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J Physiol Volume 578, Number 2, 563-568, January 15, 2007 DOI: 10.1113/jphysiol.2006.122648
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RAPID REPORT

Inhibition of myosin light-chain phosphorylation inverts the birefringence response of porcine airway smooth muscle

Alexander V. Smolensky1, Susan H. Gilbert1, Margaret Harger-Allen2 and Lincoln E. Ford1,2

1 Krannert Institute of Cardiology, Indiana University School of Medicine, 1800 N. Capitol Ave, Indianapolis, IN 46202, USA
2 Roudebush VA Medical Center, 1481 W. Tenth St, Indianapolis, IN 46202, USA

Muscle birefringence, caused mainly by parallel thick filaments, increases in smooth muscle during stimulation, signalling thick filament formation upon activation. The reverse occurs in skeletal muscle, where a decrease in birefringence has been correlated with crossbridge movement away from the thick filaments. When force generation by trachealis muscle was inhibited with wortmannin, which inhibits myosin light-chain phosphorylation and thick-filament formation, but not the calcium increase caused by stimulation, the birefringence response inverted, suggesting crossbridge movement similar to that of skeletal muscle. Resistance to quick stretches was much greater in stimulated muscle than in unstimulated muscle before wortmannin treatment and no different in stimulated and unstimulated muscle after force inhibition by wortmannin. Before wortmannin treatment, stimulation reduced thick-filament cross-sectional areas in electron micrographs by 44%. After force inhibition by wortmannin, filament areas were not significantly different in stimulated and unstimulated muscle and not significantly different from those of relaxed muscle without wortmannin treatment. These results suggest that myofibrillar-space calcium causes crossbridges to move away from the thick filaments without firmly attaching to thin filaments.

(Received 10 October 2006; accepted after revision 8 November 2006; first published online 9 November 2006)
Corresponding author L. E. Ford: Krannert Institute of Cardiology, Indiana University School of Medicine, 1800 N. Capitol Ave, Indianapolis, IN 46202, USA. Email: lieford{at}iupui.edu






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