Protein kinase C δ and ε mediate positive inotropy in adult ventricular myocytes

To examine cardiac contractile regulation and protein kinase C (PKC) translocation in parallel, the δ and ε isoforms of PKC were fused to green fluorescent protein (GFP) and expressed in adult rat ventricular myocytes maintained in short term culture. PKC-δ-GFP and PKC-ε-GFP were predominantly cytosolic until phorbol dibutyrate (PDBu) was introduced. PKC-δ-GFP redistributed preferentially to perinuclear structures that co-localized with a Golgi marker, whereas PKC-ε-GFP redistributed preferentially to the surface sarcolemma. Myocyte contractile function was assessed by monitoring twitch shortening with field stimulation at 0.5 Hz, 22 °C. In myocytes expressing PKC-δ-GFP, PDBu caused a transient negative inotropic response followed by a robust and sustained positive inotropic response that paralleled perinuclear PKC-δ accumulation. In PKC-ε-GFP myocytes, PDBu caused a sustained negative inotropic response that paralleled accumulation at the surface sarcolemma, but this response did not differ from myocytes expressing GFP alone. At higher expression levels, PKC-ε-GFP myocytes responded more like PKC-δ-GFP myocytes including perinuclear accumulation and a sustained positive inotropic response. Positive inotropic responses were markedly attenuated if PKC translocation was biased toward the surface sarcolemma by use of a more hydrophobic PKC activator, and were completely and selectively blocked by the PKC antagonist bis-indoylmaleimide. In contrast, transient and sustained negative inotropic responses were selectively blocked by the Ca2+-dependent PKC isoform antagonist Go6976. The data indicate that the novel PKC isoforms δ and ε have little effect on contractility when accumulating at the cell surface, but produce a strong positive inotropic response upon accumulation at the Golgi or other intracellular sites.