Cross-bridge kinetics in rat myocardium: effect of sarcomere length and calcium activation.
We tested the hypotheses that Ca(2+) concentration ([Ca(2+)]) and sarcomere length (SL) modulate force development via graded effects on cross-bridge kinetics in chemically permeabilized rat cardiac trabeculae. Using sinusoidal length perturbations, we derived the transfer functions of stiffness over a range of [Ca(2+)] at a constant SL of 2.1 micrometer (n = 8) and at SL of 2.0, 2.1, and 2.2 micrometer (n = 4). We found that changes in SL affected only the magnitude of stiffness, whereas [Ca(2+)] affected the magnitude and phase-frequency relations. The data were fit to complex functions of two exponential processes. The characteristic frequencies (b and c) of these processes are indexes of cross-bridge kinetics, with b relating to cross-bridge attachment to and c to detachment from certain non-force-generating states. Both were significantly affected by [Ca(2+)], with an increase in b and c of 140 and 44%, respectively, over the range of [Ca(2+)] studied (P < 0.01). In contrast, SL had no effect on the characteristic frequencies (P > 0.6). We conclude that Ca(2+) activation modulates force development in rat myocardium, at least in part, via a graded effect on cross-bridge kinetics, whereas SL effects are mediated mainly by recruitment of cross bridges.
Wannenburg
T
T
Section on Cardiology, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157, USA. twannen@wfubmc.edu
Heijne
G H
GH
Geerdink
J H
JH
Van Den Dool
H W
HW
Janssen
P M
PM
De Tombe
P P
PP
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D006321:Heart / Q000502:physiology*; D066298:In Vitro Techniques; D007700:Kinetics; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D009200:Myocardial Contraction / Q000502:physiology*; D009206:Myocardium / Q000378:metabolism* / Q000648:ultrastructure; D051381:Rats; D012518:Sarcomeres / Q000502:physiology* / Q000648:ultrastructure
D002118:Calcium