Differences between rapid and longer-term actions of angiotensin II in isolated rat hepatocytes. Effects on...

Angiotensin II stimulates phosphorylase a activity and c-fos expression in isolated rat hepatocytes. Both effects are mediated through AT1 receptors. However, the time-courses and the dose-dependencies of these responses were different. These effects of angiotensin II were blocked by Losartan when the antagonist was added to the cells before the hormone or when both the hormone and the antagonist were added simultaneously. Interestingly, when the antagonist was added 2 or 3 min after the peptide hormone, the action on phosphorylase a activity was markedly decreased but, in contrast, that on c-fos expression was not affected. The data suggest that angiotensin II-mediated activation of phosphorylase a was reversible and dependent on continuous receptor activation by the hormone whereas c-fos expression did not seem to have these characteristics, i.e., our data suggest that once protooncogene expression is triggered, the process becomes independent of the hormone-receptor interaction.