Independent regulation of cardiac Kv4.3 potassium channel expression by angiotensin II and phenylephrine

Hypertrophied cardiac myocytes exhibit prolonged action potentials and decr eased transient outward potassium current (I-to). Because Kv4.3 is a major contributor to I-to we studied regulation of its expression in neonatal rat cardiac myocytes in response to the known stimulators of cardiac myocyte h ypertrophy, angiotensin II (Ang II) and phenylephrine (PE). RNase protectio n assays and immunoblots revealed that Ang II and PE each downregulate Kv4. 3 mRNA and protein. However, although PE induces a faster and more extensiv e hypertrophic response than Ang II, the PE effect on Kv4.3 mRNA develops s lowly and is sustained, whereas Ang II rapidly and transiently decreases Kv 4.3 mRNA expression. Turnover measurements revealed that Kv4.3 mRNA is very stable, with a half-life >20 hours. This suggests that Ang II must destabi lize the channel mRNA. In contrast, PE does not affect the rate of Kv4.3 mR NA degradation. To test for transcriptional regulation, the 5' flanking reg ion of the rat Kv4.3 gene was cloned, and Kv4.3 promoter-reporter construct s were expressed in cardiac myocytes. Whereas Ang II was found to have no e ffect on transcription, PE inhibits Kv4.3 promoter activity. Pharmacologica l experiments also indicate that PE and Ang II act independently to downreg ulate Kv4.3 gene expression. Thus, regulation of Kv4.3 gene expression is n ot a simple secondary response to hypertrophy. Rather, Ang II and PE use di fferent mechanisms to decrease Kv4.3 channel expression in neonatal rat car diac myocytes.