The effect of loud sound exposure on cochlear blood flow was studied in the guinea pig by the laser Doppler method. Fourteen guinea pigs with normal Preyer reflex were anesthetized and tracheotomy was performed. A tracheotomy tube was connected to a ventilator and the experiment was performed with artificial ventilation. After exposure of the tympanic bulla and complete removal of the mucosa, a probe of a laser Doppler flowmeter was attached to the lateral wall of the basal turn of the cochlea. A specially-designed ear piece connected with a speaker was inserted into the external ear canal and loud sound (10 kHz at 120 dB SPL) was delivered to the ear for 30 min. Thirteen of the 14 exposed guinea pigs showed a prompt decrease in cochlear blood flow at the onset of the sound exposure and 12 of the 13 guinea pigs showed a prompt recovery of the cochlear blood flow after the cessation of the sound exposure.
The relationship between blood pressure and cochlear blood flow was investigated in 68 guinea pigs, using the vasoactive drugs angiotensin II, norepinephrine, phentolamine, isoproterenol, dobutamine, salbutamol, propranolol, bradykinin, papaverine, vinpocetine dilazep, and brovincamine. Cochlear blood flow increases markedly and proportionately to increases in blood pressure. By contrast, cochlear blood flow shows various responses toward a fall in blood pressure. In general, cochlear blood flow appears relatively resistant to blood pressure decrease.
Laser Doppler flowmetry demonstrates that loud sound induces a decrease of blood flow in the cochlea of the guinea pig. In this experiment, we observed the effects of frequency and intensity of sound on cochlear blood flow using 15 guinea pigs. In the first 5 guinea pigs, a Doppler probe was attached to the basal turn of the cochlea and sounds of 6, 7, 8, 9 and 10 kHz were delivered to the ear serially from lower to higher frequency, i.e. from 6 kHz to 10 kHz. In the next 5 guinea pigs, the sound was delivered from higher to lower frequency, i.e. from 10 kHz to 6 kHz. The sound intensity delivered to the ear was changed from lower to higher intensity (80 to 120 dB SPL by 10 dB width) at each frequency. In the last 5 guinea pigs, the blood flow in the basal, second, third, and fourth turns of the cochlea was measured at 120 dB SPL of 10 kHz. No change of blood flow was seen in the cochlear basal turn at 6 and 7 kHz up to 120 dB SPL, but a decrease of blood flow was found at 110 and 120 dB SPL at 8, 9, and 10 kHz. On the other hand, the sound of 120 dB SPL at 10 kHz induced a decrease of blood flow only in the basal turn of the cochlea. Our results suggest that there is a corresponding blood flow area which is sensitive to specific frequency in the cochlea.
Loud sound has been proved by means of laser Doppler flowmetry to decline cochlear blood flow. The purpose of this study was to investigate the effect of a vasodilating agent on cochlear blood flow under loud sound exposure, i.e. whether the drug can impede blood flow decrease or not. As a vasodilating agent, dilazep dihydrochloride in a dose of 5 mg/kg was used. This drug caused a stable and significant increase of cochlear blood flow when intravenously injected into guinea pigs. When guinea pigs were exposed to loud sound (120 dB SPL at 10 kHz) for 10 min, cochlear blood flow promptly declined at the onset of sound stimulation and promptly recovered at its cessation. Then, dilazep dihydrochloride 5 mg/kg was injected intravenously into the same animal and loud sound (120 dB SPL at 10 kHz) was exposed for 10 min. Dilazep did not fully block a prompt decline of cochlear blood flow. However, the blood flow level was kept much higher than at pre-injection level. This study shows that a vasodilating agent which normally enhances blood flow probably does not completely block the sound-induced drop response in cochlear blood flow.
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