Bukatina, Anna E.

By binding to a nebulin site on actin in skeletal myofibrils phalloidin (PH) causes a force decrease (phase 1) followed by an increase in force (phase 2). The initial force decrease likely reflects a disturbance in actin‐nebulin interaction. The aim of this work was to determine the role of myosin cross‐bridges (XBs) and Ca 2+ in PH‐induced force decrease in skinned rabbit psoas fibers. Force response to PH was studied in the presence of 30 mM 2,3‐Butanedione monoxime (an agent reducing a fraction of strongly bound XBs), where we found that the phase 2 response was suppressed and PH induced 60–70% force decrease. Pretreatment with PH of relaxed (pCa 9, strongly bound XBs are absent) or rigor (in the absence of ATP, all XBs are strongly bound to actin) muscles also resulted in force loss. At the same duration of pretreatment, the remaining force was higher and more sensitive to PH in the latter case. The same properties were found for dethiophalloidin (DTPH), where it was also shown that the force loss was essentially the same at pCa 4 and pCa 9. We concluded that rigor XBs, but not Ca 2+ , affect the rate of PH binding to actin in myofibrils. The XBs mediated changes in nebulin site on actin are supportive for suggested regulatory nebulin role. Supported by NIH grants HL68555 and HL37680.