Effect of hemodilution on cochlear blood flow measured by laser-Doppler flowmetry

Oral administration of an herbal medicine to prevent progressive hearing loss in a mouse model of diabetes

OBJECTIVE

Tsumura Suzuki Obese Diabetes (TSOD) mice exhibit early age-associated hearing loss. Histopathological analysis of these mice shows narrowing of capillaries in the stria vascularis and chronic reduction of blood flow in the cochlea. In this study, we investigated the effect of oral administration of a herbal medicine or calorie restriction on hearing in TSOD mice.

METHODS

TSOD mice were divided into 4 groups: CR (calorie restriction), BF and DS (treated with the herbal medicines, Bofutsushosan and Daisaikoto, respectively), and the control group. Body weight, blood glucose levels, and auditory brainstem responses (ABRs) were measured. The cochleae were excised and evaluated histopathologically.

RESULTS

Blood glucose levels were suppressed in the CR, BF, and DS groups. In addition, the elevation of ABR thresholds was inhibited in the CR, BF, and DS groups. Cochlear blood vessels remained wide in the three treatment groups compared with the control group. These results suggested that the administration of these herbal medicines improved glucose tolerance and yielded results similar to those on calorie restriction.

CONCLUSION

Oral administration of 2 herbal medicines can prevent hearing function disorder in a model mouse of diabetes. The results may clarify the possibility of clinical application.

Pathophysiology of the cochlear intrastrial fluid-blood barrier (review)

The blood-labyrinth barrier (BLB) in the stria vascularis is a highly specialized capillary network that controls exchanges between blood and the intrastitial space in the cochlea. The barrier shields the inner ear from blood-born toxic substances and selectively passes ions, fluids, and nutrients to the cochlea, playing an essential role in the maintenance of cochlear homeostasis. Anatomically, the BLB is comprised of endothelial cells (ECs) in the strial microvasculature, elaborated tight and adherens junctions, pericytes (PCs), basement membrane (BM), and perivascular resident macrophage-like melanocytes (PVM/Ms), which together form a complex "cochlear-vascular unit" in the stria vascularis. Physical interactions between the ECs, PCs, and PVM/Ms, as well as signaling between the cells, is critical for controlling vascular permeability and providing a proper environment for hearing function. Breakdown of normal interactions between components of the BLB is seen in a wide range of pathological conditions, including genetic defects and conditions engendered by inflammation, loud sound trauma, and ageing. In this review, we will discuss prevailing views of the structure and function of the strial cochlear-vascular unit (also referred to as the "intrastrial fluid-blood barrier"). We will also discuss the disrupted homeostasis seen in a variety of hearing disorders. Therapeutic targeting of the strial barrier may offer opportunities for improvement of hearing health and amelioration of auditory disorders. This article is part of a Special Issue entitled <Annual Reviews 2016>.

Expression of adhesion molecular proteins in the cochlear lateral wall of normal and PARP-1 mutant mice

Sound can damage peripheral cochlear function through a number of mechanisms, and emerging evidence suggests that inflammation may be one of them. Using immunohistochemistry and poly (ADP-ribose) polymerase-1 (PARP-1) mutant mice, we tested whether PARP-1 contributes to loud-sound induced cochlear lateral wall damage by triggering inflammatory effects, including upregulating intercellular adhesion molecule-1 (ICAM-1), P-selectin and platelet-endothelial cell-adhesion molecule-1 (PECAM-1). In control conditions, we found that there was no detectable poly-ADP-ribose (PAR) in the marginal cells and microvessels. ICAM-1 was expressed only at low levels in the vessels of the stria vascularis and the spiral ligament. P-selectin and PECAM-1 were barely detected and only in the vessels of the spiral ligament. Following loud-sound exposure, PAR was detected in numbers of marginal cells and some vessels of the spiral ligament. Also, an elevated expression of ICAM-1 was demonstrated in some vessels of the stria vascularis and spiral ligament. Increased expression of P-selectin and PECAM-1 were mainly located in the vessels of the spiral ligament, while increased populations of non-migrated and migrated leukocytes were observed in the area of the spiral ligament. However, neither increased expression of adhesion proteins nor increased population of leukocytes, were observed in the PARP-1 knockout mouse. We thus conclude that loud-sound stress activates the expression of adhesion molecular proteins in the lateral wall and that PARP-1 modulates inflammation-linked protein expression and leukocyte migration.

Pharmakotherapie des akuten Tinnitus

Hypoxia/ischemia may play an important role in the pathogenesis of sensorineural tinnitus due to the characteristics of the cochlear blood supply. In addition, hypoxia modulates molecular processes both in the acute and chronic forms of tinnitus. Transcription factor HIF-1 (hypoxia-inducible factor) may play a key role in the cells' adaptation to hypoxia and ischemia, while under hypoxic/ischemic conditions, HIF-1 induces changes in the gene expression which may contribute to the remodeling of particular structures within the cochlea. Disturbances in the cochlear blood supply may result in membrane changes, perineural edema, inflammation, disturbances in ion homeostasis and in the formation of reactive oxygen species. Thus, the pharmacotherapy of acute tinnitus may be aimed at the improvement of cochlear blood supply and the prevention of acute processes leading to cell damage. Pharmacotherapies with colloidal plasma substitutes, vasodilators, calcium antagonists, procaine, and cortisone have been described in the literature and are discussed here. Many of the pharmacological treatments have not been validated in double blind studies. Although it is impossible to deduce the cause of tinnitus from a drug's efficiency, there is some evidence that it can be effectively suppressed by improving blood supply, at least at certain stages. The aim is to achieve an improved pharmacotherapy by means of sophisticated diagnostic instruments for classifying particular types of tinnitus.

Usefulness of High Doses of Glucocorticoids and Hyperbaric Oxygen Therapy in Sudden Sensorineural Hearing Loss Treatment

OBJECTIVE

We investigated the effect of pharmacologic (steroids, vasodilators, vitamins, and Betaserc) and hyperbaric oxygen therapy on patients with sudden sensorineural hearing loss.

METHODS

The pharmacologic arm of the study consisted of 52 patients with defined sudden sensorineural hearing loss treated simultaneously in the ENT Department and National Center for Hyperbaric Medicine of the Medical University of Gdansk, Poland, from 1997 to 2000 (Group A). The hyperbaric oxygen therapy consisted of exposure to 100% oxygen at a pressure of 250 kPa for a total of 60 minutes in a multiplace hyperbaric chamber. The control group included 81 patients with defined sudden sensorineural hearing loss treated in the ENT Department, Medical University of Gdansk, from 1980 to 1996 (Group B). Both groups were comparable regarding the age of the patients, season of hearing loss occurrence, tinnitus and vestibular symptom frequency, delay before therapy, and average threshold loss before the start of treatment. The treatment results (hearing gain) were estimated using pure-tone audiometry. We retrospectively analyzed the audiograms of all patients.

RESULTS

Patients from Group A (blood flow-promoting drugs, glucocorticoids in high doses, betahistine, and hyperbaric oxygen therapy) showed significantly better recovery of hearing levels compared with those from Group B (blood flow-promoting drugs and glucocorticoids in low doses) at seven frequencies (500, 1,000, 2,000, 3,000, 4,000, 6,000, and 8,000 Hz) (p < 0.05) and four groups of frequencies (pure-tone average, high-tone average, pure middle-tone average, and overall average) (p < 0.05). Percentage hearing gain in all investigated frequencies was also better in Group A versus Group B, and the differences were statistically significant (p < 0.05).

CONCLUSION

We conclude that hyperbaric oxygen therapy with high doses of glucocorticoids improves the results of conventional sudden sensorineural hearing loss treatment and should be recommended. In addition, the best results are achieved if the treatment is started as early as possible.

Upregulated iNOS and oxidative damage to the cochlear stria vascularis due to noise stress

Our previous work has revealed increased nitric oxide (NO) production in the cochlear perilymph following noise stress. However, it is not clear if the increase of NO is related to iNOS and whether NO-related oxidative stress can cause vascular tissue damage. In this study, iNOS immunoreactivity, NO production, and reactive oxygen species (ROS) in the lateral wall were examined in normal mice and compared with similar animals exposed to 120 dBA broadband noise, 3 h/day, for 2 consecutive days. In the normal animals, iNOS expression was not observed in the vascular endothelium of the stria vascularis and only weak iNOS immunoactivity was detected in the marginal cells. However, expression of iNOS in the wall of the blood vessels of stria vascularis and marginal cells was observed after loud sound stress (LSS). Relatively low levels of NO production and low ROS activity were detected in the stria vascularis in the unstimulated condition. In contrast, NO production was increased and ROS activity was elevated in the stria vascularis after LSS. These changes were attenuated by the iNOS inhibitor, GW 274150. To explore whether noise induces apoptotic processes in the stria vascularis, we examined morphological changes in endothelial- and marginal-cells. In vitro, annexin-V phosphatidylserine (PS) (to label and detect early evidence of apoptosis) was combined with propidium iodide (PI) (to probe plasma membrane integrity). PI alone was used in fixed tissues to detect later stage apoptotic cells by morphology of the nuclei. Following LSS, PS was expressed on cell surfaces of endothelial cells of blood vessels and marginal cells of the stria vascularis. Later stage apoptosis, characterized by irregular nuclei and condensation of nuclei, was also observed in these cells. The data indicate that increased iNOS expression and production of both NO and ROS following noise stress may lead to marginal cell pathology, and the dysfunction of cochlear microcirculation by inducing blood vessel wall damage.

The demonstration of nitric oxide in cochlear blood vessels in vivo and in vitro: the role of endothelial nitric oxide in venular permeability

The objectives of the current study were to investigate the distribution and production of NO in cochlear blood vessels, and to assess whether the inhibition of basal NO production leads to vascular protein leakage of the cochlear microvasculature. Using the fluorescent dye 4,5-diaminofluorescein diacetate, NO was detected, both in vitro and in vivo, in the endothelial cells of various cochlear blood vessels, including the spiral modiolar artery, the vessel of the basilar membrane and the vessels of the spiral osseous lamina. Vessel leakage was assessed using intravital fluorescence microscopy following systemic infusion of fluorescein isothiocyanate-labeled bovine albumin. Local perfusion of the cochlear basal turn with either Ringer's solution or Ringer's containing an inactive nitric oxide synthase inhibitor (100 microM) produced minimal protein leakage. Perfusion with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (100 microM) produced significantly enhanced vascular protein leakage. The findings demonstrate the presence of endothelial NO in the cochlear blood vessels and suggest that NO protects cochlear venules against excessive vascular leakage.

The effect of blood flow promoting drugs on cochlear blood flow, perilymphatic pO(2) and auditory function in the normal and noise-damaged hypoxic and ischemic guinea pig inner ear

The effect of blood flow promoting drugs, such as hydroxyethyl starch (HES) either of low or high molecular weight (HES 70, HES 200), pentoxifylline, ginkgo biloba, naftidrofuryl and betahistine, and various combinations of the drugs was studied in unexposed and noise-exposed (broad-band noise, bandwidth 1-12 kHz, 106 dB SPL, 30 min) guinea pigs. The results were compared without therapy and placebo (isotonic saline, NaCl). The cochlear blood flow (CoBF) and the partial pressure of oxygen in the perilymph (PL-pO(2)) were continuously and simultaneously recorded over a period of 210 min. In addition, cochlear microphonics (CMs), compound action potentials of the auditory nerve (CAPs) and auditory brain stem responses (ABRs) were registered. Noise-induced hearing loss (NIHL) paralleled a decrease of PL-pO(2). Both were found to occur before evidence of reduced CoBF. PL-pO(2) and CoBF declined progressively post-exposure, while CMs, CAPs and ABRs showed no further deterioration or signs of recovery up to 180 min after cessation of noise. Treatment started 60 min post-exposure, respectively after 90 min, without manipulation in unexposed animals, and was then studied for a further 120 min. In unexposed animals, CoBF increased significantly during infusion of HES 70, HES 200, pentoxifylline and betahistine. NaCl, ginkgo biloba and naftidrofuryl did not alter CoBF. PL-pO(2) decreased significantly during infusion of all administered drugs and combinations, except for NaCl. CMs, CAPs and ABRs remained constant, with the exception of increased ABRs after infusion of HES 70 and HES 200. In noise-exposed animals, a sustained therapeutic effect on cochlear ischemia was achieved only by HES 200 and pentoxifylline. HES 70, betahistine and ginkgo biloba compensated cochlear ischemia only during infusion; however, 30-60 min after termination of therapy, no significant difference of values for CoBF was observed compared to the untreated noise-exposed groups. NaCl and naftidrofuryl showed no effect on CoBF. None of the applied drugs had a sustained compensatory effect on cochlear hypoxia. CMs, CAPs and ABRs improved significantly after HES 70, HES 200 and betahistine, resulting in partial recovery of CMs, and partial (betahistine) or even full (HES 70 and HES 200) recovery of CAPs and ABRs. In contrast, NaCl, pentoxifylline, ginkgo biloba and naftidrofuryl had no therapeutic effect on NIHL.

Influence of hypotension on cochlear blood flow in polycythemic condition

We examined the influence of hypotension by infusion of acebutolol hydrochloride (AH), a cardioselective beta-receptor antagonist, on cochlear blood flow in guinea pigs with various hematocrit values. AH infusion lowered the mean blood pressure to almost the same degree in all animals, regardless of the hematocrit level. The degree of the concomitant decrease of CBF varied with the hematocrit, being greater in animals with a higher hematocrit. In those with the highest hematocrit CBF did not return to the initial level. From these values we calculated the O2 transport capacity after AH infusion and found it to be lower than in animals without AH infusion. The difference was greater at higher hematocrits. These findings suggest that the microcirculation of the inner ear is responsive to transient decreases of perfusion pressure at high hematocrits.

Effects of posture, hypotension and locally applied vasoconstriction on the middle ear microcirculation in anaesthetized humans

Studies by laser-Doppler flowmetry of middle ear microcirculation changes induced by physical and chemical stimuli in the animal have only recently been made. This prospective study, performed in humans, was designed to compare the effects of a postural manoeuvre (headup tilt 30 degrees), hypotension and locally applied vasoconstriction on middle ear blood flow during anaesthesia. Circulatory changes provoked by a headup tilt of 30 degrees, and successive intravenous boluses of potent vasodilators, were compared with circulatory changes provoked by locally applied adrenaline, in ten healthy patients in good physical states undergoing middle ear surgical repair. Heart rate and direct arterial pressure were continuously recorded via a radial artery cannula. Middle ear blood flow was continuously recorded via a laser-Doppler probe placed on the promontorium cavi tympani. Metabolic parameters (partial pressure of O2 and CO2 in arterial blood, pH, arterial lactate concentrations) and arterial concentrations of propofol were measured just before and just after the experiment. Headup tilt did not modify heart rate, mean arterial pressure or middle ear blood flow. Vasodilators (nicardipine, nitroprusside, nitroglycerin) provoked a fall in arterial pressure (P < 0.0001, P < 0.0001, P < 0.019, respectively), but did not induce any significant variations in heart rate; variations occurred in middle ear blood flow (P > 0.05, not significant) which were different according to patients and agents. Locally applied adrenaline provoked a fall in the middle ear blood flow (P < 0.0012), with no effect on heart rate and arterial pressure. There were no significant changes in metabolic values, or propofol serum concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of experimentally elevated blood viscosity on the auditory nerve-brainstem evoked response and threshold

Blood viscosity, due to its effect on blood flow, is one of the determinants of oxygen delivery. Therefore the influence of elevated blood viscosity on hearing was studied in rats using the auditory brainstem response (ABR) threshold, wave 1 latency, brainstem transmission time (BTT) and wave 1/4 amplitude ratio. Whole blood viscosity (WBV) was elevated by 15-21% in two different ways: elevating the hematocrit (Polycythemia) by acclimation in a hypobaric chamber, or elevating the plasma viscosity by infusing a solution of Polyvinylpyrrolidone-360 (PVP). ABR was recorded before and 24 h after the blood viscosity was elevated, so that each rat served as its own control. Paired t-tests showed that there was no statistically significant difference in the ABR parameters in each of the groups as a consequence of blood viscosity elevation. In conclusion, the elevation of WBV to this degree for this duration, using two different techniques had no effect either on the function of the auditory nerve and the more peripheral sites, or on the central auditory pathway as studied by ABR.

Cochlear blood flow and microvascular resistance changes in response to hypertonic glycerol, urea, and mannitol infusions

The effect of hyperosmotic agents on cochlear blood flow (CBF) was tested in normal guinea pigs and in guinea pigs having prior unilateral operations to ablate the endolymphatic duct. Laser-Doppler-measured CBF was normalized to remove apparent changes related directly to systemic blood pressure. Hyperosmotic fluids were given via venous infusion: glycerol (20% and 40% solutions), urea (10%, 30%, and 40% solutions), and mannitol (40% solution). All agents were dissolved in 0.9% saline and the mixtures were given at a rate of 0.3 to 0.6 mL/min for 5 minutes. Control infusions were of 0.9% saline and isotonic dextran 70 (Pharmacia). All hyperosmotic infusions resulted in similar increases in normalized cochlear blood flow (nCBF) that extended to a maximum of 300% of the baseline value in a dose-dependent way during the infusion time period. Within approximately 30 minutes following infusions, nCBF had returned to baseline levels. Saline infusion alone had little effect on nCBF, but isotonic dextran 70 gave a sustained increase to 122% of the baseline levels. There was no difference between the responses of nCBF in hydropic and normal cochleas for either control or hyperosmotic solutions. Measurements of systemic hematocrit at time intervals during and following the infusions showed that transient reductions of up to approximately 8% (for the maximum osmotic challenge) occurred during the infusion. It is concluded that the hyperosmotic treatments tested here are equally effective for short-term enhancements of nCBF in both normal and hydropic cochleas. The basis of the flow increase is partially rheologic and partially due to a local vasodilation.

A randomized, double-blind, placebo-controlled study of dextran/pentoxifylline medication in acute acoustic trauma and sudden hearing loss

The effectiveness of any therapy in acute acoustic trauma or sudden hearing loss of unknown origin has not been demonstrated convincingly. The assessment is difficult because of a relatively high rate of spontaneous recovery. Nevertheless, many different forms of treatment are recommended. We tested one form, treatment with rheoactive substances, in a prospective, randomized, double-blind trial and compared treatment with (a) infusions of dextran-40 with pentoxifylline, (b) saline infusions with pentoxifylline, and (c) saline infusions with placebo medication. Pure-tone hearing thresholds served as control parameters and were taken before treatment and at 1 and 4 weeks after the onset of therapy. Three hundred eighty-two patients were included in the trial, 331 (87%) could be analyzed, 184 patients were treated because of sudden hearing loss, 147 because of acute acoustic trauma. The three treatment groups were comparable in their basic characteristics including the amount of initial hearing loss. In patients with sudden hearing loss, no significant differences of hearing recovery were detected between the three treatment groups. Hearing recovery was also similar in patients with acute acoustic trauma. A power analysis of the study revealed that possible true treatment differences of a hearing recovery of 10 dB would have lead to significance with a probability of over 90%. It is concluded that there were, in fact, no clinically relevant differences in hearing gains of sudden hearing loss or acute acoustic trauma between treatments with saline infusions together with placebo medication and treatment with dextran-40 and/or pentoxifylline.

Effects of hydroxyethyl starch, nimodipine, and propylene glycol on cochlear blood flow

A primary goal of pharmacologic treatment for otopathologies of vascular origin is to elevate cochlear blood flow (CoBF), thus facilitating the transport of oxygen and nutrients without compromising perfusion pressure in other tissues. In the present investigation, significant increases in CoBF were measured during intra-arterial infusion of the plasma expanding agent, hydroxyethyline starch (HES), and the vasodilator nimodipine, in anesthetized adult male guinea pigs. There were no changes in systemic blood pressure during the infusion of HES or nimodipine. Intra-arterial infusion of propylene glycol (PG), which is used as a nonaqueous solvent, produced inconsistent CoBF effects accompanied by initial decreases in systemic blood pressure with subsequent increases. It is concluded that nimodipine and HES are very promising agents for inducing increases in CoBF, whereas PG produced inconsistent effects on CoBF while elevating blood pressure, thus compromising its potential usefulness in the treatment of otopathologies.

The effects of Carbogen, carbon dioxide, and oxygen on noise-induced hearing loss

An investigation into the effect of Carbogen (95% O2/5% CO2), 5% CO2/air, and 100% oxygen on cochlear threshold shifts caused by noise was undertaken. Five groups of eight pigmented guinea pigs were exposed to 105 dB broad band noise for 6 h per day for five consecutive days with each group receiving the various gaseous mixtures either during noise exposure or for 1 h immediately after noise exposure. A control group received the same noise exposure but respired air. Auditory threshold shifts, as measured by the auditory evoked brainstem response, were measured at 2,4,8,12,16, 20 and 24 kHz. Recordings were taken pre-exposure and at Day 1, 3, 5, and Weeks 2 and 3 after noise exposure. Carbogen, given during noise exposure, resulted in a trend toward less post noise exposure threshold shift (as compared to controls) which reached statistical significance by Week 3 at all frequencies except 2 and 20 kHz. Subjects given Carbogen after exposure also showed a general trend toward decreased noise induced threshold shifts, as compared to controls, but this was not statistically significant. The mixture of 5% CO2/air given during noise exposure yielded no difference in threshold shifts as compared to controls. When 100% oxygen was administered during noise exposure, a marked decrease in noise induced threshold shifts could be seen as compared to controls, with differences reaching statistical significance by day 5 at most frequencies. These results indicate that oxygen (i.e. cochlear-oxygenation) is a more important factor than CO2 (i.e., as a vasodilator) in protection of the cochlea from noise induced damage.

Measurement of human cochlear blood flow

Cochlear blood flow (CBF) was measured with a laser-Doppler (L-D) flowmeter (Periflux PR2-B) in four unanesthetized human subjects with chronic tympanic membrane perforations and nine anesthetized human subjects undergoing middle ear operations. The L-D recordings were made over the promontory and/or the round window membrane during carbogen breathing and direct electrical stimulation of the cochlea in both groups and with warm water irrigation of the external ear canal in the anesthetized subjects. Carbogen led to little or no change in CBF as monitored with either measurement approach in either subject group. Electrical stimulation yielded an increase (15% to 25%) in CBF as recorded from the promontory in seven of the nine subjects tested. Warm (44 degrees C to 49 degrees C) water irrigation produced changes of 20% to 60% in CBF that were partially recoverable in the 10 minutes available for study. This study demonstrated the feasibility of direct CBF measurement in humans with the L-D method. Moreover, the data indicate that carbogen has little influence on CBF and that electrical stimulation at relatively safe levels and warm water irrigation of the ear canal produce increases in human CBF.

Pentoxifylline increases cochlear blood flow while decreasing blood pressure in guinea pigs

The effects of pentoxifylline on cochlear blood flow (CoBF) were investigated in anesthetized guinea pigs by laser Doppler flowmetry and intravital microscopy red blood cell velocity measurement. Intra-arterial infusion of pentoxifylline (3, 4, and 5 mg/kg/min) produced dose-dependent reductions in blood pressure, accompanied by significant elevations in CoBF that were not dose-dependent. These results are in general agreement with previous findings from our laboratory utilizing normotensive and spontaneously hypertensive rats, however, in contrast with rats, guinea pigs revealed an initial decrease in CoBF followed by an increase. Also, pentoxifylline produced relatively smaller elevations in CoBF in guinea pigs as compared with those previously reported in rats. Taken together these results support the hypothesis that pentoxifylline increases vascular perfusion by decreasing blood viscosity and increasing the plasticity of red blood cells.

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

In 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.

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.

The effects of pentoxifylline on cochlear blood flow in normotensive and spontaneously hypertensive rats

Normotensive and spontaneously hypertensive rats (SHRs) were given arterial infusions of saline and pentoxifylline in doses of 1.5, 2.0, and 3.0 mg/kg/min. Blood pressure decreased in a dose related fashion with increasing doses of pentoxifylline. Cochlear blood flow, measured via laser Doppler flowmeter, increased with infusion of pentoxifylline in members of both rat strains, but to a greater extent in SHRs than in normotensive animals. The mechanism of action appears to be decreased red blood cell rigidity which allows increased penetration of red blood cells into microvessels of the cochlea.

Cochlear blood flow increases after systemic hemodilution: comparison of simultaneous laser Doppler flowmetry and radioactive microsphere measurements

Guinea pig cochlear blood flow was measured before and after systemic normovolemic hemodilution with high molecular weight dextran. Absolute determinations of blood flow (in the cochlea, brain, kidney and lung) were accomplished by use of radioactive-labeled (85Sr or 141Ce) microspheres. Relative measurements of the cochlear blood flow changes were made simultaneously by the use of a laser Doppler flowmeter. The flowmeter probe was placed on the first cochlear turn. Hemodilution to an average systemic hematocrit of 20% increased cochlear blood flow by 250% as measured with microspheres. The laser Doppler instrument significantly underestimated the actual flow increase giving an indication of 148%. Furthermore, the data, when analyzed on an individual trial basis, showed a very poor correlation between the two methods. The theoretical basis for these findings in relation to the use of the laser Doppler instrument is discussed.

Blood pressure in resting, anesthetized and noise-exposed guinea pigs

This study investigated blood pressure in guinea pigs while they were 1) alert and free moving, 2) anesthetized with different anesthetics, and 3) exposed to continuous, 115 dB SPL white noise under anesthesia. The animals were prepared with a carotid artery catheter and permitted to recover for 48 h before blood pressure levels were measured. Mean arterial blood pressure in the resting, unrestrained guinea pig was 64 mmHg (+/- 1.38 S.E.). Ketamine Hydrochloride (Ketamine) significantly decreased, and Fentanyl-Citrate significantly increased, blood pressure. Fentanyl-Droperidol produced no substantial blood pressure change. Guinea pigs anesthetized with Fentanyl-Citrate and Fentanyl-Droperidol demonstrated significant blood pressure increases when exposed to noise, with the Fentanyl-Citrate group showing a greater response. Animals anesthetized with Ketamine Hydrochloride exhibited no significant blood pressure changes when exposed to the noise.

Angiotensin II-induced changes in cochlear blood flow and blood pressure in normotensive and spontaneously hypertensive rats

Previous investigations in our laboratory have measured significant increases in the circulating levels of the potent vasoconstrictive hormone, angiotensin II (AII; 26 and 64 pg/100 microliters plasma, normal and noise exposed, respectively), during and following noise exposure in the alert rat (Wright et al., 1981). In the present study, these levels were approximated through intra-arterial infusion in the anesthetized spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto (WKY) rat. Laser Doppler flowmeter measurements of cochlear blood flow (CBF) indicated that despite equivalent AII-induced elevations in systemic blood pressure, CBF in the SHR did not increase to the levels measured in the WKY. Pretreatment with the specific angiotensin receptor antagonist sarile, (Sar1,Ile8-AII), reduced AII-induced elevations in systemic blood pressure in members of both strains, but did not change the overall pattern of CBF. These results indicate that SHRs may have a compromised cochlear circulation that is refractory to increases in systemic blood pressure.

Sound-induced artifact in cochlear blood flow measurements using the laser Doppler flowmeter

The laser Doppler flowmeter has been shown to give a response from the cochlea during high intensity acoustic stimulation which is not related to blood flow through the cochlea. The magnitude of this response depends upon the intensity and frequency of stimulation and the location of the probe on the cochlea. Evidence is presented that the response is derived from the vibration of cochlear tissue and/or the bony cochlear shell during acoustic stimulation.