Role of KCNQ Potassium Channels in Control of Calcium Homeostasis in Vascular Smooth Muscle Cells

Voltage-gated Kv7 (or KCNQ) potassium channels control the activity of excitable cells, including vascular smooth muscle cells (VSMCs), by setting their resting membrane potential and controlling other excitability parameters. Calcium (Ca2+) mediates excitation-contraction coupling in smooth muscle cells (SMCs), but until now the exact role of Kv7 channels in cytosolic Ca2+ dynamics in VSMCs have not been fully elucidated.

The Ca2+ imaging experiments presented herein show that both, direct (XE991) and G protein-coupled receptor mediated (vasopressin, AVP) Kv7 channel inhibition induced robust Ca2+ oscillations, which were significantly reduced in the presence of a Kv7 channel activator (retigabine), L- and T-type Ca2+ channel blockers (nifedipine and NNC 55-0396). Plasma membrane potential measured using FluoVolt demonstrated that both XE991 and AVP induced slow depolarisation followed by a burst of sharp spikes, which were temporally well-correlated with Ca2+ oscillations. Using the inhibitors of IP3Rs (2-APB), RyRs (tetracaine) and phospholipase C (PLC; edelfosine), we found that PLC signalling is required for AVP-induced, but not XE991-induced Ca2+ oscillations.

Analysis of the Kv7 gene transcripts and protein levels identified Kv7.5 as the major Kv7 subunit expressed in rat VSMCs. CACNA1C (Cav1.2; L-type) and CACNA1G (Cav3.1; T-type) were the most abundant voltage-gated Ca2+ channel gene transcripts. Hypoxic treatment significantly reduced the expression of Kv7.5 and increased Cav3.2 T-type Ca2+ channels, and enhanced depolarisation-induced Ca2+ transients with prolonged and high-frequency Ca2+ oscillations. These changes were orchestrated by the HIF-α transcriptional signalling pathway. Importantly, even with a reduced level of Kv7.5 expression, retigabine was still efficacious to stop AVP-induced Ca2+ oscillations.

AVP and XE991 induced a large increase of [Ca2+]i in primary human internal mammary artery SMCs, which was also attenuated with retigabine and nifedipine. Interestingly, in saphenous vein SMCs, bradykinin-induced Ca2+ transients were not prevented by retigabine and nifedipine with some enhancement instead. Kv7.5 was found to be the predominant Kv7 subunit in human VSMCs as well.

This study establishes Kv7 channels as crucial regulators of Ca2+ signalling in VSMCs with Kv7.5 playing a dominant role. Retigabine or more selective activators of Kv7.5 may provide a clinically useful tool for the treatment of cardiovascular diseases, such as pulmonary hypertension.