Different 1,4-dihydropyridines exhibit discriminating effects on passive calcium uptake in rat liver plasma membrane vesicles

Factors affecting hepatocyte viability and CYPIA1 activity during encapsulation

Hepatocytes encapsulated in alginate-poly-1-lysine-alginate (APA) are used in transplantation studies and in bioartificial liver support systems. Loss of cell viability in the process of APA encapsulation is usually 20-30% while the effect on cytochrome CYP450 activity is rarely reported. This work investigates the negative influences on hepatocyte viability and CYPIA1 activity during APA encapsulation, and reports methods to alleviate these influences by incorporating certain reagents into the encapsulation solution. The results show that loss of hepatocyte viability and CYPIA1 activity was caused almost entirely by extracellular calcium toxicity rather than by mechanical damage (p < 0.05). Use of 10 mM instead of 100 mM calcium chloride (CaCl2) in the encapsulation process improved CYPIA1 activity (p < 0.05), but did not improve hepatocyte viability (p > 0.05) or result in satisfactory microcapsules. Hepatocyte viability was 25% higher (p < 0.05) in CaCl2 than in calcium lactate (CaLa) when the cells were gelled by contact with these calcium solutions at room temperature (RT). Hepatocyte viability showed little improvement by processing at 4 degrees C than at RT in CaCl2 (p > 0.05) but was 23% higher at 4 degrees C than at RT in CaLa (p < 0.05). Calcium used in the process of encapsulation caused cell necrosis rather than apoptosis. Addition of Dulbecco's modified Eagle's medium (containing 10% foetal bovine serum) or 20 mM fructose to the calcium solution did not improve cell survival. However, nifedipine at a final concentration of 25 mM modestly improved hepatocyte survival in solution containing 100 mM CaCl2 (p = 0.003). Glutathione and taurine in certain concentrations showed protective effects against loss of CYPIA1 activity (p < 0.05 and <0.01 respectively). In conclusion, to optimise the use of calcium during the process of encapsulation, CaCl2 is preferred to CaLa and inclusion of nifedipine, glutathione or taurine in 100 mM CaCl2 solution is recommended.

Hepatocellular ATP-binding cassette protein expression enhances ATP release and autocrine regulation of cell volume

In a model liver cell line, recovery from swelling is mediated by a sensitive autocrine pathway involving conductive release of ATP, P2 receptor stimulation, and opening of membrane Cl- channels (Wang, Y., Roman, R. M., Lidofsky, S. D., and Fitz, J. G. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 12020-12025). However, the mechanisms coupling changes in cell volume to ATP release are not known. Based on evidence that certain ATP-binding cassette (ABC) proteins may function as ATP channels or channel regulators, we evaluated the potential role of ABC proteins by comparing ATP release and volume regulation in rat HTC and HTC-R hepatoma cells, the latter of which overexpress Mdr proteins. In both cell types, Cl- current activation (ICl-swell) and volume recovery following swelling were dependent on conductive ATP efflux. The rate of volume recovery was approximately 6-fold faster in HTC-R cells compared with HTC cells. This effect is likely due to enhanced ABC protein-dependent ATP release since (i) ICl-swell and cell volume recovery were eliminated by inhibition of P-glycoprotein transport (20 microM verapamil and 15 microM cyclosporin A); (ii) swelling-induced Cl- current density was similar in both cell types (approximately -50 pA/pF; not significant); and (iii) ATP conductance measured by whole-cell techniques was increased approximately 3-fold in HTC-R cells compared with HTC cells. Moreover, HTC-R cells exhibited enhanced survival during hypotonic stress. By modulating ATP release, hepatic ABC proteins may play a key role in the cellular pathways coupling changes in cell volume to ion permeability and secretion.

Protective effects of calcium channel blockers on acute bromobenzene toxicity to isolated rat hepatocytes. Inhibition of phenylephrine-induced calcium oscillations

BACKGROUND AND METHODS

Protective effects of verapamil, nifedipine, diltiazem, and ethylene glycol tetraacetic acid (EGTA) on acute bromobenzene (BB) toxicity to rat hepatocytes were evaluated, and cytosolic [Ca2+]i was monitored in single BB-exposed rat hepatocytes. Additionally, the effect of nifedipine on phenylephrine-stimulated calcium oscillations was investigated.

RESULTS

BB at 0.8-2.4 mM increased the lactate dehydrogenase (LDH) leakage rate dose-dependently. Pretreatment with verapamil (25-35 microM), nifedipine (35-45 microM), diltiazem (25 microM), or EGTA (1.5-5 mM) markedly attenuated the BB-induced (1.6 mM) LDH leakage rate during 2 h of incubations. BB did not cause any detectable acute change in [Ca2+]i. BB interfered with phenylephrine-stimulated calcium oscillations, by blocking the oscillations in 58% of the cells and reducing the oscillation frequency in the rest. Nifedipine (100 and 200 microM) blocked the phenylephrine-induced calcium oscillations completely in 55% and 88% of the cells, respectively.

CONCLUSIONS

The findings demonstrate that verapamil, nifedipine, diltiazem, and EGTA significantly protect rat hepatocytes against BB toxicity. BB interferes with phenylephrine-stimulated calcium oscillations. Nifedipine inhibits the oscillations at doses higher than those exerting a protective effect.