Sympathetic nervous system and insulin resistance: from obesity to diabetes.

As the world faces an obesity "epidemic," the mechanisms by which overweight is translated into insulin resistance, hypertension, and diabetes need to be better understood. Although the processes of transition remain uncertain, overactivity of the sympathetic nervous system appears pivotal. In obesity, there is stimulation of sympathetic outflow to the kidneys, evident in increased rates of spillover of noradrenaline into the renal veins, and to skeletal muscle vasculature, demonstrated with microneurography. The cause is unclear, but possibly involves the stimulatory action of leptin released from adipose tissue, or from within the brain, for which there is recent evidence in human obesity. The high renal sympathetic tone contributes to hypertension development by stimulating renin secretion and through promoting renal tubular reabsorption of sodium. Neurally mediated skeletal muscle vasoconstriction reduces glucose delivery and uptake in muscle. Impairment of glucose uptake by skeletal muscle is a hallmark of insulin resistance syndromes. Pharmacologic sympathetic nervous suppression within the central nervous system with imidazoline receptor-binding agents such as rilmenidine is a logical therapeutic approach for lowering blood pressure (BP) in patients with essential hypertension, in whom sympathetic activity is often increased. In addition, drugs of this class appear to have the capacity to favorably modify insulin sensitivity, which has particular relevance in the treatment of hypertensive diabetic patients. In the hypertension accompanying maturity onset obesity, with recent recommendations from advisory bodies setting lower goal BP, and with these lower targets often being reached only with combinations of antihypertensive agents, it is advisable that all drugs used in combination therapy have a favorable or at least a neutral effect on insulin resistance.