

Measurements of perilymphatic oxygen tension in guinea pigs exposed to loud sound

SummaryUsing different types of custom-made oxygen-sensitive microelectrodes, the perilymphatic oxygen partial pressure (PO2) was determined in anesthetized guinea pigs. Cochlear temperature, heart rate, and arterial blood pressure and acid-base status were monitored. ThePO2 in the basal scala tympani perilymph (200 μm below the round window membrane) was found to be 53 ± 17 mmHg (mean ± SD) in 33 normal animals. In 11 guinea pigs exposed to loud sound for 15 min (10 kHz pure tone, 125 dB SPL) there was on average a continuous decline in the perilymphaticPO2, which was significant only 30 min post-exposure. A considerable variation in response was found in individual animals. Mean arterial blood pressures showed a slightly increasing time course, while heart rates did not change significantly during the whole period of the experiment. Arterial acid-base status and PO2 values remained within normal limits and did not change significantly. Cochlear microphonics and compound action potentials were substantially decreased after acoustic overstimulation. The results are discussed with due consideration of sources of error.

Laktat- und Pyruvatkonzentrationen von Perilymphe, Blut und Liquor cerebrospinalis des Meerschweinchens

Lactate and pyruvate were studied comparatively in perilymph (PL), blood, and cerebrospinal fluid (CSF) of anesthetized guinea pigs. Arterial blood pressure, heart and respiration rate (Fig. 1), and arterial blood-gas state (Table 2) were simultaneously checked in a group of the animals. The metabolites were determined enzymatically by using the fluorometric technique. The studies have shown (Table 1) that both the lactate and the pyruvate concentrations are in PL at a similar rate (about 3:1) higher than in native blood and also higher than in CSF. The metabolite values of blood, especially the lactate values, were lower when blood was taken alone, e.g., more physiological, than in the case when CSF and PL had been sampled before. The lactate/pyruvate ratios of Pl are somewhat higher than the blood ratios. The ratio of CSF was found to be lower. The high metabolite levels in PL suggest an intracochlear origin. A direct perilymphatic lactate origin could not be detected.SummaryLactate and pyruvate were studied comparatively in perilymph (PL), blood, and cerebrospinal fluid (CSF) of anesthetized guinea pigs. Arterial blood pressure, heart and respiration rate (Fig. 1), and arterial blood-gas state (Table 2) were simultaneously checked in a group of the animals. The metabolites were determined enzymatically by using the fluorometric technique. The studies have shown (Table 1) that both the lactate and the pyruvate concentrations are in PL at a similar rate (about 3∶1) higher than in native blood and also higher than in CSF. The metabolite values of blood, especially the lactate values, were lower when blood was taken alone, e.g., more physiological, than in the case when CSF and PL had been sampled before. The lactate/pyruvate ratios of PL are somewhat higher than the blood ratios. The ratio of CSF was found to be lower. The high metabolite levels in PL suggest an intracochlear origin. A direct perilymphatic lactate origin could not be detected.ZusammenfassungLaktat und Pyruvat wurden vergleichend untersucht in Perilymphe (PL), Blut und Liquor cerebrospinalis (CSF) anästhesierter Meerschweinchen. Bei einer Anzahl der Tiere wurden gleichzeitig arterieller Blutdruck, Herz- und Atemfrequenz (Abb. 1) und arterieller Blutgasstatus (Tabelle 2) kontrolliert. Die Bestimmung der Metabolite erfolgte enzymatisch mit Hilfe von Fluoreszenzmessungen. Die Untersuchungen haben gezeigt (Tabelle 1), daß die Laktat- und die Pyruvatkonzentrationen in der PL in einem ähnlichen Verhältnis (etwa 3∶1) höher sind als im nativen Blut und auch höher als im CSF. Die Metabolitwerte des Blutes, besonders die Laktatwerte, waren bei alleiniger Blutentnahme niedriger, d. h. physiologischer, als nach vorangegangener CSF- und PL-Gewinnung. Die Laktat/Pyruvatquotienten der PL sind etwas höher als die Quotienten des Blutes. Der Quotient des CSF liegt niedriger. Die hohen Metabolitspiegel in der PL sprechen für einen intrakochleären Ursprung. Ein direkter perilymphatischer Laktatursprung konnte nicht nachgewiesen werden.