Kv4 subfamily genes encode a large portion of transient outward K+ current (Ito) in cardiac myocytes. Differential expression of these genes significantly contributes to differences in action potential waveforms in myocytes located in distinct regions of the heart. For example, in left ventricular wall, action potential duration is shorter in epicardial region than endocardial region, and expression of Kv4.2 mRNA and Ito are well correlated with this difference in action potential waveform. In addition to the differential regional expression, expression of Ito is altered under pathological conditions: a significant reduction in Ito is seen in hypertrophied myocytes produced by various conditions including a chronic increase in blood pressure. Our and others' studies indicate that this decrease in Ito is likely to be due to reductions in Kv4.x channel gene expression. Thus, elucidating the molecular mechanisms controlling expression of Kv4 subfamily genes provide fundamental insights into how regional specification of myocytes is achieved and how pathological conditions produce electrical abnormality. Our preliminary results indicate that Kv4.2 gene transcription starts at distinct sites in different tissues. Specifically, skeletal muscle-type start sites are significant in right ventricle and epicardial region of the left ventricle, whereas brain-type start sites predominate in atria and endocardial region. Furthermore, in vivo experiment revealed that administration of the angiotensin-converting enzyme inhibitor captopril selectively reduces Kv4.2 mRNA in epicardial region, but not in endocardial region. Using neonatal myocyte cultures, we also found that angiotensin II (AngII) produce more dramatic decrease in Kv4.3 mRNA than does the alpha-adrenergic agonist phenylephrine. Moreover, this hormone-induced down-regulation of Kv4.3 mRNA was resistant to inhibition of protein kinase C or calcineurin. These findings suggest that expression of Kv4.x gene expression may be region-selectively controlled by distinct sets of transcription factors and that AngII may mediate hypertension-induced down-regulation of Kv4.x gene expression independently of hypertrophy. Hence, this proposal is to test these possibilities using hypertension animal models and neonatal myocyte cultures.
