Studies from our laboratory using acute pharmacologic blockade of nitric oxide synthase (NOS) activity have suggested that nitric oxide (NO) has an important role in regulating carbohydrate metabolism. We now report on insulin sensitivity in mice with targeted disruptions in endothelial NOS (eNOS) and neuronal NOS (nNOS) genes compared with their wild-type (WT) counterparts. Mice underwent hyperinsulinemic-euglycemic clamp studies after a 24-h fast, during an insulin infusion of 20 mU · k g -1 · min -1 . Glucose levels were measured at baseline and every 10 min during the clamp. Insulin levels were measured at baseline and at the end of the clamp study. Glucose infusion rates (GIRs) during the last 30 min of the clamp study were in a steady state. Tritiated glucose infusion was used to measure rates of endogenous glucose output (EGO) both at baseline and during steady-state euglycemia. Glucose disposal rates (GDRs) were computed from the GIR and EGO. Fasting and steady-state glucose and insulin levels were comparable in the 3 groups of mice. No differences in fasting EGO were noted between the groups. GIR was significantly reduced (37%, P = 0.001) in the eNOS knockout (KO) mice compared with the WT mice, with values for the nNOS mice being intermediate. EGO was completely suppressed in the nNOS and WT mice during insulin infusion, but not in the eNOS mice. Even so, the eNOS mice displayed significantly reduced whole-body GDRs compared with those of the WT mice ( 8 2 . 6 7 ± 10.77 vs. 103.67 ± 3.47 m g · k g -1 · min -1 , P = 0.03). eNOS KO mice are insulin resistant at the level of the liver and peripheral tissues, whereas the nNOS KO mice are insulin resistant only in the latter. These data indicate that NO plays a role in modulating insulin sensitivity and carbohydrate metabolism and that the eNOS isoform may play a dominant role relative to nNOS. Diabetes 49:XXX-XXX, 2000 N itric oxide (NO) has emerged as an important molecule with diverse biological functions. In the blood vessels, NO mediates endotheliumdependent vasodilation (1-3) in response to diverse stimuli such as shear stress (4-6), insulin (7), acetylcholine (8,9), and bradykinin (3,10). In the central nervous system (CNS) and peripheral nervous tissue, NO is an unusual neurotransmitter (11-13). NO is generated when the amino acid L-arginine is converted to citrulline by the enzyme NO synthase (NOS) (14,15). Three separate genes encode the known isoforms of NOS (16): endothelial NOS (eNOS or NOS III) and neuronal NOS (nNOS or NOS II) catalyze the constitutive production of NO in a calcium-dependent manner predominantly in the blood vessels and neural tissues, respect i v e l y. The third isoform, inducible NOS (iNOS or NOS I) is located in macrophages and catalyzes NO formation in i n flammatory cells.Intravenous administration of N G -m o n o m e t h y l -L-a r g i n i n e (L-NMMA), a competitive inhibitor of all NOS isoforms, acutely induces hypertension and insulin resistance in rats ( 1 7 ) . M o r e r e c e n t l y, we reported that acute pharm...
Systemic inhibition of nitric oxide synthase (NOS) with N Gmonomethyl-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 SpragueDawley 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 ( 3 H-3-glucose)-determined glucose disposal rates during 2 h euglycemic hyperinsulinemic (300 U/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. ( J. Clin. Invest.
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