CROSS-BRIDGE ATTACHMENT AND STIFFNESS DURING ISOTONIC SHORTENING OF INTACT SINGLE MUSCLE-FIBERS

Equatorial x-ray diffraction pattern intensities (I10 and I11), fiber stiffness and sarcomere length were measured in single, intact muscle fibers under isometric conditions and during constant velocity (ramp) shortening. At the velocity of unloaded shortening (V(max)) the I10 ch ange accompanying activation was reduced to 50.8% of its isometric val ue, I11 reduced to 60.7%. If the roughly linear relation between numbe rs of attached bridges and equatorial signals in the isometric state a lso applies during shortening, this would predict 51-61 % attachment. Stiffness (measured using 4 kHz sinusoidal length oscillations), anoth er putative measure of bridge attachment, was 30% of its isometric val ue at V(max). When small step length changes were applied to the prepa ration (such as used for construction of T1 curves), no equatorial int ensity changes could be detected with our present time resolution (5 m s). Therefore, unlike the isometric situation, stiffness and equatoria l signals obtained during ramp shortening are not in agreement. This m ay be a result of a changed crossbridge spatial orientation during sho rtening, a different average stiffness per attached crossbridge, or a higher proportion of single headed crossbridges during shortening.