Electrophysiological properties of isolated rat liver cells

Deranged hepatocyte intracellular Ca2+ homeostasis and the progression of non-alcoholic fatty liver disease to hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the second leading cause of cancer-related deaths in men, and the sixth in women. Non-alcoholic fatty liver disease (NAFLD) is now one of the major risk factors for HCC. NAFLD, which involves the accumulation of excess lipid in cytoplasmic lipid droplets in hepatocytes, can progress to non-alcoholic steatosis, fibrosis, and HCC. Changes in intracellular Ca²⁺ constitute important signaling pathways for the regulation of lipid and carbohydrate metabolism in normal hepatocytes. Recent studies of steatotic hepatocytes have identified lipid-induced changes in intracellular Ca²⁺, and have provided evidence that altered Ca²⁺ signaling exacerbates lipid accumulation and may promote HCC. The aims of this review are to summarise current knowledge of the lipid-induced changes in hepatocyte Ca²⁺ homeostasis, to comment on the mechanisms involved, and discuss the pathways leading from altered Ca²⁺ homeostasis to enhanced lipid accumulation and the potential promotion of HCC. In steatotic hepatocytes, lipid inhibits store-operated Ca²⁺ entry and SERCA2b, and activates Ca²⁺ efflux from the endoplasmic reticulum (ER) and its transfer to mitochondria. These changes are associated with changes in Ca²⁺ concentrations in the ER (decreased), cytoplasmic space (increased) and mitochondria (likely increased). They lead to: inhibition of lipolysis, lipid autophagy, lipid oxidation, and lipid secretion; activation of lipogenesis; increased lipid; ER stress, generation of reactive oxygen species (ROS), activation of Ca²⁺/calmodulin-dependent kinases and activation of transcription factor Nrf2. These all can potentially mediate the transition of NAFLD to HCC. It is concluded that lipid-induced changes in hepatocyte Ca²⁺ homeostasis are important in the initiation and progression of HCC. Further research is desirable to better understand the cause and effect relationships, the time courses and mechanisms involved, and the potential of Ca²⁺ transporters, channels, and binding proteins as targets for pharmacological intervention.

Regulation of bile secretion by calcium signaling in health and disease

Calcium (Ca²⁺) signaling controls secretion in many types of cells and tissues. In the liver, Ca²⁺ regulates secretion in both hepatocytes, which are responsible for primary formation of bile, and cholangiocytes, which line the biliary tree and further condition the bile before it is secreted. Cholestatic liver diseases, which are characterized by impaired bile secretion, may result from impaired Ca²⁺ signaling mechanisms in either hepatocytes or cholangiocytes. This review will discuss the Ca²⁺ signaling machinery and mechanisms responsible for regulation of secretion in both hepatocytes and cholangiocytes, and the pathophysiological changes in Ca²⁺ signaling that can occur in each of these cell types to result in cholestasis.

Effects and mechanisms of store-operated calcium channel blockade on hepatic ischemia-reperfusion injury in rats

AIM

To further investigate the important role of store-operated calcium channels (SOCs) in rat hepatocytes and to explore the effects of SOC blockers on hepatic ischemia-reperfusion injury (HIRI).

METHODS

Using freshly isolated hepatocytes from a rat model of HIRI (and controls), we measured cytosolic free Ca(2+) concentration (by calcium imaging), net Ca(2+) fluxes (by a non-invasive micro-test technique), the SOC current (I(SOC); by whole-cell patch-clamp recording), and taurocholate secretion [by high-performance liquid chromatography and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays].

RESULTS

Ca(2+) oscillations and net Ca(2+) fluxes mediated by Ca(2+) entry via SOCs were observed in rat hepatocytes. I(SOC) was significantly higher in HIRI groups than in controls (57.0 ± 7.5 pA vs 31.6 ± 2.7 pA, P < 0.05) and was inhibited by La(3+). Taurocholate secretion by hepatocytes into culture supernatant was distinctly lower in HIRI hepatocytes than in controls, an effect reversed by SOC blockers.

CONCLUSION

SOCs are pivotal in HIRI. SOC blockers protected against HIRI and assisted the recovery of secretory function in hepatocytes. Thus, they are likely to become a novel class of effective drugs for prevention or therapy of HIRI patients in the future.

Expression and function of TRP channels in liver cells

The liver plays a central role in whole body homeostasis by mediating the metabolism of carbohydrates, fats, proteins, drugs and xenobiotic compounds, and bile acid and protein secretion. Hepatocytes together with endothelial cells, Kupffer cells, smooth muscle cells, stellate and oval cells comprise the functioning liver. Many members of the TRP family of proteins are expressed in hepatocytes. However, knowledge of their cellular functions is limited. There is some evidence which suggests the involvement of TRPC1 in volume control, TRPV1 and V4 in cell migration, TRPC6 and TRPM7 in cell proliferation, and TRPPM in lysosomal Ca(2+) release. Altered expression of some TRP proteins, including TRPC6, TRPM2 and TRPV1, in tumorigenic cell lines may play roles in the development and progression of hepatocellular carcinoma and metastatic liver cancers. It is likely that future experiments will define important roles for other TRP proteins in the cellular functions of hepatocytes and other cell types of which the liver is composed.

Measuring Ca2+ influxes of TRPC1-dependent Ca2+ channels in HL-7702 cells with non-invasive micro-test technique

AIM

To explore the possibility of using the Non-invasive Micro-test Technique (NMT) to investigate the role of Transient Receptor Potential Canonical 1 (TRPC1) in regulating Ca(2+) influxes in HL-7702 cells, a normal human liver cell line.

METHODS

Net Ca(2+) fluxes were measured with NMT, a technology that can obtain dynamic information of specific/selective ionic/molecular activities on material surfaces, non-invasively. The expression levels of TRPC1 were increased by liposomal transfection, whose effectiveness was evaluated by Western-blotting and single cell reverse transcription-polymerase chain reaction.

RESULTS

Ca(2+) influxes could be elicited by adding 1 mmol/L CaCl(2) to the test solution of HL-7702 cells. They were enhanced by addition of 20 micromol/L noradrenaline and inhibited by 100 micromol/L LaCl(3) (a non-selective Ca(2+) channel blocker); 5 micromol/L nifedipine did not induce any change. Overexpression of TRPC1 caused increased Ca(2+) influx. Five micromoles per liter nifedipine did not inhibit this elevation, whereas 100 micromol/L LaCl(3) did.

CONCLUSION

In HL-7702 cells, there is a type of TRPC1-dependent Ca(2+) channel, which could be detected via NMT and inhibited by La(3+).

Ca(2+) -permeable channels in the hepatocyte plasma membrane and their roles in hepatocyte physiology

Hepatocytes are highly differentiated and spatially polarised cells which conduct a wide range of functions, including intermediary metabolism, protein synthesis and secretion, and the synthesis, transport and secretion of bile acids. Changes in the concentrations of Ca(2+) in the cytoplasmic space, endoplasmic reticulum (ER), mitochondria, and other intracellular organelles make an essential contribution to the regulation of these hepatocyte functions. While not yet fully understood, the spatial and temporal parameters of the cytoplasmic Ca(2+) signals and the entry of Ca(2+) through Ca(2+)-permeable channels in the plasma membrane are critical to the regulation by Ca(2+) of hepatocyte function. Ca(2+) entry across the hepatocyte plasma membrane has been studied in hepatocytes in situ, in isolated hepatocytes and in liver cell lines. The types of Ca(2+)-permeable channels identified are store-operated, ligand-gated, receptor-activated and stretch-activated channels, and these may vary depending on the animal species studied. Rat liver cell store-operated Ca(2+) channels (SOCs) have a high selectivity for Ca(2+) and characteristics similar to those of the Ca(2+) release activated Ca(2+) channels in lymphocytes and mast cells. Liver cell SOCs are activated by a decrease in Ca(2+) in a sub-region of the ER enriched in type1 IP(3) receptors. Activation requires stromal interaction molecule type 1 (STIM1), and G(i2alpha,) F-actin and PLCgamma1 as facilitatory proteins. P(2x) purinergic channels are the only ligand-gated Ca(2+)-permeable channels in the liver cell membrane identified so far. Several types of receptor-activated Ca(2+) channels have been identified, and some partially characterised. It is likely that TRP (transient receptor potential) polypeptides, which can form Ca(2+)- and Na(+)-permeable channels, comprise many hepatocyte receptor-activated Ca(2+)-permeable channels. A number of TRP proteins have been detected in hepatocytes and in liver cell lines. Further experiments are required to characterise the receptor-activated Ca(2+) permeable channels more fully, and to determine the molecular nature, mechanisms of activation, and precise physiological functions of each of the different hepatocyte plasma membrane Ca(2+) permeable channels.

A calcium flux is required for circadian rhythm generation in mammalian pacemaker neurons

Generation of mammalian circadian rhythms involves molecular transcriptional and translational feedback loops. It is not clear how membrane events interact with the intracellular molecular clock or whether membrane activities are involved in the actual generation of the circadian rhythm. We examined the role of membrane potential and calcium (Ca2+) influx in the expression of the circadian rhythm of the clock gene Period 1 (Per1) within the rat suprachiasmatic nucleus (SCN), the master pacemaker controlling circadian rhythmicity. Membrane hyperpolarization, caused by lowering the extracellular concentration of potassium or blocking Ca2+ influx in SCN cultures by lowering [Ca2+], reversibly abolished the rhythmic expression of Per1. In addition, the amplitude of Per1 expression was markedly decreased by voltage-gated Ca2+ channel antagonists. A similar result was observed for mouse Per1 and PER2. Together, these results strongly suggest that a transmembrane Ca2+ flux is necessary for sustained molecular rhythmicity in the SCN. We propose that periodic Ca2+ influx, resulting from circadian variations in membrane potential, is a critical process for circadian pacemaker function.

Contribution of a time-dependent and hyperpolarization-activated chloride conductance to currents of resting and hypotonically shocked rat hepatocytes

Hepatocellular Cl- flux is integral to maintaining cell volume and electroneutrality in the face of the many transport and metabolic activities that describe the multifaceted functions of these cells. Although a significant volume-regulated Cl- current (VRAC) has been well described in hepatocytes, the Cl- channels underlying the large resting anion conductance have not been identified. We used a combination of electrophysiological and molecular approaches to describe potential candidates for this conductance. Anion currents in rat hepatocytes and WIF-B and HEK293T cells were measured under patch electrode-voltage clamp. With K+-free salts of Cl- comprising the major ions externally and internally, hyperpolarizing steps between -40 and -140 mV activated a time-dependent inward current in hepatocytes. Steady-state activation was half-maximal at -63 mV and 28-38% of maximum at -30 to -45 mV, previously reported hepatocellular resting potentials. Gating was dependent on cytosolic Cl-, shifting close to 58 mV/10-fold change in Cl- concentration. Time-dependent inward Cl- currents and a ClC-2-specific RT-PCR product were also observed in WIF-B cells but not HEK293T cells. All cell types exhibited typical VRAC in response to dialysis with hypertonic solutions. DIDS (0.1 mM) inhibited the hepatocellular VRAC but not the inward time-dependent current. Antibodies against the COOH terminus of ClC-2 reacted with a protein between 90 and 100 kDa in liver plasma membranes. The results demonstrate that rat hepatocytes express a time-dependent inward Cl- channel that could provide a significant depolarizing influence in the hepatocyte.

Electrophysiological effects of anthopleurin-Q on rat hepatocytes

AIM: To study the effects of AP-Q on CCl₄-induced acute liver injury, delayed outward potassium current (I_K), inward rectifier potassium current (I_K1) and calcium release-activated calcium current (I_CRAC) in isolated rat hepatocytes.

METHODS: A single dose of CCl₄ (10 μg/mL, ip) was injected to induce acute liver injury in rats. Serum aminotransferase activities were determined. Whole cell patch-clamp techniques were used to investigate the effects of AP-Q on delayed outward potassium current (I_K), inward rectifier potassium current (I_K1) and calcium release-activated calcium current (I_CRAC).

RESULTS: AP-Q (3.5 and 7 μg/kg) pretreatment significantly reduced ALT and AST activities. AP-Q 0.1-100 nM produced a concentration-dependent increase of I_K with EC₅₀ value of 5.55±1.8 nM (n=6). AP-Q 30 nM shifted the I-V curve of I_K leftward and upward. CCl₄ 4 mM decreased I_K current 28.6±6.5% at 140 mV. After exposure to CCl₄ for 5 min, AP-Q 30 nM attenuated the decrease of I_K induced by CCl₄ close to normal amplitude. AP-Q 0.01-100 nM had no significant effect on either inward or outward components of I_K1 at any membrane potential examined. AP-Q 0.1-100 nM had no significant influence on the peak amplitude of I_CRAC, either, and did not affect the shape of its current voltage curve.

CONCLUSION: AP-Q has a protective effect on CCl₄-induced liver injury, probably through selectively increased I_K in hepatocytes.

Effects of tetrandrine on calcium and potassium currents in isolated rat hepatocytes

AIM: To study the effects of tetrandrine (Tet) on calcium release-activated calcium current (I_CRAC), delayed rectifier potassium current (I_K), and inward rectifier potassium currents (I_K1) in isolated rat hepatocytes.

METHODS: Hepatocytes of rat were isolated by using perfusion method. Whole cell patch-clamp techniques were used in our experiment.

RESULTS: The peak amplitude of I_CRAC was -508 ± 115 pA (n = 15), its reversal potential of I_CRAC was about 0 mV. At the potential of -100 mV, Tet inhibited the peak amplitude of I_CRAC from -521 ± 95 pA to -338 ± 85 pA (P < 0.01 vs control, n = 5), with the inhibitory rate of 35% at 10 µmol/L and from -504 ± 87 pA to -247 ± 82 pA (P < 0.01 vs control, n = 5), with the inhibitory rate of 49% at 100 µmol/L, without affecting its reversal potential. The amplitude of I_CRAC was dependent on extracellular Ca²⁺ concentration. The peak amplitude of I_CRAC was -205 ± 105 pA (n = 3) in tyrode’s solution with Ca²⁺ 1.8 mmol/L (P < 0.01 vs the peak amplitude of I_CRAC in external solution with Ca²⁺ 10 mmol/L). Tet at the concentration of 10 and 100 µmol/L did not markedly change the peak amplitude of delayed rectifier potassium current and inward rectifier potassium current (P > 0.05 vs control).

CONCLUSION: Tet protects hepatocytes by inhibiting I_CRAC, which is not related to I_K and I_K1.

KATP channels regulate mitogenically induced proliferation in primary rat hepatocytes and human liver cell lines. Implications for liver growth control and potential therapeutic targeting

To determine whether K(ATP) channels control liver growth, we used primary rat hepatocytes and several human cancer cell lines for assays. K(ATP) channel openers (minoxidil, cromakalim, and pinacidil) increased cellular DNA synthesis, whereas K(ATP) channel blockers (quinidine and glibenclamide) attenuated DNA synthesis. The channel inhibitor glibenclamide decreased the clonogenicity of HepG2 cells without inducing cytotoxicity or apoptosis. To demonstrate the specificity of drugs for K(+) channels, whole-cell patch-clamp recordings were made. Hepatocytes revealed K(+) currents with K(ATP) channel properties. These K(+) currents were augmented by minoxidil and pinacidil and attenuated by glibenclamide as well as tetraethylammonium, in agreement with established responses of K(ATP) channels. Reverse transcription of total cellular RNA followed by polymerase chain reaction showed expression of K(ATP) channel-specific subunits in rat hepatocytes and human liver cell lines. Calcium fluxes were unperturbed in glibenclamide-treated HepG2 cells and primary rat hepatocytes following induction with ATP and hepatocyte growth factor, respectively, suggesting that the effect of K(ATP) channel activity upon hepatocyte proliferation was not simply due to indirect modulation of intracellular calcium. The regulation of mitogen-related hepatocyte proliferation by K(ATP) channels advances our insights into liver growth control. The findings have implications in mechanisms concerning liver development, regeneration, and oncogenesis.

Transmembrane calcium influx induced by ac electric fields

Exogenous electric fields induce cellular responses including redistribution of integral membrane proteins, reorganization of microfilament structures, and changes in intracellular calcium ion concentration ([Ca2+]i). Although increases in [Ca2+]i caused by application of direct current electric fields have been documented, quantitative measurements of the effects of alternating current (ac) electric fields on [Ca2+]i are lacking and the Ca2+ pathways that mediate such effects remain to be identified. Using epifluorescence microscopy, we have examined in a model cell type the [Ca2+]i response to ac electric fields. Application of a 1 or 10 Hz electric field to human hepatoma (Hep3B) cells induces a fourfold increase in [Ca2+]i (from 50 nM to 200 nM) within 30 min of continuous field exposure. Depletion of Ca2+ in the extracellular medium prevents the electric field-induced increase in [Ca2+]i, suggesting that Ca2+ influx across the plasma membrane is responsible for the [Ca2+]i increase. Incubation of cells with the phospholipase C inhibitor U73122 does not inhibit ac electric field-induced increases in [Ca2+]i, suggesting that receptor-regulated release of intracellular Ca2+ is not important for this effect. Treatment of cells with either the stretch-activated cation channel inhibitor GdCl3 or the nonspecific calcium channel blocker CoCl2 partially inhibits the [Ca2+]i increase induced by ac electric fields, and concomitant treatment with both GdCl3 and CoCl2 completely inhibits the field-induced [Ca2+]i increase. Since neither Gd3+ nor Co2+ is efficiently transported across the plasma membrane, these data suggest that the increase in [Ca2+]i induced by ac electric fields depends entirely on Ca2+ influx from the extracellular medium.

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.

Hormone-induced rise in cytosolic Ca2+ in axolotl hepatocytes: properties of the Ca2+ influx channel

Calcium entry in nonexcitable cells occurs through Ca(2+)-selective channels activated secondarily to store depletion and/or through receptor- or second messenger-operated channels. In amphibian liver, hormones that stimulate the production of adenosine 3',5'-cyclic monophosphate (cAMP) also regulate the opening of an ion gate in the plasma membrane, which allows a noncapacitative inflow of Ca2+. To characterize this Ca2+ channel, we studied the effects of inhibitors of voltage-dependent Ca2+ channels and of nonselective cation channels on 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP)-dependent Ca2+ entry in single axolotl hepatocytes. Ca2+ entry provoked by 8-BrcAMP in the presence of physiological Ca2+ followed first-order kinetics (apparent Michaelis constant = 43 microM at the cell surface). Maximal values of cytosolic Ca2+ (increment approximately 300%) were reached within 15 s, and the effect was transient (half time of 56 s). We report a strong inhibition of cAMP-dependent Ca2+ entry by nifedipine [half-maximal inhibitory concentration (IC50) = 0.8 microM], by verapamil (IC50 = 22 microM), and by SK&F-96365 (IC50 = 1.8 microM). Depolarizing concentrations of K+ were without effect. Gadolinium and the anti-inflammatory compound niflumate, both inhibitors of nonselective cation channels, suppressed Ca2+ influx. This "profile" indicates a novel mechanism of Ca2+ entry in nonexcitable cells.

Hepatic ischemia-reperfusion injury modification during liver surgery in rats: pretreatment with nifedipine or misoprostol

The aim of the study was to determine if pretreatment with misoprostol (a prostaglandin analogue) or nifedipine (a calcium antagonist), know protectants of the whole liver, would ameliorate the ischemia-reperfusion injury (IRI) of resected liver associated with vascular occlusion. Male Wistar rats were allocated to 5 groups (n = 20 each group): sham-operated, liver resection only, liver resection plus pretreatment with 0.1 mg/kg misoprostol, 10 mg/kg, or 2 mg/kg nifedipine during the 3 days before IRI with liver resection. Fifteen percent of the liver was made ischemic by 30-minute continuous vascular occlusion, and the remaining 85% nonischemic liver was resected. The model was designed to have survival of the rats so that liver function could be studied over 3 weeks. Seventeen of 20 control resection rats survived indicating a suitable model for study. The bilirubin level was reduced by 25% on postoperative days 3 through 23 with misoprostol. The serum alanine aminotransferase (ALT) peak was significantly lower on day 1 with misoprostol and high-dose nifedipine (both reduced to half the control resection value). There was a modest but significant reduction of serum alkaline phosphatase (SAP) for low-dose nifedipine on days 1, 2, and 23. Prothrombin had a lower peak and lower values on days 1 through 4 with misoprostol. Liver histological changes were minor, being cytoplasmic vacuolization only, and was slightly more marked in the nifedipine groups. Preoperative misoprostol 0.1 mg/kg and nifedipine 10 mg/kg each ameliorate the IRI associated with liver resection, as measured by liver function tests. Different aspects of liver function were altered by the different agents. These results justify initiating a trial for human liver resections.

Hormone-regulated Ca2+ channel in rat hepatocytes revealed by whole cell patch clamp

An inward current responsible for hormone regulated Ca2+ entry has been identified in cultured rat hepatocytes using whole cell patch clamp. Addition of 20 nM vasopressin or of 100 microM ATP induced the inward current, which could be observed more clearly after blocking an outward K+ current. This large outward K+ current, which appeared after addition of vasopressin or ATP, could be blocked either by replacing K+ with Cs+ in the external medium and in the pipette solution, or by simply including 0.5 microM apamin in the K(+)-containing external medium. The outward current appears to be carried by a Ca2+ activated K+ channel. In the presence of apamin, hepatocytes pretreated with vasopressin in a Ca(2+)-free media reveal an inward current on addition of external Ca2+ (5 mM). The current could also be elicited by addition of vasopressin when cells are preincubated in the presence of 5 mM external Ca2+. No current is seen on addition of Ca2+ in the absence of vasopressin. Initially, the inward current was ca 200-300 pA at -60 mV, but it declined rapidly over 3 min to ca 20 pA. The current approached zero, as an asymptote at positive potential, and appeared to be somewhat inwardly rectifying. Additions of 5 mM Mn2+ or 5 mM Ba2+ in place of Ca2+ produced little or no current. An inhibitor of ER Ca(2+)-ATPase, thapsigargin, could also trigger the cascade of events leading to plasma membrane conductance of Ca2+. The data suggest that hormone-stimulated Ca2+ entry into hepatocytes is mediated by a Ca(2+)-release activated channel highly specific for Ca2+. This is the first demonstration of such a channel in hepatocytes, though similar ones have been described in mast cells, in vascular endothelial cells and T-lymphocytes.

Properties of single potassium channels in guinea pig hepatocytes

The patch-clamp technique of cell-attached and inside-out configurations was used to study the single potassium channels in isolated guinea pig hepatocytes. The single potassium channels in isolated guinea pig hepatocytes were recorded at different K+ concentrations. A linear single-channel current-voltage relationship was obtained at the voltage range of -80 to -20 mV with slope conductance of 70 +/- 6 pS (n = 10). Under symmetrical high K+ concentration of 148 mM in the cell-attached patch membrane, the I-V curve exhibited a mild inward rectification at potentials positive to + 20 mV. The values of reversal potential was +5 +/- 2 mV (n = 10). When the external potassium concentration ([K+]o) was decreased to 74 mM and 20 mM, the slope conductance was decreased to 48 +/- 2 pS (n = 4) and 24 +/- 3 pS (n = 3), respectively. The reversal potential was changed by 58 mV for a tenfold change in [K+]o, indicating that this channel was highly selective for K+. Open probabilities (Po) of the channel were 73-93% without apparent voltage dependence. The distributions of open time of the channels were fitted to two exponentials, while those of closed time were fitted to three exponentials, exhibiting no voltage dependence. The success rate of K+ channel activity to be recorded was 28% at room temperature, and there were no increases in the success rate nor in the channel opening probabilities at a temperature of 34-36 degrees C. Po in inside-out patches was not changed by application of 1 microM Ca2+ nor 1 mM Mg2+ to the internal side of patch membranes. It is concluded that a novel type of the K+ channels in guinea pig hepatocytes had different properties of slope conductance, channel kinetics, and sensitivity to [Ca2+]i, from those in other species.

Calcium: its modulation in liver by cross-talk between the actions of glucagon and calcium-mobilizing agonists

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Influence of calmodulin antagonists and calcium channel blockers on triiodothyronine uptake by rat hepatoma and myoblast cell lines

The influence of calcium-related mechanisms on cellular uptake of triiodothyronine (T3) has not yet been defined, although it is known that T3 can stimulate cellular entry of calcium. We therefore investigated the saturable uptake of [125I]-T3 (10(-11) mol/L) from serum-free medium in vitro by hepatoma (H4) cells and skeletal myoblast (L6) cells to establish the calcium-dependency of this process. We studied the effects of the following three structurally distinct types of calmodulin antagonists in H4 cells: the naphthalene sulfonamides W7, W12, and W13, calmidazolium, and trifluoperazine. Uptake of [125I]-T3 as a percentage of control values (n = 4, 10(-4) mol/L antagonist) was as follows: W7, 42.0% +/- 3.3% (P < .001); W12, 87.5% +/- 4.5% (NS); W13, 79.5% +/- 2.5% (P < .05); calmidazolium (10(-6) mol/L, n = 8), 55.1% +/- 2.2% (P < .001); and trifluoperazine (10(-5) mol/L, n = 6), 65.7% +/- 4.1% (P < .001). To investigate whether the calmodulin sensitivity of uptake was mediated via transmembrane calcium flux, we also studied the effects of three structurally distinct types of organic calcium channel blockers in both H4 and L6 cells. [125I]-T3 uptake as a percent of control values (10(-4) mol/L blocker, n = 4) was as follows: nifedipine, 8.6% +/- 0.9% (H4) and 16.7% +/- 7.2% (L6); verapamil, 24.6% +/- 3.2% (H4) and 61.9% +/- 4.2% (L6); diltiazem, 62.7% +/- 3.6% (H4) and 36.1% +/- 5.4% (L6); all P < .001. Eadie-Hofstee analysis indicated competitive inhibition of T3 uptake for both calmidazolium and nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)

Ion channels in human erythroblasts. Modulation by erythropoietin

To investigate the mechanism of intracellular Ca2+ ([Cai]) increase in human burst-forming unit-erythroid-derived erythroblasts by erythropoietin, we measured [Cai] with digital video imaging, cellular phosphoinositides with high performance liquid chromatography, and plasma membrane potential and currents with whole cell patch clamp. Chelation of extracellular free Ca2+ abolished [Cai] increase induced by erythropoietin. In addition, the levels of inositol-1,4,5-trisphosphate did not increase in erythropoietin-treated erythroblasts. These results indicate that in erythropoietin-stimulated cells, Ca2+ influx rather than intracellular Ca2+ mobilization was responsible for [Cai] rise. Both Ni2+ and moderately high doses of nifedipine blocked [Cai] increase, suggesting involvement of ion channels. Resting membrane potential in human erythroblasts was -10.9 +/- 1.0 mV and was not affected by erythropoietin, suggesting erythropoietin modulated a voltage-independent ion channel permeable to Ca2+. No voltage-dependent ion channel but a Ca(2+)-activated K+ channel was detected in human erythroblasts. The magnitude of erythropoietin-induced [Cai] increase, however, was insufficient to open Ca(2+)-activated K+ channels. Our data suggest erythropoietin modulated a voltage-independent ion channel permeable to Ca2+, resulting in sustained increases in [Cai].

Different 1,4-dihydropyridines exhibit discriminating effects on passive calcium uptake in rat liver plasma membrane vesicles

The effects of a number of calcium channel effectors on Ca2+ uptake by rat liver plasma membrane vesicles was examined. Nifedipine, verapamil and diltiazem had to be present at 1 mM in order to produce > 50% inhibition of Ca2+ uptake. The two structurally similar 1,4-dihydropyridines, nicardipine and nisoldipine exhibited opposite effects; nicardipine inhibited while nisoldipine stimulated Ca2+ uptake. The results show that low concentrations (microM) of calcium channel blockers of excitable cells have little effect on Ca2+ uptake by liver plasma membrane vesicles consistent with earlier findings of others that voltage-gated calcium channels are absent in hepatocytes. However, the opposite effects of higher concentrations (ca. 1 mM) of nicardipine and nisoldipine on Ca2+ uptake suggest a discriminatory action that might be useful in studying further the mechanism of passive Ca2+ uptake by these membrane vesicles.

Properties of a cell volume-sensitive potassium conductance in isolated guinea-pig and rat hepatocytes

1. Whole-cell voltage clamp and intracellular recording techniques were used to study the increase in K+ conductance that accompanies swelling in isolated guinea-pig and rat hepatocytes in short-term culture at 37 degrees C. 2. Swelling was induced (i) by the application of pressure (15 cmH2O) to the shank of the patch pipette, (ii) by exposing the cells to hypotonic solutions and (iii) as a consequence of leakage of electrolyte from an intracellular microelectrode. 3. Applying pressure to the patch pipette caused a large outward current (approximately 600 pA) to develop in guinea-pig hepatocytes voltage clamped to 0 mV. This current reversed direction at -86 mV, close to the reversal potential for K+, EK (-93 mV), and is attributable to the activation of a K+ conductance. 4. Spectral analysis of current noise during this response suggested a single-channel conductance of 7 pS, though this may well be an underestimate. The power spectrum could be fitted by the sum of two Lorentzian components, with half-power frequencies of 7 and 152 Hz. Seventy per cent of the variance was associated with the lower frequency component. 5. The steady-state current-voltage relationship for guinea-pig hepatocytes, as determined by whole-cell recording, was linear over the range -70 to +40 mV both before and during the increase in K+ conductance induced by swelling. 6. Confirming earlier work, intracellular recording using microelectrodes filled with 1 M-potassium citrate sometimes resulted in a slow hyperpolarization and a large rise in input conductance. These changes are also attributable to an increase in K+ conductance as the cell swelled because of leakage from the electrode. 7. Application of hypotonic external solutions during intracellular recording caused hyperpolarization and an increase in conductance. Conversely, hypertonic solution evoked depolarization and a fall in conductance in partly swollen cells. 8. The volume-activated K+ conductance was reversibly blocked by cetiedil, which caused half-maximal inhibition at 2.3 microM. Bepridil, quinine and barium were also effective, with IC50s (concentrations giving 50% maximal inhibition) of 2.7, 12 and 67 microM respectively. 9. Much greater concentrations of cetiedil and bepridil (IC50 approximately 1 mM and 77 microM respectively) were required to inhibit the loss of K+ which follows the application of angiotensin II (100 nM) to guinea-pig hepatocytes, and which occurs via Ca(2+)-activated K+ channels. Our evidence suggests that the activation of K+ channels by cell swelling is Ca2+ independent.(ABSTRACT TRUNCATED AT 400 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)

The liver cell plasma membrane Ca2+ inflow systems exhibit a broad specificity for divalent metal ions

1. The inflow of Mn2+ across the plasma membranes of isolated hepatocytes was monitored by measuring the quenching of the fluorescence of intracellular quin2, by atomic absorption spectroscopy and by the uptake of 54Mn2+. The inflow of other divalent metal ions was measured using quin2. 2. Under ionic conditions which resembled those present in the cytoplasmic space, Mn2+, Zn2+, Co2+, Ni2+ and Cd2+ each quenched the fluorescence of a solution of Ca2(+)-quin2. 3. The addition of Mn2+, Zn2+, Co2+, Ni2+ or Cd2+ to cells loaded with quin2 caused a time-dependent decrease in the fluorescence of intracellular quin2. Plots of the rate of decrease in fluorescence as a function of the concentration of Mn2+ reached a plateau at 100 microM-Mn2+. 4. The rate of decrease in fluorescence induced by Mn2+ was stimulated by 20% in the presence of vasopressin. The effect of vasopressin was completely inhibited by 200 microM-verapamil. Adrenaline, angiotensin II and glucagon also stimulated the rate of decrease in the fluorescence of intracellular quin2 induced by Mn2+. 5. The rate of decrease in fluorescence induced by Zn2+, Co2+, Ni2+ or Cd2+ was stimulated by between 20 and 190% in the presence of vasopressin or angiotensin II. 6. The rates of uptake of Mn2+ measured by atomic absorption spectroscopy or by using 54Mn2+ were inhibited by about 20% by 1.3 mM-Ca2+o and stimulated by 30% by vasopressin. 7. Plots of Mn2+ uptake, measured by atomic absorption spectroscopy or with 54Mn2+, as a function of the extracellular concentration of Mn2+ were biphasic over the range 0.05-1.0 mM added Mn2+ and did not reach a plateau at 1.0 mM-Mn2+. 8. It is concluded that (i) hepatocytes possess both a basal and a receptor-activated divalent cation inflow system, each of which has a broad specificity for metal ions, and (ii) the receptor-activated divalent cation inflow system is the receptor-operated Ca2+ channel.