Kir6.1-dependent K channels in lymphatic smooth muscle and vessel dysfunction in mice with Kir6.1 gain-of-function.

KEY POINTS

Spontaneous contractions are essential for normal lymph transport and these contractions are exquisitely sensitive to the K channel activator pinacidil. K channel Kir6.1 and SUR2B subunits are expressed in mouse lymphatic smooth muscle (LSM) and form functional K channels as verified by electrophysiological techniques. Global deletion of Kir6.1 or SUR2 subunits results in severely impaired lymphatic contractile responses to pinacidil. Smooth muscle-specific expression of Kir6.1 gain-of-function mutant (GoF) subunits results in profound lymphatic contractile dysfunction and LSM hyperpolarization that is partially rescued by the K inhibitor glibenclamide. In contrast, lymphatic endothelial-specific expression of Kir6.1 GoF has essentially no effect on lymphatic contractile function. The high sensitivity of LSM to K channel GoF offers an explanation for the lymphoedema observed in patients with Cantú syndrome, a disorder caused by gain-of-function mutations in genes encoding Kir6.1 or SUR2, and suggests that glibenclamide may be an appropriate therapeutic agent.

ABSTRACT

This study aimed to understand the functional expression of K channel subunits in distinct lymphatic cell types, and assess the consequences of altered K channel activity on lymphatic pump function. K channel subunits Kir6.1 and SUR2B were expressed in mouse lymphatic muscle by PCR, but only Kir6.1 was expressed in lymphatic endothelium. Spontaneous contractions of popliteal lymphatics from wild-type (WT) (C57BL/6J) mice, assessed by pressure myography, were very sensitive to inhibition by the SUR2-specific K channel activator pinacidil, which hyperpolarized both mouse and human lymphatic smooth muscle (LSM). In vessels from mice with deletion of Kir6.1 (Kir6.1 ) or SUR2 (SUR2[STOP]) subunits, contractile parameters were not significantly different from those of WT vessels, suggesting that basal K channel activity in LSM is not an essential component of the lymphatic pacemaker, and does not exert a strong influence over contractile strength. However, these vessels were >100-fold less sensitive than WT vessels to pinacidil. Smooth muscle-specific expression of a Kir6.1 gain-of-function (GoF) subunit resulted in severely impaired lymphatic contractions and hyperpolarized LSM. Membrane potential and contractile activity was partially restored by the K channel inhibitor glibenclamide. In contrast, lymphatic endothelium-specific expression of Kir6.1 GoF subunits had negligible effects on lymphatic contraction frequency or amplitude. Our results demonstrate a high sensitivity of lymphatic contractility to K channel activators through activation of Kir6.1/SUR2-dependent channels in LSM. In addition, they offer an explanation for the lymphoedema observed in patients with Cantú syndrome, a disorder caused by gain-of-function mutations in genes encoding Kir6.1/SUR2.