Instrumentation has been developed to detect rapidly the polarization of tryptophan fluorescence from single muscle fibers in rigor, relaxation, and contraction. The polarization parameter (P) obtained by exiciting the muscle tryptophans with light polarized perpendicular to the long axis of the muscle fiber had a magnitude P (relaxation) > P (contraction) > P (rigor) for the three types of muscle fibers examined (glycerinated rabbit psoas, glycerinated dorsal longitudinal flight muscle of Lethocerus americanus, and live semitendinosus of Rana pipiens). P from single psoas fibers in rigor was found to increase as the sarcomere length increased but in relaxed fibers P was independent of sarcomere length. After rigor, pyrophosphate produced little or no change in P, but following an adenosine triphosphate (ATP)-containing solution, pyrophosphate produced a value of P that fell between the contraction and relaxation values. Sinusoidal or square wave oscillations of the muscle of amplitude 0.5–2.0% of the sarcomere length and frequency 1, 2, or 5 Hz were applied in rigor when the myosin cross-bridges are considered to be firmly attached to the thin filaments. No significant changes in P were observed in either rigor or relaxation. The preceding results together with our present knowledge of tryptophan distribution in the contractile proteins has led us to the conclusion that the parameter P is a probe of the contractile state of myosin which is probably sensitive to the orientation of the myosin S1 subfragment.

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