Effects of adrenergic, cholinergic and ganglionic blockade on acute depressor responses to metformin in spontaneously hypertensive rats.

Metformin lowers blood pressure in humans and in experimental animal models. To determine the mechanism of acute metformin-induced hypotension, we measured changes in mean arterial pressure (MAP) and heart rate (HR) during metformin alone (0, 10, 50, 100 mg/kg i.v.; n = 10) and during concomitant alpha adrenergic (phentolamine, 5 mg/kg; n = 5), beta adrenergic (propranolol, 3 mg/kg; n = 6), muscarinic (atropine, 200 micrograms/kg; n = 7), ganglionic (hexamethonium, 30 mg/kg; n = 11), nitric oxide synthase (NG-methyl-L-arginine acetate salt, 15 mg/ kg; n = 9) and combination ganglionic plus alpha adrenergic plus beta adrenergic (n = 6) blockade in spontaneously hypertensive rats (SHR). Responses to metformin alone were also assessed in normotensive Wistar-Kyoto rats (n = 6). In SHRs, metformin elicited depressor responses accompanied by tachycardia (100 mg/kg; delta MAP, -26 +/- 3 mm Hg; delta HR, +49 +/- 12 bpm). Depressor responses in Wistar-Kyoto rats were significantly attenuated (100 mg/kg; delta MAP, -9 +/- 4 mm Hg; P < .01). Hypotensive actions of metformin in SHRs were abolished and reversed into pressor responses by hexamethonium (100 mg/kg; delta MAP, +24 +/- 6 mm Hg), phentolamine (100 mg/kg; delta MAP, +62 +/- 10 mm Hg) and by combination ganglionic plus adrenergic (100 mg/kg; delta MAP, +62 +/- 10 mm Hg) blockade. Neither propranolol, atropine nor NG-methyl-L-arginine acetate salt affected hypotensive responses to metformin. We conclude that acute intravenous metformin administration decreases MAP by causing withdrawal of sympathetic activity. The increase in MAP uncovered by hexamethonium and phentolamine suggests that the original depressor response to metformin is buffered by mechanisms unrelated to the autonomic nervous system.