FORCE-LOAD-VELOCITY RELATION AND THE INTERNAL LOAD OF TETANIZED FROG CARDIAC MUSCLE

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Abstract
A frog ventricular muscle strip could be fully tetanized by AC stimulation at 10 Hz and 20 V/cm in a solution containing 9 mM Ca2+. During isometric tetanus the controlled release was made and the shortening velocities against various loads were measured. The isometric force was varied by reducing the stimulus intensity in K+-rich solution, or by reducing the external Ca2+ concentration. The force-load-velocity relation was described by a simple hyperbolic equation: (P+A)(v+b)=b(F+A), A=(F/Fm)a for shortening, and (2F-P+A'')(-v+b'')=b''(F+A''), A'' = (F/Fm)a'' for lengthening, where F is the isometric force; Fm is the maximum isometric force at the optimal muscle length, Lm; P is the load; v is the velocity; a, b, a'' and b'' are constants. The values of constants were a/Fm = 0.51, b = 0.75 Lm/s for shortening and a''/Fm = 0.39, b'' - 0.75 Lm/s for lengthening at 20.degree. C. At muscle lengths shorter than 0.92 Lm, the internal load, defined as the difference between the external load and calculated load at a given velocity, increased in proportion to both the velocity and the decrease in muscle length.