Vol. 281, Issue 2, 618-623, 1997

Effects of Adrenergic, Cholinergic and Ganglionic Blockade on Acute Depressor Responses to Metformin in Spontaneously Hypertensive Rats¹

Martin S. Muntzel, Ayat Abe and Jørgen S. Petersen

Department of Biological Sciences, Lehman College, Bronx, New York and Department of Pharmacology, The Panum Institute, University of Copenhagen, DK-2200 Copenhagen, Denmark

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 µg/kg; n = 7), ganglionic (hexamethonium, 30 mg/kg; n = 11), nitric oxide synthase (N^G-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; MAP, 26 ± 3 mm Hg; HR, +49 ± 12 bpm). Depressor responses in Wistar-Kyoto rats were significantly attenuated (100 mg/kg; 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; MAP, +24 ± 6 mm Hg), phentolamine (100 mg/kg; MAP, +62 ± 10 mm Hg) and by combination ganglionic plus adrenergic (100 mg/kg; MAP, +62 ± 10 mm Hg) blockade. Neither propranolol, atropine nor N^G-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.