Synergistic stimulation of Ca2+ uptake by glucagon and Ca2+-mobilizing hormones in the perfused rat liver. A role for mitochondria in long-term Ca2+ homoeostasis

Mitochondrial Transporters as Novel Targets for Intracellular Calcium Signaling

Ca2+signaling in mitochondria is important to tune mitochondrial function to a variety of extracellular stimuli. The main mechanism is Ca2+entry in mitochondria via the Ca2+uniporter followed by Ca2+activation of three dehydrogenases in the mitochondrial matrix. This results in increases in mitochondrial NADH/NAD ratios and ATP levels and increased substrate uptake by mitochondria. We review evidence gathered more than 20 years ago and recent work indicating that substrate uptake, mitochondrial NADH/NAD ratios, and ATP levels may be also activated in response to cytosolic Ca2+signals via a mechanism that does not require the entry of Ca2+in mitochondria, a mechanism depending on the activity of Ca2+-dependent mitochondrial carriers (CaMC). CaMCs fall into two groups, the aspartate-glutamate carriers (AGC) and the ATP-Mg/Picarriers, also named SCaMC (for short CaMC). The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle, and citrin, the liver AGC, is also a member of the urea cycle. Both types of CaMCs are activated by Ca2+in the intermembrane space and function together with the Ca2+uniporter in decoding the Ca2+signal into a mitochondrial response.

Regulation of total mitochondrial Ca2+ in perfused liver is independent of the permeability transition pore

Triggering of the permeability transition pore (PTP) in isolated mitochondria causes release of matrix Ca2+, ions, and metabolites, and it has been proposed that the PTP mediates mitochondrial Ca2+ release in intact cells. To study the role of the PTP in mitochondrial energy metabolism, the mitochondrial content of Ca2+, Mg2+, ATP, and ADP was determined in hormonally stimulated rat livers perfused with cyclosporin A (CsA). Stimulation of livers perfused in the absence of CsA with glucagon and phenylephrine induced an extensive uptake of Ca2+, Mg2+, and ATP plus ADP by the mitochondria, followed by a release on omission of hormones. In the presence of CsA, the PTP was fully inhibited, but neither the hormone-induced uptake of Ca2+, ATP, or ADP by mitochondria nor their release after washout of hormones was significantly changed. We conclude that the regulation of sustained changes in mitochondrial Ca2+ content induced by hormonal stimulation is independent of the PTP.

Specific features of glycogen metabolism in the liver

Although the general pathways of glycogen synthesis and glycogenolysis are identical in all tissues, the enzymes involved are uniquely adapted to the specific role of glycogen in different cell types. In liver, where glycogen is stored as a reserve of glucose for extrahepatic tissues, the glycogen-metabolizing enzymes have properties that enable the liver to act as a sensor of blood glucose and to store or mobilize glycogen according to the peripheral needs. The prime effector of hepatic glycogen deposition is glucose, which blocks glycogenolysis and promotes glycogen synthesis in various ways. Other glycogenic stimuli for the liver are insulin, glucocorticoids, parasympathetic (vagus) nerve impulses and gluconeogenic precursors such as fructose and amino acids. The phosphorolysis of glycogen is mainly mediated by glucagon and by the orthosympathetic neurotransmitters noradrenaline and ATP. Many glycogenolytic stimuli, e.g. adenosine, nucleotides and NO, also act indirectly, via secretion of eicosanoids from non-parenchymal cells. Effectors often initiate glycogenolysis cooperatively through different mechanisms.

The Ca2+ transport mechanisms of mitochondria and Ca2+ uptake from physiological-type Ca2+ transients

Mitochondria contain a sophisticated system for transporting Ca2+. The existence of a uniporter and of both Na+-dependent and -independent efflux mechanisms has been known for years. Recently, a new mechanism, called the RaM, which seems adapted for sequestering Ca2+ from physiological transients or pulses has been discovered. The RaM shows a conductivity at the beginning of a Ca2+ pulse that is much higher than the conductivity of the uniporter. This conductivity decreases very rapidly following the increase in [Ca2+] outside the mitochondria. This decrease in the Ca2+ conductivity of the RaM is associated with binding of Ca2+ to an external regulatory site. When liver mitochondria are exposed to a sequence of pulses, uptake of labeled Ca2+ via the RaM appears additive between pulses. Ruthenium red inhibits the RaM in liver mitochondria but much larger amounts are required than for inhibition of the mitochondrial Ca2+ uniporter. Spermine, ATP and GTP increase Ca2+ uptake via the RaM. Maximum uptake via the RaM from a single Ca2+ pulse in the physiological range has been observed to be approximately 7 nmole/mg protein, suggesting that Ca2+ uptake via the RaM and uniporter from physiological pulses may be sufficient to activate the Ca2+-sensitive metabolic reactions in the mitochondrial matrix which increase the rate of ATP production. RaM-mediated Ca2+ uptake has also been observed in heart mitochondria. Evidence for Ca2+ uptake into the mitochondria in a variety of tissues described in the literature is reviewed for evidence of participation of the RaM in this uptake. Possible ways in which the differences in transport via the RaM and the uniporter may be used to differentiate between metabolic and apoptotic signaling are discussed.

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

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

Protein kinase A regulates the disposition of Ca2+ which enters the cytoplasmic space through store-activated Ca2+ channels in rat hepatocytes by diverting inflowing Ca2+ to mitochondria

The roles of a trimeric GTP-binding regulatory protein, protein kinase A and mitochondria in the regulation of store-activated (thapsigargin-stimulated) Ca2+ inflow in freshly-isolated rat hepatocytes were investigated. Rates of Ca2+ inflow were estimated by measuring the increase in the fluorescence of intracellular fura-2 following the addition of extracellular Ca2+ (Ca2+o) to cells incubated in the absence of added Ca2+o. Guanosine 5'-[gamma-thio]-triphosphate (GTP[S]) and AlF4(-) inhibited the thapsigargin-stimulated Ca2+o-induced increase in cytoplasmic free Ca2+ concentration ([Ca2+]c) and this inhibition was prevented by the Rp diastereoisomer of adenosine 3',5'-(cyclic)phosphoro[thioate]. cAMP, forskolin and glucagon (half-maximal effect at 10 nM) mimicked inhibition of the thapsigargin-stimulated Ca2+o-induced increase in [Ca2+]c by GTP[S], but had little effect on thapsigargin-induced release of Ca2+ from intracellular stores. Azide and carbonyl cyanide p-trifluoromethoxyphenylhydrazone inhibited the thapsigargin-stimulated Ca2+o-induced increase in [Ca2+]c in the presence of increased cAMP (induced by glucagon). In contrast, Ruthenium Red markedly enhanced the thapsigargin-stimulated Ca2+o-induced increase in [Ca2+]c in both the presence and absence of increased cAMP (induced by forskolin and dibutyryl cAMP). It is concluded that, in hepatocytes, protein kinase A regulates the disposition of Ca2+, which enters the cytoplasmic space through store-activated Ca2+ channels, by directing some of this Ca2+ to the mitochondria. The idea that caution should be exercised in using observed values of Ca2+o-induced increase in [Ca2+]c as estimates of rates of agonist-stimulated Ca2+ inflow is briefly discussed.

Permeable analogues of cGMP promote hepatic calcium inflow induced by the synergistic action of glucagon and vasopressin but inhibit that induced by vasopressin alone

Treatment of perfused rat liver with the nitric oxide-generating reagent molsidomine led to substantial increases in cGMP without itself affecting basal Ca2+ fluxes. Under these conditions the ability of glucagon plus vasopressin to induce Ca2+ influx was greatly enhanced. The permeable analogue of cGMP (8-bromo-cGMP) enhanced glucagon plus vasopressin-induced Ca2+ influx to a similar extent as that with molsidomine. This suggests that the effect of the latter is attributable to the generation of cGMP which itself enhances the ability of the two hormones to induce synergistic Ca2+ influx. While 8-bromo-cGMP (or molsidomine) did not influence Ca2+ fluxes induced by glucagon, these agents strongly inhibited Ca2+ influx induced by vasopressin alone. These data show that while 8-bromo-cGMP has no effect on basal Ca2+ fluxes, it is able to modify the Ca2+ influx induced by glucagon and vasopressin action in hepatic tissue.

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

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

Rapid Ca2+ influx induced by the action of dibutylhydroquinone and glucagon in the perfused rat liver

Glucagon induces a slight Ca2+ efflux when administered to the perfused rat liver. However, the hormone promotes rapid and significant Ca2+ influx after the prior administration of 2, 5-di(t-butyl)-1,4-hydroquinone (BHQ), an agent that promotes Ca2+ release from the endoplasmic reticulum (ER). The concentrations of glucagon that promote Ca2+ influx are similar to those that promote glycogenolysis and gluconeogenesis in isolated hepatocytes. The permeable analogue of cAMP, but not that of cGMP, is able to duplicate the Ca2+-mobilizing effects of glucagon. The influx of Ca2+ into liver is blocked by Ni2+. Administration of sodium azide, an inhibitor of mitochondrial electron transport, also blocks the BHQ plus glucagon-induced Ca2+ influx and this is reversed when azide administration is terminated. The actions of azide are evident within 60 s after administration or withdrawal, and also occur when either oligomycin or fructose is co-administered; this provides evidence for an effect of azide independent of cellular ATP depletion. Measurement of total calcium in mitochondria that were isolated rapidly from perfused livers after the combined administration of glucagon and BHQ confirmed that large quantities of extracellular Ca2+ had entered these organelles. These experiments provide evidence that in the perfused rat liver the artificial emptying of the ER Ca2+ pool allows glucagon to promote rapid and sustained Ca2+ influx that seems to terminate in mitochondria.

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.

Nickel: an agent for investigating the relation between hormone-induced Ca2+ influx and bile flow in the perfused rat liver

Influx of Ca2+ induced by the synergistic action of glucagon plus vasopressin in the perfused rat liver was progressively inhibited by infusing increasing concentrations of Ni2+ to the perfusion medium. The onset of Ca2+ influx following vasopressin administration was delayed and inhibition occurred of both the initial rate of Ca2+ influx as well as the total amount of Ca2+ taken up by the liver. Inhibition of the Ca2+ influx rate was almost maximal at approximately 500 microM Ni2+; half-maximal inhibition occurred at less than 250 microM. Added Ni2+ also delayed the onset of the early transient bile flow peak. In addition, the duration of the transient peak in bile flow was prolonged by approximately 2 min by all concentrations of Ni2+ between 25-500 microM, the greatest amount of bile being released in the presence of 250 microM Ni2+. Concentrations of Ni2+ at 100 microM and above also inhibit the decrease in bile flow to below baseline levels. The data identify a multiple role for Ca2+ mobilisation in bile flow.

Calcium ion-dependent signalling and mitochondrial dysfunction: mitochondrial calcium uptake during hormonal stimulation in intact liver cells and its implication for the mitochondrial permeability transition

Hormones that elevate cytosolic Ca2+ concentrations ([Ca2+]cyt) often use Ca2+ as a messenger to activate intramitochondrial metabolic processes. However, the mitochondrial Ca2+ level also regulates the activation of the mitochondrial permeability transition (MPT), a process that involves the assembly of a high conductance proteinaceous pore across the inner and outer membrane. Studies on intact liver cells indicate that the MPT is a critical step in the cell killing induced by anoxia or respiratory inhibitors. In this study, we used freshly isolated hepatocytes to investigate to what extent the elevation of [Ca2+]cyt by vasopressin or other agonists causes Ca2+ accumulation in the mitochondria and how this treatment affects the mitochondrial susceptibility to undergo the MPT. Hepatocytes were incubated with vasopressin, glucagon, or with thapsigargin (an inhibitor of the endoplasmic reticulum Ca2+ pump) prior to permeabilization with digitonin. Mitochondrial Ca2+ accumulation was determined by following the ionomycin-induced Ca2+ release in permeabilized cells and mitochondrial swelling was studied by following cyclosporin A-sensitive light scattering changes induced by phenyl-arsenoxide and rotenone. The results indicate that agents that elevate [Ca2+]cyt cause a significant Ca2+ accumulation in the mitochondria. Excessive Ca2+ accumulation (> 10-fold increase over basal levels) was obtained with the combination of vasopressin and glucagon or with incubations containing thapsigargin. These conditions were also associated with a marked increase in rotenone-induced mitochondrial swelling. However, the more modest increase in mitochondrial Ca2+ content after treating cells with vasopressin alone did not enhance the swelling response; instead, vasopressin suppressed mitochondrial swelling compared to control incubations. Vasopressin also partly suppressed the swelling associated with thapsigargin treatment, although it did not significantly affect the Ca2+ accumulation under these conditions. This effect of vasopressin was mimicked by phorbol ester, suggesting a role for protein kinase C. The data indicate that mitochondrial Ca2+ accumulation following elevation of elevation of [Ca2+]cyt enhances the susceptibility for activation of the MPT, a response that may increase cell injury during anoxia or in response to other challenges. However, hormones also activate protective responses in the cell that suppress the MPT.

Elevated intracellular cyclic AMP exerts different modulatory effects on cytosolic free Ca2+ oscillations induced by ADP and ATP in single rat hepatocytes

Single aequorin-injected hepatocytes respond to agonists acting via the phosphoinositide signalling pathway by the generation of oscillations in cytosolic free Ca2+ concentration ([Ca2+]free). The duration of [Ca2+]free transients is characteristic of the stimulating agonist. We have previously reported that ADP and ATP, which are believed to act through a single P(2y)-purinoceptor species, induce very different oscillatory [Ca2+]free responses in the majority of hepatocytes. We have interpreted these data as evidence for two separate Ca(2+)-mobilizing purinoceptors for these nucleotides. We show here that the elevation of intracellular cyclic AMP concentration, by the co-application of either dibutyryl cyclic AMP or 7 beta-desacetyl-7 beta-[gamma-(N-methylpiperazino)butyryl]- forskolin (L858051), exerts different modulatory effects on [Ca2+]free oscillations induced by ADP and ATP in single rat hepatocytes. Elevated intracellular cyclic AMP levels enhance the frequency and peak [Ca2+]free of transients induced by ADP. In contrast, the elevation of intracellular cyclic AMP levels in hepatocytes producing [Ca2+]free oscillations in response to ATP stimulates either an increase in the duration of transients or a sustained rise in [Ca2+]free. The data illustrate a further difference between the oscillatory [Ca2+]free responses of hepatocytes to ADP and ATP, thus further arguing against ADP and ATP acting via a single purinoceptor species.

The synergistic action (cross-talk) of glucagon and vasopressin induces early bile flow and plasma-membrane calcium fluxes in the perfused rat liver

A study was made of the initial responses of perfusate Ca2+ fluxes and bile flow to Ca(2+)-mobilizing agonists, following refinements to the methods for analysing these parameters in the perfused rat liver. Net Ca2+ efflux induced by vasopressin commences at 15 s, reaches a maximal rate at 35 s and declines to zero by 55 s, when Ca2+ influx commences. Vasopressin-induced increases in bile flow commence by 20 s, attain a maximal rate by 35 s and begin to decline at 50 s, to reach basal values by 90 s. Concomitant administration of glucagon modifies each of these actions of vasopressin in the following ways: it decreases by 5 s the time of onset of net Ca2+ efflux, and the time and magnitude of such efflux, and the time of onset of bile flow is decreased to 15 s, and the flow reaches maximal rates by 30 s. When the alpha 1-adrenergic agonist phenylephrine is used in place of vasopressin, Ca2+ efflux commences at 17-18 s and is greater in magnitude; little bile flow is induced by this agonist. Glucagon modifies the action of phenylephrine in the following ways: the onset of Ca2+ efflux is brought forward by 2-3 s, it is of lower magnitude and Ca2+ influx begins by 45 s; bile flow commences by 15-20 s, and reaches a maximum at 30 s, where the rate is much greater than in the absence of glucagon; this rate gradually declines to be near basal by 80 s. The onset of agonist-induced oxygen uptake was also brought forward by the co-administration of glucagon. Comparison of agonist-induced plasma-membrane Ca2+ fluxes and bile flow (with or without glucagon administration) suggests that correlations can be made between net Ca2+ fluxes and the transient increases seen in bile flow.

Cyclic AMP stimulates Ca2+ entry in rat hepatocytes by interacting with the plasma membrane carriers involved in receptor-mediated Ca2+ influx

The regulation of Ca2+ influx in rat hepatocytes by glucagon and cyclic AMP (cAMP) was investigated. Exposing hepatocytes to glucagon resulted in an increase in the initial rate of Ca2+ entry. The concentrations of glucagon producing half-maximal and maximal stimulation of Ca2+ entry were 10(-10) and 10(-8) M, respectively. A similar stimulation of Ca2+ influx was obtained in cells exposed to cAMP analogues or to forskolin. Exposing hepatocytes suspended in nominally Ca(2+)-free medium to glucagon for 3 min produced a 9% decrease in the size of the vasopressin-sensitive Ca2+ pool; in contrast, N6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate (Bt2cAMP) slightly augmented the size of this pool. Glucagon and Bt2cAMP synergized the initial vasopressin-stimulated Ca2+ and Mn2+ influx rates, but only moderately increased the initial rate of Ca2+ entry after thapsigargin addition. The glucagon- and Bt2cAMP-stimulated Ca2+ influx was inhibited by the same antagonists of the plasma membrane Ca2+ carriers that mediate Ca2+ entry during stimulation by vasopressin. Thus, cAMP does not stimulate Ca2+ entry through either a capacitative type of mechanism or inositol phosphate turnover. The authors' findings instead suggest that cAMP acts directly, or through protein kinase A on the same Ca2+ carriers that are activated by phospholipase C-linked receptor agonists.

Crosstalk between calcium- and cyclic AMP-mediated signalling systems and the short-term modulation of bile flow in normal and cholestatic rat liver

The flow of bile is subject to short-term modulation by glucagon and calcium-mobilizing hormones. Of potential relevance is the crosstalk between the second messenger-mediated signal transducing systems of these agonists. This latter point has revealed an area of investigation that should enable further insights to be made into a physiological network that interrelates bile flow, hepatocellular calcium movements and hormone action. This information in turn may provide insights into the etiology and treatment of human and animal diseases in which cholestasis is an underlying feature.

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

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Evidence that stimulation of plasma-membrane Ca2+ inflow is an early action of glucagon and dibutyryl cyclic AMP in rat hepatocytes

The ability of glucagon (1 nM) and of dibutyryl cyclic AMP (50 microM) to increase cytosolic free Ca2+ concentration ([Ca2+]i) in Fura-loaded rat hepatocytes was examined in a system wherein Ca2+ inflow was induced by the re-admission of excess Ca2+ to a nominally Ca(2+)-free medium. An increase in [Ca2+]i did not occur in the absence of either agonist, but did so after co-addition of either agonist with Ca2+. Increasing the time between addition of dibutyryl cyclic AMP (or of glucagon) and Ca2+ led to increases in [Ca2+]i; half-maximal and maximal increases were observed at 0 s (i.e. at co-addition) and 5-7 s respectively. Dibutyryl cyclic AMP and Ca2+ each exhibited a concentration-dependence when their respective concentrations were changed for a fixed time interval between additions. Half-maximal and maximal effects were obtained with 30 microM and 50 microM dibutyryl cyclic AMP and with 0.5 mM and approx. 1 mM Ca2+ respectively. The data demonstrate an early action of glucagon and dibutyryl cyclic AMP on [Ca2+]i. It is argued that the agonist-induced rise in [Ca2+]i results from an increase in plasma-membrane Ca2+ inflow, an effect that appears to occur much earlier than that on mobilization of internal stores of Ca2+.

Effects of catecholamines on plasma potassium: the role of alpha- and beta-adrenoceptors

The sympathetic nervous system plays an important role in the control of plasma potassium levels. Administration of adrenaline or noradrenaline evokes, in the majority of mammal species a dual response: first a short transient hyperkalaemia, followed by a maintained hypokalaemia. Alpha 1- and alpha 2-adrenoceptors mediate the initial hyperkalaemia through the activation of hepatic Ca(2+)-dependent-K(+)-channels. Stimulation of beta 1- and beta 2-adrenoceptors induces the late hypokalaemia by stimulation of skeletal muscle Na(+)-K(+)-ATPase. Beta 3-adrenoceptor stimulation may also have an effect on plasma potassium control since administration of selective beta 3-adrenoceptor agonists induces a decrease in plasma potassium. The simultaneous infusion of phenyleprine (alpha-adrenoceptor agonist) and isoprenaline (beta-adrenoceptor agonist) increases plasma potassium levels: this effect is several times larger than the algebric summation of the changes in plasma potassium when each agent is infused separately, thus suggesting potentiation. The physiological (changes in cell volume and function secondary to changes in ion fluxes) and clinical implications (pathophysiological conditions with hypo or hyperkalaemia, hyperkalaemic periodic paralysis, ventricular arrythmias) of these findings are discussed.

Functional involvement of α1and α2-adrenoceptors in86Rb efflux from liver slices and lipolysis in guinea-pig isolated adipocytes

1. The application of an alpha 1-adrenoceptor agonist, amidephrine, to guinea-pig liver slices increases glucose release and 86Rb efflux. Since prazosin was more potent than yohimbine in inhibiting both responses, alpha 1-adrenoceptors seem to be involved in the effects evoked by the agonist. 2. Clonidine (an alpha 2-adrenoceptor agonist) at doses unable to activate liver glycogenolysis increased 86Rb release and potentiated isoprenaline in promoting 86Rb efflux. Since yohimbine antagonized clonidine in promoting 86Rb efflux, alpha 2-adrenoceptors also seem to control plasmalemmal permeability to 86Rb. 3. The liver slice responses resulting from alpha 1- and alpha 2-adrenoceptor stimulation required extracellular calcium. Calcium absence or the administration of D-600 attenuated the effects of amidephrine on glucose release and 86Rb outflow and Ca2+ excess re-established both responses. D-600 and apamin blocked clonidine-induced 86Rb efflux, suggesting that alpha 2-adrenoceptor stimulation activates calcium dependent K+ channels. 4. alpha 2-adrenoceptors do not appear to mediate antilipolytic effects in guinea-pig fat cells.

Acute effects of cholestatic and choleretic bile salts on vasopressin- and glucagon-induced hepato-biliary calcium fluxes in the perfused rat liver

The effects were investigated of the choleretic bile salt glycoursodeoxycholate (G-UDCA) and of the cholestatic bile salt taurochenodeoxycholate (T-CDCA) on changes in perfusate Ca2+, glucose and oxygen and in bile calcium and bile flow induced by the administration of (a) vasopressin, (b) glucagon and (c) glucagon plus vasopressin together to the perfused rat liver [Hamada, Karjalainen, Setchell, Millard & Bygrave (1992) Biochem. J. 281, 387-392]. G-UDCA itself increased the secretion of calcium in the bile several-fold, but its principal effect was to augment each of the above-mentioned metabolic events except glucose and oxygen output; particularly noteworthy was its ability to augment the 'transients' in bile calcium and bile flow seen immediately after the administration of vasopressin with or without glucagon. T-CDCA, by contrast, produced opposite effects and attenuated all of the parameters measured, and in particular the transients in bile calcium and bile flow. The data provide evidence of a strong correlation between calcium fluxes occurring on both the sinusoidal and the bile-canalicular membranes and that all are modifiable by glucagon, Ca(2+)-mobilizing hormones and bile salts.

Concomitant stimulation by vasopressin of biliary and perfusate calcium fluxes in the perfused rat liver

Changes in perfusate Ca2+ (measured with a Ca(2+)-selective electrode) and changes in bile calcium (measured by atomic absorption spectroscopy) were continuously and simultaneously monitored after infusion of (a) vasopressin, (b) glucagon and (c) both vasopressin and glucagon together to the perfused rat liver. Also monitored were perfusate glucose and oxygen concentrations and bile flow. Vasopressin induces a sharp, transient, pulse of increased bile flow and increased bile calcium within 1 min of infusion, concomitant with rapid changes in perfusate Ca2+ fluxes, glucose output and oxygen uptake. This is immediately followed by a decrease in both bile flow and bile calcium for as long as the hormone is administered. Changes induced by glucagon are a relatively slow onset of perfusate Ca2+ efflux and oxygen uptake, but rapid glucose output, and a small but significant and transient decrease in bile flow and bile calcium which, despite the continued infusion of the hormone, spontaneously and rapidly returns to normality. However, the greatest responses are observed after co-administration of both hormones. Coincident with the augmented perfusate Ca2+ fluxes (influx) seen in earlier work, there occurs within 1 min of vasopressin infusion a sharp increase in bile secretion and bile calcium greater in magnitude than that produced by vasopressin alone. Immediately thereafter bile secretion and bile calcium decline below basal values and remain there for as long as the hormones are administered. Glucagon and vasopressin therefore each have opposing effects on bile flow and bile calcium. However, the action of vasopressin is enhanced by the prior administration of glucagon. The data thus reveal features about the actions of glucagon and Ca(2+)-mobilizing hormones on bile flow and bile calcium not previously recorded and provide a novel framework around which the whole issue of hepato-biliary Ca2+ homoeostasis can be assessed in normal and diseased liver.

Beneficial effect of dexamethasone on attenuated hormone-induced uptake of calcium and glycogenolysis by perfused liver of rats infected with Fasciola hepatica

Infection of rats with the liver fluke, Fasciola hepatica, impaired the responses of the perfused liver to calcium uptake and glucose release induced by the synergistic action of glucagon and vasopressin. Treatment of infected rats with dexamethasone prevented the impairment of each of these two responses.

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.

Frequency and amplitude enhancement of calcium transients by cyclic AMP in hepatocytes

Interactions between signalling pathways such as the cyclic AMP and the Ca2+/phosphatidylinositol pathway may occur and be of major relevance in the regulation of cell function. We demonstrate here that cyclic-AMP-dependent mechanisms cause a marked increase in frequency and peak free Ca2+ of alpha 1-receptor-induced Ca2+ transients in single hepatocytes and lower the threshold for alpha 1-receptor agonists. Adrenaline at low physiological concentrations generates alpha 1-receptor-induced Ca2+ transients, which requires activation of the beta 2-receptor signalling pathway. We conclude that an interaction between the alpha 1-receptor signalling pathway and cyclic-AMP-dependent mechanisms activated by beta 2-receptor occupation is crucial to elicit a complete adrenergic response to adrenaline at physiological concentrations in rat hepatocytes.

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

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

Nafenopin, a hypolipidemic and non-genotoxic hepatocarcinogen increases intracellular calcium and transiently decreases intracellular pH in hepatocytes without generation of inositol phosphates

Addition of nafenopin (30-300 microM to 45Ca2+ preloaded cultured hepatocytes caused a rapid and concentration-dependent increase in 45Ca2+ efflux in a manner similar to vasopressin, as evidenced by the loss of radioactivity from the cells. In contrast to vasopressin, addition of nafenopin to [3H]inositol prelabelled hepatocytes in culture did not increase [3H]inositol phosphate production. When added simultaneously with vasopressin, nafenopin inhibited the vasopressin-stimulated [3H]inositol phosphate production. In hepatocyte suspensions isolated from rats treated for 1 week with a carcinogenic dose of nafenopin (1000 ppm in their daily food) the incorporation of [3H]inositol into the phosphoinositide fraction, particularly phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, was much less than that in hepatocytes isolated from untreated rats. The vasopressin-stimulated [3H]inositol phosphate production was also decreased. Experiments with hepatocyte suspensions preloaded with Ca2+ or pH sensitive fluorescent indicators demonstrated that addition of nafenopin caused an increase in intracellular free Ca2+ and transient acidification of the cells. The increase in [Ca2+]i was decreased by only about 25% when extracellular calcium was removed indicating that nafenopin mainly mobilizes Ca2+ from intracellular stores. The recovery to basal pH was amiloride-sensitive indicating the importance of Na+/H+ exchange in pH recovery after intracellular acidification. Amiloride also inhibited DNA synthesis induced by nafenopin and by epidermal growth factor in cultured hepatocytes; but this effect occurred concomitantly with inhibition of basal DNA synthesis. We suggest that hepatic Ca2+ mobilization induced by nafenopin may play an important role in the mechanism by which nafenopin exerts its physiological as well as its tumour promotive activity upon chronic treatment with carcinogenic doses.

Regional differences in the activation of synaptosomal mitochondrial Ca2+ uptake by spermine in rat brain

Several properties of Ca2+ uptake by isolated synaptosomal mitochondria were characterized by using a repetitive Ca2+ loading technique. Synaptosomal mitochondria maintained extramitochondrial Ca2+ concentration at submicromolar levels when challenged with successive additions of small amounts of Ca2+. Ca2+ uptake under these conditions was markedly stimulated by the presence of spermine, a polyamine found in high concentrations in brain. Moreover, mitochondria isolated from telencephalic areas of rat brain were activated to a greater extent by spermine than were mitochondria from non-telencephalic brain regions. The present results support the idea that brain mitochondria could play a significant role in limiting the intraneuronal rise in Ca2+ that follows stimulation or injury. In addition, telencephalic mitochondria exhibit more flexibility in the regulation of Ca2+ uptake than do mitochondria from phylogenetically older non-telencephalic brain regions and this could be related to differences in Ca2+ influx mechanisms in these brain regions.

Conditions that result in the mobilization and influx of Ca2+ into rat hepatocytes induce the rapid loss of 3-hydroxy-3-methylglutaryl-CoA reductase activity that is not reversed by phosphatase treatment

We investigated the effects of conditions that induce Ca2+ mobilization from intracellular stores and Ca2+ influx into hepatocytes on the expressed and total (fully dephosphorylated) activities of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase. Vasopressin and phenylephrine when added alone had small or negligible effects on the phosphorylation state of the enzyme, as judged from the expressed/total activity ratio. However, when added in combination with glucagon, they elicited appreciable increases in the phosphorylation of the enzyme. Glucagon on its own had no effect either on phosphorylation state or on total HMG-CoA reductase activity during 40 min of incubation. Under conditions of sustained Ca2+ influx (i.e. vasopressin or phenylephrine plus glucagon), there was a marked loss of total HMG-CoA reductase activity. This effect was more pronounced when vasopressin was used; 50% of the enzyme activity was lost within 40 min. The involvement of Ca2+ in these effects was verified directly by the use of ionophore A23187. Its addition to hepatocytes resulted both in a very pronounced increase in the phosphorylation state of the enzyme and in the loss of 50% of the total activity within 30 min. There was no correlation between the ability of any set of conditions to increase the phosphorylation of the enzyme and the subsequent loss of total HMG-CoA reductase activity. The latter parameter appeared to be directly related, however, to the maintenance of prolonged Ca2+ influx, as indicated by the continued activation of glycogen phosphorylase, measured in the same cells. The lack of a causal relationship between increased phosphorylation and loss of total activity was demonstrated directly by studies in which okadaic acid was used to induce phosphorylation of HMG-CoA reductase in hepatocytes by inhibition of phosphatase 1 and 2A activities. This was not accompanied by any loss of total enzyme activity. Neither did okadaic acid enhance the loss of reductase induced by A23187 when the two agents were added together. It is concluded that altered Ca2+ fluxes in hepatocytes in vivo, under conditions of acute or chronic stress (such as may be associated with trauma or diabetes respectively), may be involved in the regulation of the expression of HMG-CoA reductase activity through alteration of enzyme concentration in the liver.

Interaction of arginine vasopressin and angiotensin II on Ca2+ in vascular smooth muscle cells

The non-osmotic release of arginine vasopressin (AVP) is associated with the concomitant activation of the renin-angiotensin and sympathetic nervous systems. In vivo studies suggest that a positive interaction may occur between AVP and angiotensin II (Ang II), and other Ca2+ mobilizing hormones. In the present study, the cellular mechanisms of this interaction between AVP and Ang II in vascular smooth muscle cell (VSMC) were examined. These results support the existence of a positive interaction between AVP and Ang II on Ca2+ mobilization in VSMC. In fact, the challenge of VSMC with combined AVP and Ang II, in a range from 5 x 10(-11) to 10(-8) M, enhanced cytosolic free Ca2+ ([Ca2+]i) and 45Ca2+ efflux in a more than additive manner. This potentiation, which was not dependent of the presence of extracellular calcium, correlated with an increased VSMC shape change. Moreover, the combination of subthreshold doses of AVP and Ang II (5 x 10(-11) M), which do not release Ca2+ alone, evoked a Ca2+ mobilizing response. A subthreshold dose of Ang II also shifted to the left the concentration-response curve of the AVP-mediated 45Ca2+ efflux. Since there were no changes in receptor binding of either hormone by the other hormone and the interaction of the two hormones on the production of inositol phosphatides was additive, the AVP and AII positive interaction on Ca2+ mobilization on VSMC may occur at the level of the intracellular Ca2(+)-releasing mechanism itself. Such an interaction can occur at hormone concentrations below the Ca2+ release threshold and may explain an increased functional response to the combination of pressor hormones compared to that of each hormone alone.

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.

Phosphate and calcium uptake by mitochondria and by perfused rat liver induced by the synergistic action of glucagon and vasopressin

Co-administration of glucagon and vasopressin to rat liver perfused with buffer containing 1.3 mM-Ca2+ induces a 4-fold increase in Pi in the subsequently isolated mitochondria (from approx. 9 to approx. 40 nmol/mg of mitochondrial protein). This increase is not attributable to PPi hydrolysis, and is not observed if the perfusate Ca2+ is lowered from 1.3 mM to 50 microM. The increase in mitochondrial Pi closely parallels that of mitochondrial Ca2+; when the increase in Pi and Ca2+ accumulation is maximal, the molar ratio is close to that in Ca3(PO4)2. Measurement of changes in the perfusate Pi revealed that, whereas administration of glucagon or vasopressin alone brought about a rapid decline in perfusate Pi, the largest decrease (reflecting net retention of Pi by the liver) was observed when the hormone was co-administered in the presence of 1.3 mM-Ca2+. The synergistic action of glucagon plus vasopressin was nullified by lowering the perfusate Ca2+ to 50 microM. The data provide evidence that, whereas glucagon may be able to alter Pi fluxes directly in intact liver, any alterations induced by vasopressin are indirect and result only from its action of mobilizing Ca2+.

Receptor-mediated stimulation of taurocholate efflux from the rat hepatocyte and the ex vivo perfused rat liver

The peptide hormone, arginine-vasopressin[( Arg8]vasopressin, AVP), stimulates efflux of the bile salts taurocholate and glycocholate from the rat hepatocyte in suspension via its association with the V1 receptor on the hepatic cell membrane. At a concentration ratio of 5:1 (antagonist to hormone), the V1 vasopressin antagonist, (dCH2)5Tyr(Me)AVP, inhibits the vasopressin induced efflux of taurocholate by approximately 82%, and of glycocholate, by approximately 85%. In contrast, the V2 antagonist (d(CH2)5[D-Ile2,Ala4]AVP, does not interfere with the stimulation of taurocholate and glycocholate efflux by vasopressin. In the isolated perfused rat liver, vasopressin (5 X 10(-10) M) causes an immediate increase of 55 +/- 12% over baseline in [14C]taurocholate secretion and a corresponding increase in bile flow. A more gradual and prolonged increase in [14C]taurocholate secretion, reflecting an increased biliary concentration of [14C]taurocholate, is observed beginning 6 min after vasopressin, reaching a plateau of 23 +/- 12% over baseline by 14 min and returning to baseline by 30 min. The mean rate of 14C secretion during the 30 min following administration of vasopressin (non-steady state) is increased by 14.3 +/- 6.4% over pre-infusion steady-state baseline (P less than 0.05). Prior administration of the V1 receptor antagonist d(CH2)5Tyr(Me)AVP attenuates these effects of vasopressin. The combination of these in vitro and in vivo findings suggest that vasopressin may play a role in regulating bile salt efflux. Furthermore, these studies in the isolated hepatocyte and the intact liver may provide a unique approach for defining biochemical changes associated with bile salt transport from the hepatic cell.

Allosteric activation of brain mitochondrial Ca2+ uptake by spermine and by Ca2+: developmental changes

Kinetic analysis of 45Ca2+ uptake by rat brain mitochondria in Ca2+ - 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid buffers indicated that spermine both increased the apparent affinity for Ca2+ and decreased the cooperativity of uptake. Both effects are consistent with an allosteric activation of uptake by spermine. The stimulating effect of spermine on 45Ca2+ uptake was maximal with mitochondria from postnatal day 10 animals and then steadily decreased with increasing age to reach adult values by approximately 30 postnatal days; this was observed independently of the substrates used to fuel mitochondria. Mitochondrial Ca2+ buffering was also analyzed by use of a Ca2+-selective electrode. Addition of a large bolus of Ca2+ produced a decrease in the subsequent equilibrium extramitochondrial Ca2+ concentration (or a "rebound overshoot") under some conditions. It is proposed that this effect is the result of an allosteric activation of Ca2+ uptake by Ca2+. This effect was slowly reversible, or hysteretic, and was blocked by spermine. The overshoot was increased in the presence of higher concentrations of Mg2+ and was absent when mitochondria were incubated with 0.3 mM Mg2+. It was maximal in mitochondria prepared from early postnatal brain, and changes in the magnitude of the effect during development paralleled those obtained with spermine stimulation of 45Ca2+ uptake. The data suggest that spermine produces an allosteric activation of Ca2+ uptake by binding to the same regulatory sites that are involved in the Ca2+-induced activation. The results as a whole suggest that spermine could modulate mitochondrial buffering of the intracellular Ca2+ concentration in brain, particularly during the early postnatal period.

Ca2+-induced accumulation of pyrophosphate in mitochondria during acetate metabolism

The mechanism of pyrophosphate (PPi) accumulation in rat liver during acetate metabolism was investigated. Perfusion of the liver with acetate in the presence of noradrenaline and glucagon induced marked accumulation of PPi (2 mumol/g of liver, 200 times that of control). In contrast, perfusion with glutamine, which generates PPi only in the cytosol, caused little accumulation of PPi, even in the presence of the two hormones. The site of PPi accumulation was shown to be the mitochondria by the finding that isolated mitochondria from the liver perfused with acetate and the hormones contained 50 nmol of PPi/mg of protein. The addition of an uncoupler to mitochondria with accumulated PPi caused gradual decrease in their PPi content, with concomitant release of a stoichiometric amount of Ca2+. Similar accumulation of PPi was observed when isolated mitochondria were incubated with acetate and Ca2+. These results show that an increase in cytosolic Ca2+ caused by the co-administration of the two hormones induced uptake of the ion into mitochondria, and that PPi accumulated in mitochondria only when it was generated in the organelles with an elevated concentration of Ca2+. High mitochondrial concentrations of Ca2+ are considered to inhibit inorganic pyrophosphatase through the formation of a stable complex, CaPPi-. Mitochondria with accumulated PPi had normal respiratory activities, and their adenine nucleotide concentrations were increased 2-fold rather than being decreased, the increases also being considered to be caused by their high concentration of Ca2+.

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.

The regulation of the matrix volume of mammalian mitochondria in vivo and in vitro and its role in the control of mitochondrial metabolism

The purpose of this article is to describe briefly the methods by which the intra-mitochondrial volume may be measured both in vitro and in situ, to summarise the mechanisms thought to regulate the mitochondrial volume and then to review in more detail the evidence that changes in the intra-mitochondrial volume play an important part in the regulation of liver mitochondrial metabolism by glucogenic hormones such as glucagon, adrenaline and vasopressin. It will be shown that these hormones cause an increase in matrix volume sufficient to produce significant activation of fatty acid oxidation, respiration and ATP production, pyruvate carboxylation, citrulline synthesis and glutamine hydrolysis. These are all processes activated by such hormones in vivo. I will go on to demonstrate that the increase in matrix volume is brought about by an increase in mitochondrial [PPi]. This is able to stimulate K+ entry into the matrix, perhaps through an interaction with the adenine nucleotide translocase. The rise in matrix [PPi] is a consequence of an increase in cytosolic and hence mitochondrial [Ca2+] which inhibits mitochondrial pyrophosphatase. In the final section of the review I provide evidence that changes in mitochondrial volume may be important in the responses of a variety of tissues to hormones and other stimuli. I write as a metabolist with a working knowledge of bioenergetics rather than the converse, and this will certainly be reflected in the approach taken. If I cause offence to any dedicated experts in the field of bioenergetic by my ignorance or lack of understanding of their studies I can only offer my apologies and ask to be corrected.

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.

Stimulation of hepatic inositol 1,4,5-trisphosphate kinase activity by Ca2+-dependent and -independent mechanisms

A cytosolic fraction derived from rat hepatocytes was used to investigate the regulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] kinase, the enzyme which converts Ins(1,4,5)P3 to inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. The activity was doubled by raising the free Ca2+ concentration of the assay medium from 0.1 microM to 1.0 microM. A 5 min preincubation of the hepatocytes with 100 microM-dibutyryl cyclic AMP (db.cAMP) plus 100 nM-tetradecanoylphorbol acetate (TPA) resulted in a 40% increase in Ins(1,4,5)P3 kinase activity when subsequently assayed at 0.1 microM-Ca2+. This effect was smaller at [Ca2+] greater than 0.5 microM, and absent at 1.0 microM-Ca2+. Similar results were obtained after preincubation with 100 microM-db.cAMP plus 300 nM-vasopressin (20% increase at 0.1 microM-Ca2+; no effect at 1.0 microM-Ca2+). Preincubation with vasopressin, db.cAMP or TPA alone did not alter Ins(1,4,5)P3 kinase activity. It is proposed that these results, together with recent evidence implicating Ins(1,3,4,5)P4 in the control of Ca2+ influx, could be relevant to earlier findings that hepatic Ca2+ uptake is synergistically stimulated by cyclic AMP analogues and vasopressin.

Inorganic pyrophosphate is located primarily in the mitochondria of the hepatocyte and increases in parallel with the decrease in light-scattering induced by gluconeogenic hormones, butyrate and ionophore A23187

1. The effects of a variety of hormones on the PPi content and light-scattering of isolated rat liver cells was studied. 2. The basal PPi content was about 130 pmol/mg of cell protein, and increased after hormone addition, in parallel with a decrease in light-scattering which we have observed previously [Quinlan, Thomas, Armston & Halestrap (1983) Biochem. J. 214, 395-404]. 3. The mean increases in PPi content with the agonists shown (as pmol/mg of protein) were: 0.1 microM-glucagon, 25; 20 microM-phenylephrine, 30; 25 nM-vasopressin, 127; glucagon + phenylephrine, 115; glucagon + vasopressin, 382; 100 microM-ADP, 50; 15 microM-A23187, 72; 1 mM-butyrate, 80. 4. In the absence of extracellular Ca2+, vasopressin had little effect on either the PPi content or the light-scattering of hepatocytes. 5. The magnitude of the increase in PPi content correlated with that of the decrease in light-scattering irrespective of the stimulating agent, provided that the PPi did not exceed 300 pmol/mg of protein. Above this value little additional change in light-scattering was observed. 6. Subcellular fractionation showed that over 90% of the cellular PPi was intramitochondrial in both control and stimulated cells. 7. The data support the conclusions of previous experiments using isolated liver mitochondria [Davidson & Halestrap (1987) Biochem. J. 246, 715-723] that hormones increase the mitochondrial matrix volume through a Ca2+-induced rise in matrix [PPi]. 8. It is further proposed that this increase in mitochondrial [PPi] allows entry of ADP into the mitochondria in exchange for PPi and is therefore responsible for the increase in total mitochondrial adenine nucleotides observed after hormone treatment.

Non-parenchymal cells as mediators of physiological responses in liver

Parenchymal cells (hepatocytes) are the sites at which the principal metabolic functions of the liver are located. In the perfused liver, responses (e.g. vasoconstriction and glycogenolysis) to stimulating agents such as zymosan, platelet-activating factor and arachidonic acid, are inhibited by indomethacin and bromophenacyl bromide, inhibitors of cyclo-oxygenase and phospholipase A2, respectively. Since cultured Kupffer and endothelial cells but not hepatocytes, produce eicosanoids, and since eicosanoids and especially prostaglandins induce similar patterns of responses when added directly to the perfused liver, an involvement of these non-parenchymal cells in mediating the above responses is considered likely. We propose that in most situations the responses induced by these stimulating agents are mediated through a combination of pathways that include interaction of the agents directly with hepatocytes or with vasoactive cells (endothelial and/or smooth muscle cells), or interaction of agents initially with non-parenchymal cells to produce and release eicosanoids, which then subsequently interact with hepatocytes or with vasoactive cells.