Norepinephrine in cochlear microcirculation of guinea pigs

The blood flow in the radiating arteriole through a small cochlear fenestra was recorded with motion pictures in anesthetized guinea pigs, before and after norepinephrine injection into the ipsilateral carotid artery. The flow velocity was determined by measuring the displacement of plasma space running through the radiating arteriole per second. Norepinephrine injection of 0.01 and 0.15 mg/kg increased both flow velocity and arterial blood pressure. The flow velocity increase related directly to the increase in arterial blood pressure. However, a larger norepinephrine dose (1.2 and 2.5 mg/kg) led to transient cessation in flow, which was independent of the blood pressure increase. Dilatation of vessel diameter was always observed with the rise in blood pressure, irrespective of norepinephrine doses. When blood flow in the radiating arteriole stopped after a large norepinephrine injection, the arteriole's vascular lumen was completely obstructed by the aggregated red blood cells. These results suggested that cochlear microcirculation is disturbed by microemboli formed by norepinephrine-induced platelet hyperaggregation.