Transcriptional Regulation of L-type Calcium Channel Expression in Cardiac Myocytes

Abstract

The L-type calcium channel is a heteromultimeric protein complex, which is expressed in the cardiac sarcolemma. Although post-translational regulation of its subunits by protein kinase A (PKA) has been widely reported, little is known about molecular processes that regulate expression of calcium channel subunits (α1C,α2 - δ, and β2Asubunits). Previous studies from our group demonstrate that the steady-state mRNA level of the α1Cunit is increased by treatment of myocytes with β -adrenergic agonists. The current study is designed to determine whether the mRNA levels for all subunits of the L-type calcium channel are coordinately controlled by a β -adrenergic agonist, and whether this occurs predominantly through control of rate of transcription. Nuclear run-on assays were used to determine the transcription initiation rate of these genes in cultured neonatal rat cardiac myocytes. In isoproterenol (10−7m)-treated myocytes, transcription of genes encoding the α1C, α2- δ, and β2Asubunits was enhanced. The increases in transcription initiation rate for α1C, α2- δ, andβ2A subunits genes were 404%, 367%, and 240% of control, respectively. Pretreatment with the β -adrenergic antagonist propranolol (10−5m) or PKA inhibitor H-89 (10−6m) blocked the effects of isoproterenol, while either drug alone did not affect the gene transcription rate significantly. Steady state mRNA levels of the subunits increased following isoproterenol treatment. These results suggest that β -adrenergic stimulation and the PKA signaling pathway play an important role in transcriptional regulation of the L-type calcium channel in myocyte. The expression of all the subunits of this ion channel is under coordinate transcriptional control.