Enhanced isradipine sensitivity in vascular smooth muscle cells due to hypoxia-induced Cav1.2 splicing and RbFox1/Fox2 downregulation

Calcium influx via the L-type voltage-gated Ca_v1.2 calcium channel in smooth muscle cells regulates vascular contraction. Calcium channel blockers (CCBs) are widely used to treat hypertension by inhibiting Ca_v1.2 channels. Using the vascular smooth muscle cell line, A7r5 and primary culture of cerebral vascular smooth muscle cells, we found that the expression and function of Ca_v1.2 channels are downregulated during hypoxia. Furthermore, hypoxia induces structural changes in Ca_v1.2 channels via alternative splicing. The expression of exon 9* is upregulated, whereas exon 33 is downregulated. Such structural alterations of Ca_v1.2 channels are caused by the decreased expression of RNA-binding proteins RNA-binding protein fox-1 homolog 1 and 2 (RbFox1 and RbFox2). Overexpression of RbFox1 and RbFox2 prevents hypoxia-induced exon 9* inclusion and exon 33 exclusion. Importantly, such structural alterations of the Ca_v1.2 channel partly contribute to the enhanced sensitivity of Ca_v1.2 to isradipine (a CCB) under hypoxia. Overexpression of RbFox1 and RbFox2 successfully reduces isradipine sensitivity in hypoxic smooth muscle cells. Our results suggest a new strategy to manage ischemic diseases such as stroke and myocardial infarction.