s-nitro-n-acetylpenicillamine and Hypoxia

In mammals, the in vivo coronary blood flow and myocardial oxygen consumption are closely related via changes in coronary resistance in response to the metabolic demands of the myocardium. A fine neurohumoral regulation of coronary resistance holds true also in fish, and particularly in teleosts, where several vasoconstrictive and vasodilative mechanisms have been described, with numerous putative effectors, including prostanoids, acetylcholine, adrenaline, serotonin, adenosine, steroid hormones. Here, a resume is reported of the available evidence on the involvement of nitric oxide (NO) in the control of coronary resistance in teleosts and particularly in salmonids. Most of the evidence reported is from a comprehensive study performed on a Langedorff-type preparation of the isolated trout heart. Using a physio-pharmacological approach, the experiments performed on this preparation have demonstrated that trout coronary resistance is reduced by l-arginine (NOS substrate), nitroprusside and SNAP (NO donors) and is increased by the NOS inhibitors l-NNA and l-NAME. The vasodilation induced by nitroprusside is blocked by the guanylate cyclase inhibitor methylene blue. l-arginine increases NO release in the perfusate, while l-NNA reduces the release. NO release is inversely related with the coronary resistance. l-NNA inhibits the vasodilatory effects of acetylcholine, serotonin and adenosine. The vasodilation induced by adenosine is accompanied by NO release and involves stretch receptors. Hypoxia induces vasodilation and both adenosine and NO release in the preparation; the NO release under hypoxia is blocked by theophylline. On the whole these data indicate that NO plays a central role in the control of coronary resistance in trout. In particular, a main role for NO as an amplifier of the adenosine-mediated vasodilation under hypoxia can be hypothesized.

Topics: Adenosine; Animals; Arginine; Coronary Circulation; Electrodes; Fishes; Guanylate Cyclase; Heart; Hypoxia; Models, Biological; NG-Nitroarginine Methyl Ester; Nitric Oxide; Nitric Oxide Synthase Type III; Nitroprusside; Oxygen; Oxygen Consumption; Penicillamine; Theophylline; Time Factors; Trout; Vasodilator Agents

The activation of ATP-sensitive potassium (K ATP) ion channels in the heart is thought to exert a cardioprotective effect under low oxygen conditions, possibly enhancing tolerance of environmental hypoxia in aquatic vertebrates. The purpose of this study was to examine the possibility that hypoxia-induced activation of cardiac K ATP channels, whether in the sarcolemma (sarcK ATP) or mitochondria (mitoK ATP), enhances viability in cardiac muscle cells from a species highly tolerant of low oxygen environments, the goldfish Carassius auratus. During moderate hypoxia (6-7 kPa), the activation of sarcK ATP channels was indicated by a reduction in transmembrane action potential duration (APD). This response to hypoxia was mimicked by the NO-donor SNAP (100 micromol l(-1)) and the stable cGMP analog 8-Br-cGMP, but abolished by glibenclamide or l-NAME, an inhibitor of NO synthesis. The mitoK ATP channel opener diazoxide did not affect APD. Isolated ventricular muscle cells were then incubated under normoxic and hypoxic conditions. Cell viability was decreased in hypoxia; however, the negative effects of low oxygen were reduced during simultaneous exposure to SNAP, 8-Br-cGMP, and diazoxide. The cardioprotective effect of diazoxide, but not 8-Br-cGMP, was reduced by the mitoK ATP channel blocker 5-HD. These data suggest that hypoxia-induced activation of sarcK ATP or mitoK ATP channels could enhance tolerance of low-oxygen environments in this species, and that sarcK ATP activity is increased through a NO and cGMP-dependent pathway.

Topics: Acclimatization; Action Potentials; Analysis of Variance; Animals; Cyclic GMP; Diazoxide; Glyburide; Goldfish; Hypoxia; Mitochondria; Models, Biological; Myocardium; NG-Nitroarginine Methyl Ester; Penicillamine; Potassium Channels; Sarcolemma

Hypoxia-induced shortening of cardiac action potential duration (APD) has been attributed in mammalian hearts to the activation of ATP-sensitive potassium (KATP) channels. Since KATP channels are also present at high densities in the hearts of vertebrate ectotherms, speculation arises as to their function during periods of reduced environmental oxygen. The purpose of the present study was to determine whether nitric oxide (NO) plays a role in cardiac sarcolemmal KATP channel activation during hypoxia in a species with a high degree of tolerance to low oxygen environments: the goldfish (Carassius auratus). Conventional intracellular and patch-clamp recording techniques were used to record responses from excised ventricles or isolated ventricular myocytes and inside-out patches, respectively, from fish acclimated at 21 degrees C. During moderate, substrate-free hypoxia (6.1 +/- 0.2 kPa), ventricular APD was significantly shortened at 50% and 90% of full repolarization, a response that was reversible upon reoxygenation and blocked by the KATP channel antagonist BDM. Under normoxic conditions, APD was also reduced in the presence of the NO-donor SNAP (100 micromol l(-1)). In cell-attached membrane patches, sarcolemmal KATP channel activity was enhanced after 10 min hypoxia, an effect that was reduced or eliminated by simultaneous exposure to BDM, to the guanylate cyclase inhibitor ODQ or to the NO synthase inhibitor L-NAME. In cell-free patches, KATP channel activity was abolished by 2 mmol l(-1) ATP but increased by SNAP; the cGMP analog 8-Br-cGMP (200 micromol l(-1)) also enhanced activity, an effect that was eliminated by BDM. Our data indicate that NO synthesized in cardiac myocytes could enhance sarcolemmal KATP channel activation during moderate hypoxia in goldfish. This response may serve a cardioprotective role by helping to conserve ATP or by reducing intracellular Ca2+ accumulation.

Topics: Action Potentials; Analysis of Variance; Animals; Diacetyl; Goldfish; Hypoxia; Myocardium; NG-Nitroarginine Methyl Ester; Nitric Oxide; Oxadiazoles; Patch-Clamp Techniques; Penicillamine; Potassium Channels; Quinoxalines; Sarcolemma

Female rats develop less severe pulmonary hypertension (PH) in response to chronic hypoxia compared with males, thus implicating a potential role for ovarian hormones in mediating this gender difference. Considering that estrogen upregulates endothelial nitric oxide (NO) synthase (eNOS) in systemic vascular tissue, we hypothesized that estrogen inhibits hypoxic PH by increasing eNOS expression and activity. To test this hypothesis, we examined responses to the endothelium-derived NO-dependent dilator ionomycin and the NO donors S-nitroso-N-acetylpenicillamine and spermine NONOate in U-46619-constricted, isolated, saline-perfused lungs from the following groups: 1) normoxic rats with intact ovaries, 2) chronic hypoxic (CH) rats with intact ovaries, 3) CH ovariectomized rats given 17 beta-estradiol (E(2)beta), and 4) CH ovariectomized rats given vehicle. Additional experiments assessed pulmonary eNOS levels in each group by Western blotting. Our findings indicate that E(2)beta attenuated chronic hypoxia-induced right ventricular hypertrophy, pulmonary arterial remodeling, and polycythemia. Furthermore, although CH augmented vasodilatory responsiveness to ionomycin and increased pulmonary eNOS expression, these responses were not potentiated by E(2)beta. Finally, responses to S-nitroso-N-acetylpenicillamine and spermine NONOate were similarly attenuated in all CH groups compared with normoxic control groups. We conclude that the inhibitory influence of E(2)beta on chronic hypoxia-induced PH is not associated with increased eNOS expression or activity.

Topics: 15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5,13-dienoic Acid; Animals; Chronic Disease; Endothelium, Vascular; Enzyme Inhibitors; Estradiol; Female; Hypertension, Pulmonary; Hypertrophy, Right Ventricular; Hypoxia; Ionomycin; Ionophores; Nitric Oxide; Nitric Oxide Donors; Nitric Oxide Synthase; Nitric Oxide Synthase Type III; Nitroarginine; Nitrogen Oxides; Ovariectomy; Penicillamine; Polycythemia; Pulmonary Circulation; Rats; Rats, Sprague-Dawley; Spermine; Vascular Resistance; Vasoconstrictor Agents; Vasodilation

The fundamental, yet poorly understood, physiological mechanism known as 'acidic-metabolic' vasodilation, contributes to local blood flow regulation during hypoxia/ischaemia and increased metabolic activity. The vasodilator nitric oxide (NO) has been suggested to be involved in this event. Besides enzymatic production by NO synthases, a novel mechanism for generation of this gas in vivo was recently described. This involves non-enzymatic reduction of inorganic nitrite to NO, a reaction that takes place predominantly during acidic/reducing conditions. We have studied the effects of physiological amounts of nitrite on NO generation and relaxation of rat aorta in vitro in a situation where environmental pH was reduced to levels seen in tissues during hypoxia/ischaemia. The relaxatory effect of nitrite was increased in an acidic buffer solution (pH 6.6) compared with neutral pH; EC50 for nitrite was reduced from 200 to 40 microM. Nitrite-evoked relaxation was effectively prevented by coadministration of an inhibitor of soluble guanylyl cyclase. The relaxation was further potentiated by the addition of ascorbic acid. In parallel, NO was generated from nitrite in a pH dependent manner with even larger amounts seen after addition of ascorbic acid. NO generation from nitrite correlated to the the degree of relaxation of rat aorta. These results illustrate non-enzymatic release of NO from nitrite at physiological concentrations. This may be an important auto-regulated physiological mechanism involved in the regulation of vascular tone during hypoxia/ischaemia.

Topics: Acidosis; Animals; Aorta; Dose-Response Relationship, Drug; Enzyme Inhibitors; Hypoxia; In Vitro Techniques; Indicators and Reagents; Muscle, Smooth, Vascular; Nitric Oxide; Nitric Oxide Donors; Oxadiazoles; Penicillamine; Quinoxalines; Rats; Rats, Wistar; Regional Blood Flow; Sodium Nitrite; Vasodilation