As the primary interface between the blood and various tissues of the body, the vascular endothelium exhibits a diverse range of roles and activities, all of which contribute to the overall health and function of the cardiovascular system. In this focused review, we discuss several key aspects of endothelial function, how this may be compromised and subsequent consequences. Specifically, we examine the dynamic regulation of arterial contractility and distribution of blood flow through the generation of chemical and electrical signaling events that impinge upon vascular smooth muscle. The endothelium can generate a diverse range of vasoactive compounds and signals, most of which act locally to adjust blood flow in a dynamic fashion to match tissue metabolism. Disruption of these vascular signaling processes (e.g. reduced nitric oxide bioavailability) is typically referred to as endothelial dysfunction, which is a recognized risk factor for cardiovascular disease in patients and occurs early in the development and progression of hypertension, atherosclerosis and tissue ischemia. Endothelial dysfunction is also associated with type-2 Diabetes and aging and increased mechanistic knowledge of the cellular changes contributing to these effects may provide important clues for interventional strategies. The endothelium also serves as the initial site of interaction for immune cells entering tissues in response to damage and acts to facilitate the actions of both the innate and acquired immune systems to interact with the vascular wall. In addition to representing the main cell type responsible for the formation of new blood vessels (i.e. angiogenesis) within the vasculature, the endothelium is also emerging as a source of extracellular vesicle or microparticles for the transport of signaling molecules and other cellular materials to nearby, or remote, sites in the body. The characteristics of released microparticles appear to change with the functional status of the endothelium; thus, these microparticles may represent novel biomarkers of endothelial health and more serious cardiovascular disease.

Large conductance, Ca2+-activated K+ (BK) channels represent an important pathway for the outward flux of K+ ions from the intracellular compartment in response to membrane depolarization, and/or an elevation in cytosolic free [Ca2+]. They are functionally expressed in a range of mammalian tissues (e.g. nerve and smooth muscles), where they can either enhance or dampen membrane excitability. The diversity of BK channel activity results from the considerable alternative mRNA splicing and post-translational modification (e.g. phosphorylation) of key domains within the pore-forming alpha subunit of the channel complex. Most of these modifications are regulated by distinct upstream cell signaling pathways that influence the structure and/or gating properties of the holo-channel and ultimately, cellular function. The channel complex may also contain auxiliary subunits that further affect channel gating and behavior, often in a tissue-specific manner. Recent studies in human and animal models have provided strong evidence that abnormal BK channel expression/function contributes to a range of pathologies in nerve and smooth muscle. By targeting the upstream regulatory events modulating BK channel behavior, it may be possible to therapeutically intervene and alter BK channel expression/function in a beneficial manner.

Recent data have led us to hypothesize that selective activation of endothelial small‐ and/or intermediate‐conductance, calcium‐activated potassium channels (SK _Ca and IK _Ca channels, respectively) by the opener compounds NS309 and DCEBIO would augment stimulated nitric oxide (NO) synthesis and vasodilation in resistance arteries. Experimentally, ATP‐evoked changes in membrane potential, cytosolic Ca ²⁺ , and NO synthesis were recorded by patch clamp and microfluorimetry in single human endothelial cells. Agonist‐evoked inhibition of myogenic tone in isolated, pressurized arterioles from rat cremaster skeletal muscle was analyzed by video microscopy. NS309 and DCEBIO enhanced ATP‐evoked membrane hyperpolarization and cytosolic Ca ²⁺ transients, along with acute NO synthesis in isolated endothelial cells. The acetylcholine‐mediated inhibition of myogenic tone (IC ₅₀ = 237 nM) was left‐shifted in the presence of NS309 and DCEBIO (10, 100, and 1000 nM) to IC ₅₀ values of 101, 78, and 43 nM;endothelial denudation inhibited this drug effect. l ‐NAME attenuated the acetylcholine‐induced inhibition of myogenic tone but did not interfere with NS309 and DCEBIO‐evoked vasodilation. Collectively, our data demonstrate that drug‐induced enhancement of endothelial SK _Ca and IK _Ca channel activities represents a novel cellular mechanism to increase vasodilation of small‐resistance arterioles, thereby highlighting these channels as potential therapeutic targets in cardiovascular disease states associated with compromised NO signaling.—Sheng, J.‐S., Ella, S., Davis, M. J., Hill, M. A., Braun, A. P. Openers of SK _Ca and IK _Ca channels enhance agonist‐evoked endothelial nitric oxide synthesis and arteriolar vasodilation. FASEB J . 23, 1138–1145 (2009)

Smooth muscle contractility and neuronal excitability are regulated by large conductance, Ca ²⁺ ‐activated K ⁺ (BK _Ca ) channels, the activity of which can be increased after modulation by type I cGMP‐dependent protein kinase (cGKI) via nitric oxide (NO)/cGMP signaling. Our study focused on identifying key phosphorylation sites within the BK _Ca channel underlying functional enhancement of channel activity by cGKI. BK _Ca channel phosphorylation by cGKIα was characterized biochemically using radiolabeled ATP, and regulation of channel activity by NO/cGMP signaling was quantified in rat aortic A7r5 smooth muscle cells by cell‐attached patch‐clamp recording. Serine to alanine substitutions at 3 of 6 putative cGKI phosphorylation sites (Ser691, Ser873, and Ser1112) in the BK _Ca α subunit individually reduced direct channel phosphorylation by 25–60% and blocked BK _Ca activation by either an NO donor or a membrane‐permeable cGMP by 80–100%. Acute inhibition of cGKI prevented stimulus‐evoked enhancement of BK _Ca channel activity. Our data further suggest that augmentation of BK _Ca activity by NO/cGMP/cGKI signaling requires phosphorylation at all 3 sites and is independent of elevations in [Ca ²⁺ ] _i . Phosphorylation of 3 specific Ser residues within the murine BK _Ca α subunit by cGKIα accounts for the enhanced BK _Ca channel activity induced by elevated [cGMP] _i in situ. —Kyle, B.D., Hurst, S., Swayze, R. D., Sheng, J., Braun, A.P. Specific phosphorylation sites underlie the stimulation of a large conductance, Ca ²⁺ ‐activated K ⁺ channel by cGMP‐dependent protein kinase. FASEB J. 27, 2027–2038 (2013). www.fasebj.org