Depression of endothelium-dependent relaxation in aorta from rats with Brugia pahangi lymphatic filariasis

Hypoxically Induced Nitric Oxide: Potential Role as a Vasodilator in Mytilus edulis Gills

Intertidal Mytilus edulis experience rapid transgression to hypoxia when they close their valves during low tide. This induces a physiological stress response aiming to stabilize tissue perfusion against declining oxygen partial pressure in shell water. We hypothesized that nitric oxide (NO) accumulation supports blood vessel opening in hypoxia and used live imaging techniques to measure NO and superoxide anion (O2∙-) formation in hypoxia-exposed gill filaments. Thirty minutes of moderate (7 kPa pO₂) and severe hypoxia (1 kPa pO₂) caused 1.6- and 2.4-fold increase, respectively, of NO accumulation in the endothelial muscle cells of the hemolymphatic vessels of the gill filaments. This led to a dilatation of blood vessel diameter by 43% (7 kPa) and 56% (1 kPa), which facilitates blood flow. Experiments in which we applied the chemical NO-donor Spermine NONOate (concentrations ranging from 1 to 6 mM) under normoxic conditions corroborate the dilatational effect of NO on the blood vessel. The formation of O2∙- within the filament epithelial cells increased 1.5 (7 kPa) and 2-fold (1 kPa) upon treatment. Biochemical analysis of mitochondrial electron transport complexes in hypoxia-exposed gill tissue indicates decreased activity of complexes I and III in both hypoxic conditions; whereas complex IV (cytochrome-c oxidase) activity increased at 7 kPa and decreased at 1 kPa compared to normoxic exposure conditions. This corresponds to the pattern of pO₂-dependent gill respiration rates recorded in ex-vivo experiments. Severe hypoxia (1 kPa) appears to have a stabilizing effect on NO accumulation in gill cells, since less O₂ is available for NO oxidation to nitrite/nitrate. Hypoxia thus supports the NO dependent inhibition of complex IV activity, a mechanism that could fine tune mitochondrial respiration to the local O₂ availability in a tissue. Our study highlights a basal function of NO in improving perfusion of hypoxic invertebrate tissues, which could be a key mechanism of tolerance toward environmental O₂ variations.

Heartworm extract induces relaxation of isolated rat thoracic aorta

Isolated rat thoracic aortic strips undergoing noradrenaline-induced contraction were treated with an adult heartworm (HW) crude extract and then examined for isometric changes in tension. HW extract caused relaxation of endothelium-intact strips, but not endothelium-denuded strips. This effect was inhibited by treatment with N^G-nitro-L-arginine methyl ester hydrochloride (L-NAME) and could be reversed by additional treatment with L-arginine. However, HW extract at a high concentration caused slight relaxation of endothelium-denuded strips, and relaxation persisted after L-NAME treatment in endothelium intact-strips. These data suggested that the relaxation induced by HW extract was mainly endothelium-dependent, nitric oxide-mediated, but in part, also endothelium-independent. In addition, a bioassay using isolated rat thoracic aortas may be a useful tool for investigating vasoactive substances in the HW extract.

Does segmental difference in alpha 1-adrenoceptor subtype explain contractile difference in rat abdominal and thoracic aortae?

The cyclooxygenase inhibitor, indomethacin, depresses adrenergic agonist constriction of endothelium-denuded rat abdominal, but not thoracic, aorta. In order to explain this finding, we explored the possibility of segmental differences in the population of alpha 1-adrenoceptor (AR) subtypes. In endothelium-denuded tissues, phenylephrine elicited concentration-dependent contractions in the thoracic and abdominal aortic rings with potencies and maximal effects that, respectively, did not differ significantly (P > .05). Indomethacin (1 x 10(-5) M) inhibited phenylephrine-induced contractions only in abdominal aorta. The subtype-selective alpha 1D-AR antagonist, BMY 7378, was found to antagonize contractions to phenylephrine competitively in abdominal (pA2 8.44) and thoracic (pA2 8.56) aortic rings. These data are consistent with published alpha 1D-AR functional potency and clonal alpha 1D-AR binding affinity. In addition, cumulative concentration-contraction curves for phenylephrine were competitively antagonized in the rat abdominal and thoracic aortae by prazosin, 5-methylurapidil and WB 4101, with pA2 values of 9.39 and 9.61, 7.64 and 7.85, and 9.43 and 9.58, respectively. These compounds with varying degrees of subtype selectivity inhibited contractions of the thoracic and abdominal aortae with affinities consistent with those determined at the alpha 1D-AR subtype. The results of this study suggest that the contraction to phenylephrine of the rat abdominal and thoracic aorta is mediated via the same alpha 1D-AR subtype.

Dirofilaria immitis: heartworm infection alters pulmonary artery endothelial cell behavior

The pathogenesis of filariasis has generally been attributed to either physical presence of the adult parasites or the host's immune response to the parasites. However, the spectrum of filariasis cannot be entirely explained by these causes, and other mechanisms must be operative. It is now evident that factors released by filarial parasites likely contribute to the pathogenesis of filarial diseases. Adult heartworms (Dirofilaria immitis) reside in the right heart and pulmonary artery, so the pulmonary artery should be exposed to the highest concentration of filarial factors. We tested the hypothesis that endothelium-dependent relaxation is altered in the in vitro pulmonary artery from heartworm-infected dogs. Relaxation responses to endothelium-dependent vasodilators (methacholine, bradykinin, substance P, and A-23187) and the nonendothelium-dependent vasodilator nitroglycerin and contractile responses were measured in rings of pulmonary artery from control and heartworm-infected dogs. Endothelium-dependent relaxation was assessed in the presence and absence of inhibitors of nitric oxide synthase, cyclooxygenase, and guanylate cyclase. Responses to methacholine, substance P, and A-23187, but not to bradykinin, nitroglycerin, norepinephrine, or KCl, were depressed in pulmonary artery from heartworm-infected dogs when compared with control, suggesting that changes in endothelial cell and not vascular smooth muscle behavior are involved in altered relaxation. The mechanism of endothelium-dependent relaxation in control pulmonary artery appears to involve nitric oxide in the case of methacholine and both nitric oxide and a cyclooxygenase product in the case of bradykinin and A-23187. The mechanism of endothelium-dependent relaxation in pulmonary artery from heartworm-infected dogs was not clearly elucidated. These data provide no evidence that heartworm infection globally influences either endothelial cell receptor function or the vascular smooth muscle guanylate cyclase guanosine 3',5'-cyclic monophosphate system, making it likely that changes in intracellular signaling are primarily responsible for the observed alteration of endothelium-mediated relaxation. Alteration of endothelial cell function by filarial parasites may be an important component in the pathology associated with filariasis.

Role of nitric oxide in parasitic infections

Nitric oxide is produced by a number of different cell types in response to cytokine stimulation and thus has been found to play a role in immunologically mediated protection against a growing list of protozoan and helminth parasites in vitro and in animal models. The biochemical basis of its effects on the parasite targets appears to involve primarily inactivation of enzymes crucial to energy metabolism and growth, although it has other biologic activities as well. NO is produced not only by macrophages and macrophage-like cells commonly associated with the effector arm of cell-mediated immune reactivity but also by cells commonly considered to lie outside the immunologic network, such as hepatocytes and endothelial cells, which are intimately involved in the life cycle of a number of parasites. NO production is stimulated by gamma interferon in combination with tumor necrosis factor alpha or other secondary activation signals and is regulated by a number of cytokines (especially interleukin-4, interleukin-10, and transforming growth factor beta) and other mediators, as well as through its own inherent inhibitory activity. The potential for design of prevention and/or intervention approaches against parasitic infection (e.g., vaccination or combination chemo- and immunotherapy strategies) on the basis of induction of cell-mediated immunity and NO production appears to be great, but the possible pathogenic consequences of overproduction of NO must be taken into account. Moreover, more research on the role and regulation of NO in human parasitic infection is needed before its possible clinical relevance can be determined.

Cyclooxygenase inhibitors depress norepinephrine constriction of rat abdominal, but not thoracic, aorta

Experiments were done on aortic rings (thoracic and abdominal) from young and retired breeder Lewis and Sprague-Dawley male rats. Constriction responses to norepinephrine, 5-hydroxytryptamine (5-HT), and prostaglandin F2 alpha, were done +/- the cyclooxygenase blockers, indomethacin or mefenamic acid. Indomethacin significantly depressed norepinephrine constriction in abdominal (but not thoracic) aorta of all groups. In additional studies of abdominal aorta from Lewis retired breeders, indomethacin and mefenamic acid depressed norepinephrine (but not 5-HT or prostaglandin F2 alpha) construction. Furthermore, indomethacin depressed norepinephrine constriction in vessels denuded of endothelial cells. The thromboxane receptor antagonist SQ 29548 did not alter norepinephrine constriction. Thus, in rat abdominal aorta, norepinephrine constriction is mediated by a constrictor prostanoid of vascular smooth muscle origin that is not thromboxane A2.