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

Calcium Signaling in Liver Injury and Regeneration

The liver fulfills central roles in metabolic control and detoxification and, as such, is continuously exposed to a plethora of insults. Importantly, the liver has a unique ability to regenerate and can completely recoup from most acute, non-iterative insults. However, multiple conditions, including viral hepatitis, non-alcoholic fatty liver disease (NAFLD), long-term alcohol abuse and chronic use of certain medications, can cause persistent injury in which the regenerative capacity eventually becomes dysfunctional, resulting in hepatic scaring and cirrhosis. Calcium is a versatile secondary messenger that regulates multiple hepatic functions, including lipid and carbohydrate metabolism, as well as bile secretion and choleresis. Accordingly, dysregulation of calcium signaling is a hallmark of both acute and chronic liver diseases. In addition, recent research implicates calcium transients as essential components of liver regeneration. In this review, we provide a comprehensive overview of the role of calcium signaling in liver health and disease and discuss the importance of calcium in the orchestration of the ensuing regenerative response. Furthermore, we highlight similarities and differences in spatiotemporal calcium regulation between liver insults of different etiologies. Finally, we discuss intracellular calcium control as an emerging therapeutic target for liver injury and summarize recent clinical findings of calcium modulation for the treatment of ischemic-reperfusion injury, cholestasis and NAFLD.

Hepatic ischemia-reperfusion injury: roles of Ca2+ and other intracellular mediators of impaired bile flow and hepatocyte damage

Liver resection and liver transplantation have been successful in the treatment of liver tumors and end-stage liver disease. This success has led to an expansion in the pool of patients potentially treatable by liver surgery and, in the case of transplantation, to a shortage of liver donors. At present, there are significant numbers of potential candidates for liver resection and liver donation who have fatty livers, are aged, or have livers damaged by chemotherapy. All of these are at high risk for ischemic reperfusion (IR) injury. The aims of this review are to assess current knowledge of the clinical effectiveness of ischemic preconditioning and intermittent ischemia in reducing IR damage in liver surgery; to evaluate the use of bile flow as a sensitive indicator of IR liver damage; and to analyze the molecular mechanisms, especially intracellular Ca2+, involved in IR injury and ischemic preconditioning. It is concluded that bile flow is a sensitive indicator of IR injury. Together with reactive oxygen species (ROS) and other extracellular and intracellular signaling molecules, intracellular Ca2+ in hepatocytes plays a key role in the normal regulation of bile flow and in IR-induced injury and cell death. Ischemic preconditioning is an effective strategy to reduce IR injury but there is considerable scope for improvement, especially in patients with fatty and aged livers. The development of effective new strategies to reduce IR injury will depend on improved understanding of the molecular mechanisms involved, especially by gaining a better perspective of the relative importance of the various intrahepatocyte signaling pathways involved.

Induction of cholestasis in the perfused rat liver by 2-aminoethyl diphenylborate, an inhibitor of the hepatocyte plasma membrane Ca2+ channels

BACKGROUND AND AIMS

An increase in the cytoplasmic free Ca2+ concentration in hepatocytes as a result of the release of Ca2+ from intracellular stores and Ca2+ inflow from the extracellular space is a necessary part of the mechanism by which bile acids are moved along the bile cannaliculus by contraction of the cannaliculus. 2-Aminoethyl diphenylborate (2-APB) is a recently discovered inhibitor of store-operated plasma membrane Ca2+ channels in hepatocytes. The aim of the present study was to test the ability of 2-APB to inhibit bile flow.

METHODS

Bile flow was measured in the isolated perfused rat liver using cannulation of the common bile duct. Measurements were carried out in the presence or absence of 2-APB in either the presence of taurocholic acid (to enhance basal bile flow) or in the absence of taurocholic acid and in the presence of the hormones vasopressin and glucagon, which are known to stimulate bile flow.

RESULTS

In livers perfused in the presence of taurocholic acid, 2-APB reversibly inhibited bile flow with a slow time of onset. The time of onset of inhibition was reduced by prior addition of the endoplasmic reticulum (Ca(2+) + Mg2+)adenosine triphosphatase inhibitor, 2,5-di-t-butylhydroquinone. In livers perfused in the absence of taurocholate, 2-APB had little effect on the basal rate of bile flow, but inhibited the ability of vasopressin and glucagon to stimulate bile flow.

CONCLUSIONS

It is concluded that an inhibitor of hepatocyte plasma membrane Ca2+ channels can induce cholestasis. The results provide evidence that suggests that, over a period of time, the normal function of hepatocyte store-operated Ca2+ channels is required to maintain bile flow. Future strategies directed at the regulation of bile flow might include pharmacological or other interventions that modulate Ca2+ inflow to hepatocytes.

Evidence that store-operated Ca2+ channels are more effective than intracellular messenger-activated non-selective cation channels in refilling rat hepatocyte intracellular Ca2+ stores

Liver cells possess store-operated Ca2+ channels (SOCs) with a high selectivity for Ca2+ compared with Na+, and several types of intracellular messenger-activated non-selective cation channels with a lower selectivity for Ca2+ (NSCCs). The main role of SOCs is thought to be in refilling depleted endoplasmic reticulum Ca2+ stores [Cell Calcium 7 (1986) 1]. NSCCs may be involved in refilling intracellular stores but are also thought to have other roles in regulating the cytoplasmic-free Ca2+ and Na+ concentrations. The ability of SOCs to refill the endoplasmic reticulum Ca2+ stores in hepatocytes has not previously been compared with that of NSCCs. The aim of the present studies was to compare the ability of SOCs and maitotoxin-activated NSCCs to refill the endoplasmic reticulum in rat hepatocytes. The experiments were performed using fura-2FF and fura-2 to monitor the free Ca2+ concentrations in the endoplasmic reticulum and cytoplasmic space, respectively, a Ca2+ add-back protocol, and 2-aminoethyl diphenylborate (2-APB) to inhibit Ca2+ inflow through SOCs. In cells treated with 2,5-di-t-butylhydroquinone (DBHQ) or vasopressin to deplete the endoplasmic reticulum Ca2+ stores, then washed to remove DBHQ or vasopressin, the addition of Ca2+ caused a substantial increase in the concentration of Ca2+ in the endoplasmic reticulum and cytoplasmic space due to the activation of SOCs. These increases were inhibited 80% by 2-APB, indicating that Ca2+ inflow is predominantly through SOCs. In the presence of 2-APB (to block SOCs), maitotoxin induced a substantial increase in [Ca2+](cyt), but only a modest and slower increase in [Ca2+](er). Under these conditions, Ca2+ inflow is predominantly through maitotoxin-activated NSCCs. It is concluded that SOCs are more effective than maitotoxin-activated NSCCs in refilling the endoplasmic reticulum Ca2+ stores. The previously developed concept of a specific role for SOCs in refilling the endoplasmic reticulum is consistent with the results reported here.

Efflux of hepatic ascorbate: a potential contributor to the maintenance of plasma vitamin C

Ascorbate (AH, the reduced form of vitamin C) is an important radical scavenger and antioxidant in human plasma; the resulting ascorbyl radical can disproportionate to AH and dehydroascorbic acid (DHA). Here we address potential maintenance mechanism(s) for extracellular AH by examining the ability of cells to convert extracellularly presented DHA to AH. DHA was rapidly transported into human liver (HepG2), endothelial and whole blood cells in vitro by plasma membrane glucose transporters and reduced intracellularly. Liver cells displayed the highest capacity to release the intracellularly accumulated AH. The proteins responsible for DHA uptake and AH release could be distinguished by inhibitor studies. Thus, unlike DHA uptake, AH efflux was largely insensitive to cytochalasin B and thiol-reactive agents but was inhibited by phloretin, 4,4'-di-isothiocyanostilbene-2,2'-disulphonate and isoascorbate. Efflux of AH from cells was temperature-sensitive and saturable with a low affinity (millimolar, intracellular) for AH. In addition to isolated liver cells, perfusion of intact rat and guinea-pig liver with DHA resulted in AH in the circulating perfusate. Our results show that hepatocytes take up and reduce DHA and subsequently release part of the AH formed, probably via a membrane transporter. By converting extracellular DHA to extracellular AH, the liver might contribute to the maintenance of plasma AH, a process that could be important under conditions of oxidative stress.

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.

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.

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.