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Articles in PresS, published online ahead of print September 26, 2002
Am J Physiol Heart Circ Physiol, 10.1152/ajpheart.00698.2002
Submitted on August 7, 2002
Accepted on September 23, 2002
1 Weis Center for Research, Geisinger Medical Center, Danville, PA, USA
2 Weis Center for Research, Geisinger Medical Center, Danville, PA, USA; Department of Medicine, Geisinger Medical Center, Danville, PA, USA
* To whom correspondence should be addressed. E-mail: jcheung{at}geisinger.edu.
Previous studies showed that overexpression of phospholemman (PLM) affected contractile function and Ca2+ homeostasis in adult rat myocytes. We tested the hypothesis that PLM modulated Na+/Ca2+ exchange (NCX1) activity. PLM was overexpressed in adult rat myocytes by adenovirus-mediated gene transfer. After 72h, half-time of relaxation from caffeine-induced contracture, an estimate of forward NCX1 activity, was prolonged 1.8-fold (p<0.003) in myocytes overexpressing PLM when compared to control myocytes overexpressing green fluorescent protein alone. Reverse NCX1 current (3 Na+ out: 1 Ca2+ in) was significantly (p<0.0001) lower in PLM myocytes, especially at more positive voltages. Immunofluorescence demonstrated co-localization of PLM and NCX1 to the plasma membrane and T-tubules. Resting membrane potential, action potential amplitude and duration, myocyte size, NCX1 and calsequestrin protein levels were not affected by PLM overexpression. At 5mM extracellular [Ca2+] ([Ca2+]o), the depressed contraction amplitudes in PLM myocytes were increased towards normal by co-overexpression with NCX1. At 0.6 mM [Ca2+]o, the supranormal contraction amplitudes in PLM myocytes were reduced by co-overexpression with NCX1. We conclude that PLM modulated myocyte contractility partly by inhibiting Na+/Ca2+ exchange.
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