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A Magid

Publications and source records attributed to A Magid.

4 recordsLinked to original sources
PubMed1985

Myofibrils bear most of the resting tension in frog skeletal muscle.

The tension that develops when relaxed muscles are stretched is the resting (or passive) tension. It has recently been shown that the resting tension of intact skeletal muscle fibers is equivalent to that of mechanically skinned skeletal muscle fibers. Laser diffraction measurements of sarcomere length have now been used to show that the exponential relation between resting tension and sarcomere length for whole frog semitendinosus muscle is similar to that of single fibers. Slack sarcomere lengths and the rates of stress relaxation in these muscles were similar to those in skinned fibers, and sarcomere length remained unchanged during stress relaxation, as in skinned fibers. Thus, in intact semitendinosus muscle of the frog up to a sarcomere length of about 3.8 micrometers, resting tension arises, not in the connective tissue as is commonly thought, but in the elastic resistance of the myofibrils.

Animals↗
PubMed1984

Connecting filaments, core filaments, and side-struts: a proposal to add three new load-bearing structures to the sliding filament model.

This report concerns structural forces in resting muscle and proposes three additions to the sliding filament model to account for these mechanical properties. The proposal includes: connecting filaments (C-filaments) which connect the ends of each thick filament to the neighboring Z-lines, core filaments which support the myosin of the thick filament and which attach to the C-filaments, and side-struts which bind the thick filaments together along their length and restrict their radial movement. C-filaments would act as the parallel elastic element and transmit the passive tension to the thick filaments. Isolated myofibrils (mechanically-skinned and detergent-treated frog semitendinosus fibers) when stretched progressively showed exponentially-increasing passive tension which did not disappear when filament overlap was exceeded, but continued to rise. SL was monitored with a HeNe laser. Passive tension phasically exceeded 3 X 10(5) N/M2. Electron microscopy (thin-sectioned and freeze-fracture/deep-etch specimens) of non-overlap fibers showed orderly fibril structure with clear separation of A- and I-bands. In the gap between them could be seen filaments, 40-50 A in diameter, connected to the thick filament ends. Unlike actin, these filaments did not become decorated by myosin S-1. Equatorial X-ray measurements showed that stretching relaxed skinned muscles squeezed the thick filaments closer; this radial compression continued beyond filament overlap. Extreme stretch of fibers caused the thick filaments to strain several-fold. Treatment of non-overlap fibers with a high ionic strength pyrophosphate myosin solvent caused a large drop in passive tension and stiffness, but no change in SL was detected nor was myofibril continuity detectably affected. Non-overlap fibrils, when treated with elastase, released A-segments which retain three-dimensional coherency . Deep-etch EM's of non-overlap fibers disclosed abundant structures (about 75 A) wide attaching adjacent thick filaments.

Animals↗
PubMed1980

X-ray diffraction observations of chemically skinned frog skeletal muscle processed by an improved method.

Whole frog sartorius muscles can be chemically skinned in approximately 2 h by relaxing solutions containing 0.5% Triton X-100. The intensity and order of the X-ray diffraction pattern from living muscle is largely retained after such skinning, indicating good retention of native structure in fibrils and filaments. Best X-ray results were obtained using a solution with (mM): 75 K acetate; 5 Mg acetate; 5 ATP; 5 EGTA; 15 K phosphate, 2% PVP, pH 7.0. Equatorial X-ray patterns showed that myofibrils swell after detergent skinning, as also observed after mechanical skinning. This swelling could be reversed by adding high molecular weight colloids (PVP or dextran) to the extracting solution. By finding the colloid osmotic pressure needed to restore the in vivo interfilament spacing (3% PVP, 4 X 10(4) mol wt) the swelling pressure was estimated as 35 Torr in a standard KCl-based relaxing solution. The swelling pressure and the extent of swelling were less than acetate replaced chloride as the major anion. Detergent-skinned muscle lost the constant-volume relation between sarcomere length and lattice spacing seen in intact muscle. Changes in A band spacing were paralleled by changes in I and band-Z line spacing at a constant sarcomere length. After detergent skinning, I1,0 rose while I1,1 fell, a change in the relaxing direction. Since raising the calcium ion concentrations from pCa 9 to PCa 6.7 was without effect on equatorial or axial X-ray patterns, we concluded that these intensity changes were not due to calcium-dependent cross-bridge movement but rather to disordering of thin filaments in the A band.

Animals↗