Increased insulin sensitivity in the high sodium one-kidney, one figure-8 hypertensive rat

Mechanisms of insulin resistance in rat models of hypertension and their relationships with salt sensitivity

Several lines of evidence suggest that insulin resistance and the resultant hyperinsulinaemia are causally related to hypertension. Insulin actions are initiated by binding to a high-affinity transmembrane protein receptor which is present in all mammalian cells. These effects are predominant in skeletal muscle, liver, and fat and involve a number of tissue-specific and biochemically diverse events. Less well known are effects of insulin occurring in tissues not usually considered as insulin targets, which are hypothetical contributors to the pro-hypertensive action of the hormone. These effects include activation of renal sodium reabsorption, stimulation of the sympathetic nervous system, growth-promoting activity on vascular smooth muscle cells, and modulation of transmembrane cation transport. Epidemiological investigations have implicated sodium intake in the pathogenesis of hypertension. Because of the sodium-retaining effects of insulin, it has been postulated that insulin resistance with associated hyperinsulinaemia may be critical for the pathogenesis of salt-sensitivity in essential hypertensive subjects. Insulin resistance is present also in strains of rats with genetic hypertension that can be utilized as models to study the molecular mechanisms of this abnormality. In the present article, we summarize the current knowledge of the mechanisms of insulin resistance in rat models of arterial hypertension in which decreased sensitivity to insulin occurs and propose a rationale hypothesis that links insulin resistance with salt-sensitivity and hypertension.

Effect of nifedipine and enalapril on insulin-induced glucose disposal in spontaneous hypertensive and diabetic rats

Hypertension and diabetes mellitus are associated with hyperinsulinemia and insulin resistance. The present work was undertaken to study the effects of enalapril and nifedipine on insulin sensitivity in spontaneously hypertensive (SH) rats and diabetic rats. Insulin sensitivity was measured by insulin tolerance test using K(ITT) as an index of insulin mediated glucose metabolism. The time to produce 50% fall in initial blood sugar level (T1/2) was significantly higher in non-insulin dependent diabetes mellitus (NIDDM) and SH rats as compared to Wistar control. The mean K(ITT) values were significantly lower in NIDDM and SH rats as compared to Wistar control. Treatment with nifedipine (10 mg/kg) and enalapril (5 mg/kg) for 15 days produced a significant reduction in T1/2. Further, K(ITT) value was found to be significantly increased in SH rats treated with nifedipine or enalapril as compared to control. Our data indicate that NIDDM and SH rats are not only hyperinsulinemic but also insulin resistant. Nifedipine and enalapril treatment produced increase in insulin sensitivity in these animals.

Central nervous system nitric oxide synthase activity regulates insulin secretion and insulin action

Systemic inhibition of nitric oxide synthase (NOS) with NG-monomethyl-L-arginine (L-NMMA) causes acute insulin resistance (IR), but the mechanism is unknown. We tested whether L-NMMA-induced IR occurs via NOS blockade in the central nervous system (CNS). Six groups of Sprague-Dawley rats were studied after chronic implantation of an intracerebroventricular (ICV) catheter into the lateral ventricle and catheters into the carotid artery and jugular vein. Animals were studied after overnight food deprivation, awake, unrestrained, and unstressed; all ICV infusion of L-NMMA or D-NMMA (control) were performed with artificial cerebrospinal fluid. ICV administration of L-NMMA resulted in a 30% rise in the basal glucose level after 2 h, while ICV D-NMMA had no effect on glucose levels. Insulin, epinephrine, and norepinephrine levels were unchanged from baseline in both groups. Tracer (3H-3-glucose)-determined glucose disposal rates during 2 h euglycemic hyperinsulinemic (300 microU/ml) clamps performed after ICV administration of L-NMMA were reduced by 22% compared with D-NMMA. Insulin secretory responses to a hyperglycemic clamp and to a superimposed arginine bolus were reduced by 28% in L-NMMA-infused rats compared with D-NMMA. In conclusion, ICV administration of L-NMMA causes hyperglycemia via the induction of defects in insulin secretion and insulin action, thus recapitulating abnormalities observed in type 2 diabetes. The data suggest the novel concept that central NOS-dependent pathways may control peripheral insulin action and secretion. This control is not likely to be mediated via adrenergic mechanisms and could occur via nonadrenergic, noncholinergic nitrergic neural and/or endocrine pathways. These data support previously published data suggesting that CNS mechanisms may be involved in the pathogenesis of some forms of insulin resistance and type 2 diabetes independent of adiposity.

Short term effects of leptin on hepatic gluconeogenesis and in vivo insulin action

Long term administration of leptin decreases caloric intake and fat mass and improves glucose tolerance. Here we examine whether leptin acutely regulates peripheral and hepatic insulin action. Recombinant mouse leptin (0.3 mg/kg.h, Leptin +) or vehicle (Leptin -) were administered for 6 h to 4-month-old rats (n = 20), and insulin (3 milliunits/kg.min) clamp studies were performed. During physiologic hyperinsulinemia (plasma insulin approximately 65 microunits/ml), the rates of whole body glucose uptake, glycolysis, and glycogen synthesis and the rates of 2-deoxyglucose uptake in individual tissues were similar in Leptin - and Leptin +. Post-absorptive hepatic glucose production (HGP) was similar in the two groups. However, leptin enhanced insulin's inhibition of HGP (4.1 +/- 0.7 and 6.2 +/- 0.7 mg/kg.min; p < 0.05). The decreased HGP in the Leptin + group was due to a marked suppression of hepatic glycogenolysis (0.7 +/- 0.1 versus 4.1 +/- 0.6 mg/kg.min, in Leptin + versus Leptin -, respectively; p < 0.001), whereas the % contribution of gluconeogenesis to HGP was markedly increased (82 +/- 3% versus 36 +/- 4% in Leptin + and Leptin -, respectively; p < 0.001). At the end of the 6-h leptin infusion, the hepatic abundance of glucokinase mRNA was decreased, whereas that of phosphoenolpyruvate carboxykinase mRNA was increased compared with Leptin -. We conclude that an acute increase in plasma leptin 1) enhances insulin's ability to inhibit HGP, 2) does not affect peripheral insulin action, and 3) induces a redistribution of intrahepatic glucose fluxes and changes in the gene expression of hepatic enzymes that closely resemble those of fasting.

Abnormalities of insulin receptors in spontaneously hypertensive rats

Insulin resistance is present in some strains of rats with genetic hypertension. To determine whether this abnormality is present at the level of the insulin receptor, we compared insulin sensitivity, insulin receptor binding, and mRNA levels in tissues of 10-week-old spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto (WKY) controls. Because we have previously demonstrated an inverse relationship between dietary sodium intake and renal insulin receptor density and mRNA levels in normal Sprague-Dawley rats, the two rat strains in the current experiment were fed either low salt (0.07% NaCl) or high salt (7.5% NaCl) chow until the SHR became hypertensive. Fasting plasma glucose and plasma insulin levels did not differ between SHR and WKY and were not affected by salt intake. When the rats were maintained on the low salt diet, the rate of glucose infusion required to main euglycemia during a hyperinsulinemic clamp was significantly lower in SHR than WKY. High salt diet decreased the rate of glucose utilization during the hyperinsulinemic clamp in WKY but not SHR. During the low salt diet, insulin infusion decreased sodium excretion in both WKY and SHR. When the rats were maintained on the high salt diet, the antinatriuretic response to insulin was blunted in WKY but not SHR. Both the density and mRNA levels of insulin receptor were comparable in the kidney of WKY and SHR, but only WKY had the previously demonstrated decrease in receptor number and mRNA levels when fed the high salt chow. Hepatic insulin receptor mRNA levels were significantly lower in SHR than WKY fed the low salt diet. High salt diet decreased significantly insulin receptor mRNA levels in the liver of WKY but not of SHR. Thus, SHR appear to have lost the feedback mechanism that normally limits insulin-induced sodium retention when extracellular volume is expanded. A decreased expression of insulin receptor in the liver of SHR provides a possible explanation for the insulin resistance and decreased insulin clearance present in this strain.

Insulin resistance after hypertension induced by the nitric oxide synthesis inhibitor L-NMMA in rats

To explore the relationship between insulin resistance and hypertension, we examined whether acute induction of hypertension can engender insulin resistance. For this purpose we measured rates of insulin-mediated glucose uptake in awake unstressed rats with the euglycemic hyperinsulinemic (12 microns.kg-1.min-1) clamp technique during infusions of saline alone or after induction of hypertension by bolus administration of NG-monomethyl-L-arginine (L-NMMA, 30 and 15 mg/kg), a competitive inhibitor of nitric oxide synthase. Arterial pressure was approximately 20% greater with L-NMMA bolus than with saline alone. Isotopically determined steady-state rates of glucose uptake were 36 +/- 1 mg.kg-1.min-1 during saline alone and 26 +/- 2 and 19 +/- 1 mg.kg-1.min-1 with low- and high-dose L-NMMA (P < 0.001 vs. saline), respectively. To rule out that insulin resistance induced by L-NMMA was adrenergically mediated, clamp studies were repeated with alpha- and beta-blockade. Rates of glucose uptake remained approximately 20% below those observed with saline alone (P < 0.001). A significant inverse correlation was observed between the height of the blood pressure and the rate of glucose uptake (r = 0.32, P = 0.04). In conclusion, acute induction of hypertension with L-NMMA can cause marked insulin resistance. We postulate that reduced skeletal muscle perfusion and/or sympathetic nervous system activation may contribute to insulin resistance induced by L-NMMA.