EFFECTS OF INORGANIC-PHOSPHATE ANALOGS ON STIFFNESS AND UNLOADED SHORTENING OF SKINNED MUSCLE-FIBERS FROM RABBIT

1. We examined the effects of aluminofluoride (AIF(x)) and orthovanadate (V(i)), tightly binding analogues of orthophosphate (P(i)), on the mechanical properties of glycerinated fibres from rabbit psoas muscle. Maximum Ca2+-activated force, stiffness, and unloaded shortening velocity (V(us)) were measured under conditions of steady-state inhibition (up to 1 mm of inhibitor) and during the recovery from inhibition. 2. Stiffness was measured using either step or sinusoidal (1 kHz) changes in fibre length. Sarcomere length was monitored continuously by helium-neon laser diffraction during maximum Ca2+ activation. Stiffness was determined from the changes in sarcomere length and the corresponding changes in force. V(us) was measured using the slack test method. 3. AIF(x) and V(i) each reversibly inhibited force, stiffness and V(us). Actively cycling cross-bridges were required for reversal of these inhibitory effects. Recovery from inhibition by AIF(x) was 3- to 4-fold slower than that following removal of V(i). 4. At various degrees of inhibition, AlF(x), and V(i) both inhibited steady-state isometric force more than either V(i) or stiffness. For both AlF(x) and V(i), the relatively greater inhibition of force over stiffness persisted during recovery from steady-state inhibition. We interpret these results to indicate that the cross-bridges with AlF(x) or V(i) bound are analogous to those which occur early in the cross-bridge cycle.