Effect of thyrocalcitonin on calcium concentration in liver of intact and thyroparathyroidectomized rats

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.

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.

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.

Decrease in protein kinase and phosphatase activities in the liver nuclei of rats exposed to carbon tetrachloride

The alteration in protein kinase and phosphatase activities in the liver nuclei of rats administered carbon tetrachloride (CCl(4)) was investigated. Rats received a single oral administration of CCl(4) (1 ml/100 g body wt of 5, 10, and 25% CCl(4) in corn oil), and 5, 24, and 48 h later they were euthanized by bleeding. The administration of CCl(4) (10 and 25%) caused a significant decrease in protein kinase activity in the liver nuclei. The enzyme activity in the liver nuclei from normal and CCl(4)-administered rats was significantly increased by the addition of Ca(2+) (0.5 mM) and calmodulin (10 microg/ml) in the reaction mixture, suggesting that Ca(2+)/calmodulin-dependent protein kinase activation is not suppressed by CCl(4) treatment. Liver nuclear phosphatase activity toward phosphotyrosine, but not phosphoserine and phosphothreonine, was markedly decreased by CCl(4) (5, 10, and 25%) administration. This decrease was seen 5 h after CCl(4) administration. The presence of vanadate (10(-4) M) in the reaction mixture caused a significant decrease in phosphotyrosine phosphatase activity in the liver nuclei from normal and CCl(4)-administered rats, whereas the enzyme activity was not decreased by okadaic acid (10(-5) M) or sodium fluoride (10(-3) M). The effect of anti-regucalcin antibody (100 ng/ml) in increasing phosphotyrosine phosphatase activity was seen in the liver nuclei of CCl(4)-administered rats, suggesting that regucalcin-sensitive phosphatase activity is decreased by CCl(4) administration. The present study demonstrates that CCl(4) administration induces a decrease in protein kinase and tyrosine phosphatase activities, which are involved in signaling factors in the liver nuclei of rats.

Inhibitory effect of calcium-binding protein regucalcin on protein kinase activity in the nuclei of regenerating rat liver

The effect of Ca2+-binding protein regucalcin on protein kinase activity in the nuclei of normal and regenerating rat livers was investigated. Protein kinase activity in the nuclei isolated from normal rat liver was significantly increased by addition of Ca2+ (500 microM) and calmodulin (10 microg/ml) in the enzyme reaction mixture. Nuclear protein kinase activity was significantly decreased in the presence of EGTA (1.0 mM), trifluoperazine (TFP; 20 microM), dibucaine (10(-4) M), or staurosporine (10(-7) M), indicating that Ca2+-dependent protein kinases are present in the nuclei. Protein kinase activity was significantly elevated in the liver nuclei obtained at 6 to 48 h after a partial hepatectomy. Hepatectomy-increased nuclear protein kinase activity was significantly decreased in the presence of EGTA (1.0 mM), TFP (20 microM), or staurosporine (10(-7) M) in the enzyme reaction mixture. The presence of regucalcin (0.1-0.5 microM) caused a significant decrease in protein kinase activity in the nuclei obtained from normal and regenerating rat livers. Meanwhile, the nuclear protein kinase activity from normal and regenerating livers was significantly elevated in the presence of anti-regucalcin monoclonal antibody (50-200 ng/ml). The present study suggests that regucalcin plays a role in the regulation of protein kinase activity in the nuclei of proliferative liver cells.

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.

Alteration in calcium content and Ca2+-ATPase activity in the liver nuclei of rats orally administered carbon tetrachloride

The alteration in calcium transport in the liver nuclei of rats orally administered carbon tetrachloride (CCl4) was investigated. Rats received a single oral administration of CCl4 (5, 10, and 25%, 1.0 ml/100 g body weight), and 5, 24 and 48 h later the animals were sacrificed. The administration of CCl4 (25%) caused a remarkable elevation of calcium content in the liver tissues and the nuclei of rats. Liver nuclear Ca2+-ATPase activity was markedly decreased by CCl4 (25%) administration. The presence of dibutyryl cyclic AMP(10(-4) and 10(-3) M) or inositol 1,4,5-trisphosphate (10(-6) and 10(-5) M) in the enzyme reaction mixture caused a significant decrease in Ca2+-ATPase activity in the liver nuclei obtained from normal rat, while the enzyme activity was significantly increased by calmodulin (1.0 and 2.0 microg/ml). These signaling factor's effects were completely impaired in the liver nuclei obtained from CCl4 (25%)-administered rats. DNA fragmentation in the liver nuclei obtained from CCl4-administered rats was significantly decreased by the presence of EGTA (2 mM) in the reaction mixture, suggesting that the endogenous calcium activates nuclear DNA fragmentation. The present study demonstrates that calcium transport system in the liver nuclei is impaired by liver injury with CCl4 administration in rats.

Increase of Ca2+-ATPase activity in the brain microsomes of rats with increasing ages: involvement of protein kinase C

A possible mechanism of aging-induced increase in brain microsomal Ca2+-adenosine triphosphatase (ATPase) activity of rats was investigated. Calcium content in the brain tissues and Ca2+-ATPase activity in the brain microsomes of aging rats (50 weeks of age) increased significantly as compared with those of young rats (5 weeks of age). Brain microsomal Ca2+-ATPase activity in aging rats was decreased significantly by treatment of ethyleneglycol-bis-(aminoethylether) N,N,N',N'-tetraacetic acid (EGTA) (2.7 mM) or digitonin (10(-3)%), while such decrease was not seen in the enzyme activity of young rats. Microsomal Ca2+-ATPase activity in aging rats was markedly decreased by the presence of staurosporine (10(-8) and 10(-7) M), an inhibitor of protein kinase C, in the enzyme reaction mixture, although the enzyme activity of young rats was not inhibited. Meanwhile, dibucaine (10(-6) and 10(-5) M), an inhibitor of Ca2+/calmodulin-dependent protein kinase, did not have an effect on Ca2+-ATPase activity in the brain microsomes of young and aging rats. The addition of protein kinase C (100 and 200 mU/ml) in the reaction mixture caused a significant increase in brain microsomal Ca2+-ATPase activity of young rats. These results suggest that protein kinase C is partly involved in the elevation of brain microsomal Ca2+-ATPase activity in rats with increasing ages.

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.

Streptozotocin-induced diabetes increases (Ca2+-Mg2+)-ATPase activity in hepatic plasma membranes of rats: involvement of protein kinase C

The alteration in calcium transport in the liver of rats with streptozocin(STZ)-diabetic state was investigated. STZ (6 mg/100 g body weight) was subcutaneously administered in rats, and 1 or 2 weeks later they were sacrificed by bleeding. STZ administration caused a remarkable elevation of serum glucose concentration. Liver calcium content was significantly increased by STZ administration. Hepatic plasma membrane (Ca2+-Mg2+)-ATPase activity was markedly elevated by STZ administration. This increase was completely abolished by the presence of staurosporine (10(-7)-10(-5) M), an inhibitor of protein kinase C, in the enzyme reaction mixture, suggesting an involvement of protein kinase C signalling. Moreover, the STZ-induced increase in liver plasma membrane (Ca2+-Mg2+)-ATPase activity was significantly raised by the presence of okadaic acid (10(-5) and 10(-4) M). Meanwhile, the STZ-increased (Ca2+-Mg2+)-ATPase activity was not appreciably altered by the presence of anti-regucalcin IgG in the reaction mixture, indicating that the activatory protein regucalcin does not participate in the elevation of the enzyme activity. The present study demonstrates that STZ-induced diabetes causes the increase in hepatic plasma membrane (Ca2+-Mg2+)-ATPase activity of rats.

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.

Alterations in Ca2+-ATPase activity and calcium-binding protein regucalcin mRNA expression in the kidney cortex of rats with saline ingestion

The alteration in calcium metabolism in rats ingested with saline was investigated. Rats were freely given saline as drinking water for 2 and 7 days. Calcium concentration in the serum was significantly elevated by saline ingestion for 2 and 7 days, while serum inorganic phosphorus concentration was not altered. Serum urea nitrogen concentration was significantly increased by saline ingestion for 7 days. Calcium content in the femoral-diaphyseal and metaphyseal tissues was not altered by saline ingestion for 7 days. Calcium content in the kidney cortex was significantly elevated by saline ingestion for 7 days. Ca2+-ATPase activity in the basolsateral membranes of kidney cortex was clearly increased by saline ingestion for 2 and 7 days. The enzyme activity was not altered by the addition of sodium chloride (10(-3) and 10(-2) M), parathyroid hormone (10(-7) and 10(-6) M), and calcitonin (3 x 10(-8) and 3 x 10(-7) M) in the enzyme reaction mixture. A calcium-binding protein regucalcin mRNA expression in the kidney cortex was markedly suppressed by saline ingestion for 7 days, although such a suppression was not seen for 2 days. These results suggest that saline ingestion causes the disturbance of calcium transport system in the kidney cortex of rats, and that the renal disorder may induce hypercalcemia.

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.

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.

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.

Enhanced expression of calcium-binding protein regucalcin mRNA in regenerating rat liver

The expression of hepatic calcium-binding protein regucalcin mRNA was investigated in regenerating rat liver. The change of regucalcin mRNA levels was analyzed by Northern blotting, using liver regucalcin cDNA (0.9 kb with complete open reading frame). The reduced liver weight by partial hepatectomy (about 70%) was completely restored at 3 days after surgery. Regenerating liver significantly increased calcium content. Liver regucalcin mRNA levels clearly increased 1-5 days after hepatectomy, in comparison with that of sham-operated rats, although the increase was not seen 12 hr after the surgery. Increased regucalcin mRNA levels in regenerating liver were appreciably reduced by single intraperitoneal administration of actinomycin D (100 micrograms/100 g body weight), an inhibitor of transcriptional process. Moreover, the increased regucalcin mRNA levels by hepatectomy was weakened by a single intraperitoneal administration of trifluoperazine (2.5 mg/100 g), an inhibitor of Ca2+/calmodulin. These findings demonstrate that the expression of hepatic regucalcin mRNA is enhanced in regenerating rat liver.

Expression of calcium-binding protein regucalcin mRNA in rat liver is stimulated by calcitonin: the hormonal effect is mediated through calcium

The involvement of a hypocalcemic hormone calcitonin (CT) in the expression of hepatic Ca(2+)-binding protein regucalcin mRNA was investigated. The change of regucalcin mRNA levels was analyzed by Northern blotting using liver regucalcin complementary DNA (0.9 kb). A single oral administration of calcium chloride (100 mg Ca/100 g body weight) to rats induced a remarkable increase in the serum calcium concentration and a corresponding elevation of the liver calcium content during 120 min after the administration. Thyroparathyroidectomy (TPTX) did not cause a significant increase in the liver calcium content after calcium administration. Hepatic regucalcin mRNA level was markedly elevated by calcium administration; the level was about 180% of controls at 60 min after the administration. This increase was completely abolished by TPTX. A single subcutaneous administration of CT (synthetic eel CT; 25-100 MRC mU/100 g) to TPTX rats received oral administration of calcium (100 mg/100 g) produced a remarkable increase in hepatic regucalcin mRNA levels; the level was about 280% of controls with the dose of 25 MRC mU CT/100 g. The present finding suggests that the expression of hepatic mRNA is stimulated by CT, and that the hormonal effect is mediated through Ca2+ in rat liver.

Stimulatory effect of calcitonin on Ca2+ inflow in isolated rat hepatocytes

The effects of synthetic [Asu1,7]eel calcitonin (CT) on the unidirectional inflow of Ca2+ were investigated in isolated rat liver cells by measuring the initial rate of 45Ca2+ uptake. CT increased Ca2+ inflow, with EC50 values (concentrations giving half-maximal effect) of 10(-10) M. The action of CT was in evidence within 15 s after the addition of 45Ca2+ to the cells. CT-activated Ca2+ inflow was completely blocked by the presence of the Ca2(+)-antagonist verapamil at a concentration of 10(-8) M. Meanwhile, epinephrine (10(-8) to 10(-4) M) or phenylephrine (10(-8) to 10(-4) M) increased Ca2+ inflow within 60 s after the addition of 45Ca2+ to the cells. Those hormonal effects were additively enhanced by the presence of CT (10(-8) M). Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, increased Ca2+ inflow at a concentration of 10(-9) to 10(-5) M. The presence of CT (10(-8) M) synergistically enhanced PMA-increased Ca2+ inflow at concentrations of 10(-7) to 10(-5) M. The present results suggest that CT can stimulate the rate of Ca2+ inflow in rat liver cells.

Effect of calcitonin on exchangeable calcium transport in isolated rat hepatocytes

The effect of calcitonin (synthetic [Asu1,7]eel) on the exchangeable Ca2+ transport was investigated in isolated rat hepatocytes by measuring 45Ca2+ uptake. Calcitonin (CT) increased the uptake of Ca2+ with 74.3 pM giving a half-maximal effect. The action of CT was evident within 5 min after the hormone addition to the cells, and the increase in Ca2+ uptake was maintained during 60 min. The increase in Ca2+ uptake caused by CT was dependent on extracellular Ca2+ concentration. On the other hand, the activity of mitochondrial succinate dehydrogenase in hepatocytes was significantly increased by addition of CT (74.3 pM) to the cells in the presence of 1.3 mM Ca2+, while the hormonal effect was not seen in the absence of added Ca2+. The presence of Ca2+ ionophore A23187 (0.1 and 1.0 microM) abolished the increase in enzyme activity caused by CT addition to the cells. It is proposed that CT can increase the rate of uptake of exchangeable CA2+ by hepatocytes, and that the hormone causes the elevation of mitochondrial succinate dehydrogenase activity which is mediated through Ca2+.

Calcium/calmodulin-mediated action of calcitonin on lipid metabolism in rats

The effects of calcitonin on lipid metabolism were investigated in three kinds of rats, one strain of rabbits, and a primary culture of rat hepatocytes. In a short-term experiment, calcitonin decreased serum cholesterol and triglycerides after injection in rats on either an ordinary or high-fat diet. In a long-term experiment, calcitonin decreased the serum cholesterol and triglycerides in uremic rats, hypothalamic obese rats, and Watanabe-heritable hyperlipidemic rabbits. In cultured hepatocytes, calcitonin reduced the incorporation of [14C]acetate into cholesterol and triglycerides in a dose-dependent way. Treatment with W7, a calmodulin inhibitor, overcame the decrease caused by calcitonin in serum lipids in rats and in the synthesis of triglycerides from acetate or palmitate in the hepatocytes, but did not alter the intracellular cAMP level or incorporation of [32P]Pi into PI in the cells. The results suggest that calcitonin lowers serum lipid levels and lipogenesis in hepatocytes in a calcium/calmodulin-dependent way.

The effects of calcitonin on calcium uptake and respiration in rat-liver mitochondria

The action of calcitonin on both the transport of calcium across the mitochondrial membrane and cellular respiration has been studied in the presence and absence of added phosphate. In the presence of phosphate, both the rate of calcium entry and the amount of calcium accumulated was stimulated by calcitonin, above a threshold concentration, in a saturable manner. In the absence of phosphate, calcitonin enhanced the rate of calcium entry, but had no appreciable effect on the levels of total calcium accumulated. The minimum concentration of calcitonin necessary to produce these effects was in all cases dependent on the external calcium concentration. Mitochondrial respiration was inhibited only at calcitonin levels much higher than those affecting calcium uptake. These results are consistent with the idea that the action of calcitonin is directly related to the mechanism of calcium uptake, and not to the respiratory process.

The synthesis of metallothionein-like protein containing zinc in liver of rat after administration of calcium and calcitonin

Subcutaneous (s.c.) and intraperitoneal (i.p.) injection and oral administration of calcium (Ca) into rats increased Ca content of liver compared with control rats. When liver cytosol was filtered through a Sephadex G-75 column, zinc (Zn) content of metallothionein (MT)-like protein fraction was several times higher in Ca treatment than in control. Zn and copper (Cu) content of high-molecular (HM) and moderate-molecular (MM) fractions and Cu content of MT fraction was unaffected by Ca treatment. Zn in MT fraction showed two peaks in the direct-connection method of high-performance liquid chromatography to atomic absorption spectrophotometer (AAS). The retention time of these two peaks agreed with those of Zn or cadmium (Cd) in MT fraction of liver cytosol obtained from Zn- or Cd-administered rat. These results show that MT-like protein containing Zn is induced by Ca. In conditions showing a slight increase in liver Ca and a significant decrease in serum Ca by synthetic [Asu1,7] eel calcitonin (CT) injection, gel filtration of liver cytosol obtained from CT-treated rats showed a higher content of Zn and a higher radioactivity of [3H]cystine than that from control injection. This suggests that CT causes an increase in liver Ca and results in induction of MT-like protein containing Zn by Ca.