Hypoxic induction of T‐type Ca²⁺ channels in rat cardiac myocytes: role of HIF‐1α and RhoA/ROCK signalling

Abstract

Key points T‐type Ca 2+ channels are expressed in the ventricular myocytes of the fetal and perinatal heart, but are downregulated as development progresses. However, these channels are re‐expressed in adult cardiomyocytes under pathological conditions. Hypoxia induces the upregulation of the T‐type Ca 2+ channel Ca v 3.2 mRNA in cardiac myocytes, whereas Ca v 3.1 mRNA is not significantly altered. The effect of hypoxia on Ca v 3.2 mRNA requires hypoxia inducible factor‐1α (HIF‐1α) stabilization and involves the small monomeric G‐protein RhoA and its effector ROCKI. Our results suggest that the hypoxic regulation of the Ca v 3.2 channels may be involved in the increased probability of developing arrhythmias observed in ischemic situations, and in the pathogenesis of diseases associated with hypoxic Ca 2+ overload. Abstract T‐type Ca 2+ channels are expressed in the ventricular myocytes of the fetal and perinatal heart, but are normally downregulated as development progresses. Interestingly, however, these channels are re‐expressed in adult cardiomyocytes under pathological conditions. We investigated low voltage‐activated T‐type Ca 2+ channel regulation in hypoxia in rat cardiomyocytes. Molecular studies revealed that hypoxia induces the upregulation of Ca v 3.2 mRNA, whereas Ca v 3.1 mRNA is not significantly altered. The effect of hypoxia on Ca v 3.2 mRNA was time‐ and dose‐dependent, and required hypoxia inducible factor‐1α (HIF‐1α) stabilization. Patch‐clamp recordings confirmed that T‐type Ca 2+ channel currents were upregulated in hypoxic conditions, and the addition of 50 μ m NiCl 2 (a T‐type channel blocker) demonstrated that the Ca v 3.2 channel is responsible for this upregulation. This increase in current density was not accompanied by significant changes in the Ca v 3.2 channel electrophysiological properties. The small monomeric G‐protein RhoA and its effector Rho‐associated kinase I (ROCKI), which are known to play important roles in cardiovascular physiology, were also upregulated in neonatal rat ventricular myocytes subjected to hypoxia. Pharmacological experiments indicated that both proteins were involved in the observed upregulation of the Ca v 3.2 channel and the stabilization of HIF‐1α that occurred in response to hypoxia. These results suggest a possible role for Ca v 3.2 channels in the increased probability of developing arrhythmias observed in ischaemic situations, and in the pathogenesis of diseases associated with hypoxic Ca 2+ overload.