Effects of chronic administration of L-arginine on vasoactive responses induced by endothelin-1 and its plasma level in streptozotocin-induced diabetic rats

Metabolic effects of L-citrulline in type 2 diabetes

The prevalence of type 2 diabetes (T2D) is increasing worldwide. Decreased nitric oxide (NO) bioavailability is involved in the pathophysiology of T2D and its complications. L-citrulline (Cit), a precursor of NO production, has been suggested as a novel therapeutic agent for T2D. Available data from human and animal studies indicate that Cit supplementation in T2D increases circulating levels of Cit and L-arginine while decreasing circulating glucose and free fatty acids and improving dyslipidemia. The underlying mechanisms for these beneficial effects of Cit include increased insulin secretion from the pancreatic β cells, increased glucose uptake by the skeletal muscle, as well as increased lipolysis and β-oxidation, and decreased glyceroneogenesis in the adipose tissue. Thus, Cit has antihyperglycemic, antidyslipidemic, and antioxidant effects and has the potential to be used as a new therapeutic agent in the management of T2D. This review summarizes available literature from human and animal studies to explore the effects of Cit on metabolic parameters in T2D. It also discusses the possible mechanisms underlying Cit-induced improved metabolic parameters in T2D.

Impairment of α1-adrenoceptor-mediated contractile activity in caudal arterial smooth muscle from type 2 diabetic Goto-Kakizaki rats

1. In the present study, we compared the responsiveness of de-endothelialized caudal artery smooth muscle strips, isolated from Type 2 diabetic Goto-Kakizaki (GK) and normal Wistar rats, to alpha(1)-adrenoceptor stimulation (cirazoline) and membrane depolarization (K(+)). 2. The contractile and myosin 20 kDa light chain (LC(20)) phosphorylation responses to 0.3 micromol/L cirazoline of caudal artery strips isolated from 12-week-old GK rats were significantly reduced compared with those of age-matched Wistar rats, whereas the contractile and LC(20) phosphorylation responses to 60 mmol/L K(+) were unaltered. 3. Stimulation of fura 2-AM-loaded strips from GK rats with 0.3 micromol/L cirazoline induced a significantly smaller rise in [Ca(2+)](i) (by approximately 20%) compared with that in strips from Wistar rats, whereas comparable Ca(2+) transients were evoked by K(+) in both. 4. Using quantitative polymerase chain reaction, no significant differences were detected in the mRNA expression of alpha(1A)-, alpha(1B)- and alpha(1D)-adrenoceptor subtypes between GK and Wistar rats. 5. Cirazoline (1 micromol/L)- and caffeine (20 mmol/L)-induced contractions in the absence of extracellular Ca(2+) were unaltered in GK rats, suggesting that the release of Ca(2+) from the sarcoplasmic reticulum in response to cirazoline does not differ between GK and Wistar rats. 6. The results of the present study suggest that Ca(2+) entry from the extracellular space via alpha(1)-adrenoceptor-activated, Ca(2+)-permeable channels, but not via membrane depolarization and voltage-gated L-type Ca(2+) channels, is impaired in caudal artery smooth muscle of GK rats.

Effect of chronic and selective endothelin receptor antagonism on microvascular function in type 2 diabetes

Vascular dysfunction, which presents either as an increased response to vasoconstrictors or an impaired relaxation to dilator agents, results in worsened cardiovascular outcomes in diabetes. We have established that the mesenteric circulation in Type 2 diabetes is hyperreactive to the potent vasoconstrictor endothelin-1 (ET-1) and displays increased nitric oxide-dependent vasodilation. The current study examined the individual and/or the relative roles of the ET receptors governing vascular function in the Goto-Kakizaki rat, a mildly hyperglycemic, normotensive, and nonobese model of Type 2 diabetes. Diabetic and control rats received an antagonist to either the ET type A (ETA; atrasentan; 5 mg x kg(-1) x day(-1)) or type B (ET(B); A-192621; 15 or 30 mg x kg(-1) x day(-1)) receptors for 4 wk. Third-order mesenteric arteries were isolated, and vascular function was assessed with a wire myograph. Maximum response to ET-1 was increased in diabetes and attenuated by ETA antagonism. ETB blockade with 15 mg/kg A-192621 augmented vasoconstriction in controls, whereas it had no further effect on ET-1 hyperreactivity in diabetes. The higher dose of A-192621 showed an ETA-like effect and decreased vasoconstriction in diabetes. Maximum relaxation to acetylcholine (ACh) was similar across groups and treatments. ETB antagonism at either dose had no effect on vasorelaxation in control rats, whereas in diabetes the dose-response curve to ACh was shifted to the right, indicating a decreased relaxation at 15 mg/kg A-192621. These results suggest that ETA receptor blockade attenuates vascular dysfunction and that ETB receptor antagonism exhibits differential effects depending on the dose of the antagonists and the disease state.

Effects of anthocyanidin derivative (HK-008) on relaxation in rat perfused mesenterial bed

Anthocyanins, which are responsible for a variety of bright colors (including red, blue, and purple) in fruits, vegetables, and flowers, are consumed as dietary polyphenols. Anthocyanin-containing fruits are thought to decrease coronary heart disease and are used in anti-diabetic preparations. Diabetes is associated with a variety of cardiovascular complications that may be mediated by endothelial dysfunction, and so this study was designed mainly to characterize the influence of a synthesized anthocyanidin derivative (HK-008) over acetylcholine (ACh)-induced relaxation in mesenteric arterial beds isolated from rats. In a glucose-tolerance test in intact rats, HK-008 (30 mg/kg) reduced the glucose level as effectively as the same dose of glibenclamide. The aortic relaxation induced by pinacidil (an ATP-sensitive potassium channel opener) was greatly inhibited by glibenclamide (10 microM), and also significantly inhibited by HK-008 (10 microM). Interestingly, the ACh-induced relaxation in the perfused, preconstricted mesenteric arterial bed was significantly enhanced by HK-008 (10 microM), and this enhancement was significantly attenuated by indomethacin (10 microM). The ACh-induced mesenteric relaxation was impaired by an increase in oxidative stress, viz. superoxide-generating treatment [xanthine oxidase (XO; 0.1 U/ml) plus hypoxanthine (HX; 10 microM)]. However, this impairment was strongly suppressed by HK-008 (10 microM). These results suggest that HK-008 increases endothelium-induced relaxation by suppressing oxidative stress or modulating prostanoids signaling. This compound may therefore be useful against certain cardiovascular disorders.

Altered arachidonic acid-mediated responses in the perfused kidney of the streptozotocin-induced diabetic rat

Using perfused kidneys isolated from age-matched controls and streptozotocin (STZ)-induced diabetic rats, we investigated the effects of arachidonic acid (AA) on perfusion pressure in the presence of methoxamine. AA elicited a transient contraction followed by a sustained relaxation in each group. The amplitude of contraction was smaller in the diabetic group than in the control group, whereas the amplitude of the sustained relaxation was greater in the former than in the latter group. In the diabetic group, the AA-induced sustained relaxation was completely inhibited by indomethacin [cyclooxygenase (COX) inhibitor], SKF525A [cytochrome P450 (CYP450) inhibitor], or clotrimazole (epoxygenase inhibitor), but not by furegrelate [thromboxane A(2) (TXA(2))-synthase inhibitor], SQ29548 (TXA(2)-receptor antagonist), or baicalein [lipoxygenase (LOX) inhibitor]. In the diabetic kidney, more-or-less additive inhibitions of the AA-induced relaxation were seen when indomethacin was given with either SKF525A or clotrimazole. These results suggest that in the STZ-induced diabetic perfused kidney, vasorelaxant metabolites derived from AA (probably COX and/or CYP450 metabolites) are increased, and may serve to regulate vascular tone.

Diabetes-related changes in contractile responses of stomach fundus to endothelin-1 in streptozotocin-induced diabetic rats

The contractile response of the stomach fundus to endothelin-1 (ET-1) was examined in streptozotocin (STZ)-induced diabetic rats. In STZ-diabetic rats (versus age-matched control rats) (a) ET-1 caused a longer-lasting contraction of stomach fundus strips, and (b) in the dose-response curve, the ET-1-induced contraction was significantly greater for a given concentration (3 x 10(-7) to 10(-7) M). Although repeated application of ET-1 led to desensitization, the desensitization was less pronounced in STZ-diabetic rats than in the controls. The density of the binding sites for [(125)I]-ET-1 was increased in the diabetic stomach fundus (versus the controls), but Kd values were similar between the two groups. The ET(B) receptor mRNA expression level was significantly increased in the diabetic stomach fundus. These results suggest that the diabetes-related enhancement of the ET-1-induced contraction of the stomach fundus may be due to an increase in the ET(B) receptor population.

The PI3-K/Akt pathway: roles related to alterations in vasomotor responses in diabetic models

Macro- and microvascular disease states currently represent the principal causes of morbidity and mortality in patients with type I or type II diabetes mellitus. Abnormal vasomotor responses and impaired endothelium-dependent vasodilation have been demonstrated in various beds in different animal models of diabetes and in humans with type I or type II diabetes. Several mechanisms leading to endothelial dysfunction have been reported, including changes in substrate avail ability, impaired release of NO, and increased destruction of NO. The principal mediators of diabetes-associated endothelial dysfunction are (a) increases in oxidized low density lipoprotein, endothelin-1, angiotensin II, oxidative stress, and (b) decreases in the actions of insulin or growth factors in endothelial cells. An accumulating body of evidence indicates that abnormal regulation of the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway may be one of several factors contributing to vascular dysfunction in diabetes. The PI3-K pathway, which activates serine/threonine protein kinase Akt, enhances NO synthase phosphorylation and NO production. Several studies suggest that in diabetes the relative ineffectiveness of insulin and the hyperglycemia act together to reduce activity in the insulin-receptor substrates (IRS)/PI3-K/Akt pathway, resulting in impairments of both IRS/PI3-K/Akt-mediated endothelial function and NO production. This article summarizes the PI3-K/Akt pathway-mediated contraction and relaxation responses induced by various agents in the blood vessels of diabetic animals.