The isolated in vitro perfused spiral modiolar artery: pressure dependence of vasoconstriction
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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2015, Bioorganic and Medicinal ChemistryCitation Excerpt :The final concentration of DMSO did not exceed 0.1 vol %. The vascular diameter was continuously monitored via the microscope as described previously.11,12 Briefly, the magnified image of the spiral modiolar was monitored by a video camera (WV-1410, Panasonic), displayed on a monitor (PVM-122, Sony), and recorded on video tape (AG-1960 Panasonic).