Ca2+-independent myosin II phosphorylation and contraction in chicken embryo fibroblasts.

1. Non-muscle contraction is widely believed to be mediated through Ca2+-stimulated myosin II regulatory light chain (LC20) phosphorylation, similar to the contractile regulation of smooth muscle. However, this hypothesis lacks conclusive experimental support. 2. By modulating chicken embryo fibroblast cytosolic Ca2+ concentration ([Ca2+]i), we investigated the putative role of [Ca2+]i in fetal bovine serum (FBS)-stimulated LC20 phosphorylation and force development in these cells. 3. Eliminating the FBS-stimulated rise in [Ca2+]i with the Ca2+ chelator BAPTA only partially inhibited FBS-stimulated LC20 phosphorylation and did not significantly alter the magnitude of FBS-stimulated isometric contraction. 4. Ionomycin (1 microM) produced a larger but shorter lasting rise in [Ca2+]i relative to FBS. However, ionomycin only stimulated a small and transient increase in LC20 phosphorylation and did not cause contraction. 5. We conclude that fibroblasts differ from smooth muscle in that LC20 phosphorylation and contraction are predominantly regulated independently of [Ca2+]i.