Comparative effect of tropolone and diosmin on venous COMT and sympathetic activity in rat.

: The ability of diosmin to inhibit venous catechol-O-methyltransferase (COMT) activity was studied comparing it with tropolone in rat superior mesenteric vein (MV), inferior vena cava (IVC) and saphenous vein (SV). Diosmin inhibited COMT activity in the mesenteric vein after 200 and 400 mg/kg i.p., but only at 400 mg/kg in IVC. The COMT inhibitory effect of diosmin (400 mg/kg) was smaller than that of tropolone (50 mg/kg). Furthermore, diosmin, but not tropolone, increased urinary normetanephrine (NMN) excretion (+56%) at 400 mg/kg and that of 3-methoxy 4-hydroxyphenylglycol (MHPG) in a dose-dependent way, thus suggesting that diosmin may exert an activating effect on sympathetic activity. Both mechanism--local inhibition of COMT and enhanced sympathetic activity--may contribute to increase NE levels in the synaptic clefts of the vascular wall and explain the venoconstrictor effect of this drug.