Insulin effects on the sympathetic contraction of rabbit ear arteries

A novel endothelium-independent effect of insulin on basal vascular tone in cafeteria diet-induced hypertensive rats

Insulin vasorelaxant effect in metabolic syndrome has been shown on precontracted vessels. However, the insulin effects on basal vascular tone and its interrelationship with nitric oxide (NO) and K-channels are unknown. To test the effect of insulin on the basal vascular tone in isolated aortic rings from the cafeteria diet-induced hypertensive rats and to determine the role of NO and K-channels on this insulin effect. Male Wistar rats were randomized into two groups: one group fed with a cafeteria diet (CafR) and another fed with a standard chow diet (control rats: CR). Then, in isolated aortic rings, the insulin effect on the basal tone and the role of K-channels were evaluated. Also, the endothelial function, NO levels, and resting membrane potential were measured. CafR increased blood pressure (138 ± 6.2 mmHg; n = 9 vs. CR: 109 ± 1.4 mmHg; n = 9; p < 0.001) and vascular basal tone. Insulin 400 mU/ml reduced basal tone in aortic rings (-284 ± 47 mg; n = 9). This effect was unaffected by endothelium removal or N^G-nitro-l-arginine methyl ester (L-NAME) treatment. Likewise, CafR showed low NO levels and a hyperpolarized resting membrane potential. Insulin decreased the resting membrane potential and the K_Ca and Kv channels blockers abolished this effect. In CafR, endothelial dysfunction is accompanied by an increased basal tone. Insulin reduced it by Kv and KCa channels dependent mechanisms, using an endothelium-independent pathway. These results highlight a novel insulin effect on basal tone of aortic rings from animals with metabolic syndrome and endothelial dysfunction, pathophysiological conditions associated with human hypertension.

Immediate direct peripheral vasoconstriction in response to hyperinsulinemia and metformin in the anesthetized pig

Elevated levels of insulin have been reported to induce both an arterial vasodilation mediated by nitric oxide (NO), and vasoconstriction mediated by endothelin and reactive oxygen radicals. Metformin, used to control blood glucose levels in type 2 diabetes, has also been shown to cause NO-mediated dilation of conduit arteries. It is possible that these contradictory vascular effects are due to a non-direct action on arteries. Therefore, the direct effect of high levels of insulin and metformin infusion on resistance artery diameter was evaluated. Experiments were carried out on the anesthetized pig; blood flow and pressure were measured in the iliac artery. An adjustable snare was applied to the iliac above the pressure and flow measurement site to induce step decreases (3-4 occlusions at 5 min intervals were performed for each infusion) in blood flow, and hence iliac pressure, and the conductance (deltaflow / deltapressure) calculated. Saline, insulin (20 and 40 mUSP/l/min), and metformin (1 microg/ml/min) were infused separately downstream of the adjustable snare and their effect on arterial conductance assessed. Insulin at both infusion rates and metformin caused a significant reduction in peripheral vascular conductance. In conclusion, hyperinsulinemia and metformin infusion constrict resistance arterial vessels in vivo.