Acetylcholine caused a concentration‐dependent smooth muscle hyperpolarization and relaxation in rat small mesenteric arteries (diameter at 100 mmHg 250–450 mm) stimulated with noradrenaline (3 μm). Nitric oxide (NO), generated from either NO‐gas or from acidified sodium nitrite, also induced smooth muscle hyperpolarization but only in the absence of active force. However, unlike the hyperpolarizations to acetylcholine, those to NO were abolished either by prior smooth muscle depolarization caused by noradrenaline, or by the K⁺ channel blocker, glibenclamide (3 μm). Hyperpolarization and relaxation to acetylcholine were unaffected by prior exposure of the mesenteric artery to either the cyclo‐oxygenase inhibitor, indomethacin (10 μm), or the nitric oxide synthase inhibitor, N^G‐nitro‐l‐arginine (l‐NNA, 100 μm). Haemoglobin (1.5 μm), which binds and inactivates NO, blocked the hyperpolarizing and vasorelaxant response to NO, but did not alter either response to acetylcholine. These data show that, in the rat small mesenteric artery, membrane hyperpolarizations to NO and acetylcholine are mediated by different mechanisms, and that the hyperpolarization induced by NO is not involved in the responses to acetylcholine. In addition, they provide evidence that the acetylcholine responses in this artery, which are endothelium‐dependent, are not mediated by the release of NO.

Among the 28 identified and unique mammalian TRP (transient receptor potential) channel isoforms, at least 19 are expressed in vascular endothelial cells. These channels appear to participate in a diverse range of vascular functions, including control of vascular tone, regulation of vascular permeability, mechanosensing, secretion, angiogenesis, endothelial cell proliferation, and endothelial cell apoptosis and death. Malfunction of these channels may result in disorders of the human cardiovascular system. All TRP channels, except for TRPM4 and TRPM5, are cation channels that allow Ca ²⁺ influx. However, there is a daunting diversity in the mode of activation and regulation in each case. Specific TRP channels may be activated by different stimuli such as vasoactive agents, oxidative stress, mechanical stimuli, and heat. TRP channels may then transform these stimuli into changes in the cytosolic Ca ²⁺ , which are eventually coupled to various vascular responses. Evidence has been provided to suggest the involvement of at least the following TRP channels in vascular function: TRPC1, TRPC4, TRPC6, and TRPV1 in the control of vascular permeability; TRPC4, TRPV1, and TRPV4 in the regulation of vascular tone; TRPC4 in hypoxia-induced vascular remodeling; and TRPC3, TRPC4, and TRPM2 in oxidative stress–induced responses. However, in spite of the large body of data available, the functional role of many endothelial TRP channels is still poorly understood. Elucidating the mechanisms regulating the different endothelial TRP channels, and the associated development of drugs selectively to target the different isoforms, as a means to treat cardiovascular disease should, therefore, be a high priority.

Endothelial cell (EC) Ca ²⁺ -activated K channels (SK _Ca and IK _Ca channels) generate hyperpolarization that passes to the adjacent smooth muscle cells causing vasodilation. IK _Ca channels focused within EC projections toward the smooth muscle cells are activated by spontaneous Ca ²⁺ events (Ca ²⁺ puffs/pulsars). We now show that transient receptor potential, vanilloid 4 channels (TRPV4 channels) also cluster within this microdomain and are selectively activated at low intravascular pressure. In arterioles pressurized to 80 mmHg, ECs generated low-frequency (∼2 min ⁻¹ ) inositol 1,4,5-trisphosphate receptor-based Ca ²⁺ events. Decreasing intraluminal pressure below 50 mmHg increased the frequency of EC Ca ²⁺ events twofold to threefold, an effect blocked with the TRPV4 antagonist RN1734. These discrete events represent both TRPV4-sparklet- and nonsparklet-evoked Ca ²⁺ increases, which on occasion led to intracellular Ca ²⁺ waves. The concurrent vasodilation associated with increases in Ca ²⁺ event frequency was inhibited, and basal myogenic tone was increased, by either RN1734 or TRAM-34 (IK _Ca channel blocker), but not by apamin (SK _Ca channel blocker). These data show that intraluminal pressure influences an endothelial microdomain inversely to alter Ca ²⁺ event frequency; at low pressures the consequence is activation of EC IK _Ca channels and vasodilation, reducing the myogenic tone that underpins tissue blood-flow autoregulation.