PKC activation increases Ca2+ sensitivity of permeabilized lymphatic muscle via myosin light chain 20 phosphorylation-dependent and -independent mechanisms

The contractile activity of muscle cells lining the walls of collecting lymphatics is responsible for generating and regulating flow within the lymphatic system. Activation of PKC signaling contributes to the regulation of smooth muscle contraction by enhancing sensitivity of the contractile apparatus to Ca2+. It is currently unknown whether PKC signaling contributes to the regulation of lymphatic muscle contraction. We hypothesized that the activation of PKC signaling would increase the sensitivity of the lymphatic myofilament to Ca2+. To test this hypothesis, we determined the effects of PKC activation with phorbol esters [PMA or phorbol dibutyrate (PDBu)] on the contractile behavior of alpha-toxin-permeabilized rat mesenteric and cervical lymphatics or the thoracic duct. The addition of PMA or PDBu induced a significant increase in the contractile force of submaximally activated alpha-toxin-permeabilized lymphatic muscle independent of a change in intracellular Ca2+ concentration, and the Ca2+ -force relationship of lymphatic muscle was significantly left shifted, indicating greater myofilament Ca2+ sensitivity. Phorbol esters increased the maximal rate of force development, whereas the rate of relaxation was reduced. Western blot and immunohistochemistry data indicated that the initial rapid increase in tension development after stimulation by PDBu was associated with myosin light chain ( MLC)(20) phosphorylation; however, the later, steady-state Ca2+ sensitization of permeabilized lymphatic muscle was not associated with increased phosphorylation of MLC20 at Ser(19), 17-kDa C-kinase- potentiated protein phosphatase-1 inhibitor at Thr(38), or caldesmon at Ser(789). Thus, these data indicate that PKC-dependent Ca2+ sensitization of lymphatic muscle may involve MLC20 phosphorylation-dependent and -independent mechanism(s).