The effect of the hepatic nerves on the disposition of a mixed meal by conscious dogs

Impaired hepatic counterregulatory response to insulin-induced hypoglycemia in hepatic denervated pigs

•

Hepatic denervation results in a blunted counterregulatory response during insulin-induced hypoglycemia.

•

Fasting glucose concentration, glucose production and uptake are unaffected by hepatic denervation.

•

Insulin action and extrahepatic glucose uptake are unaffected by hepatic denervation.

Objective

The liver reacts to hypoglycemia by increasing its glucose output. This response is assumed to depend both on glucose sensing at the liver and the brain, as well as efferent impulses from the brain to the liver. We tested the importance of this signaling pathway by studying the hepatic response to insulin-induced hypoglycemia in hepatic complete denervated pigs.

Materials/methods

Two weeks prior to the metabolic study, 36-kg pigs underwent either total hepatic denervation (DN; n = 12) or sham operation (sham; n = 12). On the metabolic study day, measurements were performed at baseline conditions and during a hypoglycemic hyperinsulinemic (5 mU/kg/min) clamp. Endogenous insulin and glucagon secretions were inhibited by somatostatin, and glucagon was replaced at baseline levels. Endogenous glucose production (EGP) and glucose utilization (Rd) were determined by [3-³H] glucose infusion.

Results

Baseline plasma concentrations of glucose, insulin, EGP and Rd did not differ significantly between the two groups of animals. During insulin infusion, the plasma glucose concentration was clamped at ~3 mmol/L in both groups of animals resulting in an increase in plasma concentrations of epinephrine and norepinephrine in sham pigs (both P < 0.05), while this effect was abolished in DN pigs. While insulin action (P = 0.09) and glucose utilization (P = 0.44) were similar, EGP was markedly decreased in the DN pigs (P < 0.05).

Conclusion

The findings indicate a blunted hepatic counterregulatory response to hypoglycemia following complete hepatic denervation. This implies that intact neural impulses to and from the liver are necessary to maintain the increase in EGP that protects the organism against hypoglycemia.

Interaction between the central and peripheral effects of insulin in controlling hepatic glucose metabolism in the conscious dog

The importance of hypothalamic insulin action to the regulation of hepatic glucose metabolism in the presence of a normal liver/brain insulin ratio (3:1) is unknown. Thus, we assessed the role of central insulin action in the response of the liver to normal physiologic hyperinsulinemia over 4 h. Using a pancreatic clamp, hepatic portal vein insulin delivery was increased three- or eightfold in the conscious dog. Insulin action was studied in the presence or absence of intracerebroventricularly mediated blockade of hypothalamic insulin action. Euglycemia was maintained, and glucagon was clamped at basal. Both the molecular and metabolic aspects of insulin action were assessed. Blockade of hypothalamic insulin signaling did not alter the insulin-mediated suppression of hepatic gluconeogenic gene transcription but blunted the induction of glucokinase gene transcription and completely blocked the inhibition of glycogen synthase kinase-3β gene transcription. Thus, central and peripheral insulin action combined to control some, but not other, hepatic enzyme programs. Nevertheless, inhibition of hypothalamic insulin action did not alter the effects of the hormone on hepatic glucose flux (production or uptake). These data indicate that brain insulin action is not a determinant of the rapid (<4 h) inhibition of hepatic glucose metabolism caused by normal physiologic hyperinsulinemia in this large animal model.

Evidence against a physiologic role for acute changes in CNS insulin action in the rapid regulation of hepatic glucose production

This Perspective will discuss the physiologic relevance of data that suggest CNS insulin action is required for the rapid suppression of hepatic glucose production. It will also review data from experiments on the conscious dog, which show that although the canine brain can sense insulin and, thereby, regulate hepatic glucoregulatory enzyme expression, CNS insulin action is not essential for the rapid suppression of glucose production caused by the hormone. Insulin's direct hepatic effects are dominant, thus it appears that insulin's central effects are redundant in the acute regulation of hepatic glucose metabolism.

Intraportal administration of neuropeptide Y and hepatic glucose metabolism

We examined whether intraportal delivery of neuropeptide Y (NPY) affects glucose metabolism in 42-h-fasted conscious dogs using arteriovenous difference methodology. The experimental period was divided into three subperiods (P1, P2, and P3). During all subperiods, the dogs received infusions of somatostatin, intraportal insulin (threefold basal), intraportal glucagon (basal), and peripheral intravenous glucose to increase the hepatic glucose load twofold basal. Following P1, in the NPY group (n = 7), NPY was infused intraportally at 0.2 and 5.1 pmol.kg(-1).min(-1) during P2 and P3, respectively. The control group (n = 7) received intraportal saline infusion without NPY. There were no significant changes in hepatic blood flow in NPY vs. control. The lower infusion rate of NPY (P2) did not enhance net hepatic glucose uptake. During P3, the increment in net hepatic glucose uptake (compared with P1) was 4 +/- 1 and 10 +/- 2 micromol.kg(-1).min(-1) in control and NPY, respectively (P < 0.05). The increment in net hepatic fractional glucose extraction during P3 was 0.015 +/- 0.005 and 0.039 +/- 0.008 in control and NPY, respectively (P < 0.05). Net hepatic carbon retention was enhanced in NPY vs. control (22 +/- 2 vs. 14 +/- 2 micromol.kg(-1).min(-1), P < 0.05). There were no significant differences between groups in the total glucose infusion rate. Thus, intraportal NPY stimulates net hepatic glucose uptake without significantly altering whole body glucose disposal in dogs.

Effect of hepatic denervation on peripheral insulin sensitivity in conscious dogs

We tested the hypothesis that the loss of hepatic nerves decreases peripheral insulin sensitivity. Surgical hepatic denervation (DN) was performed in 22 dogs approximately 16 days before study; 7 dogs (Sham-Sal) had a sham procedure. A euglycemic hyperinsulinemic (1 mU x kg(-1) x min(-1); arterial insulin 35 +/- 1 microU/ml in all dogs) clamp was performed in conscious dogs. From 0 to 90 min of the clamp, all dogs received the same treatment; then the DN dogs were divided into three groups. From 90 to 180 min, DN-PeA (n = 7) and DN-PoA (n = 7) groups received acetylcholine 2.5 microg x kg(-1) x min(-1) via peripheral or portal vein, respectively, and DN-Sal (n = 8) received no acetylcholine. During 150-180 min, the Sham-Sal, DN-Sal, DN-PeA, and DN-PoA groups exhibited glucose infusion rates of 12.4 +/- 0.8, 9.3 +/- 0.8 (P < 0.05 vs. Sham-Sal), 9.1 +/- 0.1 (P < 0.05 vs. Sham-Sal), and 12.7 +/- 1.6 mg. kg(-1) x min(-1); nonhepatic glucose uptakes of 11.5 +/- 0.9, 8.9 +/- 0.7 (P < 0.05 vs. Sham-Sal), 8.6 +/- 0.9 (P < 0.05 vs. Sham-Sal), and 11.9 +/- 1.7 mg. kg(-1). min(-1); net hindlimb glucose uptakes of 18.4 +/- 2.1, 13.7 +/- 1.1 (P < 0.05 vs. Sham-Sal), 17.5 +/- 1.9, and 16.7 +/- 3.2 mg/min; and glucose utilization rates of 14.4 +/- 1.4, 10.4 +/- 0.8 (P < 0.05 vs. Sham-Sal), 9.8 +/- 0.9 (P < 0.05 vs. Sham-Sal), and 13.6 +/- 1.8 mg. kg(-1) x min(-1), respectively. DN caused peripheral insulin resistance, and intraportal but not peripheral acetylcholine restored insulin sensitivity.