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.endothelial cell calcium | cremaster arterioles | mesenteric arteries C a 2+ -activated K + (K Ca ) channels in arteriolar endothelial cells (ECs) are activated by intrinsic spontaneous or receptormediated Ca 2+ events, each leading to hyperpolarization of smooth muscle cells (SMCs) and vasodilation independent of nitric oxide or prostacyclin-the endothelium-dependent hyperpolarization (EDH) response. This hyperpolarization spreads both radially and longitudinally through the vascular wall via patent gap junctions to evoke local and conducted dilation, and it is central to cardiovascular function (1, 2).EDH is the predominant endothelium-dependent mechanism in smaller "resistance" arteries and arterioles. The underlying hyperpolarization is generated by two subtypes of K Ca channels found in the EC, but not SMC, membrane, the small (SK Ca ,K Ca 2.3) and intermediate (IK Ca, K Ca 3.1) conductance forms that may be activated independently of each other (3). The physiological importance of independent activation is apparent from studies with K Ca 3.1-deficient mice in which the mean blood pressure is raised by ∼7 mmHg, but further elevated by disrupting both K Ca channels (4). In mesenteric resistance arteries, IK Ca channels are focused within EC projections through the internal elastic lamina (IEL) termed myoendothelial junctions (MEJs). MEJs can contain gap junctions (MEGJs) coupling ECs to SMCs, and EDH can spread by direct electrical coupling and/or a diffusible factor (5, 6). The IK Ca channels enriched within MEJs can be activated by spontaneous inositol 1,4,5-trisphosphate (IP 3 ...

, via competing with nitric oxide (NO) synthase for the substrate L-arginine, may interfere with NO-mediated vascular responses. We tested the hypothesis that arginase 1 contributes to coronary vasomotor dysfunction in patients with diabetes mellitus (DM). Coronary arterioles were dissected from the right atrial appendages of 41 consecutive patients with or without DM (the 2 groups suffered from similar comorbidities), and agonist-induced changes in diameter were measured with videomicroscopy. We found that the endothelium-dependent agonist ACh elicited a diminished vasodilation and caused constriction to the highest ACh concentration (0.1 M) with a similar magnitude in patients with (18 Ϯ 8%) and without (17 Ϯ 9%) DM. Responses to ACh were not significantly affected by the inhibition of NO synthesis with N G -nitro-L-arginine methyl ester in either group. The NO donor sodium nitroprusside-dependent dilations were not different in patients with or without DM. Interestingly, we found that the presence of N G -hydroxy-L-arginine (10 M), a selective inhibitor of arginase or application of L-arginine (3 mM), restored ACh-induced coronary dilations only in patients with DM (to 47 Ϯ 6% and to 40 Ϯ 19%, respectively) but not in subjects without DM. Correspondingly, the protein expression of arginase 1 was increased in coronary arterioles of patients with DM compared with subjects without diabetes. Moreover, using immunocytochemistry, we detected an abundant immunostaining of arginase 1 in coronary endothelial cells of patients with DM, which was colocalized with NO synthase. Collectively, we provided evidence for a distinct upregulation of arginase 1 in coronary arterioles of patients with DM, which contributes to a reduced NO production and consequently diminished vasodilation. coronary microvessel; endothelium; endothelial nitric oxide synthase PATIENTS WITH DIABETES exhibit endothelial dysfunction, which is characterized by an impaired flow-and acetylcholine (ACh)-induced, endothelium-dependent relaxation of brachial artery (8) and forearm resistance vessels (36). Kaneda et al. (17) performed a study in which 165 patients underwent intracoronary injection of ACh and found that diabetes was the strongest predictor for ACh-induced vasospasm, a response that indicates coronary endothelial dysfunction. This and other studies indicated that diabetes is also associated with an impaired dilator function of coronary arteries, and this is manifested and measured as a reduced vasodilator or even vasoconstrictor response to ACh (5, 23). Our previous studies have demonstrated that coronary arterioles isolated from animals with experimental diabetes also exhibit impaired ACh-induced dilation, which is primarily due to the reduced synthesis and/or availability of nitric oxide (NO) (1, 2, 9, 16). The exact mechanism(s) responsible for the diminished NO production in human diabetes is still incompletely understood.L-Arginine, the substrate for NO synthase, is the precursor for NO synthesis in the vascular endothelium. Experimenta...

Thus, under normal conditions, Cav-1 limits the contribution of the BK(Ca) channel to EDHF-mediated arteriolar dilation. In obesity, a reduced expression of Cav-1 leads to greater contribution of the BK(Ca) channel to EDHF-mediated response, which seems essential for maintained coronary dilation.