Cross-bridges and sarcomere stiffness in single intact frog muscle fibers.

The number of cross-bridges formed in activated skeletal muscles is a key information for both energetics and mechanics of muscle contraction. In this study we determined the cross-bridge number in single fibers by measuring the tension Pc which forcibly detached the cross-bridge by fast stretches (Bagni et al. 2005, J. Physiol 565). Fibers, isolated from the tibialis anterior muscle of Rana esculenta, were mounted between an electromagnetic motor and a fast force transducer. Sarcomere length was measured by means of a striation follower device. Measurements were made during tetanus rise in normal Ringer and in sub-maximal tetanic contractions in Ringer-BTS (N-benzyl-p-toluene sulphonamide, 1 μM) at 5°C at sarcomere length of 2.1 µm. The results were compared with fiber stiffness, another indicator of cross-bridge number, measured with 4 kHz sinusoidal length oscillations (1 nmhs-1 p-p amplitude). The stiffness-tension relation was the same both during the tetanus rise and Ringer-BTS and showed the non-linearity expected from the myofilament compliance. However, the data could not be fitted satisfactorily with a simple model made of cross-bridge and linear filament compliances in series. A good fit was obtained by assuming that a fraction (~14%) of attached bridges at tetanus plateau was generating no-force. Relative myofilament and cross-bridge compliance resulted 0.37 and 0.63 respectively. The stretch data showed a linear relation between Pc and tension with a slope consistent with the presence of the non-force generating bridges suggested by stiffness data. These results suggests the existence of a possible non-linearity between cross-bridge force and stiffness and show that the relation between fiber stiffness and cross-bridge number is not as simple as usually assumed.