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Evidence for absence of latch-bridge formation in muscular saphenous arteries

¹Departments of Biochemistry and Pediatrics, ²Department of Physiology, ³Department of Mechanical Engineering, Virginia Commonwealth University School of Medicine, Richmond, Virginia; and ⁴Department of Biological Sciences, Marquette University, Milwaukee, Wisconsin

Submitted 12 September 2005 ; accepted in final form 31 January 2006

Large-diameter elastic arteries can produce strong contractions indefinitely at a high-energy economy by the formation of latch bridges. Whether downstream blood vessels also use latch bridges remains unknown. The zero-pressure medial thickness and lumen diameter of rabbit saphenous artery (SA), a muscular branch of the elastic femoral artery (FA), were, respectively, approximately twofold and half-fold that of the FA. In isolated FA and SA rings, KCl rapidly (<16 s) caused strong increases in isometric stress (1.2 x 10⁵ N/m²) and intracellular Ca²⁺ concentration ([Ca²⁺]_i; 250 nM). By 10 min, [Ca²⁺]_i declined to 175 nM in both tissues, but stress was sustained in FA (1.3 x 10⁵ N/m²) and reduced by 40% in SA (0.8 x 10⁵ N/m²). Reduced tonic stress correlated with reduced myosin light chain (MLC) phosphorylation in SA (28 vs. 42% in FA), and simulations with the use of the four-state kinetic latch-bridge model supported the hypothesis that latch-bridge formation in FA, but not SA, permitted maintenance of high stress values at steady state. SA expressed more MLC phosphatase than FA, and permeabilized SA relaxed more rapidly than FA, suggesting that MLC phosphatase activity was greater in SA than in FA. The ratio of fast-to-slow myosin isoforms was greater for SA than FA, and on quick release, SA redeveloped isometric force faster than FA. These data support the hypothesis that maintained isometric force was 40% less in SA than in FA because expressed motor proteins in SA do not support latch-bridge formation.

vascular smooth muscle; myosin light chain phosphorylation; KCl; myosin isoforms; calcium

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