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

Induction of hepatic cytochrome P450s responsible for the metabolism of xenobiotics by nicardipine and other calcium channel antagonists in the male rat

1. The effects of nicardipine and three other calcium channel antagonists, nifedipine, diltiazem and verapamil, on hepatic gene expression of cytochrome P450s (P450), CYP1A1, CYP1A2, CYP2B1, CYP2B2, CYP3A1 and CYP3A2 in male rats were examined by an RT-PCR method. 2. Treatment of rats with nicardipine resulted in a significant increase in hepatic expression of all the P450 genes examined. Other calcium channel antagonists, nifedipine, diltiazem and verapamil, also enhanced the gene expression of CYP2B1, CYP2B2, CYP3A1 and CYP3A2, although these showed no capacity for activating CYP1A1 and CYP1A2 genes. 3. We have demonstrated for the first time that nicardipine activated not only the genes of CYP2B1, CYP2B2, CYP3A1 and CYP3A2, but also those of CYP1A1 and CYP1A2 in the rat liver and have further suggested that calcium channel antagonists may show a common capacity for activating the genes of CYP2B1, CYP2B2, CYP3A1 and CYP3A2. Furthermore, this increased expression of P450 genes was demonstrated to contribute to increase in the protein level of the corresponding P450s.

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.

The effect of boromycin on the Ca2+ homeostasis

A boron-containing antibiotic, boromycin (BM), was found to influence the Ca2+ homeostasis in both excitable and non-excitable cells. In non-excitable cells (human erythrocytes and leucocytes) it inhibited the resting passive 45Ca2+ transport in 10(-6)-10(-5) mol/L concentrations. In human erythrocytes, the passive 15Ca2+ transport induced by the presence of 1 mmol/L NaVO3 was inhibited by boromycin (90% inhibition) as well. The inhibitory effect of BM on the NaVO3-induced passive 45Ca2+ transport was diminished in the presence of inhibitory concentrations of nifedipine (10 micromol/L -60% inhibition) or of those of K+o (75 mmol/L -20% inhibition). On the other hand, in rat brain synaptosomes, and rat cardiomyocytes, BM stimulated the passive 45Ca2+ transport in 'resting' cells at similar concentrations. In rat cardiomyocytes the stimulation was transient. The stimulatory effect on the passive 45Ca2+ transport in rat brain synaptosomes was accompanied with the increase of cytoplasmic Ca2+ concentration measured by means of the entrapped fluorescent Ca2+ chelator fura-2. The stimulatory effect of BM was diminished when synaptosomes were pre-treated with veratridine (10 micromol/L) which itself stimulated the passive 45Ca2+ transport. At saturating concentrations of veratridine, no stimulatory effect of BM was observed. These results could be explained by the indirect interaction of BM with both Ca2+ and Na+ transport systems via transmembrane ionic gradients of monovalent cations and could be useful in determining whether the cells belong to excitable, or non-excitable cells.

Store-operated Ca(2+) inflow in Reuber hepatoma cells is inhibited by voltage-operated Ca(2+) channel antagonists and, in contrast to freshly isolated hepatocytes, does not require a pertussis toxin-sensitive trimeric GTP-binding protein

The treatment of H4-IIE cells (an immortalised liver cell line derived from the Reuber rat hepatoma) with thapsigargin, 2, 5-di-(tert-butyl)-1,4-benzohydroquinone, cyclopiazonic acid, or pretreatment with EGTA, stimulated Ca(2+) inflow (assayed using intracellular fluo-3 and a Ca(2+) add-back protocol). No stimulation of Mn(2+) inflow by thapsigargin was detected. Thapsigargin-stimulated Ca(2+) inflow was inhibited by Gd(3+) (maximal inhibition at 2 microM Gd(3+)), the imidazole derivative SK&F 96365, and by relatively high concentrations of the voltage-operated Ca(2+) channel antagonists, verapamil, nifedipine, nicardipine and the novel dihydropyridine analogues AN406 and AN1043. The calmodulin antagonists W7, W13 and calmidazolium also inhibited thapsigargin-induced Ca(2+) inflow and release of Ca(2+) from intracellular stores. No inhibition of either Ca(2+) inflow or Ca(2+) release was observed with calmodulin antagonist KN62. Substantial inhibition of Ca(2+) inflow by calmidazolium was only observed when the inhibitor was added before thapsigargin. Pretreatment of H4-IIE cells with pertussis toxin, or treatment with brefeldin A, did not inhibit thapsigargin-stimulated Ca(2+) inflow. Compared with freshly isolated rat hepatocytes, H4-IIE cells exhibited a more diffuse actin cytoskeleton, and a more granular arrangement of the endoplasmic reticulum (ER). In contrast to freshly isolated hepatocytes, the arrangement of the ER in H4-IIE cells was not affected by pertussis toxin treatment. Western blot analysis of lysates of freshly isolated rat hepatocytes revealed two forms of G(i2(alpha)) with apparent molecular weights of 41 and 43 kDa. Analysis of H4-IIE cell lysates showed only the 41 kDa form of G(i2(alpha)) and substantially less total G(i2(alpha)) than that present in rat hepatocytes. It is concluded that H4-IIE cells possess store-operated Ca(2+) channels which do not require calmodulin for activation and exhibit properties similar to those in freshly isolated rat hepatocytes, including susceptibility to inhibition by relatively high concentrations of voltage-operated Ca(2+) channel antagonists. In contrast to rat hepatocytes, SOCs in H4-IIE cells do not require G(i2(alpha)) for activation. Possible explanations for differences in the requirement for G(i2(alpha)) in the activation of Ca(2+) inflow are briefly discussed.

Effect of lead exposure and accumulation on copper homeostasis in cultured C6 rat glioma cells

C6 rat glioma cells resemble rat astroglia in culture in that both cell types accumulate lead (Pb) intracellularly from the medium. As such, C6 cells are a model for Pb accumulation by the brain. In this study, an increase in intracellular Pb accumulation induced by p-chloromercuribenzoate (PCMB) after exposure to 10 microM Pb acetate suggests a role for sulfhydryl groups in Pb retention. Stimulation of Pb accumulation by nifedipine suggests the entry of Pb into these cells by a novel path. Most of the intracellular Pb from exposure for 7 days to 1 microM Pb was associated with high-molecular weight components in cytosol. Pb exposure increased the abundance of three proteins with the following characteristics on two-dimensional gels: 81 kDa with pI of 5.6, 81 kDa with pI of 4. 9, and 71 kDa with pI of 5.6. The levels of five other proteins, ranging in size from 37-41 kDa with pIs of 6.0-6.8 declined. Exposed C6 cells accumulated copper (Cu) intracellularly, and Cu accumulation after Pb exposure was shown by kinetic analysis with 67Cu to result from an increased uptake and a decreased efflux for Cu. Pb-exposed cells also showed increased Cu binding to membranes, which is consistent with the increase of Cu uptake. These data indicate that intracellular Pb interacts with high molecular weight proteins in C6 cells, and exposure also alters membrane transport properties for copper.

Oxytocin-induced Ca2+ responses in human myometrial cells

Complex spatiotemporal changes in intracellular Ca2+ were monitored in an immortalized human myometrial cell line (PHM1-41) and first-passage human myometrial cells after oxytocin stimulation (1. 0-1000 nM). Laser cytometry revealed intracellular Ca2+ oscillations in both culture systems starting at 1.0 nM, which were followed by repetitive Ca2+ transients by 10-15 min that lasted for at least 90 min. The amplitude of the initial Ca2+ spike was dose dependent, while the frequency of Ca2+ oscillations identified by Fast Fourier Transform (FFT) tended to increase with dose. Removal of oxytocin resulted in termination of oscillations. Analysis of the sources of the Ca2+ involved in oscillations indicated that the major contribution to oscillation frequencies of </= 6 mHz in cells was from the inositol 1,4,5-trisphosphate-sensitive pool, accounting for about 60% of the frequencies. Most of the remaining frequencies were attributable to extracellular Ca2+, which presumably comes from plasma membrane channels other than L-type channels. When oscillation frequencies exceeded 6 mHz, a significant contribution from a ryanodine-sensitive Ca2+ pool was detected. Eight-bromo-cAMP suppressed both the initial Ca2+ spike and the long-term oscillations. Prostaglandin E1 and E2 caused a significant increase in the frequency of oxytocin-induced Ca2+ oscillations. FFT analysis may be of considerable value for study of the mechanisms of rhythmic Ca2+ transients and their function in myometrial cells, as well as the mechanisms by which uterotonins and tocolytic agents impact myometrial Ca2+ regulation.

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.

Inhibition of calcium influx during hypoxia/reoxygenation in primary cultured rat hepatocytes

Calcium has been demonstrated to play an important role in hepatocyte damage during ischemia/reperfusion phases. Calcium influx was determined in primary cultured rat hepatocytes submitted to a succession of warm hypoxia and reoxygenation phases in the presence of diltiazem, gallopamil and a Na+/H+ antiport inhibitor, HOE-694. Only diltiazem significantly inhibited calcium influx with higher potency after reoxygenation than after hypoxia only, suggesting a complex mechanism of action of diltiazem which could act on different physiological functions involved in Ca2+ invasion of hepatocytes after hypoxic insult.

Methapyrilene hepatotoxicity is associated with oxidative stress, mitochondrial disfunction and is prevented by the Ca2+ channel blocker verapamil

Methapyrilene (MP) is an unusual hepatotoxin in that it causes periportal necrosis in rats. The mechanism of acute methapyrilene hepatotoxicity has, therefore, been investigated in cultured male rat hepatocytes. Addition of methapyrilene to rat hepatocytes resulted in a time- and dose-dependent loss in cell viability between 4 and 8 h of incubation as judged by cellular enzyme leakage. The cytochrome P450 (CYP) inhibitor metyrapone protected against methapyrilene-mediated toxicity suggesting that MP is metabolised by CYP for toxicity. The concentration-dependent protection from methapyrilene toxicity afforded by metyrapone correlated with an inhibition of microsomal CYP2C11-associated androstenedione 16alpha hydroxylase activity, and hepatocytes prepared from hypophysectomised rats (containing reduced levels of microsomal immunodetectable CYP2C11 and associated androstenedione 16alpha hydroxylase activity) showed resistance to the toxic effects of methapyrilene. These data suggest that the toxicity of methapyrilene is predominantly dependent on the CYP2C11 isoform. Treatment of hepatocytes with a toxic concentration of MP caused oxidative stress as indicated by increases in NADP+ levels within 2 h and cellular thiol oxidation as evidenced by a reduction--but not complete loss--in glutathione levels. Methapyrilene hepatotoxicity was associated with an early loss in mitochondrial function, as indicated by mitochondrial swelling and significant losses in cellular ATP within 2 h. Co-incubation of methapyrilene-treated hepatocytes with inhibitors of inner mitochondrial transition permeability pore opening--cyclosporin A or the thiol reductant dithiothreitol--abrogated cell death suggesting that pore opening and loss of mitochondrial Ca2+ homeostasis play a significant role in methapyrilene-mediated cell death. Co-incubation of methapyrilene-treated hepatocytes with the phenylalkylamine calcium channel blocker verapamil--but not by treating cells in a nominally calcium-free medium--also abrogated cell death, suggesting that if Ca2+ is involved in cell killing then it is dependent on an intracellular Ca2+ pool. Pre-treatment of hepatocytes for 1 h with verapamil--to inhibit intracellular Ca2+ pool filling--increased the potency of verapamil protection against methapyrilene toxicity by approximately 100-fold. Taken together, these data indicate that methapyrilene intoxication leads to mitochondrial disfunction and suggest a critical role for a loss of mitochondrial Ca2+ homeostasis in this model of hepatocyte death.

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.

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.

Hepatotoxicity of ethanol: protective effect of calcium channel blockers in isolated hepatocytes

This study examines the effects of three calcium channel blockers (verapamil, nifedipine and diltiazem) on isolated rat hepatocytes exposed to ethanol. In the first part of our study, hepatocytes were incubated with increasing concentrations of ethanol (100, 300, 500, 1000 mM) for varying times. Alanine aminotransferase (ALT), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) release were measured to evaluate the cytotoxic effects of ethanol. The concentration of 300 mM and time of incubation of 45 min were chosen for cytoprotection experiments in which calcium channel blockers, at two different concentrations, were added to the medium 30 min prior to the addition of ethanol. ALT, AST and LDH release as well as lipid peroxidation and cellular reduced glutathione (GSH) were measured. Nifedipine and verapamil (25 microM) reduced ALT, AST and LDH activities. The highest dose of diltiazem (50 microM) was more effective than the lowest one (25 microM). Ethanol caused a significant depletion of cellular GSH content as well as a moderate enhancement of lipid peroxidation. While none of the three calcium channel blockers was able to restore the decrease in GSH levels, diltiazem (25 microM) and nifedipine (50 microM) showed the greatest effect, significantly reducing lipid peroxidation.

Calcium inflow in cells transfected with cloned rat and porcine calcitonin receptors

Ca2+ fluxes were examined in HEK 293 cells stably expressing the rat or porcine calcitonin receptors (CTRs). Calcitonin (CT) rapidly increased cytosolic Ca2+ ([Ca2+]i) concentrations in these cells in a manner which was sustained in the presence of extracellular Ca2+ ([Ca2+]e). In cells pretreated with CT, elevation of the [Ca2+]e concentration resulted in a further increase in [Ca2+]i which was concentration-dependent with respect to both the concentration of CT and the increment of [Ca2+]e. Untransfected cells, cells transfected with vector alone, and CTR-transfected cells not treated with CT, were unresponsive to [Ca2+]e. The microsomal Ca(2+)-ATPase inhibitor thapsigargin was able to mimic both the acute [Ca2+]i fluxes and responsiveness to [Ca2+]e mediated by CT in these cells. The CT-induced responsiveness to [Ca2+]e was neither mimicked by, nor affected by, activators of the cAMP or protein kinase C pathways. Treatment of cells with pertussis toxin influenced neither the primary Ca2+ fluxes in response to CT or thapsigargin nor the agonist-induced [Ca2+]e influx. Nifedipine failed to block responses to either CT or thapsigargin. These results lead to the important conclusion that the CTR participates in receptor-activated Ca2+ inflow, in which depletion of intracellular Ca2+ pools leads secondarily to influx of extracellular Ca2+.

Vasopressin-induced calcium signaling in cultured hippocampal neurons

We recently demonstrated that the neural peptide vasopressin (AVP) can act as a neurotrophic factor for hippocampal nerve cells in culture. Because the neurotrophic effect of vasopressin is mediated by the V1 receptor [11], we investigated AVP activation of calcium signaling pathways in cultured hippocampal neurons. Results of this investigation demonstrate that exposure of cultured hippocampal neurons prelabeled with [3H]myo-inositol to vasopressin induced a significant accumulation of [3H]inositol-1-phosphate ([3H]IP1). The selective V1 vasopressin receptor agonist, [Phe2, Orn2]vasotocin, induced a significant accumulation of [3H]IP1 whereas a selective V2 vasopressin receptor agonist, [deamino1, D-Arg8]-vasopressin, did not. Moreover, V1 agonist-induced accumulation of [3H]IP1 was blocked by the selective V1 vasopressin receptor antagonist d(CH2)5[Tyr(Me)2]-vasopressin. V1 agonist-induced accumulation of [3H]IP1 was concentration dependent and exhibited a steep inverted U-shaped curve that included both stimulation and inhibition of [3H]IP1 accumulation. Time course analysis of V1 agonist-induced accumulation of [3H]IP1 revealed significant increase by 20 min which continued to be significantly elevated for 60 min. Investigation of the effect of closely related peptides on [3H]IP1 accumulation indicated that the vasopressin metabolite peptide AVP4-9 and oxytocin significantly increased [3H]IP1 accumulation whereas the vasopressin metabolite peptide AVP4-8 did not. AVP4-9 and oxytocin induced [3H]IP1 accumulation were blocked by the V1 vasopressin receptor antagonist d(CH2)5[Tyr(Me)2]-vasopressin. V1 receptor activation was associated with a pronounced rise in intracellular calcium. Results of calcium fluorometry studies indicated that V1 agonist exposure induced a marked and sustained rise in intracellular calcium that exhibited oscillations.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1B-receptors and intracellular calcium mediate sympathetic nerve induced constriction of rat irideal blood vessels

The present study has investigated the receptors involved in the non-cholinergic nerve mediated constriction of the larger blood vessels (30-50 microns) within the rat iris. This response was blocked by the alpha-adrenoceptor antagonist, benextramine (10(5) M). Furthermore, the response was more sensitive to blockade by the alpha 1 antagonist, prazosin (IC50 9 x 10(-10) M), than to blockade by the alpha 2 antagonist, yohimbine (IC50 2 x 10(-7) M), or the adrenergic antagonist, WB4101 (IC50 2 x 10(-8) M), and was abolished by chloroethylclonidine (10(-5) M). These results suggest the involvement of alpha 1B-adrenoceptors. The nerve mediated constriction was not blocked by the voltage-dependent calcium channel blocking drugs, nifedipine (10(-6) M), verapamil (10(-6) M) or diltiazem (10(-6) M), but was completely abolished by the intracellular calcium mobilizer, caffeine (10(-3) M), supporting the hypothesis that alpha 1B-adrenoceptors are activated following nerve stimulation. Dantrolene (10(-4) M), which interferes with calcium release from the sarcoplasmic reticulum, reduced the nerve mediated constriction by 40% as did thapsigargin (2 x 10(-6) M), which inhibits the calcium ATPase responsible for uptake of calcium into intracellular stores. When influx of calcium was blocked by verapamil (10(-6) M), thapsigargin, but not dantrolene, completely abolished the response. Noradrenaline (10(-5) M) produced a vasoconstriction in the presence or absence of external calcium although the latter response was significantly smaller than the former. Vasoconstriction produced by a submaximal concentration of noradrenaline (10(-6) M), was completely prevented by pretreatment with chloroethylclonidine. The data indicate that noradrenaline released from sympathetic nerves causes a constriction of arterioles in the iris by activating alpha 1B-adrenoceptors and releasing calcium from dantrolene sensitive and insensitive intracellular stores, followed by inflow of calcium through verapamil sensitive calcium channels. Applied noradrenaline also activates chloroethylclonidine sensitive receptors on the arteriolar surface.

Effects of acoustic prepulses on the startle reflex in rats: a parametric analysis

Small changes in the sensory environment, called prepulses, prior to a startle-eliciting stimulus can either inhibit or facilitate the startle reaction. To investigate this apparent discrepancy, a number of characteristics of the acoustic prepulse were varied and the effects on the startle reaction were studied. The results showed that increasing the intensity of the prepulse (81-85 dB) resulted in an increased inhibition and could even turn facilitation into inhibition (at 3-13 ms prepulse-startle interval). Varying prepulse lengths (1-45 ms) did not change the observed startle modification. Only when the prepulse offset was close to the startle onset, changes could be observed. Confronting the animal with the same test session for several days resulted in increased inhibition and a change from facilitation to inhibition (at 3-13 ms prepulse-startle interval). The results demonstrate that the characteristics of the prepulse determine its effect on the startle reaction. An hypothetical model is proposed which might explain the observed data.

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.

Expression of a growth arrest specific gene (gas-6) during liver regeneration: molecular mechanisms and signalling pathways

A set of growth arrest-specific (gas) genes negatively regulated by serum has been identified. To define the role of gas genes in a model of cell proliferation in vivo we analyzed the expression of one of these genes (gas-6) during liver regeneration after partial hepatectomy (PH). We found that gas-6 mRNA was down-regulated 4 hours after PH, within the G0 to G1 transition. Later on, gas-6 mRNA increased over the level found in normal liver with a peak at 16 hours, before the onset of DNA synthesis. This surge was probably triggered by an inflammatory response caused by the surgical trauma, because an increase of similar extent occurring with the same time course was present in livers of sham-operated and turpentine-treated rats. Comparison of mRNA steady state levels with nuclear transcription rates indicated that gas-6 expression is post-transcriptionally regulated. As we found that down-regulation of gas-6 expression was prevented by treatment with Actinomycin D, a labile protein might be involved in the determination of gas-6 mRNA stability. To investigate the mitogenic signals controlling gas-6 expression during liver regeneration we treated hepatectomized rats with a specific alpha-1-adrenoceptor blocker (prazosin) as well as with drugs which modify intracellular calcium levels. The decrease of gas-6 mRNA 4 hours after PH was prevented by prazosin and by neomycin, an inhibitor of calcium release from endogenous stores. These findings suggest that down-regulation of gas-6 expression during hepatic regeneration is triggered by catecholamines interaction with alpha-1-adrenergic receptors and by subsequent calcium release. In addition we found that the rise of gas-6 gene expression occurring at 16 hours after PH was not affected by prazosin but was inhibited by trifluoperazine. Therefore, we suggest that up-regulation of gas-6 gene expression is mediated by the interaction of calcium with calmodulin, independently of catecholamines.

Protective effect of various calcium antagonists against an experimentally induced calcium overload in isolated hepatocytes

The effect of the hepatotoxic substance diamidinothionaphthene (98/202) on cytosolic, mitochondrial and extra-mitochondrial calcium distribution was measured in isolated rat hepatocytes. The drastic disturbance of the intracellular calcium homeostasis caused by this substance (increase of the cytosolic and mitochondrial calcium contents and depletion of extra-mitochondrial calcium stores, which at last lead to cell death) gave rise to an investigation of the possible cytoprotective effect of calcium antagonists of various chemical classes: verapamil, diltiazem, and nifedipine on isolated hepatocytes. Our results show that all three calcium antagonists prevented cell death caused by 98/202. The 98/202-induced increase of cytosolic and mitochondrial calcium content was inhibited by all three calcium antagonists. However, only verapamil was able to inhibit the depletion of extra-mitochondrial calcium stores. Since 98/202-induced cell death occurs only in the presence of extracellular calcium, it is concluded that calcium antagonists are also able to inhibit the influx of extracellular calcium in liver cells, which leads to a calcium overload of the cytosol and mitochondria. The various ways of interfering with the calcium homeostasis of liver cells qualifies the hepatotoxic substance 98/202 as a suitable in vitro hepatotoxicity model for testing the hepatoprotective effect of different calcium antagonists.

Influence of aging on the beta- and glucagon-receptor-mediated glycogenolysis in rat hepatocytes

The influence of aging on beta-receptor and glucagon-receptor control of glycogenolysis was investigated in rat hepatocytes. The beta-receptor-induced glucose output was detectable only in senescent rats, was partly dependent on extracellular Ca2+, and was inhibited by 4 beta-phorbol 12-myristate 13-acetate (PMA), insulin, and the Ca(2+)-antagonists, verapamil and nifedipine. Chelation of extracellular Ca2+ potentiated the effect of nifedipine only. In contrast, glucagon-stimulated glycogenolysis, similar in mature and senescent rats, was independent on extracellular Ca2+ and was unaffected by PMA. Verapamil, in senescent rats only, and nifedipine, in mature and senescent rats, inhibited glucagon-stimulated glucose output only in the presence of Ca2+. Insulin inhibited glucagon-induced glucose output, irrespective of the age of the rat and the presence of Ca2+. We conclude that the beta-receptor component in the adrenergic regulation of glycogenolysis in senescent rats consists of a major Ca(2+)-independent and a minor Ca(2+)-dependent part, displaying different sensitivity towards protein kinase C (PKC), Ca(2+)-antagonists, and insulin. Aging does not change the capacity of glucagon to induce a full glycogenolytic response in the absence of extracellular Ca2+; Ca(2+)-influx, however, seems to be involved when extracellular Ca2+ is present, and this sensitivity is increased on aging.

Ca2+ antagonists do not protect isolated perfused rat hepatocytes from anoxic injury

Ca2+ antagonists were studied during anoxia in perfused isolated rat hepatocytes. Cytosolic free calcium (Ca2+i) was measured with aequorin. Anoxia was induced for 2 h by saturating the perfusate with 95% N2/5+ CO2. Anoxia increased Ca2+i in two distinct phases reaching a maximum of 1.5 microM. The increase in Ca2+i was caused by Ca2+ influx from the extracellular fluids because the main Ca2+i surge was totally abolished in Ca(2+)-free media. LDH release increased 6-fold during the second hour of anoxia, but when Ca2+ was removed from the perfusate during the anoxic period, LDH rose only 2.7-fold. Ca2+ antagonists (10(-7) to 10(-5) M) did not prevent the increase in Ca2+i and the rise in LDH release. On the contrary, high concentrations (10(-6) and 10(-5) M) of the blockers nifedipine and diltiazem significantly increased anoxic cell injury. The observation that the increase in LDH and the rise in Ca2+i were not suppressed by Ca2+ antagonists suggests that (i) Ca2+ antagonists protect the whole liver from anoxic injury by acting on cells other than parenchymal cells; (ii) the influx of Ca2+ responsible for the massive increase in hepatocyte Ca2+i evoked by anoxia did not take place through voltage-sensitive Ca2+ channels but must have occurred via the Na(+)-Ca2+ antiporter operating in the reverse mode (Ca2+ influx vs. Na+ efflux), and (iii) high concentrations of Ca2+ antagonists may be deleterious to the parenchymal cells of the liver.

Verapamil and diltiazem inhibit receptor-operated calcium channels and intracellular calcium oscillations in rat hepatocytes

Fura-2 loaded rat hepatocytes were used to determine whether the L-type channel blockers, verapamil and diltiazem, affect receptor-operated calcium channels (ROCCs). The flux through ROCCs was followed by quenching of fura-2 fluorescence due to the influx of extracellular Mn2+ induced by vasopressin. Verapamil as well as diltiazem inhibited vasopressin-stimulated Mn2+ influx in a dose-dependent manner up to 60% at concentrations of 200-400 microM. Furthermore, both inhibitors decreased significantly the frequency of phenylephrine-induced oscillation of [Ca2+]i. The experimental findings indicate that L-type channel blockers inhibit ROCCs in rat hepatocytes.

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)

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.

Effects of Ca2+ agonists on cytosolic Ca2+ in isolated hepatocytes and on bile secretion in the isolated perfused rat liver

The effects of increases in cytosolic Ca2+ on hepatocyte bile secretion are unknown. A number of agents that alter levels of cytosolic Ca2+ in the hepatocyte also produce hepatic vasoconstriction and activate protein kinase C, which complicates interpretations of their effects on bile secretion. To better understand the role of cytosolic Ca2+ in bile secretion, we examined the effect of the Ca2+ ionophore A23187 (0.1 mumol/L), the Ca2+ agonist vasopressin (10 nmol/L) and the Ca(2+)-mobilizing agent, 2,5-di(tert-butyl)-1,4-benzohydroquinone (25 mumol/L) on cytosolic Ca2+ in isolated hepatocytes and on bile flow in the isolated perfused rat liver, using vasodilators and inhibitors of protein kinase C and Ca2+ influx. Single-pass perfused livers were used, and cytosolic Ca2+ was measured by luminescent photometry in isolated hepatocytes loaded with the Ca(2+)-sensitive photoprotein aequorin. After A23187 perfusion, a sustained 74% +/- 10% (mean +/- S.D.) decrease in bile flow and a sustained 271% +/- 50% increase in perfusion pressure was observed. Simultaneous pretreatment with the vasodilator papaverine (25 mumol/L) and the protein kinase C inhibitor H-7 (50 mumol/L) abolished the pressure increase but not the decrease in bile flow, whereas pretreatment with Ni2+ (25 mumol/L) to block the influx of extracellular Ca2+ markedly reduced both the pressure increase and the decrease in bile flow. Vasopressin produced a transient (mean = 6 min) 75% +/- 4% decrease in bile flow and a sustained 7% +/- 4% increase in perfusion pressure. Pretreatment with H-7 alone corrected the vasopressin-induced pressure increase but also failed to eliminate the decrease in bile flow, whereas pretreatment with Ni2+ decreased the magnitude of the decrease by two-thirds without affecting the increase in perfusion pressure, 2,5'-di(tert-butyl)-1,4-benzohydroquinone produced a transient 65% +/- 20% decrease in bile flow and a transient 56% +/- 15% increase in perfusion pressure. In isolated hepatocytes, bromo-A23187, the nonfluorescent form of the ionophore, produced a sustained 56% +/- 32% increase in the cytosolic Ca2+ signal, whereas vasopressin resulted in a transient 241% +/- 75% increase and 2,5-di(tert-butyl)-1,4-benzohydroquinone resulted in a sustained 149% +/- 66% increase. The ionophore-induced increase in Ca2+ was abolished completely by pretreatment of the hepatocytes with Ni2+, whereas the vasopressin-induced increase was reduced by 38%.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of 2-acetylaminofluorene on intracellular free Ca2+ in isolated rat hepatocytes

The effect of 2-acetylaminofluorene (2-AAF) on the intracellular free Ca2+ ([Ca2+]i) and viability of isolated rat hepatocytes has been investigated using the fluorescent probes quin 2 and propidium iodide respectively. At the highest concentration tested (224 microM), 2-AAF produces an elevation of [Ca2+]i which shows a biphasic profile. A small initial increase is observed during the first 5 min; this is followed by a considerable rise which reaches up to 2.5 times the control value at 15 min. These changes in intracellular calcium are not accompanied by detectable alterations in cell viability. In order to determine the mechanisms by which this effect of 2-AAF takes place, three calcium antagonists, namely verapamil, TMB-8 (8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate) and ruthenium red (RuR), have been used. The results suggest that the first phase is dependent upon internal Ca2+ store mobilization, while the second phase seems to be related to Ca2+ entry from the extracellular space. The data obtained with RuR further indicate that mitochondria may be involved in the perturbation of calcium homeostasis caused by 2-AAF. In addition, in the experiments involving antagonists, no consistent pattern emerges that suggests a close relationship between intracellular Ca2+ levels and cell viability. The present study provides further information on the mechanisms by which these well-known hepatotoxin 2-AAF may interact with liver cells. It also shows that when these cells are exposed to a toxin, short-term changes in [Ca2+]i may not be accompanied by loss of cell viability, and conversely, that changes in cell viability may occur without alterations in [Ca2+]i.

Differences in cadmium and mercury uptakes by hepatocytes: role of calcium channels

Calcium uptake in cells occurs through specific membrane channels. Since cadmium and mercury inhibit calcium uptake, this study examined whether the calcium channels may also be involved in the uptake of these metals. Primary cultures of rat hepatocytes were incubated with 3 microM CdCl2 or HgCl2 in the absence or presence of four different organic calcium channel blockers or a calcium agonist. The calcium channel blockers had no significant effect on mercury accumulation. In comparison, the uptake of cadmium was inhibited by diltiazem and verapmil (50-250 microM) as well as by nifedipine and nitrendipine (25-100 microM), with a maximum inhibition of 31% after 30 min incubation with 250 microM verapamil. The calcium agonist vasopressin (20 nM) increased cadmium accumulation by 15%. This effect was blocked by 250 microM verapamil. Kinetic analysis showed that verapamil decreased the Vmax of cadmium uptake, without altering the Km, indicating a noncompetitive inhibition. The calcium channel blockers were ineffective at 4 degrees C. These data suggest that about a third of the cadmium enters hepatocytes through the calcium channels. The mechanism of mercury uptake, on the other hand, is very different as it does not appear to involve the calcium channels.

Stimulation of lipid peroxidation increases the intracellular calcium content of isolated hepatocytes

Lipid peroxidation induced in isolated rat hepatocytes by FeCl3 (0.1 mM) was associated with an increase in the cytosolic free Ca2+ and in the ionophore-mobilizable Ca2+ content of both mitochondrial and extramitochondrial (endoplasmic reticular) pools. Ca2+ accumulation was completely prevented by the antioxidants promethazine and vitamin E succinate and was not linked to the inhibition of plasma membrane (Ca2+ + Mg2+)-ATPase and Ca2+ transport or to the depletion of intracellular ATP content. Moreover, preincubation of the hepatocytes with the Ca2+ channel blockers verapamil and nifedipine inhibited the elevation of cytosolic Ca2+, as well as the ion accumulation without interfering with the stimulation of lipid peroxidation by iron. These results suggest that peroxidative alterations of the hepatocyte plasma membranes might perturb the functions of verapamil- and nifedipine-sensitive Ca2+ channels resulting in a net influx of Ca2+, which is subsequently sequestrated in the intracellular compartments.

Mechanisms of metal transport across liver cell plasma membranes

The liver's pivotal role in the homeostasis of essential trace metals and detoxification of exogenous metals is attributed to its ability to efficiently extract metals from plasma, metabolize, store, and redistribute them in various forms either into bile or back into the bloodstream. Bidirectional transport across the sinusoidal plasma membrane allows the liver to control plasma concentrations and therefore availability to other tissues. In contrast, transport across the canalicular membrane is largely, but not exclusively, unidirectional and is a major excretory pathway. Although each metal has relatively distinct hepatic transport characteristics, some generalizations can be made. First, movement of metals from plasma to bile follows primarily a transcellular route. The roles of the paracellular pathway and of ductular secretion appear minimal. Second, intracellular binding proteins and in particular metallothionein play only indirect roles in transmembrane flux. The amounts of metallothionein normally secreted into plasma and bile are quite small and cannot account for total metal efflux. Third, metals traverse liver cell plasma membranes largely by facilitated diffusion, and by fluid-phase, adsorptive, and receptor-mediated endocytosis/exocytosis. There is currently no evidence for primary active transport. Because of the high rate of hepatocellular membrane turnover, metal transport via endocytic vesicles probably makes a larger contribution than previously recognized. Finally, there is significant overlap in substrate specificity on the putative membrane carriers for the essential trace metals. For example, zinc and copper share many transport characteristics and apparently compete for at least one common transport pathway. Similarly, canalicular transport of five of the metals discussed in this overview (Cu, Zn, Cd, Hg, and Pb) is linked to biliary GSH excretion. These metals may be transported as GSH complexes by the canalicular glutathione transport system(s). Unfortunately, none of the putative membrane carrier proteins have been studied at the subcellular or molecular level. Our knowledge of their biochemical properties is rudimentary and rests almost entirely on indirect evidence obtained in vivo or in intact cell systems. The challenge for the future is to isolate and characterize these putative metal carriers, and to determine how they are functionally regulated.

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.

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)

Effect of adrenergic and Ca2+ antagonists on increased ornithine decarboxylase expression in regenerating rat liver

Partial hepatectomy (PH) (70% resection) causes within 4 hr an accumulation of ornithine decarboxylase (EC 4.1.1.17, ODC) mRNAs concomitant with an increase in ODC activity, maximum values being observed at 8 and 16 hr, respectively. In the early hours of hepatic regeneration, enhancement of transcriptional-rate of ODC gene, demonstrated by nuclear run-on analysis, can account for the accumulation of ODC mRNAs. The involvement of catecholamines in these processes is demonstrated by using prazosin and propranolol, specific antagonists of alpha 1 and beta adrenoceptors, respectively. Prazosin reduces almost completely the rise of ODC activity at 4 hr, without affecting mRNA levels. At 16 hr, enzyme activity and mRNAs increase, however, over the values observed in regenerating liver of prazosin-untreated animals. These findings suggest that alpha 1-receptor activation triggers positive control signals for ODC gene expression at the early time of liver regeneration and, on the contrary, negative signals at later times by mainly post-transcriptional and transcriptional mechanisms, respectively. Propranolol reduces similarly the initial 4 hr-rise of ODC activity. These results indicate that activation of both alpha 1- and beta-adrenoceptors causes the large increase in ODC activity. Pharmacological manipulation of intracellular Ca2+ levels by verapamil, a Ca2(+)-channel blocker, or neomycin, an inhibitor of Ca2+ release from endogenous stores, diminishes ODC activity at 4 and 16 hr after PH. ODC mRNA levels, which are not modified at 4 hr, increase over the values of partially hepatectomized rat liver at 16 hr. Trifluoperazine inhibits both ODC activity and mRNA accumulation at the times studied. As a working hypothesis it is proposed that Ca2(+)-mediated processes induced by catecholamines are involved in ODC gene expression during the prereplicative phase of liver regeneration.

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.

Differentiation of alpha adrenoceptors mediating increase of oxygen consumption in rat submandibular salivary gland slices

Clonidine, noradrenaline and adrenaline (in the presence of propranolol), but not phenylephrine and methoxamine, stimulated an increase in the oxygen consumption of these slices that was blocked by yohimbine but not by prazosin. The stimulation was inhibited by ouabain and required the presence of Ca2+ in the incubation medium. The calcium ionophore A 23187 stimulated oxygen consumption in the tissue slices and enhanced the respiratory effect of clonidine. Atropine and (D-Pro2, D-Trp7.9)-substance P failed to block the respiratory response to clonidine in concentrations that inhibited the respiratory effects of carbachol and substance P, respectively. Release of acetylcholine from the unstimulated gland slices was reduced by clonidine or Ca2+ omission. Yohimbine prevented the clonidine effect and stimulated acetylcholine resting release. Nifedipine did not affect either the release of acetylcholine or the clonidine-induced reduction of acetylcholine release but blocked the oxygen uptake due to clonidine or to release acetylcholine.

The measurement of Ca2+ inflow across the liver cell plasma membrane by using quin2 and studies of the roles of Na+ and extracellular Ca2+ in the mechanism of Ca2+ inflow

1. Rates of Ca2+ inflow across the hepatocyte plasma membrane in the presence of vasopressin were estimated by using quin2. 2. Plots of the rate of Ca2+ inflow as a function of the intracellular quin2 concentration reached a plateau at about 1.7 mM intracellular quin2. Ca2+ inflow was inhibited by 60% in the presence of 400 microM-verapamil. 3. A plot of the rate of Ca2+ inflow as a function of the concentration of extracellular Ca2+ ([Ca2+]o) was biphasic. The second (slower) phase showed no sign of saturation at values of [Ca2+]o up to 5 mM. It is concluded that, in the presence of vasopressin, Ca2+ flows into the liver cell by two different processes, one of which is not readily saturated by Ca2+o. 4. The effect of the replacement of extracellular NaCl by choline or tetramethylammonium chloride on cellular Ca2+ movement was found to depend on the presence or absence of intracellular quin2. 5. In cells loaded with quin2 and incubated in the presence of choline or tetramethylammonium chloride, a small decrease in the basal intracellular free Ca2+ concentration ([Ca2+]i) was observed, and the increase in [Ca2+]i caused by the addition of vasopressin was considerably diminished when compared with cells incubated in the presence of NaCl. In cells loaded with quin2, replacement of NaCl by choline chloride caused a decrease in Ca2+ inflow in the presence of vasopressin, as measured by using quin2 or 45Ca2+ exchange, whereas no change in Ca2+ inflow was observed in the absence of vasopressin. 6. In cells not loaded with quin2, replacement of NaCl by choline chloride did not alter Ca2+ inflow either in the presence or in the absence of vasopressin. 7. It is concluded that (i) Ca2+ inflow through the basal and receptor-activated Ca2+ inflow systems does not involve the inward movement of Ca2+ in exchange for Na+ or the induction of Ca2+ inflow by intracellular Na+, and (ii) the presence of both intracellular quin2 and extracellular choline or tetramethylammonium chloride (in place of NaCl) inhibits Ca2+ inflow through the receptor-activated Ca2+ inflow system but not through the basal Ca2+ inflow system, and inhibits the release of Ca2+ from intracellular stores.

Inhibition of the liver cell receptor-activated Ca2+ inflow system by metal ion inhibitors of voltage-operated Ca2+ channels but not by other inhibitors of Ca2+ inflow

The properties of the receptor-activated Ca2+ inflow system in the liver cell plasma membrane were compared with those of voltage-operated Ca2+ channels and receptor-operated Ca2+ channels present in other cell types by testing the susceptibility of the Ca2+ inflow system to inhibition by other metal ions and known inhibitors of Ca2+ movement across membranes. Co2+ inhibited Ca2+ inflow through the receptor-activated Ca2+ inflow system, as assessed by measurement of (a) the activation by extracellular Ca2+ (Cao2+) of glycogen phosphorylase in the presence of vasopressin and (b) 45Ca2+ exchange in the presence of the hormone. The concentration of Co2+ which gave half-maximal inhibition was 280 microM. The inhibition by Co2+ was reversed by high Cao2+. Co2+ did not inhibit basal Ca2+ inflow as measured by 45Ca2+ exchange in the absence of vasopressin. Zn2+, Cd2+, Ni2+ and Mn2+ each inhibited Ca2+ inflow through the receptor-activated Ca2+ inflow system. The concentrations of these ions which gave half-maximal inhibition were 10, 50, 220 and 400 microM, respectively. Little inhibition of receptor-activated Ca2+ inflow was observed in the presence of Sr2+ or Ba2+. However, substantial amounts of 90Sr2+ were taken up by hepatocytes. Rates of 90Sr2+ uptake increased from 0.5-8 nmol per min per mg wet wt. when the extracellular concentration of Sr2+ was varied from 0.25 to 2.5 mM. Sr2+ uptake was inhibited 50% by Cao2+ with half-maximal inhibition at 100 microM Cao2+, but was not inhibited by verapamil and was not stimulated by vasopressin. The movement of Ca2+ through the receptor-activated Ca2+ inflow system was not inhibited by high concentrations of each of a number of inhibitors of voltage-operated and receptor-operated Ca2+ channels and intracellular Ca2+ movement. It is concluded that while the susceptibility to inhibition by metal ions of the receptor-activated Ca2+ inflow system in the liver cell plasma membrane is similar to that of voltage-operated Ca2+ channels, there are significant differences between the liver cell receptor-activated Ca2+ inflow system and both voltage-operated Ca2+ channels and some other receptor-operated Ca2+ channels with respect to inhibition by organic compounds.

Vasopressin-stimulated Ca2+ influx in rat hepatocytes is inhibited in high-K+ medium

We examined the effects of K+ substitution for Na+ on the response of hepatocytes to vasopressin, and on the hepatocyte plasma-membrane potential. (1) High K+ (114 mM) had no effect on the initial increase in phosphorylase a activity in response to vasopressin, but abolished the ability of the hormone to maintain increased activity beyond 10 min. With increasing concentrations a decrease in the vasopressin response was first observed at 30-50 mM-K+. (2) High K+ (114 mM) had no effect on basal 45Ca2+ influx, but abolished the ability of vasopressin to stimulate influx. This effect was also first observed at a concentration of 30-50 mM-K+. (3) Increasing K+ had little effect on the plasma-membrane potential until a concentration of 40 mM was reached. With further increases in concentration the plasma membrane was progressively depolarized. (4) Replacement of Na+ with N-methyl-D-glucamine+ depolarized the plasma membrane to a much smaller extent than did replacement with K+, and was also much less effective in inhibiting the vasopressin response. (5) The plasma-membrane potential was restored to near the control value by resuspending cells in normal-K+ medium after exposure to high-K+ medium. The effects of vasopressin on phosphorylase activity were also restored. (6) We conclude that the Ca2+ channels responsible for vasopressin-stimulated Ca2+ influx are closed by depolarization of the plasma membrane.

Electrophysiological properties of isolated rat liver cells

The electrophysiological properties of isolated rat liver cells were studied using the patch clamp method in whole-cell configuration. The membrane potential in isolated hepatocytes was -42 +/- 7 mV (n = 20). The input resistance (Rin) and the time constant (tau m) were 51 +/- 17 M (the range of 34 to 180 M omega) (n = 20) and 4.2 +/- 1.0 msec (the range of 3 to 16.5 ms) (n = 20). Assuming that the specific membrane capacitance is 1 microF/cm2, the membrane resistance and membrane capacitance were 42. +/- 9.0 K omega cm2 and 87 +/- 27 pF. These values indicate that isolated rat hepatocytes are not abnormally permeable or leaky. The current-voltage relationship was linear with no rectification. The depolarizing pulse from the resting potential did not induce fast or slow inward currents even when norepinephrine or high Ca2 (3.6 mM) were applied. This indicates that there is no voltage-sensitive Ca2+ channel in the isolated hepatocytes.

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+.

Evidence that guanosine 5'-[gamma-thio]triphosphate stimulates plasma membrane Ca2+ inflow when introduced into hepatocytes

1. Slowly hydrolysable analogues of GTP were introduced into hepatocytes by incubating the cells in the absence of Mg2+ and in the presence of ATP4-. Experiments using guanosine 5'-[gamma-[35S]thio]triphosphate (GTP[35S])indicated that about 50% of the GTP[S] loaded into the cells was subsequently hydrolysed. 2. In cells loaded with GTP[S] and incubated in the absence of added extracellular Ca2+ (Ca2+o), the rate of activation of glycogen phosphorylase observed after addition of 1.3 mM-Ca2+o was 250% greater than the rate observed in unloaded cells. Smaller effects (130%) were observed in cells loaded with either guanyl-5'-yl imidodiphosphate or guanosine 5-[beta-thio]diphosphate (GDP[S]). Cells loaded with adenosine 5'-[gamma-thio]triphosphate showed no increase in glycogen phosphorylase activity on addition of Ca2+o. 3. The effect of a submaximal concentration of GTP[S] on the Ca2+-induced activation of glycogen phosphorylase was additive with that of a half-maximally effective concentration of vasopressin. GTP[S] did not increase the effect of a maximally effective concentration of the hormone. 4. Cells loaded with GTP[S] exhibited an increased initial rate of 45Ca2+ exchange measured at 1.3 mM-Ca2+o. 5. GTP[S] did not affect the amount of 45Ca2+ exchanged by cells incubated at 0.1 mM-Ca2+o or the ability of vasopressin to release 45Ca2+ from these cells. 6. It is concluded that the introduction of slowly hydrolysable analogues of GTP to the liver cell cytoplasmic space stimulates the inflow of Ca2+ across the plasma membrane through a channel similar to that activated by vasopressin.