Effects of Hyperventilation and Hypoventilation on Cochlear Blood Flow and Endocochlear Direct-Current Potential

Abstract

To understand the importance of oxygen transport to the inner ear tissue, we studied, in guinea pigs, the relationship between cochlear blood flow and endocochlear direct-current potential (EP) under different respiratory conditions. EP, a functional parameter of the stria vascularis, was recorded by a microelectrode inserted into the lateral wall of the chochlea. To measure the cochlear blood flow (CoBF), we employed laser Doppler flowmetry and recorded the flow with a probe placed on the same spot on the lateral wall. During 3 min of asphyxia, CoBF and systemic blood pressure showed irregular biphasic increases, while the EP decreased to reach a negative value. In the hypoxemia experiment, which was induced by stepwise reduction of the respiratory rate to 60%, increases in CoBF and blood pressure were evident during hypoventilation with an intermediate position of EP in the positive range. The mechanisms of these increases in two parameters are discussed from the viewpoints of sympathicotonic activity in the autonomic nervous system and the vasodilating action of CO₂ during hypercapnia. In the hyperoxemia experiment, which was induced by stepwise increase in the respiratory rate to 140%, CoBF and blood pressure were found to decrease during hyperventilation with no significant change of EP. The decrease in blood pressure was considered to be due to the increase in intrathoracic pressure caused by the increased rate of artificial respiration. As for the concomitant decrease in CoBF, chemical regulation of PCO₂ in the vascular bed of the lateral wall of the cochlea was thought to be a contributory factor.

Functional expression of P2X4 receptor in capillary endothelial cells of the cochlear spiral ligament and its role in regulating the capillary diameter

“…However, the cellular mechanism underlying the ATP-induced regional blood flow increase has not been studied. The SL capillaries are exposed to perilymph and perform an important function of controlling the blood flow in the SV, which is critical to generate EP (46,31,63,62,37,32). In this study, we found that topical perilymphatic superfusion of ATP (1 mM) dilated a single segment of the SL capillaries by 11.5%, which is a substantial change for the SL capillary network considering the power relationship of diameter change to flow resistance for the capillaries.…”

“…The cochlear lateral wall expends energy to generate the endocochlear potential (EP), an important component of the driving force for the hair cell transduction current, which is essential for normal hearing. EP generation is highly dependent on the oxygen that is provided by blood flow through microcirculation in the cochlear lateral wall (57,14,46,31,63,62,37,32). The vulnerability of the EP to hypoxia and ischemia suggests that the blood flow in the cochlear lateral wall is dynamically and precisely regulated to meet the changing metabolic needs of the cochlear lateral wall.…”

Sensitivity of the Endocochlear Potential Level to Cochlear Blood Flow during Hypoventilation

“…5 At the point 5 minutes after the beginning of hypoventilation, angiotensin II (5 |ig/kg or 1 mL/kg in saline solution) was infused for 60 seconds to raise the EP. For the analysis of changes in CoBF and EP after angiotensin II infusion, we made the following definitions.…”

“…As illustrated in Fig 1, in response to hypoventilation loading, the BP, CoBF, and EP generally showed some increases and/or decreases. 5 At the point 5 minutes after the beginning of hypoventilation, angiotensin II (5 |ig/kg or 1 mL/kg in saline solution) was infused for 60 seconds to raise the EP. For the analysis of changes in CoBF and EP after angiotensin II infusion, we made the following definitions.…”