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This Article Full Text Full Text (PDF) All Versions of this Article: 582/3/991    most recent jphysiol.2007.132712v2 jphysiol.2007.132712v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Yaradanakul, A. Articles by Hilgemann, D. W. Search for Related Content PubMed PubMed Citation Articles by Yaradanakul, A. Articles by Hilgemann, D. W. Related Collections Cellular

Departments of
¹ Physiology
² Pharmacology, University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75390, USA

Cardiac Na⁺–Ca²⁺ exchange (NCX1) inactivates in excised membrane patches when cytoplasmic Ca²⁺ is removed or cytoplasmic Na⁺ is increased. Exogenous phosphatidylinositol-4,5-bis-phosphate (PIP₂) can ablate both inactivation mechanisms, while it has no effect on inward exchange current in the absence of cytoplasmic Na⁺. To probe PIP₂ effects in intact cells, we manipulated PIP₂ metabolism by several means. First, we used cell lines with M1 (muscarinic) receptors that couple to phospholipase C's (PLCs). As expected, outward NCX1 current (i.e. Ca²⁺ influx) can be strongly inhibited when M1 agonists induce PIP₂ depletion. However, inward currents (i.e. Ca²⁺ extrusion) without cytoplasmic Na⁺ can be increased markedly in parallel with an increase of cell capacitance (i.e. membrane area). Similar effects are incurred by cytoplasmic perfusion of GTPS or the actin cytoskeleton disruptor latrunculin, even in the presence of non-hydrolysable ATP (AMP-PNP). Thus, G-protein signalling may increase NCX1 currents by destabilizing membrane cytoskeleton–PIP₂ interactions. Second, to increase PIP₂ we directly perfused PIP₂ into cells. Outward NCX1 currents increase as expected. But over minutes currents decline substantially, and cell capacitance usually decreases in parallel. Third, using BHK cells with stable NCX1 expression, we increased PIP₂ by transient expression of a phosphatidylinositol-4-phosphate-5-kinase (hPIP5KI) and a PI4-kinase (PI4KII). NCX1 current densities were decreased by > 80 and 40%, respectively. Fourth, we generated transgenic mice with 10-fold cardiac-specific overexpression of PI4KII. This wortmannin-insensitive PI4KII was chosen because basal cardiac phosphoinositides are nearly insensitive to wortmannin, and surface membrane PI4-kinase activity, defined functionally in excised patches, is not blocked by wortmannin. Both phosphatidylinositol-4-phosphate (PIP) and PIP₂ were increased significantly, while NCX1 current densities were decreased by 78% with no loss of NCX1 expression. Most mice developed cardiac hypertrophy, and immunohistochemical analysis suggests that NCX1 is redistributed away from the outer sarcolemma. Cholera toxin uptake was increased 3-fold, suggesting that clathrin-independent endocytosis is enhanced. We conclude that direct effects of PIP₂ to activate NCX1 can be strongly modulated by opposing mechanisms in intact cells that probably involve membrane cytoskeleton remodelling and membrane trafficking.

(Received 18 March 2007; accepted after revision 24 May 2007; first published online 31 May 2007)
Corresponding author D. W. Hilgemann: Department of Physiology, UTSouthwestern Medical Center, Dallas, TX 75390-9040, USA. Email: donald.hilgemann{at}utsouthwestern.edu

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