Role of calcium in the hormonal regulation of liver metabolism

Mitochondrial redox state, bioenergetics, and calcium transport in caloric restriction: A metabolic nexus

Mitochondria congregate central reactions in energy metabolism, many of which involve electron transfer. As such, they are expected to both respond to changes in nutrient supply and demand and also provide signals that integrate energy metabolism intracellularly. In this review, we discuss how mitochondrial bioenergetics and reactive oxygen species production is impacted by dietary interventions that change nutrient availability and impact on aging, such as calorie restriction. We also discuss how dietary interventions alter mitochondrial Ca2+ transport, regulating both mitochondrial and cytosolic processes modulated by this ion. Overall, a plethora of literature data support the idea that mitochondrial oxidants and calcium transport act as integrating signals coordinating the response to changes in nutritional supply and demand in cells, tissues, and animals.

The Effect of Adrenaline on the Mineral and Trace Element Status in Rats

Up until now, changes in biochemical and physiological parameters occurring a long time after stress are not yet elucidated. This is particularly the case for metals, some of which may considerably influence other branches of metabolism, such as bioenergetics and antioxidant defense. The aim of the current study was to investigate changes in levels of minerals (calcium and magnesium) and trace elements (copper and zinc) in erythrocytes and the liver of rats injected once or twice (modeling repeated stress) with adrenaline. The tissues were sampled 0.5 and 24 hours after the injection. A single injection of adrenaline in rats led to a dramatic increase in the levels of zinc (Zn), magnesium (Mg), and calcium (Ca) in their erythrocytes and liver, without a return to the control level (unstressed animals) after 24 hours. The levels of copper (Cu) increased 0.5 hour after a single adrenaline injection in erythrocytes and the liver, but returned to the control level after 24 hours. Double injection of rats with adrenaline led to an increase in the levels of Cu and Zn in their erythrocytes, and Mg in the liver, without a return to the control level after 24 hours. On the other hand, the double injection led to a drastic but transient increase in levels of Mg and Ca in erythrocytes, and Cu, Zn, and Ca in the liver. Thus, injection with adrenaline results in dramatic changes in levels of minerals and trace elements, which do not return to the control level after stress. Low doses of adrenaline lead to more stable changes in levels of essential metals.

Comparative proteomic analysis of hepatic mechanisms of Megalobrama amblycephala infected by Aeromonas hydrophila

Hemorrhage syndrome is one of the most prevalent and epidemic diseases that is mainly caused by Aeromonas hydrophila invasion in Megalobrama amblycephala. Recent studies have uncovered a number of immune enzymes and transcripts that are differently expressed in this disease, but the molecular mechanism elicited still remain largely unknown. Here, we constructed an in vivo A. hydrophila infection to investigate the immune mechanism in M. amblycephala using comparative proteomic approach at the one day after infection. 30 altered protein spots were found to undergo differential expression against A. hydrophila infection in the hepatopancreas of M. amblycephala based on 2-DE and were all successfully identified using MALDI-TOF/TOF, representing 18 unique proteins. These proteins were functionally classified into metabolism, antioxidant, cofactors and vitamins, chaperone and signal transduction. Network interaction and Gene Ontology annotation indicated 13 unique proteins were closely related to immune response and directly regulated by each other. Compared with the control group, A. hydrophila infection significantly decreased the metabolism-related mRNA expressions of ENO3, APOA1, CAT and FASN, but increased the mRNA expressions of MDH, ALDOB and RSP12, which was consistent with the protein expression. Nevertheless, FAH was down-regulated at both levels but had no significant difference in mRNA level, ALDH8a1 was down-regulated at protein level but non-significantly up-regulated at the mRNA level. GSTm was up-regulated at protein level but down-regulated at the mRNA level. Consequently, these results revealed that A. hydrophila infection altered the related antioxidative proteins via complex regulatory mechanisms and reduced the immune ability of M. amblycephala at the one day after infection.

The transcriptional regulation of regucalcin gene expression

Regucalcin, which is discovered as a calcium-binding protein in 1978, has been shown to play a multifunctional role in many tissues and cell types; regucalcin has been proposed to play a pivotal role in keeping cell homeostasis and function for cell response. Regucalcin and its gene are identified in over 15 species consisting of regucalcin family. Comparison of the nucleotide sequences of regucalcin from vertebrate species is highly conserved in their coding region with throughout evolution. The regucalcin gene is localized on the chromosome X in rat and human. The organization of rat regucalcin gene consists of seven exons and six introns and several consensus regulatory elements exist upstream of the 5'-flanking region. AP-1, NF1-A1, RGPR-p117, β-catenin, and other factors have been found to be a transcription factor in the enhancement of regucalcin gene promoter activity. The transcription activity of regucalcin gene is enhanced through intracellular signaling factors that are mediated through the phosphorylation and dephosphorylation of nuclear protein in vitro. Regucalcin mRNA and its protein are markedly expressed in the liver and kidney cortex of rats. The expression of regucalcin mRNA in the liver and kidney cortex has been shown to stimulate by hormonal factors (including calcium, calcitonin, parathyroid hormone, insulin, estrogen, and dexamethasone) in vivo. Regucalcin mRNA expression is enhanced in the regenerating liver after partial hepatectomy of rats in vivo. The expression of regucalcin mRNA in the liver and kidney with pathophysiological state has been shown to suppress, suggesting an involvement of regucalcin in disease. Liver regucalcin expression is down-regulated in tumor cells, suggesting a suppressive role in the development of carcinogenesis. Liver regucalcin is markedly released into the serum of rats with chemically induced liver injury in vivo. Serum regucalcin has a potential sensitivity as a specific biochemical marker of chronic liver injury with hepatitis. Regucalcin has been proposed to be a key molecule in cellular regulation and metabolic disease.

Regulation of mitochondrial dehydrogenases by calcium ions

Studies in Bristol in the 1960s and 1970s, led to the recognition that four mitochondrial dehydrogenases are activated by calcium ions. These are FAD-glycerol phosphate dehydrogenase, pyruvate dehydrogenase, NAD-isocitrate dehydrogenase and oxoglutarate dehydrogenase. FAD-glycerol phosphate dehydrogenase is located on the outer surface of the inner mitochondrial membrane and is influenced by changes in cytoplasmic calcium ion concentration. The other three enzymes are located within mitochondria and are regulated by changes in mitochondrial matrix calcium ion concentration. These and subsequent studies on purified enzymes, mitochondria and intact cell preparations have led to the widely accepted view that the activation of these enzymes is important in the stimulation of the respiratory chain and hence ATP supply under conditions of increased ATP demand in many stimulated mammalian cells. The effects of calcium ions on FAD-isocitrate dehydrogenase involve binding to an EF-hand binding motif within this enzyme but the binding sites involved in the effects of calcium ions on the three intramitochondrial dehydrogenases remain to be fully established. It is also emphasised in this article that these three dehydrogenases appear only to be regulated by calcium ions in vertebrates and that this raises some interesting and potentially important developmental issues.

Suppressive effect of endogenous regucalcin on nitric oxide synthase activity in cloned rat hepatoma H4-II-E cells overexpressing regucalcin

The role of endogenous regucalcin, which is a regulatory protein in calcium signaling, in the regulation of nitric oxide (NO) synthase activity in the cloned rat hepatoma H4-II-E cells was investigated. Hepatoma cells were cultured for 24-72 h in the presence of fetal bovine serum (FBS; 10%). NO synthase activity in the 5,500 g supernatant of cell homogenate was significantly increased by the addition of calcium chloride (10 microM) and calmodulin (2.5 microg/ml) in the enzyme reaction mixture. The presence of trifluoperazine (TFP; 50 microM), an antagonist of calmodulin, inhibited the effect of calcium (10 microM) addition in increasing NO synthase activity, indicating the existence of Ca(2+)/calmodulin-dependent NO synthase in hepatoma cells. NO synthase activity was significantly decreased by the addition of regucalcin (10(-8) or 10(-7) M) in the reaction mixture without or with Ca(2+)/calmodulin addition. The effect of regucalcin (10(-7) M) in decreasing NO synthase activity was also seen in the presence of TFP (50 microM) or EGTA (1 mM). The presence of anti-regucalcin monoclonal antibody (10-50 ng/ml) in the reaction mixture caused a significant elevation of NO synthase activity. NO synthase activity was significantly suppressed in the hepatoma cells (transfectants) overexpressing regucalcin. This decrease was completely abolished in the presence of anti-regucalcin monoclonal antibody (50 ng/ml) in the reaction mixture. Moreover, the effect of Ca(2+)/calmodulin addition in increasing NO synthase activity in the hepatoma cells (wild-type) was completely prevented in transfectants. The present study demonstrates that endogenous regucalcin has a suppressive effect on NO synthase activity in the cloned rat hepatoma H4-II-E cells.

Role of endogenous regucalcin in nuclear regulation of regenerating rat liver: suppression of the enhanced ribonucleic acid synthesis activity

The role of endogenous regucalcin in the regulation of ribonucleic acid (RNA) synthesis activity in the nucleus of normal and regenerating rat livers was investigated. Nuclear RNA synthesis was measured by the incorporation of [(3)H]-uridine 5'-triphosphate into the nuclear RNA in vitro. The presence of regucalcin (0.25 or 0.5 microM) in the reaction mixture caused a significant decrease in nuclear RNA synthesis of normal rat liver. alpha-Amanitin (10(-8)-10(-6) M), an inhibitor of RNA polymerase II and III, decreased significantly nuclear RNA synthesis activity. The effect of regucalcin (0.25 microM) in decreasing nuclear RNA synthesis activity was not seen in the presence of alpha-amanitin (10(-6) M). The calcium chloride (10 microM)-increased nuclear RNA synthesis activity was significantly suppressed by the addition of regucalcin (0.25 microM). RNA synthesis activity was significantly enhanced in the nuclei of regenating rat liver obtained at 24, 48, or 72 h after partial hepatectomy. This enhancement was significantly inhibited in the presence of PD98059 (10(-5) M), staurosporine (10(-6) M), or vanadate (10(-3) M). Western analysis of the nuclei of regenerating liver obtained at 24, 48, or 72 h after partial hepatectomy showed a significant increase in regucalcin protein as compared with that of sham-operated rats. The presence of anti-regucalcin monoclonal antibody (25 or 50 ng/ml) in the reaction mixture caused a significant increase in nuclear RNA synthesis activity of normal rat liver. This increase was completely blocked by the addition of regucalcin (1.0 microM). The effect of anti-regucalcin monoclonal antibody (50 ng/ml) in increasing nuclear RNA synthesis activity was significantly enhanced in the nuclei of regenerating liver obtained at 24, 48, or 72 h after partial hepatectomy. This enhancement was significantly suppressed by the addition of alpha-amanitin (10(-6) M), PD98059 (10(-5) M), staurosporine (10(-6) M), or vanadate (10(-3) M) in the reaction mixture. The present study demonstrates that endogenous regucalcin has a suppressive effect on the enhancement of RNA synthesis activity in the nucleus of regenerating rat liver with proliferative cells.

Suppressive role of endogenous regucalcin in the enhancement of nitric oxide synthase activity in liver cytosol of normal and regucalcin transgenic rats

The suppressive role of endogenous regucalcin, which is a regulatory protein of calcium signaling, in the enhancement of nitric oxide (NO) synthase activity in the liver cytosol of rats was investigated. The enzyme activity was measured in a reaction mixture containing either vehicle or calcium chloride (1-20 microM) in the absence or presence of regucalcin (0.1, 0.25, or 0.5 microM). NO synthase activity was significantly increased by the addition of calcium (5-20 microM). This increase was completely abolished in the presence of trifluoperazine (TFP; 10-50 microM), an antagonist of Ca(2+)/calmodulin. The addition of regucalcin (0.1-0.5 microM) caused a significant fall in the calcium-increased enzyme activity. The effect of regucalcin (0.25 microM) in decreasing NO synthase activity was seen in the presence of ethylene glycol bis-(2-aminoethylether) N,N,N',N'-tetraacetic acid (EGTA, 1 mM) or TFP (20 microM), indicating that regucalcin acts independent on Ca(2+)/calmodulin. NO synthase activity was significantly raised in the presence of anti-regucalcin monoclonal antibody (10-50 ng/ml) in the reaction mixture. The effect of the antibody (50 ng/ml) or calcium (10 microM) in elevating NO synthase activity in the liver cytosol of normal rats was not seen in the liver cytosol obtained from regucalcin transgenic rats. Moreover, the increase in NO synthase activity in the liver cytosol of normal rats induced by a single intraperitoneal administration of calcium (5.0 mg/100 g body weight) was significantly enhanced in the presence of anti-regucalcin monoclonal antibody (50 ng/ml) in the reaction mixture. The administration of calcium caused a significant increase in regucalcin level in the liver cytosol of normal rats. The present study demonstrated that endogenous regucalcin plays a suppressive role in the enhancement of NO synthase activity in the liver cytosol of rats.

Potential role of regucalcin as a specific biochemical marker of chronic liver injury with carbon tetrachloride administration in rats

The potential sensitivity of liver specific protein regucalcin as a biochemical marker of chronic liver injury with carbon tetrachloride (CCl4) administration in rats was investigated. CCl4 (10%; 1.0 ml/100 g body wt) was orally given 5 times at 3-day intervals to rats, and the animals were killed by bleeding at 3, 6, 18, and 30 days after the first administration of CCl4. The body weight of rats was significantly lowered 3 and 6 days after CCI4 administration as compared with that of control rats administered with corn oil, and then the weight was restored at 18 and 30 days. Serum glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) activities were significantly increased 3 days after the administration, while a significant increase in serum y-glutamyltranspeptidase (gamma-GTP) activity was seen at 3 and 6 days after the administration. Serum GOT, GPT, and gamma-GTP activities were restored to control levels at 18 and 30 days after CCl4 administration. Serum albumin, alpha-fetoprotein, and ammonium levels were not changed by CCl4 administration. Meanwhile, serum regucalcin concentration was markedly increased 3 and 6 days after CCl4 administration, and a significant increase in serum regucalcin concentration was observed 18 and 30 days after the administration. Liver regucalcin mRNA and liver cytosolic regucalcin levels were significantly decreased 18 and 30 days after CCl4 administration. Liver content of calcium, which intracellular calcium homeostasis is maintained, was significantly increased between 3 and 30 days after CCl4 administration. Hepatic mitochondrial succinate dehydrogenase activity was significantly increased 30 days after the administration. The present study demonstrates that serum regucalcin has a potential sensitivity as a specific biochemical marker of chronic liver injury with CCl4 administration in rats.

The role of regucalcin in nuclear regulation of regenerating liver

Regucalcin was discovered in 1978 as a Ca(2+)-binding protein that does not contain EF-hand motif of Ca(2+)-binding domain [Yamaguchi, M., and Yamamoto T., Chem. Pharm. Bull. 26, 1915-1918, 1978]. The name regucalcin was proposed for this Ca(2+)-binding protein, which can regulate liver cell functions related to Ca(2+). Regucalcin has been demonstrated to play a multifunctional role in liver and kidney cells, for which regucalcin mRNA expression and its protein content are pronounced. Hepatic regucalcin mRNA expression has been shown to be mediated through signaling pathway of Ca(2+)/calmodulin-dependent protein kinase, protein kinase C, and tyrosine kinase. AP-1- and NF-1-like factors can bind to the promotor region of the rat regucalcin gene to mediate the Ca(2+) response for transcriptional activation. Growing evidence supports the view, moreover, that regucalcin plays an important role in the regulation of Ca(2+) signaling from the cytoplasm to nuclei in the proliferative cells of regenerating rat liver. Also, regucalcin has been demonstrated to be transported to liver nucleus, and it can inhibit nuclear protein kinase, protein phosphatase, and DNA and RNA synthesis in regenerating liver. Regucalcin plays a physiologic role in the control for overexpression of proliferative cells. Regucalcin has been proposed to be an important regulatory protein in nuclear signaling system.

Expression of Ca(2+)-binding protein regucalcin in rat brain neurons: inhibitory effect on protein phosphatase activity

The expression of Ca(2+)-binding protein regucalcin and its role in the regulation of protein phosphatase activity in rat brain neuronal cells obtained with primary culture was investigated. The expression of regucalcin mRNA was demonstrated by reverse transcription-polymerase chain reaction (RT-PCR) analysis in brain neuronal cells using rat regucalcin-specific primers. Moreover, regucalcin protein in brain neuronal cells was detected by Western blot analysis using a polyclonal rabbit anti-regucalcin antibody. The presence of anti-regucalcin monoclonal antibody (20 or 50 ng/ml) in the enzyme reaction mixture caused a significant increase in protein phosphatase activity toward phosphotyrosine, phosphoserine and phosphothreonine in the reaction mixture containing the cytosol of neuronal cell homogenates. This increase was completely prevented by the addition of regucalcin (10(-8) M). Protein phosphatase activity toward three phosphoaminoacids was significantly elevated by the addition of Ca(2+) (100 microM) and calmodulin (5 microg/ml). This elevation was completely blocked by the addition of regucalcin (10(-8) M). The present study demonstrates that regucalcin is expressed in rat brain neuronal cells, and that it has an inhibitory effect on protein phosphatase activity in the cells.

Role of regucalcin in calcium signaling

Regucalcin was discovered in 1978 as a calcium-binding protein that does not contain EF-hand motif of Ca(2+)-binding domain [M. Yamaguchi and T. Yamamoto, Chem. Pharm. Bull. 26 1915-1918 (1978)]. In recent years, regucalcin has been demonstrated to play an important role as a regulatory protein in Ca2+ signaling in rat liver and kidney cells. The organization of the rat regucalcin gene consists of seven exons and six introns. The mRNA is mainly present in liver and kidney with a size of 1.8 kb. Hepatic regucalcin mRNA expression has been shown to be stimulated by various factors including calcium, calcitonin, insulin, and estrogen in rats. The mRNA is also expressed in hepatoma cells (Morris hepatoma, HepG2, and rat hepatoma H4-II-E cells). Regucalcin plays a role in the maintenance of intracellular Ca2+ homeostasis due to activating Ca2+ pump enzymes in the plasma membrane (basolateral membrane) and microsomes of liver and renal cortex cells. Moreover, regucalcin has an inhibitory effect on the activation of Ca2+/calmodulin-dependent enzymes and protein kinase C. Also, regucalcin has been demonstrated to regulate nuclear function in liver cells; it can inhibit Ca(2+)-activated DNA fragmentation, DNA and RNA synthesis, protein kinase and protein phosphatase activities in the nuclei. Such an effect is also seen in the nuclei of regenerating rat liver. Regucalcin may play a physiological role in the control for overexpression of proliferative cells. Regucalcin has been proposed to be an important regulatory protein in Ca2+ signaling system, and it plays a multifunctional role in liver and kidney cells.

Stimulatory effect of regucalcin on proteolytic activity is impaired in the kidney cortex cytosol of rats with saline ingestion

The effect of regucalcin (RC) on neutral proteolytic activity in the cytosol of rat kidney cortex was investigated. Proteolytic activity was significantly increased by the presence of RC (0.01-0.10 microM) in the enzyme reaction mixture. This increase was completely abolished by the addition of anti-RC monoclonal antibody (150 ng/ml). When the renal cortex cytosol was incubated without RC addition, the degradation of globin of substrate was demonstrated by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. This degradation was clearly inhibited by the addition of anti-RC antibody (150 ng/ml), indicating that protein degradation results partly from the cytosolic endogenous RC. Meanwhile, proteolytic activity was significantly decreased in the renal cortex cytosol of rats with saline ingestion for 2, 7, and 14 days. The effect of RC (0.1 microM) in increasing proteolytic activity was weakened in the kidney cortex cytosol of saline-ingested rats. The present study suggests that endogenous RC plays a role in the activation of proteases in the renal cortex cytosol, and that the RC effect is impaired in saline-ingested rats.

Expression of calcium-binding protein regucalcin and microsomal Ca2+-ATPase regulation in rat brain: attenuation with increasing age

The expression of calcium-binding protein regucalcin and its effect on the microsomal Ca2+-ATPase activity in rat brain tissues was investigated. The expression of regucalcin mRNA was demonstrated by reverse transcription-polymerase chain reaction (RT-PCR) analysis in brain tissues using rat regucalcin-specific primers. Regucalcin concentration in the brain tissues was about 5 x 10(-9)) M as measured using enzyme-linked immunoadsorbent assay (ELISA), and this level was lowered with increasing age (50 weeks old). The presence of regucalcin (10(-9) to 10(-7) M) in the enzyme reaction mixture caused a significant decrease in Ca2+-ATPase activity in the brain microsomes of young rats (5 weeks old). Meanwhile, the enzyme activity was not significantly altered by the addition of calmodulin (1 or 50 microg/ml), calbindin (1 or 10 microg/ml), and S-100 A protein (5 or 25 microg/ml), which are other Ca2+-binding proteins in rat brain. The effect of regucalcin to inhibit microsomal Ca2+-ATPase activity was weakened in the brain of rats with increasing age (50 weeks old). The present study demonstrates that regucalcin is expressed in the brain, and that it can uniquely inhibit Ca2+-ATPase activity in the brain microsomes of rats. The findings suggest that regucalcin plays a role in the regulation of microsomal Ca2+-ATPase activity in rat brain tissues.

Stimulatory effect of regucalcin on proteolytic activity in rat renal cortex cytosol: involvement of thiol proteases

The effect of regucalcin, a calcium-binding protein, on neutral proteolytic activity in the cytosol of rat kidney cortex was investigated. Proteolytic activity was significantly increased in the presence of regucalcin (0.01-0.25 microM) in the enzyme reaction mixture. This increase was not significantly altered by the addition of CaCl, (0.01 and 1.0 mM) or EGTA (1.0 mM), indicating that the effect of regucalcin was independent on Ca2+. The effect of regucalcin to increase proteolytic activity was completely prevented in the presence of N-ethylmaleimide (5 mM), a modifying reagent of thiol groups. Proteolytic activity was clearly elevated by dithiothreitol (5 mM). This elevation was further enhanced by regucalcin (0.1 microM). Meanwhile, the stimulatory effect of regucalcin on proteolytic activity was not significantly altered in the presence of diisopropylfluorophosphate (2.5 mM), an inhibitor of serine proteases. Also, the regucalcin effect was not appreciably changed by the addition of EDTA (2.5 mM), a chelator of metal ions, indicating that it is not involved in metal-related proteases. These results demonstrate that regucalcin can increase proteolytic activity in the cytosol of rat kidney cortex. Regucalcin may activate thiol proteases independent on Ca2+.

Role of calcium-binding protein regucalcin in regenerating rat liver

Regucalcin is a novel calcium-binding protein which does not contain EF-hand motif as a Ca²⁺ -binding domain. The organization of the rat regucalcin gene consists of seven exons and six introns. Its mRNA is mainly present in liver but slightly in kidney with a size of 1.8 kb. Hepatic regucalcin mRNA expression is stimulated by various factors including calcium, calcitonin, insulin, and oestrogen in rats. The mRNA is also expressed in hepatoma cells (Morris hepatoma and HepG2). Regucalcin plays a role in the maintenance of cytosolic Ca²⁺ homeostasis in liver cells. Moreover, regucalcin has an inhibitory effect on Ca²⁺ /calmodulin-dependent enzyme activation, protein kinase C activation, and many Ca²⁺ -activated enzymes, indicating a role in the regulation of the Ca²⁺ -signalling system. Recently, regucalcin has been demonstrated to regulate nuclear function in liver cells. Regucalcin can inhibit Ca²⁺ -activated nuclear DNA fragmentation in rat isolated liver nuclei. Furthermore, the liver nuclear DNA and RNA syntheses are inhibited by regucalcin. Such an effect of regucalcin is also seen in the nuclei of regenerating rat liver. The regucalcin mRNA level is increased in regenerating liver. These findings suggest that regucalcin plays a regulatory role in the suppression for overexpression of proliferative cells.

Relative contribution of Ca2+-dependent mechanism in glucagon-induced glucose output from the liver

Divalent cations are known to affect the activity of the cAMP-generating system. By observing the effects of the addition of cobalt (Co2+) and the depletion of calcium (Ca2+), this study tried to determine the relative contribution of Ca2+-dependent mechanism in glucagon-induced glucose output from the isolated perfused rat liver. Co2+ (1 mM) completely suppressed glucose and cAMP output induced by 0.1 nM glucagon and partly suppressed those induced by 1 to 10 nM glucagon. Co2+ (1-5 mM) did not inhibit 125I-labeled glucagon binding to hepatic cell membrane. Phenylephrine- or angiotensin II-induced glucose output was not affected by 1 mM Co2+. Co2+ (1 mM) inhibited a glucagon-induced increase in [Ca2+]i in isolated rat hepatocytes but did not inhibit a phenylephrine-induced increase in [Ca2+]i. The removal of Ca2+ from the perfusion medium impaired phenylephrine- or angiotensin II-induced glucose output, but did not impair glucagon-induced glucose output. When glucagon-induced cAMP production was inhibited by Co2+, the glucose output produced by 1 to 10 nM glucagon was impaired further in the Ca2+-free perfusion. Addition of 0.1 mM IBMX increased the glucose output produced by 1 nM glucagon but did not increase that produced by 10 nM glucagon in the Co2+-containing Ca2+-free perfusion. These results suggest that Co2+ inhibits the glucagon-responsive adenylyl cyclase system directly, resulting in impaired glucose output. Glucagon increases [Ca2+]i through a mechanism different from that of phenylephrine. Glucagon (0.01-10 nM)-induced glucose output from the liver is derived mainly through a cAMP-dependent mechanism. Only when glucagon-induced cAMP production was inhibited by Co2+ was the Ca2+ dependency observed in high concentrations (>/=1 nM) of glucagon-induced glucose output, and it approximated 30% of the glucose output produced by 10 nM glucagon.

Stimulatory effect of calcium administration on regucalcin mRNA expression is attenuated in the kidney cortex of rats ingested with saline

The expression of calcium-binding protein regucalcin mRNA in the kidney cortex of rats ingested with saline was investigated. The alteration in regucalcin mRNA levels was analyzed by Northern blotting using liver regucalcin complementary DNA (0.9 kb of open reading frame). Rats were freely given saline as drinking water for 7 days. Regucalcin mRNA levels in the kidney cortex were suppressed by saline ingestion. When calcium chloride (10 mg Ca/100 g body weight) was intraperitoneally administered to rats ingested with saline for 7 days, the effect of calcium administration to increase regucalcin mRNA levels was weakened by saline ingestion. Such effect was also seen by the administration of 2.5 and 5 mg Ca/100 g. Regucalcin mRNA levels in the kidney cortex of spontaneous hypertensive rats (SHR) were not appreciably increased by the administration of calcium (10 mg/100 g). Meanwhile, calcium content in the kidney cortex was significantly elevated by the administration of calcium (10 mg/100 g) to normal rats. This increase was weakened in saline-ingested rats. Moreover, Ca2+/calmodulin-dependent protein kinase activity in the cytosol of kidney cortex was significantly decreased by saline ingestion. These results suggest the possibility that saline ingestion-induced suppression of regucalcin mRNA expression in the kidney cortex is partly involved in the attenuation of Ca2+ signalling.

Expression of calcium-binding protein regucalcin mRNA in fetal rat liver is stimulated by calcium administration

The expression of hepatic calcium-binding protein regucalcin mRNA in fetal rats was investigated. The alteration in regucalcin mRNA levels was analyzed by Northern blotting using liver regucalcin cDNA (0.9 kb with complete open reading frame). Hepatic regucalcin mRNA levels were progressively increased with fetal development; the mRNA was clearly expressed at 15 and 21 days of pregnancy but only slightly at the 8 days. Meanwhile, beta-actin mRNA levels in the fetal liver were remarkable at 8 and 15 days of pregnancy. The fetal liver regucalcin mRNA levels at 15 days of pregnancy were significantly decreased by overnight-fasting of maternal rats. The oral administration of calcium chloride (50 mg Ca/100 g body weight) to maternal rats at 15 days of pregnancy caused a remarkable elevation (about 2 fold) of regucalcin mRNA levels in the fetal liver; this increase was seen 60 and 180 min after the calcium administration. After birth, regucalcin mRNA was increasingly expressed in the livers of newborn and weanling rats, while hepatic beta-actin mRNA expression was not appreciably altered with increasing ages. These findings demonstrate that the expression of hepatic regucalcin mRNA is increased with fetal development, and that the gene expression may be stimulated by the ingestion of dietary calcium.

Inhibitory effect of regucalcin on Ca2+/calmodulin-dependent cyclic AMP phosphodiesterase activity in rat kidney cytosol

The effect of regucalcin, a novel Ca2+-binding protein, on Ca2+/calmodulin-dependent cyclic adenosine monophosphate (AMP) phosphodiesterase activity in the cytosol of rat renal cortex was investigated. Regucalcin with physiologic concentration (10[-7] M) in rat kidney had no effect on cyclic AMP phosphodiesterase activity in the absence of CaCl2 and calmodulin. However, the activatory effect of both CaCl2 (10 microM) and calmodulin (20 U/ml) on cyclic AMP phosphodiesterase was markedly inhibited by the addition of regucalcin (10[-8] to 10[-6] M) in the enzyme reaction mixture. The inhibitory effect of regucalcin on the enzyme activity was also seen in the presence of CaCl2 (5-50 microM) or calmodulin (5-50 U/ml) with increasing concentrations. The presence of trifluoperazine (10 microM), an antagonist of calmodulin, caused a partial inhibition of Ca2+/calmodulin-dependent cyclic AMP phosphodiesterase activity. This inhibition was further enhanced by the addition of regucalcin (10[-7] M). The inhibitory effect of regucalcin (10[-7] M) was not seen in the presence of 20 microM trifluoperazine. Moreover, the activatory effect of calmodulin (20 U/ml) on cyclic AMP phosphodiesterase was not entirely seen, when calmodulin was added 10 min after incubation in the presence of CaCl2 (10 microM) and regucalcin (10[-7] M). The present results demonstrates that regucalcin has an inhibitory effect on Ca2+/calmodulin-dependent cyclic AMP phosphodiesterase activation in the cytosol of rat renal cortex.

Inhibitory effect of calcium-binding protein regucalcin on ribonucleic acid synthesis in isolated rat liver nuclei

The effect of regucalcin, a Ca2+-binding protein isolated from rat liver cytosol, on ribonucleic acid (RNA) synthesis in the nuclei of normal rat liver and of regenerating rat liver was investigated. The liver weight at 1 day after partial hepatectomy was increased about 50% of that of sham-operated (control) rats. Calcium chloride (1.0-20 microM Ca2+ as final concentration) was added into the reaction mixture of nuclear RNA synthesis. RNA synthesis was established by incorporation of [3H]-uridine 5'-triphosphate (UTP) into the nuclear RNA. Addition of Ca2+ (5 and 10 microM) caused a significant increase of RNA synthesis in the nuclei from control rat liver. Such effect of Ca2+ was potentiated in the nuclei of regenerating liver; nuclear RNA synthesis was increased about 2 fold by the 1.0 and 2.5 microM Ca2+ addition. The stimulatory effect of Ca2+ was significantly inhibited by the presence of alpha-amanitin (10(-8) M), an inhibitor of RNA polymerase II. The presence of regucalcin (0.25 and 0.5 microM) significantly inhibited RNA synthesis in the nuclei from control rat liver and from regenerating rat liver. The inhibitory effect of regucalcin was remarkable in the presence of EGTA (0.5 mM), and it was weakened by the addition of Ca2+ (5 microM). Such regucalcin effect was not seen in the presence of alpha-amanitin. The presence of anti-regucalcin IgG in the reaction mixture significantly increased RNA synthesis in the nuclei from control rat liver, indicating that the endogenous regucalcin may be involved in nuclear RNA synthesis. The present results demonstrate that regucalcin can inhibit nuclear RNA synthesis in rat liver. Regucalcin may have an inhibitory role in the regulation of liver nuclear RNA synthesis.

Increase in calcium content and Ca(2+)-ATPase activity in the brain of fasted rats: comparison with different ages

The effect of fasting on calcium content and Ca(2+)-ATPase activity in the brain tissues of 5 weeks and 50 weeks old rats was investigated. Brain calcium content and Ca(2+)-ATPase activity in the microsomal and mitochondrial functions of the brain homogenate from young and elderly rats were significantly increased by overnight-fasting. These increases were appreciably restored by a single oral administration of glucose solution (400 mg/100 g body weight) to fasted rats. In comparison with young and elderly rats, brain calcium content and microsomal Ca(2+)-ATPase activity were significantly elevated by increasing ages. The effect of ageing was not seen in the brain mitochondrial Ca(2+)-ATPase activity. When calcium (50 mg/100 g) was orally administered to young and elderly rats, brain calcium content was significantly elevated. The calcium administration-induced increase in brain calcium content was greater in elderly rats than in young rats. Also, calcium administration caused a significant increase in Ca(2+)-ATPase activity in the microsomal and mitochondrial fractions of brain homogenates from young rats. In aged rats, the microsomal Ca(2+)-ATPase activity was not further enhanced by calcium administration, although the mitochondrial enzyme activity was significantly raised. The present study demonstrates that the fasting-induced increase in brain calcium content is involved in Ca(2+)-ATPase activity raised in the brain microsomes and mitochondria of rats with different ages, supporting a energy-dependent mechanism in brain calcium accumulation.

Activatory effect of calcium-binding protein regucalcin on ATP-dependent calcium transport in the basolateral membranes of rat kidney cortex

The effect of regucalcin, a calcium-binding protein, on ATP-dependent Ca2+ transport in the basolateral membranes isolated from rat kidney cortex was investigated. The prepared membranes were in inside-out oriented and membrane vesicles. Ca(2+)-ATPase activity in the basolateral membranes was progressively elevated by increasing concentrations of regucalcin (10(-8) to 10(-6) M) in the reaction mixture. This increase was dependent on Ca2+ addition. The activatory effect of regucalcin on the enzyme is inhibited by the presence of digitonin (5 x 10(-3)%) which can solubilize the membranous lipids. Moreover, the regucalcin effect was clearly abolished by the presence of vanadate (0.1 mM) or N-ethylmaleimide (5.0 mM). However, the effect of calmodulin (6 x 10(-7) M) to increase Ca(2+)-ATPase activity was not significantly inhibited by vanadate or N-ethylmaleimide, indicating that the action mode of regucalcin differs from that of calmodulin. Also, the activatory effect of regucalcin on Ca(2+)-ATPase was appreciably inhibited by addition of dibutyryl cAMP (10(-5) and 10(-3) M), while inositol 1,4,5-trisphosphate (10(-7) and 10(-5) M) had no effect. Dibutyryl cAMP itself did not have an effect on the enzyme activity. Furthermore, the 45Ca2+ uptake by the basolateral membranes was clearly increased by the presence of regucalcin (10(-7) and 10(-6) M). This increase was completely blocked by the presence of vanadate (0.1 mM), N-ethylmaleimide (5.0 mM) or dibutyryl cAMP (10(-4) and 10(-3) M) in the reaction mixture. These results clearly demonstrate that regucalcin, which is expressed in rat kidney cortex, can increase Ca(2+)-ATPase activity and Ca2+ uptake in the basolateral membranes. Regucalcin may play a cell physiologic role as an activator in the ATP-dependent Ca2+ pumps in the basolateral membranes from rat kidney cortex.

Hepatic calcium-binding protein regucalcin concentration is decreased by streptozotocin-diabetic state and ethanol ingestion in rats

The alteration in calcium-binding protein regucalcin in the liver and serum of rats with streptozotocin (STZ)-diabetic state or ethanol ingestion was investigated. STZ (6.0 mg/100 g body weight) was subcutaneously administered in rats, and 1 or 3 weeks later they were sacrificed by bleeding. Liver regucalcin mRNA levels were not clearly altered by the diabetic state, as evidenced by Northern blotting using regucalcin cDNA (0.9 kb of open reading frame). Based on enzyme-linked immunoadsorbent assay (ELISA) with rabbit-anti-regucalcin IgG, hepatic regucalcin concentration was decreased about 50% of control levels by STZ treatment. However, serum regucalcin concentration was not significantly altered by STZ treatment. Meanwhile, when rats ingested ethanol (10 and 30%) in the drinking water for 2 weeks, liver regucalcin mRNA levels were clearly increased, although hepatic regucalcin concentration was significantly decreased. Serum regucalcin concentration was not appreciably altered. Serum transaminases (GOT and GPT) activities were significantly increased at 1 or 3 weeks after STZ administration in rats, while their activities were not altered by ethanol ingestion. The present study demonstrates that hepatic regucalcin concentration is decreased independent of mRNA expression in the STZ-diabetes and during ethanol ingestion in rats.

Stimulatory effect of regucalcin on ATP-dependent calcium transport in rat liver plasma membranes

The effect of regucalcin, a calcium-binding protein isolated from rat liver cytoplasm, on ATP-dependent calcium transport in the plasma membrane vesicles of rat liver was investigated. (Ca(2+)- Mg2+)-ATPase activity in the liver plasma membranes was significantly increased by the presence of regucalcin (0.1-0.5 microM) in the enzyme reaction mixture. This increase was completely inhibited by the presence of sulfhydryl group modifying reagent Nethylmaleimide (5.0 mM NEM) or digitonin (0.04%), which can solubilize the membranous lipids. When ATP-dependent calcium uptake by liver plasma membrane vesicles was measured by using 45CaCl2, the presence of regucalcin (0.1-0.5 microM) in the reaction mixture caused a significant increase in the 45Ca2+ uptake. This increase was about 2-fold with 0.5 microM regucalcin addition. An appreciable increase was seen by 5 min incubation with regucalcin addition. The regucalcin-enhanced ATP-dependent 45Ca2+ uptake by the plasma membrane vesicles was completely inhibited by the presence of NEM (5.0 mM) or digitonin (0.04%). These results demonstrate that regucalcin activates (Ca(2+)-Mg2+)-ATPase in the liver plasma membranes and that it can stimulate ATP-dependent calcium transport across the plasma membranes.

Potential sensitivity of hepatic specific protein regucalcin as a marker of chronic liver injury

A novel calcium-binding protein regucalcin has been shown to be specifically expressed in the liver of various specifies including human. Regucalcin concentration in the serum of patients with chronic liver injury was estimated by enzyme-linked immunoadsorbent assay (ELISA) with rabbit-anti-regucalcin IgG. Serum samples were obtained from 42 persons who were diagnosed as liver disorder. Serum regucalcin concentration in all patients was in the range of 3.7-69.6 ng/ml, although regucalcin was not entirely seen in the serum of normal subjects (10 persons) without hepatitis. Meanwhile, in 18 patients with liver injury, serum glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) activities were normal value (less than 40 I.U./I). Serum GOT and GPT activities from 24 patients showed a comparatively higher level (50-234 I.U./I). The present results demonstrate the potential sensitivity of regucalcin as a marker of chronic liver injury.

Effect of apoptosis-related compounds on Ca2+ transport system in isolated rat liver nuclei

The effect of various inhibitors of DNA topoisomerase II, which has been shown to induce apoptotic cell death, on Ca2+ transport in isolated rat liver nuclei was investigated. Ca2+ uptake and release were determined with a Ca2+ electrode. The presence of aurintricarboxylic acid (ATA; 10(-6) to 10(-4) M), etoposide (10(-4) M), genistein (10(-5) and 10(-4) M) or amsacrine (10(-4) M) in the reaction mixture caused a significant increase in Ca2+ release from the nuclei. Also, these compounds (10(-4) M) significantly inhibited Ca2+ uptake by the nuclei. However, the presence of ATA (10(-5) and 10(-4) M) in the enzyme reaction mixture did not significantly inhibit Ca2+-ATPase activity, which is involved in the nuclear Ca2+ uptake, in the liver nuclei, while etoposide (10(-4) M), genistein (10(-4) M) and amsacrine (10(-4) M) appreciably decreased the enzyme activity. Meanwhile, addition of Ca2+ clearly activated DNA fragmentation in the liver nuclei. The Ca2+ activated DNA fragmentation was significantly prevented by the presence of etoposide, genistein and amsacrine with the concentrations of 10(-5) and 10(-4) M in the reaction mixture, although ATA (10(-5) and 10(-4) M) had no effect. The present study demonstrates that some apoptosis inducible compounds used can influence on Ca2+ transport system in isolated rat liver nuclei, suggesting a decrease of nuclear Ca2+ level involved in nuclear functions.

The 5' end sequences and exon organization in rat regucalcin gene

The 5'-flanking region of the gene for a Ca(2+)-binding protein regucalcin was cloned from a rat genomic library which was constructed in lambda EMBL3 SP6/T7 vector. The genomic library was screened by using the radiolabeled probe with the 5' region (0.5 kb) of rat regucalcin complementary deoxyribonucleic acid (cDNA). Positive clone had the 5.5 kb fragment which was hybridized with the 5'-probe. This fragment contained three exons (I-III) of the gene coding for a rat regucalcin. The nucleotide sequence of exons completely agreed with that of a rat regucalcin cDNA clone. A supposed translational initiation site existed in the exon II. Homology analysis showed that a putative transcription start site in the rat regucalcin gene was located at position 26 downstream from a TATA-box. Another upstream element, a CCAAT box-like sequence, was located at -170. Moreover, there were many regulatory elements (Hox, AP-1, AP-2 and AP-4) in the 5'-flanking region of the rat regucalcin gene. The organization of rat regucalcin gene seemed to be about 18 kb in size and consisted of seven exons and six introns.

Calcium-binding protein regucalcin mRNA expression in the kidney cortex is suppressed by saline ingestion in rats

The effect of adrenalectomy (ADX) or saline ingestion, which is a hypertensive factor, on the expression of calcium-binding protein regucalcin mRNA in the kidney cortex of rats was investigated. The change of regucalcin mRNA levels was analyzed by Northern blotting using rat liver regucalcin complementary DNA (0.9 kb of open-reading frame). Regucalcin mRNA was expressed in the kidney cortex but not the medulla. Rats were adrenalectomized, and 48 h later they were sacrificed. ADX caused a reduction of regucalcin mRNA levels in the kidney cortex, suggesting that adrenal glands participate in the regulation of the mRNA expression. This reduction was not restored by the subcutaneous administration of dexamethasone with an effective dose (1 mg/kg body weight), which can stimulate kidney regucalcin mRNA expression. Regucalcin mRNA levels in the kidney cortex of rats were markedly suppressed by the ingestion of saline for 7 days. The ADX-induced decrease of renal cortex regucalcin mRNA levels was not appreciably restored by saline ingestion. Moreover, regucalcin mRNA levels in the kidney cortex of spontaneous hypertensive rats (SHR) were clearly decreased as compared with that of control (Wistar-Kyoto) rats. Meanwhile, calcium content in the kidney cortex was not significantly decreased by ADX or saline ingestion. The present study suggests that the expression of regucalcin mRNA in the kidney cortex of rats is suppressed by saline administration.

Enhancement of plasma membrane (Ca(2+)-Mg2+)-ATPase activity in regenerating rat liver: involvement of endogenous activating protein regucalcin

The alteration of (Ca(2+)-Mg2+)-ATPase activity in the plasma membranes of regenerating rat liver after a partial hepatectomy was investigated. Liver was surgically removed about two thirds of that of sham-operated rats. The reduced liver weight by partial hepatectomy was restored about 50% at 24 h after the surgery, and it was completely restored at 72 h. Regenerating liver significantly increased calcium content and plasma membrane (Ca(2+)-Mg2+)-ATPase activity between 12-48 h after hepatectomy. Those increases were maximum at 24 h after the surgery. The regenerating liver-induced increase in hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase activity was completely abolished by the presence of anti-regucalcin IgG (1.0-4.0 micrograms/ml). The regenerating liver-induced increase in hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase activity was clearly inhibited by N-ethylmaleimide (2.5 and 5.0 mM) addition into the enzyme reaction mixture. This NEM effect was also seen for the activatory effect with regucalcin (0.25 microM) addition on the enzyme activity in the plasma membranes from normal rat liver. The endogenous regucalcin may play a cell physiological role in the activation of the plasma membrane (Ca(2+)-Mg2+)-ATPase to maintain the intracellular calcium level in regenerating rat liver.

Calcium-binding protein regucalcin inhibits deoxyribonucleic acid synthesis in the nuclei of regenerating rat liver

The effect of regucalcin, a Ca(2+)-binding protein isolated from rat liver cytosol, on deoxyribonucleic acid (DNA) synthesis in the nuclei of regenerating rat liver was investigated. At 1 day after partial hepatectomy, the liver weight was increased about 50% of that of sham-operated rats, and it reached to the same levels as sham operation at 3 days after hepatectomy. Nuclear DNA synthesis was markedly increased at 1 day after hepatectomy, and this increase was also seen at 3 days. Nuclear DNA synthesis was clearly enhanced in the presence of EGTA (0.4 mM) in the incubation mixture. The presence of Ca2+ (1.0-25 microM) caused a significant decrease in the nuclear DNA synthesis of normal rat liver. Regucalcin (0.25 and 0.5 microM) clearly inhibited the nuclear DNA synthesis of normal rat liver. This inhibition was also seen in the presence of Ca2+ (1.0 microM). Moreover, in the liver nuclei obtained at 1 day after partial hepatectomy, the presence of regucalcin (0.05-0.5 microM) caused a remarkable inhibition of nuclear DNA synthesis. This effect was also revealed in the presence of EGTA (0.4 mM). Thus, the inhibitory effect of regucalcin was remarkable in regenerating rat liver nuclei in comparison with that of normal rat liver. The present results demonstrate that regucalcin can suppress nuclear DNA synthesis in regenerating rat liver. We suppose that regucalcin may have a role in the regulation of nuclear DNA synthesis in liver cell proliferation.

Characterization of calcium accumulation in the brain of rats administered orally calcium: the significance of energy-dependent mechanism

The characterization of calcium accumulation in the brain of rats administered orally calcium chloride solution was investigated. Rats received a single oral administration of calcium (15-50 mg/100 g body weight), and they were sacrificed by bleeding between 15 and 120 min after the administration. The administration of calcium (50 mg/100 g) produced a significant increase in serum calcium concentration and a corresponding elevation of brain calcium content, indicating that the transport of calcium into the brain is associated with the elevation of serum calcium levels. The increase in brain calcium content by calcium administration was not appreciably altered by the pretreatment with Ca2+ channel blockers (verapamil or diltiazem with the doses of 1.5 and 3.0 mg/100 g). In thyroparathyroidectomized rats, the administration of calcium (50 mg/100 g) caused a significant increase in brain calcium content, indicating that calcium-regulating hormones do not participate in the brain calcium transport. Now, brain calcium content was clearly elevated by fasting (overnight), although serum calcium level was not significantly altered. Calcium administration to fasted rats induced a further elevation of brain calcium content as compared with that of control (fasted) rats. The fasting-induced increase in brain calcium content was appreciably restored by refeeding. This restoration was also seen by the oral administration of glucose (0.4 g/100 g) to fasted rats. The present study demonstrates that serum calcium is transported to brain, and that the increased brain calcium is released promptly. The release of calcium from brain may be involved in energy metabolism, and this release may be weakened by the reduction of glucose supply into brain. The finding suggests a physiological significance of energy-dependent mechanism in the regulation of brain calcium.

Activatory effect of regucalcin on hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase is impaired by liver injury with carbon tetrachloride administration in rats

The alteration of the plasma membrane (Ca(2+)-Mg2+)-ATPase activity in the liver of rats administered orally carbon tetrachloride (CCl4) solution was investigated. Rats received a single oral administration of CCl4 (10, 25 and 50%, 1.0 ml/100 g body weight), and 3 or 24 h later they were sacrificed. CCl4 administration caused a remarkable elevation of liver calcium content and a corresponding increase in liver plasma membrane (Ca(2+)-Mg2+)-ATPase activity, indicating that the increased Ca2+ pump activity is partly involved in calcium accumulation in liver cells. Moreover, the participation in regucalcin, which is an intracellular activating factor on the enzyme, was examined by using anti-regucalcin IgG. The plasma membrane (Ca(2+)-Mg2+)-ATPase activity increased by CCl4 administration was not entirely inhibited by the presence of anti-regucalcin IgG (1.0 and 2.5 ug/ml) in the enzyme reaction mixture. However, the effect of regucalcin (0.25-1.0 uM) to activate (Ca(2+)-Mg2+)-ATPase in the liver plasma membranes of normal rats was not revealed in the liver plasma membranes obtained from CCl4-administered rats. Also, the effect of regucalcin was not seen when the plasma membranes were washed with 1.0 mM EGTA, indicating that the disappearance of regucalcin effect is not dependent on calcium binding to the plasma membranes due to liver calcium accumulation. Now, the presence of dithiothreitol (5 mM) or heparin (20 ug/ml) caused a remarkable elevation of the plasma membrane (Ca(2+)-Mg2+)-ATPase activity in the liver obtained from CCl4-administered rats. Thus, the regucalcin effect differed from that of dithiothreitol or heparin. The present study suggests that the impairment of regucalcin effect on Ca2+ pump activity in liver plasma membranes is partly contribute to hepatic calcium accumulation induced by liver injury with CCl4 administration.

Stimulation of microsomal cholesterol ester hydrolase by glucagon, cyclic AMP analogues, and vasopressin in isolated rat hepatocytes

Short-term activation of microsomal cholesterol ester hydrolase by glucagon, cAMP analogues, and vasopressin in isolated rat hepatocytes is described. Glucagon led to a dose- and time-dependent activation of cholesteryl oleate hydrolysis, but values returned to basal levels within 120 min. Exposure of isolated hepatocytes to 0.5 mM concentrations of dibutyryl-cAMP or 8-[4-chlorophenylthio]-cAMP, or 25 microM forskolin caused persistent activation of cholesterol ester hydrolase activity after a lag period of 30 min. The three agents resulted in early marked intracellular accumulation of cAMP that declined progressively, and moderate and sustained reductions in the diacylglycerol content. The actions of glucagon on hepatocytes were inhibited by pretreatment of cells with 10 nM [8-arginine] vasopressin. Vasopressin elicited a consistent and sustained increase in cholesterol ester hydrolase activity and diacylglycerol without affecting cAMP while reducing the effect of glucagon on cAMP. Furthermore, the effects of glucagon and vasopressin on the activation of cholesterol ester hydrolase were not additive despite the similarity of their stimulation of diacylglycerol formation. Blockade of vasopressin-mediated activation of cholesterol ester hydrolase and diacylglycerol content were induced by excess prazosin. These data suggest that stimulation of microsomal cholesterol ester hydrolase in isolated liver cells may involve at least two signal transduction systems.

Expression of calcium-binding protein regucalcin mRNA in hepatoma cells

Whether the gene expression of hepatic Ca(2+)-binding protein regucalcin is altered in hepatomas was investigated. The change in regucalcin mRNA levels was analyzed by Northern blotting using liver regucalcin complementary DNA (0.9 kb). Rat hepatoma was induced by continuous feeding of basal diet containing 0.06% 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB). After 35 weeks feeding, rats were sacrificed, and the non-tumorous and tumorous tissues of the livers were removed. In individual rats, the regucalcin mRNA levels in the tumorous tissues were generally decreased in comparison with that of the non-tumorous tissues of the chemical-fed rats, although the chemical administration might decrease the mRNA expression in normal rat liver, suggesting that the chemical administration causes a suppressive effect on the mRNA expression. When the genomic DNA extracted from the liver tumorous tissues was digested with restriction enzymes (EcoRI, BamHI and HindIII) and analyzed by Southern blotting, no rear-ranged band was found in the regucalcin gene from the hepatoma. Interestingly, in the transplantable Morris hepatoma cells, the regucalcin mRNA was markedly expressed, while the albumin mRNA was expressed only slightly. The present study demonstrates that regucalcin mRNA is clearly expressed in the transformed cells (Morris hepatoma cells).

Genomic cloning and chromosomal assignment of rat regucalcin gene

The gene for a Ca(2+)-binding protein regucalcin was cloned from a rat genomic library which was constructed in lambda FIX II by screening with radiolabeled probe (complementary DNA of rat liver regucalcin). Positive clone had 19.9 kb insert of size and contained four exons of the gene coding for a rat regucalcin. These exons included the partial coding sequence (61.2% of open reading frame) and the entire 3'-untranslated region of the gene. The nucleotide sequence of exons completely agreed with that of a rat regucalcin cDNA clone. The sequence analysis of the clone showed that the identifier sequence and two simple repeated sequences exist in the intron of the gene. Moreover, chromosomal location of the rat regucalcin gene was determined by direct R-banding fluorescence in situ hybridization (FISH) method with the 19.9 kb clone containing four exons. The regucalcin gene was localized on rat chromosome Xq11.1-12 proximal end.

Stimulatory effect of hormonal signaling factors on (Ca(2+)-Mg2+)-ATPase activity in rat liver plasma membranes: cross talk with regucalcin

The effect of hormonal signaling factors on (Ca(2+)-Mg2+)-ATPase activity in rat liver plasma membranes was investigated. The presence of inositol-glycan (10(-7)-10(-5) M), dibutyryl cAMP (10(-4) and 10(-3) M) or inositol 1,4,5-trisphosphate (IP3; 10(-6) and 10(-5) M) in the enzyme reaction mixture produced a significant increase in (Ca(2+)-Mg2+)-ATPase activity. These effects were completely inhibited by the presence of vanadate (10(-4) M), an inhibitor of the enzyme phosphorylation, and N-ethylmaleimide (5 x 10(-3) M), a SH group modifying reagent. Meanwhile, regucalcin, a Ca(2+)-binding protein isolated from rat liver cytosol, increased the enzyme activity by binding to the SH groups of (Ca(2+)-Mg2+)-ATPase in liver plasma membranes. The presence of regucalcin (0.25 microM) with an effective concentration completely inhibited the effect of inositol-glycan (10(-5) M) to increase (Ca(2+)-Mg2+)-ATPase activity, while the effect of dibutyryl cAMP (10(-3) M) or IP3 (10(-5) M) was not altered. The inositol-glycan effect was not modulated by the presence of dibutyryl cAMP or IP3. Now, the preincubation of the plasma membranes with regucalcin did not modify the effect of inositol-glycan on the enzyme activity, suggesting that regucalcin competes with inositol-glycan for the binding to the plasma membranes. The present results suggest that there may be a cross talk with regucalcin and hormonal signaling factors in the regulation of (Ca(2+)-Mg2+)-ATPase activity in liver plasma membranes.

Suppressed expression of calcium-binding protein regucalcin mRNA in the renal cortex of rats with chemically induced kidney damage

The alteration of Ca(2+)-binding protein regucalcin mRNA expression in the kidney cortex of rats administered cisplatin and cephaloridine, which can induce kidney damage, was investigated. Cisplatin (0.25, 0.5 and 1.0 mg/100 g body weight) or cephaloridine (25, 50 and 100 mg/100 g) was intraperitoneally administered in rats, and 1, 2 and 3 days later they were sacrificed. The alteration in serum findings after the administration of cisplatin (1.0 mg/100 g) or cephaloridine (50 and 100 mg/100 g) demonstrated chemically induced kidney damage; blood urea nitrogen (BUN) concentration increased markedly and serum inorganic phosphorus or calcium concentration decreased significantly. Moreover, the administration of cisplatin (1.0 mg/100 g) or cephaloridine (100 mg/100 g) caused a remarkable increase of calcium content in the kidney cortex of rats, indicating kidney damage. The expression of regucalcin mRNA in the kidney cortex was markedly reduced by the administration of cisplatin or cephaloridine in rats, when the mRNA levels were analyzed by Northern blotting using rat liver regucalcin cDNA (0.9 kb). The mRNA decreases were seen with the used lowest dose of cisplatin or cephaloridine. The present study clearly demonstrates that the mRNA expression of Ca(2+)-binding protein regucalcin in the kidney cortex of rats is decreased by chemically induced kidney damage.

Characterization of regucalcin effect on proteolytic activity in rat liver cytosol: relation to cysteinyl-proteases

The increasing effect of regucalcin, isolated from rat liver cytosol, on neutral proteolytic activity in the hepatic cytosol was characterized. The proteolytic activity was markedly elevated by the addition of regucalcin (0.1-0.5 microM) in the absence of Ca2+. This increase was not significantly altered by the presence of diisopropylfluorophosphate (DPF; 2.5 mM)--although DFP caused a significant decrease in the proteolytic activity. Regucalcin (0.25 microM) additively enhanced the dithiothreitol (DTT; 1.0 mM)--increased proteolytic activity, while the regucalcin or DTT effect was completely abolished by NEM (5 mM), indicating that regucalcin may act on the SH group in proteases. Also, regucalcin (0.25 microM) enhanced the effect of Ca2+ (10 microM) increasing liver proteolytic activity, suggesting that regucalcin does not influence on the active sites for Ca2+ in proteases. Moreover, the proteolytic activity of regucalcin (0.25 microM) was significantly decreased by the presence of calpastatin (24 micrograms/ml), an inhibitor of Ca(2+)-activated neutral protease (calpain). Now, regucalcin (0.25 microM) increased about 7-fold the activity of m-calpain isolated from rabbit skeletal muscle. These observations demonstrate that regucalcin directly activates cysteinyl-proteases. Regucalcin may have a role as a potent proteolytic activator in the cytoplasm of liver cells.

Effect of nuclear Ca2+ uptake inhibitors on Ca(2+)-activated DNA fragmentation in rat liver nuclei

The effect of nuclear Ca2+ uptake inhibitors on the Ca(2+)-activated DNA fragmentation in rat liver nuclei was investigated. The addition of Ca2+ (40 microM) into the reaction mixture containing liver nuclei in the presence of 2.0 mM ATP caused a remarkable increase in nuclear DNA fragmentation. This Ca(2+)-activated DNA fragmentation was not seen in the absence of ATP, because nuclear Ca2+ uptake is not initiated without ATP addition. Moreover, the presence of various reagents (10 microM arachidonic acid, 2.0 mM NAD+, 10 microM zinc sulfate and 0.2 mM N-ethylmaleimide), which could inhibit Ca(2+)-ATPase activity and Ca2+ uptake in the nuclei, produced a significant inhibition of the Ca(2+)-activated DNA fragmentation in the nuclei. The results show that the Ca(2+)-activated DNA fragmentation is involved in the uptake of Ca2+ by the nuclei, suggesting a role of Ca2+ transport system in the regulation of liver nuclear functions.

Enhancement of nuclear Ca(2+)-ATPase activity in regenerating rat liver: involvement of nuclear DNA increase

The alteration of calcium content, Ca(2+)-ATPase activity, DNA content and DNA fragmentation in the nuclei of regenerating rat liver was investigated. Liver was surgically removed about 70% of that of sham-operated rats. The reduced liver weight by partial hepatectomy was completely restored at 3 days after the surgery. Regenerating liver significantly increased Ca(2+)-ATPase activity and DNA content in the nuclei between 1 and 5 days after hepatectomy. The nuclear calcium content was clearly increased from 2 days after hepatectomy. The increase of Ca(2+)-ATPase activity in regenerating liver was clearly inhibited by the presence of trifluoperazine (10 microM), staurosporine (2.5 microM) and dibucaine (10 microM), which are inhibitors of calmodulin and protein kinase, in the enzyme reaction mixture. However, the nuclear enzyme activity in normal rat liver was not significantly altered by these inhibitors. Meanwhile, the increase of nuclear DNA content in regenerating liver was completely blocked by the administration of trifluoperazine (2.5 mg/100 g body weight), suggesting an involvement of calmodulin. Now, the nuclear DNA fragmentation was significantly decreased in regenerating liver, suggesting that this decrease is partly contributed to the increase in nuclear DNA content. The present study clearly demonstrates that regenerating liver enhances nuclear Ca(2+)-ATPase activity and induces a corresponding elevation of nuclear calcium content. This Ca(2+)-signaling system may be involved in the regulation of nuclear DNA functions in regenerating rat liver.

Expression of calcium-binding protein regucalcin mRNA in the kidney cortex of rats: the stimulation by calcium administration

The expression of calcium-binding protein regucalcin mRNA in the kidney cortex of rats was investigated. The change of regucalcin mRNA levels was analyzed by Northern blotting using liver regucalcin complementary DNA (0.9 kb of open-reading frame). Regucalcin mRNA was expressed in the kidney cortex, and this expression was clearly increased by a single intraperitoneal administration of calcium chloride solution (5-15 mg Ca/100 g body weight) in rats; this increase was remarkable at 60-120 min after the administration. Thyroparathyroidectomy (TPTX) caused a slight decrease of regucalcin mRNA levels in the kidney cortex. However, the administration of calcium (10 mg/100 g) in TPTX rats produced a clear increase of regucalcin mRNA levels in the kidney cortex. The subcutaneous administration of calcitonin (10-100 MRC mU/100 g) or parathyroid hormone [1-34] (1-10 U/100 g) in TPTX rats which received calcium (10 mg/100 g) administration did not cause an appreciable alteration of regucalcin mRNA levels in the kidney cortex, suggesting that the mRNA expression is not stimulated by calcium-regulating hormones. The administration of trifluoperazine (TFP; 5 mg/100 g), an inhibitor of Ca2+/calmodulin action, completely blocked the expression of regucalcin mRNA stimulated by calcium administration. Now, calcium content in the kidney cortex was significantly elevated by a single intraperitneal administration of calcium (10 mg/100 g) in rats. The present study clearly demonstrates that the expression of regucalcin mRNA in the kidney cortex is stimulated by calcium administration in rats. This expression may be mediated through Ca2+/calmodulin action in the kidney cortex.

Increase of (Ca(2+)-Mg2+)-ATPase activity in hepatic plasma membranes of rats administered orally calcium: the endogenous role of regucalcin

The alteration of the plasma membrane (Ca(2+)-Mg2+)-ATPase activity in the liver of rats administered orally calcium chloride solution was investigated. The plasma membrane (Ca(2+)-Mg2+)-ATPase activity was significantly increased by a single oral administration of calcium (10, 25 and 50 mg/100 g body weight) in rats. This increase was seen between 10 and 60 min after the administration. The presence of anti-regucalcin IgG (1.0-5.0 micrograms/ml) in the enzyme reaction mixture caused a complete inhibition for the elevation of the plasma membrane (Ca(2+)-Mg2+)-ATPase activity by the addition of regucalcin (0.25 microM). Also, the calcium administration-induced increase in hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase activity was completely abolished by the presence of anti-regucalcin IgG (1.0 and 2.5 micrograms/ml). Moreover, the calcium administration-induced increase in hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase activity was not inhibited by vanadate (0.1 and 0.2 mM) addition into the enzyme reaction mixture, although the inhibitory effect of vanadate was seen in the plasma membranes from normal rat liver. Now, the activating effect of regucalcin (0.25 microM) on hepatic plasma membrane (Ca(2+)-Mg2+)-ATPase was not inhibited by vanadate addition. The endogenous regucalcin may play a role in the calcium administration-induced increase of (Ca(2+)-Mg2+)-ATPase activity in the liver plasma membranes of rats.