Phalloidin suppresses force in nebulin-rich lamprey cardiac muscle

We assessed the effect of phalloidin, known to detach nebulin from actin in skeletal myofibrils, on the isometric force of skinned lamprey cardiac muscle, which has nebulin in amounts comparable to that in skeletal muscle. In contrast to mammalian cardiac muscle, which contains much less nebulin and reacts to phalloidin only by an increase in force, the lamprey cardiac muscle responds to phalloidin by a pronounced (∼ 50%) reduction in isometric force, thereby resembling the behavior of skeletal muscle. These results support our hypothesis that nebulin detachment from actin underlies phalloidin-induced force loss and suggest a role of actin-nebulin interaction in contractile function.