Mobilization of intracellular calcium by glucagon and cyclic AMP analogues in isolated rat hepatocytes

Hepatic changes during a carrageenan induced granuloma in rats

Hepatic changes during inflammation were studied in rats bearing a carrageenan induced granuloma. In spite of a decrease in the metabolic capacity of microsomes to induce lipid peroxidation during inflammation, the endogenous lipid peroxidation remained unchanged and unrelated with the hepatic activities measured. The continuous increase in hepatic cAMP observed during acute and chronic phases could be related to adenylate cyclase stimulation by mediators, and could be an initial step in the hepatocyte adaptation leading to the increased level of hepatic caeruloplasmin, to the reduction of cytochrome P-450 level and to the modifications of Ca²⁺ sequestration by microsomes.

The responses of rat hepatocytes to glucagon and adrenaline. Application of quantified elasticity analysis

The internal control of hepatocyte metabolism has been previously analysed using metabolic control analysis. The aim of this paper is to extend this analysis to include the responses of the cells to hormonal stimulus. Hepatocyte metabolism was divided into nine reaction blocks: glycogen breakdown, glucose release, glycolysis, lactate production, NADH oxidation, pyruvate oxidation, proton leak, mitochondrial phosphorylation and ATP consumption, linked by five intermediates: mitochondrial membrane potential, cytoplasmic NADH/NAD and total cellular ATP, glucose 6-phosphate and pyruvate. The kinetic responses of the reaction blocks to the intermediates were determined previously in the absence of added hormones. In this study, the changes in flux and intermediate levels that occurred upon addition of either glucagon or adrenaline were measured. From comparison of the fractional changes in fluxes and intermediate levels with the known kinetics of the system, it was possible to determine the primary sites of action of the hormones. The results show that the majority of processes in the cell are responsive to the hormones. The notable exception to this is the failure of adrenaline to have a direct effect on glycolysis. The activity change of each metabolic block observed in the presence of either hormone was quantified and compared to the indirect effects on each block caused by changes in metabolite levels. The second stage of the analysis was to use the calculated activity changes and the known control pattern of the system to give a semiquantitative analysis of the regulatory pathways employed by the hormones to achieve the changes in fluxes and metabolite levels. This was instructive in analysing, for example, how glucagon caused a decrease in flux through glycolysis and an increase in oxidative phosphorylation without large changes in metabolite levels (homeostasis). Conversely, it could be seen that the failure of adrenaline to maintain a constant glucose 6-phosphate concentration was due to the stimulation of glycogen breakdown and inhibition of glucose release.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

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

Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion

This review has considered a number of observations obtained from studies of insulin in perfused liver, hepatocytes, transformed liver cells and in vivo and each of the experimental systems offers advantages. The evaluation of insulin effects on component lipid synthesis suggests that overall, lipid synthesis is positively influenced by insulin. Short-term high levels of insulin through stimulation of intracellular degradation of freshly translated apo B and effects on synthesis limit the ability of hepatocytes to form and secrete TRL. The intracellular site of apo B degradation may involve membrane-bound apo B, cytoplasmic apo B and apo B which has entered the ER lumen. How insulin favors intracellular apo B degradation is not known. An area of recent investigation is in insulin-stimulated phosphorylation of intracellular substrates such as IRS-1 which activates insulin specific cellular signaling molecules [245]. Candidate molecules to study insulin action on apo B include IRS-1 and SH2-containing signaling molecules. Insulin dysregulation in carbohydrate metabolism occurs in non-insulin-dependent diabetes mellitus due to an imbalance between insulin sensitivity of tissue and pancreatic insulin secretion (reviewed in Refs. [307,308]). Insulin resistance in the liver results in the inability to suppress hepatic glucose production; in muscle, in impaired glucose uptake and oxidation and in adipose tissue, in the inability to suppress release of free FA. This lack of appropriate sensitivity towards insulin action leads to hyperglycemia which in turn stimulates compensatory insulin secretion by the pancreas leading to hyperinsulinemia. Ultimately, there may be failure of the pancreas to fully compensate, hyperglycemia worsens and diabetes develops. The etiology of insulin resistance is being intensively studied for the primary defect may be over secretion of insulin by the pancreas or tissue insulin resistance and both of these defects may be genetically predetermined. We suggest that, in addition to effects in carbohydrate metabolism, insulin resistance in liver results in the inability of first phase insulin to suppress hepatic TRL production which results in hypertriglyceridemia leading to high levels of plasma FA which accentuate insulin resistance in other target organs. As recently reviewed [17,254] the role of insulin as a stimulator of hepatic lipogenesis and TRL production has been long established. Several lines of evidence support that insulin is stimulatory to the production of hepatic TRL in vivo. First, population based studies support a positive relationship between plasma insulin and total TG and VLDL [253]. Second, there is a strong association between chronic hyperinsulinemia and VLDL overproduction [309].(ABSTRACT TRUNCATED AT 400 WORDS)

Hormone-mediated Ca2+ transients in isolated renal cortical thick ascending limb cells

Peptide hormones control salt reabsorption in cortical thick ascending limb (cTAL) cells of the loop of Henle. These agonists act, in part, through alterations on intracellular Ca2+ ([Ca2+]i). Primary cell cultures were prepared from porcine kidneys using a double antibody technique (goat antihuman Tamm-Horsfall and rabbit antigoat IgG antibodies). [Ca2+]i was determined in single cells with fluorescent techniques using fura-2. Parathyroid hormone (PTH) and arginine vasopressin (AVP) transiently increased [Ca2+]i in a dose-dependent manner. [Ca2+]i maximally increased from 85 +/- 5 nmol/l to 608 +/- 99 nmol/l with PTH, 10(-6) M, and to 766 +/- 162 nmol/l with AVP, 10(-7) M. The increment in [Ca2+]i by both hormones was by intracellular Ca2+ release and entry through plasma membrane Ca2+ channels. 8-Bromo-adenosine-3',5'-cyclic monophosphate (8-BrcAMP), 10(-4) M, increased [Ca2+]i (basal 83 +/- 3 to 427 +/- 121 nmol/l) but only from internal sources as nifedipine (10 mumol), ([Ca2+]i changes: 86 +/- 4 to 390 +/- 29 nmol/l) and removal of bath Ca/+o, ([Ca2+]i changes: 84 +/- 6 to 517 +/- 142 nmol/l), were without effect on agonist-induced [Ca2+]i. Thapsigargin, 1.5 mumol, completely abolished the AVP- and cyclic adenosine monophosphate-(cAMP)-induced Ca2+ transients, and partially inhibited PTH-mediated Ca2+ transients by about 50%. Pretreatment with 8-BrcAMP inhibited the PTH and AVP responses likely through depletion of cAMP-sensitive Ca2+ stores. Activation of protein kinase C (PKC) with phorbol esters inhibited PTH and AVP responses and 8-BrcAMP-induced [Ca2+]i transients.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of VLDL secretion in primary culture of rat hepatocytes: involvement of cAMP and cAMP-dependent protein kinases

When hepatocytes were cultured for 24 h in the presence of forskolin (10(-4) mol l-1) or isobutylmethylxanthine (IBMX, 10(-3) mol l-1), the intracellular cAMP concentration peaked (320-380 pmol mg-1 protein) after 10-20 min of culture. This increase was accompanied by a decrease in the secretion of triacylglycerol, cholesterol and apoprotein B associated with VLDL. After 4 h cAMP levels had returned almost to basal values but the inhibition of VLDL secretion persisted. There was a small intracellular accumulation of triacylglycerol but not of apoprotein B. Addition of forskolin and IBMX together led to a further increase in intracellular cAMP and a further suppression of VLDL output. Similar effects on the secretion of VLDL were also observed after addition of Bt2cAMP. Exposure of cell cultures to glucagon (10(-7) mol l-1) for only 10 min raised cellular cAMP levels to > 200 pmol mg-1 protein, and suppressed VLDL secretion during the next 24 h to < 40% of control. All of the substances tested inhibited de novo synthesis of fatty acids but had little or no effect on cholesterol synthesis and did not inhibit oleate esterification to triacylglycerol. The cAMP-dependent protein kinase antagonist Rp-cAMPS prevented suppression of VLDL triacylglycerol secretion induced by glucagon (10(-7) mol l-1) and abolished glucagon-induced ketogenesis. Rp-cAMPS also inhibited Bt2cAMP (7.5 x 10(-6) mol l-1)-induced suppression of VLDL secretion and enhancement of ketogenesis. It is concluded that rat hepatic VLDL metabolism can be regulated by cAMP and cAMP-dependent protein kinases, and that the initial transient rise in cellular cAMP levels induced by glucagon is sufficient to maintain a long-term inhibitory effect on assembly and secretion of VLDL.

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

Images

Cyclic-AMP-dependent Ca2+ influx elicited by prostaglandin D2 in freshly isolated nonchromaffin cells from bovine adrenal medulla

We previously reported that prostaglandin D2 (PGD2) specifically elevates intracellular cyclic AMP in nonchromaffin cells isolated from bovine adrenal medulla (Biochim. Biophys. Acta (1989) 1011, 75-80). Here we again found that PGD2 increased intracellular Ca2+ concentration ([Ca2+]i) in freshly isolated nonchromaffin cells and investigated the cellular mechanisms of PGD2-induced [Ca2+]i increase using the Ca2+ indicator fura-2 and a fluorescence microscopic imaging system. Treatment of the cells with PGD2 receptor agonists BW245C and ZK110841 resulted in both marked stimulation of cyclic AMP formation and an increase in [Ca2+]i. The [Ca2+]i response was also induced by bypassing of the receptor with forskolin, a direct activator of adenylate cyclase, but not by PGE2 or PGF2 alpha both of which are devoid of the ability to generate cyclic AMP in the cells. These cyclic AMP and [Ca2+]i responses induced by PGD2 were completely blocked by the PGD2 receptor antagonist BWA868C. The time-course of cyclic AMP production stimulated by PGD2 coincided with that of the [Ca2+]i increase. While the Ca(2+)-mobilizing hormone bradykinin stimulated a rapid inositol phosphate accumulation in nonchromaffin cells, PGD2 did not stimulate it significantly. Removal of extracellular Ca2+ markedly reduced the Ca2+ response to PGD2 in magnitude and duration, but did not alter the peak [Ca2+]i response to bradykinin. These results demonstrate that PGD2 receptor activation induces the increase in [Ca2+]i via cyclic AMP mainly by increasing the Ca2+ influx from the outside, unlike inositol trisphosphate which causes release of Ca2+ from internal stores.

Intracellular signalling of epinephrine in rat hepatocytes during fetal development and hepatic regeneration

The changes in intracellular calcium concentration and IP3 production after the addition of epinephrine were analysed in adult, fetal (20th-22nd day of intrauterine life), and regenerating rat hepatocytes (4 h-24 h after partial hepatectomy) to determine whether the signal transduction is the same in quiescent proliferating and differentiating cells. The epinephrine treatment causes a significative cytosolic calcium transient in hepatocytes isolated in the last day of fetal life (22-day old) and in the early stage of regeneration (4 h). This effect is not significant in the previous stage of fetal life (20-day old) and at the onset of M phase of cell cycle after partial hepatectomy (24 h). [3H]myo inositol incorporation into IP3 and IP4 is higher in 20 day fetal and regenerating hepatocytes with respect to the control. In these cells the epinephrine does not affect basal level of IP3 and IP4, while it causes a substantial increase of these inositol phosphates in adult hepatocytes. [3H]myo inositol incorporation into PIP2 is very low at the 20th day of fetal life. Epinephrine has no effect on this parameter in fetal and regenerating hepatocytes. Our results show that the epinephrine signal is mediated differently in proliferating and in quiescent hepatocytes.

The role of pancreatic hormones in the regulation of lipid storage, oxidation and secretion in primary cultures of rat hepatocytes. Short- and long-term effects

Exposure of cultured rat hepatocytes to a high concentration of insulin (78 nM) for 24 h in the presence of extracellular oleate (0.75 mM) resulted in a decrease in the secretion of apoprotein B (apoB) and triacylglycerol associated with very-low-density lipoprotein (VLDL). However, continuous exposure of the cells to insulin for longer periods (72 h) stimulated the secretion of apoB and triacylglycerol. Treatment of hepatocytes with glucagon (0.1 microM) for 24 h also suppressed the secretion of VLDL apoB, cholesterol and triacylglycerol. The cells remained responsive to the inhibitory effect of glucagon for at least 3 days. In contrast with insulin, however, exposure of the cells to glucagon for a continuous period of 72 h did not lead to a reversal of the initial inhibition. Glucagon also stimulated ketogenesis, and in this regard the cells were responsive for at least 3 days in culture. These changes were accompanied by a transient increase in intracellular cyclic AMP (cAMP) concentration, which reached a peak 10 min after addition of glucagon. Between 12 h and 24 h after glucagon addition, cAMP levels had returned almost to normal, but the secretion of VLDL remained suppressed during this period.

Hormone-mediated down-regulation of hepatic glutathione synthesis in the rat

Our present work characterized the role of hormone-mediated signal transduction pathways in regulating hepatic reduced glutathione (GSH) synthesis. Cholera toxin, dibutyryl cAMP (DBcAMP), and glucagon inhibited GSH synthesis in cultured hepatocytes by 25-43%. Cellular cAMP levels exhibited a lower threshold for stimulation of the GSH efflux than inhibition of its synthesis. The effect of DBcAMP was independent of the type of sulfur amino acid precursor and cellular ATP levels and unassociated with increased GSH mixed disulfide formation or altered GSH/oxidized glutathione ratio. In liver cytosols, addition of DBcAMP and cAMP-dependent protein kinase (A-kinase) inhibited GSH synthesis from substrates (cysteine, ATP, glutamate, and glycine) by approximately 20% which was prevented by the A-kinase inhibitor. However, if only substrates of the second step in GSH synthesis were used (gamma-glutamylcysteine, glycine, and ATP), DBcAMP and A-kinase exerted no inhibitory effect. Phenylephrine, vasopressin, and phorbol ester also inhibited GSH synthesis in cultured cells by approximately 20%, and depleted cell GSH independent of the type of sulfur amino acid precursor. Cellular cysteine level was unchanged despite the significant fall in GSH after glucagon or phenylephrine treatment. Pretreatment with either staurosporine, C-kinase inhibitor, or calmidazolium, a calmodulin inhibitor, partially prevented but, together, completely prevented the inhibitory effect of phenylephrine. The same combination had no effect on the inhibitory effect of glucagon. The effects of hormones were confirmed in both the intact perfused liver and after in vivo administration. Thus, two classes of hormones acting through distinct signal transduction pathways may down-regulate hepatic GSH synthesis by phosphorylation of gamma-glutamylcysteine synthetase.

Role of a nonmitochondrial Ca2+ pool in the synergistic stimulation by cyclic AMP and vasopressin of Ca2+ uptake in isolated rat hepatocytes

The subcellular distribution of 45Ca2+ accumulated by isolated rat hepatocytes exposed to dibutyryl cyclic AMP (dbcAMP) followed by vasopressin (Vp) was studied by means of a nondisruptive technique. When treated with dbcAMP followed by vasopressin, hepatocytes obtained from fed rats accumulated an amount of Ca2+ approximately fivefold higher than that attained under control conditions. Ca2+ released from the mitochondrial compartment by the uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) accounted for only a minor portion of the accumulated Ca2+. The largest portion was released by the Ca2+ ionophore A23187 and was attributable to a nonmitochondrial compartment. DbcAMP + Vp-treatment also caused a maximal stimulation of glucose production and a twofold increase in cellular glucose 6-phosphate levels. In hepatocytes obtained from fasted rats, dbcAMP + Vp-stimulated Ca2+ accumulation was lower, although with the same subcellular distribution, and was associated with a minimal glucose production. In the presence of gluconeogenetic substrates (lactate plus pyruvate) hepatocytes from fasted rats were comparable to cells isolated from fed animals. However, Ca2+ accumulation and glucose 6-phosphate production could be dissociated in the absence of dbcAMP, in the presence of lactate/pyruvate alone. Under this condition in fact Vp induced only a minimal accumulation of Ca2+ in hepatocytes isolated from fasted rats, although glucose production was markedly increased. Moreover, treatment of fed rat hepatocytes with 1 mM ATP caused a maximal activation of glycogenolysis, but only a moderate stimulation of cellular Ca2+ accumulation. In this case, sequestration of Ca2+ occurred mainly in the mitochondrial compartment. By contrast, the addition of ATP to dbcAMP-pretreated hepatocytes induced a large accumulation of Ca2+ in a nonmitochondrial pool. Additional experiments using the fluorescent Ca2+ indicator Fura-2 showed that dbcAMP pretreatment can enlarge and prolong the elevation of cytosolic free Ca2+ caused by Vp. A nonmitochondrial Ca2+ pool thus appears mainly responsible for the Ca2+ accumulation stimulated by dbcAMP and Vp in isolated hepatocytes, and cyclic AMP seems able to activate Ca2+ uptake in such a nonmitochondrial pool.

Cyclic AMP-evoked oscillations of intracellular [Ca2+] in guinea-pig hepatocytes

The effects of the beta-adrenoceptor agonist isoprenaline and cyclic AMP (cAMP) on cytosolic free Ca2+ ([Ca2+]i) were studied in the single guinea-pig hepatocyte. In common with InsP3-dependent agonists such as noradrenaline or angiotensin II, isoprenaline (0.5-10 microM) and cAMP (50-100 mM, perfused into the cell via the patch-pipette), were able to generate fast and slow fluctuations of [Ca2+]i. Responses to isoprenaline and cAMP also were observed in the absence of external Ca2+. Isoprenaline-evoked [Ca2+]i rises were not blocked by the intracellular perfusion of heparin, suggesting that these fluctuations are independent of the binding of InsP3 to its receptor.

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.

Modulation of glucagon actions by phorbol myristate acetate in isolated hepatocytes. Effect of hypothyroidism

Phorbol myristate acetate (PMA) inhibits glucagon-stimulated cyclic AMP accumulation and shifts to the right the dose-response curve to glucagon for ureagenesis. In cells from hypothyroid rats the effect of PMA on glucagon-stimulated ureagenesis was much more pronounced, but its effect on cyclic AMP accumulation was similar to that observed in the control cells. The stimulations of ureagenesis by the glucagon analogue THG and dibutyryl cyclic AMP (But2-cAMP) were also diminished by PMA, to a greater extent in cells from hypothyroid rats than in those from euthyroid rats. PMA inhibited the increases in cytoplasmic [Ca2+] induced by glucagon. THG or But2-cAMP; the effect of PMA was much more marked in cells from hypothyroid rats than in the controls. Treatment of the cells with glucagon or THG increased the production of citrulline by subsequently isolated mitochondria, whereas PMA diminished their effects. The results suggest that PMA alters glucagon actions at least at two levels; (i) cyclic AMP production and (ii) elevation of cytosol calcium. The increased sensitivity to PMA of some glucagon effects in hypothyroid rats seems to be related to the latter action.

Ca2+ mobilization by vasopressin and glucagon in perfused livers. Effect of prior intoxication with bromotrichloromethane

Perfused livers isolated from rats treated with BrCCl3 for up to 15 min were used as an experimental tool to investigate the role of the hepatic endoplasmic reticulum in Ca2+ mobilization elicited by vasopressin and glucagon. BrCCl3-treatment caused extensive impairment (37 to 92%) of Ca2+ pumps of isolated liver microsomes, while Ca2+ pumps of mitochondria and plasma membrane vesicles remained undamaged. In perfused livers of BrCCl3-treated rats, the efflux of Ca2+ and the concomitant stimulation of O2 consumption and glucose release induced by vasopressin were decreased. The extent of the decrease paralleled the duration of BrCCl3-treatment. The decrease of Ca2+ efflux following vasopressin addition was closely correlated with the decrease of active Ca2+ accumulation by isolated microsomes (r = 0.99, P less than 0.001). The Ca2+ efflux elicited by glucagon was also decreased after BrCCl3-treatment, whereas stimulation of O2 consumption and glucose release were retained. The possibility that BrCCl3-treatment might impair the production of the intracellular Ca2+-mobilizing messenger IP3 is unlikely, since vasopressin still induced the formation of inositol phosphates, including IP3, in isolated hepatocytes obtained from BrCCl3-treated rats. Thus, this work supports the hypothesis that the Ca2+ stored in the liver ER is the major pool of intracellular Ca2+ available for mobilization by vasopressin, glucagon and other effectors.

Quantitative interactions between Pb2+ and Ca2+ homeostasis in cultured osteoclastic bone cells

Cellular calcium homeostasis and calcium-mediated cell functions are conceptually attractive processes to be involved in the manifestation(s) of lead toxicity including impaired skeletal growth and cardiovascular and neurological dysfunction. Knowledge of Ca:Pb and Pb:Ca ratios in different structural and functional compartments of cells is essential for identifying, characterizing, and understanding the significance of Pb2+-Ca2+ interactions. Experiments were conducted to characterize the steady-state kinetic distribution and behavior of 45Ca in primary cultures of murine osteoclastic bone cells. Bone cells, derived from mouse calvaria, were enriched for osteoclasts by a sequential collagenase digestion and maintained in primary culture for 1 week. Cultures were labeled with 45Ca for two or 24 hr and the kinetic parameters were obtained by analysis of 45Ca washout curves. Cellular metabolism was based upon a model with three kinetic pools of intracellular Ca2+ containing approximately 45, 25, and 30% of the total cell calcium. In addition, we describe quantitative measurements of Ca:Pb and Pb:Ca ratios at important functional cell sites of Ca2+ transport and storage in intact cells. The intracellular relationships of Ca2+ and Pb2+ were calculated concurrently in individual cultures, using kinetic analysis of dual-label 45Ca and 203Pb washout curves. The Ca:Pb ratios of the rate constants and half-times were approximately 1:1, supporting the concept of similar cellular metabolism of the two elements. The Ca:Pb ratios for the kinetic pools and fluxes were considerably higher than 1:1. These in situ Ca:Pb relationships should be useful for designing and evaluating Ca-Pb studies with calmodulin, isolated mitochondria, and other individual components of the calcium messenger system. Moreover, these data demonstrate both similarities and differences in the kinetic distribution and behavior of Ca2+ and Pb2+ in osteoclastic bone cells.

Cyclic AMP raises cytoplasmic calcium in pancreatic alpha 2-cells by mobilizing calcium incorporated in response to glucose

The cytoplasmic Ca2+ concentration ([Ca2+]i) was monitored in individual guinea-pig pancreatic alpha 2-cells exposed to modulators of glucagon release. Addition of the stimulatory amino acid arginine resulted in a sustained increase in [Ca2+]i, whereas the inhibitor glucose had the opposite effect. Epinephrine, the beta-adrenergic agonist isoproterenol, the adenylate cyclase activator forskolin and 8-bromo-cAMP transiently raised [Ca2+]i provided that the cells had been pretreated with glucose. However, simultaneous presence of glucose was not required and the effect occurred even in the absence of extracellular Ca2+. Carbachol, the alpha 2-adrenergic agonist clonidine and the sulfonylurea tolbutamide lacked effects on [Ca2+]i. In addition to providing support for the concept that glucagon release is positively modulated by [Ca2+]i, the results demonstrate that cAMP raises [Ca2+]i in the alpha 2-cells by mobilizing calcium incorporated in response to glucose.

Exposure of cultured hepatocytes to cyclic AMP enhances the vasopressin-mediated stimulation of inositol phosphate production

Isolated rat hepatocytes in primary monolayer culture were maintained for 18-24 h in the presence of 10% (v/v) serum and [3H]inositol. Vasopressin (100 nM) stimulated the production of inositol mono-, bis- and tris-phosphates (IP1, IP2, and IP3). Prior exposure of hepatocytes to 8-bromo cyclic AMP (8Br-cAMP; 100 microM), but not 8-bromo cyclic GMP, enhanced the vasopressin-mediated stimulation of inositol phosphate accumulation, but had no significant effect on their formation in the absence of vasopressin. The effect of the cyclic AMP analogue was mimicked by glucagon (10 nM), and was seen whether cyclic AMP or glucagon was added 5 min or 12 h before the addition of vasopressin. An 8 h incubation with dexamethasone (100 nM) enhanced the accumulation of IP3, but not that of IP2 or IP1, in the presence of 8Br-cAMP and vasopressin. Cycloheximide or actinomycin D had little effect on the vasopressin stimulation of inositol phosphate accumulation, after an 8 h incubation in the presence or absence of 8Br-cAMP.

Evidence of cyclic AMP-independent action of glucagon on calcium mobilization in rat hepatocytes

Glucagon increases the cytoplasmic free calcium concentration as measured by aequorin bioluminescence. It has been proposed by Wakelam et al. (Nature 323 (1986) 68-71) that low concentrations of glucagon mobilize calcium from an intracellular pool by causing polyphosphoinositide breakdown. To identify whether cyclic AMP mediates changes in the cytoplasmic free calcium concentration ([Ca2+]c) induced by glucagon, the effects of forskolin and exogenous cyclic AMP on [Ca2+]c were compared with that of glucagon in aequorin-loaded hepatocytes. Although the magnitudes of the [Ca2+]c responses to 250 microM forskolin and 1 mM 8-bromo cyclic AMP were identical to that of 5 nM glucagon, these two agents induced a more prolonged elevation of [Ca2+]c. Glucagon-induced elevation of [Ca2+]c was accompanied by a smaller increase in cyclic AMP than that induced by forskolin. When the cyclic AMP response to glucagon was potentiated by an inhibitor of phosphodiesterase, 3-isobutyl-1-methylxanthine, the glucagon-induced increase in [Ca2+]c was not affected. Conversely, when the cyclic AMP response to glucagon was reduced by pretreatment of the cells with angiotensin II, glucagon-induced changes in [Ca2+]c were rather enhanced. Furthermore, vasopressin potentiated glucagon-induced changes in [Ca2+]c despite the reduction of the cyclic AMP response to glucagon. In the presence of 1 microM extracellular calcium, angiotensin II did not enhance glucagon-induced changes in [Ca2+]c. These results suggest that at least part of the action of 5 nM glucagon on calcium mobilization is independent of cyclic AMP.

Modulation of glucagon effects by changes in extracellular pH and calcium

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

Action of phenylephrine on protein synthesis in liver cells

The alpha-adrenergic agonist phenylephrine was found to inhibit protein labelling from [3H]valine in isolated liver cells. This effect is only observable under conditions of partial Ca2+ depletion and in cells displaying maximal rates of protein labelling, i.e. cells isolated from fed animals or from starved animals when incubated in the presence of alanine. The ability of phenylephrine to inhibit protein labelling at near-saturating concentrations of the amino acid precursor indicates that this alpha-agonist actually decreases the rate of protein synthesis. The possibility that phenylephrine acts by making cellular Ca2+ availability further limiting can be ruled out, since alanine stimulates protein labelling under conditions of severe Ca2+ depletion obtained by pretreatment of the cells with EGTA. The following observations indicate that the phenylephrine action may be mediated by an increase in cellular cyclic AMP content: (1) a close relationship was found between the abilities of phenylephrine to inhibit protein labelling and to increase cyclic AMP content; (2) cyclic AMP mimics the phenylephrine action only in cells partially depleted of Ca2+; (3) the alpha 1-antagonist prazosin, which inhibited the phenylephrine-mediated increase in cyclic AMP, also abolished the effect on protein synthesis.

The effect of ionophore A23187, verapamil, and dibutyryl cyclic AMP on bile acid synthesis in isolated rat hepatocytes

The effect of the divalent cation ionophore A23187 and the calcium channel-blocker verapamil on bile acid synthesis in isolated hepatocytes in the presence and absence of dibutyryl cyclic AMP was studied. Both A23187 (1 microM) and verapamil (0.04 mM) caused a small (approximately 15-20%) but consistent decrease in total bile acid synthesis in the cells. When hepatocytes were incubated with dibutyryl cyclic AMP (1 mM) production of total bile acid was increased by about 25%, and this effect was unchanged by A23187 but abolished by verapamil. The relative proportions of the individual bile acids produced were not affected by either A23187 or verapamil. Dibutyryl cyclic AMP (1 mM) lowered the ratio of the amount of conjugated cholic acid to conjugated chenodeoxycholic + beta-muricholic acid formed in the cells by about 50%. Neither A23187 nor verapamil was able to prevent this change. These results suggest that the stimulatory effect of dibutyryl cyclic AMP on total bile acid synthesis is dependent on mobilisation of calcium from intracellular stores, but its effect on the relative proportions of bile acid formed via the cholic acid versus the chenodeoxycholic acid pathway is independent of calcium movement.

Liver mitochondrial pyrophosphate concentration is increased by Ca2+ and regulates the intramitochondrial volume and adenine nucleotide content

1. The matrix pyrophosphate (PPi) content of isolated energized rat liver mitochondria incubated in the presence of ATP, Mg2+, Pi and respiratory substrate was about 100 pmol/mg of protein. 2. After incubation with sub-micromolar [Ca2+], this was increased by as much as 300%. There was a correlation between the effects of Ca2+ on PPi and on the increase in matrix volume reported previously [Halestrap, Quinlan, Whipps & Armston (1986) Biochem. J. 236, 779-787]. Half-maximal effects were seen at 0.3 microM-Ca2+. 3. Coincident with these effects, the total adenine nucleotide content increased in a carboxyatractyloside-sensitive manner. 4. Incubation with 0.2-0.5 mM-butyrate induced similar but smaller effects on mitochondrial swelling and matrix PPi and total adenine nucleotide content. Addition of butyrate after Ca2+, or vice versa, caused Ca2+-induced mitochondrial swelling to stop or reverse, while matrix PPi increased 30-fold. 5. Addition of atractyloside or the omission of ATP from incubations greatly enhanced swelling induced by Ca2+ without increasing matrix PPi. 6. Swelling of mitochondria incubated under de-energized conditions in iso-osmotic KSCN was progressively enhanced by the addition of increasing concentrations of PPi (1-20 mM) or valinomycin. 7. In iso-osmotic potassium pyrophosphate swelling was slow initially, but accelerated with time. This acceleration was inhibited by ADP, whereas carboxyatractyloside induced rapid swelling. Swelling in other iso-osmotic PPi salts showed that the rate of entry decreased in the order NH4+ greater than K+ greater than Na+ greater than Li+, whereas choline, tetramethylammonium and Tris did not enter. It is suggested that the adenine nucleotide translocase transports small univalent cations when PPi is bound and that PPi can also be transported when the transporter is in the conformation induced by carboxyatractyloside. 8. It is concluded that Ca2+ and butyrate cause swelling of energized mitochondria through this effect of PPi on K+ permeability of the mitochondrial inner membrane. 9. Freeze-clamped livers from rats treated with glucagon or phenylephrine show 30-50% increases in tissue PPi. It is proposed that Ca2+-mediated increases in mitochondrial PPi are responsible for the increase in matrix volume and total adenine nucleotide content observed after hormone treatment.

Parathyroid hormone acutely elevates intracellular calcium in osteoblastlike cells

Changes in cytoplasmic calcium concentration ([Ca2+]i) activate numerous cellular processes thus mediating the effects of a number of hormones, but whether this mechanism is involved in the activation of osteoblasts by parathyroid hormone (PTH) remains uncertain. To examine this question, [Ca2+]i has been measured in suspensions of UMR 106 cells, a rodent osteosarcoma cell line with an osteoblastic phenotype. Basal [Ca2+]i was 137 +/- 3.7 nM (n = 60) and after the addition of rat PTH-(1-34) [rPTH-(1-34)] there was a rapid, dose-related increase with return to base line within 1 min. Half-maximal stimulation was produced by 5 X 10(-8) M rPTH-(1-34). Complexing of intracellular calcium by EGTA addition immediately before that of rPTH did not affect the calcium transient; neither did MnCl2 (10(-4) M) nor diltiazem (10(-4) M). Verapamil (10(-5) M) reduced the [Ca2+]i peak height after rPTH to 0.48 +/- 0.14 of control (n = 7). 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoic acid and dantrolene both reduced the [Ca2+]i response to rPTH (0.65 +/- 0.08 and 0.29 +/- 0.13 of control, respectively). Forskolin (10(-6) and 10(-5) M) produced a slight [Ca2+]i transient smaller in amplitude than seen with PTH. It is concluded that PTH mobilizes an intracellular calcium pool in these osteoblastlike cells, and the predominant mechanism for this is independent of cAMP.

Stimulatory effect of glucose 6-phosphate on the non-mitochondrial Ca2+ uptake in permeabilized hepatocytes and Ca2+ release by inositol trisphosphate

The relationships between Ca2+ transport and glucose-6-phosphatase activity, previously studied in isolated liver microsomes, were investigated in permeabilized hepatocytes in the presence of mitochondrial inhibitors. It was found that the addition of glucose 6-phosphate to the cells markedly stimulates the MgATP-dependent Ca2+ uptake. A progressive increase in the stimulation of Ca2+ uptake was seen with increasing amounts of glucose 6-phosphate up to 5 mM concentrations. Vanadate, when added in adequate concentrations (20-40 microM) to the hepatocytes inhibits both the glucose-6-phosphatase activity and the stimulation of Ca2+ uptake by glucose 6-phosphate, while not affecting the MgATP-dependent Ca2+ uptake. The addition of inositol 1,4,5-trisphosphate to permeabilized hepatocytes in which Ca2+ had been accumulated in the presence of MgATP and glucose 6-phosphate, results in a rapid release of Ca2+.

The rapid desensitization of glucagon-stimulated adenylate cyclase is a cyclic AMP-independent process that can be mimicked by hormones which stimulate inositol phospholipid metabolism

Treatment of intact hepatocytes with glucagon, TH-glucagon [( 1-N-alpha-trinitrophenylhistidine, 12-homoarginine]glucagon), angiotensin or vasopressin led to a rapid time- and dose-dependent loss of the glucagon-stimulated response of the adenylate cyclase activity seen in membrane fractions isolated from these cells. Intracellular cyclic AMP concentrations were only elevated with glucagon. All ligands were capable of causing both desensitization/loss of glucagon-stimulated adenylate cyclase activity and stimulation of inositol phospholipid metabolism in the intact hepatocytes. Maximally effective doses of angiotensin precluded any further inhibition/desensitizing action when either glucagon or TH-glucagon was subsequently added to these intact cells, as has been shown previously for the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate) [Heyworth, Wilson, Gawler & Houslay (1985) FEBS Lett. 187, 196-200]. Treatment of intact hepatocytes with these various ligands caused a selective loss of the glucagon-stimulated adenylate cyclase activity in a washed membrane fraction and did not alter the basal, GTP-, NaF- and forskolin-stimulated responses. Angiotensin failed to inhibit glucagon-stimulated adenylate cyclase activity when added directly to a washed membrane fraction from control cells. Glucagon GR2 receptor-stimulated adenylate cyclase is suggested to undergo desensitization/uncoupling through a cyclic AMP-independent process, which involves the stimulation of inositol phospholipid metabolism by glucagon acting through GR1 receptors. This action can be mimicked by other hormones which act on the liver to stimulate inositol phospholipid metabolism. As the phorbol ester TPA also mimics this process, it is proposed that protein kinase C activation plays a pivotal role in the molecular mechanism of desensitization of glucagon-stimulated adenylate cyclase. The site of the lesion in desensitization is shown to be at the level of coupling between the glucagon receptor and the stimulatory guanine nucleotide regulatory protein Gs, and it is suggested that one or both of these components may provide a target for phosphorylation by protein kinase C.

Effects of glucagon and Ca2+ on the metabolism of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate in isolated rat hepatocytes and plasma membranes

Rat hepatocytes whose phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) had been labelled for 60 min with 32P were treated with glucagon for 10 min or phenylephrine for 2 min. Glucagon caused a 20% increase in PIP but no change in PIP2 whereas phenylephrine caused a similar increase in PIP but a 15% decrease in PIP2. Addition of both hormones together for 10 min produced a 40% increase in PIP. A crude liver mitochondrial fraction incubated with [32P]Pi and ADP incorporated label into PIP, PIP2 and phosphatidic acid. The PIP2 was shown to be in contaminating plasma membranes and PIP in both lysosomal and plasma-membrane contamination. A minor but definitely mitochondrial phospholipid, more polar than PIP2, was shown to be labelled with 32P both in vitro and in hepatocytes. The rate of 32P incorporation into PIP was faster in mitochondrial/plasma-membrane preparations from rats treated with glucagon or if 3 microM-Ca2+ and Ruthenium Red were present in the incubation buffer. Loss of 32P from membranes labelled in vitro was shown to be accompanied by formation of inositol 1,4,5-trisphosphate (IP3) and inositol 1,4-bisphosphate, and was faster in preparations from glucagon-treated rats or in the presence of 3 microM-Ca2+. It is concluded that glucagon stimulates both PIP2 phosphodiesterase and phosphatidylinositol kinase activities, as does the presence of 3 microM-Ca2+. The resulting formation of IP3 may be responsible for the observed release of intracellular Ca2+ stores. The roles of a guanine nucleotide regulatory protein and phosphorylation in mediating these effects are discussed.

The effect of dibutyryl cyclic AMP on the excretion of taurocholic acid from isolated rat liver cells

Dibutyryl cyclic AMP (50-1000 microM) was found to increase the initial rate of efflux of taurocholic acid from isolated rat hepatocytes. Efflux of the bile acid was inhibited by sodium, and in the absence of sodium dibutyryl cyclic AMP failed to stimulate the rate. Increasing the concentration of calcium from 0 to 1.2 mM had no effect on the initial rate of taurocholic acid efflux from the cells, but the absence of calcium markedly reduced the effect of dibutyryl cyclic AMP. The results suggest that changes in the fluxes of sodium and calcium are involved in the effect of the cyclic nucleotide on taurocholic acid efflux from the cells.

Effects of glucagon and vasopressin on hepatic Ca2+ release

The effects of physiological levels of glucagon on Ca2+ efflux were examined in the perfused rat liver. Two methods were used to estimate Ca2+ efflux: prior labeling of the calcium pools with 45Ca2+ and measurement of perfusate Ca2+ with atomic absorption. According to both methods, glucagon administration at the physiological level evoked Ca2+ release. The released Ca2+ originated mostly from a carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP)-depletable pool and also from an FCCP-insensitive pool from which Ca2+ could be released with A23187. Maximally effective doses of glucagon and vasopressin had no additive effect on Ca2+ release. Prior administration of vasopressin resulted in markedly reduced Ca2+ release by glucagon. These results indicate that glucagon releases Ca2+ from the same pool that vasopressin does.

Activation of two signal-transduction systems in hepatocytes by glucagon

The ability of glucagon to stimulate glycogen breakdown in liver played a key part in the classic identification of cyclic AMP and hormonally stimulated adenylate cyclase. But several observations indicate that glucagon can exert effects independent of elevating intracellular cAMP concentrations. These effects are probably mediated by an elevation of the intracellular concentration of free Ca2+ although the mechanism by which this occurs is unknown. We show here that glucagon, at the low concentrations found physiologically, causes both a breakdown of inositol phospholipids and the production of inositol phosphates. Indeed, we show that the glucagon analogue, (1-N-alpha-trinitrophenylhistidine,12-homoarginine)glucagon (TH-glucagon), which does not activate adenylate cyclase or cause any increase in cAMP in hepatocytes yet can fully stimulate glycogenolysis, gluconeogenesis and urea synthesis, stimulates the production of inositol phosphates. This stimulation of inositol phospholipid metabolism by low concentrations of glucagon provides a mechanism whereby glucagon can exert cAMP-independent actions on target cells. We suggest that hepatocytes possess two distinct receptors for glucagon, a GR-1 receptor coupled to stimulate inositol phospholipid breakdown and a GR-2 receptor coupled to stimulate adenylate cyclase activity.

Glucagon and vasopressin interactions on Ca2+ movements in isolated hepatocytes

The effects of glucagon and vasopressin, singly or together, on cytosolic free Ca2+ concentration [( Ca2+]i) and on the 45Ca2+ efflux were studied in isolated rat liver cells. In the presence of 1 mM external Ca2+, glucagon and vasopressin added singly induced sustained increases in [Ca2+]i. The rate of the initial fast phase of the [Ca2+]i increase and the magnitude of the final plateau were dependent on the concentrations (50 pm-0.1 microM) of glucagon and vasopressin. Preincubating the cells with a low concentration of glucagon (0.1 nM) for 2 min markedly accelerated the fast phase and elevated the plateau of the [Ca2+]i increase caused by vasopressin. In the absence of external free Ca2+, glucagon and vasopressin transiently increased [Ca2+]i and stimulated the 45Ca2+ efflux from the cells, indicating mobilization of Ca2+ from internal store(s). Preincubating the cells with 0.1 nM-glucagon accelerated the rate of the fast phase of the [Ca2+]i rise caused by the subsequent addition of vasopressin. However, unlike what was observed in the presence of 1 mM-Ca2+, glucagon no longer enhanced the maximal [Ca2+]i response to vasopressin. In the absence of external free Ca2+, higher concentrations (1 nM-0.1 microM) of glucagon, which initiated larger increases in [Ca2+]i, drastically decreased the subsequent Ca2+ response to vasopressin (10 nM). At these concentrations, glucagon also decreased the vasopressin-stimulated 45Ca2+ efflux from the cells. It is suggested that, in the liver, glucagon accelerates the fast phase and elevates the plateau of the vasopressin-mediated [Ca2+]i increase respectively by releasing Ca2+ from the same internal store as that permeabilized by vasopressin, probably the endoplasmic reticulum, and potentiating the influx of extracellular Ca2+ caused by this hormone.