Vascular effects of metformin. Possible mechanisms for its antihypertensive action in the spontaneously hypertensive rat.

Metformin, an antidiabetic agent, potentiates insulin action and reduces insulin resistance. We examined the antihypertensive effects and vascular effects of metformin in spontaneously hypertensive rats (SHR). Wistar-Kyoto normotensive (WKY) and SHR were injected with metformin (100 mg/kg) or saline subcutaneously twice daily for 4 weeks. Blood pressure was recorded by a tail-cuff plethesmographic method. Metformin treatment significantly attenuated (P < .05) the increase in blood pressure in metformin treated SHR versus untreated control SHR. At the end of the experimental period of 4 weeks, metformin-treated SHR had a mean blood pressure that was 34 mm lower than that of untreated SHR. Metformin treatment had no significant effect on blood pressure in WKY rats. Treatment of SHR aortic smooth muscle (SM) cells with metformin (2 micrograms/mL) for 24 h significantly decreased (P < .05) arginine vasopressin- and thrombin- stimulated increase in [Ca2+]i. However, metformin treatment did not have a significant effect on the basal [Ca+]i. Incubation of SHR aortic SM cells with OH-L-arginine (25 to 100 mumol/L) for 24 h increased nitrite production in a dose dependent manner. Metformin (5 micrograms/mL) treatment of SM cells increased nitrite production at all concentrations of OH-L-arginine; however, differences were significant (P < .05) only at 25 and 50 mumol/L OH-L-arginine. These results suggest that metformin may be decreasing arterial pressure in the SHR, at least in part, by attenuating the agonist-stimulated [Ca2+]i response in SHR vascular smooth muscle cells.