FORCE-VELOCITY RELATIONS OF RAT CARDIAC MYOSIN ISOZYMES SLIDING ON ALGAL CELL ACTIN CABLES IN-VITRO

The difference in kinetic properties between two myosin isozymes (V-1 and V-3) in rat ventricular myocardium was studied by determining the steady-state force-velocity (P-V) relations in the ATP-dependent movem ent of V-1 and V-3-coated polystyrene beads on actin cables of giant a lgal cells mounted on a centrifuge microscope. The maximum unloaded ve locity of bead movement was larger for V-1 than for V-3. The velocity of bead movement decreased with increasing external load applied by th e centrifuge microscope, and eventually reached zero when the load was equal to the maximum isometric force (P-0) generated by the myosin he ads. The maximum isometric force P-0 was less than 10 pN, and did not differ significantly between V-1 and V-3. The P-V curves consisted of a hyperbolic part in the low force range and a non-hyperbolic part in the high force range. The critical force above which the curve deviate d from the hyperbola was much smaller for V-1 than for V-3. An analysi s using a model with an extremely small number of myosin heads involve d in the bead movement suggested a marked difference in kinetic proper ties between V-1 and V-3.