Endothelium-derived contractile factors

Nitric Oxide Donors as Potential Drugs for the Treatment of Vascular Diseases Due to Endothelium Dysfunction

Endothelial dysfunction and consequent vasoconstriction are a common condition in patients with hypertension and other cardiovascular diseases. Endothelial cells produce and release vasodilator substances that play a pivotal role in normal vascular tone. The mechanisms underlying endothelial dysfunction are multifactorial. However, enhanced reactive oxygen species (ROS) production and consequent vasoconstriction instead of endothelium-derived relaxant generation and consequent vasodilatation contribute to this dysfunction considerably. The main targets of the drugs that are currently used to treat vascular diseases concerning enzyme activities and protein functions that are impaired by endothelial nitric oxide synthase (eNOS) uncoupling and ROS production. Nitric oxide (NO) bioavailability can decrease due to deficient NO production by eNOS and/or NO release to vascular smooth muscle cells, which impairs endothelial function. Considering the NO cellular mechanisms, tackling the issue of eNOS uncoupling could avoid endothelial dysfunction: provision of the enzyme cofactor tetrahydrobiopterin (BH4) should elicit NO release from NO donors, to activate soluble guanylyl cyclase. This should increase cyclic guanosine-monophosphate (cGMP) generation and inhibit phosphodiesterases (especially PDE5) that selectively degrade cGMP. Consequently, protein kinase-G should be activated, and K+ channels should be phosphorylated and activated, which is crucial for cell membrane hyperpolarization and vasodilation and/or inhibition of ROS production. The present review summarizes the current concepts about the vascular cellular mechanisms that underlie endothelial dysfunction and which could be the target of drugs for the treatment of patients with cardiovascular disease.

Vasoactive androgens: Vasorelaxing effects and their potential regulation of blood pressure

BACKGROUND

Testosterone, 5α- and 5β-dihydrotestosterone (-DHT) induce an acute in vitro vasorelaxation and in vivo vasodepressor, hypotensive and antihypertensive responses. Our aim was to study whether androgen-induced blood pressure (BP) reduction is involved with a blockade of Ca²⁺ influx through L-type voltage-operated calcium channels (L-VOCCs) and/or the signaling pathways of α₁-adrenoceptors to induce vasoconstriction, which are one of the major mechanisms of BP maintenance.

MATERIALS AND METHODS

The relaxing potency and efficacy of each androgen in large conduit (thoracic aorta) and resistance (mesenteric) arteries from male hypertensive (SHR) and normotensive (WKY) rats were established. Blood vessels were isometrically recorded and precontracted with KCl or phenylephrine (Phe).

RESULTS

Androgens induced concentration-dependent vasorelaxation in precontracted arteries from SHR and WKY rats. 5β-DHT was always the most potent vasorelaxant in arteries from SHR. The KCl-induced contraction resulted significantly more sensitive to androgen-induced vasorelaxation than the Phe-induced contraction. On Phe-induced contraction, 5β-DHT was more potent in the mesenteric artery than in the thoracic aorta.

CONCLUSIONS

The vasorelaxation induced by androgens is mainly mediated by blocking L-VOCCs and in lesser extent by the blockade of multiple signaling pathways operative during α-adrenoceptor-induced vasoconstriction. 5β-DHT regulates vascular resistance and BP by mainly acting in the mesenteric arterial bed, which may explain its outstanding antihypertensive response previously reported.

Orchidectomy modulates alpha2-adrenoceptor reactivity in rat mesenteric artery through increased thromboxane A2 formation

The present study evaluates the effect of endogenous male sex hormones on the reactivity to alpha2-adrenoceptor activation, and to analyze the role of the endothelium in this response in intact and endothelially denuded superior mesenteric arteries from control and orchidectomized male Sprague-Dawley rats. The concentration-dependent constriction induced by clonidine was analyzed in the absence and presence of the nitric oxide synthase (NOS) inhibitor, Nomega-nitro-L-arginine (L-NAME), cyclooxygenase (COX-1 and COX-2) inhibitors, indomethacin, the specific COX-2 inhibitor NS-398, the thromboxane-prostanoid receptor antagonist SQ29,548 and the thromboxane A2 (TXA2) synthase inhibitor, furegrelate. Endothelial NOS (eNOS), COX-2 and TXA2 synthase protein expression was studied by Western blot analysis. In addition, the basal and clonidine-stimulated production of TXB2, the stable TXA2 metabolite, was also measured. In intact vessels from control male rats, the concentration-dependent constriction induced by clonidine was increased by both L-NAME or endothelial removal, unaltered by indomethacin and decreased by NS-398; in denuded vessels, the clonidine response was decreased by NS-398 and unaltered by L-NAME, indomethacin, SQ29,548 or furegrelate. In intact vessels from orchidectomized rats, the constriction induced by clonidine was increased by L-NAME but practically abolished by indomethacin or NS-398; in endothelially denuded segments the clonidine response was unaltered by L-NAME, but was decreased by indomethacin, NS-398, SQ29,548 or furegrelate. Orchidectomy failed to modify eNOS,COX-2 and TXA2 synthase expression, and increased basal and clonidine-stimulated TXB2 release. These results show that TXA2 produced in smooth muscle cells is increased in mesenteric arteries from orchidectomized rats compared to their controls, and that this prostanoid is functionally involved in the vasoconstrictor response to clonidine only in arteries from the orchidectomized rats.

Cyclooxygenase contracting factors and altered pulmonary vascular responses in chronically hypoxic newborn pigs

Pulmonary hypertension and blunted pulmonary vascular responses to ACh develop when newborn pigs are exposed to chronic hypoxia for 3 days. To determine whether a cyclooxygenase (COX)-dependent contracting factor, such as thromboxane, is involved with altered pulmonary vascular responses to ACh, newborn piglets were raised in 11% O(2) (hypoxic) or room air (control) for 3 days. Small pulmonary arteries (100-400 microm diameter) were cannulated and pressurized, and their responses to ACh were measured before and after either the COX inhibitor indomethacin; a thromboxane synthesis inhibitor, dazoxiben or feregrelate; or the thromboxane-PGH(2)-receptor antagonist SQ-29548. In control arteries, indomethacin reversed ACh responses from dilation to constriction. In contrast, hypoxic arteries constricted to ACh before indomethacin and dilated to ACh after indomethacin. Furthermore, ACh constriction in hypoxic arteries was nearly abolished by either dazoxiben, feregrelate, or SQ-29548. These findings suggest that thromboxane is the COX-dependent contracting factor that underlies the constrictor response to ACh that develops in small pulmonary arteries of piglets exposed to 3 days of hypoxia. The early development of thromboxane-mediated constriction may contribute to the pathogenesis of chronic hypoxia-induced pulmonary hypertension in newborns.

Diabetes alters neuronal nitric oxide release from rat mesenteric arteries. Role of protein kinase C

The objective of the present study was to assess the influence of diabetes in the neuronal nitric oxide (NO) release elicited by electrical field stimulation (EFS, 200 mA, 0.3 ms, 1-16 Hz, for 30 s, at 1 min interval) in endothelium-denuded mesenteric artery segments from control and streptozotocin-induced diabetic rats, assessing the influence of protein kinase C (PKC) in this release. N(G)-nitro-L-arginine-methyl ester (L-NAME, 10 microM, a NO synthase inhibitor) enhanced EFS-elicited contractions in control, and specially in diabetic rats, whereas they were unaltered by AMT (5 nM, an inducible NO synthase inhibitor) and capsaicin (0.5 microM, a sensory neurone toxin). Calphostin C (0.1 microM, a PKC inhibitor) increased the contraction elicited by EFS in both types of arteries. This increase was further enhanced by calphostin C + L-NAME in diabetic rats. Phorbol 12,13-dibutyrate (PDBu, 1 microM) reduced and unaltered EFS-induced contractions in control and diabetic rats, respectively. The further addition of L-NAME reversed the reduction obtained in control rats, and enhanced the response observed in diabetic rats. These results suggest that the EFS-induced NO release from perivascular nitrergic nerves, that negatively modulates the contraction, which is synthesized by neuronal constitutive NO synthase. The NO synthesis is positively stimulated by PKC. This NO release is increased in diabetes, likely due to an increase in the activity of this enzyme. The sensory nerves of these arteries do not seem to be involved in the contractile response.

Role of vascular nitric oxide in physiological and pathological conditions

This review describes the ability of certain diseases, such as essential hypertension, atherosclerosis, angina, and vasospasm, to reduce vascular nitric oxide (NO) formation or to increase its metabolism. In contrast, others, such as hypotension, sepsis, stroke, myocardial depression, and inflammatory responses, increase NO synthesis. The mechanism implicated in the changes in the formation and metabolism of NO are described. To prevent or treat these pathological processes, in which a deficiency in vascular NO formation plays a causative role, NO may be provided through methods such as direct NO administration or indirect NO supply through either NO donors or L-arginine, which facilitates NO formation.

Endothelial modulation of alpha 1-adrenoceptor contractile responses in the tail artery of spontaneously hypertensive rats

1. Vascular contraction induced by phenylephrine was studied in tail artery rings from spontaneously hypertensive (SHR) and Wistar Kyoto rats (WKY) with particular focus on the role of endothelium. The influence of receptor reserve and the density of alpha 1-adrenoceptors on the possible differences observed were also analysed. 2. Phenylephrine (0.01-100 microM) induced concentration-dependent vasoconstrictions. The maximum response (alpha, P < 0.001) was greater but the pEC50 (P < 0.05) smaller in rings from SHR than from WKY rats irrespective of the presence or absence of endothelium. 3. Removal of endothelial cells resulted in a decrease of the maximum contraction with no modification in the pEC50 in arteries from both WKY and SHR. 4. The density of alpha 1-adrenoceptors (Bmax) and the dissociation constant (KD) were found to be the same for preparations from SHR and WKY rats in [3H]-prazosin binding experiments. 5. The apparent affinity (pKA) determined by the nested hyperbolic method and the operational model was similar in tail arteries from the two rat strains, irrespective of the presence or absence of endothelium. However, in endothelium-denuded rings, the pKA value was enhanced when compared with intact rings, in both SHR and WKY rats. 6. In rings from hypertensive rats, the operational parameter maximum possible effect (Em) was greater and the agonist efficacy (tau) was smaller than in rings from normotensive rats. When the endothelium was removed log tau and Em diminished in preparations from both rat strains. 7. In summary, the increased maximum responsiveness to phenylephrine in rings from SHR could be due to enhancement in Em. The log tau values indicate a deterioration in the transduction of the stimulus provided by the agonist in tail arteries from hypertensive animals. This study also suggests that the absence of endothelium modifies the alpha 1-adrenoceptor-mediated vasoconstriction probably by altering the transduction signalling mechanisms. The importance of analysing the degree of endothelium functionality when comparing results from different groups of rats is stated.

Effect of NG-nitro-L-arginine methylester (L-NAME) on functional and biochemical alpha 1-adrenoceptor-mediated responses in rat blood vessels

1. The modulation by NG-nitro-L-arginine methylester (L-NAME) of alpha 1-adrenoceptor-mediated contraction was investigated on isolated segments of rat tail artery and aorta. The influence of L-NAME on inositol phosphates accumulation by alpha 1-adrenoceptor agonists was also investigated to elucidate the intracellular mechanism responsible for this modulation. 2. In aorta but not in tail artery L-NAME (30 microM) enhanced the sensitivity (3.3 times) and the maximum contraction (Emax) induced by the full agonist, phenylephrine. 3. St-587, a partial alpha 1-adrenoceptor agonist, behaved as a weak agonist in the aorta (22.2% of phenylephrine Emax). However, when the same agonist was studied in tail artery rings a maximum contraction that was 78.4% of the phenylephrine induced Emax was reached. 4. L-NAME increased (3.3 times) the Emax for St-587 contraction in the aorta but not in the tail artery. Sensitivity to St-587 was slightly but significantly (P < 0.001) enhanced (1.9 times) by L-NAME in tail artery segments. 5. Contractile responses to phenylephrine after partial alkylation with phenoxybenzamine were analyzed by the nested hyperbolic null method. To elicit 50% of Emax for contraction only 1.1% of the receptors in the tail artery and 21% of the receptors in the aorta need to be occupied. These results indicate a higher receptor reserve for the tail artery than the aorta. 6. In the tail artery but not in the aorta, St-587 activates phosphoinositide turnover. The presence of L-NAME was without effect on inositol phosphates accumulation induced by this partial alpha 1-adrenoceptor agonist. 7. The maximum contraction induced by phenylephrine, after partial alpha-adrenoceptor alkylation, was enhanced by L-NAME in tail artery rings. However, the NO synthase inhibitor was unable to modify the phenylephrine-induced accumulation of inositol phosphates in the presence of phenoxybenzamine. 8. These results indicate that the differences in St-587-induced contraction and the modulation by L-NAME of alpha 1-adrenoceptor-mediated contraction observed between the tail artery and aorta are associated with differences in receptor reserve. In addition, our biochemical studies indicate that the potentiating effect of L-NAME is independent of intracellular calcium release via phosphatidylinositol turnover.

Effects of Bay K 8644 in aorta from spontaneously hypertensive and Wistar Kyoto rats of different ages

1. The Ca(2+)-channel agonist, Bay K 8644, induced small contractions in aortae from Wistar-Kyoto (WKY) rats of 5-week-, 3-month-, 1-year- and 1.5-year-old, which were unaltered with age. These contractions were increased by partial depolarization with 15 mM K+. 2. In segments from spontaneously hypertensive rats (SHR), the contractions obtained in both situations were similar and equivalent to those observed in segments from normotensive animals partially depolarized. Responses to Bay K 8644 were modified by age only in tissues from the SHR, the responses to this agent in basal conditions being increased in tissues from 3-month- and 1-year-old animals and depressed in those from 1.5-year SHR. 3. A reduction of the response to Bay K 8644 was observed in partial depolarized endothelium denuded segments from WKY of all ages, and no modification in basal situation. However, the direct contractions induced by Bay K 8644 in aortae from 3-month- and 1.5-year-old SHR were reduced by endothelium removal. 4. These results suggest that: (a) in the hypertensive strain the voltage-gated Ca2+ channels seem to be partially activated; (b) the direct contractions induced by Bay K 8644 were unaltered by age in aortae from WKY but increased in tissues from SHR of 3-month-and-1-year old and depressed in those from 1.5 years, and (c) the contractions evoked by Bay K 8644 seem to involve an endothelium-derived contracting factor in aortae from both strains, or the endothelium produces a partial depolarization of vascular smooth muscle that increases the responsiveness to Bay K 8644.

Age-related changes in vascular responses: a review

Normal aging is associated with different changes in the cardiovascular system that lead to an increase in pathological processes, such as hypertension, coronary artery disease, heart failure, and postural hypotension with enhancement of both morbidity and mortality. The vascular alterations consist of changes in the function and structure of the arteries, and increasing vascular stiffness, mainly when atherosclerosis is present, whose incidence is increased with age. The arteries accumulate lipids, collagen, and minerals. Cerebral perfusion may be reduced in the elderly, mainly regional cerebral blood flow, which leads to a deterioration of mental and physical functions. The degree of deterioration is increased when aging is associated with hypertension. Aging alters endothelial cells, which play an important role in vascular tone regulation. Such a process tends to reduce endothelium-dependent relaxations, and clearly reduces the vasodilation elicited by beta-adrenoceptor agonists. The contractions induced by different agents, such as 5-hydroxytryptamine, histamine, high potassium and angiotensin are barely affected with aging, whereas those elicited by noradrenaline or endothelin are usually reduced. However, plasma noradrenaline levels are increased with age, mainly due to a reduction in the sensitivity of presynaptic alpha 2-adrenoceptors and also of noradrenaline uptake. Sodium pump activity, that controls cellular ionic homeostasis, may be altered depending on animal species. Finally, vascular Ca2+ regulation appears to be altered and the extracellular Ca2+ dependence of contractile responses elicited by agonists is increased, which justifies the enhanced sensitivity to Ca2+ antagonists in senescence.

Influence of endothelium on cultured vascular smooth muscle cell proliferation

The endothelium exerts a large influence on the underlying vascular smooth muscle, not only by the release of both contracting and relaxing factors but also by its ability to synthesize a large number of molecules that influence vascular smooth muscle growth. In addition to well-characterized growth promoters or growth inhibitors, some endothelium-derived factors, originally described as vasoactive compounds, seem to possess growth-regulatory properties. The vasoconstrictor endothelin-1 elicited a dose-dependent increase of cultured vascular smooth muscle cell DNA synthesis with a maximal effect of 57 +/- 14% over basal levels, whereas vasodilators such as prostacyclin, sodium nitroprusside, and 8-bromoguanosine 3':5'-cyclic monophosphate reduced DNA synthesis by 19 +/- 5%, 22 +/- 2%, and 31 +/- 3%, respectively. Medium conditioned by cultured bovine aortic endothelial cells markedly stimulated both DNA synthesis and proliferation of smooth muscle cells. When medium was conditioned in the presence of the endothelin-converting enzyme inhibitor phosphoramidon, the mitogenic effect was significantly reduced, thus indicating a role for endothelin in the stimulation of smooth muscle cell growth by endothelial cells. However, when both cell types were maintained in a coculture system, a 13 +/- 2% decrease of DNA synthesis was observed in smooth muscle cultures. The addition of the nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester, the cyclooxygenase inhibitor indomethacin, or both during the coculture period did not revert the antiproliferative effect of endothelial cells in coculture, thereby indicating it is not likely due to these unstable endothelium-derived vasorelaxant molecules.

Microvasculature of the anterior optic nerve

The traditional definition of glaucoma as a disorder of increased intraocular pressure (IOP) oversimplifies the clinical situation. Some glaucoma patients never have higher than normal IOP and others continue to develop optic nerve damage despite maximal lowering of IOP. Another possible factor in the etiology of glaucoma may be regulation of the regional microvasculature of the anterior optic nerve. One reason to believe that microvascular factors are important is that many microvascular diseases are associated with glaucomatous optic neuropathy. In this study, endothelin-1 was used to produce vasoconstriction of the ocular vasculature following both intravitreal and perineural delivery. Intravitreal injections (1.25 and 2.50 micrograms) were given to six rabbits to assess the vascular response of the posterior segment. In addition, endothelin (0.0-4.2 micrograms) was delivered with an osmotically driven minipump system to the perineural region of six rabbits. Vasoconstriction of the anterior optic nerve vasculature was observed following both methods of endothelin delivery. A better understanding of the microvasculature of the anterior optic nerve is needed to evaluate its relationship to glaucomatous damage. Modulation of this vascular supply, with agents that mimic autoregulatory controls, may allow the sites of vasomotor control to be established and, in the future, their status to be manipulated.

Vasoconstrictive responses elicited by endothelin in bovine cerebral arteries

1. Endothelin (ET-1) induced concentration-dependent contractions, which were slowly developed in segments of bovine cerebral arteries. Furthermore, this agent produced tachyphylaxis. 2. The contractions evoked by ET-1 were markedly reduced in Ca-free medium containing 1 mM EGTA and by the Ca channel antagonist, nifedipine (1 microM), but increased by the Ca channel agonist, BAY K 8644 (10 nM). 3. The contractions caused by ET-1 were significantly reduced by the protein kinase C (PKC) inhibitor, staurosporine (1 and 10 nM). 4. These results indicate that ET-1 induced potent vasoconstrictive responses, probably mediated by PKC activation, which were mainly dependent on extracellular Ca; this Ca enters the smooth muscle cells via dihydropyridine sensitive Ca channels.