Endothelin-1 (ET-1) is known to increase the mitotic response of different growth factors but also to different vasoactive hormones already at threshold concentrations. The aim of this study was to investigate the influence of chronic elevated ET-1 concentrations on protein kinase C (PKC) isoforms in vitro and in vivo. Smooth muscle cells were incubated with ET-1 at a concentration of 10 mol/L. The incubation lasted for 1-96 hours. For in vivo studies, rats were chronically infused with ET-1 for 4 weeks using minipumps. Specific antibodies were used to determine the amount of PKC isoform in western blots. Incubation of smooth muscle cells with ET-1 revealed an increase in PKC-alpha (48 hours, 314 +/- 31.7%). In vivo PKC-alpha was augmented to 166 +/- 17.5%. In vitro PKC-epsilon showed an elevated concentration after 18 hours of 213 +/- 15.9% (maximum, 339 +/- 4.5% at 72 hours). In vivo PKC-epsilon was elevated to 162 +/- 4.2%. In the immunofluorescence microscopy after 48 hours of ET-1 incubation PKC-alpha was localized in the cytoplasm. Addition of angiotensin II resulted in a translocation of it into the nucleus. These data show that chronic elevated ET-1 concentrations modulate PKC isoforms. This might explain the increased response to different vasoactive hormones after ET-1 incubation.