PKCβ _II modulation of myocyte contractile performance

Significant up-regulation of the protein kinase Cβ _II (PKCβ _II) develops during heart failure and yet divergent functional outcomes are reported in animal models. The goal here is to investigate PKCβ _II modulation of contractile function and gain insights into downstream targets in adult cardiac myocytes. Increased PKCβ _II protein expression and phosphorylation developed after gene transfer into adult myocytes while expression remained undetectable in controls. The PKCβ _II was distributed in a peri-nuclear pattern and this expression resulted in diminished rates and amplitude of shortening and re-lengthening compared to controls and myocytes expressing dominant negative PKCβ _II (PKCβDN). Similar decreases were observed in the Ca ²⁺ transient and the Ca ²⁺ decay rate slowed in response to caffeine in PKCβ _II-expressing myocytes. Parallel phosphorylation studies indicated PKCβ _II targets phosphatase activity to reduce phospholamban (PLB) phosphorylation at residue Thr17 (pThr17-PLB). The PKCβ inhibitor, LY379196 (LY) restored pThr17-PLB to control levels. In contrast, myofilament protein phosphorylation was enhanced by PKCβ _II expression, and individually, LY and the phosphatase inhibitor, calyculin A each failed to block this response. Further work showed PKCβ _II increased Ca ²⁺-activated, calmodulin-dependent kinase IIδ (CaMKIIδ) expression and enhanced both CaMKIIδ and protein kinase D (PKD) phosphorylation. Phosphorylation of both signaling targets also was resistant to acute inhibition by LY. These later results provide evidence PKCβ _II modulates contractile function via intermediate downstream pathway(s) in cardiac myocytes.