Cardiac Na+-Ca2+ exchanger current induced by tyrphostin tyrosine kinase inhibitors

Regulation of the Na+/Ca2+ exchanger by pyridine nucleotide redox potential in ventricular myocytes

The cardiac Na(+)/Ca(2+) exchanger (NCX) is the major Ca(2+) efflux pathway on the sarcolemma, counterbalancing Ca(2+) influx via L-type Ca(2+) current during excitation-contraction coupling. Altered NCX activity modulates the sarcoplastic reticulum Ca(2+) load and can contribute to abnormal Ca(2+) handling and arrhythmias. NADH/NAD(+) is the main redox couple controlling mitochondrial energy production, glycolysis, and other redox reactions. Here, we tested whether cytosolic NADH/NAD(+) redox potential regulates NCX activity in adult cardiomyocytes. NCX current (INCX), measured with whole cell patch clamp, was inhibited in response to cytosolic NADH loaded directly via pipette or increased by extracellular lactate perfusion, whereas an increase of mitochondrial NADH had no effect. Reactive oxygen species (ROS) accumulation was enhanced by increasing cytosolic NADH, and NADH-induced INCX inhibition was abolished by the H2O2 scavenger catalase. NADH-induced ROS accumulation was independent of mitochondrial respiration (rotenone-insensitive) but was inhibited by the flavoenzyme blocker diphenylene iodonium. NADPH oxidase was ruled out as the effector because INCX was insensitive to cytosolic NADPH, and NADH-induced ROS and INCX inhibition were not abrogated by the specific NADPH oxidase inhibitor gp91ds-tat. This study reveals a novel mechanism of NCX regulation by cytosolic NADH/NAD(+) redox potential through a ROS-generating NADH-driven flavoprotein oxidase. The mechanism is likely to play a key role in Ca(2+) homeostasis and the response to alterations in the cytosolic pyridine nucleotide redox state during ischemia-reperfusion or other cardiovascular diseases.

Phosphorylation and Other Conundrums of Na/Ca Exchanger, NCX1

The Na+/Ca2+ exchanger (NCX) is an important Ca2+ transport mechanism in virtually all cells in the body. There are three genes that control the expression of NCX in mammals. There are at least 16 alternatively spliced isoforms of NCX1 that target muscle and nerve and other tissues. Here we briefly discuss three remarkable regulatory issues or "conundrums" that involve the most prevalently expressed gene, NCX1. (1) How is NCX1 regulated by phosphorylation? We suggest that the macromolecular complex of NCX1 plays a critical role in the regulation of NCX. The role of the macromolecular complex and evidence supporting its existence and functional importance is presented. (2) Can there be transport block of a single "mode" of NCX1 transport by drugs or therapeutic agents? The simple answer is "no." A brief explanation is provided. (3) How can NCX1 knockout mice live? The answer is "by other compensatory regulatory mechanisms." These conundrums highlight important features in NCX1 and lay the foundation for new experiments to elucidate function and regulation of NCX1 and provide a context for investigations that seek to understand novel therapeutic agents.