New ATP-Sensitive Potassium Channel Opener, Iptakalim, Protects Against Insulin Resistance And Its Mechanism

Cumulative clinical and experimental evidence suggests insulin resistance has been proposed as a possible metabolic link among hypertension, obesity, dyslipidemia, and atherosclerosis cardiovascular disease. Humans with a genetic predisposition to hypertension tend to develop insulin resistance and hyperinsulinemia. Therefore, in addition to hypotensive effects, one has to take into account the effects of antihypertensive agents on insulin sensitivity. However, current first-line agents, such asβ-adrenergic receptor blockers and diuretics, have been reported to impair insulin sensitivity. Calcium channel blockers are well-known, highly effectious drugs for lowing blood pressure, but have neutral effect on insulin sensitivity. Thus, it is very important to find a promising drug to simultaneously improve insulin sensitivity and lower blood pressure.Endothelial dysfunction has been reported that it plays an important role in the development of hypertension. On the other hand, great progress has been made in understanding nonclassical cardiovascular actions of insulin and the importance of the vascular endothelium. In healthy conditions, the vascular action of insulin in stimulating the production of nitric oxide (NO) enhances capillary recruitment and vasodilation. This leads to increased blood flow to the skeletal muscle, resulting in increased delivery of glucose and insulin to the metabolic targets of insulin action. These vasodilatory actions of insulin contribute significantly to insulin-mediated glucose uptake. However, in insulin-resistant conditions, impairment of the shared insulin-signaling pathways in metabolic and cardiovascular tissues contributes to a reciprocal relationship between insulin resistance and endothelial dysfunction, which underlies the close association between metabolic and cardiovascular diseases. ATP-sensitive potassium channels (K_ATP channels) are present in the endothelial cells of the vascular system. The activation of endothelial K_ATP channels plays an important role in the process of endothelial dysfunction by activating of eNOS and inhibiting the release and synthesis of ET-1. The endothelium K_ATP channel is composed of SUR2B/Kir6.1. Iptakalim is a novel ATP-sensitive potassium channel opener with antihypertensive properties. SUR2B/Kir6.1 channels are activated by iptakalim. We have previously reported that iptakalim enhances the release of NO and inhibits ET-1 release and synthesis in cultured aortic endothelia cells. Iptakalim also inhibits the overexpression of adhesion molecules in aortic endothelial cells during metabolic disturbances induced by low-density lipoprotein, homocysteine, or hyperglycemia. Thus, the improvement of endothelial dysfunction by activating endothelial K_ATP channels could be an important strategy in the treatment of patients at high risk of cardiovascular diseases. Because iptakalim has antihypertensive and endothelial protective effects, we hypothesized that iptakalim has therapeutic effects on hypertension and endothelial dysfunction, thereby alleviating insulin resistance.In this study, we first investigated the effects of iptakalim on blood pressure, insulin resistance, and endothelial dysfunction in rats on a high-fructose diet. We also investigated the effects of iptakalim on endothelial dysfunction in cell model of insulin resistance. Finally, we investigated the effects of natakalim, a SUR2B/Kir6.1 subtype selective potassium opener, on blood pressure, insulin resistance, and endothelial dysfunction in rats on a high-fructose diet. The results of our study have important implications for the development of novel therapeutic strategies for hypertensive patients with insulin resistance.1. Effects of iptakalim on insulin resistance in fructose-fed rats1.1 Effects of iptakalim on SBP, insulin sensitivity and metabolic parameters in fructose-fed ratsThe animals maintained on the fructose diet exhibited modest but significant hypertension, hyperglycemia, hyperinsulinemia, and hypertriglyceridemia compared with the control group. Conversely, heart rates and plasma cholesterol levels did not differ between the control and fructose-fed animals. After the last 4 weeks of treatment, iptakalim (1, 3, or 9 mg/kg per day) effectively prevented the development of hypertension, hyperglycemia, hyperinsulinemia and increased the GIR and the insulin sensitive index in the fructose-fed rats. Treatment of the control group with iptakalim (9 mg/kg per day) did not alter any of these parameters and insulin sensitivity.1.2 Improvement of endothelial function by iptakalim in fructose-fed rats1.2.1 Increased this endothelium-dependent vascular relaxationThe acetylcholine-induced endothelium-dependent vascular relaxation of isolated aortic rings precontracted with norepinephrine derived from the fructose-fed rats decreased significantly. After the fructose-fed rats had been treated with iptakalim (1, 3, or 9 mg/kg per day) for 4 weeks, this endothelium-dependent vascular relaxation increased markedly.1.2.2 Effects of iptakalim on cardiovascular-active substances in fructose-fed ratsThe serum NO and 6-keto-PGF1αlevels of fructose-fed rats were significantly lower than those of the control rats. Iptakalim at 3 mg/kg per day increased the serum NO and 6-keto-PGF1αlevels significantly compared with those of model rats. The serum ET-1 and angiotensinâ…¡levels in the fructose-fed rats were significantly elevated compared with those of control rats. Iptakalim at 3 mg/kg per day significantly reduced the ET-1 level but had no effect on serum Angâ…¡level compared with that of model rats.1.2.3 Effects of iptakalim on adiponectin in fructose-fed ratsThe level of adiponectin was significantly reduced in fructose-fed rats compared with that in the control rats. Treatment with iptakalim (3 mg/kg per day) increased the serum adiponectin levels in the fructose-fed rats.2. Iptakalim protects against endothelial dysfunction in cell model of insulin resistance2.1 Enhanced the NO production and NOS production and increased eNOS protein expressionPretreatment of ECV304 with 10 nM-10μM iptakalim for 6 h increased NO and NOS levels, and increased eNOS protein expression in a concentration-dependent manner. The effects of iptakalim (0.1-10μM) on NO release were abolished by glibenclamide (10μM), an ATP-sensitive potassium channel blocker. 2.2 Inhibited the ET-1 secretion and increased the 6-Keto-PGF1αproductionIptakalim (10 nM-10μM) reduced ET-1 levels in a concentration-dependent manner in endothelial cells in cell model of insulin resistance. Iptakalim at 10μM increased the release of 6-Keto-PGF1α, whereas iptakalim at 10 nM-10μM had no effect on PAI-1 levels.2.3 Suppressed endothelium-monocytes adhesion in cell model of insulin resistancePretreatment of ECV304 with 10 nM-10μM iptakalim for 6 h reduced the adhesion of U937 cells to ECV304 in a concentration-dependent manner.3. Effects of iptakalim on glucose uptake in vitroWe investigated the effects of iptakalim on basal glucose metabolism. The incubation of HepG2 cells, 3T3-L1 adipocytes, or L6 myoblasts with iptakalim (10 nM-10μM) had no effect on basal glucose uptake.4. Effects of natakalim,selective SUR2B/Kir6.1 subtype ATP-sensitive potassium opener,on insulin resistance in fructose-fed rats4.1 Effects of natakalim on SBP, insulin sensitivity and metabolic parameters in fructose-fed ratsAfter the last 4 weeks of treatment, natakalim 3 mg/kg per day effectively prevented the development of hypertension, hyperglycemia, hyperinsulinemia, and increased the GIR and the insulin sensitive index in the fructose-fed rats.4.2 Improvement of endothelial function by natakalim in fructose-fed ratsNatakalim at 3 mg/kg per day significantly increased the serum NO, 6-keto-PGF1α, adiponectin levels and reduced the ET-1 level compared with those of model rats. Natakalim at 3 mg/kg per day had no effect on serum Angâ…¡level compared with that of model rats.Therefore, we come to the conclusion:1. We have confirmed for the first time in the fructose-fed rats that treatment with iptakalim lowers blood pressure, improves insulin sensitivity, decreases insulin levels, as well as improving endothelial function. 2. Iptakalim improves the endothelial dysfunction in animal model and cell model of insulin resistance by balancing endothelial-derived vasodilators (e.g., nitric oxide and 6-Keto-PGF1α) and vasoconstrictors (e.g., endothelin-1). This protective effect was significantly inhibited by glibenclamide, a K_ATP channel blocker, indicating that the protective role of iptakalim may be associated with the activation of K_ATP channels.3. The endothelium K_ATP channel is composed of SUR2B/Kir6.1. We use natakalim, a selective SUR2B/Kir6.1 opener, to further investigate the effect on insulin resistance through activation of this subtype. We also found natakalim lowers blood pressure, improves insulin sensitivity, decreases insulin levels, as well as improving endothelial function in the fructose-fed rats. Natakalim significantly increases the serum NO, 6-keto-PGF_1α, adiponectin levels and reduces the ET-1 level.4. Endothelial dysfunction plays an important role in the development of insulin resistance. Iptakalim and natakalim can activate the SUR2B/Kir6.1 subtype of K_ATP in endothelium. Iptakalim can increase the NO and PGI₂ production, inhibits the secretion of ET-1, and improves the endothelial dysfunction, which may be associated with the improvement of insulin resistance. Iptakalim has no direct effect on RAS system; its mechanism on improving insulin resistance is different from ACEI. Iptakalim has no effect on basal glucose uptake in HepG2 cells, 3T3-L1 adipocytes, or L6 myoblasts, it indicates its mechanism on improving insulin resistance is different from rosiglitazone.