Observing cycling of a few cross-bridges during isometric contraction of skeletal muscle.
During muscle contraction a myosin cross-bridge imparts periodic force impulses to actin. It is possible to visualize those impulses by observing a few molecules of actin or myosin. We have followed the time course of orientation change of a few actin molecules during isometric contraction by measuring parallel polarized intensity of its fluorescence. The orientation of actin reflects local bending of a thin filament and is different when a cross-bridge binds to, or is detached from, F-actin. The changes in orientation were characterized by periods of activity during which myosin cross-bridges interacted normally with actin, interspersed with periods of inactivity during which actin and myosin were unable to interact. The periods of activity lasted on average 1.2 +/- 0.4 s and were separated on average by 2.3 +/- 1.0 s. During active period, actin orientation oscillated between the two extreme values with the ON and OFF times of 0.4 +/- 0.2 and 0.7 +/- 0.4 s, respectively. When the contraction was induced by a low concentration of ATP both active and inactive times were longer and approximately equal. These results imply that cross-bridges interact with actin in bursts and suggest that during active period, on average 36% of cross-bridges are involved in force generation.
Department of Molecular Biology & Immunology, Center for Commercialization of Fluorescence Technology, University of North Texas HSC, Fort Worth, Texas 76107, USA.
P
P
Mettikolla
N
N
Calander
R
R
Luchowski
I
I
Gryczynski
Z
Z
Gryczynski
J
J
Borejdo
D000199:Actins / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D005453:Fluorescence; D021621:Imaging, Three-Dimensional; D007537:Isometric Contraction / Q000187:drug effects / Q000502:physiology*; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D009210:Myofibrils / Q000187:drug effects / Q000378:metabolism; D010590:Phalloidine / Q000378:metabolism; D011817:Rabbits; D013997:Time Factors
D000199:Actins; D010590:Phalloidine; D000255:Adenosine Triphosphate