Phorbol ester down-regulates glucagon stimulated taurocholate uptake in hepatocytes

Glucagon (G) enhances bile flow and stimulates bile acid uptake by hepatocytes, although the mechanism of action is unclear. G may stimulate bile acid uptake by increasing intracellular cAMP concentration. Cyclic cAMP dependent cell functions may be modulated by protein kinase C. The authors evaluated the interaction of G-induced cAMP formation and PKC activation on bile acid uptake by rat hepatocytes. The effect of PKC activation by Phorbol-12,13-dibutyrate (PDBu) on intracellular cAMP or taurocholate (TC) uptake in unstimulated or G-stimulated isolated rat hepatocytes was investigated. {sup 3}H-TC uptake was measured 5, 10, 20 and 30 min after the addition of G. The effect of db-cAMP {plus minus} PDBu was also evaluated. Intracellular cAMP was measured by RIA. G stimulation increased TC uptake at all time points, with maximum stimulation from 10-20 min. PDBu virtually abolished the G effect. Uptake kinetics showed only a change in Vmax. PDBu had no effect on unstimulated TC uptake. PDBu did not alter the G-stimulated intracellular cAMP rise. PDBu decreased TC uptake even when hepatocytes were treated with db-cAMP. G-stimulation of hepatic TC uptake coincided with a rise in intracellular cAMP, although only low levels of cAMP were necessary to achieve maximum TC uptake. Calciummore » depletion caused 20% decrease in basal TC uptake, but did not alter the pattern of G-stimulation of TC uptake, but did not alter the pattern of G-stimulation of TC uptake nor the inhibitory effects of PDBu. PKC down-regulates G-stimulated TC uptake via mechanisms which appear to be independent of changes in either intracellular cAMP or calcium.« less