Response of EP and cochlear blood flow to angiotensin II during hypoxic condition.

To understand the importance of oxygen transport to the inner ear tissue, we studied, using guinea pigs, the relationship between endocochlear DC potential (EP) and cochlear blood flow (CoBF) under hypoxic conditions. 33 guinea pigs weighing 200-400 g were used. EP and CoBF were recorded from the basal turn of the cochlea. Hypoxia (10 min) was induced by inspiring a mixture gas (2-10% oxygen in nitrogen). By inducing hypoxia, the EP declined to a intermediate level of the positive range in a few minutes. In the midpoint of 10-min loading of hypoxia, Angiotensin II (1 microg/kg, 1 ml/kg) was infused in 60 s, resulting in raising of systemic blood pressure (BP). Responding to this, CoBF and EP showed generally transient elevations (deltaCoBF, deltaEP) in accordance with BP change. Obtained data were analyzed for the value of deltaEP/deltaCoBF, which means the sensitivity of EP to CoBF change. Distribution of the value of deltaEP/deltaCoBF was found to be in a correlation with the EP level just before Angiotensin II infusion (EPi). That is, the sensitivity of EP to CoBF change became greater as a intermediate level of EP (EPi) was lowered in the range from 70 to 100% of EPi. Among several factors, we thought the hyperactivity of glycolysis in the stria vascularis may occur in such a mild hypoxemic condition. In addition to this, it could be presumable that cyclic AMP in the stria vascularis may increase by the sympathetic stimulation during hypoxemia, and increased cyclic AMP may produce more EP with a given O2 consumption.