Glucose-6-Phosphate Dehydrogenase and NADPH Redox Regulates Cardiac Myocyte L-Type Calcium Channel Activity and Myocardial Contractile Function |
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| Authors: | Dhwajbahadur K Rawat Peter Hecker Makino Watanabe Sukrutha Chettimada Richard J Levy Takao Okada John G Edwards Sachin A Gupte |
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| Institution: | 1. Department of Biochemistry and Molecular Biology, University of South Alabama, Mobile, Alabama, United States of America.; 2. Department of Medicine, Division of Cardiology, University of Maryland, Baltimore, Maryland, United States of America.; 3. Departments of Physiology, Juntendo University School of Medicine, Tokyo, Japan.; 4. Department of Physiology, New York Medical College, Valhalla, New York, United States of America.; Instituto Nacional de Cardiologia, Mexico, |
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| Abstract: | We recently demonstrated that a 17-ketosteroid, epiandrosterone, attenuates L-type Ca2+ currents (ICa-L) in cardiac myocytes and inhibits myocardial contractility. Because 17-ketosteroids are known to inhibit glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in the pentose phosphate pathway, and to reduce intracellular NADPH levels, we hypothesized that inhibition of G6PD could be a novel signaling mechanism which inhibit ICa-L and, therefore, cardiac contractile function. We tested this idea by examining myocardial function in isolated hearts and Ca2+ channel activity in isolated cardiac myocytes. Myocardial function was tested in Langendorff perfused hearts and ICa-L were recorded in the whole-cell patch configuration by applying double pulses from a holding potential of −80 mV and then normalized to the peak amplitudes of control currents. 6-Aminonicotinamide, a competitive inhibitor of G6PD, increased pCO2 and decreased pH. Additionally, 6-aminonicotinamide inhibited G6PD activity, reduced NADPH levels, attenuated peak ICa-L amplitudes, and decreased left ventricular developed pressure and ±dp/dt. Finally, dialyzing NADPH into cells from the patch pipette solution attenuated the suppression of ICa-L by 6-aminonicotinamide. Likewise, in G6PD-deficient mice, G6PD insufficiency in the heart decreased GSH-to-GSSG ratio, superoxide, cholesterol and acetyl CoA. In these mice, M-mode echocardiographic findings showed increased diastolic volume and end-diastolic diameter without changes in the fraction shortening. Taken together, these findings suggest that inhibiting G6PD activity and reducing NADPH levels alters metabolism and leads to inhibition of L-type Ca2+ channel activity. Notably, this pathway may be involved in modulating myocardial contractility under physiological and pathophysiological conditions during which the pentose phosphate pathway-derived NADPH redox is modulated (e.g., ischemia-reperfusion and heart failure). |
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