Inositol trisphosphate and diacylglycerol as intracellular second messengers in liver

Egg Yolk Fat Deposition Is Regulated by Diacylglycerol and Ceramide Enriched by Adipocytokine Signaling Pathway in Laying Hens

The mechanism which regulates differential fat deposition in egg yolk from the indigenous breeds and commercial laying hens is still unclear. In this research, Chinese indigenous Huainan Partridge chickens and Nongda III commercial laying hens were used for egg collection and liver sampling. The weight of eggs and yolk were recorded. Yolk fatty acids were determined by gas chromatography-mass spectrometry. Lipid metabolites in the liver were detected by liquid chromatography-mass spectrometry. Yolk weight, yolk ratio and yolk fat ratio exhibited higher in the Huainan Partridge chicken than that of the Nongda III. Compared to the Nongda III, the content of total saturated fatty acid was lower, while the unsaturated fatty acid was higher in the yolk of the Huainan Partridge chicken. Metabolites of phosphatidylinositol and phosphatidylserine from glycerolphospholipids, and metabolites of diacylglycerol from glycerolipids showed higher enrichment in the Huainan Partridge chicken than that of the Nongda III, which promoted the activation of the adipocytokine signaling pathway. However, metabolites of phosphatidic acid, phosphatidylcholine, phosphatidylethanolamine and lysophosphatidylcholine from glycerol phospholipids, and metabolites of triacylglycerol from glycerolipids showed lower enrichment in the Huainan Partridge chicken than that of the Nongda III. The high level of yolk fat deposition in the Huainan Partridge chicken is regulated by the activation of the adipocytokine signaling pathway which can promote the accumulation of diacylglycerol and ceramide in the liver.

Quantitative live-cell imaging of GPCR downstream signaling dynamics

G-protein-coupled receptors (GPCRs) play an important role in sensing various extracellular stimuli, such as neurotransmitters, hormones, and tastants, and transducing the input information into the cell. While the human genome encodes more than 800 GPCR genes, only four Gα-proteins (Gαs, Gαi/o, Gαq/11, and Gα12/13) are known to couple with GPCRs. It remains unclear how such divergent GPCR information is translated into the downstream G-protein signaling dynamics. To answer this question, we report a live-cell fluorescence imaging system for monitoring GPCR downstream signaling dynamics. Genetically encoded biosensors for cAMP, Ca2+, RhoA, and ERK were selected as markers for GPCR downstream signaling, and were stably expressed in HeLa cells. GPCR was further transiently overexpressed in the cells. As a proof-of-concept, we visualized GPCR signaling dynamics of 5 dopamine receptors and 12 serotonin receptors, and found heterogeneity between GPCRs and between cells. Even when the same Gα proteins were known to be coupled, the patterns of dynamics in GPCR downstream signaling, including the signal strength and duration, were substantially distinct among GPCRs. These results suggest the importance of dynamical encoding in GPCR signaling.

Intake of Bifidobacterium lactis Probio-M8 fermented milk protects against alcoholic liver disease

Alcoholic liver disease (ALD) is a liver disease caused by long-term heavy drinking, which is characterized by increased inflammation and oxidative stress in the liver and gut dysbiosis. The purpose of this study was to investigate the protective effect of administering ordinary and probiotic- (containing the Bifidobacterium animalis ssp. lactis Probio-M8 strain; M8) fermented milk to rats. Several biochemical parameters and the fecal metagenomes were monitored before (d 0) and after (d 42) the intervention. Our results confirmed that alcohol could cause significant changes in the liver levels of the proinflammatory cytokine IL-1β, antioxidation indicators, and liver function-related indicators; meanwhile, the gut bacterial and viral microbiota were disrupted with significant reduction in microbial diversity and richness. Feeding the rats with Probio-M8-fermented milk effectively maintained the gut microbiota stability, reduced liver inflammation and oxidative stress, and mitigated liver damages in ALD. Moreover, the Probio-M8-fermented milk reversed alcohol-induced dysbiosis by restoring the gut microbiota diversity, richness, and composition. Four predicted fecal metabolites (inositol, tryptophan, cortisol, and vitamin K2) increased after the intervention, which might help regulate liver metabolism and alleviate ALD-related symptoms. In short, our data supported that consuming Probio-M8-fermented milk effectively mitigated ALD. The protective effect against ALD could be related to changes in the gut microbiome after probiotic-fermented milk consumption. However, such observation and the causal relationship among probiotic milk consumption, changes in gut microbiome, and disease alleviation would still need to be further confirmed. Nevertheless, this study has shown in a rat model that consuming probiotic-fermented milk could protect against ALD.

Montelukast: The New Therapeutic Option for the Treatment of Epilepsy

Currently, there is no definitive cure for epilepsy. The available medications relieve symptoms and reduce seizure attacks. The major challenge with the available antiepileptic medication is safety and affordability. The repurposing of montelukast for epilepsy can be an alternative medication with a better safety profile. Montelukast is a leukotriene receptor antagonist that binds to the cysteinyl leukotrienes (CysLT) receptors used in the treatment of bronchial asthma and seasonal allergies. Emerging evidence suggests that montelukast's anti-inflammatory effect can help to maintain BBB integrity. The drug has also neuroprotective and anti-oxidative activities to reduce seizure incidence and epilepsy. The present review summarizes the neuropharmacological actions of montelukast in epilepsy with an emphasis on the recent findings associated with CysLT and cell-specific effects.

Hepatic Fatty Acid Balance and Hepatic Fat Content in Humans With Severe Obesity

OBJECTIVE

Nonalcoholic fatty liver disease can lead to hepatic inflammation/damage. Understanding the physiological mechanisms that contribute to excess hepatic lipid accumulation may help identify effective treatments.

DESIGN

We recruited 25 nondiabetic patients with severe obesity scheduled for bariatric surgery. To evaluate liver export of triglyceride fatty acids, we measured very-low-density lipoprotein (VLDL)-triglyceride secretion rates the day prior to surgery using an infusion of autologous [1-14C]triolein-labeled VLDL particles. Ketone body response to fasting and intrahepatic long-chain acylcarnitine concentrations were used as indices of hepatic fatty acid oxidation. We measured intraoperative hepatic uptake rates of plasma free fatty acids using a continuous infusion of [U-13C]palmitate, combined with a bolus dose of [9,10-3H]palmitate and carefully timed liver biopsies. Total intrahepatic lipids were measured in liver biopsy samples to determine fatty liver status. The hepatic concentrations and enrichment from [U-13C]palmitate in diacylglycerols, sphingolipids, and acyl-carnitines were measured using liquid chromatography/tandem mass spectrometry.

RESULTS

Among study participants with fatty liver disease, intrahepatic lipid was negatively correlated with VLDL-triglyceride secretion rates (r = -0.92, P = 0.01) but unrelated to hepatic free fatty acid uptake or indices of hepatic fatty acid oxidation. VLDL-triglyceride secretion rates were positively correlated with hepatic concentrations of saturated diacylglycerol (r = 0.46, P = 0.02) and sphingosine-1-phosphate (r = 0.44, P = 0.03).

CONCLUSION

We conclude that in nondiabetic humans with severe obesity, excess intrahepatic lipid is associated with limited export of triglyceride in VLDL particles rather than increased uptake of systemic free fatty acids.

Synthesis of Biotinylated Inositol Hexakisphosphate To Study DNA Double-Strand Break Repair and Affinity Capture of IP6-Binding Proteins

Inositol hexakisphosphate (IP6) is a soluble inositol polyphosphate, which is abundant in mammalian cells. Despite the participation of IP6 in critical cellular functions, few IP6-binding proteins have been characterized. We report on the synthesis, characterization, and application of biotin-labeled IP6 (IP6-biotin), which has biotin attached at position 2 of the myo-inositol ring via an aminohexyl linker. Like natural IP6, IP6-biotin stimulated DNA ligation by nonhomologous end joining (NHEJ) in vitro. The Ku protein is a required NHEJ factor that has been shown to bind IP6. We found that IP6-biotin could affinity capture Ku and other required NHEJ factors from human cell extracts, including the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), XRCC4, and XLF. Direct binding studies with recombinant proteins show that Ku is the only NHEJ factor with affinity for IP6-biotin. DNA-PKcs, XLF, and the XRCC4:ligase IV complex interact with Ku in cell extracts and likely interact indirectly with IP6-biotin. IP6-biotin was used to tether streptavidin to Ku, which inhibited NHEJ in vitro. These proof-of-concept experiments suggest that molecules like IP6-biotin might be used to molecularly target biologically important proteins that bind IP6. IP6-biotin affinity capture experiments show that numerous proteins specifically bind IP6-biotin, including casein kinase 2, which is known to bind IP6, and nucleolin. Protein binding to IP6-biotin is selective, as IP3, IP4, and IP5 did not compete for binding of proteins to IP6-biotin. Our results document IP6-biotin as a useful tool for investigating the role of IP6 in biological systems.

Onion (Allium cepa L.) peel extract has anti-platelet effects in rat platelets

The effects of onion peel extract (OPE) in collagen (5 μg/mL)-stimulated washed rat platelet aggregation were investigated. OPE inhibited platelet aggregation via inhibition of aggregation-inducing molecules, intracellular Ca(2+) and thromboxane A2 (TXA2) by blocking cyclooxygenase-1 (COX-1) and TXA2 synthase (TXAS) activities in a dose-dependent manner. In addition, OPE elevated the formation of cyclic adenosine monophosphate (cAMP), aggregation-inhibiting molecule, but not cyclic guanosine monophosphate (cGMP). High performance liquid chromatography (HPLC) analysis of OPE revealed that OPE contains quercetin, one of the major flavonoids, which has anti-platelet effect. In conclusion, we suggest that OPE is an effective inhibitor of collagen-stimulated platelet aggregation in vitro. Therefore, it can be a promising and safe strategy for anti-cardiovascular diseases.

The impact of age-related dysregulation of the angiotensin system on mitochondrial redox balance

Aging is associated with the accumulation of various deleterious changes in cells. According to the free radical and mitochondrial theory of aging, mitochondria initiate most of the deleterious changes in aging and govern life span. The failure of mitochondrial reduction-oxidation (redox) homeostasis and the formation of excessive free radicals are tightly linked to dysregulation in the Renin Angiotensin System (RAS). A main rate-controlling step in RAS is renin, an enzyme that hydrolyzes angiotensinogen to generate angiotensin I. Angiotensin I is further converted to Angiotensin II (Ang II) by angiotensin-converting enzyme (ACE). Ang II binds with equal affinity to two main angiotensin receptors—type 1 (AT₁R) and type 2 (AT₂R). The binding of Ang II to AT₁R activates NADPH oxidase, which leads to increased generation of cytoplasmic reactive oxygen species (ROS). This Ang II-AT₁R–NADPH-ROS signal triggers the opening of mitochondrial K_ATP channels and mitochondrial ROS production in a positive feedback loop. Furthermore, RAS has been implicated in the decrease of many of ROS scavenging enzymes, thereby leading to detrimental levels of free radicals in the cell. AT₂R is less understood, but evidence supports an anti-oxidative and mitochondria-protective function for AT₂R. The overlap between age related changes in RAS and mitochondria, and the consequences of this overlap on age-related diseases are quite complex. RAS dysregulation has been implicated in many pathological conditions due to its contribution to mitochondrial dysfunction. Decreased age-related, renal and cardiac mitochondrial dysfunction was seen in patients treated with angiotensin receptor blockers. The aim of this review is to: (a) report the most recent information elucidating the role of RAS in mitochondrial redox hemostasis and (b) discuss the effect of age-related activation of RAS on generation of free radicals.

Embryonic lethality in mice lacking the nuclear factor of activated T cells 5 protein due to impaired cardiac development and function

Nuclear factor of activated T cells 5 protein (NFAT5) is thought to be important for cellular adaptation to osmotic stress by regulating the transcription of genes responsible for the synthesis or transport of organic osmolytes. It is also thought to play a role in immune function, myogenesis and cancer invasion. To better understand the function of NFAT5, we developed NFAT5 gene knockout mice. Homozygous NFAT5 null (NFAT5^−/−) mouse embryos failed to develop normally and died after 14.5 days of embryonic development (E14.5). The embryos showed peripheral edema, and abnormal heart development as indicated by thinner ventricular wall and reduced cell density at the compact and trabecular areas of myocardium. This is associated with reduced level of proliferating cell nuclear antigen and increased caspase-3 in these tissues. Cardiomyocytes from E14.5 NFAT5^−/− embryos showed a significant reduction of beating rate and abnormal Ca²⁺ signaling profile as a consequence of reduced sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) and ryanodine receptor (RyR) expressions. Expression of NFAT5 target genes, such as HSP 70 and SMIT were reduced in NFAT5^−/− cardiomyocytes. Our findings demonstrated an essential role of NFAT5 in cardiac development and Ca²⁺ signaling. Cardiac failure is most likely responsible for the peripheral edema and death of NFAT5^−/− embryos at E14.5 days.

The calcium pump of plasma membranes

The calcium pump of plasma membranes is an ATPase of the E1E2 type; that is, it forms a phosphoenzyme during the reaction cycle and is inhibited by vanadate. It differs from the Ca2+-transporting ATPase of sarcoplasmic reticulum in molecular mass, immunological properties and Ca2+/ATP stoichiometry. Its affinity for calcium, which is low in the absence of calmodulin (Km, 10-20 microM), is increased by the latter (to a Km of about 0.5 microM). The effect of calmodulin is mimicked by acidic phospholipids (including the phosphorylated products of phosphatidylinositol), long-chain polyunsaturated fatty acids, and controlled treatment with a number of proteases. The ATPase has been purified to homogeneity from a number of plasma membranes using calmodulin affinity chromatography. The purified enzyme (a single polypeptide of molecular mass 138 kDa) pumps calcium into reconstituted liposomes in exchange for protons. Controlled trypsin proteolysis has shown that about one-third of the enzyme mass can be removed without impairing calcium transport. It has also indicated that the ability to bind calmodulin and to respond to it resides in a 9 kDa sequence of the enzyme molecule. The sequence contains a 4 kDa domain that binds calmodulin, and a 5 kDa domain which is essential for the stimulation.

Inositol trisphosphate and calcium mobilization

Calcium-mobilizing agonists act by stimulating the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2) to inositol 1,4,5-trisphosphate and diacylglycerol (DG). In response to such agonists cells also produce inositol 1,3,4-trisphosphate but this isomer is unlikely to influence calcium mobilization. Application of inositol 1,4,5-trisphosphate (Ins1,4,5P3) to permeabilized cells results in a rapid release of calcium from the endoplasmic reticulum. Structure-activity studies reveal that the vicinal phosphates on the 4- and 5-positions are essential for releasing calcium whereas the phosphate on the opposite side enhances the affinity of Ins1,4,5P3 for its putative receptor. The flow of calcium across the endoplasmic reticulum appears to be electrogenic and requires an opposite flow of potassium to neutralize charge movements. Diacylglycerol, acting through protein kinase C, does not play a direct role in calcium signalling but it does modulate various aspects of the InsP3/Ca2+ pathway. The DG/protein kinase C pathway can influence both the formation and hydrolysis of PtdIns4,5P2 and can alter the responsiveness of various processes to the action of calcium. The Ins1,4,5P3/Ca2+ signal pathway functions throughout the life history of cells to regulate such diverse activities as egg maturation and fertilization, growth, secretion, metabolism, neural activity, and perhaps excitation-contraction coupling in skeletal muscle.

Sodium/myo-inositol cotransporter-1 is essential for the development and function of the peripheral nerves

Sodium/myo-inositol cotransporter-1 (SMIT-1) is one of the transporters responsible for importing myo-inositol (MI) into the cells. MI is a precursor for a family of signal transduction molecules, phosphatidylinositol, and its derivatives that regulates many cellular functions. SMIT-1 null mice died soon after birth due to respiratory failure, but neonatal lethality was prevented by prenatal maternal MI supplement. Although the lung air sacs were closed, lung development was not significantly affected in the SMIT-1 null mice. The development of the peripheral nerves, including the brachial plexus, facial, vagus, and intercostal nerves, and the phrenic nerve that innervates the diaphragm was severely affected. All of these peripheral nerve abnormalities were corrected by prenatal MI supplement, indicating that MI is essential for the development of peripheral nerve and that neonatal lethality of the SMIT-1 knockout mice is most likely due to abnormal development of the nerves that control breathing. In the adult SMIT-1 deficient mice rescued by MI supplement, MI content in their brain, kidney, skeletal muscle, liver, and sciatic nerve was greatly reduced. The sciatic nerve, in particular, was most dependent on SMIT-1 for the accumulation of MI, and nerve conduction velocity and protein kinase C activity in this tissue were significantly reduced by SMIT-1 deficiency.

Two types of protease-activated receptors (PAR-1 and PAR-2) mediate calcium signaling in rat retinal ganglion cells RGC-5

Protease-activated receptors (PARs), G-protein-coupled receptors, are widely expressed in various tissues, where they participate in physiological and pathological processes, such as hemostasis, proliferation, tissue repair, and inflammation. Recently, we found that PARs were upregulated in the rat retina following optic nerve crush injury. However, the role of PAR in retinal ganglion cells following optic nerve crush still remains unknown. Here, we studied PAR-mediated calcium signaling in retinal ganglion cells, RGC-5. Using reverse transcription-polymerase chain reaction, we demonstrate that RGC-5 cells mainly express PAR-1 and to a lower extent PAR-2, which was further confirmed by indirect immunofluorescence. Short-term stimulation of RGC-5 cells with thrombin (0.001-1 U/ml) and trypsin (1-100 nM) concentration-dependently induced a transient increase in intracellular calcium concentration ([Ca(2+)](i)). An increase in [Ca(2+)](i) was also induced by both TRag (PAR-1 activating peptide) and PAR-2 activating peptide (PAR-2 AP). The EC(50) values were 0.3 nM for thrombin, 12.0 nM for trypsin, 1.3 microM for TRag, and 1.6 microM for PAR-2 AP, respectively. Desensitization was studied using two successive pulses of agonists. The thrombin-induced calcium response was significantly reduced by PAR-1 desensitization caused by pre-challenging RGC-5 cells with thrombin or TRag, but not by PAR-2 desensitization. On the other hand, pretreatment with trypsin, TRag or PAR-2 AP desensitized the cells since the calcium response to a second exposure to trypsin was significantly reduced. Calcium source studies revealed that PAR-induced [Ca(2+)](i) rise mainly comes from intracellular stores in RGC-5 cells. Thus, we demonstrate that PAR-1 and PAR-2 are functionally expressed in retinal ganglion cells, mediating calcium mobilization mainly from intracellular stores.

Inositol 1,4,5-trisphosphate receptors as signal integrators

The inositol 1,4,5 trisphosphate (IP3) receptor (IP3R) is a Ca2+ release channel that responds to the second messenger IP3. Exquisite modulation of intracellular Ca2+ release via IP3Rs is achieved by the ability of IP3R to integrate signals from numerous small molecules and proteins including nucleotides, kinases, and phosphatases, as well as nonenzyme proteins. Because the ion conduction pore composes only approximately 5% of the IP3R, the great bulk of this large protein contains recognition sites for these substances. Through these regulatory mechanisms, IP3R modulates diverse cellular functions, which include, but are not limited to, contraction/excitation, secretion, gene expression, and cellular growth. We review the unique properties of the IP3R that facilitate cell-type and stimulus-dependent control of function, with special emphasis on protein-binding partners.

Stress and radiation-induced activation of multiple intracellular signaling pathways

Exposure of cells to a variety of stresses induces compensatory activations of multiple intracellular signaling pathways. These activations can play critical roles in controlling cell survival and repopulation effects in a stress-specific and cell type-dependent manner. Some stress-induced signaling pathways are those normally activated by mitogens such as the EGFR/RAS/PI3K-MAPK pathway. Other pathways activated by stresses such as ionizing radiation include those downstream of death receptors, including pro-caspases and the transcription factor NFKB. This review will attempt to describe some of the complex network of signals induced by ionizing radiation and other cellular stresses in animal cells, with particular attention to signaling by growth factor and death receptors. This includes radiation-induced signaling via the EGFR and IGFI-R to the PI3K, MAPK, JNK, and p38 pathways as well as FAS-R and TNF-R signaling to pro-caspases and NFKB. The roles of autocrine ligands in the responses of cells and bystander cells to radiation and cellular stresses will also be discussed. Based on the data currently available, it appears that radiation can simultaneously activate multiple signaling pathways in cells. Reactive oxygen and nitrogen species may play an important role in this process by inhibiting protein tyrosine phosphatase activity. The ability of radiation to activate signaling pathways may depend on the expression of growth factor receptors, autocrine factors, RAS mutation, and PTEN expression. In other words, just because pathway X is activated by radiation in one cell type does not mean that pathway X will be activated in a different cell type. Radiation-induced signaling through growth factor receptors such as the EGFR may provide radioprotective signals through multiple downstream pathways. In some cell types, enhanced basal signaling by proto-oncogenes such as RAS may provide a radioprotective signal. In many cell types, this may be through PI3K, in others potentially by NFKB or MAPK. Receptor signaling is often dependent on autocrine factors, and synthesis of autocrine factors will have an impact on the amount of radiation-induced pathway activity. For example, cells expressing TGFalpha and HB-EGF will generate protection primarily through EGFR. Heregulin and neuregulins will generate protective signals through ERBB4/ERBB3. The impact on radiation-induced signaling of other autocrine and paracrine ligands such as TGFbeta and interleukin 6 is likely to be as complicated as described above for the ERBB receptors.

Green tea catechins evoke a phasic contraction in rat aorta via H2O2-mediated multiple-signalling pathways

1. The contractile effects of tea polyphenols (TP) and its four principle catechins, namely (-)-epicatechin (EC), (-)-epicatechin-3-gallate (ECG), (-)-epigallocatechin (EGC) and (-)-epigallocatechin-3-gallate (EGCG), on rat aorta contractility were investigated using the isometric tension recording technique. 2. At concentrations of 5-100 mg/L, TP evoked phasic contraction of rat aorta in a concentration-dependent but endothelium-independent manner. Of the four catechins tested, EGCG and EGC (3-300 micromol/L), but not EC and ECG, mimicked the contractile response to TP, suggesting that the epigallol moiety in the B ring may be associated with the contractile effect. 3. Contractions in response to EGCG and EGC were not affected by several endogenous vasoconstrictor receptor antagonists, but could be abolished by 10 micro mol/L BAPTA-AM, a membrane-permeable Ca2+ chelator, or attenuated by removal of extracellular Ca2+, suggesting the involvement of both intracellular and extracellular Ca2+ in evoking the contraction. 4. Pretreatment with non-selective Ca2+ channel antagonists mefenamic acid (10 micro mol/L), tetrandrine (30 micro mol/L) and SKF 96365 (30 micromol/L), but not nifedipine (1 micromol/L), the selective inhibitor of voltage-dependent Ca2+ channels, inhibited the contractile responses to EGC and EGCG, indicating the involvement of Ca2+ influx via non-voltage dependent Ca2+ channels. 5. Several intracellular Ca2+ channel modulators, including procaine (5 mmol/L), dantrolene (30 micromol/L) and 2-amino ethoxydiphenyl borate (50 micromol/L; an inositol 1,4,5-trisphosphate receptor inhibitor), also inhibited EGCG- and EGC-induced contractions, thus suggesting a role of intracellular Ca2+ release in these contractions. 6. Both EGCG- and EGC-induced contractions were depressed, to different degrees, by inhibitors of several receptor-coupled enzymes, including phospholipase C, protein kinase C, phospholipase A2 and tyrosine kinase. Furthermore, both EGCG- and EGC-induced contractions were completely abolished by catalase, but not by superoxide dismutase or mannitol/dimethyl sulphoxide. 7. Taken together, these data show, for the first time, that TP and its related catechins that contain an epigallol structure in the B ring, as in EGCG and EGC, exert direct contractile effects on rat aortic smooth muscle via a H2O2-mediated pathway.

Protease-activated receptor-2-mediated Ca2+ signaling in guinea pig tracheal epithelial cells

The protease-activated receptor-2 (PAR-2), a G protein-coupled receptor activated by trypsin, contributes to the pathogenesis of inflammatory disease including asthma. Here, we examined the mechanisms by which stimulation of PAR-2 induces an increase in intracellular Ca2+ concentration ([Ca2+]i) in guinea pig tracheal epithelial cells. Trypsin (0.01-3 units/ml) dose-dependently induced a transient increase in [Ca2+]i, the increase being blocked by soybean trypsin inhibitor (SBTI 1 microM). An increase in [Ca2+]i was also induced by an agonist peptide for PAR-2 (SLIGRL-NH2, 0.001-10 microM) but not by thrombin (3 units/ml, an activator for PAR-1, PAR-3 or PAR-4). Repeated or cross stimulation of trypsin or SLIGRL-NH2 caused marked desensitization of the [Ca2+]i response. These responses of [Ca2+]i to trypsin and SLIGRL-NH2 were attenuated by a phospholipase C inhibitor, U-73122, and a Ca2+-ATPase inhibitor, thapsigargin (100 nM), while removal of Ca2+ and a L-type Ca2+-channel blocker, verapamil, were without significant effects. Further, trypsin was without effect on the rate of fura 2 quenching by Mn2+ entry as an indicator of Ca2+ influx. Thus, stimulation of PAR-2 appears to increase [Ca2+]i through the mobilization of Ca2+ from intracellular stores probably via phospholipase Cbeta-linked generation of a second messenger.

Vasopressin-activated calcium-mobilizing (VACM-1) receptor mRNA is present in peripheral organs and the central nervous system of the laboratory rat

The distribution and physiological role of the neuropeptide, arginine vasopressin (AVP), and its three receptor subtypes, V1a, V1b and V2, has been well described. A fourth AVP receptor, VACM-1, was recently discovered and appears to be a member of the cullin gene family. The objective of this research is to characterize VACM-1 receptor mRNA expression in the CNS as well as in various tissues and organs of the laboratory rat. Northern blotting of poly(A) + RNA from various tissues demonstrated the size of VACM-1 MRNA in the rat is approximately 6.3 kb. RT-PCR indicated the transcript is present in all twelve tissues examined: brainstem, cerebral cortex, cerebellum, hypothalamus, aorta, gastrointestinal tract, heart, kidney medulla, liver, lung, skeletal muscle, and spleen. Quantitative realtime PCR confirmed RT-PCR results that VACM-1 mRNA is in all organs and tissues and expression levels are similar in all tissues examined. The transcript encoding VACM-1, a novel AVP receptor, appears to be ubiquitously expressed in various tissues of the laboratory rat. VACM-1 shares some similarities with both V1 and V2 receptors, as it binds AVP analogues that independently recognized either of these receptors. Therefore, many functions ascribed to activation of the previously known AVP receptors could at least in part be mediated by VACM-1.

A23187 causes release of inositol phosphates from cultured rat Kupffer cells

The Ca2+ ionophore A23187 is routinely used to illustrate the extracellular Ca2+-dependence of a variety of cellular reactions. We found that A23187-induced hydrolysis of phosphoinositides to various inositol phosphates in rat Kupffer cells was accompanied by their release from the cells. The synthesis and release of inositol phosphates was A23187 concentration-dependent (0.5-10 microM), and was apparent at the lowest concentration tested. A23187-induced release of inositol phosphates increased time-dependently, was apparent at 5 s of stimulation and maximal at 20 min. The effects of A23187 were reversed by EGTA. The integrity of the cells was not affected by A23187 treatment as indicated by their exclusion of trypan blue and the lack of release of lactate dehydrogenase. We propose that such effects should be considered while evaluating the Ca2+-dependence of biological processes based on the actions of A23187.

Pepsinogens: physiology, pharmacology pathophysiology and exercise

Human gastric mucosa contains aspartic proteinases that can be separated electrophoretically on the basis of their physical properties into two major groups: Pepsinogen I (PGA, PGI); and Pepsinogen II (PGC, PGII). Pepsinogens consist of a single polypeptide chain with molecular weight of approximately 42,000 Da. Pepsinogens are mainly synthesized and secreted by the gastric chief cells of the human stomach before being converted into the proteolytic enzyme pepsin, which is crucial for the digestive processes in the stomach. Pepsinogen synthesis and secretion are regulated by positive and negative feed-back mechanisms. In the resting state pepsinogens are stored in granules, which inhibit further synthesis. After appropriate physiological or external chemical stimuli, pepsinogens are secreted in the stomach lumen where hydrochloric acid, secreted by the parietal cells, converts them into the corresponding active enzyme pepsins. The stimulus-secreting coupling mechanisms of pepsinogens appear to include at least two major pathways: one involving cAMP as a mediator, the other involving modification of intracellular Ca(2+)concentration. Physiological or external chemical stimuli acting through the intracellular metabolic adenyl cyclase are more effective in inducing ' de novo ' pepsinogen synthesis than those acting through intracellular Ca(2+). The activation of protein kinase C (PK-C) would appear to be involved in regulatory processes. The measurement of pepsinogens A and C in the serum is considered to be one of the non-invasive biochemical markers for monitoring peptic secretion and obtaining information on the gastric mucosa status of healthy subjects. Recently, pepsinogen measurements have been used as an effective biochemical method for evaluating and monitoring patients with gastrointestinal diseases and for checking the effects of drug treatment. The level of PGA in the serum is always high in normal gastritis, while in atrophic gastritis it is always low. In both cases the PGC level in the serum is high. In most gastrointestinal pathologies the ratio between the PGA/PGC decreases. Various reports concerning hormone and/or enzyme modification as well as gastrointestinal distress in the case of long distance exercise have been reported. It has been suggested that the origin of the gastrointestinal distress experienced by long distance runners is a transient ischaemia of the gastric mucosa; it is also suggested that a hypobaric-hypoxic environment could contribute to induce gastric mucosa necrosis. Interrelation between gastrointestinal distress, hypobaric-hypoxic environment and modifications of PGA and PGC, gastrin and cortisol was evaluated in 13 athletes after a marathon performed at 4300 m. Gastrointestinal symptoms occurred in approximately 40% of the athletes. After the race the athletes showed a significant increase of gastrin and cortisol, while the ratio between PGA/PGC decreased. No relationship was observed between gastrointestinal symptoms and hormonal changes after the race. A control group of five subjects, who had been exposed to the same environmental conditions, showed no gastrointestinal or hormonal alteration. Conversely, control subjects presented a significant decrease of cortisol related to the circadian rhythm. The same incidence of gastrointestinal symptoms at high altitude and at sea level and the absence of pathological alteration of PGA and PGC in the serum of the athletes indicates that running a marathon and living for 6 days at 4300 m does not induce gastric mucosa necrosis. Cortisol and gastrin alteration observed in the athletes at this altitude would seem to be related to an activation of the mesopontine and forebrain structures involved in the behavioural and metabolic integration of the autonomic control and arousal and psychophysical-exercise stress. 2000 Academic Press@p$hr

Phospholipase C-delta3 binds with high specificity to phosphatidylinositol 4,5-bisphosphate and phosphatidic acid in bilayer membranes

In order to acquire an understanding of phospholipase C-delta3 (PLC-delta3) action on substrate localized in lipid membrane we have studied the binding of human recombinant PLC-delta3 to large, unilamellar phospholipid vesicles (LUVs). PLC-delta3 bound weakly to vesicles composed of phosphatidylcholine (PtdCho) or PtdCho plus phosphatidylethanolamine (PtdEtn) or phosphatidylinositol (PtdIns). The enzyme bound strongly to LUVs composed of PtdEtn + PtdCho and phosphatidylinositol 4,5-bisphosphate (PtdInsP2). The binding affinity (molar partition coefficient) of PLC-delta3 to PtdEtn + PtdCho + PtdInsP2 vesicles was 7.7 x 105 m-1. High binding of PLC-delta3 was also observed for LUVs composed of phosphatidic acid (PA). Binding of PLC-delta3 to phosphatidylserine (PtdSer) vesicles was less efficient. Calculated molar partition coefficient for binding of PLC-delta3 to PA and PtdSer vesicles was 1.6 x 104 m-1 and 9.4 x 102 m-1, respectively. Presence of PA in the LUVs containing PtdInsP2 considerably enhanced the binding of PLC-delta3 to the phospholipid membrane. Binding of PLC-delta3 to phospholipid vesicles was not dependent on Ca2+ presence. In the liposome assay PA caused a concentration-dependent increase in activity of PLC-delta3. The stimulatory effect of PA on PLC-delta3 was calcium-dependent. At Ca2+ concentrations lower than 1 microm, no effect of PA on the activity of PLC-delta3 was observed. PA enhanced PLC-delta3 activity by increasing the Vmax and lowering Km for PtdInsP2. As the mol fraction of PA increased from 0-40 mol% the enzyme Vmax increased 2.3-fold and Km decreased threefold. Based on the results presented, we assume that PA supports binding of PLC-delta3 to lipid membranes by interaction with the PH domain of the enzyme. The stimulatory effect of PA depends on calcium-dependent interaction with the C2 domain of PLC-delta3. We propose that binding of PLC-delta3 to PA may serve as a mechanism for dynamic membrane association and modulation of PLC-delta3 activity.

Alterations in hepatic gluconeogenesis, prostanoid, and intracellular calcium during sepsis

OBJECTIVE

The metabolic alterations observed during sepsis may be associated with changes in local concentrations of intracellular calcium (Ca2+) and prostanoid synthesis in the liver. The authors studied hepatocyte intracellular Ca2+ and the release of glucose and prostanoid in an in-vivo murine liver perfusion model.

METHODS

Sepsis was induced in anesthetized, fasted rats by cecal ligation and puncture (CLP, n = 42). Hepatic glucose release was studied in control (n = 10) and CLP (n = 10) groups using a non-recirculating liver perfusion model with and without lactate as gluconeogenic substrate. Hepatocyte intracellular Ca2+ (n = 11) was measured using the selective indicator Fura-2 under basal and epinephrine (10(-5) M) stimulated conditions. 6-Keto-prostaglandin F1alpha (6-Keto) and thromboxane B2 (TxB2) were determined from liver perfusate by radioimmunassay (n = 11). Data were analyzed using t-tests and repeated-measures ANOVA.

RESULTS

Plasma glucose was significantly lower in CLP groups compared with controls (74.9+/-6.6 vs 115.7+/-4.6 mg/dL, p < 0.05). Plasma lactate was significantly higher in CLP vs controls (3.7+/-0.4 vs 1.4+/-0.1 mM, p < 0.05). Glucose release in isolated perfused livers was significantly lower in CLP vs controls (8.5 vs 16+/-1.2 microM/g/hr, p < 0.001). With the addition of lactate + pyruvate to the perfusate, glucose output in CLP livers was significantly lower following 5 (9.9+/-0.7 vs 17.7+/-1.1 microM/g/hr, p < 0.05) and 10 (11.9+/-1.2 vs 20.6+/-1.3 microM/g/hr, p < 0.001) minutes of perfusion. The basal level of intracellular calcium ([Ca2+]i) in CLP rats (460.1+/-91.6 nM) was significantly higher than in control rats (196.3+/-35.5 nM) (p < 0.05). A significant increase (p < 0.05) in [Ca2+]i occurred after the addition of epinephrine in hepatocytes in control (196.3+/-35.5 vs 331.8+/-41.4 nM) but not CLP (460.1+/-91.6 vs 489.4+/-105 nM) rats. 6-Keto was significantly lower in CLP compared with controls at 30 minutes (25.7+/-3.9 vs 33.4+/-5.5 pg/mL, p < 0.05), whereas TxB2 was not significantly altered (52.1+/-34.7 vs 87.5+/-43.2 pg/mL).

CONCLUSION

These results demonstrate that CLP sepsis is associated with an increase in hepatocyte intracellular free Ca2+ concentration along with attenuation of hormone-mediated Ca2+ mobilization and hepatic gluconeogenesis.

Activation of phospholipase C and protein kinase C is required for expression of anthrax lethal toxin cytotoxicity in J774A.1 cells

Anthrax lethal toxin (LT) comprises two proteins: the protective antigen (PA) and the lethal factor (LF). The LT is cytotoxic to macrophage-like cell line J774A.1. Pre-treatment of these cells with neomycin, a phospholipase C inhibitor, protected them against anthrax LT cytotoxicity. Protection obtained with neomycin indicated that LT stimulates phospholipase C in these cells. It was found that levels of inositol 1,4,5-triphosphate (IP3) dramatically increased in toxin-treated cells. The rise in IP3 levels was proportional to the dose of LF that was allowed to bind to receptor-bound PA. By using protein kinase C (PKC) inhibitors, we found that the activation of PKC is required for mediating anthrax LT cytotoxicity. Activation of phospholipase C or PKC is not required for the binding of PA to the cell surface receptors or for the uptake or internalisation of the toxin. In this study, we demonstrate that the IP3 signalling cascade is initiated by anthrax lethal toxin in J774A.1 cells. The second messengers generated during the cascade aid LF in mediating lethality only after its translocation into the cytosol.

The contractile mechanism of sodium metavanadate in isolated rat aortae

The mechanism of contractile effect of vanadate was investigated in rat aortae. Sodium metavanadate (NaVO3; 10(-5)-3 x 10(-3) M) induced contractile responses in a concentration-dependent manner. Removal of endothelium did not affect the response to NaVO3. The response to NaVO3 was inhibited by nifedipine, a voltage-operated Ca2+ channel (VOC) inhibitor; NCDC, a phospholipase C inhibitor; and H-7, a protein kinase C inhibitor, but not by prazosin, an alpha1-adrenoceptor antagonist; methysergide, a serotonin-receptor antagonist; tripelennamine, a histamine-receptor antagonist; glibenclamide, an adenosine triphosphate (ATP)-dependent K+-channel inhibitor; or iberiotoxin, a large-conductance Ca2+-activated K+-channel inhibitor. In addition, genistein or tyrphostin A48, tyrosine kinase inhibitors, did not affect the contraction induced by NaVO3. Mg2+ removal or antimycin A, a Ca2+-ATPase inhibitor, did not cause any contraction. Ouabain, a Na+, K+-ATPase inhibitor, or K+-free medium caused the contraction of the aortae. The maximal contraction induced by NaVO3 plus ouabain was similar to that induced by NaVO3 alone. In addition, the response to NaVO3 was inhibited by AA861, a 5-lipoxygenase inhibitor, and RHC-80267, a diacylglycerol (DAG) lipase inhibitor. In the presence of AA861, either H-7 or nifedipine further inhibited the residual response to NaVO3. In the presence of NCDC, however, AA861 failed further to affect the residual response to NaVO3. In rat aortae, NaVO3 increased the levels of inositol monophosphate (IP) and prostaglandin F2alpha (PGF2alpha). AA861 and NCDC inhibited the IP increase. In addition, NCDC inhibited the PGF2alpha increase. These results suggest that the response to NaVO3 in rat aortae may be mainly the result of the increased phosphoinositide metabolism.

Hemodynamic and metabolic effects of vasopressin blockade in endotoxin shock

BACKGROUND

Arginine vasopressin V1 receptor antagonist (AVPRA) was administered to investigate the influence of vasopressin blockade on hemodynamics and metabolism during endotoxin shock.

METHODS

Anesthetized rats were divided into four groups: control (0.9% saline solution, n = 5), drug control (AVPRA, n = 5), endotoxin (endotoxin, 5 mg/kg, n = 10), and pretreatment (AVPRA and endotoxin, n = 10). Hemodynamics and oxygen transport were evaluated for 2 hours. Terminal arterial and portal venous concentrations of endotoxin, pyruvate, lactate, and ketone bodies were determined.

RESULTS

The endotoxin group maintained blood pressure levels similar to those of control animals. AVPRA pretreatment decreased vascular resistance and resulted in lower blood pressure than endotoxin alone. Endotoxin decreased oxygen consumption and the oxygen extraction ratio and increased arterial lactate concentration and the lactate/pyruvate ratio. Endotoxin also decreased arterial ketone body concentration and markedly decreased ketone body availability in the mesenteric circulation. AVPRA pretreatment improved oxygen consumption, oxygen extraction ratio, and ketone body availability; arterial lactate concentration, lactate/pyruvate ratio, and arterial ketone body concentration were not affected. Pretreatment with AVPRA also decreased arterial and portal venous concentrations of endotoxin.

CONCLUSIONS

Vasopressin receptor blockade during endotoxemia resulted in lower blood pressure than endotoxin alone. Vasopressin receptor blockade also maintained oxygen extraction ratio and ketone body availability in the mesenteric circulation. Vasopressin may play a key role in the response to endotoxemia.

Altered signal transduction system in hypertrophied myocardium: angiotensin II stimulates collagen synthesis in hypertrophied hearts

Hypertensive cardiac hypertrophy is associated with the accumulation of collagen in the myocardial interstitium. Previous studies have demonstrated that this myocardial fibrosis accounts for impaired myocardial stiffness and ventricular dysfunction. Although cardiac fibroblasts are responsible for the synthesis of fibrillar collagen, the factors that regulate collagen synthesis in cardiac fibroblasts are not fully understood. We investigated the effects of angiotensin II on cardiac collagen synthesis in cardiac fibroblasts of 10-week-old spontaneously hypertensive rats and age-matched WKY rats. Basal collagen synthesis in cardiac fibroblasts from spontaneously hypertensive rats was 1.6-fold greater than that in the cell of WKY rats. Angiotensin II stimulated collagen synthesis in cardiac fibroblasts in a dose-dependent manner. The responsiveness of collagen production to angiotensin II was significantly enhanced in cardiac fibroblasts from spontaneously hypertensive rats (100 nM angiotensin II resulted in 185 +/- 18% increase above basal levels, 185 +/- 18 vs 128 +/- 19% in WKY rats, P < .01). This effect was receptor-specific, because it was blocked by the competitive inhibitors saralasin and MK 954. These results indicate that collagen production is enhanced in cardiac fibroblasts from spontaneously hypertensive rats, that angiotensin II has a stimulatory effect on collagen synthesis in cardiac fibroblasts, and that cardiac fibroblasts from spontaneously hypertensive rats are hyper-responsive to stimulation by angiotensin II. In the hearts of spontaneously hypertensive rats, mRNA of the renin-angiotensin system (renin, angiotensinogen, angiotensin converting enzyme) was expressed. Levels of angiotensinogen and renin mRNA expressed in ventricles, and angiotensinogen mRNA expressed in fibroblasts from SHR were higher than those from WKY. ACE mRNA was also more strongly expressed in the ventricles and fibroblasts from SHR compared with those of WKY. These findings suggest that the cardiac reninangiotensin system may play an important role in collagen accumulation in hypertensive cardiac hypertrophy (fig.4).

Chronic ethanol administration to rats decreases receptor-operated mobilization of intracellular ionic calcium in cultured hepatocytes and inhibits 1,4,5-inositol trisphosphate production: relevance to impaired liver regeneration

We tested the hypothesis that ethanol impairs liver regeneration by abrogating receptor-mediated elevation of cytosolic free calcium ([Ca2+]i). In rats fed for 16 weeks with ethanol, hepatocellular proliferation induced by partial hepatectomy was greatly impaired. Similarly, EGF-induced DNA synthesis was reduced in cultured hepatocytes from ethanol-fed rats. There was no change in the number or affinity of EGF receptors on hepatocytes from ethanol-fed rats. Despite this, EGF-mediated production of inositol 1,4,5-trisphosphate (Ins[1,4,5]P3) was lower in hepatocytes from ethanol-fed rats, and the EGF-induced [Ca2+]i transient appeared to be abrogated. When vasopressin or phenylephrine were used as cell surface receptor ligands, hepatocytes cultured from ethanol-fed rats exhibited major reductions in Ins(1,4,5)P3 synthesis. This was associated with greatly truncated [Ca2+]i transients. These changes were not due to an effect on the Ins(1,4,5)P3 receptor on the endoplasmic reticulum or to a decrease in the size of the Ins(1,4,5)P3-mobilizable intracellular Ca+2 store. Further, mobilization of the same Ca2+ store by 2,5-di-tert-butylhydroquinone or thapsigargin restored the ability of hepatocytes from ethanol-fed rats to proliferate when exposed to EGF. It is concluded that chronic ethanol consumption inhibits liver regeneration by a mechanism that is, at least partly, the result of impaired receptor-operated [Ca2+]i signaling due to reduced generation of Ins(1,4,5)P3.

In vitro and in vivo effects of lead, methyl mercury and mercury on inositol 1,4,5-trisphosphate and 1,3,4,5-tetrakisphosphate receptor bindings in rat brain

In vitro and in vivo effects of mercury (Hg), methyl mercury (MM) and lead (Pb) on [3H]inositol 1,4,5-trisphosphate (IP3) and [3H]inositol 1,3,4,5-tetrakisphosphate (IP4) receptor binding in the Sprague-Dawley rat brain cerebellar membranes were studied. In vitro studies indicate that binding of [3H]IP3 and [3H]IP4 to cerebellar membranes was inhibited by Hg while they were stimulated by MM or Pb in a concentration-dependent manner. MM was more potent (EC50 3.4 microM) than Pb (EC50 18.2 microM) in stimulating the [3H]IP3 receptor binding activity whereas Pb (IC50 30 microM) was more potent than MM (IC50 133 microM) in stimulating the [3H]IP4 receptor binding. When the rats were treated (i.p) with Hg (5 mg/kg body wt.) or MM (5 mg/kg body wt.) or Pb (25 mg/kg body wt.) for 3 or 24 h, no significant alterations in [3H]IP3 receptor binding were observed in cerebellum and cerebral cortex. But the above treatment of Pb or MM for 3 or 24 h to rats resulted in an increase of [3H]IP4 receptor binding in the membranes of cerebral cortex. However, the rats treated with Hg (1 mg/kg body wt./day) or Pb (25 mg/kg body wt./day) for 7 days did not show any alteration in binding of [3H]IP3 to its receptors in cerebellar membranes but an increase in this receptor binding was noticed with the treatment of MM (2.5 mg/kg body wt./day) for 7 days. The cerebellum and cerebral cortex of rats with the above treatment of MM or Pb for 7 days exhibited an increase in [3H]IP4 receptor binding. These in vitro and in vivo data suggest that alterations in inositol polyphosphate receptor binding by metals could result in alterations in intracellular calcium levels which may influence neuronal activity.

Enhancing effects of vasoconstrictors on bile flow and bile acid excretion in the isolated perfused rat liver

The effects of vasoconstrictors on bile flow and bile acid excretion were examined in single-pass isolated perfused rat livers. Administration of norepinephrine (NE), 4 nmol/min, plus continuous infusion of taurocholate (TC) (1.0 mumol/min) rapidly increased bile flow in 1 min, and from min 5 until the end of NE administration (late period) bile flow remained above the basal level (111.7 +/- 2.2%), as did bile acid output (114.6 +/- 1.8%). Without TC infusion, administration of NE produced no increase in the late period. Administration of NE plus taurochenodeoxycholate (1.0 mumol/min) increased bile flow and bile acid output in the late period to 121.9 +/- 7.0 and 137.1 +/- 6.8%, respectively. With NE plus taurodehydrocholate, the respective values were only 105.4 +/- 1.6 and 104.1 +/- 4.0%. When horseradish peroxidase (HRP) (25 mg) was infused over 1 min with continuous NE, the late peak (20-25 min) of HRP elimination into bile significantly exceeded that of untreated controls (P < 0.01). These observations suggest that vasoconstrictors enhance biliary excretion of more hydrophobic bile acids, in part by stimulating vesicular transport.

Modulation of the hepatic alpha 1-adrenoceptor responsiveness by colchicine: dissociation of free cytosolic Ca(2+)-dependent and independent responses

1. The cytoskeletal depolymerizing agent, colchicine, prevents the hepatic alpha 1-adrenoceptor-mediated stimulation of respiration, H+ and Ca2+ release to the effluent perfusate, intracellular alkalosis, and glycogenolysis. Unlike the other parameters, colchicine does not perturb the alpha 1-agonist-induced stimulation of gluconeogenesis or phosphorylase 'a' activation, and enhances the increase in portal pressure response. The lack of effect of colchicine on the hepatic alpha 2-adrenoceptor-mediated effects indicates that its actions are alpha 1-specific. 2. Colchicine enhances the acute alpha 1-adrenoceptor-mediated intracellular Ca2+ mobilization and prevents the activation of protein kinase C. This differential effect on the two branches of the alpha 1-adrenoceptor signalling pathway is a distinctive feature of the colchicine action. 3. The lack of effect of colchicine in altering the alpha 1-adrenoceptor ligand binding affinity suggests that it might interact with some receptor-coupled regulatory element(s). 4. The acuteness of the colchicine effect and the ability of its isomer beta-lumicolchicine to prevent all the alpha 1-adrenoceptor-mediated responses but the increase in vascular resistance, indicate that its action cannot be merely ascribed to its effects in depolymerizing tubulin. 5. Colchicine perturbs the hepatic responses to vasoactive peptides. It enhances the vasopressin-induced rise of cytosolic free Ca2+ in isolated hepatocytes and prevents the sustained decrease of Ca2+ in the effluent perfusate. It also inhibits the stimulation of glycogenolysis, without altering the stimulation of gluconeogenesis. 6. It is concluded that there are at least two major alpha 1-adrenoceptor signalling pathways. One is colchicine-sensitive, independent of variations in free cytosolic Ca2+, and protein kinase C-dependent; the other one is colchicine-insensitive, dependent on variations in free cytosolic Ca2+, and protein kinase C-independent.

Effect of sympathoadrenal blockade on the hyperglycemic action of angiotensin II

The present experiments were designed to investigate the influence of the sympathoadrenal system on the hyperglycemic action of angiotensin II in freely moving rats divided into four experimental groups: (1) sham-operated animals submitted to intravenous administration of angiotensin II (1.9 nmol) which caused a rapid increase in plasma glucose reaching the highest values at 5 min after the injection (26.5% of the initial values; P < 0.01); (2) Sham-operated animals submitted to blockade of sympathetic noradrenergic pathways by treatment with guanethidine (10 mg/100 g body weight), which greatly decreased the baseline value of plasma glucose (85 +/- 5.5 mg% vs 136 +/- 5.1 mg% P < 0.01), and increased the hyperglycemic response to angiotensin II by 96% (P < 0.01); (3) Animals submitted to adrenodemedullation that did not alter the hyperglycemic response to angiotensin II; (4) Adrenodemedullated animals submitted to sympathetic blockade by guanethidine treatment which caused a 40.5% decrease in the hyperglycemic response to angiotensin II (P < 0.05). These data indicate that angiotensin II has a direct hyperglycemic effect in addition to its action on sympathetic nervous system activation and adrenomedullary secretion.

Effect of [1-Sar,8-Thr]-angiotensin II on the hyperglycemic response to hemorrhage in adrenodemedullated and guanethidine-treated rats

The present experiments were designed to further investigate the action of an angiotensin II antagonist on the hyperglycemic response to hemorrhage (1.2 ml/100 g b.wt./2 min). The animals were divided into 3 experimental groups; (1) sham-operated animals submitted to intravenous administration of [1-Sar,8-Thr]-angiotensin II (sarthran), an antagonist of angiotensin II (750 ng/100 g b.wt. as a bolus plus an infusion of 25 ng/100 g b.wt./min over 30 min), which greatly attenuated (51.8% lower than controls; P < 0.01) the hyperglycemic response to hemorrhage; (2) animals submitted to adrenodemedullation which decreased the hyperglycemic response to hemorrhage by 64% (P < 0.01). However, sarthran infusion into adrenodemedullated rats caused a 38.5% further decrease in hyperglycemic response to hemorrhage (P < 0.01); and (3) intact animals submitted to blockade of sympathetic noradrenergic pathways by treatment with guanethidine (10 mg/100 g b.wt.), which greatly decreased the baseline value of plasma glucose (64.1 +/- 3.5 mg% vs. 125.3 +/- 4.5 mg%, P < 0.01), and reduced the hyperglycemic response to hemorrhage by 34% (P < 0.01). Sarthran infusion into guanethidine-treated rats caused a further 34% decrease in hyperglycemic response to hemorrhage (P < 0.01). These data indicate that angiotensin II has a direct hyperglycemic effect in addition to its action on sympathetic nervous system activation and adrenomedullary secretion.

Regulation of phospholamban and troponin-I phosphorylation in the intact rat cardiomyocytes by adrenergic and cholinergic stimuli: roles of cyclic nucleotides, calcium, protein kinases and phosphatases and depolarization

Protein phosphorylation was investigated in [32P]-labeled cardiomyocytes isolated from adult rat heart ventricles. The beta-adrenergic stimulation (by isoproterenol, ISO) increased the phosphorylation of inhibitory subunit of troponin (TN-I), C-protein and phospholamban (PLN). Such stimulation was largely mediated by increased adenylyl cyclase (AC) activity, increased myoplasmic cyclic AMP and increased cyclic AMP dependent protein kinase (A-kinase)-catalyzed phosphorylation of these proteins in view of the following observations: (a) dibutyryl-and bromo-derivatives of cyclic AMP mimicked the stimulatory effect of ISO on protein phosphorylation while (b) Rp-cyclic AMP was found to attenuate ISO-dependent stimulation. Unexpectedly, 8-bromo cyclic GMP was found to markedly increase TN-I and PLN phosphorylation. Both beta 1- and beta 2-adrenoceptors were present and ISO binding to either receptor was found to stimulate myocyte AC. However, the stimulation of the beta 2-AR only marginally increased while the stimulation of beta 1-AR markedly increased PLN phosphorylation. Other stimuli that increase tissue cyclic AMP levels also increased PLN and TN-I phosphorylation and these included isobutylmethylxanthine (non-specific phosphodiesterase inhibitor), milrinone (inhibits cardiotonic inhibitable phosphodiesterase, sometimes called type III or IV) and forskolin (which directly stimulates adenylyl cyclase). Cholinergic agonists acting on cardiomyocyte M2-muscarinic receptors that are coupled to AC via pertussis toxin(PT)-sensitive G proteins inhibited AC and attenuated ISO-dependent increases in PLN and TN-I phosphorylation. The in vivo PT treatment, which ADP-ribosylated Gi-like protein(s) in the myocytes, markedly attenuated muscarinic inhibitory effect on PLN and TN-I phosphorylation on one hand and, increased the beta-adrenergic stimulation, on the other. Controlled exposure of isolated myocytes to N-ethyl maleimide, also led to the findings similar to those seen following the PT treatment. Exposure of myocytes to phorbol, 12-myristate, 13-acetate (PMA) increased the protein phosphorylation, augmenting the stimulation by ISO, and such augmentation was antagonized by propranolol suggesting modulation of the beta-adrenoceptor coupled AC pathway by PMA. Okadaic acid (OA) exposure of myocytes also increased protein phosphorylation with the results supporting the roles for type 1 and 2A protein phosphatases in the dephosphorylation of PLN and TN-I. Interestingly OA treatment attenuated the muscarinic inhibitory effect which was restored by subsequent brief exposure of myocytes to PMA. While the stimulation of alpha adrenoceptors exerted little effect on the phosphorylation of PLN and TN-I, inactivation of alpha adrenoceptors by chloroethylclonidine (CEC), augmented beta-adrenergically stimulated phosphorylation. KCl-dependent depolarization of myocytes was observed to potentiate ISO-dependent increase in phosphorylation (incubation period 15 sec to 1 min) as well as to accelerate the time-dependent decline in this phosphorylation seen upon longer incubation. Verapamil decreased ISO-stimulated protein phosphorylation in the depolarized myocytes. Depolarization was found to have little effect on the muscarinic inhibitory action on phosphorylation. Prior treatment of myocytes with PMA, was found to augment ISO-stimulated protein phosphorylation in the depolarized myocytes. Such augmented increases were completely blocked by propranolol. Forskolin also stimulated PLN and TN-I phosphorylation. Prior exposure of myocytes to forskolin followed by incubation in the depolarized and polarized media showed that PLN was dephosphorylated more rapidly in the depolarized myocytes. The results support the view that both cyclic AMP and calcium signals cooperatively increase the rates of phosphorylation of TN-I and PLN in the depolarized cardiomyocytes during beta-adrenergic stimulation. (ABSTRACT TRUNCATED)

Oscillations in cytosolic free Ca2+ induced by ADP and ATP in single rat hepatocytes display differential sensitivity to application of phorbol ester

We have previously described differences in the oscillatory responses of cytosolic free Ca2+ concentration ([Ca2+]i) in hepatocytes to ADP and ATP, which we have interpreted as evidence that these two nucleotides are acting at distinct receptors. We show here that ADP- and ATP-induced oscillations are differentially sensitive to application of the phorbol ester 4 beta-phorbol 12,13-dibutyrate (PDB). ADP-induced [Ca2+]i oscillations are abolished by low concentrations of PDB (5-10 nM), whereas ATP-induced oscillations of long duration are refractory to PDB, even at greatly elevated concentrations (100 nM). The data illustrate a further difference in the actions of ADP and ATP, strengthening the argument that these agonists are not acting at the same receptor on rat hepatocytes.

Vanadate inhibition of hepatocytic autophagy. Calcium-modulated and osmolality-modulated antagonism by asparagine

The phosphate analogue vanadate, at 10 mM, strongly (approximately 90%) inhibited the autophagic sequestration of endogenous lactate dehydrogenase in isolated rat hepatocytes. The effect of vanadate was markedly (approximately 80%) antagonized by asparagine (20 mM), and to a lesser extent by glutamine, glycine, and alanine. The antagonism was only observed in the presence of Ca2+ when an isotonic standard incubation medium was used, but by increasing the medium osmolality this Ca2+ requirement could be eliminated. Asparagine induced a cell swelling (17% at 20 mM) that might account for at least part of its vanadate antagonism, since hypotonic cell swelling by itself stimulated autophagy (with a maximal effect at approximately 200 mosM). Conversely, hypertonic media inhibited autophagy and were additive to vanadate. In a strongly hypotonic medium (less than 200 mosM), both asparagine and vanadate were inhibitory. However, since vanadate alone had no effect on cell volume, the vanadate-asparagine antagonism could not be exerted exclusively at the level of cell volume regulation. An additional mechanism might be a partial deamination of asparagine, generating ammonia, which was found to oppose the vanadate inhibition of autophagy while having no effect on cell volume. Other metabolizable amino acids, like alanine and glycine, were moderately vanadate-antagonistic while failing to induce cell swelling. These results are compatible with a vanadate-antagonistic effect of asparagine mediated partly through an unknown mechanism (possibly pH change) by its deamination product, ammonia, partly through cell swelling and a secondary Ca2+ influx that could compensate for a vanadate-induced depletion of intracellular calcium stores.

Effect of acute bile acid pool depletion on total and ionized calcium concentrations in human bile

Although calcium salts are important components of gallstones, there are few data on the total and ionized calcium content of human bile. Therefore, in 14 fasting T-tube patients studied 7-11 days after cholecystectomy, we measured bile flow, bile acid [BA], total [CaTOT] and free ionized [Ca++] calcium concentrations, in 20-30 min bile collections during acute BA pool depletion induced by 6-8 h of continuous bile drainage. During washout of the BA pool there were parallel falls in bile flow, BA output and total calcium output (correlation coefficients ranging from 0.59 to 0.99; P < 0.02-0.001). In 12 of the 14 patients, [CaTOT] also fell (from 1.84 +/- 0.29 to 1.32 +/- 0.34 mmol L-1) in parallel with [BA] (from 34.0 +/- 14.0 to 8.2 +/- 8.0 mmol L-1; r = 0.75-0.98; P < 0.005). In contrast, biliary [Ca++] remained virtually unchanged. These data suggest that the BAs are linked to the bound, rather than to the free, ionized, fraction of biliary calcium, which is consistent with in vivo calcium binding by BAs. A model is proposed in which BA-induced biliary calcium secretion results from (i) bile acid-induced water flow via solvent drag; and (ii) calcium binding in the bile canaliculus by bile acids, which induces paracellular diffusion of Ca++, thereby maintaining [Ca++] independent of [BA].

A role for protein kinase C-mediated phosphorylation in eliciting glucagon desensitization in rat hepatocytes

An immobilized hepatocyte preparation was used to show that both vasopressin and glucagon could desensitize the ability of glucagon to increase intracellular cyclic AMP concentrations. This process was not dependent on any influx of extracellular Ca2+ and was not mediated by any rise in the intracellular level of Ca2+. The protein kinase C-selective inhibitors chelerythrine, staurosporine and calphostin C acted as potent inhibitors of the desensitization process but with various degrees of selectivity regarding their ability to inhibit the desensitizing actions of glucagon and vasopressin. The protein phosphatase inhibitor okadaic acid was just as potent as vasopressin and glucagon in causing desensitization. Treatment of hepatocyte membranes with alkaline phosphatase restored to near control levels the ability of glucagon to stimulate adenylate cyclase activity in membranes from both glucagon- and vasopressin-treated (desensitized) hepatocytes. It is suggested that the desensitization of glucagon-stimulated adenylate cyclase activity involves a reversible phosphorylation reaction with the likely target being the glucagon receptor itself.

Nitric oxide blocks bile canalicular contraction by inhibiting inositol trisphosphate-dependent calcium mobilization

BACKGROUND/AIMS

The biochemical mechanism of bile canalicular contraction is similar to that of smooth muscle contraction. Contraction follows inositol-1,4,5-trisphosphate (InsP3)-dependent Ca2+ release, which activates actin-myosin interactions. Nitric oxide is a myorelaxant through the actions of 5'-cyclic guanosine monophosphate (cGMP) and is produced in hepatocytes exposed to endotoxin and cytokines. The aim of this study was to investigate the effect of nitric oxide on canalicular contraction and to determine the mechanism by which cGMP interferes with the contractile signal.

METHODS

The canalicular motility in rat hepatocyte doublets was measured by microscopic image analysis, and intracellular Ca2+ was measured by fluorescence microscopy. cGMP and InsP3 were determined by radio-immunoassay and high-pressure liquid chromatography. Ca2+ release from liver homogenate was measured by filtration and superfusion assays.

RESULTS

Compounds that release nitric oxide stimulated hepatocellular production of cGMP and prevented agonist-induced contraction by inhibiting the increase in intracellular Ca2+. The cGMP analogue bromo-cGMP prevented contraction and the increase in Ca2+. Bromo-cGMP marginally decreased InsP3 production. cGMP blocked InsP3-dependent Ca2+ release from internal stores.

CONCLUSIONS

These findings suggest that nitric oxide interferes with Ca2+ signals by cGMP-mediated inhibition of the InsP3 receptor/Ca2+ channel and that hepatocellular production of nitric oxide may be cholestatic by impairing canalicular motility.

Regulation of hepatocyte plasma membrane alpha 1-adrenergic receptors by 4 beta-phorbol 12-myristate 13-acetate

The effect of phorbol 12-myristate 13-acetate (PMA) on hepatocyte alpha 1-adrenergic receptors was determined by [3H]prazosin binding to plasma membranes from control and PMA-treated hepatocytes. Membranes from hepatocytes incubated with PMA (1 microgram/ml) for 1 h exhibited a 40% decrease in alpha 1-adrenergic receptors (481 +/- 10 fmol/mg of protein; mean +/- S.E.M. for three separate experiments) relative to vehicle-treated (dimethylformamide) hepatocytes (802 +/- 91 fmol/mg of protein; n = 3), with no significant effect on the KD. The PMA-induced decrease in alpha 1-adrenergic receptors was maximal by 30 min and half-maximal inhibition of [3H]prazosin binding occurred with a PMA concentration of approx. 15 ng/ml. Pretreatment of hepatocytes with staurosporine (5 microM) blocked the effect of PMA, and 4 beta-phorbol 13-monoacetate was ineffective, suggesting the involvement of protein kinase C (PKC). Treatment of hepatocytes with primaquine (300 microM) for 15 min decreased hepatocyte plasma membrane alpha 1-adrenergic receptors by 34.0 +/- 2.4% (mean +/- S.E.M. of three experiments). Removal of primaquine allowed essentially complete recovery (98 +/- 4%; mean +/- S.E.M. for five separate experiments) of plasma membrane [3H]prazosin binding within 20 min, suggesting that the alpha 1-adrenergic receptor undergoes endocytotic recycling. Addition of PMA (1 microgram/ml) to hepatocytes immediately after removal of primaquine, completely inhibited the increase in plasma membrane alpha 1-adrenergic receptors relative to control cells, but had no effect on hepatocytes whose cell surface alpha 1-receptors remaining after primaquine treatment had been inactivated by alkylation. These observations suggested that activation of PKC may facilitate the internalization of the alpha 1-adrenergic receptor in hepatocytes.

Hyperglycemic action of angiotensin II in freely moving rats

Angiotensin II has been implicated in the regulation of liver glycogen phosphorylase. Although it has been suggested that angiotensin II can raise blood glucose levels during hemorrhage, experimental data have not been presented. In the present study, the effect of angiotensin II on blood glucose levels was studied in freely moving rats, divided in three experimental groups: 1) intravenous administration of angiotensin II (0.48, 1.9, or 4.8 nmol) caused a dose-dependence response; 2) intracerebroventricular administration of angiotensin II (1.9 or 4.8 nmol) did not cause any significant change in glycemia compared with saline-treated controls; 3) intravenous administration of [Sar1,Thr8]angiotensin II, an antagonist of angiotensin II (750 ng/100 g b. wt. as a bolus plus a continuous injection of 25 ng/100 g b. wt./min over 30 min), greatly attenuated (39.2% lower than controls; p < 0.01) the hyperglycemic response to hemorrhage (1.2 ml/100 g b.wt.). These data indicate an in vivo involvement of angiotensin II in blood glucose regulation.

Characterization of signaling pathways to Na+/H+ exchanger activation with epidermal growth factor in hepatocytes

To investigate the signaling pathways to Na+/H+ exchanger activation with epidermal growth factor in hepatocytes, we measured changes in cytosolic free calcium and intracellular pH levels at the single-cell level using digital imaging fluorescence microscopy of fura-2- or BCECF-loaded hepatocytes in primary culture. Epidermal growth factor induced cytosolic free calcium oscillations consisting of periodic trains of spikes with a latency period of up to several minutes. These calcium responses were inhibited by tyrosine kinase inhibitor genistein (100 mumol/L) and abolished by emptying of intracellular Ca2+ pools with 3 mumol/L thapsigargin, an inhibitor of Ca(2+)-ATPase on the endoplasmic reticulum. Epidermal growth factor (1 nmol/L) induced an intracellular pH increase of 0.12 +/- 0.07 units from the basal level of 7.25 +/- 0.09 units after several minutes of latency. This effect was completely abolished by 1 mmol/L amiloride, an inhibitor of the Na+/H+ exchanger. The epidermal growth factor-induced intracellular pH increase was inhibited by pretreatment of hepatocytes with genistein (100 mumol/L), thapsigargin (3 mumol/L) or calmodulin inhibitor W-7 (25 mumol/L), but not with protein kinase C inhibitor H-7 (50 mumol/L) or with cyclic AMP-dependent kinase inhibitor H-8 (60 mumol/L). Phorbol ester PMA (phorbol 12-myristate 13-acetate), a potent activator of protein kinase C, induced a slight intracellular pH increase significantly smaller than that with epidermal growth factor, whereas this effect was completely blocked by pretreatment with H-7, indicating that PMA-induced intracellular pH increase is mediated by protein kinase C pathways, unlike epidermal growth factor.(ABSTRACT TRUNCATED AT 250 WORDS)

Lead alters inositol polyphosphate receptor activities: protection by ATP

Receptor-mediated phosphoinositide signaling pathway which generates a variety of second messengers is regulated by intracellular free Ca2+ concentrations. Since toxic metal cations like Pb2+ are known to alter Ca(2+)-dependent processes, the present study was initiated to study the effects of Pb2+ on inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) receptor binding and InsP3-mediated Ca(2+)-release. Rat cerebellar membrane and microsomal fractions were incubated with various concentrations of Pb2+ (0.01-100 microM). Pb2+ significantly stimulated [3H]-InsP3 and [3H]-InsP4 receptor binding (EC50 22.7 and 13.5 microM respectively) as a function of metal concentrations. However, InsP3-mediated Ca2+ release, determined by measuring the changes in fluorescence intensity of Fura-2, was significantly inhibited by varying concentrations of Pb2+. Re-uptake of Ca2+ into the microsomes was also inhibited by Pb2+. A significant inhibition of microsomal Ca(2+)-pump by micromolar concentration of Pb2+ was also observed. ATP at 5-1000 microM concentration range inhibited [3H]-InsP3 and [3H]-InsP4 binding to the specific receptors. [3H]-InsP4 receptor binding was more sensitive to ATP inhibition as compared to [3H]-InsP3 receptor binding. Furthermore, varying concentrations of ATP also inhibited Pb(2+)-mediated increase in [3H]-InsP3 and [3H]-InsP4 receptor binding. The kinetic analysis of ATP effect on Pb(2+)-stimulated [3H]-InsP4 receptor binding revealed non-competitive type of interaction. The results of the present study suggest that Pb2+ may be increasing the binding of [3H]-InsP3 and [3H]-InsP4 to the specific receptors by modulating the conformation of the receptor sites. ATP may be playing a protective role in Pb2+ induced alteration of the receptor sites.