Hepatic parasympathetic role in insulin resistance on an animal model of hypertension R.T. Ribeiro a , R.A. Afonso a,b , M. Paula Macedo a,c, 4 a Department of Physiology, Faculty of Medical Sciences, New University of Lisbon, Campo Ma ´rtires da Pa ´tria, 130, 1169-056 Lisbon, Portugal b Department of Biochemistry, Faculty of Medical Sciences, New University of Lisbon, Campo Ma ´rtires da Pa ´tria, 130, 1169-056 Lisbon, Portugal c Portuguese Diabetes Association, Rua do Salitre 118-120, 1250-203 Lisbon, Portugal Received 9 May 2006; accepted 12 September 2006 Abstract The hepatic insulin sensitizing substance (HISS) pathway, which includes the hepatic parasympathetic nerves and hepatic nitric oxide (HNO), has been shown to be crucial to the action of insulin on glucose metabolism. Insulin resistance in essential hypertension has been related to parasympathetic dysfunction; thus, we tested the hypothesis that the HISS pathway is impaired in spontaneously hypertensive rats (SHR) when compared with their normotensive controls, Wistar (WIS) and Wistar Kyoto (WKY) rats. A modified euglycemic clamp quantified insulin sensitivity. Differentiation of the HISS-dependent and HISS-independent components of insulin action was achieved by administration of a muscarinic receptor antagonist (atropine, 3 mg/kg) or of a nitric oxide synthase inhibitor (N g -methyl-arginine, 0.73 mg/ kg). Both SHR and WKY had lower postprandial total insulin action when compared with WIS (209.1 F 13.6 for WKY and 217.8 F 19.8 for SHR vs 296.1 F 16.9 mg glucose/kg body weight for WIS, P b .05). Furthermore, we observed that this is due to a decrease of the HISS- dependent component of insulin action (154.8 F 16.4 for WIS vs 87.1 F 14.5 for WKY and 55.9 F 15.6 mg glucose/kg body weight for SHR; P b .05 and P b .001, respectively; data concerning the atropine protocol). Blockade of HISS action by inhibition of hepatic nitric oxide synthase with N g -methyl-arginine showed similar results to those obtained with atropine, suggesting that they indeed act through the same pathway. In conclusion, our results support our hypothesis that impairment of the HISS pathway is responsible for the development of insulin resistance between WIS and SHR. D 2007 Elsevier Inc. All rights reserved. 1. Introduction A pathway that includes both the hepatic parasympa- thetic nerves (HPNs) and hepatic nitric oxide (HNO) has been shown to be crucial to the action of insulin on glucose metabolism. The activation of the HPNs during the immediate postprandial state leads to the release, by the liver, of a putative hormone referred to as hepatic insulin sensitizing substance (HISS), in a mechanism mediated by acetylcholine, hepatic glutathione, and HNO [1,2]. The HPN-HNO pathway determines the ability of insulin to cause the release of HISS. Hepatic insulin sensitizing substance then travels through the bloodstream to enhance skeletal muscle insulin-stimulated glucose uptake, conse- quently accounting for 50% to 60% of whole-body glucose uptake [2]. Hepatic insulin sensitizing substance action is highest in the immediate postprandial state and decreases with the duration of fasting [3]. By blockade of HISS release through hepatic parasympathetic denervation or pharmacologic manipulation, by muscarinic receptor blockade or nitric oxide (NO) synthase inhibitors, it is possible to distinguish the HISS-dependent and HISS-independent components of insulin action [1,4]. Impairment of the HISS pathway as the primary cause for the development of insulin resistance has been recently reported for several pathologies usually related to the metabolic syndrome [2,5], in which hypertension is included. Furthermore, insulin resistance in essential hyper- tension seems to be related to parasympathetic [6] and NO [7] dysfunction. This prompted us to study the spontaneously hyperten- sive rat (SHR), a genetic pathologic model widely used for 0026-0495/$ – see front matter D 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.metabol.2006.09.018 4 Corresponding author. Department of Physiology, Faculty of Medical Sciences, New University of Lisbon, Campo Ma ´rtires da Pa ´tria, 130, 1169- 056 Lisbon, Portugal. Tel.: +351 218803017; fax: +351 218803028. E-mail address: mpmacedo.biot@fcm.unl.pt (M.P. Macedo). Metabolism Clinical and Experimental 56 (2007) 227 – 233 www.elsevier.com/locate/metabol