HIGH IONIC-STRENGTH AND LOW PH DETAIN ACTIVATED SKINNED RABBIT SKELETAL-MUSCLE CROSSBRIDGES IN A LOW FORCE STATE

The effects of varying pH and ionic strength on the force-velocity rel ations and tension transients of skinned rabbit skeletal muscle were s tudied at 1-2-degrees-C. Both decreasing pH from 7.35 to 6.35 and rais ing ionic strength from 125 to 360 mM reduced isometric force by about half and decreased sarcomere stiffness by about one-fourth, so that t he stiffness/force ratio was increased by half. Lowering pH also decre ased maximum shortening velocity by approximately 29%, while increasin g ionic strength had little effect on velocity. These effects on veloc ity were correlated with asymmetrical effects on stiffness. The increa se in the stiffness/force ratio with both interventions was manifest a s a greater relative force change associated with a sarcomere length s tep. This force difference persisted for a variable time after the ste p. At the high ionic strength the force difference was long-lasting af ter stretches but relaxed quickly after releases, suggesting that the structures responsible would not impose much resistance to steady-stat e shortening. The opposite was found in the low pH experiments. The fo rce difference relaxed quickly after stretches but persisted for a lon g time after releases. Furthermore, this force difference reached a co nstant value of approximately 8% of isometric force with intermediate sizes of release, and was not increased with larger releases. This val ue was almost identical to the value of an internal load that would be sufficient to account for the reduction in maximum velocity seen at t he low pH. The results are interpreted as showing that both low pH and high ionic strength inhibit the movement of crossbridges into the for ce-generating parts of their cycle after they have attached to the act in filaments, with very few other effects on the cycle. The two interv entions are different, however, in that detained bridges can be detach ed readily by shortening when the detention is caused by high ionic st rength but not when it is caused by low pH.