Intracellular calcium in the isolated rat liver: correlation to glucose release, K(+) balance and bile flow

Calcium signalling in hepatic metabolism: Health and diseases

The flexibility between the wide array of hepatic functions relies on calcium (Ca2+) signalling. Indeed, Ca2+ is implicated in the control of many intracellular functions as well as intercellular communication. Thus, hepatocytes adapt their Ca2+ signalling depending on their nutritional and hormonal environment, leading to opposite cellular functions, such as glucose storage or synthesis. Interestingly, hepatic metabolic diseases, such as obesity, type 2 diabetes and non-alcoholic fatty liver diseases, are associated with impaired Ca2+ signalling. Here, we present the hepatocytes' toolkit for Ca2+ signalling, complete with regulation systems and signalling pathways activated by nutrients and hormones. We further discuss the current knowledge on the molecular mechanisms leading to alterations of Ca2+ signalling in hepatic metabolic diseases, and review the literature on the clinical impact of Ca2+-targeting therapeutics.

Hepatic glucose production and insulin resistance

Insulin resistance relates to hepatic glucose production (HGP) and hepatic triglyceride content (HTG). Elevation of free fatty acids (FFA) and imbalance of adipocytokines are major mechanisms involved in insulin resistance. Using isolated perfused rat livers we examined metabolic effects of hormones, FFA and leptin. Not only insulin, but also insulin-like growth factor-I similarly decreased epinephrine-induced HGP. Likewise, leptin not only reduced epinephrine-induced HGP, but also decreased fasting HGP by inhibiting gluconeogenesis from lactate. This resulted from the stimulation of the insulin receptor substrate (IRS)-2 pathway and the synthesis of phosphoenolpyruvate carboxykinase, whereas the IRS-1 pathway is inhibited. In dietary-induced obesity, leptin receptors and signalling were downregulated and its cross-talk with insulin signalling was differentially regulated depending on nutritional status. Leptin further increased HTG and intrahepatic FFA. A short-term increase in circulating FFA (palmitate and oleate) augmented lactate uptake, but not HGP. This early effect was paralleled by protein phosphorylation at different sites resulting in impaired insulin signalling.

Limited utility of acetoxymethyl (AM)-based intracellular delivery systems, in vivo: interference by extracellular esterases

The use of acetoxymethyl (AM) groups to deliver and trap exogenous optical probes inside cells is an established tool in cell biology/physiology, however, these probes have not been used extensively in vivo. In this study, the use of the acetoxymethyl delivery system for optical probes was evaluated, in vivo. Initial studies revealed very little trapped probe in intact tissues even when near saturating levels of probe were injected in living animals. We tested the hypothesis that extracellular esterases rapidly cleave the acetoxymethyl groups preventing the probes from entering cells, in vivo. The rates of hydrolysis of 11 acetoxymethyl probes in diluted porcine plasma revealed an essentially first order high rate dye cleavage with half times on the order of minutes or less. Studies on mice and rabbits revealed rates 10- to 2-fold higher, respectively. These plasma studies suggested that the acetoxymethyl probes were being cleaved before having a chance to enter cells in tissues in vivo. This was confirmed using intravital 2-photon excitation microscopy in muscle tissue where several acetoxymethyl probes were found to rapidly cleave in the vascular space during infusion and not be trapped in the muscle cells. Studies with succinimidyl esters that should quickly bind to proteins on cleavage also failed to enter cells, in vivo, consistent with the notion that the cleavage was occurring in the extracellular space. These data suggest that the high level of plasma and extracellular esterase activity render the classical acetoxymethyl probes ineffective for monitoring intracellular events, in vivo. Different approaches to trapping exogenous probes will need to be explored for physiological studies using intravital microscopy.

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.

Effect of NPC-14686 (Fmoc-L-homophenylalanine) on intracellular Ca2+ levels in human hepatoma cells

The effect of NPC-14686, a potential anti-inflammatory drug, on cytosolic free Ca(2+) levels ([Ca(2+)](i)) in HA22/VGH human hepatoma cells was explored by using fura-2 as a fluorescent Ca(2+) indicator. NPC-14686 at concentrations above 10 microM increased [Ca(2+)](i) in a concentration-dependent manner with an EC(50) value of 100 microM. The Ca(2+) signal was reduced by removing extracellular Ca(2+) or by 10 microM nifedipine and was not changed by verapamil or diltiazem. Pretreatment with 1 microM thapsigargin (an endoplasmic reticulum Ca(2+) pump inhibitor) to deplete the endoplasmic reticulum Ca(2+) abolished 200 microM NPC-14686-induced Ca(2+) release; and conversely pretreatment with NPC-14686 abolished thapsigargin-induced Ca(2+) release. The Ca(2+) release induced by 200 microM NPC-14686 was not changed by inhibiting phospholipase C with 2 microM U73122. Together, the results suggest that in human hepatoma cells, NPC-14686 induced a [Ca(2+)](i) increase by causing store Ca(2+) release from the endoplasmic reticulum in an phospholipase C-independent manner, and by inducing nifedipine-sensitive Ca(2+) influx.