Increase in glucose and lactate output and perfusion resistance by stimulation of hepatic nerves in isolated perfused rat liver: role of alpha 1-, alpha 2-, beta 1- and beta 2-receptors.

Rat liver was perfused in situ via the portal vein without recirculation: 1) Electrical stimulation of the nerve bundles around hepatic artery and portal vein increased glucose and lactate output, reduced flow and caused an overflow of noradrenaline into the hepatic vein. The alpha-agonist phenylephrine also augmented glucose and lactate output and lowered flow with an ED50 of about 1 microM, while the beta-agonist isoproterenol increased glucose output but reduced lactate output with an ED50 of about 0.2 microM and left flow unaltered. 2) The alpha 1-receptor antagonist prazosin (KI at alpha 1-sites approximately 1 nM, at alpha 2-sites approximately 100 nM) inhibited the nerve stimulation-dependent increase in glucose and lactate output and reduction of flow with an ID50 of about 1 nM, while the alpha 2-receptor antagonist yohimbine (KI at alpha 2-sites approximately 10 nM, at alpha 1-sites approximately 1500 nM) was inhibitory only with an ID50 of about 400 nM. 10 nM prazosin clearly reduced the nerve actions, completely blocked the effects of 1 microM phenylephrine and left the effects of 0.2 microM isoproterenol unaltered. 10 nM yohimbine did not affect the nerve actions nor the effects of phenylephrine or isoproterenol. 3) The beta 1-receptor antagonist metoprolol (KI at beta 1-sites approximately 100 nM, at beta 2-sites approximately 1.2 microM) at 10 microM concentrations did not interfere with the nerve stimulation-dependent increase in glucose and lactate output or the decrease in flow. It did not have any specific alpha-antagonistic influence either on the changes brought about by 1 microM phenylephrine; however, it blocked the beta 2-mediated increase in glucose output by isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

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)     

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

A study on the metabolic and hemodynamic actions of hepatic nerve stimulation in the perfused liver of guinea pig and tree shrew as compared to rat was performed, since the density of liver innervation was reported to be different. 1) Nerve stimulation resulted in an increase in glucose release and decrease in lactate uptake or in a shift to output as well as a decrease in portal flow in all three species. The change in glucose output was very similar, that in lactate balance and flow was smaller in tree shrew than in guinea pig and rat. Apparently, the metabolic and hemodynamic changes did not reflect the different densities of liver innervation. 2) The overflow of the neurotransmitter noradrenaline into the hepatic vein differed very clearly in the three animals. In the guinea pig and tree shrew the maximal increase in noradrenaline concentration measured in the effluent was about 6-7-fold higher than in the rat. 3) The content of noradrenaline in the liver in vivo was about five-fold higher in the guinea pig and again another four-fold higher in the tree shrew than in the rat. The contents of adrenaline and dopamine were very low in comparison to those of noradrenaline. The different hepatic noradrenaline contents of the three species investigated are in line with the anatomical findings on the different innervation density. 4) Inhibitors of eicosanoid synthesis reduced the nerve stimulation-dependent metabolic and hemodynamic alterations in guinea pig liver as in rat liver indicating a similar mechanism in these species. Apparently, prostaglandins might be involved as mediators or modulators of nerve actions also in the more densely innervated guinea pig liver and not only in the less densely innervated rat liver.(ABSTRACT TRUNCATED AT 250 WORDS)     

Control of glucose balance in the perfused rat liver by the parasympathetic innervation

Electrical stimulation of the nerve bundles around the hepatic artery and the portal vein activates both the sympathetic and parasympathetic liver nerves; the sympathetic effects clearly predominate. Parasympathetic effects were therefore studied in the rat liver perfused in situ by perivascular nerve stimulation in the presence of both an alpha- and a beta-blocker. In the presence of the alpha-blocker phentolamine and the beta-blocker propranolol all sympathetic nerve effects were prevented; the remaining parasympathetic stimulation had no influence on the basal glucose and lactate metabolism nor on the hemodynamics. Insulin alone, with both alpha- and beta-blockade, provoked a small, parasympathetic nerve stimulation in the presence of insulin a more pronounced enhancement of glucose utilization. In the presence of an alpha- and beta-blocker perivascular nerve stimulation antagonized the glucagon stimulated glucose release, but did not affect lactate exchange. The nerve effect was abolished by the parasympathetic antagonist atropine. Acetylcholine or insulin, with both an alpha- and beta-blocker present, mimicked the effects of nerve stimulation antagonizing the glucagon-stimulated glucose release. Nerve stimulation in the presence of insulin was more effective than either stimulus alone. The present results show that in rat liver stimulation of the parasympathetic hepatic nerves has direct effects on glucose metabolism synergistic with insulin and antagonistic to glucagon.     

Role of extracellular calcium in the metabolic and hemodynamic actions of sympathetic nerve stimulation, noradrenaline and prostaglandin F2 alpha in perfused rat liver. Differential inhibition by nifedipine and verapamil

In perfused rat liver hepatic nerve stimulation (10 Hz, 2 ms) caused an increase in glucose and lactate output, a decrease in flow and an overflow of noradrenaline into the hepatic vein. Noradrenaline (1 microM) (NA) and prostaglandin F2 alpha (5 microM) (PGF2 alpha), which are implicated as mediators of nerve action, elicited similar effects. 1) All actions of nerve stimulation and the hemodynamic but not the metabolic effects of noradrenaline and PGF2 alpha were largely dependent on extracellular calcium. 2) The dihydropyridine type calcium antagonist nifedipine (5 microM) inhibited the hemodynamic but not the metabolic actions of nerve stimulation, NA and PGF2 alpha, while the phenylalkylamine type calcium antagonist verapamil (5 microM) had no effect. These findings allow the following conclusions: Calcium influx into I nerve endings, necessary for the release of neurotransmitter, II parenchymal cells, for the display of metabolic effects induced by nerve stimulation, and III the actions of NA and PGF2 alpha, do not appear to be mediated by the normal affinity nifedipine- or the verapamil-sensitive channels. Calcium influx into vascular smooth muscle and/or endothelial cells for the display of hemodynamic action induced by nerve stimulation and the NA and PGF2 alpha effects, appear to occur through nifedipine-sensitive but verapamil-insensitive channels.     

Unspecific inhibition by the calmodulin antagonist calmidazolium and the intracellular calcium antagonist TMB-8 of the actions of sympathetic hepatic nerves and noradrenaline on glucose balance and flow in perfused rat liver

In perfused rat liver hepatic nerve stimulation (10 Hz, 2 ms) (NS) increased glucose and lactate output, decreased flow and was accompanied by an overflow of noradrenaline into the hepatic vein. These effects were dependent on extracellular and partly on intracellular calcium. Infusion of noradrenaline (1 microM) (NA) elicited similar effects. 1) Calmidazolium at 1, 2 and 5 microM caused an increase in basal glucose output and a decrease and intrahepatic redistribution of flow after a lag of 30, 20 and 5 min, respectively. 2) After 5 min of 1 microM calmidazolium, i.e. before it altered basal metabolism and flow, the actions of NS and NA remained unaltered. 3) After 40 min of 1 microM calmidazolium, i.e. after it had just begun to alter basal metabolism and flow, NS caused a decrease in glucose and lactate output rather than an increase and the metabolic effects of NA were strongly reduced whereas the hemodynamic changes of both stimuli were not altered. 4) TMB-8 at 25, 50 and 100 microM caused a transient increase in lactate output and a decrease and intrahepatic redistribution of flow after a lag of 5 min only at 100 microM concentrations. 5) The effects of NS were inhibited already by 25 microM TMB-8 which reduced NA release whereas the effects of NA were not influenced. Thus, calmidazolium and TMB-8 did not act as a calmodulin and intracellular calcium antagonist, respectively, but had unspecific "side effects" in the complex system of the perfused liver. The antagonists cannot be used to study the role of intracellular calcium in intact organs.     

Activation of inositol phosphate formation by circulating noradrenaline but not by sympathetic nerve stimulation with a similar increase of glucose release in perfused rat liver

In the isolated rat liver perfused in situ, stimulation of the nerve bundles around the hepatic artery and portal vein caused an increase of glucose and lactate output and a reduction of perfusion flow. These changes could be inhibited completely by alpha-receptor blockers. The possible involvement of inositol phosphates in the intracellular signal transmission was studied. 1. In cell-suspension experiments, which were performed as a positive control, noradrenaline caused an increase in glucose output and, in the presence of 10 mM LiCl, a dose-dependent and time-dependent increase of inositol mono, bis and trisphosphate. 2. In the perfused rat liver 1 microM noradrenaline caused an increase of glucose and lactate output and in the presence of 10 mM LiCl a time-dependent increase of inositol mono, bis and trisphosphate that was comparable to that observed in cell suspensions. 3. In the perfused rat liver stimulation of the nerve bundles around the portal vein and hepatic artery caused a similar increase in glucose and lactate output to that produced by noradrenaline, but in the presence of 10 mM LiCl there was a smaller increase of inositol monophosphate and no increase of inositol bis and trisphosphate. These findings are in line with the proposal that circulating noradrenaline reaches every hepatocyte, causing a clear overall increase of inositol phosphate formation and thus calcium release from the endoplasmic reticulum, while the hepatic nerves reach only a few cells causing there a small local change of inositol phosphate metabolism and thence a propagation of the signal via gap junctions.     

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.     

Modulation by somatostatin of nerve-mediated activation of glycogenolysis in the perfused rat liver

Perivascular nerve stimulation of rat livers perfused in situ with erythrocyte-free Krebs-Henseleit buffer at constant pressure in a non-recirculating system resulted in an increase of glucose and lactate production and in a decrease of portal flow. Infusion of somatostatin in different concentrations (2 × 10⁻⁷, 10⁻⁸, 10⁻⁹ mol·l⁻¹) reduced the nerve-mediated activation of glucose release maximally to 66%. There was only a slight effect on the lactate output, the nerve-mediated reduction of portal flow was unaltered. In controls, somatostatin alone had no effect on the metabolic and hemodynamic parameters. In order to differentiate between a presynaptic and postsynaptic mechanism, the noradrenaline overflow was calculated. The unaltered release of the neurotransmitter in the presence or absence of somatostatin excluded a presynaptic mechanism. To mimic the nerve effects on the carbohydrate metabolism and on the hemodynamics, noradrenaline (2 × 10⁻⁷ mol·l⁻¹) was infused instead of the nerve stimulation over a period of 5 min. Somatostatin did not change the endocrine effects of the catecholamine under these conditions. The nerve-dependent effect of somatostatin suggests that other neurotransmitters (e.g. VIP) or mediators (e.g. prostanoids) may be influenced by somatostatin.     

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.     

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.     

Facilitation of nerve stimulation evoked noradrenaline overflow by isoprenaline but not by circulating adrenaline in the dog in vivo

The influence of isoprenaline and adrenaline on the overflow of endogenous noradrenaline evoked by sympathetic nerve stimulation was studied in canine blood perfused gracilis muscle in situ. Neuronal uptake was inhibited by desipramine. Local i.a. infusions of isoprenaline enhanced stimulation evoked noradrenaline overflow by 32 +/- 10% (P less than 0.05), indicating the existence of prejunctional facilitatory beta-adrenoceptors. This effect of isoprenaline was not antagonized by beta 1-adrenoceptor blockade and does not seem to be related to the vasodilatation caused by isoprenaline. In a second series of experiments circulating adrenaline levels were raised by i.v. infusions from basal levels of 0.4 +/- 0.2 nM to 1.7 +/- 0.2 and 6.3 +/- 0.6 nM, respectively, in arterial plasma. Adrenaline elicited vasodilatation in the gracilis muscle (19 +/- 3 and 28 +/- 5% increases in vascular conductance, respectively), indicating activation of postjunctional beta 2-adrenoceptors, without influencing nerve stimulation evoked noradrenaline overflow. Thus, our results support the existence of a prejunctional beta 2-adrenoceptor mediated mechanism facilitating noradrenaline release in vivo, but provide no evidence to support the idea that physiologically relevant increases in circulating adrenaline levels enhance noradrenergic neurotransmission in skeletal muscle.     

Control of glycogenolysis and hemodynamics in perfused rat liver by the sympathetic innervation. Dependence on stimulation frequency and duration

Perivascular stimulation of the hepatic nerves in the in situ perfused rat liver with a constant frequency of 20 Hz over a constant period of 5 min had previously been shown to cause an increase of glucose output, a shift from lactate uptake to release, a reduction in perfusion flow (Hartmann et al. (1982) Eur. J. Biochem. 123, 521-526) and an overflow of noradrenaline into the hepatic vein (Beckh et al. (1982) FEBS Lett. 149, 261-265). In the present study the dependence of the metabolic and hemodynamic effects on the frequency between 1 and 30 Hz and duration of stimulation between 0.5 and 5 min was investigated. Over a constant stimulation period of 5 min the alteration in glucose exchange was maximal with a frequency of 10 Hz and half-maximal with 4 Hz. The corresponding values for the exchange of lactate were 5 Hz and 2 Hz, respectively, and for the perfusion flow 2.5 Hz and 1.5 Hz, respectively. An increase of noradrenaline overflow was not observed with the lower frequencies of 1 and 2.5 Hz; it was maximal at 10 Hz and half-maximal at 6.5 Hz. At a constant frequency of 20 Hz the increase in glucose release was maximal with a total stimulation period of 1 min and half-maximal with a period of 0.4 min. An essentially maximal alteration of lactate exchange and perfusion flow as well as of noradrenaline overflow was also effected by a stimulation period of 1 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5′-triphosphate, adenosine diphosphate, uridine 5′-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.     

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.     

Neuropeptide Y (NPY) and the pig heart: release and coronary vasoconstrictor effects

The effects of electrical stimulation of the stellate ganglia on the arterio-venous concentration differences of neuropeptide Y (NPY)-like immunoreactivity (LI) over the pig heart were studied in vivo in relation to changes in heart rate and left ventricular pressure. Furthermore, the effects of NPY on coronary vascular tone were analysed in vivo and in vitro. Stellate ganglion stimulation at a high frequency (10 Hz) caused a clear-cut, long lasting increase in plasma levels of NPY-LI in the coronary sinus compared to the aorta, suggesting release of this peptide from sympathetic terminals within the heart. The stimulation-evoked overflow of NPY-LI from the heart was enhanced about 3-fold by alpha-adrenoceptor blockade using phenoxybenzamine, suggesting that NPY release is under prejunctional inhibitory control by noradrenaline (NA). Combined alpha- and beta-adrenoceptor blockade abolished most of the positive inotropic response of the heart upon stellate ganglion stimulation, while a considerable positive chronotropic effect remained. After guanethidine treatment, stellate ganglion stimulation still produced a small positive inotropic and chronotropic effect on the heart. The stimulation evoked NPY overflow was markedly reduced by guanethidine indicating an origin from sympathetic nerve terminals. Injection of NPY into the constantly perfused left anterior descending artery in vivo caused a long lasting, adrenoceptor antagonist resistant increase in perfusion pressure, suggesting coronary vasoconstriction. NPY contracted coronary arteries in vitro via a nifedipine-sensitive mechanism. NA dilated coronary vessels both in vivo and in vitro via beta-adrenoceptor activation. It is concluded that sympathetic nerve stimulation increases overflow of NPY-LI from the heart suggesting release from cardiac nerves in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

The canine sympathetic neuropeptide galanin: a neurotransmitter in pancreas, a neuromodulator in liver.

Our laboratory has investigated the role of the neuropeptide galanin in the sympathetic neural control of both the canine endocrine pancreas and liver. Galanin mRNA and peptide were found in the neuronal cell bodies of the celiac ganglion, which projects fibers to both organs. Galanin fibers formed dense networks around the islets. Galanin was released from these nerves and the amount released appeared sufficient to markedly inhibit basal insulin secretion. We therefore propose that galanin is a sympathetic neurotransmitter in canine endocrine pancreas. Galanin was also found in hepatic nerves usually co-localized with tyrosine hydroxylase, a sympathetic marker. Further, intraportal administration of the sympathetic neurotoxin, 6-hydroxydopamine, abolished galanin staining in the hepatic parenchyma. We evaluated the role of galanin in mediating the actions of sympathetic nerves to increase hepatic glucose production and decrease hepatic arterial conductance. Local infusion of synthetic galanin had little effect by itself, but it did potentiate the action of norepinephrine to stimulate hepatic glucose production, demonstrating a neuromodulatory action. In contrast, galanin had no effect on hepatic arterial blood flow. We therefore propose that in the liver galanin functions as a neuromodulator of norepinephrine's metabolic action.     

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.