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.     

Chain-shortening of prostaglandin F2 alpha by rat liver peroxisomes

Liver peroxisomes were isolated from di(2-ethylhexyl)phthalate treated rats by isopycnic sucrose gradient centrifugation of a light mitochondrial fraction. Incubation of prostaglandin F2 alpha with purified peroxisomes resulted in conversion into a more polar product(s). In contrast, incubation with mitochondrial fractions and microsomal fractions under the same conditions did not result in any detectable conversion. The polar material obtained from a preparative incubation was purified by high performance liquid chromatography and characterized by radio-gas chromatography and gas chromatography-mass spectrometry. The structure of the polar compound was shown to be 5,7,11-trihydroxy-tetranorprost-9-enoic acid (tetranor-prostaglandin F1 alpha). Prostaglandin F2 alpha was thus chain-shortened by four carbon atoms.     

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.     

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.     

Role of prostaglandin F2alpha in ovulation.

Using radioimmunoassay procedures, the levels of plasma, uterine and ovarian prostaglandin (PG) F2alpha, and those of plasma estradiol and progesterone were measured in intact, hysterectomized or ovariectomized immature female rats pretreated with PMS and subsequent HCG. Occurrence of ovulation was confirmed at 8 hours after the HCG administration not only in the intact rats but also in the hysterectomzied rats. The levels of plasma estradiol and progesterone, and of uterine and ovarian PGF2alpha rose with the PMS injection alone, but they did not reach the peaks before the HCG administration. Both plasma estradiol and uterine PGF2alpha showed a peak at 2 hours after the HCG injection. These peaks were antecedent 2 or 6 hours before the peaks of ovarian and plasma PGF2alpha, respectively. However, such increase of uterine PGF2alpha does not seem to be indispensable for ovulation, because ovulation could occur in the hysterectomized rats. The levels of ovarian PGF2alpha showed a high plateau from 4 to 8 hours after the HCG injection, and then rapidly decreased after ovulation. The levels of plasma PGF2alpha peaked not only in the intact rats but also in the hysterectomized rats at 8 hours after the HCG treatment. But in the ovariectomized rats, this plasma PGF2alpha peak at 8 hours disappeared and there was no statistical change of plasma PGF2alpha throughout the PMS-HCG treatment. Plasma progesterone gradually increased and reached the maximum at 10 hours after the HCG injection. These results conclude that the main source of increased plasma PGF2alpha during the ovulatory process induced with the PMS-HCG treatment is the ovary, and it is strongly suggested that a rapid increase of PGF2alpha in the ovary may play some important role(s) in the ovulatory process.     

Evaluation of prostaglandin F2 alpha and prostacyclin interactions in the isolated perfused rat lung.

To characterize the interactions between prostaglandin F2 alpha and prostacyclin in controlling tone in the pulmonary circulation, isolated rat lungs were ventilated, perfused with blood, and subjected to challenge by prostaglandin F2 alpha in increasing doses. The pulmonary resistance was evaluated using occlusion techniques that separate the resistance into segments of large and small arteries and veins. The total vascular compliance was evaluated using outflow occlusion. Resistance increased after prostaglandin F2 alpha, and this resistance change was primarily in the small artery segment. The maximum resistance increase by prostaglandin F2 alpha (Rmax,PGF2 alpha), calculated from the Michaelis-Menton equation, was 16.6 +/- 3.6 cmH2O.l-1.min.100 g-1 for total vascular resistance with a concentration required to produce 50% Rmax (K0.5) of 5.26 +/- 3.57 nM. The Rmax,PGF2 alpha for small artery resistance was 13.5 +/- 2.4 cmH2O.l-1.min.100 g-1 with a K0.5 of 2.35 +/- 1.57 nM. The vascular compliance decreased during vasoconstriction by prostaglandin F2 alpha, and the maximum decrease in compliance (Cmin,PGF2 alpha) was -0.43 +/- 0.12 ml/cmH2O with a K0.5 of 2.84 +/- 2.99 nM. At each dose of prostaglandin F2 alpha, prostacyclin was administered in increasing doses to reverse the vasoconstriction caused by prostaglandin F2 alpha. For each concentration of prostaglandin F2 alpha, prostacyclin almost completely reversed the resistance increases and approximately one-half the compliance decrease. The maximum change in vascular resistance or compliance produced by prostacyclin was dependent on the dose of prostaglandin F2 alpha; yet the K0.5 for prostacyclin was within the picomolar range for all doses of prostaglandin F2 alpha.(ABSTRACT TRUNCATED AT 250 WORDS)     

Down-regulation of prostaglandin F2 alpha receptors in rat liver during chronic endotoxemia.

Prostaglandin (PG) F2 alpha binding parameters were measured in purified plasma membrane preparations isolated from livers of chronically endotoxin-(ET) treated rats and corresponding controls. Two classes of binding sites were detected in both groups: high affinity, low capacity, with a KD of 44.4 +/- 8.8 nM for saline- and 28.6 +/- 11.3 nM for ET-treated rats (n = 5 for both, p greater than 0.05) and low affinity, high capacity with a KD of 1.12 +/- 0.49 microM for saline- and 1.24 +/- 0.43 microM for ET-treated rats (p greater than 0.05). Bmax values for high affinity sites were 1.01 +/- 0.18 fmol.mg-1 protein for saline- and 1.02 +/- 0.54 (same units) for ET-treated rats (p greater than 0.05). There was a significant difference (p less than 0.01) between the Bmax values for low affinity sites in saline- (675 +/- 332 fmol.mg-1 protein) and ET-treated rats (12 +/- 1, same units). This decrease in the amount of PGF2 alpha low affinity high capacity binding sites may underlie the depression of the PGF2 alpha stimulatory effect on hepatic gluconeogenesis induced by non-lethal, chronic ET treatment of rats, recently described by us (9).     

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)     

Localization of prostaglandin F2 alpha inhibition of lipoprotein use by bovine luteal cells.

Prostaglandin F2 alpha (PGF2 alpha) inhibits lipoprotein-stimulated progesterone production by bovine luteal cells in vitro and the objective of this study was to localize the site of action of PGF2 alpha. Cultured bovine luteal cells were treated with PGF2 alpha for seven days, and then with either lipoproteins or 25-hydroxycholesterol in the presence of aminoglutethimide (which inhibits cholesterol side-chain cleavage) for the final 48 h. The effects of PGF2 alpha on progesterone production, cellular cholesterol content, mitochondrial cholesterol content and cholesterol side-chain cleavage activity were determined. As expected, PGF2 alpha inhibited (P less than 0.05) lipoprotein-stimulated progesterone production. However, PGF2 alpha did not inhibit low-density lipoprotein-stimulated, or high density lipoprotein-stimulated, increases in cellular cholesterol (P less than 0.05) or inhibit lipoprotein-induced increases in mitochondrial cholesterol content (P less than 0.05). Additionally, cholesterol content of mitochondria increased (P less than 0.05) in the presence of PGF2 alpha alone. To determine if the PGF2 alpha-induced inhibition of steroidogenesis occurred at, or after, the side-chain cleavage reaction, we treated cells with the readily diffusable sterol, 25-hydroxycholesterol. Prostaglandin F2 alpha did not inhibit 25-hydroxycholesterol-stimulated progesterone production (P less than 0.05). Prostaglandin F2 alpha may therefore exert its luteolytic effect at a site after cholesterol transport to the mitochondria but before cholesterol side-chain cleavage.     

Role of liquid membrane phenomenon in the biological actions of prostaglandins: studies on prostaglandin E1 and prostaglandin F2 alpha

Modifications in the transport of relevant permeants to their respective sites of action due to the liquid membranes formed by prostaglandins in association with lecithin and cholesterol have been discussed in the light of biological actions of prostaglandins. It has been shown that the phenomenon of liquid membrane formation is likely to make a significant contribution to the biological actions.     

Glucose dependent stimulation by prostaglandin D2 of glucagon and insulin in perfused rat pancreas

Effects of prostaglandin D2 on pancreatic islet function in perfused rat pancreas were examined in comparison with those of prostaglandin E2, which has hitherto been suggested to be a modifier of pancreatic hormone release. In the presence of 2.8 mM glucose, only glucagon release was strongly stimulated by 14 microM of prostaglandin D2, while release of both glucagon and insulin was augmented by 14 microM of prostaglandin E2. When the glucose concentration was elevated to 11.2 mM, insulin release was accelerated by 14 microM of prostaglandin D2 but there was no effect upon glucagon release. Again, release of both glucagon and insulin was augmented by 14 microM of prostaglandin E2 in the presence of 11.2 mM of glucose. The regulation of glucagon and insulin release through prostaglandin D2 is apparently adapted to glycemic changes, and may be a physiological modulator of pancreatic islet function.     

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)     

Prostaglandin responses in isolated perfused rat liver: Ca2+ and K+ fluxes, hemodynamic and metabolic effects

Addition of prostaglandin F2 alpha and prostaglandin E2 to isolated perfused rat liver led to a dose-dependent, transient net Ca2+ release, which was completed within 3 min. Withdrawal of the prostaglandins resulted in a Ca2+ re-uptake over a period of about 10 min. Simultaneously, these prostaglandins induced an increase of portal pressure, stimulated hepatic glucose output and 14CO2 production from [1-14C]glutamate and led to K+ movements across the hepatocyte plasma membrane similar to those observed with other Ca2+-mobilizing agents. With prostaglandin F2 alpha there was a close correlation between the net Ca2+ release and the maximal rate of initial net K+ uptake by the liver (linear regression coefficient r = 0.902; n = 20). Prostaglandin F2 alpha was more effective than prostaglandin E2 or D2. Because prostaglandins are known to be produced by hepatic non-parenchymal cells during stimulation by phagocytosis or by addition of extracellular ATP or UTP, these data suggest an interaction between non-parenchymal and parenchymal liver cells and point to a modulating role of prostaglandins in hepatic metabolism and microcirculation, which is mediated by Ca2+-mobilizing mechanisms.     

Correlation between endogenous noradrenaline and glucose released from the liver upon hepatic sympathetic nerve stimulation in anesthetized dogs

The metabolic role of neurally released noradrenaline (NA) was studied in the liver of anesthetized dogs. Sustained stimulation with various frequencies was directly applied on the anterior plexus of hepatic nerves. Stimulation-induced changes in plasma concentrations of endogenous catecholamines in hepatic venous blood were determined in correlation with concomitant changes in those of glucose (GL). Mean basal values for hepatic venous NA, adrenaline, dopamine, and GL were 0.062, 0.022, 0.032 ng/mL, and 97.9 mg%, respectively. Among these catecholamines, NA was the only one being released significantly during stimulation. While hepatic venous NA increased rapidly during stimulation, being maximum within 3 min, hepatic venous GL increased gradually, reaching a maximum value 5 min after the onset of stimulation. A highly significant correlation (r = 0.90, P less than 0.001) was found between changes in hepatic venous NA and GL concentrations observed during stimulation at various frequencies (2-16 Hz). However, hepatic vasoconstricting responses to stimulation were not correlated with increased hepatic venous GL. An alpha-blockade with phentolamine (2 mg/kg, iv) resulted in diminished release of GL by approximately 50% (P less than 0.05) and reduced hepatic arterial vasoconstriction by approximately 47% (P less than 0.01) upon stimulation (8 Hz, 5 min), even though NA release was markedly enhanced. We conclude that in the dog, NA is the sole catecholamine released within the liver in response to direct hepatic nerve stimulation, and NA thus released mediates the hepatic glycogenolysis via alpha-adrenoceptors.     

Stimulation by prostaglandin F2 alpha of phosphatidic acid-phosphatidylinositol turnover in rat luteal cells

Prostaglandin F2 alpha (PGF 2 alpha) causes a rapid and marked increase of [32P]-orthophosphate incorporation into phosphatidylinositol (PI) and phosphatidic acid (PA) in rat luteal cells in culture. The incorporation of radioactivity is increased as early as 2 and 5 min after PGF 2 alpha addition into PA and PI, respectively, and by 10 min has reached a 2-fold stimulation over control in both lipid moieties. The labeling of other phospholipids is not affected. PGF 2 alpha exerts its stimulatory effect at an ED50 value of approximately 200 and 60 nM on PI and PA labeling, respectively. By contrast, human chorionic gonadotropin has no effect alone and does not interfere with the PGF 2 alpha-induced stimulation of PA-PI labeling. The striking similarity between the effects of PGF 2 alpha and LHRH on PA-PI labeling suggests that the two agents may exert their direct action on the corpus luteum via a common intracellular mechanism involving acidic phospholipid metabolism.     

Signal transduction in endotoxin-stimulated synthesis of TNF-alpha and prostaglandin E2 by rat Kupffer cells. Role of extracellular calcium ions and protein kinase C.

Endotoxin is a well established elicitor of cytokine production in mononuclear cells. Nevertheless, the path of signal transduction between the crucial contact of the cells with endotoxin (lipopolysaccharide) and the synthesis and release of the mediators is yet poorly understood. In particular, the involvement of Ca2+ and protein kinase C in this process is still a matter of controversy. Here, it will be demonstrated that removal of extracellular Ca2+ by EGTA does not have a significant effect on the endotoxin-stimulated production of tumor necrosis factor-alpha (TNF-alpha) and on total protein synthesis in rat Kupffer cells. However, the release of prostaglandin E2 could not be raised above the basal level under these conditions. Treatment with inhibitors of protein kinase C such as the isoquinoline derivative, H-7, or staurosporin is without influence on TNF-alpha synthesis. The depletion of protein kinase C through preincubation of rat Kupffer cells with phorbol 12-myristate 13-acetate for 24 h was also without effect on TNF-alpha production. The effectiveness of these inhibitors under the conditions used was ascertained by measurement of the O2- release from the same cell batches. Superoxide production known as protein kinase C-dependent in Kupffer cells (Dieter et al. (1986) Eur. J. Biochem. 86, 451-457) was suppressed in a dose-dependent manner by staurosporin or after prolonged pretreatment with the phorbol ester. H-7 decreased superoxide production only slightly in high doses that severely harm the Kupffer cells. Prostaglandin E2 release, although clearly protein-kinase C-dependent in phagocytosing rat Kupffer cells, is not decreased following exposure to lipopolysaccharide in the presence of protein kinase C inhibitors.