Elsevier

Hearing Research

Volume 115, Issues 1–2, January 1998, Pages 113-118
Hearing Research

The isolated in vitro perfused spiral modiolar artery: pressure dependence of vasoconstriction

https://doi.org/10.1016/S0378-5955(97)00184-6Get rights and content

Abstract

We developed a new technique, the isolated in vitro perfused spiral modiolar artery, which allowed the continuous measurement of the vascular diameter and control of the intravascular pressure. An isolated section of the spiral modiolar artery from the gerbil was perfused on one end with a set of concentric pipettes and occluded on the other end in order to apply a defined intravascular pressure in the range between 10 and 230 cm H2O. The preparation was continuously superfused with a NaCl solution. The diameter of the spiral modiolar artery in NaCl solution displayed little dependence on the applied intravascular pressure. The diameter was 73±10 μm (n=5) at 10 cm H2O and increased with pressure to 85±7 μm (n=5) at the highest applied pressure (220 or 230 cm H2O). Elevation of the K+ concentration from 3.6 to 150 mM in the superfusate caused a transient vasoconstriction. The amplitude of the K+-induced vasoconstriction depended strongly on the applied intravascular pressure. At 10 cm H2O the amplitude was maximal and the outer diameter decreased transiently by 49±9% (from 73±10 to 38±9 μm; n=5). The amplitude of K+-induced vasoconstriction was nearly maximal at pressures lower than 30 cm H2O, declined at higher pressures, and was not significantly different from zero at pressures larger than 100 cm H2O. These observations in conjunction with an estimation of the intravascular pressures in vivo suggest that cochlear blood flow can be regulated on two levels: (1) cochlear blood flow can be regulated by controlling the vascular diameter of the spiral modiolar artery (intracochlear blood flow regulation) and (2) intracochlear blood flow regulation can be modulated by altering the perfusion pressure which is controlled by the vasculature upstream of the cochlea.

Introduction

The regulation of cochlear blood flow has until now almost exclusively been studied in situ using techniques such as laser-Doppler flowmetry (Miller et al., 1984), microsphere injection (Angelborg et al., 1977), impedance plethysmography (Snow and Suga, 1973), intravital microscopy (Weille, 1954) and hydrogen clearance (Maass and Kellner, 1984). Although in situ studies have led to many important observations, the interpretation of the data is often limited by systemic effects confounding the results. Effects observed in the cochlea may not necessarily originate there but rather may be the result of changes in the systemic blood pressure which may even involve changes in the plasma concentrations of vasoactive hormones. Further, effects observed in the cochlea may be the result of vascular diameter changes up- and/or downstream of the cochlea leading to localized pressure changes. Such localized pressure changes may not be detectable by measuring the systemic blood pressure but may result in the observation of cochlear blood flow changes which did not originate in the cochlea. Even if effects on cochlear blood flow originated in the cochlea, it may remain obscure whether they occurred in the vessels of the modiolus or the lateral wall. Thus, effects on cochlear blood flow monitored in situ cannot be localized with confidence. In contrast, unambiguous localization of observations as well as elimination of systemic effects are intrinsic features of measurements in vitro.

The goal of the present study was to develop an in vitro preparation for the study of factors which control cochlear blood flow. We chose the spiral modiolar artery since this vessels provides the main blood supply to the cochlea. Blood flow through a single vessel depends, according to Ohm's law, on the intravascular pressure difference and the vascular resistance. Vascular resistance, according to the law of Hagen-Poiseuille, is chiefly controlled by the vascular diameter. Thus, the second goal of the present study was to develop a technique to measure the vascular diameter of the spiral modiolar artery in vitro.

This paper describes a new technique, the in vitro perfused spiral modiolar artery, as well as data obtained with this method. This paper has been presented in abstract form at a recent meeting (Wangemann and Gruber, 1996).

Section snippets

Preparation

Gerbils were anesthetized with pentobarbital sodium (50 mg/kg i.p.) and decapitated under a protocol approved by the Institutional Animal Care and Use Committee at Boys Town National Research Hospital. The temporal bones were removed from the head, opened and placed into a dissection chamber for microdissection at 4°C. The dissection solution contained (mM) 150 NaCl, 1.6 K2HPO4, 0.4 KH2PO4, 0.7 CaCl2, 1.0 MgCl2 and 5.0 glucose, pH 7.4. This Cl free medium was chosen since it has been proven to

Results

We developed a new technique, the in vitro perfused spiral modiolar artery, which allows continuous measurement of the vascular diameter and control of the intravascular pressure. This preparation was developed to study the regulation of cochlear blood flow in vitro (Fig. 1, Fig. 2). In the present study, we describe and discuss a set of experiments obtained with this new technique.

Discussion

The diameter of the spiral modiolar artery in NaCl solution displayed little dependence on the applied intravascular pressure (Fig. 3B). Similar observations have been made in other arterioles such as from the retina (Yu et al., 1994) and the cortex (Kimura et al., 1993). It is most likely that the extracellular matrix surrounding the spiral modiolar artery limited dilation of the vessel at high pressures.

Elevation of the K+ concentration in the superfusate induced a transient vasoconstriction

Acknowledgements

Programs in aid of data acquisition and analysis were written by P. Wangemann. Supported by Research Grant P01-DC-00215 from the National Institute on Deafness and Other Communication Disorders, National Institutes of Health.

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