Measurement of pulse propagation in single permeabilized muscle fibers by optical diffraction.

Abstract

The advent of tissue engineering has necessitated the characterization of the viscoelastic behavior of structural tissues of the body, including skeletal muscle. The purpose was to test the hypotheses that the viscoelastic behavior of relaxed and partially activated single muscle fibers is dependent upon strain pulse amplitude and strain rate. The instrumentation to test this hypothesis was designed and built. Chemically permeabilized single muscle fibers from soleus muscles of male F344 rats were tested in the relaxed and partially activated condition at 15$\sp\circ$C. The first order optical diffraction pattern was detected at two positions along single muscle fibers. A servo motor imposed single longitudinal strain pulses on the fibers. Data were analyzed to determine the pulse propagation velocity and attenuation coefficient. The pulse velocity and the attenuation coefficient were measured for strain pulse amplitudes from 0.5% to 10%, and pulse frequencies from 250 Hz to 2 kHz. During shortening pulses of both relaxed and partially activated fibers, buckling of fibers prevented analysis of the data. Under all other conditions, the attenuation coefficient was negligible. For both relaxed and partially activated muscle fibers, the maximum pulse propagation velocity occurred at strain amplitudes of from 1% to 5%. The peak velocity represents a 9- to 25-fold increase in the elastic modulus when compared with the elastic modulus for strain pulse amplitudes both below and above 1% to 5% strain. In both relaxed and partially activated muscle fibers, the repeated emergence and disappearance of a peak stiffness with increasing strain pulse amplitude suggested a phenomenon of recoverable yield. The recoverable yield was hypothesized to result from the attachment and detachment under strain of weakly-bound cross-bridges. The phenomenon of steps and pauses observed in every sarcomere length-time record was hypothesized to be caused by an optical artifact. The source of the artifact was identified as interference from scattering sources in the optical path. The instrumentation was modified by employing a diode laser module with reduced coherence length. The modification eliminated the artifact and permitted more accurate measurements of strain pulse amplitude.