AJP - Cell Physiology, Vol 262, Issue 6 C1388-C1391, Copyright © 1992 by American Physiological Society
Steady-state dependence of stress on cross-bridge phosphorylation in the swine carotid media
P. Di Blasi, D. Van Riper, R. Kaiser, C. M. Rembold and R. A. Murphy
Department of Physiology, University of Virginia, Charlottesville 22908.
Tonic contractions of the swine carotid media are typically characterized
by initial transients in myoplasmic [Ca2+] and cross-bridge phosphorylation
followed by force maintenance with reduced intracellular [Ca2+] and
cross-bridge phosphorylation ("latch"). The presence of effective
mechanisms in the carotid media to limit steady-state myoplasmic [Ca2+] and
cross-bridge phosphorylation to modest increases over resting values has
limited experimental attempts to determine the dependence of active stress
(force/tissue cross-sectional area) on cross-bridge phosphorylation. In
this study, we employed stimulation protocols that combined effective
contractile agonists with inhibitors of Ca2+ extrusion or sequestration to
achieve high steady-state levels of cross-bridge phosphorylation (up to
60%). Increases in cross-bridge phosphorylation from 30 to 60% were not
associated with significant increases in stress in agreement with the
predictions of Hai and Murphy [Am. J. Physiol. 254 (Cell Physiol. 23):
C99-C106, 1988] four-state cross-bridge model for the carotid media. Thus
cross-bridge phosphorylation may suffice to determine force generation in
vascular smooth muscle if both phosphorylated and dephosphorylated attached
cross bridges (or latch bridges) contribute to active stress.
