Alpha- and beta-adrenergic stimulation of parenchymal cell Ca2+ influx. Influence of extracellular pH

Aspartic Acid in Health and Disease

Aspartic acid exists in L- and D-isoforms (L-Asp and D-Asp). Most L-Asp is synthesized by mitochondrial aspartate aminotransferase from oxaloacetate and glutamate acquired by glutamine deamidation, particularly in the liver and tumor cells, and transamination of branched-chain amino acids (BCAAs), particularly in muscles. The main source of D-Asp is the racemization of L-Asp. L-Asp transported via aspartate-glutamate carrier to the cytosol is used in protein and nucleotide synthesis, gluconeogenesis, urea, and purine-nucleotide cycles, and neurotransmission and via the malate-aspartate shuttle maintains NADH delivery to mitochondria and redox balance. L-Asp released from neurons connects with the glutamate-glutamine cycle and ensures glycolysis and ammonia detoxification in astrocytes. D-Asp has a role in brain development and hypothalamus regulation. The hereditary disorders in L-Asp metabolism include citrullinemia, asparagine synthetase deficiency, Canavan disease, and dicarboxylic aminoaciduria. L-Asp plays a role in the pathogenesis of psychiatric and neurologic disorders and alterations in BCAA levels in diabetes and hyperammonemia. Further research is needed to examine the targeting of L-Asp metabolism as a strategy to fight cancer, the use of L-Asp as a dietary supplement, and the risks of increased L-Asp consumption. The role of D-Asp in the brain warrants studies on its therapeutic potential in psychiatric and neurologic disorders.

Aspartate-glutamate carrier 2 (citrin): a role in glucose and amino acid metabolism in the liver

Aspartate-glutamate carrier 2 (AGC2, citrin) is a mitochondrial carrier expressed in the liver that transports aspartate from mitochondria into the cytosol in exchange for glutamate. The AGC2 is the main component of the malate-aspartate shuttle (MAS) that ensures indirect transport of NADH produced in the cytosol during glycolysis, lactate oxidation to pyruvate, and ethanol oxidation to acetaldehyde into mitochondria. Through MAS, AGC2 is necessary to maintain intracellular redox balance, mitochondrial respiration, and ATP synthesis. Through elevated cytosolic Ca2+ level, the AGC2 is stimulated by catecholamines and glucagon during starvation, exercise, and muscle wasting disorders. In these conditions, AGC2 increases aspartate input to the urea cycle, where aspartate is a source of one of two nitrogen atoms in the urea molecule (the other is ammonia), and a substrate for the synthesis of fumarate that is gradually converted to oxaloacetate, the starting substrate for gluconeogenesis. Furthermore, aspartate is a substrate for the synthesis of asparagine, nucleotides, and proteins. It is concluded that AGC2 plays a fundamental role in the compartmentalization of aspartate and glutamate metabolism and linkage of the reactions of MAS, glycolysis, gluconeogenesis, amino acid catabolism, urea cycle, protein synthesis, and cell proliferation. Targeting of AGC genes may represent a new therapeutic strategy to fight cancer. [BMB Reports 2023; 56(7): 385-391].

Aspartate-glutamate carrier 2 (citrin): a role in glucose and amino acid metabolism in the liver

Aspartate-glutamate carrier 2 (AGC2, citrin) is a mitochondrial carrier expressed in the liver that transports aspartate from mitochondria into the cytosol in exchange for glutamate. The AGC2 is the main component of the malate-aspartate shuttle (MAS) that ensures indirect transport of NADH produced in the cytosol during glycolysis, lactate oxidation to pyruvate, and ethanol oxidation to acetaldehyde into mitochondria. Through MAS, AGC2 is necessary to maintain intracellular redox balance, mitochondrial respiration, and ATP synthesis. Through elevated cytosolic Ca2+ level, the AGC2 is stimulated by catecholamines and glucagon during starvation, exercise, and muscle wasting disorders. In these conditions, AGC2 increases aspartate input to the urea cycle, where aspartate is a source of one of two nitrogen atoms in the urea molecule (the other is ammonia), and a substrate for the synthesis of fumarate that is gradually converted to oxaloacetate, the starting substrate for gluconeogenesis. Furthermore, aspartate is a substrate for the synthesis of asparagine, nucleotides, and proteins. It is concluded that AGC2 plays a fundamental role in the compartmentalization of aspartate and glutamate metabolism and linkage of the reactions of MAS, glycolysis, gluconeogenesis, amino acid catabolism, urea cycle, protein synthesis, and cell proliferation. Targeting of AGC genes may represent a new therapeutic strategy to fight cancer. [BMB Reports 2023; 56(7): 385-391].

Glycine blocks the increase in intracellular free Ca2+ due to vasoactive mediators in hepatic parenchymal cells

Recently, glycine has been shown to prevent liver injury after endotoxin treatment in vivo. We demonstrated that ethanol and endotoxin stimulated Kupffer cells to release PGE(2), which elevated oxygen consumption in parenchymal cells. Because glycine has been reported to protect renal tubular cells, isolated hepatocytes, and perfused livers against hypoxic injury, the purpose of this study was to determine whether glycine prevents increases in intracellular free Ca(2+) concentration ([Ca(2+)](i)) in hepatic parenchymal cells by agonists released during stress, such as with PGE(2) and adrenergic hormones. Liver parenchymal cells isolated from female Sprague-Dawley rats were cultured for 4 h in DMEM/F12 medium, and [Ca(2+)](i) in individual cells was assessed fluorometrically using the fluorescent calcium indicator fura 2. PGE(2) caused a dose-dependent increase in [Ca(2+)](i) from basal values of 130 +/- 10 to maximal levels of 434 +/- 55 nM. EGTA partially prevented this increase, indicating that either extracellular calcium or agonist binding is Ca(2+) dependent. 8-(Diethylamino)octyl 3,4,5-trimethoxybenzoate (TMB-8), an agent that prevents the release of Ca(2+) from intracellular stores, also partially blocked the increase in [Ca(2+)](i) caused by PGE(2), suggesting that intracellular Ca(2+) pools are involved. Together, these results are consistent with the hypothesis that both the intracellular and extracellular Ca(2+) pools are involved in the increase in [Ca(2+)](i) caused by PGE(2). Interestingly, glycine, which activates anion (i.e., chloride) channels, blocked the increase in [Ca(2+)](i) due to PGE(2) in a dose-dependent manner. Low-dose strychnine, an antagonist of glycine-gated chloride channel in the central nervous system, partially reversed the inhibition by glycine. When extracellular Cl(-) was omitted, glycine was much less effective in preventing the increase in [Ca(2+)](i) due to PGE(2). Phenylephrine, an alpha(1)-type adrenergic receptor agonist, also increased [Ca(2+)](i), as expected, from 159 +/- 20 to 432 +/- 43 nM. Glycine also blocked the increase in [Ca(2+)](i) due to phenylephrine, and the effect was also reversed by low-dose strychnine. Together, these data indicate that glycine rapidly blocks the increase in [Ca(2+)](i) in hepatic parenchymal cells due to agonists released during stress, most likely by actions on a glycine-sensitive anion channel and that this may be a major aspect of glycine-induced hepatoprotection.

Evidence for norepinephrine-activated Ca2+ permeable channels in guinea-pig hepatocytes using a patch clamp technique

To determine whether the hepatocyte plasma membrane possesses a Ca2+ channel. we applied a patch clamp technique to isolated guinea-pig hepatocytes. In a cell-attached configuration, using an internal pipette solution of 110 mM BaCl2 or CaCl2, we observed sporadic inward single channel currents (Po = 0.004 +/- 0.002, n = 6) at various membrane potentials. The unit amplitude was 0.60 +/- 0.15 pA (n = 6) at resting membrane potential. The single channel conductance was 20.4 +/- 4.6 pS (n = 6) and this channel showed no rectification and no voltage dependence. Bay K 8644, a dihydropyridine Ca2+ channel activator, did not affect this channel activity. Although norepinephrine in the pipette solution did not activate this channel, its external application increased channel activity. These observations suggest that guinea-pig hepatocytes possess Ca2+ permeable channels that differ from the voltage-operated Ca2+ channels found in excitable cells and that such channels are responsible for the agonist-stimulated Ca2+ entry in hepatocytes.

The influence of Ca2+ on the effects of glucagon on hepatic glycolysis

1. The influence of Ca2+ on the effects of glucagon on glycolysis was investigated in the isolated perfused rat liver. Livers from fed rats were perfused in an open system with Krebs/Henseleit-bicarbonate buffer (pH 7.4). Glucose release, lactate plus pyruvate production (glycolysis) and oxygen uptake were measured. The following results were obtained: 2. In livers perfused with Ca(2+)-free Krebs/Henseleit-bicarbonate buffer and after depletion of the intracellular pools, the initial and transient stimulation of glycolysis, which is normally observed shortly after the onset of glucagon infusion, was more pronounced when compared to livers perfused with normal perfusion fluid (2.5 mM Ca2+) and without previous depletion of the intracellular pools (controls); the subsequent inhibition of glycolysis was delayed in Ca(2+)-free perfused livers and was less pronounced in comparison with the controls at the end of the glucagon infusion period (20 min). 3. Perfusion with a Ca(2+)-free medium supplemented with EDTA, without previous depletion of the intracellular pools, also produced a substantial reduction in the effects of glucagon on glycolysis. 4. Ca(2+)-free perfusion did not affect the stimulative action of glucagon on glucose release (glycogenolysis) and oxygen uptake. 5. Glycolysis inhibition by cAMP also was abolished in Ca(2+)-free perfused livers, and the initial stimulation was enhanced. 6. Mn2+, a metal ion known as a competitor of Ca2+, considerably reduced the action of glucagon on glycolysis; Mn2+ did not affect the basal rates of glycolysis. 7. Sr2+, a metal ion that is often recognized as Ca2+ by several biological structures and processes, increased the inhibitory action of glucagon on glycolysis. 8. Several organic compounds, which directly or indirectly take part in Ca2+ fluxes, were also able to diminish (e.g., verapamil) or even to abolish (carbenoxolone) the inhibitory action of glucagon on glycolysis. 9. It was concluded that, under the conditions of the living cell, Ca2+ is important for glycolysis inhibition by glucagon. In principle at least, the results can be explained in terms of the known Ca2+ dependencies of several protein kinases and protein phosphatases.

Co-ordinated control of apical calcium influx and basolateral calcium efflux in rabbit cortical collecting system

Transcellular Ca2+ transport in the distal nephron involves passive Ca2+ influx at the apical membrane, diffusion through the cytosol and active extrusion across the opposing basolateral membrane. The molecular identity of the apical Ca2+ entry step is still elusive, but its regulatory aspects have been analyzed in the present study. To this end, rabbit connecting and cortical collecting tubular cells were cultured on permeable and transparent supports and the apical Ca2+ influx was deduced from Mn2+ quenching of Ca2+ independent Fura-2 fluorescence, while the intracellular Ca2+ concentration ([Ca2+]i) was measured simultaneously. In parallel experiments, transcellular Ca2+ transport was determined isotopically as 45Ca2+ flux from the apical to basolateral compartment. Decreasing the apical pH from 7.4 to 5.9 inhibited transcellular Ca2+ transport by 53 +/- 1%, whereas apical Ca2+ influx was reduced by 39 +/- 7% and [Ca2+]i decreased by 18 +/- 3%. Reversal of the Na+/Ca2+ exchanger by iso-osmotic replacement of Na+ by N-methyl-D-glucamine in the basolateral compartment resulted in 50 +/- 5% inhibition of Ca2+ transport, 46 +/- 3% reduction of apical Ca2+ influx and 60 +/- 3% increase in [Ca2+]i. In the absence of basolateral Ca2+, however, this manoeuvre decreased [Ca2+]i by 21 +/- 8%, while Ca2+ transport and apical Ca2+ influx were reduced by the same magnitude as in the presence of Ca2+, that is by 53 +/- 6% and 45 +/- 4%, respectively. Stimulation of adenylyl cyclase with forskolin (10(-5) M) increased transcellular Ca2+ transport by 108 +/- 40%, stimulated apical Ca2+ influx by 120 +/- 17% and increased [Ca2+]i by 110 +/- 2%. In conclusion, the apical Ca2+ influx is regulated by apical pH, intracellular cAMP and basolateral Na+/Ca2+ exchanger activity, and is coupled in an 1:1 fashion to the rate of transepithelial Ca2+ transport.

Ca2+ dependence of gluconeogenesis stimulation by glucagon at different cytosolic NAD(+)-NADH redox potentials

The influence of Ca2+ on hepatic gluconeogenesis was measured in the isolated perfused rat liver at different cytosolic NAD(+)-NADH potentials. Lactate and pyruvate were the gluconeogenic substrates and the cytosolic NAD(+)-NADH potentials were changed by varying the lactate to pyruvate ratios from 0.01 to 100. The following results were obtained: a) gluconeogenesis from lactate plus pyruvate was not affected by Ca(2+)-free perfusion (no Ca2+ in the perfusion fluid combined with previous depletion of the intracellular pools); gluconeogenesis was also poorly dependent on the lactate to pyruvate ratios in the range of 0.1 to 100; only for a ratio equal to 0.01 was a significantly smaller gluconeogenic activity observed in comparison to the other ratios. b) In the presence of Ca2+, the increase in oxygen uptake caused by the infusion of lactate plus pyruvate at a ratio equal to 10 was the most pronounced one; in Ca(2+)-free perfusion the increase in oxygen uptake caused by lactate plus pyruvate infusion tended to be higher for all lactate to pyruvate ratios; the most pronounced difference was observed for lactate/pyruvate ratio equal to 1. c) In the presence of Ca2+ the effects of glucagon on gluconeogenesis showed a positive correlation with the lactate to pyruvate ratios; for a ratio equal to 0.01 no stimulation occurred, but in the 0.1 to 100 range stimulation increased progressively, producing a clear parabolic dependence between the effects of glucagon and the lactate to pyruvate ratio. d) In the absence of Ca2+ the relationship between the changes caused by glucagon in gluconeogenesis and the lactate to pyruvate ratio was substantially changed; the dependence curve was no longer parabolic but sigmoidal in shape with a plateau beginning at a lactate/pyruvate ratio equal to 1; there was inhibition at the lactate to pyruvate ratios of 0.01 and 0.1 and a constant stimulation starting with a ratio equal to 1; for the lactate to pyruvate ratios of 10 and 100, stimulation caused by glucagon was much smaller than that found when Ca2+ was present. e) The effects of glucagon on oxygen uptake in the presence of Ca2+ showed a parabolic relationship with the lactate to pyruvate ratios which was closely similar to that found in the case of gluconeogenesis; the only difference was that inhibition rather than stimulation of oxygen uptake was observed for a lactate to pyruvate ratio equal to 0.01; progressive stimulation was observed in the 0.1 to 100 range. f) In the absence of Ca2+ the effects of glucagon on oxygen uptake were different; the dependence curve was sigmoidal at the onset, with a well-defined maximum at a lactate to pyruvate ratio equal to 1; this maximum was followed by a steady decline at higher ratios; at the ratios of 0.01 and 0.1 inhibition took place; oxygen uptake stimulation caused by glucagon was generally lower in the absence of Ca2+ except when the lactate to pyruvate ratio was equal to 1. The results of the present study demonstrate that stimulation of gluconeogenesis by glucagon depends on Ca2+. However, Ca2+ is only effective in helping gluconeogenesis stimulation by glucagon at highly negative redox potentials of the cytosolic NAD(+)-NADH system. The triple interdependence of glucagon-Ca(2+)-NAD(+)-NADH redox potential reveals highly complex interrelations that can only be partially understood at the present stage of knowledge.

Expression of photoreceptor cyclic nucleotide-gated cation channel alpha subunit (CNGCalpha) in the liver and skeletal muscle

Glucagon and beta-adrenergic agents increase cAMP levels and stimulate Ca2+ influx in liver cells. There is no consensus as to the mechanism by which these hormones stimulate the influx of Ca2+. Using mouse retinal rod CNGCalpha cDNA probes, we cloned rat liver and skeletal muscle, and human hepatic CNGCalpha subunit sequences showing 97-100% identity with the human rod channel. In order to assess channel activity, the effect of cyclic nucleotides on free intracellular Ca2+ levels of isolated hepatocytes was measured. Dibutyryl-cAMP was more effective in increasing free Ca2+ levels than dibutyryl-cGMP. These data indicate that the CNGCalpha subunit is expressed in both the liver and skeletal muscle possibly mediating hormonal effects on ion fluxes.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

A hypothesis for the hormonal regulation of gluconeogenesis, in which increases in cytosolic free-Ca2+ levels ([Ca2+]i) play a major role, is presented. This hypothesis is based on the observation that gluconeogenic hormones evoke a common pattern of Ca2+ redistribution, resulting in increases in [Ca2+]i. Current concepts of hormonally evoked Ca2+ fluxes are presented and discussed. It is suggested that the increase in [Ca2+]i is functionally linked to stimulation of gluconeogenesis. The stimulation of gluconeogenesis is accomplished in two ways: (1) by increasing the activities of the Krebs cycle and the electron-transfer chain, thereby supplying adenosine triphosphates (ATP) and reducing equivalents to the process; and (2) by stimulating the activities of key gluconeogenic enzymes, such as pyruvate carboxylase. The hypothesis presents a conceptual framework that ties together two interrelated manifestations of hormone action: signal transduction and metabolism.

Characterisation of the divalent cation channels of the hepatocyte plasma membrane receptor-activated Ca2+ inflow system using lanthanide ions

The ability of Gd3+ to inhibit vasopressin-stimulated Ca2+ inflow to hepatocytes was compared with its effect on Mn2+ inflow. In the absence of Gd3+, the stimulation of Mn2+ inflow by vasopressin increased with increasing pH of the extracellular medium. Maximal inhibition of vasopressin-stimulated Ca2+ and Mn2+ inflow by saturating concentrations of Gd3+ was 70 and 30%, respectively. Gd3+ also inhibited thapsigargin-stimulated Ca2+ and Mn2+ inflow with maximal inhibition of 70 and 40%, respectively. It is concluded that vasopressin and thapsigargin each activate two types of Ca2+ inflow processes, one which is sensitive and one which is insensitive to lanthanides. The nature of the pore of the lanthanide-sensitive Ca2+ channel was investigated further using different lanthanides as inhibitors. Tm3+, Gd3+, Eu3+, Nd3+ and La3+ each inhibited vasopressin-stimulated Ca2+ and Mn2+ inflow but had no effect on Ca2+ inflow in the absence of an agonist, or on vasopressin-stimulated release of Ca2+ from intracellular stores. Maximal inhibition of vasopressin-stimulated Ca2+ inflow in the presence of a saturating concentration of each lanthanide ranged from 70-90%. An equation which describes a 1:1 interaction of the lanthanide with a putative binding site in the Ca2+ channel gave a good fit to dose-response curves for the inhibition of vasopressin-stimulated Ca2+ inflow by each lanthanide. Lanthanides in the middle of the series exhibited the lowest dissociation constant (Kd) values. The Kd for Gd3+ increased with increasing extracellular Ca2+ concentration, suggesting competitive inhibition of Ca2+ binding by Gd3+. In the absence of lanthanide, vasopressin-stimulated Mn2+ inflow was substantially reduced when the plasma membrane was depolarised by increasing the extracellular K+ concentration. Changing the membrane potential had little effect on the maximum inhibition by Gd3+ of vasopressin-stimulated Mn2+ inflow. The Kd for inhibition of vasopressin-stimulated Ca2+ inflow by Gd3+, measured at the lowest attainable membrane potential, was about 6-fold lower than the Kd measured at the highest attainable membrane potential. The idea that there is a site in the vasopressin-stimulated lanthanide-sensitive Ca2+ channel composed of carboxylic acid groups which bind Ca2+, Mn2+ or a lanthanide ion is consistent with the data obtained using the different lanthanides.

Inhibition of the increase of intrahepatic Ca2+ by diltiazem in rats with liver ischemia

The effects of a continuous infusion of a calcium entry blocker, 1, 5-benzothiazepine derivative (diltiazem), on ischemic liver cell damage were studied using quantitative 45Ca-autoradiographic and liquid scintillation techniques. The drug was administered to male Wistar rats as a continuous infusion for 3 h, beginning 30 min before ischemia. Autoradiographic studies showed that 45Ca accumulated in the liver lobuli after 1 h of liver ischemia and 3 h of reperfusion, but the level of 45Ca accumulation was significantly lower in drug-treated rats than in untreated animals. In addition, liquid scintillation studies showed significant differences in the intrahepatic 45Ca contents. These results suggest that diltiazem may inhibit the rise of intracellular Ca2+ due to the flow of extracellular Ca2+ into the cytosol, and may protect the ischemic liver from damage.

Two separate plasma membrane Ca2+ carriers participate in receptor-mediated Ca2+ influx in rat hepatocytes

The plasma membrane Ca2+ carrier system involved in receptor-mediated Ca2+ entry was studied. Using the Ca2+ readdition protocol, the rate of cytosolic free Ca2+ concentration ([Ca2+]i) increase in vasopressin-pretreated hepatocytes was significantly higher than in thapsigargin- or 2,5-di(tert-butyl)hydroquinone-pretreated cells. The addition of Mn2+ to unstimulated hepatocytes resulted in a biphasic quench of fura-2 fluorescence. After an initial phase that was fast in rate but of short duration, the rate of fura-2 quench by Mn2+ became much slower and lasted until all the cellular fura-2 was quenched. Pretreatment of the cells with vasopressin only accelerated the rate of the latter phase but not of the initial one. In agonist-stimulated cells, acidification of the extracellular medium or the presence of ruthenium red, econazole or SK&F 96365 decreased the rates of both [Ca2+]i increase and Mn2+ entry upon addition of the respective cation. By contrast, neomycin and N-tosyl-L-phenylalanine chloromethyl ketone markedly decreased the rate of [Ca2+]i increase upon Ca2+ readdition but had no effect on vasopressin-stimulated Mn2+ entry. None of the treatments affected the ability of vasopressin and thapsigargin to mobilize the internal Ca2+ store. It is concluded that in hepatocytes the two pathways of receptor-mediated Ca2+ entry control two distinct yet pharmacologically related cation carriers.

Adrenergic modulation of Ca2+ homeostasis in isolated fish hepatocytes

Isolated hepatocytes of the American eel (Anguilla rostrata LeSueur) and brown bullhead (Ictaluras nebulosus) have been used to characterize the effects of alpha- and beta-adrenergic agonists (epinephrine, phenylephrine, isoproterenol) and antagonists (phentolamine, propranolol) on calcium flux (influx, efflux) and cytosolic free calcium concentrations ([Ca2+]i). Bullhead hepatocytes have higher influx but lower efflux of Ca2+ than eel hepatocytes, which may relate to the primary source for changes in [Ca2+]i. Adrenergic agonists did not affect influx, but significantly enhanced efflux in eel hepatocytes (not bullhead) and [Ca2+]i in both species. Increases in efflux and in [Ca2+]i were blocked by alpha-antagonists (phentolamine) but not beta-antagonists (propranolol) when present in 100-fold excess of the agonist. Isoproterenol had no significant effect on either parameter tested. This study supports our hypothesis that liver cell Ca2+ homeostasis is modulated by alpha-adrenoceptor-linked pathways in these two fish species as has been previously demonstrated for the rat.

Regulation of plasma membrane permeability to calcium in primary cultures of rat hepatocytes

Experiments were designed to characterize the hormone sensitive transport of Ca2+ from the external media into rat hepatocytes maintained in culture. In the absence of added vasopressin, hepatocytes were nearly impermeable to Ca2+, whereas a significant and rapid influx of Ca2+ could be detected when external Ca2+ was added to hepatocytes after the addition of 20 nM vasopressin. The transport was measured as the initial rate of increase of free intracellular Ca2+ [( Ca2+]i) after Ca2+ addition to the external media. Most data were obtained from the majority of cells on a coverslip immersed in a spectrophotometric cuvette, but selected data were obtained by measuring Ca2+ changes in single cells. Ca2+ influx measured using a large number of cells was similar to data obtained using single cells. The Vmax of Ca2+ influx was 140 nM/s. Ca2+ transport was competitive with H+ so that the Km was 17.4 mM at pH 6.8, 3.7 mM at pH 7.4 and 1.8 mM at pH 7.8. Ca2+ influx was insensitive to external K+ (1 to 70 mM) and to the presence of 5 nM valinomycin, suggesting that it was independent of the electrical potential gradient across the plasma membrane. Transport also appeared to be insensitive to the activity of protein kinase C, which was varied by addition of the activator, 12-myristate 13-acetate phorbol ester, and by addition of the kinase inhibitor, staurosporine. Stimulation of transport following vasopressin addition exhibited a delay with a t1/2 of approximately 30 s. A vasopressin antagonist blocked the activation of transport, if added prior to vasopressin. However, experiments designed to determine the effect of hormone occupancy per se on transport activity indicated that continued hormone occupancy was not required. When the external medium was nominally Ca2+ free and an antagonist was added 1 min after vasopressin, Ca2+ entry, even 8 min after antagonist addition, was rapid. Conversely, preincubation with vasopressin antagonist in medium containing 0.5 mM Ca2+ dramatically lowered plasma membrane Ca2+ permeability. The ER Ca2+ pool emptied by vasopressin was refilled in the presence of vasopressin antagonist plus 0.5 mM Ca2+, but did not refill when the medium contained no added Ca2+. Under the conditions of these experiments, the Ca2+ levels of the ER hormone-sensitive Ca2+ pool were estimated as well as intracellular concentrations of inositol-1,4,5-trisphosphate. The Ca2+ levels of the endoplasmic reticulum correlated inversely with plasma membrane Ca2+ permeability, whereas cellular concentrations of inositol-1,4,5-trisphosphate did not correlate with Ca2+ permeability.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of intracellular pH on cytosolic free [Ca2+] in human epidermoid A-431 cells

This study characterizes the correlation between intracellular pH (pHi) and the cytosolic free Ca2+ concentration ([Ca2+]i) in suspended and adherent human epidermoid A-431 cells. Using the fluorescent dyes 2,7-bis(carboxyethyl)carboxyfluorescein acetoxymethyl ester (BCECF) and fura-2, the resting pHi and [Ca2+]i in suspended cells were 7.23 +/- 0.03 and 209 +/- 30 nM; those in adherent cells were 7.28 +/- 0.02 and 87 +/- 5 nM. Removal of external Ca2+ did not change the resting pHi but reduced the resting [Ca2+]i, indicating the resting level of [Ca2+]i is in part maintained by an influx of Ca2+ from the external medium. When both suspended and adherent cells were acidified or alkalinized, resting [Ca2+]i was altered. An intracellular acidification induced a fall in [Ca2+]i, and a rise in pHi induced a rise in [Ca2+]i. These changes in [Ca2+]i were correlated with an uptake of 45Ca2+ from the external medium, whereas no Ca2+ efflux occurred. The alteration in [Ca2+]i induced by modification of pHi was abolished in the absence of external Ca2+ or by adding 2 mM CoCl2, LaCl3, and attenuated by the addition of 2 mM MnCl2 to the bathing medium. It was insensitive to the voltage-gated Ca2+ channel blockers nifedipine or verapamil (1 mM). CoCl2, LaCl3, and MnCl2 each induced changes in pHi and [Ca2+]i but verapamil and nifedipine did not. Because CoCl2, LaCl3, and MnCl2 are also known to block Na+/Ca2+ exchange, intracellular Na+ ([Na+]i) was measured by flame photometry in acidified or alkalinized cells. In either case no change in [Na+]i was observed. Furthermore, treatment with amiloride (100 microM), a blocker of the Na+/Ca2+ exchanger, did not inhibit the pH-induced changes in [Ca2+]i. 1,2-bis(o-Aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) (100 microM), a Ca2+ chelator, induced a decrease in pHi as well as a reduction of [Ca2+]i, also supporting the direct relation between pHi and [Ca2+]i. 3,4,5-Trimethoxybenzoic acid 8-(diethylamino)ocytl ester HCl (TMB-8) (100 microM), a known blocker of intracellular Ca2+ mobilization, did not change the resting pHi and [Ca2+]i in normal cells or cells acidified or alkalinized. This observation, taken together with data from cells incubated in the absence of external Ca2+, suggests intracellular Ca2+ pools are not involved in changes in [Ca2+]i that result from a modification of pHi. Resting pHi and [Ca2+]i in cells treated with either 8-bromo-dibutyryl cAMP (1 mM) or forskolin (150 microM) are not changed.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+ oscillations induced by histamine H1 receptor stimulation in HeLa cells: Fura-2 and patch clamp analysis

The response of HeLa cells to histamine H1 receptor stimulation is characterized by periodic increases in cytosolic free Ca2+ concentration. The mechanisms underlying this oscillatory behaviour are not well understood. Fura-2 and patch clamp experiments carried out on HeLa cells have previously shown: (a) that Ca2+ oscillations are not initially dependent on the presence of external Ca2+, that external Ca2+ is required to maintain the oscillatory activity; (b) that a depolarization of the cell membrane leads to an inhibition of Ca2+ oscillations during the external Ca2+ dependent phase of the process; and (c) that Ca2+ oscillations can be abolished during this latter phase by the exogenous addition of Ca2+ channel blocking agents, such as Co2+ or La3+. The contribution of the inositol phosphate pathway to Ca2+ oscillations was more recently investigated in whole cell experiments performed with patch pipettes containing IP3 or the non-hydrolysable GTP analogue GTP-gamma S. Clear periodic current fluctuations were recorded using both patch pipette solutions. Assuming that the intracellular IP3 level remained constant under these conditions, these findings provide direct evidence that the Ca2+ oscillations in HeLa cells do not arise from a periodic production of IP3. The effect of the internal and external cell pH on the oscillatory process was also investigated in Fura-2 and patch clamp experiments. It was found that an increase in intracellular pH from 7.4 to 7.7 during the external Ca2+ dependent phase of the histamine stimulation abolishes the appearance of Ca2+ spikes whereas, a cellular acidification to pH 7.2 maintains or stimulates the Ca2+ oscillatory activity. The former effect was observed in the absence of Ca2+ in the bathing medium, indicating that the inhibitory action of alkaline pH was not related to a reduced Ca2+ entry. An increase in extracellular pH from 7.3 to 9.0 in contrast elicited an intracellular Ca2+ accumulation which resulted in most cases in an inhibition of the oscillatory process. This effect was dependent on external Ca2+ and was observed in alkaline internal pH conditions (pH 7.7). These observations suggest: (a) that the net Ca2+ influx in HeLa cells is strongly dependent on the cell internal and external pH; and (b) that the magnitude of this Ca2+ influx controls to a large extent the oscillation frequency. Finally, an inhibition of the histamine induced Ca2+ oscillatory activity was observed following the addition of the Ca(2+)-induced Ca(2+)-release (CICR) inhibitor adenine to the external medium.(ABSTRACT TRUNCATED AT 250 WORDS)

The nature and mechanism of activation of the hepatocyte receptor-activated Ca2+ inflow system

Progress in elucidation of the properties of the hepatocyte receptor-activated Ca2+ inflow system (RACIS) has been hampered by difficulties in measuring rates of Ca2+ inflow to hepatocytes. These difficulties have led, for example, to different conclusions about the relationship between the extracellular Ca2+ concentration and the movement of Ca2+ through the RACIS. The hepatocyte RACIS admits Mn2+ and a number of other divalent cations as well as Ca2+. Many of these cations also inhibit the movement of Ca2+ through this system. While the RACIS is inhibited by high concentrations of verapamil and by some other Ca2+ antagonists, it is relatively insensitive to inhibition by organic compounds which inhibit other Ca2+ channels and Ca2+ transporters. There is circumstantial evidence which suggests that the hepatocyte RACIS is an exchange system, possibly one which catalyses Ca(2+)-H+ exchange or the co-transport of Ca2+ and OH-. Other circumstantial evidence suggests that the RACIS is a channel, with some similarities to voltage-operated Ca2+ channels in excitable cells. However, experiments using the patch-clamp technique have not yet detected agonist-stimulated Ca2+ movement across the hepatocyte plasma membrane. The molecular components of the RACIS probably differ from those which facilitate the large inflow of Ca2+ to hepatocytes which occurs in the absence of an agonist. The mechanism by which agonists activate the RACIS has not been elucidated.(ABSTRACT TRUNCATED AT 250 WORDS)

Lithium distribution in mania: single-dose pharmacokinetics and sympathoadrenal function

We examined lithium distribution after a single dose of 25 mEq in 14 drug-free manic patients. Lithium concentrations were measured in plasma, red blood cells, and urine. Maximum concentrations of lithium, times at which they were attained, and influx and efflux rate constants for extracellular fluid, red blood cell, and muscle-like compartments were estimated using a three-compartment pharmacokinetic model. Tissue lithium concentrations may continue to increase for hours after plasma lithium concentrations have peaked. Rate constants for absorption, excretion, and influx and efflux for the tissue compartments were similar to those previously reported for normal subjects. Rate constants for transport into and out of the tissue compartments correlated negatively with norepinephrine or epinephrine excretion and positively with the plasma/red cell Na+ gradient. Rate constants for efflux from red blood cell and muscle compartments correlated with measures of adrenocortical function and were higher in dexamethasone nonsuppressors than in suppressors. These data show that distribution of lithium may be related to sympathodrenal activity and Na+ distribution in manic patients.

Relationship between intracellular pH (pHi) and calcium (Cai2+) in avian heart fibroblasts

Measurements of pHi and Cai2+ were made in single isolated avian heart fibroblasts using the fluorescent dyes 2,3-dicyanohydroquinone (DCH) and Indo-1. The resting level of Cai2+ is in part maintained by an influx of Ca2+ from the external medium. This flux was reduced in the absence of Ca0(2+) or by adding 2 mM LaCl3 or CoCl2 to the bathing medium; however, it was insensitive to calcium channel blockers nifedipine and verapamil. BAPTA (25 microM), a calcium chelator, also reduced Cai2+. Changes in Cai2+ brought about by any of these methods were found to be accompanied by an intracellular acidification. Experiments were carried out altering pHi using trimethylamine, propionate, and ammonium chloride to determine whether pHi could influence Cai2+. It was found that an intracellular acidification induced a fall in Cai2+ and any rise in pHi induced a rise in Cai2+. These results suggest a direct interaction between Cai2+ and pHi. Various models are described which may account for the experimental observations. The findings are discussed in terms of the possible roles for pHi and Cai2+ and their interactions to influence cell motility and adhesion.

Passive calcium influx by plasma membrane vesicles isolated from rat liver

Passive Ca2+ influx independent of ATP addition to the incubation medium, took place in plasma membrane vesicles isolated from rat liver. The rate of Ca2+ influx was found to depend on the concentration of added Ca2+, and on the incubation temperature, and was inhibited by La3+, Hg2+ and by p-chloromercuribenzoate. Influx was not blocked by calcium channel blockers, or affected by a range of uncouplers. Addition of the Ca2+ ionophore A23187 to vesicles that had taken up the ion induced a rapid efflux of Ca2+ especially when EGTA also was added to the incubation medium. A number of divalent cations inhibited Ca2+ influx. The vesicles could be frozen and stored overnight with little loss in activity. The kinetics of Ca2+ influx could be related to that which occurs in the unstimulated perfused rat liver. The data suggest that the plasma membrane vesicle preparation may be useful for further studies on the basal liver cell Ca2+ influx system in vitro.

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+.

Hormonal modulation of cytosolic free calcium

Many hormones are capable of increasing [Ca2+]i in many different tissues by mobilizing Ca2+ from internal stores, primarily the endoplasmic reticulum, and by promoting Ca2+ influx across the plasma membrane. Recent studies suggest that both processes involve G-proteins. In one case, a G-protein activates a PI 4,5-P2 specific phospholipase C, resulting in the formation of Ins 1,4,5-P3, which then releases Ca2+ from the endoplasmic reticulum. Ca2+ influx can be stimulated by low concentrations of agonist, which do not promote internal Ca2+ mobilization. This process appears to be stimulated directly by a G-protein.

Exposure to depolarizing concentrations of K+ inhibits hormonally-induced calcium influx in rat liver

The exposure of perfused rat livers to depolarizing concentrations of K+ (60 mM) by partial substitution of the NaCl in the medium with KCl induces glycogenolysis, respiratory changes and vasoconstriction. These responses were found to be inhibited 70-80% by 20 microM indomethacin and by 20 microM bromophenacyl bromide. This suggests that eicosanoids, namely prostaglandins, are involved in mediating these effects, and hence that the action of K+ involves primarily an effect on eicosanoid-producing cells (Kupffer and endothelial cells) within the liver. A 5 min pre-exposure of perfused livers to depolarizing concentrations of K+ (in the presence of indomethacin) was found to inhibit (by approx. 85%) the influx of Ca2+ induced by the co-administration of 10 nM glucagon and 10 nM vasopressin. A similar result was observed in isolated hepatocytes. The inhibition was probably not due to a decrease in the concentration of Na+ in the medium since the substitution of 80 mM NaCl with 80 mM choline chloride resulted in significantly less inhibition (30-40%). These results suggest that under these conditions the influx of Ca2+ in liver occurs through a pathway that is inhibited by high K+ concentration and/or a depolarization of the plasma membrane.

Modulation of glucagon effects by changes in extracellular pH and calcium

We have examined the influence of extracellular pH and calcium concentration on the action of glucagon on isolated rat hepatocytes, perfused liver or plasma membrane preparations. Incubation of rat hepatocytes with 10 nM glucagon at pH 7.4 caused an immediate increase in cAMP concentrations (8-fold), and this rise was almost 50% lower at acidic extracellular pH (6.9). This effect of pH could not be explained by an alteration of the hormone binding to its receptor for glucagon concentrations higher than 1 nM. The effect of acidosis on cAMP production was still present with non-hormonal effectors, such as 10 microM Gpp[NH]p, 30 microM forskolin or 10 mM NaF. This suggests a direct action of acidosis on the regulatory component Ns and/or on the catalytic subunit of adenylate cyclase. Acidic pH also depressed mitochondrial processes responsive to glucagon (NAD(P)H fluorescence, glutamine breakdown). Whatever the experimental model, calcium appeared to be required for maximal stimulation of cAMP production by glucagon. On perfused rat liver, glycogenolysis was depressed in the absence of extracellular calcium in the perfusate. In isolated hepatocytes, the stimulation of phosphorylase alpha activity by glucagon was modulated by extracellular calcium concentrations lower than 0.2 mM. This suggests that, although glucagon action is chiefly cAMP-mediated, its effect on calcium mobilization (affecting various cellular process, including cAMP production itself) should also be taken into account. This work also confirmed the importance of calcium in the stimulation of mitochondrial metabolism of glutamine by glucagon.

A new immunoblotting assay for thymidylate synthetase and its application to the regulation of enzyme activity in regenerating rat liver

A highly sensitive and specific immunoblot assay has been developed to quantitate the content of rat liver thymidylate synthetase (EC 2.1.1.45). Applying the method, it is demonstrated that the increase of the activity of thymidylate synthetase in liver regeneration after partial hepatectomy is due to the de novo synthesis of the enzyme protein. Administration of cycloheximide, phenoxybenzamine, phorbol 12-myristate 13-acetate, nifedipine, dexamethasone or indomethacin to partially hepatectomized rats prevented the synthesis of thymidylate synthetase in regenerating liver. Thyroparathyroidectomy also inhibited the increase of the enzyme in liver regeneration. These observations are discussed in relation to the signal transduction concerning the alpha 1-receptor, which was shown to regulate liver regeneration in our previous papers.

Effect of calcium channel blockers and trifluoperazine on rat liver regeneration

The activity of hepatic thymidylate synthetase and thymidine kinase at 24 h after 70% partial hepatectomy of rats was suppressed significantly compared with that in the control group by the administration of calcium channel blockers (verapamil, diltiazem and nifedipine) 8 h after partial hepatectomy. The decrease of thymidylate synthetase and thymidine kinase activities was accompanied by a reduction of DNA content in 24 h regenerating liver. Trifluoperazine showed an effect similar to that of the calcium channel blockers on DNA synthesis during liver regeneration. These results suggest that calcium entry into the hepatic cell is an essential event in liver regeneration.

Effect of extracellular Ca2+ on plasma membrane Ca2+ inflow and cytoplasmic free Ca2+ in isolated hepatocytes

An initial rapid phase and a subsequent slow phase of 45Ca2+ uptake were observed following the addition of 45Ca2+ to Ca2+-deprived hepatocytes. The magnitude of the rapid phase increased 15-fold over the range 0.1-11 mM extracellular Ca2+ (Ca2+o) and was a linear function of [Ca2+]o. The increases in the rate of 45Ca2+ uptake were accompanied by only small increases in the intracellular free Ca2+ concentration. In cells made permeable to Ca2+ by treatment with saponin, the rate of 45Ca2+ uptake (measured at free Ca2+ concentrations equal to those in the cytoplasm of intact cells) increased as the concentration of saponin increased from 1.4 to 2.5 micrograms per mg wet weight cells. Rates of 45Ca2+ uptake by cells permeabilized with an optimal concentration of saponin were comparable with those of intact cells incubated at physiological [Ca2+o], but were substantially lower than those for intact cells incubated at high [Ca2+o]. It is concluded that Ca2+ which enters the hepatocyte across the plasma membrane is rapidly removed by binding and transport to intracellular sites and by the plasma membrane (Ca2+ + Mg2+)-ATPase and the plasma membrane Ca2+ inflow transporter is not readily saturated with Ca2+o.

Studies with verapamil and nifedipine provide evidence for the presence in the liver cell plasma membrane of two types of Ca2+ inflow transporter which are dissimilar to potential-operated Ca2+ channels

The addition of 500 microM verapamil or nifedipine to isolated hepatocytes incubated in the presence of 1.3 mM Ca2+ caused 20% inhibition of Ca2+ inflow as measured by the initial rate of 45Ca2+ exchange. No stimulation of 45Ca2+ exchange was observed in the presence of the Ca2+ agonist CGP 28392. An increase in the concentration of extracellular K+ from 6 to 60 mM (to depolarize the plasma membrane) increased the initial rate of 45Ca2+ exchange by 30%. In the presence of 60 mM K+, 400 microM verapamil inhibited the initiate rate of 45Ca2+ exchange by 50%. Verapamil and nifedipine completely inhibited vasopressin-induced Ca2+ inflow as determined by measurement of the initial rate of 45Ca2+ exchange and of glycogen phosphorylase a activity. This effect of verapamil was completely reversed by increasing the extracellular concentration of Ca2+. The concentrations of Ca2+ antagonist which gave 50% inhibition of vasopressin- or K+-stimulated Ca2+ inflow were in the range 50-100 microM, about 50-fold greater than the concentration which gave 50% inhibition of the beating of electrically-stimulated myocardial muscle cells. In the absence of vasopressin, verapamil caused a transient increase in glycogen phosphorylase a activity by a process which is largely independent of Ca2+. It is concluded that verapamil and nifedipine inhibit the transport of Ca2+ across the hepatocyte plasma membrane through a putative Ca2+ transporter which is activated by vasopressin and which differs in nature from potential-operated Ca2+ channels in excitable cells and from the Ca2+ transporter present in hepatocytes in the absence of hormone.

Role of inositol lipid breakdown in the generation of intracellular signals. State of the art lecture

Many hormones, neurotransmitters, and secretagogues act by increasing the intracellular free Ca2+ concentration in target cells. The initial event following binding of agonists to specific receptors in the plasma membrane involves a receptor-mediated activation of a guanosine nucleotide-binding protein (G protein), which induces a Ca2+-independent activation of phospholipase C. This novel, presently uncharacterized G protein is inactivated by pertussis toxin-catalyzed adenosine 5'-diphosphate ribosylation in some but not all cell types. Phospholipase C catalyzes the breakdown of inositol lipids, notably phosphatidylinositol 4,5-bisphosphate, with the production of inositol phosphates and 1,2-diacylglycerol. Inositol 1,4,5-trisphosphate (IP3) is responsible for a rapid mobilization of intracellular Ca2+ by activating Ca2+ efflux from a subpopulation of the endoplasmic reticulum. The properties of this process are consistent with its being a ligand-activated ion channel with electrogenic Ca2+ efflux being charge-compensated by K+ influx. Sustained hormonal responses require extracellular Ca2+ and a prolonged elevation of the cytosolic free Ca2+. This is brought about by hormone-mediated changes of Ca2+ flux across the plasma membrane involving both an inhibition of Ca2+ efflux and an activation of Ca2+ influx. This review summarizes recent findings concerning the role of G proteins in receptor coupling to phospholipase C; the regulation of enzymes of phosphoinositide metabolism; the evidence for IP3 being a Ca2+-mobilizing second messenger and its mechanism of action; the formation of new inositol phosphates and their possible significance; the relation of intracellular Ca2+ mobilization and plasma membrane Ca2+ fluxes to the kinetics of the hormone-induced cytosolic free Ca2+ transient; and the possible roles of protein kinase C in influencing the hormone-mediated functional response.

Mechanisms involved in alpha-adrenergic phenomena

Epinephrine and norepinephrine exert many important actions by interacting with alpha 1- and alpha 2-adrenergic receptors in their target cells. Activation of alpha 2-adrenergic receptors causes platelet aggregation and other inhibitory cellular responses. Some of these responses are attributable to a decrease in cAMP due to inhibition of adenylate cyclase. Activation of alpha 2-adrenergic receptors promotes their coupling to an inhibitory guanine nucleotide binding protein (Ni). This coupling promotes the binding of GTP to Ni, causing it to dissociate into subunits. This results in inhibition of the catalytic component of adenylate cyclase. Activation of alpha 1-adrenergic receptors stimulates the contraction of most smooth muscles and alters secretion and metabolism in several tissues. The primary event is a breakdown of phosphatidylinositol-4,5-bisphosphate in the plasma membrane to produce two intracellular "messengers": myo-inositol-1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DAG). IP3 causes the release of Ca2+ from endoplasmic reticulum, producing a rapid rise in cytosolic Ca2+. Ca2+ binds to the regulatory protein calmodulin, and the resulting complex interacts with specific or multifunctional calmodulin-dependent protein kinases and other calmodulin-responsive proteins, altering their activities and thereby producing a variety of physiological responses. DAG also produces effects by activating a Ca2+-phospholipid-dependent protein kinase (protein kinase C) that phosphorylates and alters the activity of certain cellular proteins. Frequently there is synergism between the IP3 and DAG mechanisms.