Stretch activation and nonlinear elasticity of muscle cross-bridges.

When active insect fibrillar flight muscle is stretched, its ATPase rate increases and it develops "negative viscosity," which allows it to perform oscillatory work. We use a six-state model for the cross-bridge cycle to show that such "stretch activation" may arise naturally as a nonlinear property of a cross-bridge interacting with a single attachment site on a thin filament. Attachment is treated as a thermally activated process in which elastic energy must be supplied to stretch or compress the cross-bridge spring. We find that stretch activation occurs at filament displacements where, before the power stroke, the spring is initially in compression rather than in tension. In that case, pulling the filaments relieves the initial compression and reduces the elastic energy required for attachment. The result is that the attachment rate is enhanced by stretching. The model also displays the "delayed tension" effect observed in length-step experiments. When the muscle is stretched suddenly, the power stroke responds very quickly, but there is a time lag before dissociation at the end of the cycle catches up with the increased attachment rate. This lag is responsible for the delayed tension and hence also for the negative viscosity.

A theory of tension fluctuations due to muscle cross-bridges.

We present an analytical theory for the spectrum of tension fluctuations due to muscle cross-bridges. The theory is based upon the Langevin theory of brownian motion, and is illustrated using a simplified three-state model for the cross-bridge cycle, which is intended to model cross-bridges in fibrillar insect flight muscle. Langevin white-noise sources, representing fluctuations in the net transition rates for each step in the cycle, are introduced into the rate equations, and their strengths are adjusted to give the correct mean-square fluctuations in the occupation probabilities. The Langevin theory shows that the noise is closely related to the elastic properties of cross-bridges, and it also shows in detail how each step in the cross-bridge cycle contributes differently to the noise spectrum. We find that the total noise increases with filament displacement. For small filament displacements, the noise is dominated by the power stroke and by dissociation at the end of the cycle. These contributions increase in the region of stretch activation, whilst at larger displacements, where the cross-bridge becomes locked in the strong-binding state, the noise is much larger and is dominated by attachment and detachment at the beginning of the cycle. The cross-bridge properties in this regime are strongly affected by free inorganic phosphate. Finally, we show how the noise spectrum is modified by the inclusion of a series compliance representing a practical force transducer.

Penetration, excretion and metabolism of 14C malathion in susceptible and resistant strains of Plutella xylostella.

1. The rate of penetration 14C malathion into the larvae of the diamondback moth, Plutella xylostella L., was found to be significantly different in an R-strain and S-strain. The rate of penetration was more rapid in the R-strain during the first hour after treatment. 2. The rate of metabolism in vivo and the rate of excretion were also higher in the R-strain compared with the S-strain. 3. The main metabolites produced in both strains were malathion dicarboxylic acid and desmethyl malathion. 4. The mechanism of insecticide resistance in Plutella xylostella L. is multifactorial, and involves a higher rate of penetration into the larvae of the R-strain, a higher activity of enzymes involved in its metabolism, and a higher level of excretion of the toxic compounds from the body of the R-strain compared with the S-strain.

Cholinergic nicotinic receptors in the vestibular epithelia.

Receptor binding studies specific for nicotinic cholinergic receptors have been carried out on isolated vestibular epithelia of the frogs Rana catesbiana and Rana temporaria. Evidence is presented for the presence of nicotinic-like cholinergic receptors specifically associated with the sensory areas.

Optical diffraction by well-ordered muscle fibres.

We have studied the diffraction of a focussed laser beam by single fibres of glycerinated rabbit psoas muscle as a function of the angle of incidence. Diffraction efficiencies as high as 34% were observed at the first-order Bragg angle, indicative of well-ordered striated fibres with a strong periodic modulation of the refractive index. A theory is presented to account for our observations based upon the coupled-wave model developed by Kogelnik (1967) and Magnusson and Gaylord (1977) for the description of thick phase gratings in holography. We have solved the coupled-wave equations on a computer using a realistic index modulation taken from the measurements of Huxley and Hanson (1957). Comparison of theory with experiment shows that coupled-wave effects are indeed present in well-ordered muscle fibres, and the observed diffraction efficiency is in quite good agreement with what would be expected theoretically. Most importantly, the computer model allows us to calculate the diffraction efficiency for curved striations, which are observed for real muscle fibres under a microscope. The sensitivity of the diffraction efficiency to curvature of the striations may have implications for the interpretation of other optical experiments on muscle. We also consider the effects on our measurements of the focussing lens and refraction by the cylindrical fibre.

Observation of Brillouin scattering from single muscle fibres.

The propagation of sound waves along relaxed single fibres of glycerinated rabbit psoas muscle has been observed using Brillouin scattering at frequencies up to 1.6 GHz. Two types of waves were observed: one with a velocity of 1508 +/- 7 m s-1, which is attributed to sound waves in intra-cellular saline, the other with a velocity of 912 +/- 25 m s-1, which is attributed to waves propagating along the protein filaments within individual sarcomeres. The latter sound velocity is much higher than that which has been reported by Stienen and Blangé (1985) for 50 microseconds tension transients, and the difference is attributed to the much higher stiffness of the protein filaments compared to the cross-bridges which determine the low-frequency elasticity of muscle fibres.

Metabolic fate of [14C]dieldrin in Schistocerca gregaria with particular reference to the nervous system.

1. In the metabolism of [14C]dieldrin in Schistocerca gregaria, four metabolites of dieldrin were detected. Three of these have been identified as cis-aldrindiol, trans-aldrindiol and seco-aldrin dicarboxylic acid. 2. Apart from the formation of cis and trans-aldrindiol, another metabolite, M1, was detected in the nervous system. 3. The uptake of the dose of dieldrin was maximal in nerve cords followed by thoracic muscles and gut. Degradation of dieldrin was, however, not detected in thoracic muscles.