DIFFERENT FEATURES OF CA2+ OSCILLATIONS IN DIFFERENTIATED AND UNDIFFERENTIATED HEPATOCYTE DOUBLETS

Cytosolic free Ca2+ ([Ca2+](i)) oscillations are postulated to play a critical role in cellular proliferation. By using doublets of normal r ats (NR) and those 18 hours after two-thirds hepatectomy (PHR), we inv estigated cytosolic free Ca2+ ([Ca2+](i)) responses in Liver regenerat ion. Normal rat hepatocyte doublets that retain their bite canaliculi are polarized and well differentiated PHR doublets, which also retain their bile canaliculi, were characterized as undifferentiated by (1) d ecreased canalicular secretion of fluorescein-isothiocyanate-labeled g lycocholate; (2) increased labeling;index of hepatocytes in BrdU stain ing (similar to 30%); and (3) impaired transfer of fluorescent dye inj ected into one cell of the pair to the other. Addition of phenylephrin e to NR and PHR doublets in the presence of extracellular Ca2+ resulte d in [Ca2+](i) oscillations or a nonoscillatory-sustained increase in [Ca2+](i) followed by a gradual return to the baseline. Extracellular Ca2+ was not required for [Ca2+](i) oscillations but was necessary for a sustained increase in [Ca2+](i). Simultaneous addition of prazosin, alpha 1-receptor blocker, to doublets immediately abolished these [Ca 2+](i) responses. The [Ca2+](i) level in each of the adjacent cells wa s synchronous in sustained increase in [Ca2+](i), but asynchronous in [Ca2+](i) oscillations. As the phenylephrine concentration was increas ed (1 to 100 mu mol/L), oscillations were replaced by a sustained incr ease in [Ca2+](i) in NR doublets. In contrast, in PHR doublets, oscill ations remained, whereas the frequency of oscillations increased in a dose-dependent manner. These results indicate that the mechanisms of p henylephrine-evoked [Ca2+](i) responses are different in differentiate d and undifferentiated doublets and that the frequency modulation of [ Ca2+](i) oscillations mag be involved in the intracellular signal tran sduction in the cellular proliferation process during Liver regenerati on