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.     

Modulation of the sympathetic nerve action on carbohydrate and ketone body metabolism by fatty acids, glucagon und insulin in perfused rat liver

Rat liver was perfused in situ via the portal vein without recirculation: 1) Nerve stimulation (20 Hz, 2 ms, 20 V) increased glucose output and shifted lactate uptake to output; the alterations were diminished by oleate but not octanoate. 2) Glucagon (1nM) stimulated glucose output maximally also in the presence of the fatty acids, so that nerve stimulation could not increase it further. The hormone also enhanced lactate uptake and nerve stimulation counteracted this effect. The counteraction was diminished by oleate but not octanoate. 3) Insulin (100nM) slightly lowered glucose output and had no effect on lactate balance. It antagonized the increase of glucose output by nerve stimulation, but left the shift of lactate uptake to release unaffected. These events were not influenced by the fatty acids. 4) Nerve stimulation decreased ketone body production from oleate and octanoate. 5) Glucagon increased ketogenesis from oleate, but not octanoate. In the presence of glucagon nerve stimulation also lowered ketogenesis. This decrease was diminished in the presence of oleate. 6) Insulin lowered ketogenesis from oleate but not octanoate. In the presence of insulin nerve stimulation decreased ketogenesis; the relative change was independent of the fatty acids. The complex interactions between fatty acids, glucagon and insulin in the modulation of sympathetic nerve actions can be summarized as follows: Oleate, which enters the mitochondria via the carnitine system, but not octanoate, which enters independently from this system, as well as insulin but not glucagon effectively modulated the nerve actions on carbohydrate metabolism. Glucagon but not insulin modulated the nerve effects on ketogenesis from oleate but not octanoate. The regulatory interactions between substrates, hormones and nerves can best be explained on the basis of the model of metabolic zonation.     

Sympathoadrenal system in neuroendocrine control of glucose: mechanisms involved in the liver, pancreas, and adrenal gland under hemorrhagic and hypoglycemic stress.

Glucose homeostasis is maintained by complex neuroendocrine control mechanisms, involving three peripheral organs: the liver, pancreas, and adrenal gland, all of which are under control of the autonomic nervous system. During the past decade, abundant results from various studies on neuroendocrine control of glucose have been accumulated. The principal objective of this review is to provide overviews of basic adrenergic mechanisms closely related to glucose control in the three peripheral organs, and then to discuss the integrated glucoregulatory mechanisms in hemorrhage-induced hypotension and insulin-induced hypoglycemia with special reference to sympathoadrenal control mechanisms. The liver is richly innervated by sympathetic and parasympathetic nerves. The functional implication in glucoregulation of sympathetic nerves has been well-documented, while that of parasympathetic nerves remains less understood. More recently, hepatic glucoreceptors have been postulated to be coupled with capsaicin-sensitive afferent nerves, conveying sensory signals of blood glucose concentration to the central nervous system. The pancreas is also richly supplied by the autonomic nervous system. Besides the well documented adrenergic and cholinergic mechanisms, the potential implication of peptidergic neurotransmission by neuropeptide Y and neuromodulation by galanin has recently been postulated in the endocrine secretory function. Presynaptic interactions of these putative peptidergic neurotransmitters with the classic transmitters, noradrenaline and acetylcholine, in the pancreas remain to be clarified. It may be of particular interest that it was vagus nerve stimulation that caused a dominant release of neuropeptide Y over that caused by sympathetic nerve stimulation in the pig pancreas. The adrenal medulla receives its main nerve supply from the greater and lesser splanchnic nerves. Adrenal medullary catecholamine secretion appears to be regulated by three distinct local mechanisms: adrenoceptor-mediated, dihydropyridine-sensitive Ca2+ channel-mediated, and capsaicin-sensitive sensory nerve-mediated mechanisms. In response to hemorrhagic hypotension and insulin-induced hypoglycemia, the sympathoadrenal system is activated resulting in increases of adrenal catecholamine and pancreatic glucagon secretions, both of which are significantly implicated in glucoregulatory mechanisms. An increase in sympathetic nerve activity occurs in the liver during hemorrhagic hypotension and is also likely to occur in the pancreas in response to insulin-induced hypoglycemia. The functional implication of hepatic and central glucoreceptors has been suggested in the increased secretion of glucose counterregulatory hormones, particularly catecholamines and glucagon.     

Inhibition of glucose production during hepatic nerve stimulation in regenerating rat liver perfused in situ. Possible involvement of gap junctions in the action of sympathetic nerves.

To explore the possible role of gap junctions in neural regulation of hepatic glucose metabolism, the effects of hepatic nerve stimulation on metabolic and hemodynamic changes were examined in normal and regenerating rat liver which was perfused in situ at constant pressure via the portal vein with a medium containing 5 mM glucose, 2 mM lactate and 0.2 mM pyruvate. 1. The content of connexin 32, a major component of gap junctions in rat liver, decreased transiently to about 25% of the control level in regenerating liver 48-72 h after partial hepatectomy and recovered to normal by the 11th day after the operation. 2. In normal liver, electrical stimulation of the hepatic nerves (10 Hz, 20 V, 2 ms) and infusion of noradrenaline (1 microM) both increased glucose and lactate output and reduced perfusion flow. 3. In early stage of regenerating liver 48 h and 72 h after partial hepatectomy, the increase in glucose output in response to nerve stimulation was almost completely inhibited, whereas the change in lactate balance was partially suppressed and the reduction of flow rate was retained. The response of glucose output to nerve stimulation recovered by the 11th day after partial hepatectomy. In contrast, exogenous application of noradrenaline increased glucose output even in the early stage of regenerating liver. 4. The increase in noradrenaline overflow during hepatic nerve stimulation in the early stage of regenerating liver was approximately the same as in normal liver. Liver glycogen was sufficiently preserved in the early stage of regenerating liver. However, noradrenaline infusion could no more increase glucose output both in normal and in regenerating livers after 24 h of fasting that depleted liver glycogen. These results suggest that the impaired effects of sympathetic nerve stimulation on glucose metabolism observed in regenerating liver are derived neither from reduced release of noradrenaline nor from depletion of liver glycogen, but rather from transient reduction of gap junctions which assist signal propagation of the nerve action through intercellular communication in rat liver.     

Involvement of the autonomic nervous system in the in vivo TRH-induced increases in the plasma levels of glucagon, insulin and glucose in rabbits

Thyrotropin-releasing hormone, TRH, increases the plasma levels of glucagon, insulin, glucose and free fatty acids in rabbits. However, TRH has no direct effects on the release of hormones neither from the endocrine pancreas in humans nor from the isolated perfused rat pancreas. The aim of the present study was to investigate if the effects of TRH in rabbits were mediated by the autonomic nervous system. The TRH "Roche"-induced hyperglucagonemia was inhibited by phentolamine (an alpha-receptor blocking drug), yohimbine (an alpha-2 -receptor blocking drug) and atropine. The TRH "Roche"-induced hyperinsulinemia was inhibited by propranolol (a beta-receptor blocking drug). The TRH "Roche"-induced hyperglycemia was inhibited by all four drugs. The TRH "Roche"-induced increases in the plasma levels of free fatty acids were not inhibited by the sympathetic and parasympathetic blocking drugs. The effects of TRH "Roche" on the plasma levels of glucagon, insulin and glucose cannot be explained by increases in the plasma levels of catecholamines. TRH, given intravenously into rabbits, may possibly act on regions in the central nervous system which control carbohydrate metabolism and the release of glucagon and insulin from the endocrine pancreas by sympathetic and parasympathetic mechanisms.     

Different accessibility from the artery and the portal vein of alpha- and beta-receptors involved in the sympathetic nerve action on glycogenolysis and hemodynamics in perfused rat liver

In perfused rat liver perivascular nerve stimulation (7.5 Hz, 20 V, 2 ms, 5 min) at the liver hilus caused an increase in glucose and lactate output and a decrease in flow. The influence of the alpha 1-receptor blocker prazosine and the beta-blocker propranolol on these nerve effects was studied in the isolated rat liver perfused classically via the portal vein only and, as developed recently, via both the hepatic artery and the portal vein. 1) In livers perfused via the portal vein only the nerve stimulation-dependent metabolic alterations were nearly completely inhibited by prazosine (5 microM), but not influenced by propranolol (10 microM). The hemodynamic changes were lowered to only 33% by prazosine and not altered by propranolol either. 2) In livers perfused via the hepatic artery (100 mm Hg, 20-40% of flow) and the portal vein (10 mm Hg, 80-60% of flow)--similar to portal perfusions--the nerve stimulation--dependent metabolic alterations were almost completely blocked by arterial, portal or simultaneously applied arterial and portal prazosine. However--in contrast to portal perfusions--the metabolic alterations were reduced to about 20% (glucose) and 50% (lactate) also by propranolol independently of its site of application. The decrease in flow was reduced by prazosine to about 60%, 50% and 30% when applied via the artery, the portal vein or via both vessels, respectively. The hemodynamic alterations were not influenced by propranolol. These results allow the following conclusions: A subpopulation of beta-receptors can play a permissive role in the alpha 1-receptor-mediated sympathetic nerve action on glucose and lactate metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)     

Potential role for prostaglandin F2 alpha, D2, E2 and thromboxane A2 in mediating the metabolic and hemodynamic actions of sympathetic nerves in perfused rat liver

In isolated rat liver perfused at constant pressure perivascular nerve stimulation caused an increase of glucose and lactate output and a reduction of perfusion flow. The metabolic and hemodynamic nerve effects could be inhibited by inhibitors of prostanoid synthesis, which led to the suggestion that the effects of nerve stimulation were, at least partially, mediated by prostanoids [Iwai, M. & Jungermann, K. (1987) FEBS Lett. 221, 155-160]. This suggestion is corroborated by the present study. 1. Prostaglandin D2, E2 and F2 alpha as well as the thromboxane A2 analogue U46619 enhanced glucose and lactate release and lowered perfusion flow similar to nerve stimulation. 2. The extents, the kinetics and the concentration dependencies of the metabolic and hemodynamic actions of the various prostanoids were different. Prostaglandin F2 alpha and D2 caused relatively stronger changes of metabolism, while prostaglandin E2 and U46619 had stronger effects on hemodynamics. Prostaglandin F2 alpha elicited greater maximal alterations than D2 with similar half-maximally effective concentrations. Prostaglandin F2 alpha mimicked the nerve actions on both metabolism and hemodynamics best with respect to the relative extents and the kinetics of the alterations. 3. The hemodynamic effects of prostaglandin F2 alpha could be prevented completely by the calcium antagonist nifedipine without impairing the metabolic actions of the prostanoid. Apparently, prostaglandin F2 alpha influenced metabolism directly rather than indirectly via hemodynamic changes. The present results, together with the previously described effects of prostanoid synthesis inhibitors, suggest that prostanoids, probably prostaglandin F2 alpha and/or D2, could be involved in the actions of sympathetic hepatic nerves on liver carbohydrate metabolism. Since prostanoids are synthesized only in non-parenchymal cells, nervous control of metabolism appears to depend on complex intra-organ cell-cell interactions between the nerve, non-parenchymal and parenchymal cells.     

Neural activation of alpha-adrenoreceptors in glucose mobilization from liver.

Using a newly described method for obtaining pure, mixed hepatic venous blood samples, it was demonstrated that glucose mobilization from the liver of the anesthetized cat in response to hepatic nerve stimulation is via alpha-adrenergic receptors. Neither the elevation of portal pressure nor the amount of glucose generated by the liver was affected by intraportal administration of 1 mg propranolol/kg (beta blockade). In the presence of alpha-receptor blockade (3 mg phentolamine/kg) the portal venous pressure change was minor and the glucose output actually decreased slightly upon nerve stimulation, a response consistent with our previously demonstrated reduction of glucose output by parasympathetic nerve stimulation. The present responses to nerve stimulation were not due to activation of pancreatic nerves since these nerves were routinely ligated.     

Mechanism of action of sympathetic hepatic nerves on carbohydrate metabolism in perfused rat liver

In the perfused rat liver stimulation of the hepatic nerves around the portal vein and the hepatic artery was previously shown to increase glucose output, to shift lactate uptake to output, to decrease and re-distribute intrahepatic perfusion flow and to cause an overflow of noradrenaline into the hepatic vein. The metabolic effects could be caused directly via nerve hepatocyte contacts or indirectly by the hemodynamic changes and/or by noradrenaline overflow from the afferent vasculature into the sinusoids. Evidence against the indirect modes of nerve action is presented. Reduction of perfusion flow by lowering the perfusion pressure from 2 to 1 ml X min-1 X g-1--as after nerve stimulation--or to 0.35 ml X min-1 X g-1--far beyond the nerve stimulation-dependent effect--did not change glucose output and lowered lactate uptake only slightly. Only re-increase of flow to 2 ml X min-1 X g-1 enhanced glucose and lactate release transiently due to washout of glucose and lactate accumulated in parenchymal areas not perfused during low perfusion flow. In chemically sympathectomized livers nerve stimulation decreased perfusion flow almost normally but without changing the intrahepatic microcirculation; yet it enhanced glucose and lactate output only insignificantly and caused noradrenaline overflow of less than 10% of normal. Conversely, in the presence of nitroprussiate (III) nerve stimulation reduced overall flow only slightly without intrahepatic redistribution but still increased glucose and lactate output strongly and caused normal noradrenaline overflow.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glucose disposal by insulin, but not IGF-1, is dependent on the hepatic parasympathetic nerves

Insulin-like growth factor-1 (IGF-1) has many insulin-like activities, including stimulation of glucose uptake in skeletal muscle. However, those with diabetes or chronic liver disease are insulin resistant but show a normal hypoglycemic response to IGF-1. We have previously shown that insulin sensitivity depends on a hepatic parasympathetic reflex release of a hormone from the liver. The hypothesis was tested that insulin action, but not IGF-1 action, is dependent on the hepatic parasympathetic reflex. Glucose disposal in response to three doses of IGF-1 (25, 100, 200 microg/kg) was determined in rats. IGF-1 at 200 microg/kg had similar effect on glucose disposal as did 50 mU/kg of insulin. Interruption of the hepatic parasympathetic reflex either by surgical ablation of the anterior nerve plexus or by atropine (1.0 mg/kg) resulted in insulin, but not IGF-1, resistance. Sixteen hours of fasting resulted in insulin, but not IGF-1, resistance. In conclusion, insulin, but not IGF-1, triggers the hepatic parasympathetic dependent release of a putative hepatic insulin sensitizing substance (HISS) that stimulates glucose uptake in skeletal muscle.     

Opposite excitatory and inhibitory effects of central administration of glucagon and of insulin upon the sympathetic cervical and adrenal nerve activities in the rat

The central effects of pancreatic glucagon and insulin given intracerebroventriculary (i.c.v.) upon sympathetic activity in the cervical trunk and adrenal nerve were examined in Wistar Kyoto rats. Glucagon i.c.v. administration led to an increase in sympathetic nerve activity in both nerves. Insulin injected into lateral ventricle caused opposite to glucagon inhibitory influence on sympathetic discharge in the cervical trunk and adrenal nerve. This two different central effects of glucagon and insulin on sympatho-adrenal system may contribute to glycemia homesthasis.     

Autonomic nervous control of gastric somatostatin secretion from the perfused rat stomach

The effect of parasympathetic and sympathetic nerve stimulation on the secretion of gastric somatostatin and gastrin has been studied in an isolated perfused rat stomach preparation. Stimulation of the vagus nerve inhibited somatostatin secretion and increased gastrin release. Splanchnic nerve stimulation increased somatostatin release during simultaneous atropine perfusion, but not in its absence, whereas gastrin secretion was unchanged. The secretory activity of the gastric D-cell was therefore reciprocally influenced by the sympathetic and parasympathetic nerves but sympathetic stimulation was only effective during muscarinic blockade.     

Direct control of glycogen metabolism in the perfused rat liver by the sympathetic innervation

The mode of action of hepatic nerves on the metabolism of carbohydrates was studied in the rat liver perfused in situ. 1. Electrical stimulation of the nerve bundles around the hepatic artery and the portal vein resulted in an increase of glucose and lactate output, an enhancement of phosphorylase a activity and a decrease of portal flow. 2. Sodium nitroprusside prevented the hemodynamic changes after nerve stimulation without affecting the metabolic alterations. 3. Phentolamine or an extracellular calcium level below 300 mumol x 1(-1) abolished both hemodynamic and metabolic changes after nerve stimulation, while propranolol or atropine were without effect. 4. Norepinephrine infusion mimicked nerve stimulation only at the highly unphysiological concentration of 0.1 microM; it was not effective at a concentration of 0.01 microM, which might be reached in the sinusoidal blood due to an overflow from intrahepatic synapses. The present results suggest that, in rat liver, glycogen breakdown is regulated by alpha-sympathetic nerves directly rather than indirectly via hemodynamic changes or via norepinephrine overflow.     

Effects of varying frequency of sympathetic stimulation on chloride and amylase levels of saliva elicited from rat parotid gland with electrical stimulation of both autonomic nerves.

Simultaneous stimulation of the parasympathetic and sympathetic nerves to the parotid gland of rats elicited saliva at a rate dependent on the frequency of sympathetic stimulation when parasympathetic frequency was maintained at 16 Hz. The flow rate was lowest at 2 Hz (sympathetic), moderate at 5 Hz, and highest at 16 Hz. Cl concentration of the saliva evoked with stimulation of both nerves was highest at the highest frequency and flow rate (maintained at the level of 102 mEq/liter, for 35 min) and lowest at 2 Hz (declining from 40 mEq/liter initially to 28 mEq/liter). With sympathetic nerve stimulation alone, Cl concentration ranged from 27 to 58 mEq/liter when frequency was varied from 2 to 16 Hz, and with parasympathetic stimulation alone (16 Hz), it ranged from 132 to 124 mEq/liter. Amylase concentration of sympathetically elicited saliva was, in contrast, highest at 2 Hz (1.5 times the level at 5 Hz, and twice the level at 16 Hz), and nearly 18-38 times that seen with parasympathetic stimulation alone. The same pattern was found when both nerves were stimulated, and at 2 Hz (sympathetic), amylase concentration was 1.6 times the level at 5 Hz and 2.6 times the level at 16 Hz. When the two nerves were simultaneously stimulated, the total amount of amylase secreted over 35 min was twice as high as that observed with sympathetic nerve stimulation alone, at any frequency. The relation of frequency to norepinephrine concentration was examined. There was no consistent difference in norepinephrine concentration related to variation in frequency of sympathetic stimulation. Only when both nerves were stimulated at 16 Hz was there a statistically significant reduction in norepinephrine concentration of 46%. A relation between frequency of sympathetic stimulation, flow rate, amylase concentration, and Cl concentration was established, but these changes could not be directly correlated with quantitative differences in norepinephrine concentration.     

Effects of chemical sympathectomy by means of 6-hydroxydopamine on insulin secretion and islet morphology in alloxan-diabetic mice

Activation of sympathetic nerves increases circulating glucose and inhibits insulin release from the islet beta-cells, which might contribute to stress-related diabetes. Accordingly, we have shown previously that blockade of parasympathetic activity aggravates diabetes in alloxan-treated mice, suggesting that unopposed sympathetic activity impairs diabetes. In this study, we tested whether elimination of sympathetic nerve activity by chemical sympathectomy with 6-hydroxydopamine (6-OHDA; 60 mg/kg) ameliorates the diabetogenic effects of alloxan (50 mg/kg) in NMRI mice. Mice given alloxan alone developed manifest diabetes after 2 days, as indicated by hyperglycemia. The diabetes persisted throughout the 35-day study period. Pretreatment with 6-OHDA did not, however, affect the glucose levels or the low, 2-min in vivo insulin response to glucose (1 g/kg) after alloxan. In situ hybridization at day 35 revealed a significantly reduced grain area of insulin-mRNA in the alloxan-treated animals, which was not affected by 6-OHDA, and an altered islet architecture, with accumulation of glucagon cells in the central portion. Also 6-OHDA alone reduced the insulin mRNA area, but this was accompanied by an increase in the total islet area. We conclude that, in contrast to cholinergic inhibition, sympathectomy does not perturb the development of chemically induced diabetes in mice. Alone, however, sympathectomy reduces insulin gene expression and induces increased islet size, suggesting that sympathetic nerves are of importance for long-term islet function.     

Central adrenergic suppression augments the insulin and glucagon secretory, and the glycogenolytic responses in streptozotocin-diabetic rats.

It has been suggested that the increased activity of the sympathetic nervous system and the resultant increase in the tissue catecholamine levels contribute to the pathogenesis of diabetes. In this study we evaluated the effect of clonidine, a central adrenergic agonist that decreases sympathetic tone, on the serum levels of glucose, insulin, glucagon and norepinephrine and on the hepatic glycogen content in normal and streptozotocin-diabetic rats. The animals were treated with clonidine 25 micrograms/kg/day interperitoneally for 3 weeks to suppress the central adrenergic impulses. Clonidine treatment significantly increased the weight gain, but did not affect plasma glucose, insulin, glucagon and norepinephrine in the diabetic animals. Pancreatic insulin and liver glycogen contents were significantly higher in the clonidine-treated than in the untreated diabetic rats. However, clonidine did not affect pancreatic insulin and liver glycogen content of nondiabetic animals. The intravenous administration of glucagon increased plasma glucose in the clonidine-treated, but not in the saline-treated diabetic rats. Insulin-induced hypoglycemia significantly enhanced glucagon release in clonidine-treated but not in saline-treated diabetic rats. We conclude that the suppression of central adrenergic activity may ameliorate the effects of insulin insufficiency on pancreatic hormone secretion and hepatic glycogen content.     

Evaluation of topical phenol as a means of producing autonomic denervation of the liver

Topical application of 90% phenol around the bile duct, portal vein, and hepatic artery, as well as along each of the three hepatic ligaments was tested for effectiveness of rapid and chronic denervation in cats. Because phenol produces nonselective nerve degeneration, it was assumed that proof of functional sympathectomy was adequate proof of disruption of parasympathetic and afferent nerves as well. Functional sympathetic neurons were evaluated by measuring physiological responses to direct electrical stimulation of the anterior hepatic plexus. Acute or rapid denervation was assessed by the degree of rise in portal blood pressure produced by nerve stimulation. Complete denervation appeared within 20 min and was still present by 80 min postapplication. Chronic denervation was tested by applying the phenol and recovering the cats for 6-14 days. An equal number (n = 6) of sham-denervated cats were compared. Phenol denervation did not alter basal glucose, insulin or glucagon levels, hematocrit, blood pressure, or hepatic glycogen levels. These variables are a good index of stress and metabolic status. Nerve stimulation in the chronic sham group raised portal pressure, arterial pressure, and blood glucose levels, whereas the chronic-denervated group showed no responses. The health of the two groups appeared normal with the sole difference being that the painted tissues were mildly discolored and more adhesions appeared in the phenol-denervated set. Thus phenol is a useful tool for producing hepatic denervation. It is less traumatic, faster, and more certain than surgical denervation. In addition, the hepatic lymphatics can be preserved using the topical application of phenol.     

Control of ketogenesis in the perfused rat liver by the sympathetic innervation

The regulation of ketogenesis by the hepatic nerves was investigated in the rat liver perfused in situ. Electrical stimulation of the hepatic nerves around the portal vein and the hepatic artery caused a reduction of basal ketogenesis owing to a decrease in acetoacetate release to 30% with essentially no change in 3-hydroxybutyrate release. At the same time, as observed before [Hartmann et al. (1982) Eur. J. Biochem. 123, 521-526], nerve stimulation increased glucose output, shifted lactate uptake to output and decreased perfusion flow. Ketogenesis from oleate, which enters the mitochondria via the carnitine system, was also lowered after nerve stimulation owing to a decrease of acetoacetate release to 30% with no alteration in 3-hydroxybutyrate release. Ketogenesis from octanoate, which enters the mitochondria independently of the carnitine system, was decreased after nerve stimulation as a result of a drastic decrease of acetoacetate output to 15% and a less pronounced decrease of 3-hydroxybutyrate release to 65%. Noradrenaline mimicked the metabolic nerve effects on ketogenesis only at the highly unphysiological concentration of 0.1 microM under basal conditions and in the presence of oleate as well as partly in the presence of octanoate. It was essentially not effective at a concentration of 0.01 microM, which might be reached in the sinusoids owing to overflow from the hepatic vasculature. Sodium nitroprusside prevented the hemodynamic changes after nerve stimulation; it did not affect the nerve-dependent reduction of ketogenesis under basal conditions and in the presence of oleate, yet it diminished the nerve effect on octanoate-dependent ketogenesis. Phentolamine clearly reduced the metabolic and hemodynamic nerve effects, while propranolol was without effect. The present data suggest that hepatic ketogenesis was inhibited by stimulation of alpha-sympathetic liver nerves directly rather than indirectly via hemodynamic changes or noradrenaline overflow from the vessels and that the site of regulation should be mainly intramitochondrial.     

Metabolic responses of perfused rat livers to alpha- and beta-adrenergic agonists, glucagon and cyclic AMP.

1. The mechanism of action of glucagon and epinephrine was studied in perfused rat livers. Hormone-induced transitions from one metabolic steady state to another were followed in a non-recirculating perfusion system. Glucose and lactate production rates, oxygen uptake and K+ redistribution were measured. 2. Glucagon (3 nM), cyclic AMP (0.2 mM) and epinephrine (0.5 muM) had similar effects on K+ concentrations in the perfusate. Glycogenolysis responded more rapidly and O2 uptake was enhanced to a larger extent with epinephrine than with the other agents. alpha- and beta-receptor responses were differentiated by the use of phenylephrine (0.5 muM), isoproterenol (0.5 muM) and adrenergic blocking agents (phentolamine and beta-blocker Ro 3-4787 at 0.1 mM). 3. alpha-receptors mediated an activation of glucose production that was very rapid and was paralleled by a transient decrease of K+ concentrations in the effluent from the liver, lactate production rose gradually. Respiration was also enhanced, but fell again as lactate production increased. 4. beta-receptor stimulation was followed by an increase of glucose production that was less drastic and was paralleled by a K+ release, lactate production and respiration were only slightly enhanced. beta stimulation and glucagon both resulted in an inhibition of the alpha-adrenergic effect on lactate release and simultaneously increased O2 uptake. 5. We concluded that in perfused rat livers alpha- as well as beta-adrenergic receptor stimulation resulted in an activation of glycogenolysis, possibly by two different mechanisms.     

Effects of secretory nerve stimulation on acid phosphatase and peroxidase in submandibular saliva and acini in cats

Synopsis The effects of parasympathetic or sympathetic nerve stimulation either alone, or in combination, on the acid phosphatase-containing central acinar cells and the peroxidase-containing demilunar cells of the cat submandibular salivary gland have been investigated by histochemical and cytochemical techniques. The results obtained with these techniques were correlated with biochemical assays for both enzymes in the saliva secreted. The results indicate that, although both sets of nerves probably affect both sets of cells, the predominant secretory effect of parasympathetic stimulation is on the central cells and, conversely, the predominant secretory effect of sympathetic stimulation is on the demilunes. Sympathetic stimulation appeared also to have initiated synthesis of peroxidase in the demilunar cells, especially when it was superimposed upon parasympathetic stimulation.