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It has been reported that metformin reduces blood pressure although the mechanisms have not been described. Indeed, several mechanisms could be implicated including the interaction with α-adrenoceptors or inhibition of sympathetic outflow. Therefore, this study was designed to determine the capability of metformin to block the vasopressor responses induced by α1/2-adrenoceptor agonists or selective electrical stimulation of sympathetic outflow. For this purpose, Wistar male rats were anesthetized, pithed and cannulated for selective preganglionic stimulation of the vasopressor sympathetic outflow or drugs administration. The effect of i.v. bolus injection of metformin (180 and 310mg/kg) or its vehicle (bidistilled water) was studied on the vasopressor responses induced by: (1) selective sympathetic stimulation (0.03-3Hz); (2) exogenous noradrenaline (0.03-3µg/kg); (3) methoxamine (1-100µg/kg); and (4) UK 14,304 (0.1-30µg/kg). The tachycardic responses to noradrenaline were also investigated in presence of metformin. The vasopressor responses induced by selective electrical stimulation of sympathetic outflow were diminished by metformin (180 and 310mg/kg) and remained unchanged in presence of vehicle. Moreover, the vasopressor responses induced by exogenous noradrenaline, methoxamine and UK 14,304 were dose-dependently inhibited by i.v. bolus injections of metformin (180 and 310mg/kg) and were not affected by vehicle. Metformin practically did not block the tachycardic responses to noradrenaline except at the dose of 3µg/kg. Taken together, these results demonstrate that metformin is capable to block vascular α1/2-adrenoceptors but not cardiac ß-adrenoceptors. Thus, this mechanism could contribute, at least in part, on the hypotensive responses induced by metformin.

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Metformin has been associated with cardioprotection, vasorelaxation and normalization of endothelial function during type 2 Diabetes Mellitus. However, few studies have analysed its effects on vascular adrenergic system. Our study has evaluated the vasopressor responses induced by sympathetic stimulation or by i.v. bolus injections of the agonists noradrenaline (α1/2), methoxamine (α1) and UK 14,304 (α2) in rats with fructose-induced insulin resistance chronically pretreated with either metformin or EGL-6M (N-benzylbiguanide), a novel analogue of metformin. Rats were treated with fructose (15%) or tap water (control) during 16 weeks. Next, both groups were treated daily during 4 weeks with: (1) vehicle; (2) metformin (50mg/kg); or (3) EGL-6M (50mg/kg). Blood glucose and plasma insulin were determined before and after administration of glucose during oral glucose tolerance test. Animals treated with fructose showed hyperinsulinemia and insulin resistance, which were decreased by metformin and EGL-6M. In animals treated with fructose, the vasopressor responses induced by: (1) sympathetic stimulation were decreased; (2) noradrenaline were increased; and (3) methoxamine and UK 14,304 remained unaffected compared with control group. In control animals, metformin failed to modify the vasopressor responses analysed, while EGL-6M increased the vasopressor responses to sympathetic stimulation. In rats treated with fructose, metformin decreased vasopressor response to noradrenaline but did not modify the sympathetic stimulation responses. EGL-6M increased the vasopressor responses to sympathetic stimulation without modifying those to noradrenaline, methoxamine or UK 14,304. Collectively, these data suggest that EGL-6M is capable to increase insulin sensitivity and the vasopressor sympathetic outflow in rats.

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Multiple mechanisms have been suggested to be responsible for the insulinotropic and blood glucose lowering effects of imidazoline compounds. This study was to unravel which mechanism predominantly accounts for glucose lowering by the prototypical imidazolines idazoxan and phentolamine. To this end, an α2-adrenoceptor agonist (UK14,304) and a KATP channel opener (diazoxide) were used to inhibit insulin release from isolated perifused mouse islets and to induce hyperglycaemia in conscious mice. Potentials of idazoxan and phentolamine to counteract these effects were examined in a comparative manner. In perifused islets, idazoxan increased insulin release only in the presence of the α2-agonist, whereas phentolamine strongly counteracted both inhibitors of insulin release. In vivo, a lower dose of idazoxan was necessary to ameliorate hyperglycaemia induced by the α2-agonist than by the KATP channel opener, indicating α2A-antagonism as the predominant mechanism of action (decrease in incremental area under the glucose curve induced by 0.1mg/kg idazoxan: under diazoxide, -3±7%, vs. under UK14,304, -34±9%, P<0.02). In contrast, identical doses of phentolamine were required to counteract hyperglycaemia induced by the two inhibitors of insulin release, implicating involvement of another mechanism beside α2A-antagonism (2mg/kg phentolamine: diazoxide, -11±8%, vs. UK14,304, -15±9%, ns; 4mg/kg phentolamine: diazoxide, -48±6%, vs. UK14,304, -48±8%, ns). The results show that imidazolines can lower blood glucose via more than one mechanism of action, with the relative contributions of the mechanisms varying considerably between individual compounds. Dissection of the involved mechanisms could help to develop imidazoline drugs for the treatment of type 2 diabetes.

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The second messenger cyclic AMP (cAMP) plays a vital role in the physiology of the cardiovascular system, including vasodilation of large blood vessels. This study focused on cAMP signaling in peripheral blood vessels, specifically in human vascular smooth muscle (microVSM) cells explanted from skin punch biopsy arterioles (also known as resistance vessels) of healthy volunteers. Using these human microVSM we recently demonstrated cAMP activation of exchange protein activated by cAMP (Epac), the Ras-related small GTPase Rap1A, and RhoA-ROCK-F-actin signaling in human microVSM to increase expression and cell surface translocation of functional α2C-adrenoceptors (α2C-ARs) that mediate vasoconstriction. Protein-protein association with the actin-binding protein filamin-2 and phosphorylation of filamin-2 Ser2113 by cAMP-Rap1A-Rho-ROCK signaling were necessary for receptor translocation in these cells. Although cAMP activated A-kinase in these cells, these effects were independent of A-kinase, and suggested compartmentalized A-kinase local signaling facilitated by A-kinase anchoring proteins (AKAPs). In this study we globally disrupted A-kinase-AKAP interactions by the anchoring inhibitor decoy peptide Ht31 and examined the effect on α2C-AR expression, translocation, and function in quiescent microVSM treated with the adenylyl cyclase activator and cAMP elevating agent forskolin. The results show that Ht31, but not the control peptide Ht31-P, reduced forskolin-stimulated Ser133 phosphorylation of A-kinase substrate CREB, reduced α2C-AR mRNA levels, reduced cell surface translocated receptors, and attenuated agonist-triggered receptor functional responses. Together, the results suggest that compartmentalized cAMP signaling elicits a selective cellular response in microVSM, which may have relevance to arteriole physiological function and responses.