Activation of glycogenolysis by stimulation of the hepatic nerves in perfused livers of guinea pig and tree shrew as compared to rat: differences in the mode of action

Role of the sympathetic nervous system in cardiometabolic control: implications for targeted multiorgan neuromodulation approaches

Sympathetic overdrive plays a key role in the perturbation of cardiometabolic homeostasis. Diet-induced and exercise-induced weight loss remains a key strategy to combat metabolic disorders, but is often difficult to achieve. Current pharmacological approaches result in variable responses in different patient cohorts and long-term efficacy may be limited by medication intolerance and nonadherence. A clinical need exists for complementary therapies to curb the burden of cardiometabolic diseases. One such approach may include interventional sympathetic neuromodulation of organs relevant to cardiometabolic control. The experience from catheter-based renal denervation studies clearly demonstrates the feasibility, safety and efficacy of such an approach. In analogy, denervation of the common hepatic artery is now feasible in humans and may prove to be similarly useful in modulating sympathetic overdrive directed towards the liver, pancreas and duodenum. Such a targeted multiorgan neuromodulation strategy may beneficially influence multiple aspects of the cardiometabolic disease continuum offering a holistic approach.

Hepatic nervous system and neurobiology of the liver

The liver has a nervous system containing both afferent and efferent neurons that are involved in a number of processes. The afferent arm includes the sensation of lipids, glucose, and metabolites (after eating and drinking) and triggers the nervous system to make appropriate physiological changes. The efferent arm is essential for metabolic regulation, modulation of fibrosis and biliary function and the control of a number of other processes. Experimental models have helped us to establish how: (i) the liver is innervated by the autonomic nervous system; and (ii) the cell types that are involved in these processes. Thus, the liver acts as both a sensor and effector that is influenced by neurological signals and ablation. Understanding these processes hold significant implications in disease processes such as diabetes and obesity, which are influenced by appetite and hormonal signals.

The role of the autonomic nervous liver innervation in the control of energy metabolism

Despite a longstanding research interest ever since the early work by Claude Bernard, the functional significance of autonomic liver innervation, either sympathetic or parasympathetic, is still ill defined. This scarcity of information not only holds for the brain control of hepatic metabolism, but also for the metabolic sensing function of the liver and the way in which this metabolic information from the liver affects the brain. Clinical information from the bedside suggests that successful human liver transplantation (implying a complete autonomic liver denervation) causes no life threatening metabolic derangements, at least in the absence of severe metabolic challenges such as hypoglycemia. However, from the benchside, data are accumulating that interference with the neuronal brain-liver connection does cause pronounced changes in liver metabolism. This review provides an extensive overview on how metabolic information is sensed by the liver, and how this information is processed via neuronal pathways to the brain. With this information the brain controls liver metabolism and that of other organs and tissues. We will pay special attention to the hypothalamic pathways involved in these liver-brain-liver circuits. At this stage, we still do not know the final destination and processing of the metabolic information that is transferred from the liver to the brain. On the other hand, in recent years, there has been a considerable increase in the understanding which brain areas are involved in the control of liver metabolism via its autonomic innervation. However, in view of the ever rising prevalence of type 2 diabetes, this potentially highly relevant knowledge is still by far too limited. Thus the autonomic innervation of the liver and its role in the control of metabolism needs our continued and devoted attention.

Control of hepatocyte metabolism by sympathetic and parasympathetic hepatic nerves

More than any other organ, the liver contributes to maintaining metabolic equilibrium of the body, most importantly of glucose homeostasis. It can store or release large quantities of glucose according to changing demands. This homeostasis is controlled by circulating hormones and direct innervation of the liver by autonomous hepatic nerves. Sympathetic hepatic nerves can increase hepatic glucose output; they appear, however, to contribute little to the stimulation of hepatic glucose output under physiological conditions. Parasympathetic hepatic nerves potentiate the insulin-dependent hepatic glucose extraction when a portal glucose sensor detects prandial glucose delivery from the gut. In addition, they might coordinate the hepatic and extrahepatic glucose utilization to prevent hypoglycemia and, at the same time, warrant efficient disposal of excess glucose.

Signal propagation via gap junctions, a key step in the regulation of liver metabolism by the sympathetic hepatic nerves

Cell-to-cell communication via gap junctions has been proposed to be involved in the metabolic actions of sympathetic liver nerves in the rat. The effects of hepatic nerve stimulation and noradrenaline-, PGF2 alpha- and glucagon infusion on glucose metabolism and perfusion flow were studied in perfused rat liver in the absence and presence of the gap junctional inhibitors, heptanol, carbenoxolone and (4 beta)phorbol 12-myristate 13-acetate (4 beta PMA). (i) Stimulation of the hepatic nerve plexus increased glucose output, decreased flow and caused an overflow of noradrenaline into the hepatic vein. (ii) Heptanol completely inhibited not only the nerve stimulation-dependent metabolic and hemodynamic alterations but also the noradrenaline overflow. Thus the heptanol-dependent inhibitions were caused primarily by a strong impairment of transmitter release. (iii) Carbenoxolone inhibited the effects of neurostimulation on glucose metabolism partially by about 50%, whereas it left perfusion flow and noradrenaline overflow essentially unaltered. (iv) 4 beta PMA reduced the nerve stimulation-dependent enhancement of glucose release by about 80% but the noradrenaline-dependent increase in glucose output only by about 30%; the increase in glucose release by PGF2 alpha and by glucagon remained essentially unaltered. 4 beta PMA reduced the nerve stimulation-dependent decrease in portal flow by about 35% but did not affect the noradrenaline-and PGF2 alpha-elicited alterations, nor did it alter noradrenaline overflow. The results allow the conclusion that gap junctional communication plays a major role in the regulation of hepatic carbohydrate metabolism by sympathetic liver nerves, but not by circulating noradrenaline, PGF2 alpha or glucagon.

Differential control of glycogenolysis and flow by arterial and portal acetylcholine in perfused rat liver

The effects of acetylcholine on glucose and lactate balance and on perfusion flow were studied in isolated rat livers perfused simultaneously via the hepatic artery (100 mmHg, 25-35% of flow) and the portal vein (10 mmHg, 75-65% of flow) with a Krebs-Henseleit bicarbonate buffer containing 5 mM-glucose, 2 mM-lactate and 0.2 mM-pyruvate. Arterial acetylcholine (10 microM sinusoidal concentration) caused an increase in glucose and lactate output and a slight decrease in arterial and portal flow. These effects were accompanied by an output of noradrenaline and adrenaline into the hepatic vein. Portal acetylcholine elicited only minor increases in glucose and lactate output, a slight decrease in portal flow and a small increase in arterial flow, and no noradrenaline and adrenaline release. The metabolic and haemodynamic effects of arterial acetylcholine and the output of noradrenaline and adrenaline were strongly inhibited by the muscarinic antagonist atropine (10 microM). The acetylcholine-dependent alterations of metabolism and the output of noradrenaline were not influenced by the alpha 1-blocker prazosin (5 microM), whereas the output of adrenaline was increased. The acetylcholine-dependent metabolic alterations were not inhibited by the beta 2-antagonist butoxamine (10 microM), although the overflow of noradrenaline was nearly completely blocked and the output of adrenaline was slightly decreased. These results allow the conclusion that arterial, but not portal, acetylcholine caused sympathomimetic metabolic effects, without noradrenaline or adrenaline being involved in signal transduction.