Activation of cellular signaling systems is increasingly recognized as a potential modulator of not only the function of plasma membrane proteins, but also their trafficking into and out of the cell membrane. In the heart, Na+ channels are pivotal to both normal electrophysiology and the genesis of life-threatening cardiac arrhythmias. While cardiac Na+ current is modulated by protein kinase(pKA), the nature of this regulation is controversial and the mechanisms unknown. Based on data we have generated, the goal of this research is to test the hypothesis that PKA activation regulates trafficking of cardiac Na+ channels and to investigate the molecular mechanisms. In cells expressing the human cardiac Na+ channel hHl and in rat ventricular myocytes, the effects of pKA activation on N a+ current, plasma membrane channel density, and the cellular distribution of Na+ channels will be determined using electrophysiologic and biochemical methods, immunolocalization, and a fluorescent channel fusion protein to visualize trafficking in living cells. Our preliminary data have implicated the I-II interdomain linker as a region contributing to this effect. To investigate the structural and molecular components required for channel recycling under basal and stimulated states, structure- function hypotheses will be explored for this region as well as the carboxy terminus, which interacts with other proteins (e.g., those with PDZ-domains). Evidence for interacting or adaptor proteins will be sought by both overexpression and binding studies using the channel region(s) involved in the PKA effect. To explore the cellular mechanisms that mediate regulated trafficking of Na+ channels, the role of specific molecular components in both exocytotic and endocytotic pathways will be investigated. The knowledge gained from these studies will improve our understanding of the molecular mechanisms whereby cardiac cells regulate Na+ channel activity and could identify novel strategies to modulate cardiac Na+ currents.
