title: Z/I and A-band lattice spacings in frog skeletal muscle: effects of contraction and osmolarity.
abstract: A-band and Z-line/I-band lattice spacings were measured by small-angle X-ray diffraction from relaxed and isometrically-contracting whole frog sartorius muscles with lattice spacings reduced or swollen by changing the osmolarity of the bathing solution. A-band spacing increased by approximately 3% upon isometric contraction at reduced lattice spacings (245-356 mOsm) and decreased by approximately 1% at swollen spacings (172 mOsm), similarly to the behaviour of skinned muscles upon changing from the relaxed state to rigor. The Z/I lattice underwent a significant lattice expansion (3-8%) upon isometric contraction at all osmolarities, in qualitative agreement (but quantitative disagreement) with results from electron microscopy on mammalian skeletal muscle. Lattice areas calculated for the Z/I and A-band lattices indicate a barrel-shaped sarcomere in the resting state, which may provide a partial explanation for how longitudinal forces produced in the A-band can produce a radial expansive force in the Z-line during contraction. The radial component of cross-bridge stiffness was calculated from the A-band data for contracting muscle, using a lattice stability model incorporating structural, osmotic and electrostatic forces. The calculations gave a radial cross-bridge stiffness during contraction of about 9 x 10(5) N m-2, and outward radial force per thick filament in normal Ringer's solution of 6 x 10(-9) N, corresponding to a radial force per cross-bridge of 10(-11) N.
authors.affiliation: Department of Physics, University of Guelph, Ontario, Canada.
authors.forename: T C
authors.initials: TC
authors.lastname: Irving
authors.forename: Q
authors.initials: Q
authors.lastname: Li
authors.forename: B A
authors.initials: BA
authors.lastname: Williams
authors.forename: B M
authors.initials: BM
authors.lastname: Millman
mesh_terms: D000818:Animals; D004558:Electric Stimulation; D006982:Hypertonic Solutions / Q000494:pharmacology*; D007038:Hypotonic Solutions / Q000494:pharmacology*; D007537:Isometric Contraction; D007552:Isotonic Solutions / Q000737:chemistry / Q000494:pharmacology*; D008322:Mammals / Q000502:physiology; D018482:Muscle, Skeletal / Q000187:drug effects / Q000648:ultrastructure*; D009994:Osmolar Concentration; D011894:Rana pipiens / Q000502:physiology*; D000077331:Ringer's Solution; D012518:Sarcomeres / Q000648:ultrastructure; D055672:Static Electricity; D013314:Stress, Mechanical; D014961:X-Ray Diffraction
chemical_list: D006982:Hypertonic Solutions; D007038:Hypotonic Solutions; D007552:Isotonic Solutions; D000077331:Ringer's Solution