MECHANICAL STRETCH STIMULATES THE ACTIVITY OF THE POTASSIUM CHANNEL KV1.5

Abstract

Cardiomyocytes continuously undergo mechanical load. During diastole of the cardiac cycle, blood fills the cardiac chambers and stretches the cardiomyocytes. This stretch is influenced by other conditions such as increased blood pressure and fluid retention. Evidence suggests that mechanical activities can regulate cardiac electrical properties. However, the linkage between mechanical activities and electrophysiological properties is not well understood. The cardiac activity is controlled by movement of ions such as Na+, Ca2+, and K+ through distinct ion channels. These channels are often voltage-gated, however some channels are also regulated by mechanical force. The voltage-gated potassium channel, Kv1.5 generates the ultra-rapid delayed rectifier current (IKur) in the atrial myocytes. Dysfunction of Kv1.5 channels has been linked to atrial fibrillation, one of the most common cardiac arrhythmias, prevalent in seniors. Since hypertension and cardiac dilation cause atrial fibrillation, we aim to study the effects of mechanical stretch on Kv1.5 channel function. To study the effect of mechanical force on Kv1.5 channels, we used whole cell voltage clamp in human embryonic kidney cells stably expressing Kv1.5. The expression levels of Kv1.5 channel proteins were analyzed using Western blot analysis. Low speed centrifugation (62 X g, 5 min) and low extracellular osmolarity (0.67 of normal cell culture medium; or 213 vs 320 osmolarity) were used to simulate an increase in mechanical force. While centrifugation provides direct stretch of the cells, low osmolarity in extracellular solution causes cell swelling which stretch the cell membranes. Our data shows that both low-speed centrifugation for 5 minutes and low osmolarity treatment of cells for 60 min significantly increase Kv1.5 currents. However, acute application of low osmolarity solution to the patch clamp perfusion chamber during recording did not affect Kv1.5 current. These results suggest that a mechanical stretch-activated signaling pathway is involved. Indeed, our data revealed that cell swelling increased mature Kv1.5 channel density and our co-IP experiments indicated that the membrane protein integrin associates with Kv1.5. Inhibition of focal adhesion kinase (FAK) with a selective inhibitor (FAK inhibitor 14) abolished cell swelling induced increase in Kv1.5 expression and current. Mechanical stretch stimulates Kv1.5 channel activity via a signalling pathway involving membrane protein integrin, a mechanosensor, and the activation of FAK. This study provides a potential mechanism for atrial dilation-assisted atrial fibrillation, and a foundation to further examine downstream effectors in the mechanotransduction pathway linking mechanical forces to electrical activities in the heart.