In obese patients with type 2 diabetes, insulin delivery to and insulin-dependent glucose uptake by skeletal muscle are delayed and impaired. The mechanisms underlying the delay and impairment are unclear. We demonstrate that impaired insulin signaling in endothelial cells, due to reduced Irs2 expression and insulin-induced eNOS phosphorylation, causes attenuation of insulin-induced capillary recruitment and insulin delivery, which in turn reduces glucose uptake by skeletal muscle. Moreover, restoration of insulin-induced eNOS phosphorylation in endothelial cells completely reverses the reduction in capillary recruitment and insulin delivery in tissue-specific knockout mice lacking Irs2 in endothelial cells and fed a high-fat diet. As a result, glucose uptake by skeletal muscle is restored in these mice. Taken together, our results show that insulin signaling in endothelial cells plays a pivotal role in the regulation of glucose uptake by skeletal muscle. Furthermore, improving endothelial insulin signaling may serve as a therapeutic strategy for ameliorating skeletal muscle insulin resistance.

Lysophosphatidylcholine (LPC) is a bioactive proinflammatory lipid generated by pathological activities. LPC is also a major phospholipid component of oxidized low-density lipoprotein (Ox-LDL) and is implicated as a critical factor in the atherogenic activity of Ox-LDL. LPC is believed to play an important role in atherosclerosis and inflammatory diseases by altering various functions in a number of cell-types, including endothelial cells, smooth muscle cells, monocytes, macrophages, and T-cells. LPC activates several second messengers -- including protein kinase C, extracellular-signal-regulated kinases, protein tyrosine kinases, and Ca(2+) -- implicating the engagement of transduction mechanisms in its observed actions. Moreover, recent evidence suggests that in several cell-types, cloned orphan G-protein-coupled receptors may serve as the specific receptors via which LPC modulates second messenger pathways (although LPC may not be a direct ligand of such receptors). In addition, current evidence suggests that LPC impairs the endothelium-dependent relaxations mediated by endothelium-derived relaxing factors and directly modulates contractile responses in vascular smooth muscle. However, despite all this, and although elevated levels of LPC have been linked to the cardiovascular complications associated with atherosclerosis, ischemia, and diabetes, the precise pathophysiological roles played by LPC in several states remain to be established. In this review, we focus in some detail on the entirety of the signal-transduction system for LPC, its pathophysiological implications, and the vascular abnormalities associated with it.

1 Acetylcholine (ACh)-induced relaxation of aortic strips with endothelium and production of cyclic GMP between streptozotocin-induced diabetic and age-matched control rats were compared. 2 The concentration-response curve for ACh-induced relaxation was shifted to the right in diabetic rats. IC50 values for ACh were 4.57 + 0.67 x 10-8IM and 1.00 + 0.87 x 10-7M in aortic strips from age-matched control and diabetic rats, respectively (n = 6, P < 0.05). 3 Relaxations produced by atrial natriuretic peptide (ANP) in diabetic aortae were similar to those in age-matched vessels. 4 Relaxations produced by sodium nitroprusside (SNP) in diabetic aortae were similar to those in age-matched vessels. 5 Basal levels of cyclic GMP and ACh-induced production of cyclic GMP were significantly decreased in diabetic rats. 6 These results suggest that functional changes in endothelium but not in guanylate cyclase activity in the aorta may occur in diabetes, and, thus, spontaneous and ACh-induced formation of cyclic GMP may be decreased. This decrease in production of cyclic GMP may be responsible for the decreased response of the aorta to the relaxant effect of ACh.

Abstract-The phosphatidylinositol 3-kinase (PI3-K) pathway, which activates serine/threonine protein kinase Akt, enhances endothelial nitric oxide synthase (eNOS) phosphorylation and nitric oxide (NO) production. We investigated the involvement of the PI3-K/Akt pathway in the relaxation responses to acetylcholine (ACh) and clonidine in a new type 2 diabetic model (streptozotocin plus nicotinamide-induced diabetic mice). Plasma glucose and insulin levels were significantly elevated in our model, and intravenous glucose tolerance tests revealed clear abnormalities in glucose tolerance and insulin responsiveness. Although in our model the ACh-induced relaxation and NO x Ϫ (NO 2 Ϫ ϩNO 3 Ϫ )/cGMP production were unchanged, the clonidine-induced and insulin-induced relaxations and NO x Ϫ /cGMP production were all greatly attenuated. In control mice, the clonidine-induced and insulin-induced relaxations were each abolished by LY294002 and by Wortmannin (inhibitors of PI3-K), and also by Akt-inhibitor treatment. The ACh-induced relaxation was unaffected by such treatments in either group of mice. The expression level of total Akt protein was significantly decreased in the diabetic mice aorta, but those for the p85 and p110␥ subunits of PI3-K were not. The clonidine-induced Ser-473 phosphorylation of Akt through PI3-K was significantly decreased in our model; however, that induced by ACh was not. These results suggest that relaxation responses and NO production mediated via the PI3-K/Akt pathway are decreased in this type 2 diabetic model. This may be a major cause of endothelial dysfunction (and the resulting hypertension) in type 2 diabetes. Key Words: diabetes mellitus Ⅲ hyperinsulinism Ⅲ aorta Ⅲ endothelium-derived relaxing factor Ⅲ nitric oxide N umerous epidemiological studies have indicated that the insulin resistance and hyperinsulinemia associated with type 2 diabetes make important contributions to the development of hypertension and cardiovascular diseases, and impaired endothelium-dependent vasodilation has been described in humans and in animal models of the disease. 1,2 We and others have demonstrated that both aortic endothelial dysfunction and hypertension are present in type 2 spontaneously diabetic (db/db Ϫ/Ϫ ) mice and in fructose-fed insulinresistance mice. [3][4][5][6] Our recent observation that endothelial function and nitric oxide (NO) production are impaired in aortic strips from spontaneously type 2 diabetic GotoKakizaki rats seemed to conflict with our finding that the expressions of the mRNA and protein for endothelial NO synthase (eNOS) were increased in such aortas. 7

Matsumoto, Takayuki, Tsuneo Kobayashi, and Katsuo Kamata. Alterations in EDHF-type relaxation and phosphodiesterase activity in mesenteric arteries from diabetic rats. Am J Physiol Heart Circ Physiol 285: H283-H291, 2003; 10.1152/ajpheart.00954.2002.-In isolated superior mesenteric artery rings from age-matched control rats and streptozotocin (STZ)-induced diabetic rats, we investigated the role of cAMP in endothelium-derived hyperpolarizing factor (EDHF)-type relaxation. The ACh-induced EDHFtype relaxation was significantly weaker in STZ-induced diabetic rats than in control rats, and in both groups of rats it was attenuated by 18␣-glycyrrhetinic acid (18␣-GA), an inhibitor of gap junctions, and enhanced by IBMX, a cAMP-phosphodiesterase (PDE) inhibitor. These enhanced EDHF-type responses were very similar in magnitude between diabetic and age-matched control rats. The EDHF-type relaxation was enhanced by cilostamide, a PDE3-selective inhibitor, but not by Ro 20-1724, a PDE4-selective inhibitor. The expression levels of the mRNAs and proteins for two cAMP PDEs (PDE3A, PDE3B) were significantly increased in STZ-induced diabetic rats, but those for PDE4D were not. We conclude that the impairment of EDHF-type relaxations in STZ-induced diabetic rats may be attributed to a reduction in the action of cAMP via increased PDE activity. endothelium-derived hyperpolarizing factor; adenosine 3Ј,5Ј-cyclic monophosphate; streptozotocin THE ENDOTHELIUM PLAYS a major role in the regulation of vascular tone. It is capable of exerting a profound relaxing influence on the underlying smooth muscle, an effect mediated by at least three different factors, depending on the vascular bed. These factors include nitric oxide (NO) and prostacyclin, both diffusible factors (24,31,47,52,57). In addition, after blockade of NO and prostacyclin synthesis, stimulation of the endothelium is capable of evoking a vascular smooth muscle relaxation that has been attributed to a third factor, endothelium-derived hyperpolarizing factor (EDHF) (8,21,25,67).Although the identity of EDHF is still controversial (46), there is evidence that EDHF-type relaxations involve the transfer of a mediator from the endothelium to the smooth muscle via myoendothelial gap junctions (6,20,59). Indeed, EDHF-type responses are attenuated by connexin-mimetic peptides (6,17,30) and by 18␣-and 18␤-glycyrrhetinic acids (GA), aglycones that disrupt gap junction plaques at points of cell-to-cell contact (12,66,69).It was recently reported that cAMP facilitates EDHF-type relaxation in conduit arteries by enhancing electrotonic conduction via gap junctions (26). In fact, EDHF-type relaxations are potentiated when cAMP hydrolysis is inhibited by the phosphodiesterase (PDE) inhibitor IBMX but remain susceptible to a combination of apamin plus charybdotoxin (65). In accord with such a role for gap junctions, the EDHFtype relaxations and cAMP accumulation evoked by ACh are inhibited by synthetic connexin-mimetic peptides, which interrupt intercellular communication in a connexin-s...

We previously reported that in mesenteric arteries from aged Otsuka Long-Evans Tokushima fatty (OLETF) rats (a type 2 diabetes model) endothelium-derived hyperpolarizing factor (EDHF)-type relaxation is impaired while endothelium-derived contracting factor (EDCF)-mediated contraction is enhanced (Matsumoto T, Kakami M, Noguchi E, Kobayashi T, Kamata K. Am J Physiol Heart Circ Physiol 293: H1480-H1490, 2007). Here we investigated whether acute and/or chronic treatment with metformin might improve this imbalance between the effects of the above endothelium-derived factors in mesenteric arteries isolated from OLETF rats. In acute studies on OLETF mesenteric arteries, ACh-induced relaxation was impaired and the relaxation became weaker at high ACh concentrations. Both metformin and 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside [AICAR, an AMP-activated protein kinase (AMPK) activator that is also activated by metformin] 1) diminished the tendency for the relaxation to reverse at high ACh concentrations and 2) suppressed both ACh-induced EDCF-mediated contraction and ACh-stimulated production of prostanoids (thromboxane A2 and PGE2). In studies on OLETF arteries from chronically treated animals, metformin treatment (300 mg.kg(-1).day(-1) for 4 wk) 1) improved ACh-induced nitric oxide- or EDHF-mediated relaxation and cyclooxygenase (COX)-mediated contraction, 2) reduced EDCF-mediated contraction, 3) suppressed production of prostanoids, and 4) reduced superoxide generation. Metformin did not alter the protein expressions of endothelial nitric oxide synthase (eNOS), phospho-eNOS (Ser1177), or COX-1, but it increased COX-2 protein. These results suggest that metformin improves endothelial functions in OLETF mesenteric arteries by suppressing vasoconstrictor prostanoids and by reducing oxidative stress. Our data suggest that within the timescale studied here, metformin improves endothelial function through this direct mechanism, rather than by improving metabolic abnormalities.

We investigated whether the balance between endothelium-derived relaxing factors (EDRFs) and endothelium-derived contracting factors (EDCFs) might be altered in mesenteric arteries from aged Otsuka Long-Evans Tokushima Fatty (OLETF) rats (a Type 2 diabetic model) [vs. age-matched control Long-Evans Tokushima Otsuka (LETO) rats]. ACh-induced relaxation was impaired in the OLETF group, and a tendency for the relaxation to reverse at high ACh concentrations was observed in both groups. This tendency was abolished by indomethacin. Nitric oxide- and/or endothelium-derived hypolarizing factor-mediated relaxation and the protein expressions of phospho-endothelial nitric oxide synthase (Ser1177) and extracellular superoxide dismutase were also reduced in OLETF. An ACh-induced contraction was observed at higher ACh concentrations in the presence of N(G)-nitro-L-arginine (L-NNA) but was greater in OLETF rats. This contraction in OLETF rats was reduced by cyclooxygenase (COX) inhibitors and by prostanoid-receptor antagonists. The ACh-induced productions of thromboxane A(2) and PGE(2) were greater in OLETF than LETO rats, as were the mesenteric artery COX-1 and COX-2 protein expressions. Moreover, tert-butyl hydroperoxide (t-BOOH) (membrane-permeant oxidant) induced a concentration-dependent contraction that was greater in OLETF rats. The t-BOOH-mediated contraction was increased both by L-NNA and by endothelium removal in LETO but not OLETF rats, suggesting that a negative modulatory role of the endothelium was lost in OLETF rats. These results suggest that an imbalance between EDRFs and EDCFs may be implicated in the endothelial dysfunction seen in aged OLETF mesenteric arteries, and may be attributable to increased oxidative stress.

Insulin resistance is often associated with atherosclerotic diseases in subjects with obesity and impaired glucose tolerance. This study examined the effects of insulin resistance on coronary risk factors in IRS-1 deficient mice, a nonobese animal model of insulin resistance. Blood pressure and plasma triglyceride levels were significantly higher in IRS-1 deficient mice than in normal mice. Impaired endotheliumdependent vascular relaxation was also observed in IRS-1 deficient mice. Furthermore, lipoprotein lipase activity was lower than in normal mice, suggesting impaired lipolysis to be involved in the increase in plasma triglyceride levels under insulin-resistant conditions. Thus, insulin resistance plays an important role in the clustering of coronary risk factors which may accelerate the progression of atherosclerosis in subjects with insulin resistance.