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.     

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.     

Mechanism of action of cysteinyl leukotrienes on glucose and lactate balance and on flow in perfused rat liver. Comparison with the effects of sympathetic nerve stimulation and noradrenaline

Rat livers were perfused at constant pressure via the portal vein with media containing 5 mM glucose, 2 mM lactate and 0.2 mM pyruvate. 1. Leukotrienes C4 and D4 enhanced glucose and lactate output and reduced perfusion flow to the same extent and with essentially identical kinetics. They both caused half-maximal alterations (area under the curve) of carbohydrate metabolism at a concentration of about 1 nM and of flow at about 5 nM. The leukotriene-C4/D4 antagonist CGP 35949 B inhibited the metabolic and hemodynamic effects of 5 nM leukotrienes C4 and D4 with the same efficiency, causing 50% inhibition at about 0.1 microM. 2. Leukotriene C4 elicited the same metabolic and hemodynamic alterations with the same kinetics as leukotriene D4 in livers from rats pretreated with the gamma-glutamyltransferase inhibitor, acivicin. 3. The calcium antagonist, nifedipine, at a concentration of 50 microM did not affect the metabolic and hemodynamic changes caused by 5 nM leukotriene D4. The smooth-muscle relaxant, nitroprussiate, at a concentration of 10 microM reduced flow changes, without significantly affecting the metabolic alterations. 4. Leukotriene D4 not only reduced flow; it also caused an intrahepatic redistribution of flow, restricting some areas from perfusion. Thus, leukotrienes increased glucose and lactate output directly in the accessible parenchyma and, in addition, indirectly by washout from restricted areas during their reopening upon termination of application. 5. The phospholipase A2 inhibitor, bromophenacyl bromide, but not the cyclooxygenase inhibitor, indomethacin, at a concentration of 20 microM reduced the metabolic and hemodynamic effects of 5 mM leukotriene D4. 6. Stimulation of the sympathetic hepatic nerves with 2-ms rectangular pulses at 20 Hz and infusion of 1 microM noradrenaline increased glucose and lactate output and decreased flow, similar to 10 nM leukotrienes C4 and D4. The kinetics of the metabolic and hemodynamic changes caused by the leukotrienes differed, however, from those due to nerve stimulation and noradrenaline. 7. The leukotriene-C4/D4 antagonist, CGP 35949 B, even at very high concentrations (20 microM) inhibited the metabolic and hemodynamic alterations caused by nerve stimulation or noradrenaline infusion only slightly and unspecifically.(ABSTRACT TRUNCATED AT 400 WORDS)     

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)     

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.     

Effects of nitric oxide on platelet-activating factor- and alpha-adrenergic-stimulated vasoconstriction and glycogenolysis in the perfused rat liver.

Effects of nitric oxide (NO) on hemodynamic and glycogenolytic responses to platelet-activating factor (PAF) and phenylephrine were investigated in perfused livers derived from fed rats. Infusion of NO (34 microM) into perfused livers inhibited PAF (0.22 nM)-induced increases in hepatic glucose output and portal pressure approximately 90 and 85%, respectively, and abolished effects of PAF on hepatic oxygen consumption. NO attenuated PAF-stimulated increases in glucose output and portal pressure, the latter indicative of hepatic vasoconstriction, with a similar dose dependence with an IC50 of approximately 8 microM. In contrast to its effects on PAF-induced responses in the perfused liver, NO inhibited increases in hepatic portal pressure in response to phenylephrine (10 microM) approximately 75% without altering phenylephrine-stimulated glucose output and oxygen consumption. Similarly, infusion of NO into perfused livers significantly inhibited increases in hepatic portal pressure but not in glucose output in response to a submaximal concentration of phenylephrine (0.4 microM). Like NO, sodium nitroprusside (83 microM) significantly inhibited hemodynamic but not glycogenolytic responses to phenylephrine in perfused livers. However, PAF (0.22 nM)-stimulated alterations in hepatic portal pressure, glucose output, and oxygen consumption were unaffected by infusion of sodium nitroprusside (83 microM) into perfused livers. These results provide the first evidence for regulatory effects of NO in the perfused liver and support the contention that PAF, unlike phenylephrine, stimulates glycogenolysis by mechanisms secondary to hepatic vasoconstriction. These observations raise the intriguing possibility that NO may act in liver to regulate hemodynamic responses to vasoactive mediators.     

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.     

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)     

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.     

Possible involvement of eicosanoids in the actions of sympathetic hepatic nerves on carbohydrate metabolism and hemodynamics in perfused rat liver

In isolated rat liver perfused at constant pressure with Krebs-Henseleit buffer containing 5 mM glucose, 2 mM lactate, 0.2 mM pyruvate and 0.1% bovine serum albumin, perivascular nerve stimulation (20 V, 20 Hz, 2 ms) and infusion of ATP (100 microM), noradrenaline (1 microM) or arachidonic acid (100 microM) caused an increase in glucose and lactate output and a reduction of perfusion flow. The metabolic effects of nerve stimulation but not those of ATP and noradrenaline were inhibited strongly by the phospholipase A2 inhibitor bromophenacyl bromide (BPB, 20 microM) and the cyclooxygenase inhibitor indomethacin (Indo, 20 microM) and only slightly by the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA, 20 microM). In contrast, the hemodynamic effects not only of nerve stimulation but also of ATP and noradrenaline were inhibited strongly by BPB and Indo and slightly by NDGA. The metabolic and hemodynamic actions of arachidonate were inhibited specifically by Indo. These results suggest that the effects of nerve stimulation were at least partially mediated or modulated by eicosanoids, especially by prostanoids.     

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.     

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.     

Receptor subtype mediating the action of circulating endothelin on glucose metabolism and hemodynamics in perfused rat liver.

The subtype of endothelin receptor that mediates metabolic and hemodynamic effects of circulating endothelin was explored using perfused rat liver. Infusion of endothelin (ET)-1 or ET-3 into the portal vein at a concentration of 0.3 nM increased glucose and lactate output and decreased perfusion flow, although ET-3 was less effective than ET-1. The metabolic effects of ET-1 were observed even under costant-flow perfusion. Infusion of either sarafotoxin S6b or S6c, an ET(A)- or ET(B)-receptor agonist, mimicked the actions of ET-1 to an equal extent. The flow reduction and glucose production induced by ET-1 were partly attenuated by the ET(A)-receptor antagonist BQ485. By contrast, ET(B)-receptor antagonist BQ788 enhanced glucose production caused by ET-1 and ET-3 without affecting the hemodynamic change. The effects of ET-1 and ET-3 were almost totally inhibited by the combination of BQ485 and BQ788. These results suggest that both ET(A) and ET(B) receptors are involved in the metabolic and hemodynamic effects of circulating endothelin in rat liver, while the ET(A)-receptor-mediated action appears to be dominant.     

Control of oxygen uptake, microcirculation and glucose release by circulating noradrenaline in perfused rat liver

The effect of noradrenaline on oxygen uptake, on periportal and perivenous oxygen tension at surface acini, on microcirculation and on glucose output were studied in isolated rat livers perfused at constant flow with Krebs-Henseleit-hydrogen carbonate buffer containing 5mM glucose and 2mM lactate. Noradrenaline at 1 microM concentration caused a decrease in oxygen uptake, while at 0.1 microM it led to an increase. Both high and low doses of noradrenaline decreased the tissue surface oxygen tension in periportal and - after a transient rise - in perivenous areas. Noradrenaline at an overall constant flow caused an increase of portal pressure and an alteration of the intrahepatic distribution of the perfusate: at the surface of the liver and in cross sections infused trypan blue led to only a slightly heterogeneous staining after a low dose of noradrenaline but to a clearly heterogeneous staining after a high dose. Both high and low doses of noradrenaline stimulated glucose release. All effects could be inhibited by the alpha-blocking agent phentolamine. In conclusion, control of hepatic oxygen consumption by circulating noradrenaline is a complex result of opposing hemodynamic and metabolic components: the microcirculatory changes inhibit oxygen uptake; they dominate after high catecholamine doses. The metabolic effects include a stimulation of oxygen utilization; they prevail at low catecholamine levels. The noradrenergic control of glucose release is also very complex, involving direct, metabolic and indirect, hemodynamic components.     

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.     

Hepatocyte heterogeneity in response to icosanoids. The perivenous scavenger cell hypothesis

1. The metabolic and hemodynamic effects of prostaglandin F2 alpha, leukotriene C4 and the thromboxane A2 analogue U-46619 were studied during physiologically antegrade (portal to hepatic vein) and retrograde (hepatic to portal vein) perfusion and in a system of two rat livers perfused in sequence. 2. The stimulatory effects of prostaglandin F2 alpha (3 microM) on hepatic glucose release, perfusion pressure and net Ca2+ release were diminished by 77%, 95% and 64%, respectively, during retrograde perfusion when compared to the antegrade direction, whereas the stimulation of 14CO2 production from [1-14C]glutamate by prostaglandin F2 alpha (which largely reflects the metabolism of perivenous hepatocytes) was lowered by only 20%. Ca2+ mobilization and glucose release from the liver comparable to that seen during antegrade perfusion could also be observed in retrograde perfusions; however, higher concentrations of the prostaglandin were required. 3. The glucose, Ca2+ and pressure response to leukotriene C4 (20 nM) or the thromboxane A2 analogue U-46619 (200 nM) of livers perfused in the antegrade direction were diminished by about 90% during retrograde perfusion. Sodium nitroprusside (20 microM) decreased the pressure response to leukotriene C4 (20 nM) and U-46619 (200 nM) by about 40% and 20% in antegrade perfusions, respectively, but did not affect the maximal increase of glucose output. 4. When two livers were perfused antegradely in series, such that the perfusate leaving the first liver (liver I) entered a second liver (liver II), infusion of U-46619 at concentrations below 200 nM to the influent perfusate of liver I increased the portal pressure of liver I, but not of liver II. At higher concentrations of U-46619 there was also an increase of the portal pressure of liver II and with concentrations above 800 nM the pressure responses of both livers were near-maximal [19.6 +/- 0.8 (n = 7) cm H2O and 16.5 +/- 1.1 (n = 8) cm H2O for livers I and II, respectively]. There was a similar behaviour of glucose release from livers I and II in response to U-46619 infusion. When liver I was perfused in the retrograde direction, a significant pressure or glucose response of liver II (antegrade perfusion) could not be observed even with U-46619 concentrations up to 1000 nM. 5. Similarly, the perfusion pressure increase and glucose release induced by leukotriene C4 (10 nM) observed with liver II was only about 20% of that seen with liver I.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

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.     

Presinusoidal vessels predominantly contract in response to norepinephrine, histamine, and KCl in rabbit liver.

In rabbit livers, it is not well known which segments of the hepatic vasculature are predominantly contracted by various vasoconstrictors. We determined effects of histamine, norepinephrine, and KCl on hepatic vascular resistance distribution in isolated rabbit livers perfused via the portal vein with 5% albumin-Krebs solution at a constant flow rate. Hepatic capillary pressure was measured by double vascular occlusion pressure (Pdo) and was used to determine portal (Rpv) and hepatic venous (Rhv) resistances. A bolus injection of either histamine or norepinephrine dose-dependently increased portal venous pressure but not Pdo, resulting in a dose-dependent increase in Rpv and no changes in Rhv. KCl (50 mM), when injected in anterogradely perfused livers, contracted the presinusoidal vessels selectively with liver weight loss. Although KCl significantly increased Rhv in retrogradely perfused livers, the increase in Rpv by 400% of baseline predominated over the increase in Rhv by 85% of baseline. In the retrogradely perfused livers, KCl produced an initial liver weight loss followed by a profound weight gain. We conclude that histamine and norepinephrine selectively contract the presinusoidal vessels. The results on KCl effects suggest that this selective presinusoidal constriction might be possibly due to predominant distribution of functionally active vascular smooth muscle in the presinusoidal vessels rather than the hepatic vein in rabbit livers.