NEGATIVE DEVELOPED TENSION IN RAPIDLY SHORTENING WHOLE FROG MUSCLES

High speed isovelocity shortening using a servo-controlled lever was p erformed on isolated whole frog sartorius muscles at long lengths to e nsure substantial passive tension. The tension records of unstimulated control experiments were subtracted from the tension records of fully -tetanized experiments on the same muscles to yield the developed tens ion exerted by the contractile proteins alone. There are several main results: (1) the positive developed tension had the same relation with shortening speed observed by other researchers in single fibres with no passive tension present; (2) negative developed tension was always measured at velocities of shortening above V-max, where V-max (typical ly 1.5 muscle-lengths s(-1) at 2 degrees C) is defined as the velocity of shortening observed to yield zero developed tension; (3) negative developed tension was roughly asymptotic to -0.05 T-o, where T-o is th e developed isometric tetanic tension for the muscle length at which t he developed tension was measured during steady shortening; (4) negati ve developed tension diminished in magnitude at velocities of shorteni ng above approximately 2.5 V-max; (5) a 10 degrees C increase in tempe rature from 2 degrees C to 12 degrees C had no significant effect on t he shape of the normalized force-velocity curve (%T-o versus %V-max), but did increase V-max by a factor of 2.6 in agreement with the result s of previous studies measuring V-max in the absence of passive tensio n; (6) addition of curare in the saline bath did not affect the result s.