Endothelium-derived hyperpolarizing factor but not NO reduces smooth muscle Ca2+ during acetylcholine-induced dilation of microvessels

Connexin 40-Mediated Regulation of Systemic Circulation and Arterial Blood Pressure

Vascular system is a complex network in which different cell types and vascular segments must work in concert to regulate blood flow distribution and arterial blood pressure. Although paracrine/autocrine signaling is involved in the regulation of vasomotor tone, direct intercellular communication via gap junctions plays a central role in the control and coordination of vascular function in the microvascular network. Gap junctions are made up by connexin (Cx) proteins, and among the four Cxs expressed in the cardiovascular system (Cx37, Cx40, Cx43, and Cx45), Cx40 has emerged as a critical signaling pathway in the vessel wall. This Cx is predominantly found in the endothelium, but it is involved in the development of the cardiovascular system and in the coordination of endothelial and smooth muscle cell function along the length of the vessels. In addition, Cx40 participates in the control of vasomotor tone through the transmission of electrical signals from the endothelium to the underlying smooth muscle and in the regulation of arterial blood pressure by renin-angiotensin system in afferent arterioles. In this review, we discuss the participation of Cx40-formed channels in the development of cardiovascular system, control and coordination of vascular function, and regulation of arterial blood pressure.

Chronic ethanol consumption enhances inducible endothelium-dependent hyperpolarizing factor-mediated relaxation in the rat artery

The inducible endothelium-dependent hyperpolarizing factor (iEDHF) pathway is activated as a compensatory response to adverse changes in the body. It causes vasorelaxation and maintains circulatory homeostasis in the organs. Small to moderate quantities of ethanol enhance vascular relaxation. However, its mechanism and the involvement of the iEDHF pathway in this process are unknown. Therefore, we studied iEDHF-mediated, acetylcholine-induced, endothelium-dependent relaxation in the superior mesenteric arteries (SMAs) of rats chronically fed ethanol. Rats were administered a standard diet (S-Control group), Lieber's control diet (L-Control group), or Lieber's ethanol diet (EtOH group). SMA relaxation was assessed by isometric tension measurements. Arachidonate 15-lipoxygenase (ALOX15) and soluble epoxide hydrolase (sEH) were determined by immunoblot. Acetylcholine-induced, endothelium-dependent relaxation was significantly greater in the EtOH than the control groups. These differences persisted after PGI2 and NO blockade. Thus, the increase in acetylcholine-induced relaxation was EDHF-mediated. In the EtOH group, however, it was prevented by iEDHF inhibitors. ALOX15 and sEH protein expression levels were higher in the EtOH than the L-Control group. The increase in acetylcholine-induced relaxation by chronic ethanol consumption was mediated by the iEDHF pathway. This mechanism may compensate for the blood pressure elevation induced by ethanol. This study suggests that iEDHF is induced during proper drinking and may help prevent the onset of cardiovascular conditions.

Calcium movement in smooth muscle and evaluation of graded functional intercellular coupling

Spontaneous activity of vascular smooth muscle is present in small arteries and some venous tissues like the hepatic portal vein. Whereas the ability to generate rhythmic membrane potential changes is expressed in a high number of primary oscillators, the generation of physiological tone and phasic activity requires synchronization of specialized pacemaker activity (Interstitial Cajal-like cells) by intercellular propagation and regeneration of excitation or a strong coupling mechanism of smooth muscle cells. The aim of this study was to deduce oscillator coupling by analyzing the spatiotemporal homogeneity of calcium oscillations within a native tissue preparation. Portal vein tissue was loaded with a calcium-sensitive dye (Fluo-3). By combining confocal microscopy and computation of spatial auto- and cross-correlation of the calcium signals, temporal and spatial coupling between cells was characterized. Spontaneous oscillations of calcium signals were measured at different predefined regions of interest. Cross-correlation analysis of these signals revealed that their damping was very similar in all directions of the investigated z-plane. In single experiments, improved cell-to-cell coupling was seen when noradrenaline (1-10 μM) was added to the bath-solution. With the chosen parameters of frame refresh, the velocity of signal propagation was faster than the maximum detectable velocity, but it could be estimated to exceed 0.1 mm/s. Correlative Network Analysis is a new and very useful tool to determine the functional coupling parameters of quasi-homogenous biological networks and their temporal changes. The action and significance of pharmacological modulators can be well studied on cellular and functional aspects with this newly introduced technique in biological sciences.

Connexins and Pannexins in Vascular Function and Disease

Connexins (Cxs) and pannexins (Panxs) are ubiquitous membrane channel forming proteins that are critically involved in many aspects of vascular physiology and pathology. The permeation of ions and small metabolites through Panx channels, Cx hemichannels and gap junction channels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. This review provides an overview of current knowledge with respect to the pathophysiological role of these channels in large arteries, the microcirculation, veins, the lymphatic system and platelet function. The essential nature of these membrane proteins in vascular homeostasis is further emphasized by the pathologies that are linked to mutations and polymorphisms in Cx and Panx genes.

NO Augments Endothelial Reactivity by Reducing Myoendothelial Calcium Signal Spreading: A Novel Role for Cx37 (Connexin 37) and the Protein Tyrosine Phosphatase SHP-2

OBJECTIVE

Because of its strategic position between endothelial and smooth muscle cells in microvessels, Cx37 (Connexin 37) plays an important role in myoendothelial gap junctional intercellular communication. We have shown before that NO inhibits gap junctional intercellular communication through gap junctions containing Cx37. However, the underlying mechanism is not yet identified.

APPROACH AND RESULTS

Using channel-forming Cx37 mutants exhibiting partial deletions or amino acid exchanges in their C-terminal loops, we now show that the phosphorylation state of a tyrosine residue at position 332 (Y332) in the C-terminus of Cx37 controls the gap junction-dependent spread of calcium signals. Mass spectra revealed that NO protects Cx37 from dephosphorylation at Y332 by inhibition of the protein tyrosine phosphatase SHP-2. Functionally, the inhibition of gap junctional intercellular communication by NO decreased the spread of the calcium signal (induced by mechanical stimulation of individual endothelial cells) from endothelial to smooth muscle cells in intact vessels, while, at the same time, augmenting the calcium signal spreading within the endothelium. Consequently, preincubation of small resistance arteries with exogenous NO enhanced the endothelium-dependent dilator response to acetylcholine in spite of a pharmacological blockade of NO-dependent cGMP formation by the soluable guanylyl cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one).

CONCLUSIONS

Our results identify a novel mechanism by which NO can increase the efficacy of calcium, rising vasoactive agonists in the microvascular endothelium.

Endothelial dysfunction in renal arcuate arteries of obese Zucker rats: The roles of nitric oxide, endothelium-derived hyperpolarizing factors, and calcium-activated K+ channels

The roles of nitric oxide (NO), endothelium-derived hyperpolarizing factors (EDHF), and calcium-activated K⁺ (K_Ca) channels in diabetes-associated endothelial dysfunction of small renal arteries are not clear. The present study investigated acetylcholine (ACh)-induced vasorelaxation of renal arcuate arteries from obese Zucker (OZ) rats at different diabetes durations, and the relative contribution of NO, EDHF, and K_Ca channels to the endothelial dysfunction. OZ rats of 7 weeks (prediabetic stage), 12 weeks (early diabetic stage), and 20 weeks (late diabetic stage), and time-matched lean control rats, were studied. Segments of arcuate arteries (130 to 180 μm) were isolated, cannulated and pressurized. Vascular endothelial functions were tested using ACh-induced vasodilation. Our experiments demonstrated: (1) ACh-elicited vasodilation was impaired in OZ rats of 20 weeks, but not in rats of 7 and 12 weeks; (2) inhibition of NO or EDHF (contributed by epoxyeicosatrienoic acids [EETs]) production significantly decreased ACh-induced vasodilation in both lean and OZ rats of 20 weeks. The reduction of ACh-induced vasodilation by inhibition of NO or EDHF formation was less in OZ rats, as compared to lean rats; and (3) inhibition of K_Ca channels markedly reduced ACh-induced vasodilation in lean control rats, but not in OZ rats of 20 weeks. Our observations indicated that endothelium-dependent vasodilation in renal arcuate arteries is impaired in diabetes mellitus; NO and EDHF, mainly EETs, dominate the ACh-induced vasodilation in renal arcuate arteries; the contribution of NO and EETs is impaired in diabetic rats; K_Ca channels are involved in ACh-induced vasodilation; and the activity of K_Ca channels is downregulated in diabetes mellitus.

The role of endothelial mechanosensitive genes in atherosclerosis and omics approaches

Atherosclerosis is the leading cause of morbidity and mortality in the U.S., and is a multifactorial disease that preferentially occurs in regions of the arterial tree exposed to disturbed blood flow. The detailed mechanisms by which d-flow induces atherosclerosis involve changes in the expression of genes, epigenetic patterns, and metabolites of multiple vascular cells, especially endothelial cells. This review presents an overview of endothelial mechanobiology and its relation to the pathogenesis of atherosclerosis with special reference to the anatomy of the artery and the underlying fluid mechanics, followed by a discussion of a variety of experimental models to study the role of fluid mechanics and atherosclerosis. Various in vitro and in vivo models to study the role of flow in endothelial biology and pathobiology are discussed in this review. Furthermore, strategies used for the global profiling of the genome, transcriptome, miR-nome, DNA methylome, and metabolome, as they are important to define the biological and pathophysiological mechanisms of atherosclerosis. These "omics" approaches, especially those which derive data based on a single animal model, provide unprecedented opportunities to not only better understand the pathophysiology of atherosclerosis development in a holistic and integrative manner, but also to identify novel molecular and diagnostic targets.

Arachidonic acid metabolites and endothelial dysfunction of portal hypertension

Increased resistance to portal flow and increased portal inflow due to mesenteric vasodilatation represent the main factors causing portal hypertension in cirrhosis. Endothelial cell dysfunction, defined as an imbalance between the synthesis, release, and effect of endothelial mediators of vascular tone, inflammation, thrombosis, and angiogenesis, plays a major role in the increase of resistance in portal circulation, in the decrease in the mesenteric one, in the development of collateral circulation. Reduced response to vasodilators in liver sinusoids and increased response in the mesenteric arterioles, and, viceversa, increased response to vasoconstrictors in the portal-sinusoidal circulation and decreased response in the mesenteric arterioles are also relevant to the pathophysiology of portal hypertension. Arachidonic acid (AA) metabolites through the three pathways, cyclooxygenase (COX), lipoxygenase, and cytochrome P450 monooxygenase and epoxygenase, are involved in endothelial dysfunction of portal hypertension. Increased thromboxane-A2 production by liver sinusoidal endothelial cells (LSECs) via increased COX-1 activity/expression, increased leukotriens, increased epoxyeicosatrienoic acids (EETs) (dilators of the peripheral arterial circulation, but vasoconstrictors of the portal-sinusoidal circulation), represent a major component in the increased portal resistance, in the decreased portal response to vasodilators and in the hyper-response to vasoconstrictors. Increased prostacyclin (PGI2) via COX-1 and COX-2 overexpression, and increased EETs/heme-oxygenase-1/K channels/gap junctions (endothelial derived hyperpolarizing factor system) play a major role in mesenteric vasodilatation, hyporeactivity to vasoconstrictors, and hyper-response to vasodilators. EETs, mediators of liver regeneration after hepatectomy and of angiogenesis, may play a role in the development of regenerative nodules and collateral circulation, through stimulation of vascular endothelial growth factor (VEGF) inside the liver and in the portal circulation. Pharmacological manipulation of AA metabolites may be beneficial for cirrhotic portal hypertension.

Cold-acclimation leads to differential regulation of the steelhead trout (Oncorhynchus mykiss) coronary microcirculation

The regulation of vascular resistance in fishes has largely been studied using isolated large conductance vessels, yet changes in tissue perfusion/vascular resistance are primarily mediated by the dilation/constriction of small arterioles. Thus we adapted mammalian isolated microvessel techniques for use in fish and examined how several agents affected the tone/resistance of isolated coronary arterioles (<150 μm ID) from steelhead trout (Oncorhynchus mykiss) acclimated to 1, 5, and 10°C. At 10°C, the vessels showed a concentration-dependent dilation to adenosine (ADE; 61 ± 8%), sodium nitroprusside (SNP; 35 ± 10%), and serotonin (SER; 27 ± 2%) (all values maximum responses). A biphasic response (mild contraction then dilation) was observed in vessels exposed to increasing concentrations of epinephrine (EPI; 34 ± 9% dilation) and norepinephrine (NE; 32 ± 7% dilation), whereas the effect was less pronounced with bradykinin (BK; 12.5 ± 3.5% constriction vs. 6 ± 6% dilation). Finally, a mild constriction was observed after exposure to acetylcholine (ACh; 6.5 ± 1.4%), while endothelin (ET)-1 caused a strong dose-dependent increase in tone (79 ± 5% constriction). Acclimation temperature had varying effects on the responsiveness of vessels. The dilations induced by EPI, ADE, SER, and SNP were reduced/eliminated at 5°C and/or 1°C as compared with 10°C. In contrast, acclimation to 5 and 1°C increased the maximum constriction induced by ACh and the sensitivity of vessels to ET-1 (but not the maximum response) at 1°C was greater. Acclimation temperature had no effect on the response to NE, and responsiveness to BK was variable.

Enhanced endothelin receptor type B-mediated vasodilation and underlying [Ca²⁺]i in mesenteric microvessels of pregnant rats

BACKGROUND AND PURPOSE

Normal pregnancy is associated with decreased vascular resistance and increased release of vasodilators. Endothelin-1 (ET-1) causes vasoconstriction via endothelin receptor type A (ET(A)R), but could activate ET(B)R in the endothelium and release vasodilator substances. However, the roles of ET(B)R in the regulation of vascular function during pregnancy and the vascular mediators involved are unclear.

EXPERIMENTAL APPROACH

Pressurized mesenteric microvessels from pregnant and virgin Sprague-Dawley rats were loaded with fura-2/AM for simultaneous measurement of diameter and [Ca²⁺]i.

KEY RESULTS

High KCl (51 mM) and phenylephrine (PHE) caused increases in vasoconstriction and [Ca²⁺]i that were similar in pregnant and virgin rats. ET-1 caused vasoconstriction that was less in pregnant than virgin rats, with small increases in [Ca²⁺]i. Pretreatment with the ET(B)R antagonist BQ-788 caused greater enhancement of ET-1-induced vasoconstriction in pregnant rats. ACh caused endothelium-dependent relaxation and decreased [Ca²⁺]i, and was more potent in pregnant than in virgin rats. ET-1 + ET(A)R antagonist BQ-123, and the ET(B)R agonists sarafotoxin 6c (S6c) and IRL-1620 caused greater vasodilation in pregnant than in virgin rats with no changes in [Ca²⁺]i, suggesting up-regulated ET(B)R-mediated relaxation pathways. ACh-, S6c- and IRL-1620-induced relaxation was reduced by the NO synthase inhibitor Nω-nitro-L-arginine methyl ester, and abolished by tetraethylammonium or endothelium removal. Western blots revealed greater amount of ET(B)R in intact microvessels of pregnant than virgin rats, but reduced levels in endothelium-denuded microvessels, supporting a role of endothelial ET(B)R.

CONCLUSIONS AND IMPLICATIONS

The enhanced ET(B)R-mediated microvascular relaxation may contribute to the decreased vasoconstriction and vascular resistance during pregnancy.

Calcium signalling in pericytes

Recent advances in pericyte research have contributed to our understanding of the physiology and pathophysiology of microvessels. The microvasculature consists of arteriolar and venular networks located upstream and downstream of the capillaries. Arterioles are surrounded by a monolayer of spindle-shaped myocytes, while terminal branches of precapillary arterioles, capillaries and all sections of postcapillary venules are encircled by a monolayer of morphologically diverse pericytes. There are physiological differences in the response of pericytes and myocytes to vasoactive molecules, suggesting that these two vascular cell types could have different functional roles in the regulation of local blood flow. The contractile activity of pericytes and myocytes is controlled by changes of cytosolic free Ca(2+) concentration. In this short review, we summarize our results and those of other authors on the contractility of pericytes and their Ca(2+) signalling. We describe results regarding sources of Ca(2+) and mechanisms of Ca(2+) release and Ca(2+) entry in control of the spatiotemporal characteristics of the Ca(2+) signals in pericytes.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Effects of acute transmural pressure elevation on endothelium-dependent vasodilation in isolated rat mesenteric veins

BACKGROUND/AIMS

The vascular regulatory function of the endothelium can be impaired by increases in transmural pressure (TMP). We tested the hypothesis that increasing TMP impairs the endothelial dilator function of rat mesenteric small veins (MSVs).

METHODS

In PGF2α-preconstricted MSVs, bradykinin (BK), sodium nitroprusside (SNP) and S-Nitroso-N-acetylpenicillamine (SNAP) concentration-response curves were generated at intermediate (6 mm Hg) and high (12 mm Hg) pressures. BK-induced vasodilation was examined in the absence and presence of nitric oxide synthase inhibitor [N(ω)-nitro-L-arginine (L-NNA), 100 µM], cyclooxygenase inhibitor (indomethacin, 1 µM), and large (BKCa, paxilline, 500 nM) and small (SKCa, apamin, 300 nM) conductance Ca(2+)-activated K(+) channel blockers.

RESULTS

BK, SNP and SNAP responses were not altered by TMP increases. BK-induced vasodilation was significantly reduced by L-NNA, indomethacin, apamin and paxilline at 6 mm Hg and L-NNA at 12 mm Hg, and was further reduced by coapplication of apamin and/or paxilline with L-NNA compared with responses obtained with either blocker. Endothelium removal completely abolished BK-induced vasodilation.

CONCLUSION

Venous endothelial dilator function is not affected by TMP elevation. BK-induced vasodilation is completely dependent on the presence of functional endothelial cells and mediated in part by nitric oxide, BKCa and SKCa channels, while the participation of prostacyclin may be important at intermediate pressures.

Arginase promotes skeletal muscle arteriolar endothelial dysfunction in diabetic rats

Endothelial dysfunction is a characteristic feature in diabetes that contributes to the development of vascular disease. Recently, arginase has been implicated in triggering endothelial dysfunction in diabetic patients and animals by competing with endothelial nitric oxide synthase for substrate l-arginine. While most studies have focused on the coronary circulation and large conduit blood vessels, the role of arginase in mediating diabetic endothelial dysfunction in other vascular beds has not been fully investigated. In the present study, we determined whether arginase contributes to endothelial dysfunction in skeletal muscle arterioles of diabetic rats. Diabetes was induced in male Sprague Dawley rats by streptozotocin injection. Four weeks after streptozotocin administration, blood glucose, glycated hemoglobin, and vascular arginase activity were significantly increased. In addition, a significant increase in arginase I and II mRNA expression was detected in gracilis muscle arterioles of diabetic rats compared to age-matched, vehicle control animals. To examine endothelial function, first-order gracilis muscle arterioles were isolated, cannulated in a pressure myograph system, exposed to graded levels of luminal flow, and internal vessel diameter measured. Increases in luminal flow (0–50 μL/min) caused progressive vasodilation in arterioles isolated from control, normoglycemic animals. However, flow-induced vasodilation was absent in arterioles obtained from streptozotocin-treated rats. Acute in vitro pretreatment of blood vessels with the arginase inhibitors N^ω-hydroxy-nor-l-arginine or S-(2-boronoethyl)-l-cysteine restored flow-induced responses in arterioles from diabetic rats and abolished differences between diabetic and control animals. Similarly, acute in vitro pretreatment with l-arginine returned flow-mediated vasodilation in vessels from diabetic animals to that of control rats. In contrast, d-arginine failed to restore flow-induced dilation in arterioles isolated from diabetic animals. Administration of sodium nitroprusside resulted in a similar degree of dilation in arterioles isolated from control or diabetic rats. In conclusion, the present study identifies arginase as an essential mediator of skeletal muscle arteriolar endothelial dysfunction in diabetes. The ability of arginase to induce endothelial dysfunction in skeletal muscle arterioles may further compromise glucose utilization and facilitate the development of hypertension in diabetes.

Proximal cerebral arteries develop myogenic responsiveness in heart failure via tumor necrosis factor-α-dependent activation of sphingosine-1-phosphate signaling

BACKGROUND

Heart failure is associated with neurological deficits, including cognitive dysfunction. However, the molecular mechanisms underlying reduced cerebral blood flow in the early stages of heart failure, particularly when blood pressure is minimally affected, are not known.

METHODS AND RESULTS

Using a myocardial infarction model in mice, we demonstrate a tumor necrosis factor-α (TNFα)-dependent enhancement of posterior cerebral artery tone that reduces cerebral blood flow before any overt changes in brain structure and function. TNFα expression is increased in mouse posterior cerebral artery smooth muscle cells at 6 weeks after myocardial infarction. Coordinately, isolated posterior cerebral arteries display augmented myogenic tone, which can be fully reversed in vitro by the competitive TNFα antagonist etanercept. TNFα mediates its effect via a sphingosine-1-phosphate (S1P)-dependent mechanism, requiring sphingosine kinase 1 and the S1P(2) receptor. In vivo, sphingosine kinase 1 deletion prevents and etanercept (2-week treatment initiated 6 weeks after myocardial infarction) reverses the reduction of cerebral blood flow, without improving cardiac function.

CONCLUSIONS

Cerebral artery vasoconstriction and decreased cerebral blood flow occur early in an animal model of heart failure; these perturbations are reversed by interrupting TNFα/S1P signaling. This signaling pathway may represent a potential therapeutic target to improve cognitive function in heart failure.

Effects of trypsin on cytosolic calcium levels in the rat aortic endothelium

The effect of trypsin on vascular tone and the cytosolic calcium concentration ([Ca(2+)](i)) of endothelial and smooth muscle cells were examined in the rat aorta. A calcium indicator, fura-PE3, was used to measure [Ca(2+)](i) simultaneously with vascular tone. In the endothelium-intact rat aorta, carbachol and trypsin increased [Ca(2+)](i) in a dose-dependent manner. In the endothelium-denuded rat aorta, carbachol did not change [Ca(2+)](i), but trypsin slightly increased it. Addition of trypsin to the norepinephrine-stimulated rat aorta relaxed the muscle with an additional increase in [Ca(2+)](i). Under calcium-free conditions, trypsin induced a transient increase in [Ca(2+)](i). Trypsin-induced endothelium-dependent relaxation was inhibited by preincubation with l-NMMA, an endothelial NO synthase inhibitor, U-73122, a phospholipase C inhibitor, cyclopiazonic acid, a sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase blocker, and lanthanum, a nonselective Ca(2+) channel blocker. However, indomethacin, a nonselective cyclooxygenase inhibitor, and SKF-96365, a store-operated Ca(2+)-channel blocker, had no effect on the trypsin-induced relaxation. These results suggest that trypsin increases [Ca(2+)](i) in the endothelial cells through SKF-96365-insensitive Ca(2+) channels and regulates the release of NO, which results in relaxation of the rat aorta.

A microfluidic platform for probing small artery structure and function

Although pathologic changes to the structure and function of small blood vessels are hallmarks of various cardiovascular diseases, limitations of conventional investigation methods (i.e. pressure myography) have prohibited a comprehensive understanding of the underlying mechanisms. We developed a microfluidic device to facilitate assessment of resistance artery structure and function under physiological conditions (37 degrees C, 45 mmHg transmural pressure). The platform allows for on-chip fixation, long-term culture and fully automated acquisition of up to ten dose-response sequences of intact mouse mesenteric artery segments (diameter approximately 250 micrometres and length approximately 1.5 mm) in a well-defined microenvironment. Even abluminal application of phenylephrine or acetylcholine (homogeneous condition) yielded dose-response relationships virtually identical to conventional myography. Unilateral application of phenylephrine (heterogeneous condition) limited constriction to the drug-exposed side, suggesting a lack of circumferential communication. The microfluidic platform allows us to address new fundamental biological questions, replaces a manually demanding procedure with a scalable approach and may enable organ-based screens to be routinely performed during drug development.

Connexins and gap junctions in the EDHF phenomenon and conducted vasomotor responses

It is becoming increasingly evident that electrical signaling via gap junctions plays a central role in the physiological control of vascular tone via two related mechanisms (1) the endothelium-derived hyperpolarizing factor (EDHF) phenomenon, in which radial transmission of hyperpolarization from the endothelium to subjacent smooth muscle promotes relaxation, and (2) responses that propagate longitudinally, in which electrical signaling within the intimal and medial layers of the arteriolar wall orchestrates mechanical behavior over biologically large distances. In the EDHF phenomenon, the transmitted endothelial hyperpolarization is initiated by the activation of Ca(2+)-activated K(+) channels channels by InsP(3)-induced Ca(2+) release from the endoplasmic reticulum and/or store-operated Ca(2+) entry triggered by the depletion of such stores. Pharmacological inhibitors of direct cell-cell coupling may thus attenuate EDHF-type smooth muscle hyperpolarizations and relaxations, confirming the participation of electrotonic signaling via myoendothelial and homocellular smooth muscle gap junctions. In contrast to isolated vessels, surprisingly little experimental evidence argues in favor of myoendothelial coupling acting as the EDHF mechanism in arterioles in vivo. However, it now seems established that the endothelium plays the leading role in the spatial propagation of arteriolar responses and that these involve poorly understood regenerative mechanisms. The present review will focus on the complex interactions between the diverse cellular signaling mechanisms that contribute to these phenomena.

The phosphorylation motif at serine 225 governs the localization and function of sphingosine kinase 1 in resistance arteries

OBJECTIVE

The purpose of this study was to characterize a phosphorylation motif at serine 225 as a molecular switch that regulates the pressure-dependent activation of sphingosine kinase 1 (Sk1) in resistance artery smooth muscle cells.

METHODS AND RESULTS

In isolated hamster gracilis muscle resistance arteries, pressure-dependent activation/translocation of Sk1 by ERK1/2 was critically dependent on its serine 225 phosphorylation site. Specifically, expression of Sk1(S225A) reduced resting and myogenic tone, resting Ca(2+), pressure-induced Ca(2+) elevations, and Ca(2+) sensitivity. The lack of function of the Sk1(S225A) mutant could not be entirely overcome by forced localization to the plasma membrane via a myristoylation/palmitylation motif; the membrane anchor also significantly inhibited the function of the wild-type Sk1 enzyme. In both cases, Ca(2+) sensitivity and myogenic tone were attenuated, whereas Ca(2+) handling was normalized/enhanced. These discrete effects are consistent with cell surface receptor-mediated effects (Ca(2+) sensitivity) and intracellular effects of S1P (Ca(2+) handling). Accordingly, S1P(2) receptor inhibition (1 micromol/L JTE013) attenuated myogenic tone without effect on Ca(2+).

CONCLUSIONS

Translocation and precise subcellular positioning of Sk1 is essential for full Sk1 function; and two distinct S1P pools, proposed to be intra- and extracellular, contribute to the maintenance of vascular tone.

Role of sphingosine-1-phosphate phosphohydrolase 1 in the regulation of resistance artery tone

Sphingosine-1-phosphate (S1P), which mediates pleiotropic actions within the vascular system, is a prominent regulator of microvascular tone. By virtue of its S1P-degrading function, we hypothesized that S1P-phosphohydrolase 1 (SPP1) is an important regulator of tone in resistance arteries. Hamster gracilis muscle resistance arteries express mRNA encoding SPP1. Overexpression of SPP1 (via transfection of a SPP1(wt)) reduced resting tone, Ca2+ sensitivity, and myogenic vasoconstriction, whereas reduced SPP1 expression (antisense oligonucleotides) yielded the opposite effects. Expression of a phosphatase-dead mutant of SPP1 (SPP1(H208A)) had no effect on any parameter tested, suggesting that catalytic activity of SPP1 is critical. The enhanced myogenic tone that follows overexpression of S1P-generating enzyme sphingosine kinase 1 (Sk1(wt)) was functionally antagonized by coexpression with SPP1(wt) but not SPP1(H208A). SPP1 modulated vasoconstriction in response to 1 to 100 nmol/L exogenous S1P, a concentration range that was characterized as S1P2-dependent, based on the effect of S1P(2) inhibition by antisense oligonucleotides and 1 mumol/L JTE013. Inhibition of the cystic fibrosis transmembrane regulator (CFTR) (1) restored S1P responses that were attenuated by SPP1(wt) overexpression; (2) enhanced myogenic vasoconstriction; but (3) had no effect on noradrenaline responses. We conclude that SPP1 is an endogenous regulator of resistance artery tone that functionally antagonizes the vascular effects of both Sk1(wt) and S1P2 receptor activation. SPP1 accesses extracellular S1P pools in a manner dependent on a functional CFTR transport protein. Our study assigns important roles to both SPP1 and CFTR in the physiological regulation of vascular tone, which influences both tissue perfusion and systemic blood pressure.

Acrolein induces vasodilatation of rodent mesenteric bed via an EDHF-dependent mechanism

Acrolein is generated endogenously during lipid peroxidation and inflammation and is an environmental pollutant. Protein adducts of acrolein are detected in atherosclerotic plaques and neurons of patients with Alzheimer's disease. To understand vascular effects of acrolein exposure, we studied acrolein vasoreactivity in perfused rodent mesenteric bed. Acrolein induced endothelium-dependent vasodilatation that was more robust and more sensitive than dilation induced by 4-hydroxy-trans-2-nonenal, trans-2-hexenal, or propionaldehyde. Acrolein-induced vasodilatation was mediated by K(+)-sensitive components, e.g., it was abolished in 0 [K(+)](o) buffer or in 3 mM tetrabutylammonium, inhibited 75% in 50 microM ouabain, and inhibited 64% in 20 mM K(+) buffer. Moreover, combined treatment with the Ca(2+)-activated K(+) channel inhibitors 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34, 100 nM) and apamin (5 microM) significantly reduced vasodilatation without altering sensitivity to acrolein. However, acrolein-induced % dilation was unaffected by l-NAME or indomethacin pretreatment indicating mechanistic independence of NO and prostaglandins. Moreover, acrolein induced vasodilatation in cirazoline-precontracted mesenteric bed of eNOS-null mice confirming eNOS independence. Pretreatment with 6-(2-propargyloxyphenyl) hexanoic acid (PPOH 50 microM), an epoxygenase inhibitor, or the superoxide dismutase mimetic Tempol (100 microM) significantly attenuated acrolein-induced vasodilatation. Collectively, these data indicate that acrolein stimulates mesenteric bed vasodilatation due to endothelium-derived signal(s) that is K(+)-, ouabain-, PPOH-, and Tempol-sensitive, and thus, a likely endothelium-derived hyperpolarizing factor (EDHF). These data indicate that low level acrolein exposure associated with vascular oxidative stress or inflammation stimulates vasodilatation via EDHF release in medium-sized arteries--a novel function.

Arteriolar smooth muscle Ca2+dynamics during blood flow control in hamster cheek pouch

Intracellular calcium concentration ([Ca2+]i) governs the contractile status of arteriolar smooth muscle cells (SMC). Although studied in vitro, little is known of SMC [Ca2+]idynamics during the local control of blood flow. We tested the hypothesis that the rise and fall of SMC [Ca2+]iunderlies arteriolar constriction and dilation in vivo. Aparenchymal segments of second-order arterioles (diameter 35 ± 2 μm) were prepared in the superfused cheek pouch of anesthetized hamsters ( n = 18) and perifused with the ratiometric dye fura PE-3 (AM) to load SMC (1 μM, 20 min). Resting SMC [Ca2+]iwas 406 ± 37 nM. Elevating superfusate O2from 0 to 21% produced constriction (11 ± 2 μm) that was unaffected by dye loading; [Ca2+]iincreased by 108 ± 53 nM ( n = 6, P < 0.05). Cycling of [Ca2+]iduring vasomotion (amplitude, 150 ± 53 nM; n = 4) preceded corresponding diameter changes (7 ± 1 μm) by ∼2 s. Microiontophoresis (1 μm pipette tip; 1 μA, 1 s) of phenylephrine (PE) transiently increased [Ca2+]iby 479 ± 64 nM ( n = 8, P < 0.05) with constriction (26 ± 3 μm). Flushing blood from the lumen with saline increased fluorescence at 510 nm by ∼45% during excitation at both 340 and 380 nm with no difference in resting [Ca2+]i, diameter or respective responses to PE ( n = 7). Acetylcholine microiontophoresis (1 μA, 1 s) transiently reduced resting SMC [Ca2+]iby 131 ± 21 nM ( n = 6, P < 0.05) with vasodilation (17 ± 1 μm). Superfusion of sodium nitroprusside (10 μM) transiently reduced SMC [Ca2+]iby 124 ± 18 nM ( n = 6, P < 0.05), whereas dilation (23 ± 5 μm) was sustained. Resolution of arteriolar SMC [Ca2+]iin vivo discriminates key signaling events that govern the local control of tissue blood flow.

Intracellular smooth muscle [Ca2+] in acetylcholine and nitric oxide-mediated relaxation of human small arteries

In human resistance arteries the role of intracellular calcium during receptor agonist and nitric oxide (NO)-mediated vasorelaxation is almost unknown. We examined changes in smooth muscle calcium concentration ([Ca2+]i) caused by acetylcholine and the NO donor S-nitroso-N-acetylpenicillamine (SNAP) in isolated human subcutaneous small arteries. In arteries constricted with 50 mM KCl, acetylcholine and SNAP induced relaxation without any change in [Ca2+]i, whereas in noradrenaline constricted vessels, both acetylcholine and to a lesser degree also SNAP-mediated relaxation were associated with a decrease in [Ca2+]i. Furthermore incubation with SNAP (1 microM) induced a rightward shift in the [Ca2+]i-force relationship. These results suggest that relaxation mediated by endothelium derived hyperpolarizing factors (EDHF) is associated with reduction in [Ca2+]i, whereas NO-mediated relaxation can take place without changes in [Ca2+]i. This finding seems to be, at least partly, due to NO-mediated desensitization of the contractile apparatus to calcium.

Endothelium-dependent smooth muscle hyperpolarization: do gap junctions provide a unifying hypothesis?

An endothelium-derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K(+) ions, eicosanoids, hydrogen peroxide and C-type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a 'universal EDHF'. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist-induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K(+) ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone.

Impairment of endothelium-dependent dilation response after resuscitation from hemorrhagic shock involved postreceptor mechanisms

Resuscitation from hemorrhagic shock is associated with impairment of the endothelium-dependent dilation response, whereas the dilation response induced by the endothelium-independent pathway, which is mediated by nitroprusside, a nitric oxide (NO) donor and a direct activator of guanylate cyclase, remains unaltered. Whether the impairment of the endothelium-dependent dilation response is caused by a specific receptor alteration or generally a defect in signal transduction pathway remains undetermined. Anesthetized rats were monitored for hemodynamics, and the terminal ileum was prepared for intravital videomicroscopy. Hemorrhage was 50% of mean arterial pressure for 60 min followed by resuscitation with the shed blood returned plus 2 volumes of normal saline. Intestinal microvascular reactivity to the endothelium-dependent receptor-dependent agonists acetylcholine or substance P (10(-8) or 10(-6) M), as well as the endothelium-dependent receptor-independent calcium ionophore, was determined at baseline and at 2 h postresuscitation from hemorrhagic shock. Measured vascular diameters for premucosal A3 arterioles (pA3 and dA3) were normalized and expressed as percentage of the maximal dilation capacity, as obtained from the response to the endothelium-independent NO donor sodium nitroprusside (10(-4) M). At 2 h postresuscitation, there was a marked constriction of pA3 (-70.1 +/- 20) and dA3 (-61.5 +/- 11.6) from maximal dilation capacity. Baseline premucosal arteriolar response to substance P (10(-8) M) was 30.68 +/- 4.19% and 34.66 +/- 5.82% for pA3 and dA3 arterioles, respectively. This was significantly reduced to 20.97 +/- 2.41% and 17.94 +/- 3.60% at 2 h postresuscitation. However, no significant difference between baseline and postresuscitation arteriolar responses was observed at the higher dose of substance P (10(-6) M). Postresuscitation premucosal arteriolar response to the endothelium-dependent receptor-independent calcium ionophore (10(-9) to 10(-5) M) is characterized by a marked decrease in sensitivity and an enhanced threshold for calcium ionophore-mediated dilation. The logEC50 was -7.62 +/- 0.39 and -7.75 +/- 0.32 for the pA3 and dA3 at baseline, respectively. This was significantly (P < 0.01) reduced to -5.15 +/- 0.14 and -4.39 +/- 0.71 at 2 h postresuscitation. These data suggest that impairment of the endothelium-dependent dilation response after resuscitation from hemorrhagic shock is not mediated by specific receptor alteration. Cellular mechanisms that participate in or are part of oxygen free radical formation after resuscitation from hemorrhagic shock such as Ca2+ and leukocytes, appear to have a pivotal role in the mechanism of cellular dysfunction.

Immediate exercise hyperemia: contributions of the muscle pump vs. rapid vasodilation

A striking characteristic of the blood flow adaptation at exercise onset is the immediate and substantial increase in the first few (0–5 s) seconds of exercise. The purpose of this mini-review is to put into context the present evidence regarding mechanisms responsible for this phase of exercise hyperemia. One potential mechanism that has received much attention is the mechanical effect of muscle contraction (the muscle pump). The rapid vasodilatory mechanism(s) is another possible mechanism that has recently been shown to exist. This review will provide the reader with 1) an understanding of the basic physics of blood flow and the theories of muscle pump function, 2) a critical examination of evidence both for and against the contribution of the muscle pump or rapid vasodilatory mechanisms, and 3) an awareness of the limitations and impact of experimental models and exercise modes on the contribution of each of these mechanisms to the immediate exercise hyperemia. The inability to measure microvenular pressure continues to limit investigators to indirect assessments of the muscle pump vs. vasodilatory mechanism contributions to immediate exercise hyperemia in vivo. Future research directions should include examination of muscle-contraction-induced resistance vessel distortion as a trigger for rapid smooth muscle relaxation and further investigation into the exercise mode dependency of muscle pump vs. rapid vasodilatory contributions to immediate exercise hyperemia.

Biphasic effect of hydrogen peroxide on skeletal muscle arteriolar tone via activation of endothelial and smooth muscle signaling pathways

We hypothesized that hydrogen peroxide (H2O2) has a role in the local regulation of skeletal muscle blood flow, thus significantly affecting the myogenic tone of arterioles. In our study, we investigated the effects of exogenous H2O2 on the diameter of isolated, pressurized (at 80 mmHg) rat gracilis skeletal muscle arterioles (diameter of approximately 150 microm). Lower concentrations of H2O2 (10(-6)-3 x 10(-5) M) elicited constrictions, whereas higher concentrations of H2O2 (6 x 10(-5)-3 x 10(-4) M), after initial constrictions, caused dilations of arterioles (at 10(-4) M H2O2, -19 +/- 1% constriction and 66 +/- 4% dilation). Endothelium removal reduced both constrictions (to -10 +/- 1%) and dilations (to 33 +/- 3%) due to H2O2. Constrictions due to H2O2 were completely abolished by indomethacin and the prostaglandin H2/thromboxane A2 (PGH2/TxA2) receptor antagonist SQ-29548. Dilations due to H2O2 were significantly reduced by inhibition of nitric oxide synthase (to 38 +/- 7%) but were unaffected by clotrimazole or sulfaphenazole (inhibitors of cytochrome P-450 enzymes), indomethacin, or SQ-29548. In endothelium-denuded arterioles, clotrimazole had no effect, whereas H2O2-induced dilations were significantly reduced by charybdotoxin plus apamin, inhibitors of Ca(2+)-activated K+ channels (to 24 +/- 3%), the selective blocker of ATP-sensitive K+ channels glybenclamide (to 14 +/- 2%), and the nonselective K(+)-channel inhibitor tetrabutylammonium (to -1 +/- 1%). Thus exogenous administration of H2O2 elicits 1) release of PGH2/TxA2 from both endothelium and smooth muscle, 2) release of nitric oxide from the endothelium, and 3) activation of K+ channels, such as Ca(2+)-activated and ATP-sensitive K+ channels in the smooth muscle resulting in biphasic changes of arteriolar diameter. Because H2O2 at low micromolar concentrations activates several intrinsic mechanisms, we suggest that H2O2 contributes to the local regulation of skeletal muscle blood flow in various physiological and pathophysiological conditions.

Evidence for a rapid vasodilatory contribution to immediate hyperemia in rest-to-mild and mild-to-moderate forearm exercise transitions in humans

Controversy exists regarding the contribution of a rapid vasodilatory mechanism(s) to immediate exercise hyperemia. Previous in vivo investigations have exclusively examined rest-to-exercise (R-E) transitions where both the muscle pump and early vasodilator mechanisms may be activated. To isolate vasodilatory onset, the present study investigated the onset of exercise hyperemia in an exercise-to-exercise (E-E) transition, where no further increase in muscle pump contribution would occur. Eleven subjects lay supine and performed a step increase from rest to 3 min of mild (10% maximal voluntary contraction), rhythmic, dynamic forearm handgrip exercise, followed by a further step to moderate exercise (20% maximal voluntary contraction) in each of arm above (condition A) or below (condition B) heart level. Beat-by-beat measures of brachial arterial blood flow (Doppler ultrasound) and blood pressure (arterial tonometry) were performed. We observed an immediate increase in forearm vascular conductance in E-E transitions, and the magnitude of this increase matched that of the R-E transitions within each of the arm positions (condition A: E-E, 52.8 +/- 10.7 vs. R-E, 60.3 +/- 11.7 ml.min(-1).100 mmHg(-1), P = 0.66; condition B: E-E, 43.2 +/- 12.8 vs. R-E, 33.9 +/- 8.2 ml.min(-1).100 mmHg(-1), P = 0.52). Furthermore, changes in forearm vascular conductance were identical between R-E and E-E transitions over the first nine contraction-relaxation cycles in condition A. The immediate and identical increase in forearm vascular conductance in R-E and E-E transitions within arm positions provides strong evidence that rapid vasodilation contributes to immediate exercise hyperemia in humans. Specific vasodilatory mechanisms responsible remain to be determined.

Thromboxane receptor stimulation associated with loss of SKCa activity and reduced EDHF responses in the rat isolated mesenteric artery

1. The possibility that thromboxane (TXA(2)) receptor stimulation causes differential block of the SK(Ca) and IK(Ca) channels which underlie EDHF-mediated vascular smooth muscle hyperpolarization and relaxation was investigated in the rat isolated mesenteric artery. 2. Acetylcholine (30 nm-3 microm ACh) or cyclopiazonic acid (10 microm CPA, SERCA inhibitor) were used to stimulate EDHF-evoked smooth muscle hyperpolarization. In each case, this led to maximal hyperpolarization of around 20 mV, which was sensitive to block with 50 nm apamin and abolished by repeated stimulation of mesenteric arteries with the thromboxane mimetic, U46619 (30 nm-0.1 microm), but not the alpha(1)-adrenoceptor agonist phenylephrine (PE). 3. The ability of U46619 to abolish EDHF-evoked smooth muscle hyperpolarization was prevented by prior exposure of mesenteric arteries to the TXA(2) receptor antagonist 1 microm SQ29548. 4. Similar-sized smooth muscle hyperpolarization evoked with the SK(Ca) activator 100 microm riluzole was also abolished by prior stimulation with U46619, while direct muscle hyperpolarization in response to either levcromakalim (1 microm, K(ATP) activator) or NS1619 (40 microm, BK(Ca) activator) was unaffected. 5. During smooth muscle contraction and depolarization to either PE or U46619, ACh evoked concentration-dependent hyperpolarization (to -67 mV) and complete relaxation. These responses were well maintained during repeated stimulation with PE, but with U46619 there was a progressive decline, so that during a third exposure to U46619 maximum hyperpolarization only reached -52 mV and relaxation was reduced by 20%. This relaxation could now be blocked with charybdotoxin alone. The latter responses could be mimicked with 300 microm 1-EBIO (IK(Ca) activator), an action not modified by exposure to U46619. 6. An early consequence of TXA(2) receptor stimulation is a reduction in the arterial hyperpolarization and relaxation attributed to EDHF. This effect appears to reflect a loss of SK(Ca) activity.

Metabolic control of muscle blood flow during exercise in humans

During muscle contraction, several mechanisms regulate blood flow to ensure a close coupling between muscle oxygen delivery and metabolic demand. No single factor has been identified to constitute the primary metabolic regulator, yet there are signal transduction pathways between skeletal muscle and the vasculature that induce vasodilation. A link between muscle metabolic events and microvascular control of blood flow is illustrated by local dilation of terminal arterioles during contraction of muscle fibers and conduction of vasodilation upstream. Endothelial-derived vasodilator mechanisms are known to exert control of muscle vasodilation. Adenosine, nitric oxide (NO), prostacyclin (PGI2), and endothelial-derived hyperpolarization factor (EDHF) are possible mediators of muscle vasodilation during exercise. In humans, adenosine has been shown to contribute to functional hyperemia as blood flow is reduced under nonselective adenosine-receptor blockade. No clear role has been demonstrated for either NO or PGI2(2), based on studies employing selective inhibition of these substances individually, suggesting a redundancy of vasodilator mechanisms. This is supported by recent work demonstrating that combined blockade of NOS and PGI2, and NOS and cytochrome P450, both attenuate exercise-induced hyperemia in humans. Combined vasodilator blockade studies offer the potential to uncover important interactions and compensatory vasodilator responses. The signaling pathways that link metabolic events evoked by muscle contraction to vasodilatory signals in the local vascular bed remains an important area of study.

Increase in endothelial cell Ca(2+) in response to mouse cremaster muscle contraction

We addressed the role of endothelial cells (ECs) in metabolic dilatation of skeletal muscle arterioles in anaesthetized mice in situ. Electrical field stimulation was used to contract the cremaster muscle for 15 s at 30 Hz. Diameter was observed using bright field microscopy. In controls, muscle contraction produced a 15.7 +/- 1.5 microm dilatation from a baseline of 17.4 +/- 1.6 microm. Endothelial denudation (-EC) via intraluminal perfusion of air abolished this response (1.6 +/- 1.2 microm in -EC, P < 0.05), identifying endothelium as the primary vascular cell type initiating the dilatation. To investigate the role of EC Ca(2+) in metabolic dilatation, arteriolar ECs were loaded with Fluo-4 AM or BAPTA AM by intraluminal perfusion, after which blood flow was re-established. Ca(2+) activity of individual ECs was monitored as a function of change from baseline fluorescence using confocal microscopy. In ECs, whole cell Ca(2+) increased (>10%, P < 0.05) during muscle contraction, and localized Ca(2+) transients were increased (>20%, P < 0.05) during the first minute after contraction. Chelation of EC Ca(2+) abolished the dilatations in response to muscle contraction (1.1 +/- 0.7 microm, P < 0.05). Inhibition of P(1) purinergic receptors (with xanthine amine congener) did not alter the rate of onset of the dilatation (P > 0.05) but decreased its magnitude immediately post stimulation (7.1 +/- 0.9 microm, P < 0.05) and during recovery. These findings demonstrate obligatory roles for endothelium and EC Ca(2+) during metabolic dilatation in intact arterioles. Furthermore, they suggest that at least two separate pathways mediate the local response, one of which involves stimulation of endothelial P(1) purinergic receptors via endogenous adenosine produced during muscle activity.

Small- and intermediate-conductance calcium-activated K+ channels provide different facets of endothelium-dependent hyperpolarization in rat mesenteric artery

Activation of both small-conductance (SKCa) and intermediate-conductance (IKCa) Ca2+-activated K+ channels in endothelial cells leads to vascular smooth muscle hyperpolarization and relaxation in rat mesenteric arteries. The contribution that each endothelial K+ channel type makes to the smooth muscle hyperpolarization is unknown. In the presence of a nitric oxide (NO) synthase inhibitor, ACh evoked endothelium and concentration-dependent smooth muscle hyperpolarization, increasing the resting potential (approx. -53 mV) by around 20 mV at 3 microM. Similar hyperpolarization was evoked with cyclopiazonic acid (10 microM, an inhibitor of sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA)) while 1-EBIO (300 microM, an IKCa activator) only increased the potential by a few millivolts. Hyperpolarization in response to either ACh or CPA was abolished with apamin (50 nM, an SKCa blocker) but was unaltered by 1-[(2-chlorophenyl) diphenylmethyl]-1H-pyrazole (1 microM TRAM-34, an IKCa blocker). During depolarization and contraction in response to phenylephrine (PE), ACh still increased the membrane potential to around -70 mV, but with apamin present the membrane potential only increased just beyond the original resting potential (circa -58 mV). TRAM-34 alone did not affect hyperpolarization to ACh but, in combination with apamin, ACh-evoked hyperpolarization was completely abolished. These data suggest that true endothelium-dependent hyperpolarization of smooth muscle cells in response to ACh is attributable to SKCa channels, whereas IKCa channels play an important role during the ACh-mediated repolarization phase only observed following depolarization.

Sphingosine kinase modulates microvascular tone and myogenic responses through activation of RhoA/Rho kinase

BACKGROUND

RhoA and Rho kinase are important modulators of microvascular tone.

METHODS AND RESULTS

We tested whether sphingosine kinase (Sphk1) that generates the endogenous sphingolipid mediator sphingosine-1-phosphate (S1P) is part of a signaling cascade to activate the RhoA/Rho kinase pathway. Using a new transfection model, we report that resting tone and myogenic responses of isolated resistance arteries increased with forced expression of Sphk1 in smooth muscle cells of these arteries. Overexpression of a dominant negative Sphk1 mutant or coexpression of dominant negative mutants of RhoA or Rho kinase together with Sphk1 completely inhibited development of tone and myogenic responses.

CONCLUSIONS

The tone-increasing effects of a Sphk1 overexpression suggest that Sphk1 may play an important role in the control of peripheral resistance.

Conducted dilations initiated by purines in arterioles are endothelium dependent and require endothelial Ca2+

The signaling pathways underlying the regulation of vascular resistance by purines in intact microvessels and particularly in communication of remote vasomotor responses are unclear. One process by which remote regions of arterioles communicate is via transmission of signals axially along the vessel wall. In this study, we identified a pathway for local and conducted dilations initiated by purines. Adenosine (Ado) or ATP (bind P1 and P2 purinergic receptors, respectively) was micropipette applied to arterioles (maximum diameter approximately 40 microm) in the cheek pouch of anesthetized hamsters. Observations were made at the site of stimulation (local) or approximately 1200 microm upstream along the same vessel. P2 antagonists (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid tetrasodium and suramin) inhibited local constriction to ATP, whereas local and upstream dilations were unaffected. In contrast, during inhibition of P1 receptors (with xanthine amine congener) the local constriction was unchanged, whereas both local and upstream dilations to ATP were inhibited. Hydrolysis of ATP to Ado is implicated in the dilator response as blocking 5'-ectonucleotidase (with alpha,beta-methyleneadenosine 5'-diphosphate) attenuated ATP-induced dilations. After endothelium denudation, constriction to ATP was unchanged, but dilations to both ATP and Ado were inhibited, identifying endothelial cells (ECs) as the primary target for P1-mediated dilation. Purines increased EC Ca2+ locally and upstream. Chelation of EC Ca2+ (with BAPTA) abolished the local and upstream dilations to P1 receptor stimulation. Collectively, these data demonstrate that stimulation of P1 receptors on ECs produces a vasodilation that spreads to remote regions. There is an associated increase in EC Ca2+, which is a required signaling intermediate in the manifestation of both the local and axially communicated arteriolar dilations.

Nitric oxide-induced decrease in calcium sensitivity of resistance arteries is attributable to activation of the myosin light chain phosphatase and antagonized by the RhoA/Rho kinase pathway

BACKGROUND

NO-induced dilations in resistance arteries (RAs) are not associated with decreases in vascular smooth muscle cell Ca2+. We tested whether a cGMP-dependent activation of the smooth muscle myosin light chain phosphatase (MLCP) resulting in a Ca2+ desensitization of the contractile apparatus was the underlying mechanism and whether it could be antagonized by the RhoA pathway.

METHODS AND RESULTS

The Ca2+ sensitivity of RA was assessed as the relation between changes in diameter and [Ca2+]i in depolarized RA (120 mol/L K+) exposed to stepwise increases in Ca2+ex (0 to 3 mmol/L). Effects of 10 micromol/L sodium nitroprusside (SNP) on Ca2+ sensitivity were determined before and after application of the soluble guanylate cyclase inhibitor ODQ (1 micromol/L) and the MLCP inhibitor calyculin A (120 nmol/L) and in presence of the RhoA-activating phospholipid sphingosine-1-phosphate (S1P, 12 nmol/L). SNP-induced dilations were also studied in controls and in RAs pretreated with the Rho kinase inhibitor Y27632 or transfected with a dominant-negative RhoA mutant (N19RhoA). Constrictions elicited by increasing Ca2+ex were significantly attenuated by SNP, which, however, left associated increases in [Ca2+]i unaffected. This NO-induced attenuation was blocked by ODQ, calyculin A, and S1P. The S1P-induced translocation of RhoA indicating activation of the GTPase was not reversed by SNP. Inhibition of RhoA/Rho kinase by N19RhoA or Y27632 significantly augmented SNP-induced dilations.

CONCLUSIONS

NO dilates RA by activating the MLCP in a cGMP-dependent manner, thereby reducing the apparent Ca2+ sensitivity of the contractile apparatus. MLCP inactivation via the RhoA/Rho kinase pathway antagonizes this Ca2+-desensitizing effect that, in turn, can be restored using RhoA/Rho kinase inhibitors.

Contributions of nitric oxide, EDHF, and EETs to endothelium-dependent relaxation in renal afferent arterioles

BACKGROUND

Acetylcholine-induced endothelium-dependent relaxation in the renal afferent arteriole has been ascribed to nitric oxide, but the role of endothelium-derived hyperpolarizing factors (EDHFs) and 14,15-epoxyeicosatrienoic acid (14,15-EET) are unclear.

METHODS

Single afferent arterioles were dissected from kidney of normal rabbits and microperfused in vitro at 60 mm Hg. Vessels were preconstricted submaximally with norepinephrine (10(-8) mol/L). Relaxation was assessed following cumulative addition of ACh (10(-9) to 10(-4) mol/L) alone, or in the presence of indomethacin (to inhibit cyclooxygenase), Nw-nitro-L-arginine (L-NNA) (to inhibit nitric oxide synthase), methylene blue (to inhibit soluble guanylate cyclase), or a combination of L-NNA + methylene blue. To assess contributions by EDHF, studies were repeated with either apamin + charybdotoxin [to block Ca2+-activated K+ channels (KCa)] or with 40 mmol/L KCl. To asses the role of 14,15-EET, relaxations were evaluated in the presence of its competitive inhibitor 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE).

RESULTS

Relaxation by acetylcholine was abolished following endothelial denudation. It was unaffected by indomethacin but was inhibited 54%+/- 5% (P < 0.001) by L-NNA, 57%+/- 5% by methylene blue, and 60%+/- 4% by the combination of L-NNA plus methylene blue. Relaxation was inhibited further by KCl (80%+/- 6%) or by apamin + charybdotoxin (96%+/- 2%). 14,15-EEZE, alone, inhibited acetylcholine-induced relaxation by 29%+/- 3%, and by 80%+/- 5% in the presence of L-NNA.

CONCLUSION

Acetylcholine-induced afferent arteriolar relaxation depends strongly on both nitric oxide, acting via soluble guanylate cyclase, and on an EDHF, likely 14,15-EET, acting via KCa.

Characterization of the endothelium-derived hyperpolarizing factor (EDHF) response in the human interlobar artery

BACKGROUND

In addition to nitric oxide (NO) and prostacyclin (PGI2), the vascular endothelium can influence local vascular tone by a mechanism involving the hyperpolarization of vascular smooth muscle cells. This response is attributed to the release of an endothelium-derived hyperpolarizing factor (EDHF). The present study was performed to determine the characteristics of the EDHF that mediates the NO/PGI2-independent hyperpolarization and relaxation of human renal interlobar arteries.

METHODS

Acetylcholine-induced, EDHF-mediated hyperpolarization and relaxation were assessed using sharp microelectrodes impaled into interlobar smooth muscle cells and in organ chamber experiments, respectively. All experiments were performed in the combined presence of NO synthase (NOS) and cyclooxygenase inhibitors and the thromboxane analog U46619.

RESULTS

Interlobar arteries demonstrated pronounced NO/PGI2-independent relaxations and hyperpolarizations that were sensitive to the blockade of calcium-activated K+-channels (KCa+ channels) by the combination of charybdotoxin and apamin and to the inhibition of the Na-K-ATPase by ouabain. Exogenously applied KCl also exhibited relaxations and hyperpolarizations that were sensitive to ouabain but insensitive to the combined inclusion of charybdotoxin and apamin. Relaxations induced by KCl were also observed in endothelium-denuded interlobar arteries.

CONCLUSION

These results indicate that in the human renal interlobar artery, EDHF-mediated responses display the pharmacologic characteristics of K+ ions released through endothelial KCa+ channels. Smooth muscle cell hyperpolarization and relaxation appear to be dependent on the activation of ouabain-sensitive subunits of the Na-K-ATPase.

Increased nitric oxide production following chronic hypoxia contributes to attenuated systemic vasoconstriction

Attenuated vasoconstrictor reactivity following chronic hypoxia (CH) is associated with endothelium-dependent vascular smooth muscle (VSM) cell hyperpolarization and diminished intracellular [Ca(2+)]. We tested the hypothesis that increased production of nitric oxide (NO) after CH contributes to blunted vasoconstrictor responsiveness. We found that basal NO production of mesenteric arteries from CH rats (barometric pressure = 380 Torr; 48 h) was greater than that of controls (barometric pressure = 630 Torr). In addition, studies employing pressurized mesenteric arteries (100-200 microM ID) abluminally loaded with the Ca(2+) indicator fura 2-AM demonstrated that although NO synthase (NOS) inhibition normalized agonist-induced vasoconstrictor responses between groups, VSM cell [Ca(2+)] in vessels from CH rats remained diminished compared with controls. To determine whether elevated NO production following CH results from increased NOS protein levels, we performed Western blots for NOS isoforms by using mesenteric arteries from control and CH rats. Endothelial NOS levels did not differ between groups, and other NOS isoforms were not detected in these samples. Selective endothelial loading of fura 2-AM was employed to test the hypothesis that elevated endothelial cell [Ca(2+)] following CH accounts for enhanced NOS activity. These experiments demonstrated greater endothelial cell [Ca(2+)] in mesenteric arteries isolated from CH rats compared with controls. We conclude that enhanced production of NO resulting from elevated endothelial cell [Ca(2+)] contributes to attenuated reactivity following CH by decreasing VSM cell Ca(2+) sensitivity.

An endothelium-derived factor modulates purinergic neurotransmission to mesenteric arterial smooth muscle of hamster

The interaction between the endothelium and purinergic perivascular nerves was investigated by measuring the changes in amplitude of excitatory junction potential (EJP) of smooth muscle cells in hamster mesenteric arteries (100-350 microm). Uridin-5'-triphosphate (UTP) (100 microM) applied to endothelium-intact preparations evoked a hyperpolarization of 17.0 +/- 0.7 mV (n=46). During this hyperpolarization, the amplitude of electrically evoked EJPs was inhibited to about 50% of that of the control. In endothelium-denuded preparations, UTP (100 microM) neither hyperpolarized the smooth muscle nor inhibited the amplitude of the EJP. Neither a nitric oxide (NO) synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME) (100 microM), nor a cyclooxygenase inhibitor, indomethacin (1 microM), had an effect on the UTP-evoked hyperpolarization and inhibition of the electrically evoked EJP. The UTP-evoked membrane hyperpolarization and inhibition of the EJP amplitude was antagonized by the P2Y receptor antagonist, cibacron blue (100 microM). Endothelium-derived hyperpolarizing factor (EDHF)-mediated hyperpolarization was inhibited by either adventitial or intimal application of apamin (0.1 micro and charybdotoxin (0.1 microM). However, the EJP inhibition was still present. In apamin- and charybdotoxin-treated preparations, focal application of adenosine 5'-triphosphate (ATP) (10 mM) evoked a depolarization of 15.5 +/- 1.3 mV (n=15). This postjunctional response was not modified by UTP (15.3 +/- 1.7 mV, n=4, P>0.05). These results suggest that exogenously applied UTP activates P2Y receptors of endothelium to release endothelium-derived factors, which in turn inhibit ATP release from purinergic nerves.

Cytochrome P450 2C9 plays an important role in the regulation of exercise-induced skeletal muscle blood flow and oxygen uptake in humans

Previous studies show that exercise-induced hyperaemia is unaffected by systemic inhibition of nitric oxide synthase (NOS) and it has been proposed that this may be due to compensation by other vasodilators. We studied the involvement of cytochrome P450 2C9 (CYP 2C9) in the regulation of skeletal muscle blood flow in humans and the interaction between CYP 2C9 and NOS. Seven males performed knee extensor exercise. Blood flow was measured by thermodilution and blood samples were drawn frequently from the femoral artery and vein at rest, during exercise and in recovery. The protocol was repeated three times on the same day. The first and the third protocols were controls, and in the second protocol either the CYP 2C9 inhibitor sulfaphenazole alone, or sulfaphenazole in combination with the NOS inhibitor N(omega)-monomethyl-L-arginine (L-NMMA) were infused. Compared with control there was no difference in blood flow at any time with sulfaphenazole infusion (P > 0.05) whereas with infusion of sulfaphenazole and L-NMMA, blood flow during exercise was 16 +/- 4 % lower than in control (9 min: 3.67 +/- 0.31 vs. 4.29 +/- 0.20 l min(-1); P < 0.05). Oxygen uptake during exercise was 12 +/- 3 % lower (9 min: 525 +/- 46 vs. 594 +/- 24 ml min(-1); P < 0.05) with co-infusion of sulfaphenazole and L-NMMA, whereas oxygen uptake during sulfaphenazole infusion alone was not different from that of control (P > 0.05). The results demonstrate that CYP 2C9 plays an important role in the regulation of hyperaemia and oxygen uptake during exercise. Since inhibition of neither NOS nor CYP 2C9 alone affect skeletal muscle blood flow, an interaction between CYP 2C9 and NOS appears to exist so that a CYP-dependent vasodilator mechanism takes over when NO production is compromised.

Depolarization of endothelial cells enhances platelet aggregation through oxidative inactivation of endothelial NTPDase

OBJECTIVE

The objective of this study was to investigate whether depolarization of cultured endothelial cells (human umbilical vein endothelial cells, HUVECs) affects their ectonucleotidase activity through superoxide (O2-) production.

METHODS AND RESULTS

Depolarization by the cation channel gramicidin (100 nmol/L) or tetrabutylammonium chloride (1 mmol/L) induced O2- release from HUVECs (n=4), which was decreased by superoxide dismutase (SOD, 500 U/mL). The activity of endothelial ectonucleotidases was assessed by the production of inorganic phosphate from ADP, which was decreased by chronic depolarization by 25% (n=6, P<0.05) and the amount of AMP derived from ADP in the presence of the 5'-nucleotidase inhibitor alpha,beta-methylene-5'-diphosphate (100 micromol/L). AMP was decreased by chronic depolarization from 0.54+/-0.16 to 0.39+/-0.11 micromol/min/mg protein (n=6, P<0.05). This was abolished in the continuous presence of SOD (n=6). NTPDase protein was predominantly expressed in HUVECs (n=4). Protein abundance, viability of cells, and apoptosis rates were not altered by depolarization (n=10). Only in the presence of depolarized HUVECs, but not with control cells or with HUVECs depolarized in the presence of SOD, did 5 micromol/L of ADP cause irreversible platelet aggregation. Increases in transmural pressure induced endothelial depolarization in intact hamster small arterioles.

CONCLUSIONS

Depolarization causes the endothelial production of O2-, which inhibits the activity of endothelial ectonucleotidases. Increases in transmural pressure induce endothelial depolarization. In chronically hypertensive diseases, depolarization might favor platelet aggregation.

Arteriolar occlusion causes independent cellular responses in endothelium and smooth muscle

OBJECTIVES

To test the hypothesis that arteriolar occlusion causes different cellular changes in endothelial and smooth muscle cells.

METHODS

Cheek pouch arterioles (resting diameter 41 +/- 2 microm) of anesthetized hamsters were occluded briefly (<60 seconds) either upstream or downstream from an observation site. Changes in membrane potential and intracellular calcium concentration ([Ca(2+)](i)) of the endothelial or smooth muscle cells were determined by using fluorescence microscopy (ratiometric analysis).

RESULTS

The pressure in the occluded segment decreased by 17.4 +/- 2.6 cm H(2)O during upstream occlusion and increased by 16.8 +/- 6 cm H(2)O during downstream occlusion (n = 5). Upstream occlusion caused vasoconstriction of the occluded segment by 2.4 +/- 0.4 microm, whereas downstream occlusion produced brief vasodilatation by 1.1 +/- 0.2 microm. The endothelial cells hyperpolarized during upstream or downstream occlusion (ratio change: 2.26 +/- 0.24% and 2.39 +/- 0.42%, respectively; p < 0.01, n = 5). There were no changes in endothelial [Ca(2+)](i). The smooth muscle cells depolarized (ratio change: -2.08 +/- 0.14%, n = 5) with an increase in [Ca(2+)](i) (ratio change: 2.92 +/- 0.16%, n = 6) during downstream occlusion. However, there was no detectable change in membrane potential or [Ca(2+)](i) of smooth muscle cells during upstream occlusion. All the changes rapidly recovered when occlusion was released. Responses of an in-situ isolated segment on a side branch revealed conducted dilatory signals caused by the occlusions.

CONCLUSIONS

Our results show that the endothelial and smooth muscle cells respond independently to arteriolar occlusion. The endothelial and smooth muscle cells do not effectively communicate in [Ca(2+)](i) or membrane potential during acute arteriolar occlusion. Hyperpolarizing signals in endothelium cause conducted dilation.

EDHF, but not NO or prostaglandins, is critical to evoke a conducted dilation upon ACh in hamster arterioles

Vasomotor reactions upon focal stimulation of arterioles have been shown to be conducted along the vascular wall. Such a conduction, which is assumed to reflect the spread of electrical signals, may contribute to coordination of responses within a vascular segment. We aimed to identify which endothelial autacoid(s) act as mediators of the local and conducted dilator responses, respectively. To this end, arterioles in the hamster cremaster microcirculation were locally stimulated with endothelium-dependent [acetylcholine (ACh)] or endothelium-independent dilators [sodium nitroprusside (SNP)], and the resulting changes in diameter were measured using a videomicroscopy technique at the site of application and up to 1.4 mm upstream at distant sites. Experiments were also performed after blockade of nitric oxide (NO) synthase, cyclooxygenase, P-450 monooxygenase, or K(+) channels. Dilations upon ACh (71 +/- 3%) were conducted rapidly (<1 s) to upstream sites (at 1.4 mm: 37 +/- 5%). Although the NO donor SNP induced a similar local dilation (71 +/- 7%), this response was not conducted. Maximal amplitudes of ACh-induced dilations were not attenuated after inhibition of NO synthase and cyclooxygenase at the local and remote sites. However, additional treatment with a P-450 monooxygenase blocker (sulfaphenazole) strongly attenuated the local response (from 62 +/- 9 to 17 +/- 5%) and abrogated dilations at distant sites (at 0.67 mm: from 23 +/- 4% to 4 +/- 3%). Likewise, 17-octadecynoic acid strongly attenuated local and remote responses. Blockers of Ca(2+)-dependent K(+) channels (charybdotoxin or iberiotoxin) attenuated dilations at the local and remote sites after focal application at the ACh stimulation site. In marked contrast, treatment of the upstream site with these blockers was without any effect. We conclude that upon local stimulation with ACh, a cytochrome P-450 monooxygenase product is generated that induces local dilation via the activation of Ca(2+)-dependent K(+) channels and initiates conduction of the dilation. In contrast to the local site, neither activation of these K(+) channels nor the synthesis of NO or prostaglandins is necessary to dilate the arterioles at remote, distant sites. This suggests that endothelium-derived hyperpolarizing factor serves as an important mediator to initiate conducted dilations and, by doing so, may act as a key player in the coordination of arteriolar behavior in the microcirculatory network.

Endothelium-dependent hyperpolarization as a remote anti-atherogenic mechanism

Endothelial cell injury and the loss of cytoprotective mechanisms that involve nitric oxide, prostacyclin and endothelium-dependent hyperpolarization (EDH) are thought to underlie atherosclerosis, although how these mechanisms are anti-atherogenic is unclear. This is particularly so because thrombus formation, one of the major initiators of the disease, usually occurs at discrete luminal sites; thus, only small numbers of endothelial cells can be recruited to initiate anti-inflammatory responses. However, we, and others, have demonstrated that locally generated EDH spreads to endothelial cells and smooth muscle cells throughout a vessel to cause remote vasodilatation. In this article, we propose that, in addition to a widespread inhibitory signalling mechanism, EDH produced by the endothelium also initiates remote anti-inflammatory actions that prevent large blood vessels developing atherosclerosis.

Role of calcium-activated potassium channels in impaired acetylcholine vasodilatory responses in diabetic rats

Muscarinic agonists produce endothelium-dependent vasodilatation in the presence of nitric oxide synthase (NOS) inhibition. The importance of this mechanism was assessed in the methoxamine-preconstricted perfused mesenteric vascular bed (MVB) of streptozotocin diabetic Sprague-Dawley rats. At 9 weeks of age, male rats were treated with streptozotocin (55 mg/kg in citrate buffer) or with citrate buffer alone. The superior mesenteric artery was cannulated and the MVB was detached from its intestinal borders. Concentration-response curves to acetylcholine were determined in the presence and in the absence of indomethacin, tetrabutylammonium (a calcium-activated potassium channel blocker), high extracellular potassium, or NOS inhibition with Nomega-nitro-l-arginine and l-NG-nitro-l-arginine. There was a rightward shift in the concentration-response curve with an increase in median inhibitory concentration (p < 0.05) and a reduction in acetylcholine IMAX (p < 0.05) values in 14-week streptozotocin rats. The ability of NOS inhibition to attenuate vasodilatation was reduced in the 14-week streptozotocin group relative to the 2-week streptozotocin treatment group (p < 0.05). However, the ability of tetrabutylammonium to block acetylcholine-mediated vasodilatation remained consistent in streptozotocin rats at both stages. The results demonstrate that an alternate pathway involving calcium-activated potassium channels may compensate for diminished nitric oxide bioactivity. This effect is contingent on the duration of diabetes. This study provides insight into the development and progression of altered diabetic vascular responses.

Increases in endothelial Ca(2+) activate K(Ca) channels and elicit EDHF-type arteriolar dilation via gap junctions

In skeletal muscle arterioles, the pathway leading to non-nitric oxide (NO), non-prostaglandin-mediated endothelium-derived hyperpolarizing factor (EDHF)-type dilations is not well characterized. To elucidate some of the steps in this process, simultaneous changes in endothelial intracellular Ca(2+) concentration ([Ca(2+)](i)) and the diameter of rat gracilis muscle arterioles (approximately 60 microm) to acetylcholine (ACh) were measured by fura 2 microfluorimetry (in the absence of NO and prostaglandins). ACh elicited rapid increases in endothelial [Ca(2+)](i) (101 +/- 7%), followed by substantial dilations (73 +/- 2%, coupling time: 1.3 +/- 0.2 s) that were prevented by endothelial loading of an intracellular Ca(2+) chelator [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid]. Arteriolar dilations to ACh were also inhibited by intraluminal administration of the Ca(2+)-activated K(+) (K(Ca)) channel blockers charybdotoxin plus apamin or by palmitoleic acid, an uncoupler of myoendothelial gap junctions without affecting changes in endothelial [Ca(2+)](i). The presence of large conductance K(Ca) channels on arteriolar endothelial cells was demonstrated with immunohistochemisty. We propose that in skeletal muscle arterioles, EDHF-type mediation is evoked by an increase in endothelial [Ca(2+)](i), which by activating endothelial K(Ca) channels elicits hyperpolarization that is conducted via myoendothelial gap junctions to the smooth muscle resulting in decreases in [Ca(2+)](i) and consequently dilation.

cAMP facilitates EDHF-type relaxations in conduit arteries by enhancing electrotonic conduction via gap junctions

We have investigated the role of cAMP in NO- and prostanoid-independent relaxations that are widely attributed to an endothelium-derived hyperpolarizing factor (EDHF). Under control conditions EDHF-type relaxations evoked by acetylcholine (ACh) in rabbit iliac arteries were transient, but in the presence of the cAMP phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) or the cell permeant cAMP analog 8-bromo-cAMP, relaxations became sustained with their maxima potentiated approximately 2-fold. Relaxation was associated with transient approximately 1.5-fold elevations in smooth muscle cAMP levels with both mechanical and nucleotide responses being abolished by interrupting gap junctional communication with the connexin-mimetic peptide Gap 27 and by endothelial denudation. However, IBMX induced a sustained endothelium-independent approximately 2-fold rise in cAMP levels, which was not further amplified by ACh, suggesting that the contribution of cAMP to the EDHF phenomenon is permissive. After selective loading of the endothelium with calcein AM, direct transfer of dye from the endothelium to the media was enhanced by IBMX or 8-bromo-cAMP, but not by 8-bromo-cGMP, whereas Gap 27 promoted sequestration within the intima. ACh-induced hyperpolarizations of subintimal smooth muscle in arterial strips with intact endothelium were abolished by Gap 27 and the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine but were unaffected by IBMX. By contrast, in strips partially denuded of endothelium, IBMX enhanced the transmission of hyperpolarization from the endothelium to remote smooth muscle cells. These findings support the hypothesis that endothelial hyperpolarization underpins the EDHF phenomenon, with cAMP governing subsequent electrotonic signaling via both myoendothelial and homocellular smooth muscle gap junctions.

Gap junction-dependent and -independent EDHF-type relaxations may involve smooth muscle cAMP accumulation

We have compared the mechanisms that contribute to endothelium-derived hyperpolarizing factor (EDHF)-type responses induced by ACh and the Ca(2+) ionophore A-23187 in the rabbit iliac artery. Relaxations to both agents were associated with ~1.5-fold elevations in smooth muscle cAMP levels and were attenuated by the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine (DDA) and potentiated by the cAMP phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). Mechanical responses were inhibited by coadministration of the Ca(2+)-activated K(+) channel blockers apamin and charybdotoxin, both in the absence and presence of IBMX, but were unaffected by blockade of ATP-sensitive K(+) channels with the sulphonylurea glibenclamide. Relaxations and elevations in cAMP evoked by ACh were abolished by 18alpha-glycyrrhetinic acid, which disrupts gap junction plaques, whereas the corresponding responses to A-23187 were unaffected by this agent. Consistently, in "sandwich" bioassay experiments, A-23187, but not ACh, elicited extracellular release of a factor that evoked relaxations that were inhibited by DDA and potentiated by IBMX. These findings provide evidence that EDHF-type relaxations of rabbit iliac arteries evoked by ACh and A-23187 depend on cAMP accumulation in smooth muscle, but involve signaling via myoendothelial gap junctions and the extracellular space, respectively.