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

1. Non-muscle contraction is widely believed to be mediated through Ca2+-st imulated myosin II: regulatory light chain (LC20) phosphorylation, similar to the contractile regulation of smooth muscle. However, this hypothesis la cks conclusive experimental support. 2. By modulating chicken embryo fibroblast cytosolic Ca2+ concentration ([C a2+](i))], we investigated the putative role of [Ca2+](i) in fetal bovine s erum (FBS)-stimulated LC20 phosphorylation and force development in these c ells. 3. Eliminating the FBS-stimulated rise in [Ca2+](i) with the Ca2+ chelator BAPTA only partially inhibited PBS-stimulated LC20 phosphorylation and did not significantly alter the magnitude of FBS-stimulated isometric contracti on. 4. Ionomycin (1 mu M) produced a larger but shorter lasting rise in [Ca2+]( i) relative to FBS. However, ionomycin only stimulated a small and transien t increase in LC20 phosphorylation and did not cause contraction. 5. We conclude that fibroblasts differ from smooth muscle in that LC20 phos phorylation and contraction are predominantly regulated independently of [C a2+](i).