In an effort to test the lever arm model of force generation, the effects o
f replacing magnesium with calcium as the ATP-chelated divalent cation were
determined for several myosin and actomyosin reactions, The isometric forc
e produced by glycerinated muscle fibers when CaATP is the substrate is 20%
of the value obtained with MgATP, For myosin subfragment 1 (S1), the degre
e of lever arm rotation, determined using transient electric birefringence
to measure rates of rotational Brownian motion in solution, is not signific
antly changed when calcium replaces magnesium in an S1-ADP-vanadate complex
. Actin activates S1 CaATPase activity, although less than it does MgATPase
activity, The increase in actin affinity when S1 . CaADP . P-1 is converte
d to S1 . CaADP is somewhat greater than it is for the magnesium case. The
ionic strength dependence of actin binding indicates that the change in app
arent electrostatic charge at the acto-S1 interface for the S1 . CaADP . P-
i to S1 . CaADP step is similar to the change when magnesium is bound. In g
eneral, CaATP is an inferior substrate compared to MgATP, but all the data
are consistent with force production by a lever arm mechanism for both subs
trates, Possible reasons for the reduced magnitude of force when CaATP is t
he substrate are discussed.