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Published ahead of print on September 18, 2003, doi:10.1165/rcmb.2003-0212OC

Am. J. Respir. Cell Mol. Biol., Volume 30, Number 4, April 2004, 548-554

A more recent version of this article appeared on April 1, 2004
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Submitted on June 9, 2003
Revised on September 16, 2003

ENHANCED MYOSIN PHOSPHATASE AND Ca2+-UPTAKE MEDIATE ADRENERGIC RELAXATION OF AIRWAY SMOOTH MUSCLE

Luke J Janssen1*, Tracy Tazzeo1, and Jianmin Zuo1

1 Medicine, McMaster University, Hamilton, Ontario, Canada

* To whom correspondence should be addressed. E-mail: janssenl{at}mcmaster.ca.

We examined the mechanisms underlying relaxations evoked by isoproterenol (Iso) in isolated porcine, bovine or human tracheal and bronchial tissues (TSM and BSM, respectively). Iso had little effect against contractions evoked by high KCl, indicating that it does not directly suppress voltage-dependent Ca2+-influx nor directly inhibit myosin light chain kinase. Furthermore, Iso was equally potent against carbachol (CCh) contractions in the presence versus absence of nifedipine (10-6M), establishing that the primary action of Iso is not through membrane hyperpolarization. However, Iso-relaxations in porcine/bovine BSM were significantly suppressed by inhibitors of the internal Ca2+ pump (cyclopiazonic acid; 10-5M) or of myosin light chain phosphatase (calyculin; 10-6M). Myosin light chain phosphatase activity was assayed directly (using 32P-labeled myosin) and found to be enhanced in a time- and concentration-dependent fashion by Iso. Iso-relaxations in human airway tissues, on the other hand, were not significantly affected by either calyculin or cyclopiazonic acid. Thus, we conclude that Iso acts largely in a voltage-independent fashion: in non-human airways, this involves enhanced Ca2+ pump activity (to decrease [Ca2+]i) and myosin light chain phosphatase activation (to decrease Ca2+-sensitivity of the contractile apparatus), while in human airways the underlying mechanisms are still unclear.




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