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Am J Physiol Cell Physiol 255: C401-C407, 1988;
0363-6143/88 $5.00
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AJP - Cell Physiology, Vol 255, Issue 3 C401-C407, Copyright © 1988 by American Physiological Society

ARTICLES

Sr2+ activates cross-bridge phosphorylation and latch state in smooth muscle

C. M. Hai and R. A. Murphy
Department of Physiology, School of Medicine, University of Virginia, Charlottesville 22908.

Sr2+ induced myosin phosphorylation and stress development in both skinned and K+-depolarized, Ca2+-depleted, intact swine carotid media. Although higher concentrations of Sr2+ than Ca2+ were required for phosphorylation and stress development, the dependence of stress on phosphorylation was the same in intact tissues. K+ depolarization in the presence of 5 mM Sr2+ produced a transient in phosphorylation (53.2 +/- 5.1% at 1 min, falling to a steady-state value of 21.7 +/- 2.0%) in Ca2+-depleted tissues in which intracellular stores were refilled with Sr2+. Stress developed without a transient (T1/2 = 0.70 min) to a steady state of 89.7 +/- 2.0% of the stress induced by K+ depolarization in the presence of 1.6 mM Ca2+ (K-PSS). Cross-bridge cycling rate as measured by isotonic shortening velocity was proportional to myosin phosphorylation throughout the contraction. When intracellular stores were not refilled with Sr2+, phosphorylation rose to a sustained value of 28.8 +/- 2.7% and stress developed slowly (T1/2 = 2.9 min) to a steady state of 95.9 +/- 1.5% K-PSS-induced stress. Therefore, an initial phosphorylation transient induced by intracellular Sr2+ release only accelerated stress development without significant effects on steady-state stress or phosphorylation (as was true for Ca2+- induced responses). We concluded that Sr2+ substitutes for Ca2+ in phosphorylation and regulation of the latch state in the swine carotid media.


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