Differential Ca2+ signaling in neonatal and adult rat hepatocyte doublets

Background / Aims: Intracellular Ca2+ ([Ca2+](i)) is important in various cellular functions, including cellular proliferation and differentiation. To elucidate the relationship between [Ca2+](i) oscillations and physiological hepatocyte proliferation, phenylephrine-evoked [Ca2+](i) responses were sequentially investigated using short-term cultured hepatocyte doublets obtained from 1-, 3-, 6- and 8-week-old rats. Methods / Results: DNA synthesis in hepatocytes, determined by BrdU incorporation, was similar to 20% in 1-week-old rats, and decreased to <1% as the rats aged, Correspondingly, [Ca2+](i) responses evoked by 10 mu mol/l phenylephrine in hepatocyte doublets shifted from transient to sinusoidal-type [Ca2+](i) oscillations and then to a sustained increase in [Ca2+](i), followed by a gradual return to baseline, The incidence of [Ca2+](i) oscillations was 100 +/- 0.0%, 83.3 +/- 16.7%, 38.7 +/- 0.6% and 5.5 +/- 5.0% in 1-,3-,6- and 8-week-old rats, respectively, Removal of extracellular Ca2+ did not abolish [ Ca2+](i) oscillations, indicating that [Ca2+](i) oscillations were caused primarily by Ca2+ mobilization from internal sites of the cells, The [Ca2+](i) level in each of the adjacent cells was synchronous in sustained increase in [Ca2+](i), but asynchronous in [Ca2+](i) oscillations. In proliferating doublets obtained from 1-week-old rats, the frequency of oscillations increased in a dose-dependent manner for phenylephrine concentrations of 1 to 100 mu mol/l. Conclusions: Phenylephrine-evoked [Ca2+](i) oscillations were directly related to hepatocyte proliferation and were mediated by frequency modulation, These results suggest that phenylephrine-evoked [Ca2+](i) oscillations may contribute to cell-cycle progression of hepatocytes in physiological liver growth.