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Am J Physiol Heart Circ Physiol (May 20, 2005). doi:10.1152/ajpheart.00318.2004
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Submitted on March 30, 2004
Accepted on May 12, 2005

{alpha}Modulation of myosin phosphatase targeting subunit and protein phosphatase 1 in the heart

Ravi Rajashree1, Bradford C Blunt1, and Polly A Hofmann1*

1 Physiology, University of Tennessee, Memphis, TN, USA

* To whom correspondence should be addressed. E-mail: phofmann{at}physio1.utmem.edu.

Myosin light chain 2 (LC2) phosphorylation is of both physiologic and pathologic importance to myocardial function. The phosphatase that directly dephosphorylates LC2 is a type 1 protein phosphatase (PP1) containing a catalytic subunit that complexes with a myosin-binding phosphatase targeting subunit (MYPT). The goal of the present study was to examine the role of MYPT in the regulation of PP1 in ventricular myocytes. In the first part of the study, MYPT expression and phosphorylation were determined to have a regional distribution in the unstimulated heart. The pattern of MYPT expression and phosphorylation is inversely related to the LC2 phosphorylation spatial gradient described by Epstein and colleagues (Cell 2001 107:631). In the second part of the study, adult rat isolated ventricular myocytes were exposed to an {alpha}-adrenergic receptor agonist, and properties of MYPT, PP1 and LC2 studied. We found MYPT associates with cardiac myofilaments, and this association increases upon {alpha}-adrenergic receptor stimulation. Alpha-adrenergic activation also led to a decrease in the PP1-myofilament association. Further, {alpha}-adrenergic receptor stimulation results in phosphorylation of MYPT and LC2, and an increase in myocyte Ca2 sensitivity of tension that all depend on Rho kinase activation. These data support the hypothesis that {alpha}-adrenergic receptor activation works through Rho kinase to phosphorylate MYPT, and that phospho-MYPT dissociates from PP1 so that PP1 is no longer physically associated with LC2. Hence, we propose a pathway for the dynamic modulation of LC2 phosphorylation through receptor-dependent phosphorylation of MYPT, and a spatial gradient of LC2 phosphorylation under basal conditions that occurs due to varied levels of expression and phosphorylation of MYPT in the heart.




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