Role of phosphoinositides in the regulation of liver function

PAF effects on transmembrane signaling pathways in rat Kupffer cells

Platelet activating factor (PAF) was found to stimulate the metabolism of inositol phospholipids via deacylation and phospholipase C in Kupffer cells, the resident macrophages in liver. PAF-induced phosphoinositide metabolism occurred in two phases. Within seconds after stimulation, in the absence of extracellular Ca++, platelet activating factor caused the phosphodiester hydrolysis of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate with the release of inositol 1,4,5-trisphosphate and inositol 1,4-bisphosphate. This was followed by an extracellular Ca(++)-dependent release of glycerophosphoinositol, inositol monophosphates and inositol bisphosphates. Various Ca(++)-mobilizing agonists failed to evoke hydrolysis of phosphoinositides. Platelet activating factor also stimulated the synthesis and release of prostaglandins from these cells. Platelet activating factor-stimulated phosphodiester metabolism of phosphoinositides and prostaglandin synthesis was inhibited by treatment with pertussis toxin and cholera toxin. Pertussis toxin also inhibited platelet activating factor-induced glycerophosphoinositol release. Cholera toxin, in contrast, stimulated platelet activating factor-induced glycerophosphoinositol release and prostaglandin synthesis and synergistically stimulated the effect of platelet activating factor on these processes. The results suggest that platelet activating factor-induced metabolism in the Kupffer cells occurs via specific receptors and may be mediated through the activation of different G-proteins.

Activation of phosphoinositide-specific phospholipase C delta from rat liver by polyamines and basic proteins

Phospholipase C from rat liver with a molecular weight of 87,000 (PLC delta) is stimulated by polyamines, basic proteins, and basic polyamino acids. The activation occurs in both the presence and the absence of detergents. Half-maximum activation by spermine is observed at 0.15 mM, with optimum effects between 0.2 and 0.5 mM. Spermine inhibits above 0.5 mM. Half-maximum activation by spermidine and putrescine is observed at 0.9 and 6 mM, respectively, with optimum effects at 2 and 5 mM, respectively. These polyamines also inhibit at higher concentrations. Neomycin activates the enzyme with an optimum concentration of 10 microM, but maximum activation is less than with polyamines. Half-maximum activation by histone 2B occurs at 0.5 micrograms/ml (36 nM), with maximum stimulation at 1.5 micrograms/ml. Other histones, protamine, melittin, poly-L-ornithine, poly-L-lysine, poly-D-lysine, and poly-L-arginine, activate optimally at 3-10 micrograms/ml. Myelin basic protein and lysozyme activate optimally at 50-100 micrograms/ml. Typical activations are three- to eightfold, but under some conditions the enzyme shows little or no activity in the absence of basic activators. The basic activators lower the salt concentration required for maximal activity. In the case of the detergent-micelle assay, histone shifts the optimum NaCl concentration from 350 to 200 mM for PIP2, from 260 to 100 mM for PIP, and from 150 to 0 mM for PI. Histone potentiates the activation by Ca2+, but does not shift the optimum Ca2+ concentration. The optimum salt and Ca2+ concentrations are linked, such that a decrease in the concentration of one decreases the optimum concentration of the other. Activation by histone is diminished by MgCl2 in a concentration-dependent manner.

Motility of bile canaliculi in the living animal: implications for bile flow

Modern fluorescence microscopic techniques were used to image the bile canalicular system in the intact rat liver, in vivo. By combining the use of sodium fluorescein secretion into bile, with digitally enhanced fluorescence microscopy and time-lapse video, it was possible to capture and record the canalicular motility events that accompany the secretion of bile in life. Active bile canalicular contractions were found predominantly in zone 1 (periportal) hepatocytes of the liver. The contractile movements were repetitive, forceful, and appeared unidirectional moving bile in a direction towards the portal bile ducts. Contractions were not seen in the network of canaliculi on the surface of the liver. Cytochalasin B administration resulted in reduced canalicular motility, progressive dilation of zone 1 canaliculi, and impairment of bile flow. Canalicular dilations invariably involved the branch points of the canalicular network. The findings add substantively to previous in vitro studies using couplets, and suggest that canalicular contractions contribute physiologically to bile flow in the liver.

Nafenopin, a hypolipidemic and non-genotoxic hepatocarcinogen increases intracellular calcium and transiently decreases intracellular pH in hepatocytes without generation of inositol phosphates

Addition of nafenopin (30-300 microM to 45Ca2+ preloaded cultured hepatocytes caused a rapid and concentration-dependent increase in 45Ca2+ efflux in a manner similar to vasopressin, as evidenced by the loss of radioactivity from the cells. In contrast to vasopressin, addition of nafenopin to [3H]inositol prelabelled hepatocytes in culture did not increase [3H]inositol phosphate production. When added simultaneously with vasopressin, nafenopin inhibited the vasopressin-stimulated [3H]inositol phosphate production. In hepatocyte suspensions isolated from rats treated for 1 week with a carcinogenic dose of nafenopin (1000 ppm in their daily food) the incorporation of [3H]inositol into the phosphoinositide fraction, particularly phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, was much less than that in hepatocytes isolated from untreated rats. The vasopressin-stimulated [3H]inositol phosphate production was also decreased. Experiments with hepatocyte suspensions preloaded with Ca2+ or pH sensitive fluorescent indicators demonstrated that addition of nafenopin caused an increase in intracellular free Ca2+ and transient acidification of the cells. The increase in [Ca2+]i was decreased by only about 25% when extracellular calcium was removed indicating that nafenopin mainly mobilizes Ca2+ from intracellular stores. The recovery to basal pH was amiloride-sensitive indicating the importance of Na+/H+ exchange in pH recovery after intracellular acidification. Amiloride also inhibited DNA synthesis induced by nafenopin and by epidermal growth factor in cultured hepatocytes; but this effect occurred concomitantly with inhibition of basal DNA synthesis. We suggest that hepatic Ca2+ mobilization induced by nafenopin may play an important role in the mechanism by which nafenopin exerts its physiological as well as its tumour promotive activity upon chronic treatment with carcinogenic doses.

Reduced high-affinity alpha 1-adrenoceptors in liver of senescent rats: implications of assessment at various temperatures

1. We investigated the changes occurring as a result of aging in alpha 1-adrenoceptors in the livers of Fisher 344 females rats. For comparison, we also measured beta-adrenoceptors in this tissue. Three age groups were studied, including young adults (aged 6 months), mature adults (aged 16 months) and senescent animals (aged 25 months). 2. The density of alpha 1-receptors was measured by use of [3H]-prazosin and was found to be reduced 39% (P less than 0.01) at 25 months compared with 6 months. The percentage of alpha 1-receptors displaying high affinity for adrenaline was also reduced from 85.6% at 6 months to 51.6% at 25 months (P less than 0.02). 3. In contrast, the density of beta-receptors, which was measured with [125I]-iodocyanopindolol, was increased 104% between 6 months and 25 months. The affinity of both alpha 1- and beta-adrenoceptors for antagonists was unchanged with age. 4. We found that receptor affinity for agonists may be measured accurately in binding studies conducted at 4 degrees C or 25 degrees C, but that the apparent affinity for agonist was artifactually reduced in studies conducted at 37 degrees C. This effect is poorly reversible, in that reduced agonist-affinity is also observed in tissue which has been incubated at 37 degrees C and then cooled to 4 degrees C before performing the binding studies. 5. It is concluded that liver alpha 1-adrenoceptor function is reduced and beta-adrenoceptor function increased in senescence.

Lack of metabolic effects of cholecystokinin on hepatocytes

We previously reported that the liver was the major organ that extracts small, biologically active, circulating forms of cholecystokinin. Although our work indicated extensive degradation of cholecystokinin extracted from plasma during its transit across the hepatocyte, it was unclear whether cholecystokinin might also have a physiological effect on this cell before its intracellular degradation. Therefore we tested the hypothesis that cholecystokinin has a direct biological effect on hepatocytes. Using freshly isolated or cultured hepatocytes, we studied whether cholecystokinin-octapeptide alters protein synthesis, affects amino acid transport or influences cytosolic free calcium concentrations. Using liver slices, we also determined the effect of cholecystokinin-octapeptide on cyclic nucleotide levels. Cholecystokinin-octapeptide, up to a concentration of 1 mumol/L, had no effect on the incorporation of radiolabeled amino acids into total hepatocyte protein; in contrast, comparable molar amounts of insulin stimulated protein synthesis by as much as 37% (ED50 = 1.5 x 10(-10) mol/L). Although insulin and glucagon stimulated the transport into hepatocytes of 14C-alpha-aminoisobutyric acid, a nonmetabolizable amino acid analog, cholecystokinin-octapeptide had no affect Cholecystokinin-octapeptide also did not affect either the concentration of calcium in individual hepatocytes, as measured by digitized video microscopy using Fura-2, or the levels of cyclic AMP or cyclic GMP in liver slices. Our results show that cholecystokinin has no effect on protein synthesis, on amino acid transport or on hepatocyte calcium and cyclic nucleotide levels. These and our previous data suggest that the primary outcome of hepatic extraction of cholecystokinin is hormone degradation.

Bile acids mobilise internal Ca2+ independently of external Ca2+ in rat hepatocytes

In the present study, we investigated the possible role of external Ca2+ in the rise of the cytosolic Ca+ concentration induced by the monohydroxy bile acid taurolithocholate in isolated rat liver cells. The results showed that: (a) the bile acid promotes the same dose-dependent increase in the cytosolic Ca+ concentration (half-maximal effect at 23 microM) in hepatocytes incubated in the presence of 1.2 mM Ca2+ or 6 microM Ca2+; (b) taurolithocholate is able to activate the Ca2(+)-dependent glycogen phosphorylase a by 6.3-fold and 6.0-fold in high and low Ca2+ media, respectively; (c) [14C]taurolithocholate influx is not affected by external Ca2+, and 45Ca2+ influx is not altered by taurolithocholate. These results establish that the effects of taurolithocholate on cell Ca2+ do not require extracellular Ca2+ and are consistent with the view that monohydroxy bile acids primarily release Ca2+ from the endoplasmic reticulum in the liver.

Surface dipole moments of lipids at the argon-water interface. Similarities among glycerol-ester-based lipids

Surface potential-surface pressure-area isotherms at the argon-buffer interface have been determined for 38 lipid species comprising 19 chemical classes. These lipids all exhibited a finite range of liquid-expanded surface pressure-area behavior. For most species, the linearity of surface potential with reciprocal area was excellent, but nonzero intercepts were obtained. This suggests a lipid-induced reorganization of interfacial water molecules which is area independent. The linearity of the data permits calculation of the surface dipole moment, mu perpendicular, for each lipid. The values of mu perpendicular for a series of oleoyl-containing acylglycerols, dioleoyl phosphatidylcholine, and dioleoyl phosphatidylethanolamine exhibit acylglycerol ester group mu perpendicular's which are generally consistent with known conformational properties of such lipids. The values are 132 mD for the perpendicular oleoyl glycerol-ester group and 252 mD for that in the kinked-chain conformation. Comparison of mu perpendicular's calculated using these values with homologues confirms the approximate independence of mu perpendicular from aliphatic chain length and permits identification of exceptions with possible conformational or orientational differences. Notably, diphytanoyl phosphatidylcholine shows a 45% larger mu perpendicular than predicted. Differences in mu perpendicular among lipid classes allow estimation of the electrical consequences of lipid metabolism and exchange. Calculations show that reactions such as the generation of 1,2-diacylglycerol from diacyl glycerophosphocholine or diacyl glycerophosphoinositol should produce surface potential changes of -127 and +42 mV, respectively. Thus, the two phospholipids are not simply alternative sources of diacylglycerol with respect to processes dependent on surface potential.

Regulation of inositol 1,4,5-trisphosphate-induced Ca2+ release. II. Effect of cAMP-dependent protein kinase

The effect of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) on Ca2+ loading, inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release, and [3H]IP3 binding of canine cerebellar membrane fractions was investigated. PKA in the presence of cAMP and the catalytic subunit of PKA did not change Ca2+ loading yet increased the extent of IP3-induced Ca2+ release by approximately 35%. Hill plot analysis indicated that the catalytic subunit of PKA increased the apparent Michaelis constant of IP3-induced Ca2+ release twofold, from 0.3 to 0.7 microM IP3. The protein kinase inhibitor reversed these changes. cAMP affected neither Ca2+ loading nor IP3-induced Ca2+ release. The catalytic subunit of PKA did not appreciably affect the maximum binding and dissociation constant of [3H]IP3 binding, as judged by Scatchard analysis. Thus the catalytic subunit of PKA influences the opening of Ca2+ channels by IP3 without interfering with the binding of IP3 to its receptor sites.

Kinetics of the conductance evoked by noradrenaline, inositol trisphosphate or Ca2+ in guinea-pig isolated hepatocytes

1. Guinea-pig hepatocytes respond to noradrenaline (NA, 5-10 microM) with a large membrane conductance increase to K+ and Cl-. The response has a long initial delay (range 2-30 s). Following the delay, the K+ conductance (studied in Cl(-)-free solutions) rises quickly to a peak in 1-2 s and is maintained in the continued presence of NA, though often with superimposed oscillations of conductance. The roles of intracellular Ca2+ and D-myo-inositol 1,4,5-trisphosphate (InsP3) in this complex response have been investigated by rapid photolytic release of intracellular Ca2+ (from Nitr5-Ca2+ buffers) or InsP3 from 'caged' InsP3. 2. A rapid increase of intracellular [Ca2+] produced an immediate membrane conductance increase which rose approximately exponentially to a new steady level, consistent with a direct activation of Ca2(+)-dependent ion channels. 3. Following a pulse of InsP3, conductance rose after a brief delay (range 70-1500 ms) which was shortest at high [InsP3] or if the initial cytosolic [Ca2+] had been raised above normal levels. The maximum conductance produced by InsP3 was similar in each cell to the peak recorded with NA and could be evoked by InsP3 concentrations of 0.5-1 microM. 4. The rates of rise of conductance increased with InsP3 concentration in the range 0.25-12.5 microM (range 10-90%, rise times 90-1000 ms), indicating that InsP3-evoked Ca2(+)-efflux from stores increases with InsP3 concentration in this range. 5. Photochemically released InsP3 and Ca2+ activate at physiological concentrations the same membrane conductances as NA. The results indicate that the long initial delay in NA action occurs prior to or during generation of InsP3. The mechanism of the delay and the subsequent apparently all-or-none conductance increase during NA action are discussed in terms of the high co-operativity in InsP3 and Ca2+ actions and an additional positive feedback step. 6. Evidence was found of a negative interaction between [Ca2+] and InsP3-evoked Ca2+ release. The time course of the recovery of InsP3-evoked Ca2+ release following a rise of cytosolic [Ca2+] suggests that this interaction may be important in regulating oscillatory responses of [Ca2+] during hormonal stimulation of guinea-pig hepatocytes.

G proteins and the mechanism of action of hormones, neurotransmitters, and autocrine and paracrine regulatory factors

Signal transduction by G proteins is a fundamental and widespread mechanism used by a wide variety of hormones, neurotransmitters, and autocrine and paracrine factors to regulate cellular functions. G proteins modulate not only cAMP formation, but also intracellular Ca2+ mobilization, arachidonic acid release, and, very importantly, membrane potential. The mechanism by which G proteins are activated provides for amplification, reversal of action, and continued monitoring of incoming signals. Not all G proteins are known and some are known but their functions are still unknown. More G proteins and more effector functions affected by them will surely be found. We discuss these features of signal transduction by G proteins.

Activity and distribution of protein kinase C in liver during the acute-phase response

Activity and subcellular distribution of protein kinase C were estimated in liver cytosol and membrane fractions of rats carrying a turpentine-induced inflammation. Protein kinase C activity increases significantly 8 h after treatment in the membrane fraction, with concurrent reduction in the cytosol; 10 h after treatment the membrane-associated activity returns to normal, without concomitant recovery of that detected in the cytosol. The specific binding of phorbol dibutyrate to the liver membrane fraction increases but overall the effect is less evident and delayed in time. The changes are associated to alterations in the phosphorylation pattern of some liver proteins. Liver protein kinase C activity and intracellular distribution seem to be affected by a treatment which is known to induce an acute-phase response in the liver cells.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.

Protein kinase C agonists inhibit bile secretion independently of effects on the microcirculation in the isolated perfused rat liver

The role of hormones in the regulation of bile secretion is not known; however vasoactive agents, which act via the phosphoinositide signal transduction pathway, may mediate changes in bile flow by altering the hepatic microvasculature. We therefore examined the effects of phorbol esters and diacylglycerol, agonists of the protein kinase C branch of the phosphoinositide cascade, on perfusion pressure and bile flow in a single-pass, hemoglobin-free, isolated perfused rat liver system with constant perfusate flow. The active phorbol ester, 12,13-phorbol dibutyrate, produced a dose-dependent (maximal effect at 10(-6) M), sustained and reversible decrease in bile flow from 1.09 +/- 0.18 to 0.61 +/- 0.09 microliters per min per gm liver (37.2 +/- 5.9%) while simultaneously increasing perfusion pressure from 12.3 +/- 0.7 to 21.5 +/- 2.5 cm H2O (74.0 +/- 4.3%). Both effects were inhibited by the synthetic protein kinase C antagonist H-7. 1,2-Dioctanoyl-sn-glycerol, a diacylglycerol, produced changes in bile flow and perfusion pressure that were similar to, but more marked than, those caused by 12,13-phorbol dibutyrate, whereas the inactive phorbol ester 4 alpha-phorbol didecanoate and the vehicle dimethyl sulfoxide had no effects on either parameter. 12,13-Phorbol dibutyrate infusion resulted in reversible decreases in oxygen consumption (23.3%) and a reversible vascular redistribution of trypan blue dye but did not alter hepatic venous effluent concentrations of K+.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbachol and histamine stimulation of guanine-nucleotide-dependent phosphoinositide hydrolysis in rat brain cortical membranes

Guanine nucleotides have been shown to stimulate phosphoinositide breakdown in brain membranes, but no potentiation of such an effect by agonist was demonstrated. We have studied the effect of carbachol and histamine on guanosine 5'-[gamma-thio]triphosphate (GTP[S]) stimulation of inositol phosphates formation in [3H]inositol-labelled rat brain cortical membranes. In this preparation, GTP[S] enhancement of phosphoinositide hydrolysis required the presence of MgATP and low Ca2+ concentration (100 nM). Carbachol potentiation of the GTP[S] effect was only observed when 1 mM-deoxycholate was also added. Under these conditions, stimulated production of [3H]inositol phosphates was linear for at least 15 min, and [3H]inositol bisphosphate [( 3H]IP2) accounted for approx. 80%, whereas the amount of [3H]inositol trisphosphate [( 3H]IP3) was very low. Stimulation by GTP[S] was concentration-dependent (half-maximal effect at 0.86 microM), and its maximal effect (815% over basal) was increased by 1 mM-carbachol (1.9-fold) and -histamine (1.7-fold). Both agonists decreased the slope index of the GTP[S] concentration/effect curve to values lower than unity, suggesting the appearance of some heterogeneity in the population of guanine-nucleotide-binding proteins (G-proteins) involved. The carbachol and histamine effects were also concentration-dependent, and were inhibited by atropine and mepyramine respectively. Fluoroaluminate stimulated phosphoinositide hydrolysis to a higher extent than GTP[S] plus carbachol, and these stimulations were not additive, indicating that the same polyphosphoinositide phospholipase C-coupled G-protein mediates both effects.

Physiology of the male reproductive system: endocrine, paracrine and autocrine regulation

This presentation reviews the male reproductive system, concentrating on newer advances in our knowledge of its physiology, biochemistry, and regulation, and introduces the topic of male reproductive toxicology. GnRH is the hypothalamic peptide responsible for the stimulation of LH and FSH release from the pituitary. It is synthesized as a pro-hormone, processed in the hypothalamus and released into the portal system in a pulsatile fashion. The timing of these pulses is critical to the release of LH and FSH into the general circulation. While LH and FSH are the main trophic hormones for the testis, we now realize the importance of not only endocrine control, but also of paracrine and autocrine regulation. Specifically, the local control of Leydig cells, Sertoli cells, and germ cells appears to be modulated by numerous growth factors and local regulators arising from within the testis. This point is emphasized both during a discussion of the interaction of the various cell types in the testis and during a discussion of spermatogenesis, where techniques which show stage-specific secretions are highlighted. Newest advances in the mechanism of action of steroidal and peptide hormones are also emphasized with special reference to the possible interaction between toxicants and endocrine control of the reproductive system. This update of the reproductive system "sets the stage" for an in-depth examination of the site and mechanism of action of reproductive toxicants.

Decreased loss of liver adenosine triphosphate during hypothermic preservation in rats pretreated with glucose: implications for organ donor management

Recent studies of human donor livers indicate an association between ex vivo hepatocellular adenosine triphosphate and posttransplant graft function. To test the hypothesis that prior glucose loading of donor liver would optimize its adenosine triphosphate production and adenylate energy charge during ex vivo organ preservation, adult male rats were randomized to receive either intravenous dextrose or saline for 44 h. After this infusion, a liver lobe was exposed and freeze-clamped (time 0). The remaining liver was quickly flushed, excised, and stored in Collins' II solution at 2 degrees C for 8 h. Additional lobes were freeze-clamped at 1, 4, and 8 h. Liver adenosine triphosphate, total nucleoside triphosphates, and energy charge losses were significantly reduced in the dextrose-treated rats in comparison with saline-treated rats during the first 4 h of preservation. Although the livers from rats receiving intravenous dextrose were able to generate lactate, their glycogen stores were not utilized appreciably, suggesting that exogenous glucose served as a substrate for anaerobic glycolysis. Unesterified choline levels of the fasted rat livers were significantly higher than those from the rats receiving intravenous dextrose by the first hour, indicative of increased membrane breakdown. These results indicate that prior infusion of glucose enhances the capacity of the ex vivo liver, presumably through the induction and stabilization of key glycolytic enzymes, to anaerobically generate adenosine triphosphate. Administration of glucose to liver donors before organ procurement may improve post-transplant graft function by reducing the loss of hepatocellular energy, retarding membrane damage, and fostering glycogen storage for use in the early postoperative period.