Influence of topically applied adrenergic agents on cochlear blood flow

This study was designed to assess the role of adrenergic receptors in the control of cochlear blood flow. Laser Doppler flowmetry was used to determine the effects of adrenergic drugs topically applied to the round window membrane of the cochlea. The relative influence of the various receptor types (α₁, α₂, β₁, and β₂) was examined by a selection of agonists and antagonists. The agonits norepinephrine and epinephrine, which have mixed α- and β-receptor effects, and phenylephrine, a strong α₁-agonist, all induced a dose-dependent reduction in cochlear blood flow. The agonists isoproterenol (β-active), salbutamol (β₂-active), and BHT 933 (α₂-active) had no effect on cochlear blood flow. Of the antagonists, when tested alone, only the selective α₁-antagonist prazosin had a direct effect on cochlear blood flow, demonstrating an increase in cochlear blood flow. The selective α₂-antagonist idazoxan, the β-antagonist propranolol, and the unselective α-antagonist phentolamine had no effect on cochlear blood flow. Interaction studies of agonists and antagonists were performed to specifically define the receptor subclasses responsible for the cochlear blood flow increases with norepinephrine and epinephrine. The results are consistent with the presence of an α₁-adrenergic sympathetic control of cochlear blood flow.