Differential effects of phorbol ester on phenylephrine and vasopressin-induced Ca2+ mobilization in isolated hepatocytes

Varying very low-density lipoprotein secretion of rat hepatocytes by altering cellular levels of calcium and the activity of protein kinase C

BACKGROUND

Calcium antagonists lower plasma levels of lipoproteins and suppress hepatic very low-density lipoprotein (VLDL) secretion. Similar effects have been observed with the calcium ionophore A23187. We studied further the effect of calcium on VLDL metabolism.

METHODS

Hepatocytes from male Wistar rats were isolated and cultured in the presence or absence of calcium-mobilizing hormones, or compounds that either stimulate or inhibit the activity of protein kinase C. Secreted VLDL (d < 1.006 g mL-1) was isolated by centrifugation (145,000 x g), and lipids and apolipoprotein B were analysed.

RESULTS

VLDL secretion reached maximum in hepatocytes cultured in medium containing calcium 0.8-2.4 mmolL-1. Depleting the cells of calcium by incubating in calcium-free medium or by treating the cells with the Ca(2+)-ATPase inhibitor thapsigargin (5 x 10-7 molL-1) suppressed lipid secretion to less than 15% of control, and this was accompanied by an increase in cellular levels of triacylglycerol. Calcium loading (medium calcium > 2.4 mmolL-1) suppressed both lipoprotein secretion and cellular levels of lipids, suggesting a reduced overall rate of lipid synthesis. At an extracellular calcium concentration of 0.8 mmolL-1, angiotensin II, vasopressin, endothelin-1 (10(-7) molL-1) or phenylephrine (10(-4) molL-1) suppressed VLDL secretion (maximum to 37% of control), and elevated medium calcium attenuated this effect. The protein kinase C inhibitor chelerythrine (5 x 10(-5) molL-1) and the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) (10(-6) molL-1), suppressed VLDL secretion to 18% and 60% of control, respectively, whereas the protein kinase C-inactive 4 alpha-PMA was without an effect. No effect on ketogenesis was observed by these compounds, indicating that suppressed lipid secretion was not due to an enhanced oxidation of lipids.

CONCLUSIONS

Hepatic VLDL secretion can be related to changes in hepatocyte levels of calcium and the activity of protein kinase C.

Chloroquine inhibits alpha1B-adrenergic action in hepatocytes

Noradrenaline increased phosphorylase a activity through activation of alpha1B-adrenoceptors in rat hepatocytes. Such effect was inhibited by chloroquine (Ki approximately 55 nM) and only slightly reduced by high concentrations of primaquine. Chloroquine did not inhibit the activation of phosphorylase a induced by vasopressin or angiotensin II. Binding competition experiments using [3H]prazosin showed that both chloroquine and primaquine interact with alpha1B-adrenoceptors, but only at very high concentrations. This indicates that the ability of chloroquine to block the alpha1B-adrenergic action was not due to antagonism at the receptor level. Noradrenaline increased phosphatidylinositol resynthesis and inositol trisphosphate production; these effects were inhibited by chloroquine and phorbol 12-myristate 13-acetate. Staurosporine and Ro 31-8220 (3-[1-[3-(amidinothio)propyl-1H-indol-3-yl]-3-(1-methyl-1H-indol-3 -yl)maleimide), reduced the inhibitions induced by the active phorbol ester and the antimalarial drug on adrenergic-stimulated phosphatidylinositol resynthesis. Similarly, staurosporine blocked the inhibitory actions of chloroquine and phorbol 12-myristate 13-acetate on noradrenaline-stimulated inositol trisphosphate production. These data suggest the possibility that protein kinases, such as protein kinase C, could be involved in the actions of chloroquine.

Activation of endothelin ETA receptors induces phosphorylation of alpha1b-adrenoreceptors in Rat-1 fibroblasts

The effect of endothelin-1 on the phosphorylation of alpha1b-adrenoreceptors, transfected into rat-1 fibroblasts, was studied. Basal alpha1b-adrenoreceptor phosphorylation was markedly increased by endothelin-1, norepinephrine, and phorbol esters. The effect of endothelin-1 was dose dependent (EC50 approximately 1 nM), reached its maximum 5 min after stimulation, and was inhibited by BQ-123, an antagonist selective for ETA receptors. Endothelin-1-induced alpha1b-adrenoreceptor phosphorylation was attenuated by staurosporine or genistein and essentially abolished when both inhibitors were used together. The effect of norepinephrine was not modified by either staurosporine or genistein alone, and it was only partially inhibited when both were used together. These data suggest the participation of protein kinase C and tyrosine kinase(s) in endothelin-1-induced receptor phosphorylation. However, phosphoaminoacid analysis revealed the presence of phosphoserine and traces of phosphothreonine, but not of phosphotyrosine, suggesting that the putative tyrosine kinase(s), activated by endothelin, could act in a step previous to receptor phosphorylation. The effect of endothelin-1 on alpha1b-adrenoreceptor phosphorylation was not mediated through pertussis toxin-sensitive G proteins. Calcium mobilization induced by norepinephrine was diminished by endothelin-1. Norepinephrine and endothelin-1 increased [35S]GTPgammaS binding to control membranes. The effect of norepinephrine was abolished in membranes obtained from cells pretreated with endothelin-1. Interestingly, genistein plus staurosporine inhibited this effect of the endothelial peptide. Endothelin-1 did not induce alpha1b-adrenoreceptor internalization. Our data indicate that activation of ETA receptors by endothelin-1 induces alpha1b-adrenoreceptor phosphorylation and alters G protein coupling.

Prolonged antagonism of alpha 1-adrenergic vasoconstriction in the rat liver by atrial natriuretic peptide

BACKGROUND/AIMS

Vasodilator hormones that regulate hepatic circulation at physiological concentrations have not been sufficiently identified. The presence of atrial natriuretic peptide (ANP) and its receptors in the hepatic vascular bed suggest such vasorelaxing potential.

METHODS

Livers of male Sprague-Dawley rats were perfused in a flow-constant fashion. The selective alpha 1-adrenergic agonist phenylephrine (PE) (1.5 mumol/L) was infused from 30 to 36 minutes and again from 70 to 76 minutes after starting perfusion (n = 5). ANP (0.1 pmol/L to 200 nmol/L), des-(Gln18, Ser19, Gly20, Leu21, Gly22)-ANP fragment (C-ANP) (20 nmol/L), or 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) (50 mumol/L) (each n = 4) were added from 20 to 40 minutes.

RESULTS

During the first infusion of PE, portal pressure increased from 3.7 +/- 0.5 to 12.1 +/- 0.8 cm H2O maximally (mean +/- SD) and increased again to 11.5 +/- 2.0 during the second PE infusion. ANP at physiological concentrations reduced both PE-induced increases of portal pressure in a dose-dependent fashion, reaching half-maximal effects around 20 pmol/L and maximal effects (about 50% inhibition of PE-induced vasoconstriction) at 40 pmol/L. The cGMP analogue 8-Br-cGMP showed the same long-lasting vasodilating effect as ANP. In contrast, C-ANP, which binds only to the ANP C-receptor, had no effects.

CONCLUSIONS

Physiological concentrations of ANP antagonize alpha 1-adrenergic vasoconstriction in the liver, suggesting an important function in the humoral regulation of hepatic circulation. The prolonged hemodynamic effect of ANP seems to be ANP A-receptor/guanylyl cyclase/cGMP-mediated.

Role of protein kinase C in the regulation of inositol phosphate production and Ca2+ mobilization evoked by ATP and acetylcholine in rat lacrimal acini

Stimulation of rat lacrimal acinar cells with ATP and acetylcholine (ACh) induced a rapid accumulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and its degradation products, resulting in an initial release of Ca2+ from intracellular stores. However, after pretreating the acini with U73122 no increase in the intracellular free Ca2+ concentration ([Ca2+]i) or Ins(1,4,5)P3 production was observed. A short pretreatment with the phorbol ester 4-beta-phorbol-12-beta-myristate-13-alpha-acetate (PMA) significantly attenuated the ATP- and ACh-induced increase in [Ca2+]i and overall inositol phosphate production. In contrast, staurosporine enhanced Ins(1,4,5)P3 and inositol 1,3,4-trisphosphate [Ins(1,3,4)P3] production and [Ca2+]i above control values in ATP- and ACh-stimulated cells. Stimulation of phospholipase C by ionomycin-evoked changes in [Ca2+]i were unaltered by pretreatment with staurosporine and PMA. The data show that a change in protein kinase C activity during cell stimulation affects the inositol phosphate metabolism and thereby the cellular Ca2+ signalling processes in lacrimal acinar cells.

Glycogenolytic and antiglycogenolytic prostaglandin E2 actions in rat hepatocytes are mediated via different signalling pathways

Prostaglandin E2 has been reported both to stimulate glycogen-phosphorylase activity (glycogenolytic effect) and to inhibit the glucagon-stimulated glycogen-phosphorylase activity (antiglycogenolytic effect) in rat hepatocytes. It was the purpose of this study to resolve this apparent contradiction and to characterize the signalling pathways and receptor subtypes involved in the opposing prostaglandin E2 actions. Prostaglandin E2 (10 microM) increased glucose output, glycogen-phosphorylase activity and inositol trisphosphate formation in hepatocyte cell culture and/or suspension. In the same systems, prostaglandin E2 decreased the glucagon-stimulated (1 nM) glycogen-phosphorylase activity and cAMP formation. The signalling pathway leading to the glycogenolytic effect of PGE2 was interrupted by incubation of the hepatocytes with 4 beta-phorbol 12-myristate 13-acetate (100 nM) for 10 min, while the antiglycogenolytic effect of prostaglandin E2 was not attenuated. The signalling pathway leading to the antiglycogenolytic effect of prostaglandin E2 was interrupted by an incubation of cultured hepatocytes with pertussis toxin (100 ng/ml) for 18 h, whereas the glycogenolytic effect of prostaglandin E2 was enhanced. The EP1/EP3 prostaglandin-E2-receptor-specific prostaglandin E2 analogue Sulproston had a stronger glycogenolytic potency than the EP3 prostaglandin-E2-receptor-specific prostaglandin E2 analogue Misoprostol. The antiglycogenolytic potency of both agonists was equal. It is concluded that the glycogenolytic and the antiglycogenolytic effects of prostaglandin E2 are mediated via different signalling pathways in hepatocytes possibly involving EP1 and EP3 prostaglandin E2 receptors, respectively.

Rapid and reversible uncoupling of the hepatic alpha 1-adrenergic receptor and guanine-nucleotide-binding protein by phorbol 12,13-dibutyrate

Phorbol 12,13-dibutyrate (PDBU), 1 microgram/ml for 2 min, abolished alpha 1-adrenergic receptor (AR)-mediated signal transduction in rat hepatocytes and converted 100% of alpha 1-ARs to a low-affinity state, similar to effects of a non-hydrolyzable GTP analog. Reversal of PDBU inhibition restored high-affinity alpha 1-ARs towards control values. The rapid uncoupling of the alpha 1-AR and guanine-nucleotide-binding protein by PDBU, and reversibility associated with reactivation of alpha 1-adrenergic signaling, identify this as an important inhibitory locus for PDBU.

The effects of beta-phorbol-12,13 dibutyrate on agonist-induced increases in [Ca2+]i in N1E-115 cells. Differential modulation of responses to angiotensin II and bradykinin

Addition of angiotensin II (A2; 500 nM) to populations of fura-2-loaded N1E-115 cells resulted in a transient increase in intracellular calcium which was abolished by pre-treatment with the phorbol ester, beta-phorbol-12,13 dibutyrate (PDBu) (1.5 microM). The inhibitory effects were reversed by the protein kinase C inhibitor, staurosporine (150 nM), and down-regulation of protein kinase C was observed over 48 hr. Responses to maximally effective concentrations of histamine (300 microM), ATP (100 microM), UTP (100 microM) and carbachol (100 microM) were similarly inhibited by phorbol pre-treatment but the response to bradykinin (BK) (100 nM) was unaffected. When the concentrations of BK and A2 were adjusted to produce the same-sized calcium signals, PDBu pre-treatment abolished the response to A2 but only partially inhibited the response to BK. From the data presented here we can conclude that the calcium response to BK in N1E-115 cells is less susceptible to the inhibitory effects of protein kinase C activation than the response produced by A2.

Rapid inverse changes in alpha 1B- and beta 2-adrenergic receptors and gene transcripts in acutely isolated rat liver cells

In vitro incubation of hepatocytes acutely isolated from adult male rats leads to a rapid conversion of the adrenergic activation of glycogenolysis from an alpha 1-receptor (alpha 1AR) to a beta 2-receptor (beta 2AR) mediated response within 4 h. In order to understand the underlying mechanism, we examined time-dependent changes in alpha 1- and beta 2-adrenergic activation of glycogenolysis and second messenger systems, the cellular density and affinity of alpha 1AR and beta 2AR, and the steady state levels of alpha 1BAR and beta 2AR mRNAs. Incubation of hepatocytes for 4 h resulted in a decrease in phosphorylase activation and inositol 1,4,5 trisphosphate accumulation in response to phenylephrine, a 40% decrease in alpha 1AR density, and a 70% decrease in alpha 1BAR mRNA levels. Incubation of hepatocytes for 4 h also resulted in the emergence of a phosphorylase response to isoproterenol, an increase in isoproterenol-induced but not in glucagon- or forskolin-induced cAMP accumulation, no significant change in beta 2AR density, and a twofold increase in beta 2AR mRNA levels. Exposure of cells to cycloheximide, 2 microM throughout the 4 h incubation, prevented the emergence of the phosphorylase response to isoproterenol and reduced beta 2AR densities, while the decrease in alpha 1AR density was not affected and the decrease in phosphorylase activation by phenylephrine was attenuated. The results indicate that dissociation of rat liver cells triggers a rapidly developing decrease in alpha 1BAR mRNA and increase in beta 2AR mRNA levels and corresponding inverse changes in the synthesis of alpha 1BAR and beta 2AR which account, at least in part, for the rapid conversion from alpha 1- to beta 2-adrenergic glycogenolysis.

Inhibition by noradrenaline and adrenaline of the increase in glucose and lactate output and decrease in flow after sympathetic nerve stimulation in perfused rat liver: possible involvement of protein kinase C

In perfused rat liver stimulation of the hepatic nerve plexuses increased via alpha 1-receptors glucose and lactate output decreased flow and caused an overflow of noradrenaline into the hepatic vein. Infusion of noradrenaline and adrenaline also elicited similar metabolic and hemodynamic alterations via alpha 1-receptors, whereas infusion of isoproterenol via beta 2-receptors enhanced glucose output and slightly reduced lactate release without affecting flow. The influence of circulating catecholamines on the nerve stimulation-dependent changes was investigated. Noradrenaline (100 nmol/L) or adrenaline (40 nmol/L) but not isoproterenol (1 mumol/L), which themselves caused about half-maximal alterations, strongly inhibited the nerve stimulation-induced increase in glucose and lactate output and decrease in flow but had no effect on noradrenaline overflow. The protein kinase C activator (4 beta)phorbol 12-myristate, 13-acetate (100 nmol/L) but not its analog (4 alpha)phorbol 12,13-didecanoate (100 nmol/L) strongly inhibited the metabolic and hemodynamic changes caused by nerve stimulation or noradrenaline infusion. The protein kinase C inhibitor H7 (20 mumol/L) partially prevented the inhibition of the nerve actions by noradrenaline. The results lead us to conclude that noradrenaline and adrenaline inhibited the metabolic and hemodynamic nerve actions by means of a mechanism involving protein kinase C rather than presynaptic alpha-receptors or beta-receptors. The catecholamines apparently increased via alpha 1-receptors inositol 1,4,5-trisphosphate, which in turn enhanced cytosolic Ca2+ and thus altered metabolism and in part hemodynamics, and diacylglycerol, which in turn activated protein kinase C and thus feedback inhibited the signal chain from alpha 1-receptors via G proteins to phospholipase C.

Study on the mechanism of Giardia lamblia induced diarrhoea in mice

The transmucosal fluxes of Na+ and Cl- were studied in Giardia lamblia infected mice in the presence or absence of phorbol-12-myristate-13-acetate (PMA), the activator of protein kinase C (PKC) or 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine (H-7), the inhibitor of PKC or Ca(2+)-calmodulin. There was net secretion of Na+ and Cl- in infected animals, while in control animals there was net absorption of these ions. The addition of ionophore or PMA resulted in net secretion of Na+ and Cl- in the control group while in the infected group there was no change in the fluxes of these ions. The selective potent inhibitor of protein kinase C, H-7, reversed the secretion of Na+ and Cl- in infected group to absorption. The addition of PMA and Ca(2+)-ionophore together in the infected group had a partial additive effect. This study suggests that G. lamblia induced fluid secretion involves protein kinase C and further protein kinase C acts in synergism with calcium.

Phorbol esters inhibit ionomycin-induced hydrolysis of phosphoinositides and phosphatidylcholine in bovine corneal epithelial cells

The effects of phorbol esters on phospholipase C (PLC) activity towards phosphoinositides and phosphatidylcholine (PC) in bovine corneal epithelial cells were examined. The cells were labeled with 32Pi, myo[3H]inositol or methyl[14C]choline, and PLC stimulated by incubation of the cells with Ca2+ ionophore, ionomycin. The PLC activity was assessed by monitoring the loss of radioactivity from the labeled phospholipids or the accumulation of their radioactive metabolites. The data from this study can be summarized as follows: Addition of 20 microM ionomycin to the prelabeled cells resulted in a rapid hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) and somewhat slower hydrolysis of phosphatidylinositol (PI) and phosphatidylcholine (PC) with concomitant several-fold increase in phosphatidic acid (PA). The effects of the ionophore were time- and dose-dependent. Incubation of the cells with phorbol 12,13-dibutyrate (PDBu) or phorbol 12-myristate 13-acetate (PMA) caused increased radioactivity in PC and PA, whereas the radioactivity in PI and PIP2 remained unchanged. The effects of PDBu were inhibited by staurosporine and H-7, and inactive derivatives of phorbol esters failed to exert any effect on phospholipid metabolism. Pretreatment of the corneal epithelial cells with PDBu or PMA abolished the ionomycin-induced hydrolysis of phosphoinositides and PC. The data suggest that activation of protein kinase C by phorbol esters in corneal epithelial cells results in inhibition of PLC activity towards phosphoinositides and PC through a mechanism probably involving phosphorylation of the enzyme.

Okadaic acid inhibits angiotensin II stimulation of Ins(1,4,5)P3 and calcium signalling in rat hepatocytes

OKA2 and CL-A significantly inhibit the ability of angiotensin II, ATP and vasopressin to raise [Ca2+]i in rat hepatocytes, with a partial inhibition of the initial spike, and a complete inhibition of the following plateau. In contrast, the [Ca2+]i response to thapsigargin, which releases intracellular calcium stores through a mechanism independent of inositol phosphates, is much less affected. The ability of angiotensin II to stimulate Ins(1,4,5)P3 production is also reduced by OKA, with kinetics consistent with the inhibited [Ca2+]i response. Since OKA and CL-A are potent and selective inhibitors of phosphoprotein phosphatases, these results provide further evidence that agonist-stimulated Ins(1,4,5)P3 signalling can be inhibited by protein phosphorylation.

Intracellular calcium-mediated activation of hepatic Na+/H+ exchange by arginine vasopressin and phenylephrine

The effect of Ca++ mobilizing agonists arginine vasopressin and phenylephrine on Na+/H+ exchange was studied in freshly isolated hepatocytes and isolated perfused rat livers. The activity of Na+/H+ exchange was determined from the rate of H+ efflux, 22Na uptake and pHi recovery. Arginine vasopressin and phenylephrine stimulated H+ efflux and 22Na uptake in isolated rat hepatocytes and increased the rate of pHi recovery from acid-loaded hepatocytes. These effects were inhibited by amiloride. Arginine vasopressin- and phenylephrine-induced increases in H+ efflux were also dependent on extracellular Na+. Arginine vasopressin- and phenylephrine-induced increases in intracellular Ca++ concentration, H+ efflux, 22Na uptake and intracellular pH recovery were decreased in hepatocytes preloaded with the Ca(++)-buffering agent [bis-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid] (MAPTA). Na+/H+ exchange-dependent intracellular pH recovery from cytosolic acidification was stimulated by thapsigargin, which increases intracellular calcium concentration by inhibiting endoplasmic reticulum Ca++ ATPase. Arginine vasopressin- and phenylephrine-induced increases in intracellular pH recovery were not dependent on extracellular Ca++ and were inhibited by calmidazolium, a calmodulin inhibitor. Arginine vasopressin and phenylephrine also increased H+ efflux in the absence but not in the presence of amiloride in perfused rat livers without affecting biliary HCO3- excretion. These results indicate that arginine vasopressin and phenylephrine activate Na+/H+ exchange in rat hepatocytes, an effect mediated in part by intracellular Ca++ and calmodulin kinase. Furthermore, sinusoidal Na+/H+ exchange does not appear to be involved in biliary HCO3- excretion.

Effect of okadaic acid on hormone- and mastoparan-stimulated phosphoinositide turnover in isolated rat hepatocytes

Okadaic acid is a potent and specific inhibitor of protein phosphatases 1 and 2A which seems to be useful for identifying biological processes that are controlled by reversible phosphorylation of proteins. We report here that okadaic acid inhibits in isolated hepatocytes the stimulations of phosphoinositide turnover induced by epinephrine, angiotensin II and vasopressin. Mastoparan, a peptide toxin from wasp venom that mimics receptors by activating G-proteins, also stimulates the accumulation of inositol phosphates in hepatocytes. Interestingly, this action of mastoparan was also inhibited by okadaic acid. Our data indicate that okadaic acid inhibits the phosphoinositide turnover signal transduction system in hepatocytes at a level distal to the receptors.

Effect of hypothyroidism and thyroid hormone replacement on the level of protein kinase C and protein kinase A in rat liver

We investigated the influence of the thyroid hormone status on the levels of protein kinases C (PKC) and A (PKA) in the soluble fraction of rat liver. The immunodetectable PKC level in hypothyroid liver was elevated 7.7-fold, whereas the phorbol-ester binding capacity and the immunodetectable alpha-PKC level were increased 2.4- and 2.6-fold, respectively. Conversely, in hypothyroid livers the abundance of the regulatory type I and the catalytic subunits of PKA were lowered to 42% of the euthyroid level as determined by immunoblotting and by measuring the substrate specific phosphorylation rate of PKA. These changes in the PKC and PKA levels were reversible upon treatment with 0.5 microgram T4/100 g body weight for 2-21 days. The thyroid state dependent alterations in hepatic PKC and PKA levels may be responsible for the known changes in the response of hepatocytes to other hormonal stimuli in hypothyroidism.

Prostaglandin-induced changes in calcium uptake and cAMP production in osteoblast-like cells: role of protein kinase C

Phorbol esters were used to evaluate the putative effect of protein kinase C (PKC) activation on prostaglandin E2 (PGE2)-induced increases in calcium uptake and cAMP production in the human osteoblastic osteosarcoma cell line, Saos-2. The cells were pretreated for 15 min with phorbol myristate acetate (PMA) followed by a 5 min incubation with PGE2. Calcium uptake was measured with 45Ca and cAMP by radioimmunoassay. A significant increase in calcium uptake was noted in the PGE-treated cells compared with controls and preincubation with the PMA caused a significant decrease in this response. Preincubation with PMA also inhibited the PGE2-induced increase in cAMP under identical conditions. The effect of PMA on the cAMP response was not influenced by the addition of a phosphodiesterase inhibitor. PMA had no effect on the basal levels of either calcium uptake or cAMP production. Likewise, the inactive phorbol esters, phorbol 12,13-didecanoate (PDD) and 4 alpha-phorbol 12-myristate, 13-acetate (4 alpha), had no effect on either basal levels of these parameters or on the PGE2-induced increases. These results suggest that PKC is involved in the down-regulation of PGE2-induced increases in calcium uptake and cAMP production in the Saos-2 osteoblastic cell line.

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.

Spatial and temporal organization of calcium signalling in hepatocytes

Treatment of hepatocytes with agonists which act via the second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), results in increases of cytosolic free Ca2+ [( Ca2+]i) which are manifest as a series of discrete [Ca2+]i transients or oscillations. With increasing agonist dose [Ca2+]i oscillation frequency increases and the initial latent period decreases, but the amplitude of the [Ca2+]i oscillations remains constant. Studies of these [Ca2+]i oscillations at the subcellular level have indicated that the [Ca2+]i changes do not occur synchronously throughout the cell, but initiate at a specific subcellular domain, adjacent to a region of the plasma membrane, and then propagate through the cell as a [Ca2+]i wave. For a given ceil, the locus of [Ca2+]i wave initiation is constant for every oscillation in a series and is also identical when the cell is sequentially stimulated with different agonists or when the phospholipase C-linked G protein is activated directly using AIF4-. The kinetics of the [Ca2+]i waves indicate that a Ca(2+)-activated mechanism is involved in propagating the oscillatory [Ca2+]i increases throughout the cell, and the data appear to be most consistent with a process of Ca(2+)-induced Ca2+ release. It is proposed that the ability to propagate [Ca2+]i oscillations into regions of the cell distal to the region in which the signal transduction apparatus is localized could serve an important function in allowing all parts of the cell to respond to the stimulus.

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 role of Ca2+ in the time-dependent pepsinogen secretion of frog oesophageal peptic glands stimulated by bombesin

Time- and dose-related stimulation of pepsinogen secretion by bombesin was studied in perifused dispersed peptic glands from the oesophagus of the American bullfrog Rana catesbeiana. The dose response to bombesin was monophasic between 10(-10) and 10(-7) M, with an EC50 of 10(-9) M. Time-dependent secretion was closely monitored at 1-2 min intervals. Though there was overlap, we could discriminate an early response at approximately 2 min (phase I) and a delayed or sustained response at greater than or equal to 2 min (phase II) on the basis of responses in the presence and absence of external Ca2+. Phase I was relatively independent of external [Ca2+] and coincided with 45Ca efflux following a dose-dependent increase in cytosolic [Ca2+], measured by Fura-2AM. Phase II was sustained at approximately 80% of control at an external [Ca2+] of 1-5 microM, but was eliminated by adding 0.5-1 mM EGTA. Bombesin caused a sustained Ca2+ influx and, when this was prevented by EGTA, the response to successive stimulations by bombesin and by acetylcholine was greatly attenuated. The phorbol ester, 12-O-tetradecanoyl phorbol 13-acetate, which stimulates secretion at high concentrations, was used as background at a threshold concentration of 10(-7) M, which did not by itself stimulate secretion. At this concentration, 12-O-tetradecanoyl phorbol 13-acetate potentiated the responses to bombesin and to acetylcholine. These results define the different Ca2+ dependencies of the immediate and sustained secretory responses to bombesin, but indicate a complex relationship of stimulation responses to Ca2+ homeostasis in various agonist-sensitive Ca2+ pools.

Contrasting effects of phorbol dibutyrate and phorbol myristate acetate in rabbit aorta

In rat hepatocytes, active phorbol esters inhibited the alpha 1-adrenergic stimulation of phosphatidylinositol labeling with the expected potency order: phorbol myristate acetate (PMA) greater than phorbol dibutyrate (PDB). In contrast, in rabbit aorta the alpha 1-adrenergic action was inhibited dose-dependently by PDB but not by PMA. Similarly PDB (but not PMA) induced a strong contraction in rabbit aorta. The phorbol ester-induced contraction developed slowly, was dose-dependent and independent of extracellular calcium. These effects of PDB in rabbit aorta were neither inhibited by the protein kinase inhibitor H-7 nor mimicked by the synthetic diacylglycerol, OAG. Our results raise some doubts on the mechanism(s) through which the actions of PDB take place in rabbit aorta.

Antagonizing effects of phorbol 12-myristate 13-acetate on hormonally stimulated gluconeogenesis in isolated rat hepatocytes involve activity changes of pyruvate kinase

The tumor-promoting phorbol ester phorbol 12-myristate 13-acetate partially neutralized the stimulatory effects of epinephrine (alpha 1-adrenergic actions), glucagon, and dibutyryl-cAMP on gluconeogenesis in isolated hepatocytes of fasted rats, when lactate or dihydroxyacetone was used as the substrate. By constructing metabolic crossover plots and by comparing rates of lactate production from dihydroxyacetone with K0.5 values of extracted pyruvate kinase for phosphoenolpyruvate, we obtained evidence that phorbol ester actions on hormonally stimulated gluconeogenesis were accompanied by proportionate increases in activity of pyruvate kinase. Although purified pyruvate kinase from rat liver was a substrate for protein kinase C in vitro, phosphorylation was not accompanied by modulation of kinetic parameters. Furthermore, incubation of pyruvate kinase extracted from hormone-treated hepatocytes with protein kinase C revealed no activation of the prephosphorylated enzyme. This and the absence of effects of the phorbol ester on basal rates of gluconeogenesis and lactate production suggest that effects of protein kinase C on pyruvate kinase activity in hepatocytes may result from impairment of steps at the level of hormone-induced signal transduction.

Time- and dose-dependent effects of protein kinase C on proximal bicarbonate transport

Activation of protein kinase C has been shown to cause both stimulation and inhibition of transport processes in the brush-border membrane and renal tubule. This study was designed to examine the dose-response nature and time-dependent effect of 4 beta-phorbol-12-myristate-13-acetate (PMA) on the rates of bicarbonate absorption (JHCO3) and fluid absorption (Jv) in the proximal convoluted tubule (PCT) of rat kidney. Bicarbonate flux was determined by total CO2 changes between the collected fluid and the original perfusate as analyzed by microcalorimetry. Luminal perfusion of PMA (10(-10) approximately 10(-5) M) within 10 min caused a significant increase of JHCO3 and Jv. A peaked curve of the dose response was observed with maximal effect at 10(-8) M PMA on both bicarbonate and fluid reabsorption, which could be blocked completely by amiloride (10(3) M) and EIPA (10(-5) M). On the other hand, with an increase of perfusion time beyond 15 min. PMA (10(-8) and 10(-6) M) could inhibit JHCO3 and Jv. Amiloride (10(-3) M) or EIPA (10(-5) M) significantly inhibits JHCO3 and Jv, while there is no additive effect of PMA and amiloride or EIPA on PCT transport. An inactive phorbol-ester, 4 alpha-phorbol, that does not activate protein kinase C, had no effects on JHCO3 and Jv. Capillary perfusion of PMA (10(-8) M) significantly stimulate both JHCO3 and Jv; however, PMA did not affect glucose transport from either the luminal side or basolateral side of the PCT. These results indicate that activation of endogenous protein kinase C by PMA could either stimulate or inhibit both bicarbonate and fluid reabsorption in the PCT dependent on time and dose, and these effects are through the modulation of Na+/H+ exchange mechanism.

Regulation of phosphoinositide-specific phospholipase C

The receptors involved in the regulation of phospholipase C by hormones, neurotransmitters and other ligands have seven transmembrane-spanning hydrophobic regions (seven-helix motif) and no known enzymatic activity. Furthermore these receptors can be isolated as complexes with guanine nucleotide binding (G) proteins. Guanine nucleotides affect the binding of hormones that stimulate phospholipase C and it has been possible to see activation of GTPase activity in membranes upon addition of these ligands. Further indirect evidence for a Gp (p stands for phospholipase C activation) protein is the finding that in membranes agonist activation of phospholipase C requires the presence of GTP gamma S a non-hydrolyzable analog of GTP. Furthermore, fluoride is able to activate phospholipase C but its inhibition of phosphatidylinositol-4' kinase (PI-4' kinase) can interfere with efforts to demonstrate this in intact cells. There are four major isozymes of phospholipase C that have been cloned and sequenced. Recently it was found that phospholipase C-gamma as well as PI-3'-kinase are substrates for phosphorylation on tyrosine residues by the EGF and PDGF receptors. The PI-3' kinase is able to convert phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-trisphosphate (PIP3) but the function of this lipid is unknown since it is not a substrate for any known phospholipase C. While much has been learned about the structure and regulation of the phosphoinositide specific kinases and phosphodiesterase enzymes this is a relatively new field in which we can expect many advances during the next few years.

Vasopressin and norepinephrine stimulation of inositol phosphate accumulation in rat hepatocytes are modified differently by protein f1nase C and protein kinase A

Rat hepatocytes were maintained in primary monolayer culture for 24 h in the presence of serum. Treatment of hepatocytes with 1 microM 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) for 5-15 min increased membrane-associated protein kinase C activity and concomitantly decreased soluble activity. Membrane protein kinase C activity returned to basal values within 1 h then decreased by more than 50% within 2 h. Prolonged (2-18 h) incubation with PMA did not further decrease protein kinase C activity. Pretreatment of hepatocytes with PMA for 5-15 min had little effect on the subsequent actions of 100 nM vasopressin but abolished the stimulation of inositol phosphate accumulation by 3 nM vasopressin and 20 microM norepinephrine. Long-term exposure (2-18 h) of hepatocytes to 1 microM PMA actually enhanced the effects of vasopressin and 20 microM norepinephrine. The stimulation by norepinephrine (20 microM) of inositol phosphate accumulation was abolished by the alpha 1-adrenergic antagonist prazosin (1 microM), whereas the beta-adrenergic antagonist propranolol (30 microM) had little effect. Addition of 8Br-cAMP (100 microM) or glucagon (10 nM) for 5 min or 8 h had no significant effect alone, but enhanced the subsequent vasopressin stimulation of inositol phosphate accumulation. There was no effect of 8Br-cAMP or glucagon on norepinephrine stimulation of phosphoinositide breakdown. These data indicate that the stimulation of phospholipase C activity in rat hepatocytes by 3 nM vasopressin is enhanced by cyclic AMP-dependent kinase but inhibited by protein kinase C. In contrast, down regulation of protein kinase C markedly enhanced the maximal phosphoinositide response due to both vasopressin and norepinephrine.

Phorbol ester promotes a sustained down-regulation of endothelin receptors and cellular responses to endothelin in human vascular smooth muscle cells

The effect of phorbol ester pretreatment of human vascular smooth muscle cells (hVSMC) was studied with respect to regulation of endothelin (ET)-receptor binding and cellular responses to ET. The capacity of hVSMC to bind ET was decreased (by approximately 50% at maximum) after phorbol exposure, and this reductive effect was both rapid (t 1/2 approximately 10 min.) and sustained (for up to 24 hrs. of chronic phorbol exposure). Phorbol pretreatment inhibited both inositol phosphate and diacylclycerol production responses of hVSMC to ET in a manner that was time-dependent and sustained. Phorbol pretreatment also produced a persistent reduction in the ability of ET to release isotopically-labelled arachidonic and/or its metabolites from hVSMC, but importantly ionomycin-stimulated release was similarly negatively affected. Furthermore, ET-induced accumulation of the phospholipase A2/phospholipase B-derived inositol phospholipid metabolite, glycerophosphoinositol, was not different between control and phorbol-treated hVMSC. The mechanism whereby phorbol exerts differential, but notably sustained inhibitory effects on ET-promoted signal transduction pathways are thus complex and illustrative of the selectivity of protein kinase C in regulating cellular responses.

12-O-tetradecanoylphorbol 13-acetate and forskolin modify muscarinic receptor-linked Ca2+ mobilization in SH-SY5Y neuroblastoma cells through different mechanisms

The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA), which causes differentiation of SH-SY5Y neuroblastoma cells, reduces carbachol binding and carbachol-stimulated Ca2+ mobilization in these cells. The decrease in responsiveness to carbachol is due partially to a reduction in the amount of Ca2+ released by the cells and partially to a decrease in the sensitivity of the cells to carbachol. These effects probably can be attributed to a reduction in muscarinic receptor number and a decrease in receptor affinity, respectively. Forskolin, an alkaloid known to cause an increase in cellular cyclic AMP, enhances Ca2+ influx into the cells without affecting the cytosolic free Ca2+ concentration. The alkaloid causes an apparent restoration of the reduced Ca2+ release, caused by TPA, but does not affect the sensitivity of the cells to carbachol. Forskolin increases the decay of carbachol-induced increase in cytosolic Ca2+. The effects of TPA appear to be linked directly to receptor function, whereas those of forskolin are due to the effect of cyclic AMP on cellular Ca2+ metabolism.

Calcium and pH in anoxic and toxic injury

The critical events that lead to the transition from reversible to irreversible injury remain unclear. Studies are reviewed that have suggested that a rise in cytosolic free Ca2+ initiates plasma membrane bleb formation and a sequence of events that leads ultimately to cell death. In recent studies, we have measured changes in cytosolic free Ca2+, mitochondrial membrane potential, cytosolic pH, and cell surface blebbing in relation to the onset of irreversible injury and cell death following anoxic and toxic injury to single hepatocytes utilizing multiparameter digitized video microscopy (MDVM). MDVM is an emerging new technology that permits single living cells to be labeled with multiple probes whose fluorescence is responsive to specific cellular parameters of interest. Fluorescence images specific for each probe are collected over time, and then digitized and stored. Image analysis and processing then permits quantitation of the spatial distribution of the various parameters within the single living cells. Our results indicate the following: (1) formation of plasma membrane blebs accompanies all types of injury in hepatocytes; (2) cell death is a rapid event, initiated by rupture of a plasma membrane bleb, and is coincident with the onset of irreversible injury; (3) an increase of cytosolic free Ca2+ is not the stimulus for bleb formation or the final common pathway leading to cell death; (4) a decrease of mitochondrial membrane potential precedes loss of cell viability; (5) cytosolic pH falls by more than 1 pH unit during chemical hypoxia. This acidosis protects against the onset of cell death.

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.

Cytoskeletal alterations in human platelets exposed to oxidative stress are mediated by oxidative and Ca2+-dependent mechanisms

The metabolism of the redox-active quinone, menadione (2-methyl-1,4-naphthoquinone), in human platelets was associated with superoxide anion production, oxidation and depletion of intracellular glutathione, and modification of protein thiols. The cytoskeletal fraction extracted from menadione-treated platelets exhibited a dose-dependent increase in the amount of cytoskeleton-associated protein and a concomitant loss of protein thiols. These alterations were associated with oxidative modifications of actin, including beta-mercaptoethanol-sensitive crosslinking of actin to form dimers, trimers, and high-molecular-weight aggregates which also contained other cytoskeletal proteins, i.e., alpha-actinin and actin-binding protein. In addition, analysis of the cytoskeletal fraction from platelets treated with high concentrations (greater than or equal to 100 microM) of menadione by polyacrylamide gel electrophoresis under reducing conditions revealed a net decrease in the relative abundance of the individual cytoskeletal polypeptides. Under the same incubation conditions the platelets exhibited a sustained increase in cytosolic Ca2+ concentration. The presence of glucose, or the omission of Ca2+ from the incubation medium, prevented both the increase in cytosolic Ca2+ and the decrease in the relative amounts of cytoskeletal proteins. The latter effect was also largely prevented in platelets loaded with Quin-2 tetraacetoxymethyl ester to buffer the menadione-induced elevation of cytosolic Ca2+. Finally, the presence of a protease inhibitor, leupeptin, in the incubation medium prevented the menadione-induced decrease in the amount of actin-binding protein but not the decrease in the other cytoskeletal proteins. Our findings demonstrate that the multiple effects of oxidative stress on the platelet cytoskeleton are mediated by oxidative as well as by Ca2+-dependent mechanisms.

1,2-sn-diacylglycerol accumulates in choline-deficient liver. A possible mechanism of hepatic carcinogenesis via alteration in protein kinase C activity?

Choline deficiency is associated with triacylglycerol accumulation in the liver, and is the only nutritional state known to trigger hepatic cancer spontaneously. In two different experiments, rats were pair-fed for 6 weeks with control (0.2% choline), or choline-deficient (CD) (0.002% choline) diets. Hepatic choline and phosphocholine declined in CD animals to 54% and 16% of control levels, respectively. In control livers, 1,2-sn-diacylglycerol (1,2-sn-DAG) concentration was (in nmol/g wet wt) 144 (+/- 25; mean +/- SE); while in CD livers it was 792 (+/- 140) in the first experiment. In the second experiment the values were 375 (+/- 26) and 1147 (+/- 74), respectively. 1,2-sn-DAG, a precursor of triacylglycerol, is an endogenous activator of protein kinase C (PKC). PKC is the presumed site of action of the tumor-promoting phorbol esters. We suggest that the 1,2-sn-DAG accumulating in CD liver could bind PKC, altering its activity, and thus contribute to the carcinogenic effect of CD diets.