INSECT FLIGHT MUSCLE HOT, COLD, PULLED AND PULLING

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We use the indirect flight muscle of the giant water bug Lethocerus (IFM) as a highly ordered model system for studying muscle structure and function. We have recorded synchrotron x-ray diffraction patterns from relaxed, rigor and active (isometric contraction =High Static Tension [HST] at pCa4.5) states of glycerinated Lethocerus (IFM), to explore how its structural dynamics can unify with those of vertebrate muscles including frog skeletal muscle (FSM). Using a 5m camera we were able to detect net axial motions of myosin head mass toward or away from the myosin filament midpoint by resolving relative intensity changes within the rich interference-splitting fine structure of the IFM 14.5nm meridional reflection. When ramp-stretched 1.5-2.0%, the high passive stiffness of Relaxed-State IFM sustains passive tensions to 30-60% of active isometric F0, but the 14.5nm spacing and thick filament length increase together by only 0.02%. Thus 99% of relaxed sarcomere extension must occur in the C-filament/Z-band linkages that transmit passive tension to thick filaments. No change of packing or shape within crown-shelves of relaxed myosin heads is detected interferometrically. Upon Relaxed = Isometric Activation, IFM 14.5nm meridional changes differ from those reported for FSM. Spacing elongates ~0.07% (compared to FSM 1.3%), total intensity (due to mix of attached and detached heads) drops ~40% (FSM changes ¿¿10%; detached head contribution uncertain). X-ray interferometry signals no net change in averaged IFM crossbridge angle or shape (FSM rearranges substantially). On the 19.3 nm layer line, 10-fold brightening of the first row-line spot signals crossbridge labeling of actin Target Zones, while a new outer streak signals steric unblocking of thin filaments by a tropomyosin shift. Ramp-stretch experiments on HST and rigor are being analyzed. These results can be compared and contrasted with those from vertebrate muscle to achieve a more unified picture of the fundamental force generating events in muscle.