Substance P-induced relaxation and hyperpolarization in human cerebral arteries

Putative neurochemical and cell type contributions to hemodynamic activity in the rodent caudate putamen

Functional magnetic resonance imaging (fMRI) is widely used by researchers to noninvasively monitor brain-wide activity. The traditional assumption of a uniform relationship between neuronal and hemodynamic activity throughout the brain has been increasingly challenged. This relationship is now believed to be impacted by heterogeneously distributed cell types and neurochemical signaling. To date, most cell-type- and neurotransmitter-specific influences on hemodynamics have been examined within the cortex and hippocampus of rodent models, where glutamatergic signaling is prominent. However, neurochemical influences on hemodynamics are relatively unknown in largely GABAergic brain regions such as the rodent caudate putamen (CPu). Given the extensive contribution of CPu function and dysfunction to behavior, and the increasing focus on this region in fMRI studies, improved understanding of CPu hemodynamics could have broad impacts. Here we discuss existing findings on neurochemical contributions to hemodynamics as they may relate to the CPu with special consideration for how these contributions could originate from various cell types and circuits. We hope this review can help inform the direction of future studies as well as interpretation of fMRI findings in the CPu.

Terminalia fagifolia Mart. & Zucc. elicits vasorelaxation of rat thoracic aorta through nitric oxide and K+ channels dependent mechanism

Terminalia fagifolia Mart. & Zucc. (Combretaceae) is a plant commonly found in the regions of the Brazilian cerrado, popularly used for the treatment of gastrointestinal disorders. There are no reports in the literature on the use of T. fagifolia for the treatment of the cardiovascular system conditions. Nevertheless, plants of the same genus, such as Terminaliaarjuna (Roxb.) Wight & Arn and Terminaliasuperba Engler & Diels, present cardioprotective, hypotensive and vasodilatating effects. In light of this, the aim of the study was to investigate the effect of the ethanolic extract (Tf-EE) and of its aqueous (Tf-AQF), hexanic (Tf-HEXF) and hydroethanolic (Tf-HAF) partition fractions obtained from the stem bark of T.fagifolia Mart. & Zucc. The effects of the extract and partition fractions of T. fagifolia were evaluated on isometric tensions in the thoracic aorta rings of Wistar rats (250–300 g). Tf-EE, Tf-HEXF and Tf-HAF presented a concentration-dependent vasorelaxant effect, and Tf-AQF presented a vasorelaxant effect that was more potent in the presence of endothelium. The relaxation curves of the aorta promoted by the fraction investigated were attenuated in the presence of the following pharmacological tools: L-NAME, ODQ or PTIO. The vasorelaxant effect of the aorta promoted by Tf-AQF was attenuated in the presence of TEA and 4-AP. Tf-EE induced a concentration-dependent and endothelium-independent vasorelaxation. Tf-HAF and Tf-HEXF presented concentration-dependent and vascular-endothelium-independent vasorelaxation, but did not obtain 100% of relaxation. On the other hand, Tf-AQF presented concentration-dependent vasorelaxation that was more potent in aorta rings with vascular endothelium. The relaxant mechanism induced by the Tf-AQF involves the NO/sGC/cGMP pathway and channels Kv.

Summary: The investigation of the relaxing effect of extract and fractions of the stem bark partition of Terminalia fagifolia on aortic rings is a pioneering study involving the participation of K⁺ channels, which demonstrates a potential alternative therapeutic method for the treatment of cardiovascular diseases.

The Regulation of Pulmonary Vascular Tone by Neuropeptides and the Implications for Pulmonary Hypertension

Pulmonary hypertension (PH) is an incurable, chronic disease of small pulmonary vessels. Progressive remodeling of the pulmonary vasculature results in increased pulmonary vascular resistance (PVR). This causes secondary right heart failure. PVR is tightly regulated by a range of pulmonary vasodilators and constrictors. Endothelium-derived substances form the basis of most current PH treatments. This is particularly the case for pulmonary arterial hypertension. The major limitation of current treatments is their inability to reverse morphological changes. Thus, there is an unmet need for novel therapies to reduce the morbidity and mortality in PH. Microvessels in the lungs are highly innervated by sensory C fibers. Substance P and calcitonin gene-related peptide (CGRP) are released from C-fiber nerve endings. These neuropeptides can directly regulate vascular tone. Substance P tends to act as a vasoconstrictor in the pulmonary circulation and it increases in the lungs during experimental PH. The receptor for substance P, neurokinin 1 (NK1R), mediates increased pulmonary pressure. Deactivation of NK1R with antagonists, or depletion of substance P prevents PH development. CGRP is a potent pulmonary vasodilator. CGRP receptor antagonists cause elevated pulmonary pressure. Thus, the balance of these peptides is crucial within the pulmonary circulation (Graphical Abstract). Limited progress has been made in understanding their impact on pulmonary pathophysiology. This is an intriguing area of investigation to pursue. It may lead to promising new candidate therapies to combat this fatal disease. This review provides a summary of the current knowledge in this area. It also explores possible future directions for neuropeptides in PH.

Blood-brain barrier KCa3.1 channels: evidence for a role in brain Na uptake and edema in ischemic stroke

BACKGROUND AND PURPOSE

KCa3.1, a calcium-activated potassium channel, regulates ion and fluid secretion in the lung and gastrointestinal tract. It is also expressed on vascular endothelium where it participates in blood pressure regulation. However, the expression and physiological role of KCa3.1 in blood-brain barrier (BBB) endothelium has not been investigated. BBB endothelial cells transport Na(+) and Cl(-) from the blood into the brain transcellularly through the co-operation of multiple cotransporters, exchangers, pumps, and channels. In the early stages of cerebral ischemia, when the BBB is intact, edema formation occurs by processes involving increased BBB transcellular Na(+) transport. This study evaluated whether KCa3.1 is expressed on and participates in BBB ion transport.

METHODS

The expression of KCa3.1 on cultured cerebral microvascular endothelial cells, isolated microvessels, and brain sections was evaluated by Western blot and immunohistochemistry. Activity of KCa3.1 on cerebral microvascular endothelial cells was examined by K(+) flux assays and patch-clamp. Magnetic resonance spectroscopy and MRI were used to measure brain Na(+) uptake and edema formation in rats with focal ischemic stroke after TRAM-34 treatment.

RESULTS

KCa3.1 current and channel protein were identified on bovine cerebral microvascular endothelial cells and freshly isolated rat microvessels. In situ KCa3.1 expression on BBB endothelium was confirmed in rat and human brain sections. TRAM-34 treatment significantly reduced Na(+) uptake, and cytotoxic edema in the ischemic brain.

CONCLUSIONS

BBB endothelial cells exhibit KCa3.1 protein and activity and pharmacological blockade of KCa3.1 seems to provide an effective therapeutic approach for reducing cerebral edema formation in the first 3 hours of ischemic stroke.

TRPA1

The transient receptor potential ankyrin subtype 1 protein (TRPA1) is a nonselective cation channel permeable to Ca(2+), Na(+), and K(+). TRPA1 is a promiscuous chemical nocisensor that is also involved in noxious cold and mechanical sensation. It is present in a subpopulation of Aδ- and C-fiber nociceptive sensory neurons as well as in other sensory cells including epithelial cells. In primary sensory neurons, Ca(2+) and Na(+) flowing through TRPA1 into the cell cause membrane depolarization, action potential discharge, and neurotransmitter release both at peripheral and central neural projections. In addition to being activated by cysteine and lysine reactive electrophiles and oxidants, TRPA1 is indirectly activated by pro-inflammatory agents via the phospholipase C signaling pathway, in which cytosolic Ca(2+) is an important regulator of channel gating. The finding that non-electrophilic compounds, including menthol and cannabinoids, activate TRPA1 may provide templates for the design of non-tissue damaging activators to fine-tune the activity of TRPA1 and raises the possibility that endogenous ligands sharing binding sites with such non-electrophiles exist and regulate TRPA1 channel activity. TRPA1 is promising as a drug target for novel treatments of pain, itch, and sensory hyperreactivity in visceral organs including the airways, bladder, and gastrointestinal tract.

Hemodynamic effects of substance P and its receptor antagonist RP67580 in anesthetized rats with carbon tetrachloride-induced cirrhosis

OBJECTIVE

Substance P (SP) is a vasodilator that may contribute to systemic and splanchnic vasodilatation in cirrhosis. The aim of this study was to determine the effects of SP (dose--13 pg/kg) and its specific inhibitor, RP67580 (dose--300 microg/kg) on mean arterial pressure (MAP) and portal pressure (PP) in cirrhotic rats and controls.

MATERIAL AND METHODS

MAP and PP were measured before and after administering SP and RP67580. Additionally, a small group of cirrhotic rats were pretreated with L-NAME to block the effects of nitric oxide (NO) before measurements.

RESULTS

SP produced transient systemic hypotension in both groups. SP caused a significant increase in PP in cirrhotic rats and a decrease in PP in controls. RP67580 reduced the hypotensive effect of SP, but not completely. RP67580 decreased PP in the cirrhotic group but not in controls. In cirrhotic rats pretreated with L-NAME, SP administration caused a significant decrease in MAP but no significant change in PP.

CONCLUSIONS

Exogenous SP increases PP and decreases MAP in cirrhotic rats. RP687580 decreases PP and reduces SP-induced hypotension in cirrhotic rats. NO blockade abolishes the effect of SP on PP. SP contributes to splanchnic vasodilatation in cirrhosis and this effect may be mediated by NO.

Cardiovascular responses to rat/mouse hemokinin-1, a mammalian tachykinin peptide: systemic study in anesthetized rats

Rat/mouse hemokinin-1 is a mammalian tachykinin peptide whose biological functions have not been well characterized. In the present study, an attempt has been made to investigate the effect and mechanism of action of rat/mouse hemokinin-1 on systemic arterial pressure after intravenous (i.v.) injections in anesthetized rats by comparing it with that of substance P. Our data showed that injection of rat/mouse hemokinin-1 (0.1, 0.3, 1, 3 and 10 nmol/kg) lowered systemic arterial pressure dose-dependently. This effect was significantly blocked by pretreatment with SR140333 (a selective tachykinin NK1 receptor antagonist) and the NO synthase inhibitor L-NAME (Nomega-nitro-L-arginine methyl ester hydrochloride), respectively, but was not affected by bilateral vagotomy or the muscarinic receptor blocker atropine. Compared to rat/mouse hemokinin-1, a dose of 3 nmol/kg of substance P caused biphasic changes in systemic arterial pressure (depressor and pressor responses). The results suggest that the mechanism of the depressor response caused by substance P was similar to rat/mouse hemokinin-1 in that it was inhibited by SR140333 and L-NAME, respectively, but that there was a component of the cardiovascular change induced by rat/mouse hemokinin-1 (but not substance P) that was attenuated by SR48968 (a selective tachykinin NK2 receptor antagonist). The depressor response induced by rat/mouse hemokinin-1 (i.v.) might be explained primarily by the action on endothelial tachykinin NK1 receptors to release endothelium-derived relaxing factor (NO) and this effect was not affected by vagal components. In addition, rat/mouse hemokinin-1 could not induce the pressor response through stimulation of sympathetic ganglion like substance P in anesthetized rats.

Angiotensin II-induced vasodilatation in cerebral arteries is mediated by endothelium-derived hyperpolarising factor

The angiotensin II-induced vasodilatation was evaluated in rat middle cerebral artery, especially regarding endothelium-derived hyperpolarising factor (EDHF), by use of a pressurised arteriograph. The angiotensin II dilatation was partly antagonised by inhibitors of nitric oxide synthase and cyclo-oxygenase. The remaining dilatation was inhibited by the potassium channel blockers, charybdotoxin and apamin, providing direct evidence that angiotensin II induces EDHF-mediated dilatation in cerebral arteries. The angiotensin II dilatation was blocked by the angiotensin AT1 and AT2 receptor blockers candesartan and PD 123319. Both angiotensin AT1 and AT2 receptors were detected on the endothelium by immunohistochemistry.

Possible Role for K + in Endothelium-Derived Hyperpolarizing Factor–Linked Dilatation in Rat Middle Cerebral Artery

Background and Purpose— Endothelium-derived hyperpolarizing factor (EDHF) and K + are vasodilators in the cerebral circulation. Recently, K + has been suggested to contribute to EDHF-mediated responses in peripheral vessels. The EDHF response to the protease-activated receptor 2 ligand SLIGRL was characterized in cerebral arteries and used to assess whether K + contributes as an EDHF.

Methods— Rat middle cerebral arteries were mounted in either a wire or pressure myograph. Concentration-response curves to SLIGRL and K + were constructed in the presence and absence of a variety of blocking agents. In some experiments, changes in tension and smooth muscle cell membrane potential were recorded simultaneously.

Results— SLIGRL (0.02 to 20 μmol/L) stimulated concentration and endothelium-dependent relaxation. In the presence of N G -nitro- l -arginine methyl ester, relaxation to SLIGRL was associated with hyperpolarization and sensitivity to a specific inhibitor of IK Ca , 1-[(2-chlorophenyl)diphenylmethyl]-1 H -pyrazole (1μmol/L), reflecting activation of EDHF. Combined inhibition of K IR with Ba 2+ (30μmol/L) and Na + /K + -ATPase with ouabain (1 μmol/L) markedly attenuated the relaxation to EDHF. Raising extracellular [K + ] to 15 mmol/L also stimulated smooth muscle relaxation and hyperpolarization, which was also attenuated by combined application of Ba 2+ and ouabain.

Conclusions— SLIGRL evokes EDHF-mediated relaxation in the rat middle cerebral artery, underpinned by hyperpolarization of the smooth muscle. The profile of blockade of EDHF-mediated hyperpolarization and relaxation supports a pivotal role for IK Ca channels. Furthermore, similar inhibition of responses to EDHF and exogenous K + with Ba 2+ and ouabain suggests that K + may contribute as an EDHF in the middle cerebral artery.

Effect of nitric oxide synthase inhibitor (L-NAME) on substance P-induced vasodilatation in the dental pulp

AIM

To investigate the vasodilator mechanisms of pulpal vessels, especially the involvement of nitric oxide (NO), during pulpal inflammation.

METHODOLOGY

Eleven cats were prepared for intra-arterial administration of test agents through a lingual artery. The pulpal blood flow was measured by laser Doppler flowmetry from ipsilateral mandibular canine teeth. By using the NO synthase (NOS) inhibitor N(G)-nitro L-arginine methyl ester (L-NAME), the effects of L-NAME on various vasodilators, such as Substance P (SP)-, calcitonin-gene related peptide (CGRP)-, and papaverine-induced vasodilatation, were compared in vivo in 11 feline dental pulps.

RESULTS

L-NAME pretreatment potentiates SP-induced vasodilatation for a duration of approximately 5 h. The increase of pulpal blood flow ranged from 91.47 to 109.91%, which was significantly different from SP injection alone (48.79%, P < 0.05). Other vasodilators such as CGRP and papaverine did not respond to L-NAME pretreatment.

CONCLUSIONS

This study demonstrates that NOS inhibitor L-NAME administration alone has insignificant effects on pulpal blood flow, although L-NAME pretreatment can potentiate SP-induced vasodilatation, probably via increased activity in the enzyme guanylate cyclase. CGRP and papaverine did not respond to L-NAME pretreatment, indicating that they are not mediated via an endothelium-dependent mechanism.

Essential role of Gap junctions in NO- and prostanoid-independent relaxations evoked by acetylcholine in rabbit intracerebral arteries

BACKGROUND AND PURPOSE

Direct intercellular communication via gap junctions may play a central role in endothelium-dependent relaxations that are mediated by a conducted hyperpolarization and do not involve the synthesis of NO and prostanoids. In the present study, inhibitory peptides homologous to the Gap27 domain of the second extracellular loop of connexin37/connexin43 and connexin40, designated as 37,43Gap27 and 40Gap27, respectively, were used to evaluate the role of this mechanism in intracerebral arteries.

METHODS

Isolated rings of rabbit middle cerebral artery were constricted by histamine (10 micromol/L) in the presence of N(G)-nitro-L-arginine methyl ester (300 micromol/L) and indomethacin (10 micromol/L). Concentration-relaxation curves for acetylcholine were constructed in the presence and absence of 37,43Gap27 and 40Gap27. Specific antibodies were used to delineate the distribution of connexin37, connexin40, connexin43, and connexin45 within the arterial wall.

RESULTS

Individually, 37,43Gap27 and 40Gap27 minimally affected endothelium-dependent relaxations to acetylcholine at concentrations of 300 micro mol/L, whereas their combination (at 300 micromol/L each) inhibited the maximal response by approximately 70% and increased the EC50 value for relaxation by approximately 15-fold. In endothelium-denuded rings, this peptide combination did not attenuate responses to sodium nitroprusside, an exogenous source of NO. Gap junction plaques, whose incidence was highest in endothelium, were constructed from connexin40 and connexin43 in the media and connexin37, connexin40, and connexin43 in the endothelium.

CONCLUSIONS

The findings confirm that direct communication via gap junctions contributes to agonist-induced relaxations of intracerebral arteries. More than one connexin subtype appears to participate in such responses.

Oxytocin stimulates cerebral blood flow in rainbow trout (Oncorhynchus mykiss) through a nitric oxide dependent mechanism

Our knowledge of the regulation of cerebral blood flow (CBF) in ectothermic vertebrates is still very limited. In endothermic vertebrates several peptides have been shown to affect CBF through nitric oxide (NO) dependent mechanisms. Using epi-illumination microscopy in rainbow trout in vivo, we have examined the effects of topically administered oxytocin, arginine vasopressin, substance P and bradykinin on the CBF (measured as erythrocyte velocity in venules on the optic lobes). Of these peptides, only oxytocin induced a dose dependent increase in CBF velocity. Blood pressure remained unchanged and the effect was suppressed by the NOS inhibitor, N(omega)-nitro-L-arginine. This indicates that oxytocin causes NO mediated vasodilation in rainbow trout brain.

Effect of NO on EDHF response in rat middle cerebral arteries

Whereas the actual identity of endothelium-derived hyperpolarizing factor (EDHF) is still not certain, it involves a process requiring the endothelium and eliciting hyperpolarization and relaxation of smooth muscle. It is neither nitric oxide (NO) nor prostacyclin, and its presence has been demonstrated in a variety of vessels. Recent studies in peripheral vessels report that EDHF-mediated dilations were either attenuated or blocked by NO. Studies presented here demonstrate that NO does not block EDHF-mediated dilations in cerebral vessels. Rat middle cerebral arteries were cannulated, pressurized, and luminally perfused. EDHF-mediated dilations were elicited by the luminal application of ATP in the presence of N(G)-nitro-L-arginine methyl ester (L-NAME) and indomethacin (inhibitors of NO synthase and cyclooxygenase, respectively). These dilations persisted when S-nitroso-N-acetylpenicillamine, an NO donor, was added exogenously in the presence of L-NAME, or when endogenous NO was present but its cGMP actions were blocked by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, an inhibitor of guanylate cyclase. These findings demonstrate that the EDHF response is not suppressed by NO in cerebral vessels and suggests a role for EDHF during normal physiological conditions.

Cardiovascular pharmacology of anandamide

The fatty acid amide anandamide produces hypotension and a decrease in systemic vascular resistance in vivo. A drop in blood pressure is also seen with synthetic cannabinoid (CB) receptor agonists. The hypotensive responses to anandamide and synthetic cannabinoids are absent in CB1 receptor gene knockout mice. In isolated arteries and perfused vascular beds, anandamide induces vasodilator responses, which cannot be mimicked by synthetic cannabinoids. Instead, vanilloid receptors on perivascular sensory nerves play a key role in these effects of anandamide. Activation of vanilloid receptors by anandamide triggers the release of sensory neuropeptides such as the vasodilator calcitonin gene-related peptide (CGRP). Anandamide is detected in blood and in many cells of the cardiovascular system, and macrophage-derived anandamide may be involved in several hypotensive clinical conditions. Interestingly, cannabinoid and vanilloid receptors display an overlap in ligand recognition properties, and the frequently used CB1 receptor antagonist SR141716A also inhibits vanilloid receptor-mediated responses. The presence of anandamide in endothelial cells, neurones and activated macrophages (monocytes), and its ability to activate CB and vanilloid receptors make this lipid a potential bioregulator in the cardiovascular system.

Endothelium-derived hyperpolarizing factor and potassium use different mechanisms to induce relaxation of human subcutaneous resistance arteries

This investigation examined the hypothesis that release of K(+) accounts for EDHF activity by comparing relaxant responses produced by ACh and KCl in human subcutaneous resistance arteries. Resistance arteries (internal diameter 244+/-12 microm, n=48) from human subcutaneous fat biopsies were suspended in a wire myograph. Cumulative concentration-response curves were obtained for ACh (10(-9) - 3x10(-5) M) and KCl (2.5 - 25 mM) following contraction with noradrenaline (NA; 0.1 - 3 microM). ACh (E(max) 99.07+/-9.61%; -LogIC(50) 7.03+/-0.22; n=9) and KCl (E(max) 74.14+/-5.61%; -LogIC(50) 2.12+/-0.07; n=10)-induced relaxations were attenuated (P<0.0001) by removal of the endothelium (E(max) 8.21+/-5.39% and 11.56+/-8.49%, respectively; n=6 - 7). Indomethacin (10 microM) did not alter ACh-induced relaxation whereas L-NOARG (100 microM) reduced this response (E(max) 61.7+/-3.4%, P<0.0001; n=6). The combination of ChTx (50 nM) and apamin (30 nM) attenuated the L-NOARG-insensitive component of ACh-induced relaxation (E(max): 15.2+/-10.5%, P<0.002, n=6) although these arteries retained the ability to relax in response to 100 microM SIN-1 (E(max) 127.6+/-13.0%, n=3). Exposure to BaCl(2) (30 microM) and Ouabain (1 mM) did not attenuate the L-NOARG resistant component of ACh-mediated relaxation (E(max), 76.09+/-8.92, P=0.16; n=5). KCl-mediated relaxation was unaffected by L-NOARG+indomethacin (E(max); 68.1+/-5.6%, P=0.33; n=5) or the combination of L-NOARG/indomethacin/ChTx/apamin (E(max); 86.61+/-14.02%, P=0.35; n=6). In contrast, the combination of L-NOARG, indomethacin, ouabain and BaCl(2) abolished this response (E(max), 5.67+/-2.59%, P<0.0001, n=6). The characteristics of KCl-mediated relaxation differed from those of the nitric oxide/prostaglandin-independent component of the response to ACh, and were endothelium-dependent, indicating that K(+) does not act as an EDHF in human subcutaneous resistance arteries.

Determinants of terminal mesenteric artery resistance during the first postnatal month

Experiments were conducted to delineate the vascular effector systems that contribute to setting mesenteric vascular tone in swine during the first postnatal month. Terminal mesenteric arteries (TMA), which function as resistance vessels, were studied in vitro with a microvascular perfusion system allowing independent pressure and flow manipulation. When pressure was varied 0-100 mmHg in the absence of flow, TMA from 1-day-old animals demonstrated myogenic vasoconstriction, whereas TMA from 40-day-old animals did not. In 1- but not 40-day-old TMA, the endothelin A (ET(A)) receptor antagonist BQ-610 shifted the pressure-diameter curve upward, whereas the ET(B) receptor antagonist BQ-788 and the L-arginine analog N(G)-monomethyl-L-arginine (L-NMMA) shifted the curve downward; in all instances, myogenic vasoconstriction was preserved. Flow eliminated myogenic vasoconstriction in 1-day-old TMA, i.e., diameter increased as a function of pressure. The effect of BQ-610 was lost under flow conditions; however, BQ-788 and N-acyl-L-Trp-3,5-bis-(trifluoromethyl) benzyl ester, an antagonist specific to the substance P neurokinin-1 (NK(1)) receptor, shifted the pressure-diameter curve downward in the presence of flow, whereas L-NMMA restored myogenic vasoconstriction. Adding flow had no effect on the pressure-diameter relationship in 40-day-old TMA. Other blocking agents, including prazosin, losartan, indomethacin, and charybdotoxin, had no effect on the pressure-diameter relationship in either age group under flow or no-flow conditions. Constitutive production of nitric oxide (NO) and endothelin-1 participates in setting resistance in 1-day-old TMA, and important stimulants to NO production include flow and activation of ET(B) and NK(1) receptors. In contrast, 40-day-old TMA act as passive conduits in which the elastic properties of the vessel are the primary determinant of diameter.

Endothelium-derived hyperpolarizing factor : identification and mechanisms of action in human subcutaneous resistance arteries

BACKGROUND

Both a vascular endothelial cytochrome P450 (CYP450) product of arachidonic acid metabolism and the potassium ion (K(+)) have been identified as endothelium-derived hyperpolarizing factors (EDHFs) in animal vascular tissues. We studied the relative importance of EDHF, nitric oxide (NO), and prostacyclin (PGI(2)) as vasodilators in human subcutaneous arteries. We also examined the mechanisms underlying the vasodilator action of EDHF to elucidate its identity.

METHODS AND RESULTS

Subcutaneous resistance arteries were obtained from 41 healthy volunteers. The contribution of EDHF to the vasodilation induced by acetylcholine was assessed by inhibiting production of NO, PGI(2), and membrane hyperpolarization. The mechanisms underlying the relaxation evoked by K(+) and EDHF were elucidated. EDHF was found to account for approximately 80% of acetylcholine-mediated vasorelaxation. Its action was insensitive to the combination of barium and ouabain, whereas barium and ouabain reversed K(+)-mediated vasorelaxation. EDHF-mediated vasorelaxation, however, was sensitive to the phospholipase A(2) inhibitor oleyloxyethyl phosphorylcholine and the CYP450 inhibitor ketoconazole.

CONCLUSIONS

EDHF is the major contributor to endothelium-dependent vasorelaxation in human subcutaneous resistance arteries. A product of phospholipase A(2)/CYP450-dependent metabolism of arachidonic acid and not K(+) is the likely identity of EDHF in human subcutaneous resistance arteries.

Different contribution of endothelial nitric oxide in the relaxation of human coronary arteries of ischemic and dilated cardiomyopathic hearts

Coronary artery disease and congestive heart failure (CHF) have been associated with a reduction in nitric oxide (NO) release or bioavailability from the vascular endothelium. The objectives of this study were to compare the role of NO in human coronary vessels isolated from nonischemic dilated (DCM) (n = 10) and ischemic (ICM) (n = 12) cardiomyopathic hearts. Segments were mounted on a wire myograph to record changes in isometric tension. All experiments were performed in the presence of indomethacin (10 microM). Contractions induced by angiotensin II (0.1 microM) or a depolarizing physiologic solution containing 40 mM KCl, were of similar amplitude in DCM and ICM. In vessels precontracted with angiotensin II, acetylcholine (1 microM) caused an endothelium-dependent relaxation of rings from DCM but a paradoxical contraction of rings from ICM; NO synthase inhibition with Nomega-nitro-L-arginine (L-NNA, 100 microM) did not affect acetylcholine-induced relaxation or contraction of DCM or ICM vessels, respectively. By contrast, substance P (0.1 microM) induced an endothelium-dependent relaxation in both groups of vessels; this relaxation was prevented (p < 0.05) by L-NNA in vessels from ICM hearts but only reduced (p < 0.05) by L-NNA in vessels from DCM hearts. In depolarized conditions, acetylcholine contracted (p < 0.05) whereas substance P induced a complete relaxation (p < 0.05) of vessels from both groups: substance P-induced relaxation was abolished (p < 0.05) by L-NNA. Our data suggest that in the presence of indomethacin, NO does not contribute to acetylcholine-induced relaxation of human epicardial coronary arteries isolated from DCM hearts. Furthermore, whereas NO and a secondary endothelium-derived relaxing factor sensitive to high K+ contribute to substance P-induced relaxation of rings from DCM hearts, only NO is involved in ICM hearts.

Adenovirus-mediated gene transfer to human cerebral arteries

Gene therapy is being investigated as a putative treatment option for cardiovascular diseases, including cerebral vasospasm. Because there is presently no information regarding gene transfer to human cerebral arteries, the principal objective of this study was to characterize adenovirus-mediated expression and function of recombinant endothelial nitric oxide synthase (eNOS) gene in human pial arteries. Pial arteries (outer diameter 500 to 1,000 microm) were isolated from 30 patients undergoing temporal lobectomy for intractable seizures and were studied using histologic staining, histochemistry, electron microscopy, and isometric force recording. Gene transfer experiments were performed ex vivo using adenoviral vectors encoding genes for bovine eNOS (AdCMVeNOS) and Escherichia coli beta-galactosidase (AdCMVLacZ). In transduced arteries, studied 24 hours after exposure to vectors, expression of recombinant beta-galactosidase and eNOS was detected by histochemistry, localizing mainly to the adventitia (n = 4). Immunoelectron microscopy localized recombinant eNOS in adventitial fibroblasts. During contractions to U46619, bradykinin-induced relaxations were significantly augmented in AdCMVeNOS-transduced rings compared with control and AdCMVLacZ-transduced rings (P < 0.01; n = 6). The NOS inhibitor L-nitroarginine methylester (L-NAME) caused significantly greater contraction in AdCMVeNOS-transduced rings (P < 0.001; n = 4) and inhibited bradykinin-induced relaxations in control and transduced rings (P < 0.001; n = 6). The current findings suggest that in AdCMVeNOS-transduced human pial arteries, expression of recombinant eNOS occurs mainly in adventitial fibroblasts where it augments relaxations to NO-dependent agonists such as bradykinin. Findings from the current study might be beneficial in future clinical applications of gene therapy for the treatment or prevention of cerebral vasospasm.

Nitric oxide, prostanoid and non-NO, non-prostanoid involvement in acetylcholine relaxation of isolated human small arteries

The main purpose of the study was to clarify to which extent nitric oxide (NO) contributes to acetylcholine (ACh) induced relaxation of human subcutaneous small arteries. Arterial segments were mounted in myographs for recording of isometric tension, NO concentration and smooth muscle membrane potential. In noradrenaline-contracted arteries, ACh induced endothelium-dependent relaxations. The NO synthase inhibitor, N(G)-nitro-L-arginine (L-NOARG) had a small significant effect on the concentration-response curves for ACh, and in the presence of L-NOARG, indomethacin only caused a small additional rightward shift in the ACh relaxation. The NO scavenger, oxyhaemoglobin attenuated relaxations for ACh and for the NO donor S-nitroso-N-acetylpenicillamine (SNAP). Inhibition of guanylyl cyclase with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ), and inhibition of protein kinase G with beta-phenyl-1, N2-etheno-8-bromoguanosine- 3', 5'- cyclic monophosphorothioate, Rp-isomer, slightly attenuated ACh relaxation, but abolished SNAP induced relaxation. ACh induced relaxation without increases in the free NO concentration. In contrast, for equivalent relaxation, SNAP increased the NO concentration 32+/-8 nM. ACh hyperpolarized the arterial smooth muscle cells with 11.4+/-1.3 mV and 10.5+/-1.3 mV in the absence and presence of L-NOARG, respectively. SNAP only elicited a hyperpolarization of 1.6+/-0.9 mV. In the presence of indomethacin and L-NOARG, ACh relaxation was almost unaffected by lipoxygenase inhibition with nordihydroguaiaretic acid, or cytochrome P450 inhibition with 17-octadecynoic acid or econazole. ACh relaxation was strongly reduced by the combination of charybdotoxin and apamin, but small increments in the extracellular potassium concentration induced no relaxations. The study demonstrates that the NO/L-arginine pathway is present in human subcutaneous small arteries and to a limited extent is involved in ACh induced relaxation. The study also suggests a small contribution of arachidonic acid metabolites. However, ACh relaxation is mainly dependent on a non-NO, non-prostanoid endothelium dependent hyperpolarization. British Journal of Pharmacology (2000) 129, 184 - 192

Evidence of partially preserved endothelial dilator function in diseased coronary arteries

OBJECTIVE

To examine the effects of substance P (endothelium dependent vasodilator) and glyceryl trinitrate (endothelium independent vasodilator) on epicardial coronary arteries in patients with normal coronary angiograms and patients with coronary artery disease.

DESIGN

Intracoronary infusions of normal saline, the receptor mediated nitric oxide stimulant substance P (5.6 and 27.8 pmol/min each for five minutes), and glyceryl trinitrate (250 microg bolus) were given in 24 patients with coronary artery disease and stable angina, and in nine patients with normal angiograms. The diameter of proximal and distal coronary segments was measured by computerised quantitative angiography

RESULTS

Proximal segments of patients with coronary artery disease dilated less than those of patients with normal angiograms in response to 27.8 pmol/min substance P (mean (SEM): 7.9 (1.3)% v 15 (2.3)% respectively, p < 0. 01). The proximal segments of diseased arteries also dilated less than those of "normal" arteries in response to glyceryl trinitrate (10.2 (1.6)% v 18.4 (2.9)%, respectively, p < 0.01). The responses of distal segments to substance P and glyceryl trinitrate were similar in the two patient groups. There were correlations (all p < 0.001) between the coronary diameter after substance P and after glyceryl trinitrate in normal proximal segments (r = 0.94) and normal distal segments (r = 0.64), in diseased proximal segments (r = 0.95) and diseased distal segments (r = 0.89), and for coronary stenoses (r = 0.93).

CONCLUSIONS

Proximal segments of patients with coronary disease dilated less than the proximal segments of "normal" patients in response to substance P and glyceryl trinitrate. The response to substance P is substantial and closely correlated with the response to glyceryl trinitrate in both "normal" patients and those with coronary disease. This suggests that although the proximal segments of diseased coronary arteries have a reduced capacity to dilate in response to direct stimulation of smooth muscle cell relaxation, they retain much of their endothelium dependent vasodilator function.

Human coronary arteriolar dilation to bradykinin depends on membrane hyperpolarization: contribution of nitric oxide and Ca2+-activated K+ channels

BACKGROUND

K+ channel activation in vascular smooth muscle cells (VSMCs) plays a key role in regulating vascular tone. It has been proposed that endothelium-derived hyperpolarizing factor (EDHF) contributes to microvascular dilation more than nitric oxide (NO) does. Whether hyperpolarization is important for coronary arteriolar dilation in humans is not known. Bradykinin (BK), an endogenous vasoactive substance, is released from ischemic myocardium and regulates coronary resistance. Therefore, we tested the effects of inhibiting NO synthase, cyclooxygenase, and K+ channels on the changes in diameter and membrane potential (Em) in response to BK in isolated human coronary microvessels.

METHODS AND RESULTS

Arterioles (97+/-4 micrometers; n=120) dissected from human right atrial appendages (n=78) were cannulated at a distending pressure of 60 mm Hg and zero flow. Changes in vessel diameter (video microscopy) and VSMC Em (glass microelectrodes) were measured simultaneously. In vessels constricted and depolarized (Em; -50+/-3 to -28+/-2 mV) with endothelin-1 (ET), dilation to BK was associated with greater membrane hyperpolarization (-48+/-3 mV at 10(-6) mol/L) than dilation to sodium nitroprusside (SNP) (-34+/-2 mV at 10(-4) mol/L) for similar degrees of dilation. Treatment with Nomega-nitro-L-arginine methyl ester (L-NAME; 10(-4) mol/L), an NO synthase inhibitor, partially decreased dilation to BK (maximum dilation 61+/-10% versus control 92+/-4%; P<0.05). Charybdotoxin (CTX; 10(-8) mol/L), a large-conductance Ca2+-activated K+ channel blocker, or apamin (10(-7) mol/L), a small-conductance Ca2+-activated K+ channel blocker, inhibited both dilation (CTX 22+/-6% and apamin 45+/-10% versus control 69+/-6%; P<0.05) and membrane hyperpolarization (CTX -31+/-2 mV and apamin -37+/-2 mV versus control -44+/-2 mV; P<0.05) to BK, whereas glibenclamide (10(-6) mol/L), an ATP-sensitive K+ channel blocker, was without effect.

CONCLUSIONS

Vasodilation of human coronary arterioles to BK is largely dependent on membrane hyperpolarization by Ca2+-activated K+ channel activation, with apparently less of a role for endothelium-derived NO. This suggests a role for K+ channel activation in regulating human coronary arteriolar tone.

P2u receptor-mediated release of endothelium-derived relaxing factor/nitric oxide and endothelium-derived hyperpolarizing factor from cerebrovascular endothelium in rats

BACKGROUND AND PURPOSE

Stimulation of P2u purinoceptors by UTP on endothelium dilates the rat middle cerebral artery (MCA) through the release of endothelium-derived relaxing factor/nitric oxide (EDRF/NO) and an unknown relaxing factor. The purpose of this study was to determine whether this unknown relaxing factor is endothelium-derived hyperpolarizing factor (EDHF).

METHODS

Rat MCAs were isolated, cannulated, pressurized, and luminally perfused. UTP was added to the luminal perfusate to elicit dilations.

RESULTS

Resting outside diameter of the MCAs in one study was 209+/-7 micrometer (n=10). The MCAs showed concentration-dependent dilations with UTP administration. Inhibition of NO synthase with NG-nitro-L-arginine methyl ester (L-NAME) (1 micromol/L to 1 mmol/L) did not diminish the maximum response to UTP but did shift the concentration-response curve to the right. Scavenging NO with hemoglobin (1 or 10 micromol/L) or inhibition of guanylate cyclase with ODQ (1 or 10 micromol/L) had effects on the UTP-mediated dilations similar to those of L-NAME. In the presence of L-NAME, dilations induced by 10 micromol/L UTP were accompanied by 13+/-2 mV (P<0.009) hyperpolarization of the vascular smooth muscle membrane potential (-28+/-2 to -41+/-1 mV). Iberiotoxin (100 nmol/L), blocker of the large-conductance calcium-activated K channels, sometimes blocked the dilation, but its effects were variable. Charybdotoxin (100 nmol/L), also a blocker of the large-conductance calcium-activated K channels, abolished the L-NAME-insensitive component of the dilation to UTP.

CONCLUSIONS

Stimulation of P2u purinoceptors on the endothelium of the rat MCA released EDHF, in addition to EDRF/NO, and dilated the rat MCA by opening an atypical calcium-activated K channel.

Role of substance P in the pathogenesis of spider angiomas in patients with nonalcoholic liver cirrhosis

OBJECTIVE

Cutaneous spider angioma is a common sign observed in patients with liver cirrhosis, but its pathogenesis is still unclear. Increased plasma levels of estrogen, vascular dilation, and neovascularization are possible etiologies. This study was designed to investigate the relationship of spider angiomas in patients with nonalcoholic liver cirrhosis to the plasma levels of sex hormones and various vasodilators and hemodynamic parameters.

METHODS

A total of 60 patients with nonalcoholic liver cirrhosis and 20 healthy subjects were included in this study. The number, size, and location of the spider angiomas were recorded. Plasma levels of estradiol, testosterone, substance P, calcitonin gene-related peptide, and nitrate/nitrite and forearm hemodynamics were measured.

RESULTS

Cirrhotic patients showed higher plasma estradiol/testosterone ratios (28.3+/-47.2 x 10(-3), median 10.5 x 10(-3) vs 8.2+/-8.3 x 10(-3), median 5.7 x 10(-3), p = 0.003) and levels of nitrate/ nitrite (29.9+/-17.5, median 23.8 vs 21.4+/-10.0, median 20.6 micromol/L, p = 0.01) and substance P (47.5+/-62.5, median 29.2 vs 15.2+/-7.7, median 12.3 pg/ml, p < 0.001) than healthy controls. Sixteen (27%) of the 60 cirrhotic patients had spider angiomas. Cirrhotic patients with spider angiomas disclosed higher plasma levels of substance P (84.7+/-105.3, median 53.1 vs 34.5+/-30.7, median 25.8 pg/ml, p = 0.006) and serum levels of bilirubin (3.9+/-3.8, median 1.9 vs 1.9+/-1.9, median 1.2 mg/dl, p = 0.02) than those without. Stepwise logistic regression showed substance P was the only significant and independent predictor associated with the presence of spider angiomas in cirrhotic patients (odds ratio = 3.0, 95% confidence interval = 1.4-6.6, p = 0.01).

CONCLUSION

Plasma levels of substance P are elevated in patients with nonalcoholic cirrhosis and may play an important role in the pathogenesis of spider angiomas.

Nitric oxide inhibits alpha2-adrenoceptor-mediated endothelium-dependent vasodilation

This study was designed to investigate the interaction between the NO/L-arginine pathway and the alpha2-adrenoceptor-mediated endothelium-dependent vasorelaxation. Reactivity of isolated resistance mesenteric arterial segments from mice lacking the gene for constitutive endothelial NO synthase (eNOS- mice, n=14) and from their wild-type controls (WT mice, n=46) was studied in isometric conditions in the presence of indomethacin (blocker of cyclooxygenase). Oxymetazoline (OXY, 0.01 to 30 micromol/L; a selective alpha2-adrenoceptor agonist) induced an endothelium-dependent relaxation of eNOS- but not WT arteries preconstricted either with phenylephrine or serotonin. In the presence of Nomega-nitro-L-arginine (l-NNA, 100 micromol/L), an inhibitor of NOS, OXY induced an endothelium-dependent relaxation of WT mesenteric arteries. l-NNA had no effect on the relaxation caused by OXY in eNOS- arterial rings. Therefore, the relaxation caused by OXY was independent of NO formation. To demonstrate the inhibitory role of NO on the alpha2-adrenoceptor-mediated relaxation, subthreshold (0.1 nmol/L) to threshold (1 nmol/L) concentrations of sodium nitroprusside (donor of NO) were added to l-NNA-treated arteries before OXY challenges: in these conditions, the alpha2-adrenoceptor-mediated relaxation of eNOS- and WT arteries was inhibited. OXY-induced relaxation was restored on readdition of methylene blue (1 micromol/L, inhibitor of guanylate cyclase), suggesting that cGMP may be the mechanism of inhibition of the alpha2-adrenergic pathway in the presence of NO. Finally, OXY-mediated relaxation was blocked by tetraethylammonium (1 mmol/L) but not glibenclamide (1 micromol/L), suggesting the involvement of an endothelium-derived hyperpolarizing factor that activates Ca2+-activated K+ channels. In conclusion, alpha2-adrenoceptor activation caused relaxation of isolated murine mesenteric arteries that was functionally blocked by NO through a mechanism that may involve activation of the soluble guanylate cyclase and cGMP formation. The endothelium-dependent alpha2-adrenoceptor-mediated relaxation is likely to be due to an endothelium-derived hyperpolarizing factor, whose release and/or production is reduced by concurrent NO formation.

Postnatal changes in gut hemodynamics: a possible role for substance P

Studies were conducted in young postnatal swine to determine if substance P (SP) participates in the regulation of postnatal intestinal hemodynamics and oxygenation. SP was present in homogenates of whole intestine from postnatal swine in an age-dependent manner as follows: 1 day old and never fed, 126 +/- 35; 3 days old and fasted, 148 +/- 30; and 14 days old, 51 +/- 10 pg/mg protein (P < 0.01, 14- vs. 1- or 3-day-olds). Phenylephrine-precontracted rings of mesenteric artery from 3-day-old subjects mounted for tension recording within buffer-filled myographs demonstrated brisk relaxation in response to SP (EC50, 2 x 10(-10) M). This relaxation was eliminated by mechanical removal of the endothelium or blockade with the L-arginine analog NG-monomethyl-L-arginine (L-NMMA) and was significantly attenuated by pretreatment with N-acyl-L-Trp-3,5-bis-(trifluoromethyl) benzyl ester (NATB), a highly selective NK-1 receptor antagonist (pA2 5 x 10(-10) M). Infusion of exogenous SP into the mesenteric artery of innervated in vivo gut loops reduced intestinal vascular resistance 35% and increased tissue oxygen uptake 40% in both 3- and 14-day-old subjects. By contrast, blockade of the NK-1 receptor for SP with NATB increased intestinal vascular resistance 19% in 3-day-old subjects but only 5% in 14-day-old subjects (P < 0.01). SP-induced changes in gut vascular resistance were significantly attenuated by prior coinfusion of NATB or L-NMMA, indicating that the peptide exerted this vascular effect via theNK-1 receptor, which is linked to endothelial cell nitric oxide synthase. Both NATB and L-NMMA attenuated flow-induced dilation within pump-perfused in vitro gut loops from 3-day-old subjects. SP appears to participate in the regulation of the newborn intestinal circulation, especially during the first days after birth.

Sex differences in the relative contributions of nitric oxide and EDHF to agonist-stimulated endothelium-dependent relaxations in the rat isolated mesenteric arterial bed

1. We have used the isolated, buffer-perfused, superior mesenteric arterial bed of male and female rats to assess the relative contributions of nitric oxide (NO) and the endothelium-derived hyperpolarizing factor (EDHF) to endothelium-dependent relaxations to carbachol. 2. Carbachol caused dose-related relaxations of methoxamine-induced tone in mesenteric vascular beds from male rats described by an ED50(M) of 0.43+/-0.15 nmol and a maximum relaxation (Rmax(M) of 89.6+/-1.2% (n=28) which were not significantly different from those observed in mesenteries from female rats (ED50(F)=0.72+/-0.19 nmol and Rax(F)=90.7+/-0.9%; n=22). 3. In the males, the addition of 100 microM NG-nitro-L-arginine methyl ester (L-NAME) caused the dose-response curve to carbachol to be significantly (P<0.001) shifted to the right 15 fold (ED50(M)=6.45+/-3.53 nmol) and significantly (P<0.01) reduced Rmax(M) (79.7+/-2.8%, n=13). By contrast, L-NAME had no effect on vasorelaxation to carbachol in mesenteries from female rats (ED50(f)= 0.89+/-0.19 nmol, Rmax(F)=86.9+/-2.3%, n=9). 4. Raising tone with 60 mM KCl significantly reduced the maximum relaxation to carbachol in mesenteries from male rats 2 fold (Rmax(M)=40.3+/-9.2%, n=4; P<0.001) and female rats by 1.5 fold (Rmax(F)=55.3+/-3.3%, n=6; P<0.001), compared with methoxamine-induced tone. The potency of carbachol was also significantly reduced 1.2 fold in preparations from males (ED50(M)=0.87+/-0.26 nmol; P<0.01) but not the females (ED50(F)=4.04+/-1.46 nmol). In the presence of both 60 mM KCl and L-NAME, the vasorelaxation to carbachol was completely abolished in mesenteries from both groups. 5. The cannabinoid receptor antagonist SR141716A (1 microM), which is also a putative EDHF antagonist, had no significant effect on the responses to carbachol in mesenteries from males or females (ED50(M)=1.41+/-0.74 nmol, Rmax(M)=89.4+/-2.5%, n=7; ED50(F)=2.17+/-0.95 nmol, Rmax(F)=89.9+/-1.8%, n=9). In mesenteries from male rats, in the presence of 100 microM L-NAME, SR141716A significantly (P<0.05) shifted the dose-response curve to carbachol 8 fold further to the right than that seen in the presence of L-NAME alone (ED50(M)= 53.8+/-36.8 nmol) without affecting Rmax(M) (72.4+/-4.8%, n=10). In mesenteries from female rats, the combined presence of L-NAME and SR141716A, significantly (P < 0.01) shifted the dose-response curve to carbachol 7.5 fold, (ED50(F)=6.66+/-2.46 nmol), as compared to L-NAME alone and significantly (P<0.001) decreased Rmax(F) (70.1+/-5.5%, n=8). 6. Vasorelaxations to the nitric oxide donor sodium nitroprusside (SNP), to the endogenous cannabinoid, anandamide (a putative EDHF) and to the ATP-sensitive potassium channel activator, levcromakalim, did not differ significantly between male and female mesenteric vascular beds. 7. The continuous presence of sodium nitroprusside (SNP; 20-60 nM) had no effect on vasorelaxation to carbachol in mesenteries from either males or females. In the presence of L-NAME, SNP significantly (P<0.05) reduced the potency of carbachol 6 fold, without affecting the maximal relaxation in mesenteries from male rats (ED50(M)=40.9+/-19.6 nmol, Rmax(M)=79.4+/-2.5%, n=11). Similarly in mesenteries from female rats, the ED50(F) was also significantly (P<0.01) increased 7 fold (6.24+/-2.02 nmol), while the Rmax(F) was unaffected (81.9+/-11.0%; n=4). 8 The results of the present investigation demonstrate that the relative contributions of agonist-stimulated NO and EDHF to endothelium-dependent relaxations in the rat isolated mesenteric arterial bed, differ between males and females. Specifically, although both NO and EDHF appear to contribute towards endothelium-dependent relaxations in males and females, blockade of NO synthesis alone has no effect in the female. This suggests that EDHF is functionally more important in females; one possible explanation for this is that in the absence of NO, the recently identified ability of EDHF to compensate for the loss of NO, is functio

Cerebral vasodilators

The vascular tone, vascular resistance and blood flow in the brain are regulated by neural and humoral factors in quite a different way from those of peripheral organs and tissues. In contrast to the dominant vasoconstrictor control in the periphery, the intracranial vascular tone is predominantly influenced by vasodilator mediators over vasoconstrictor ones. Recent studies have revealed that nitroxidergic vasodilator nerve and endothelium-derived hyperpolarizing factor (EDHF) or K+ channel opening substance appear to play important roles in the regulation of cerebral arterial and arteriolar tone in primate and subprimate mammals, in addition to the accepted information concerning the crucial contribution of endothelium-derived relaxing factor (EDRF) or nitric oxide (NO), polypeptides, prostanoids, etc. This article summarizes characteristic properties of vasodilator factors in controlling the cerebral arterial and arteriolar tone that undoubtedly contribute to circulatory homeostasis. The content includes vasodilator nerve, endogenous vasodilator substances, and vasodilator interventions such as hypoxia, hypercapnia and hyperosmolarity.

Control of vascular tone by endogenous endothelin-1 in human pial arteries

BACKGROUND AND PURPOSE

Endothelin-1 (ET) has been shown to be involved in human pathological conditions, but its physiological function remains to be elucidated. The aim of this work was to assess whether endothelium-derived ET was involved in the overall responsiveness of freshly isolated human pial arteries.

METHODS

Samples of cerebral cortex, otherwise discarded, were obtained during tumor or epileptic lesion resections (n = 10 donors). Arterial segments were isolated and mounted on a microvessel myograph.

RESULTS

Inhibition of nitric oxide (NO) formation with N omega-nitro-L-arginine (L-NA, 100 micromol/L) increased basal tone by 7+/-1% Emax (n=5). This increase in tone was fully abolished in the presence of BQ123 (1 micromol/L; ET(A) receptor antagonist, P<.05) but potentiated by a subthreshold concentration of exogenous ET (1 nmol/L; 33+/-8% Emax; P<.05). In the presence of L-NA, serotonin (10 micromol/L)-induced tone was doubled compared with the control response (P<.05) but reduced by 90% in the presence of BQ123 (P<.05). In the absence of L-NA, BQ123 prevented serotonin-induced tone (n=3). Oxymetazoline, a selective alpha2-adrenergic receptor agonist, induced an endothelium-dependent relaxation of preconstricted human pial arteries. The relaxation was partially sensitive to NO synthase inhibition and fully prevented by the addition of ET, whereas substance P-induced relaxation was preserved. Glibenclamide (1 micromol/L), an inhibitor of ATP-sensitive K+ channels and tetraethylammonium (1 mmol/L), an inhibitor of Ca2+-activated K+ channels had no effect on oxymetazoline-induced relaxation.

CONCLUSIONS

The results of this study suggest first that ET is involved in the tonic response induced by NO synthase inhibition; second, part of the contractile response induced by serotonin is endothelium-dependent and sensitive to BQ123; and third, the data suggest that activation of alpha2-adrenergic receptors generated an endothelium-dependent relaxation that was selectively inhibited by exogenous ET.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Reversal of endothelin-1 release by stimulation of endothelial alpha2-adrenoceptor contributes to cerebral vasorelaxation

Agonists acting on the vascular endothelium can modulate the release of a number of factors that interact with the surrounding smooth muscle cells and influence their tone. One such factor is the vasoconstricting agent endothelin-1 (ET-1), which has been implicated in several disease states, including stroke. However, very little is known about the physiological role of ET-1 in the cerebral circulation. We demonstrate that activation of alpha2-adrenoceptors in human pial artery endothelial cells reduces both constitutive and agonist-stimulated release of immunoreactive ET-1. That this has physiological relevance is supported by our demonstration that in segments of rabbit middle cerebral arteries, alpha2-adrenoceptor activation reduces the release of endothelium-derived ET-1 and causes an endothelium-dependent relaxation. The adrenoceptor-dependent relaxation was not blocked by combined addition of indomethacin and N omega-nitro-L-arginine in 25 mmol/L KCl-depolarizing physiological solution but was selectively antagonized by a subthreshold concentration of exogenous ET-1. Our data suggest that activation of endothelial alpha2-adrenoceptor would favor a decrease in ET-1 production and possibly promote vascular relaxation.

Hyperdynamic circulation of cirrhotic rats: role of substance P and its relationship to nitric oxide

BACKGROUND

It has been suggested that excessive formation of nitric oxide (NO) is responsible for the hyperdynamic circulation observed in portal hypertension. Substance P is a neuropeptide partly cleared by the liver and causes vasodilatation through the activation of the endothelial NO pathway. However, there are no previously published data concerning the plasma level of substance P in cirrhotic rats and its relationship to NO.

METHODS

Plasma concentrations of substance P and nitrate/nitrite (an index of NO production) were determined in control rats and cirrhotic rats with or without ascites using an enzyme-linked immununosorbent assay and a colorimetric assay, respectively. In addition, systemic and portal hemodynamics were evaluated by a thermodilution technique and catheterization.

RESULTS

Cirrhotic rats with and without ascites had a lower systemic vascular resistance (2.6 +/- 0.2 and 3.9 +/- 0.4 mmHg ml(-1) x min x 100 g body weight, respectively) and higher portal pressure (14.6 +/- 0.6 and 11.3 +/- 1.8 mmHg) than control rats (6.5 +/- 0.3 mmHg x ml(-1) x min x 100 g BW and 6.8 +/- 0.2 mmHg, respectively, P < 0.05), and cirrhotic rats with ascites had the lowest systemic vascular resistance. Plasma levels of nitrate/nitrite progressively increased in relation to the severity of liver dysfunction (control rats, 2.7 +/- 0.5 nmol/ml; cirrhotic rats without ascites, 5.6 +/- 1.3 nmol/ml; cirrhotic rats with ascites, 8.3 +/- 2.2 nmol/ml; P < 0.05). Cirrhotic rats with ascites displayed higher plasma values of substance P (57.7 +/- 5.9 pg/ml) than cirrhotic rats without ascites (37.9 +/- 3.1 pg/ml, P < 0.05) and control rats (30.1 +/- 1.0 pg/ml, P < 0.05). There was no significant difference in plasma substance P values between control rats and cirrhotic rats without ascites (P > 0.05). No correlation was found between plasma levels of substance P and nitrate/nitrite (r = 0.318, P > 0.05).

CONCLUSIONS

Excessive formation of NO may be responsible, at least partly, for the hemodynamic derangements in cirrhosis. Although substance P may not participate in the initiation of a hyperdynamic circulation in cirrhosis, it may contribute to the maintenance of the hyperdynamic circulation observed in cirrhotic rats with ascites.

Characterization of endothelium-derived relaxing factors released by bradykinin in human resistance arteries

1. Relaxing factors released by the endothelium and their relative contribution to the endothelium-dependent relaxation produced by bradykinin (BK) in comparison with different vasodilator agents were investigated in human omental resistance arteries. 2. BK produced an endothelium-dependent relaxation of arteries pre-contracted with the thromboxane A2 agonist, U46619. The B2 receptor antagonist, Hoe 140 (0.1, 1 and 10 microM), produced a parallel shift to the right of the concentration-response curve to BK with a pA2 of 7.75. 3. Neither the cyclo-oxygenase inhibitor, indomethacin (10 microM) alone, the nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME, 300 microM) alone, the nitric oxide scavenger, oxyhaemoglobin (Hb, 10 microM) alone, nor the combination of L-NAME plus Hb affected the concentration-response curve to BK. Conversely, the combination of indomethacin with either L-NAME or Hb attenuated but did not abolish the BK-induced relaxation. By contrast, the relaxations produced by the Ca2+ ionophore, calcimycin (A23187), and by the inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, thapsigargin (THAPS), were abolished in the presence of indomethacin plus L-NAME. Also, the presence of indomethacin plus L-NAME produced contraction of arteries with functional endothelium. 4. The indomethacin plus L-NAME resistant component of BK relaxation was abolished in physiological solution (PSS) containing 40 mM KCl and vice versa. However, in the presence of KCl 40 mM, indomethacin plus L-NAME did not affect the nitric oxide donor, S-N-acetylpenicillamine-induced relaxation. 5. The indomethacin plus L-NAME resistant component of the relaxation to BK was significantly attenuated by the K+ channel blocker tetrabutylammonium (TBA, 1 mM). However, it was not affected by other K+ channel blockers such as apamin (10 microM), 4-aminopyridine (100 microM), glibenclamide (10 microM), tetraethylammonium (10 mM) and charybdotoxin (50 nM). 6. In the presence of indomethacin plus L-NAME, the relaxation produced by BK was not affected by the phospholipase A2 inhibitor, quinacrine (10 microM) or by the inhibitor of cytochrome P450, SKF 525a (10 microM). Another cytochrome P450 inhibitor, clotrimazole (10 microM) which also inhibits K+ channels, inhibited the relaxation to BK. 7. These results show that BK induces endothelium-dependent relaxation in human small omental arteries via multiple mechanisms involving nitric oxide, cyclo-oxygenase derived prostanoid(s) and another factor (probably an endothelium-derived hyperpolarizing factor). They indicate that nitric oxide and cyclo-oxygenase derivative(s) can substitute for each other in producing relaxation and that the third component is not a metabolite of arachidonic acid, formed through the cytochrome P-450 pathway, in these arteries.

Involvement of voltage-dependent potassium channels in the EDHF-mediated relaxation of rat hepatic artery

1. In the rat hepatic artery, the acetylcholine-induced relaxation mediated by endothelium-derived hyperpolarizing factor (EDHF) is abolished by a combination of apamin and charybdotoxin, inhibitors of small (SKCa) and large (BKCa) conductance calcium-sensitive potassium (K)-channels, respectively, but not by each toxin alone. The selective BKCa inhibitor iberiotoxin cannot replace charybdotoxin in this combination. Since delayed rectifier K-channels (KV) represent another target for charybdotoxin, we explored the possible involvement of KV in EDHF-mediated relaxation in this artery. 2. The KV inhibitors, agitoxin-2 (0.3 microM), kaliotoxin (0.3 microM), beta-dendrotoxin (0.3 microM), dofetilide (1 microM) and terikalant (10 microM), each in combination with apamin (0.3 microM) had no effect on the EDHF-mediated relaxation induced by acetylcholine in the presence of N omega-nitro-L-arginine (0.3 mM) and indomethacin (10 microM), inhibitors of nitric oxide (NO) synthase and cyclo-oxygenase, respectively (n = 2-3). Although the KV inhibitor margatoxin (0.3 microM) was also without effect (n = 5), the combination of margatoxin and apamin produced a small inhibition of the response (pEC50 and Emax values were 7.5 +/- 0.0 and 95 +/- 1% in the absence and 7.0 +/- 0.1 and 81 +/- 6% in the presence of margatoxin plus apamin, respectively; n = 6; P < 0.05). 3. Ciclazindol (10 microM) partially inhibited the EDHF-mediated relaxation by shifting the acetylcholine-concentration-response curve 12 fold to the right (n = 6; P < 0.05) and abolished the response when combined with apamin (0.3 microM; n = 6). This combination did not inhibit acetylcholine-induced relaxations mediated by endothelium-derived NO (n = 5). 4. A 4-aminopyridine-sensitive delayed rectifier current (IK(V)) was identified in freshly-isolated single smooth muscle cells from rat hepatic artery. None of the cells displayed a rapidly-activating and -inactivating A-type current. Neither charybdotoxin (0.3 microM; n = 3) nor ciclazindol (10 microM; n = 5), alone or in combination with apamin (0.3 microM; n = 4-5), had an effect on IK(V). A tenfold higher concentration of ciclazindol (0.1 mM, n = 4) markedly inhibited IK(V), but this effect was not increased in the additional presence of apamin (0.3 microM; n = 2). 5. By use of membranes prepared from rat brain cortex. [125I]-charybdotoxin binding was consistent with an interaction at a single site with a KD of approximately 25 pM. [125I]-charybdotoxin binding was unaffected by iberiotoxin (0.1 microM, n = 6), but was increased by apamin in a concentration-dependent manner (Emax 43 +/- 10%, P < 0.05 and pEC50 7.1 +/- 0.2; n = 7-8). Agitoxin-2 (10 nM) displaced [125I]-charybdotoxin binding by 91 +/- 3% (n = 6) and prevented the effect of apamin (1 microM; n = 6). 6. It is concluded that the EDHF-mediated relaxation in the rat hepatic artery is not mediated by the opening of either KV or BKCa. Instead, the target K-channels for EDHF seem to be structurally related to both KV and BKCa. The possibility that a subtype of SKCa may be the target for EDHF is discussed.

Effects of a novel inhibitor of guanylyl cyclase on dilator responses of mouse cerebral arterioles

BACKGROUND AND PURPOSE

Nitric oxide-induced vasodilatation is mediated by both cGMP-dependent and -independent mechanisms. Previous studies that examined the role of soluble guanylyl cyclase in cerebral vessels have used methylene blue and LY-83583, compounds that generate superoxide anion and are not specific for inhibition of soluble guanylyl cyclase. We examined the effects of ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one), a novel and highly selective inhibitor of soluble guanylyl cyclase, on responses of cerebral arterioles.

METHODS

The effects of ODQ on responses of cerebral arterioles to acetylcholine, nitroprusside, 8-bromo-cGMP, and adenosine were examined in anesthetized mice by means of a cranial window. The effects of two concentrations of ODQ were examined in the absence and presence of superoxide dismutase. The effects of NG-nitro-L-arginine, an inhibitor of nitric oxide synthase, were also tested.

RESULTS

ODQ (3 and 10 mumol/L) produced concentration-dependent inhibition of dilatation of cerebral arterioles (control diameter = 29 +/- 1 microns) (mean +/- SE) in response to acetylcholine and nitroprusside. For example, 10 mumol/L acetylcholine and 1 mumol/L nitroprusside dilated cerebral arterioles by 28 +/- 3% and 44 +/- 2% in the absence and 6 +/- 2% and 7 +/- 1%, respectively, in the presence of 10 mumol/L ODQ (P < .05 versus control). The inhibitory effects of ODQ were not altered by superoxide dismutase. Vasodilatation in response to 8-bromo-cGMP and adenosine was not inhibited by ODQ. NG-Nitro-L-arginine (100 mumol/L), an inhibitor of nitric oxide synthase, inhibited responses to acetylcholine by approximately 80% but tended to enhance responses to nitroprusside.

CONCLUSIONS

Thus, nitric oxide-mediated dilatation of mouse cerebral arterioles is profoundly inhibited by ODQ, an inhibitor of activity of soluble guanylyl cyclase. Cerebral vasodilator responses to adenosine and 8-bromo-cGMP were preserved in the presence of ODQ, indicating that inhibition by ODQ was selective. In contrast to previously used inhibitors of soluble guanylyl cyclase (methylene blue and LY-83583), the effects of ODQ are not mediated by generation of superoxide anion.

Characterization of the potassium channels involved in EDHF-mediated relaxation in cerebral arteries

1. In the presence of NG-nitro-L-arginine (L-NOARG, 0.3 mM) and indomethacin (10 microM), the relaxations induced by acetylcholine and the calcium (Ca) ionophore A23187 are considered to be mediated by endothelium-derived hyperpolarizing factor (EDHF) in the guinea-pig basilar artery. 2. Inhibitors of adenosine 5'-triphosphate (ATP)-sensitive potassium (K)-channels (KATP; glibenclamide, 10 microM), voltage-sensitive K-channels (Kv; dendrotoxin-1, 0.1 microM or 4-aminopyridine, 1 mM), small (SKCa; apamin, 0.1 microM) and large (BKCa; iberiotoxin, 0.1 microM) conductance Ca-sensitive K-channels did not affect the L-NOARG/indomethacin-resistant relaxation induced by acetylcholine. 3. Synthetic charybdotoxin (0.1 microM), an inhibitor of BKCa and Kv, caused a rightward shift of the concentration-response curve for acetylcholine and reduced the maximal relaxation in the presence of L-NOARG and indomethacin, whereas the relaxation induced by A23187 was not significantly inhibited. 4. A combination of charybdotoxin (0.1 microM) and apamin (0.1 microM) abolished the L-NOARG/ indomethacin-resistant relaxations induced by acetylcholine and A23187. However, the acetylcholine-induced relaxation was not affected by a combination of iberiotoxin (0.1 microM) and apamin (0.1 microM). 5. Ciclazindol (10 microM), an inhibitor of Kv in rat portal vein smooth muscle, inhibited the L-NOARG/ indomethacin-resistant relaxations induced by acetylcholine and A23187, and the relaxations were abolished when ciclazindol (10 microM) was combined with apamin (0.1 microM). 6. Human pial arteries from two out of four patients displayed an L-NOARG/indomethacin-resistant relaxation in response to substance P. This relaxation was abolished in both cases by pretreatment with the combination of charybdotoxin (0.1 microM) and apamin (0.1 microM), whereas each toxin had little effect alone. 7. The results suggest that Kv, but not KATP and BKCa, is involved in the EDHF-mediated relaxation in the guinea-pig basilar artery. The synergistic action of apamin and charybdotoxin (or ciclazindol) could indicate that both Kv and SKCa are activated by EDHF. However, a single type of K-channel, which may be structurally related to Kv and allosterically regulated by apamin, could also be the target for EDHF.

Endothelium-dependent relaxation by substance P in human isolated omental arteries and veins: relative contribution of prostanoids, nitric oxide and hyperpolarization

1. The objective of the present study was to investigate human omental arteries and veins with respect to: (i) the contractile effect of the thromboxane A2 analogue U46619, (ii) endothelium-dependency and mediators of the relaxing effect of substance P (SP) and acetylcholine (ACh). 2. Changes in isometric tension in response to administration of U46619, SP and ACh were measured in human isolated omental arteries and veins with and without endothelium. To investigate the mechanism of action of SP, the SP-induced relaxation was measured in the presence of indomethacin (cyclo-oxygenase inhibitor), NG-monomethyl-L-arginine (L-NMMA, nitric oxide-synthase inhibitor), KCl (inhibitor of endothelium-dependent hyperpolarization), tetraethylammonium (TEA; non-selective inhibitor of K(+)-channels, with some preference for the high conductance Ca(2+)-activated K(+)-channel, BKCa), glibenclamide (inhibitor of the ATP-sensitive K(+)-channel) and/or clotrimazole (inhibitor of the cytochrome P450-system and the intermediate conductance Ca(2+)-activated K(+)-channel, IKCa). 3. U46619 contracted both the artery and the vein segments. Endothelium removal did not alter the contraction. 4. ACh caused neither contraction nor relaxation in artery and vein segments precontracted with U46619. 5. In both artery and vein segments precontracted with U46619, SP produced endothelium-dependent relaxation. The relaxation was unaffected by indomethacin, but was incompletely reduced by L-NMMA and KCl respectively. The L-NMMA-resistent relaxation was abolished in the presence of KCl. 6. TEA inhibited the SP-induced relaxation in artery and vein segments both in the presence and absence of L-NMMA and indomethacin, while glibenclamide and clotrimazole had no effect. 7. In conclusion, the SP-induced relaxation in human omental arteries and veins seems to be mediated via NO and endothelium-dependent hyperpolarization. KATP and IKCa are probably not involved in the hyperpolarization, but activation of BKCa may contribute to the hyperpolarization. Prostanoid synthesis and the cytochrome P450-system are probably not involved in the SP-induced relaxation in this area.

Endothelium-derived hyperpolarizing factor

1. Not all endothelium-dependent relaxations can be fully explained by the release of either nitric oxide (NO) and/or prostacyclin. Another unidentified substance(s) that hyperpolarizes the underlying vascular smooth muscle cells (endothelium-derived hyperpolarizing factor; EDHF) contributes to endothelium-dependent relaxations. 2. In blood vessels from various species these hyperpolarizations are resistant to inhibitors of NO synthase (NOS) and cyclo-oxygenase. In canine, porcine and human blood vessels the hyperpolarization cannot be mimicked by nitrovasodilators or exogeneous NO. However, in other species (rat, guinea-pig, rabbit) endothelium-dependent hyperpolarizations resistant to inhibitors of NOS and cyclo-oxygenase and hyperpolarizations to endothelium-derived or exogeneous NO can be observed in the same vascular smooth muscle cells. 3. In blood vessels where NO causes hyperpolarization, the response is blocked by glibenclamide, suggesting the involvement of ATP-dependent potassium channels. Hyperpolarizations caused by EDHF are insensitive to glibenclamide but, depending on the tissue, are inhibited by relatively small concentrations of tetraethylammonium (TEA) or by apamin or the combination of charybdotoxin plus apamin, indicating that calcium-dependent potassium channels are likely to be involved. 4. Metabolites of arachidonic acid, through the cytochrome P450 mono-oxygenase pathway (epoxyeicosatrienoic acids), are produced by the endothelial cells, increase the open-state probability of calcium-activated potassium channels sensitive to TEA or charybdotoxin, and induce the hyperpolarization of arterial smooth muscle cells, indicating that epoxyeicosatrienoic acids could be EDHF. However, in blood vessels from various species, cytochrome P450 inhibitors do not affect endothelium-dependent hyperpolarizations, indicating that EDHF is not yet identified with certainty. 5. Endothelium-derived hyperpolarizing factor released from cultured endothelial cells reduces the intracellular calcium concentration in vascular smooth muscle cells and the EDHF component of the relaxation is proportionally more important in smaller than larger arteries. In aging animals and in various models of diseases, endothelium-dependent hyperpolarizations are diminished. 6. The identification of EDHF and/or the discovery of specific inhibitors of its synthesis and its action may allow a better understanding of its physiological and pathophysiological role(s).

Potassium channels and the cerebral circulation

1. Hyperpolarization of vascular muscle in response to activation of potassium channels is a major mechanism of vasodilatation. 2. In cerebral blood vessels, two potassium channels have received considerable study recently: ATP-sensitive and calcium-dependent potassium channels. Activation of these potassium channels appears to play a major role in the relaxation of cerebral arteries and arterioles in response to diverse stimuli, including receptor-mediated agonists, intracellular second messengers, reactive oxygen species and hypoxia. 3. The functional influence of ATP-sensitive and calcium-dependent potassium channels may be altered in disease states, including hypertension, diabetes and subarachnoid haemorrhage.

Role of Ca(2+)-activated K+ channels in the regulation of membrane potential and tone of smooth muscle in human pial arteries

Smooth muscle cells (SMCs) in 58% of human pial arteries obtained during surgery showed no spontaneous contractions and displayed a stable resting membrane potential (MP) of -54.7 +/- 1.5 mV. Those that exhibited periodic spontaneous contractions associated with periodic depolarization and generation of spontaneous action potentials (APs) had a less negative MP of -43.1 +/- 0.5 mV (42%). Inhibition of calcium-activated potassium (KCa) channels in the silent arteries by charybdotoxin (CTX) and tetraethylammonium ions (TEA) induced dose-dependent depolarization, AP generation, and contraction. TEA and CTX enhanced the spontaneous depolarization and force in arteries that exhibited spontaneous activity. They also prolonged the spontaneous APs up to several times and increased their upstroke amplitude. Both TEA and CTX failed to produce significant depolarization in arteries treated with nifedipine. It is concluded that KCa channels are important regulators of human pial artery SMC resting MP and tone. They are also involved in the control of AP amplitude and duration and the associated contractions. These data suggest that alterations in the activity of SMC KCa channels could be responsible for the appearance of spontaneous activity in human pial arteries in vitro and that impaired function of these channels might be related to vasospastic phenomena in human cerebral circulation.

Effect of pregnancy on mechanisms of relaxation in human omental microvessels

We assessed mechanisms of acetylcholine- and bradykinin-induced relaxations in human omental resistance vessels. Ring segments (approximately 200 microns normalized ID) were dissected from omental biopsies obtained from women at laparotomy (nonpregnant) or at cesarean delivery (pregnant) and were studied under isometric conditions in a Mulvany-Halpern myograph. All arginine vasopressin-preconstricted vessels relaxed in a strictly endothelium-dependent manner to acetylcholine and bradykinin; maximal relaxations were not decreased by either NG-nitro-L-arginine or indomethacin. By contrast, bradykinin failed to relax vessels that had been preconstricted with potassium gluconate. In the combined presence of NG-nitro-L-arginine and indomethacin, addition of charybdotoxin, a selective antagonist of some calcium-sensitive potassium channels, did not inhibit maximal bradykinin-induced relaxation. By contrast, addition of 10 mmol/L tetraethylammonium chloride abolished relaxation in vessels from nonpregnant women but not in vessels from gravidas. We conclude that bradykinin relaxes these human resistance arteries in an endothelium-dependent but predominantly nitric oxide- and prostanoid-independent manner; relaxation likely depends on the action of an endothelium-derived hyperpolarizing vasodilator. Furthermore, in striking contrast to mechanistic insights from animal studies, human pregnancy appears to augment a mechanism of endothelium-dependent relaxation in these vessels that is insensitive to the inhibitors noted above. Whether a similar novel vasodilator mechanism in vivo contributes to the physiological vasodilation that characterizes human gestation or whether failure of such a mechanism might lead to preeclampsia remains the subject of future study.

Effects of cytochrome P450 inhibitors on EDHF-mediated relaxation in the rat hepatic artery

1. The possibility that the endothelium-derived hyperpolarising factor (EDHF) in the rat hepatic artery is a cytochrome P450 mono-oxygenase metabolite of arachidonic acid was examined in the present study. In this preparation, acetylcholine elicits EDHF-mediated relaxations in the presence of the nitric oxide (NO) synthase and cyclo-oxygenase inhibitors N omega-nitro-L-arginine (L-NOARG) and indomethacin, respectively. 2. 17-Octadecynoic acid (17-ODYA, 50 microM), a suicide-substrate inhibitor of the cytochrome P450 mono-oxygenases responsible for the production of 5,6-, 8,9-, 11,12- and 14,15-epoxyeicosatrienoic acids (EETs), had no effect on acetylcholine-induced relaxations in the presence of L-NOARG (0.3 mM) plus indomethacin (10 microM). Furthermore, 5,6-, 8,9-, 11,12- and 14,15- EETs failed to relax arteries without endothelium in the presence of L-NOARG plus indomethacin. 3. Proadifen and clotrimazole, which are inhibitors of several isoforms of cytochrome P450 mono-oxygenases, inhibited acetylcholine-induced relaxations in the presence of L-NOARG plus indomethacin. The concentration of acetylcholine which caused half-maximal relaxation was about 3 and 30 times higher in the presence than in the absence of clotrimazole (3 microM) and proadifen (10 microM), respectively. The maximal relaxation was reduced by proadifen but not by clotrimazole. Proadifen (10 microM) also inhibited acetylcholine-induced hyperpolarization in the presence of L-NOARG plus indomethacin. 4. In the presence of 30 mM K+ plus indomethacin (10 microM), acetylcholine induced an L-NOARG-sensitive relaxation mediated via release of NO. Under these conditions, proadifen (10 microM) shifted the acetylcholine concentration-response curve 6 fold to the right without affecting the maximal relaxation. Clotrimazole (3 microM) was without effect on these responses. The relaxant actions of the NO donor, 3-morpholino-sydnonimine, were unaffected by proadifen (10 microM). 5. The relaxant effects of the opener of ATP-sensitive potassium channels, levcromakalim, were abolished by proadifen (10 microM) and strongly attenuated by clotrimazole (3 microM). Proadifen (10 microM) also abolished the hyperpolarization induced by levcromakalim (1 microM). 6. The lack of effect of 17-ODYA on relaxations mediated by EDHF, together with the failure of extracellularly-applied EETs to produce relaxation, collectively suggest that EDHF is not an EET in the rat hepatic artery. It seems likely that inhibition of ion channels in the smooth muscle rather than reduced EDHF formation in the endothelium offers a better explanation for the actions of the cytochrome P450 inhibitors proadifen and clotrimazole.