Selective and complete blockade of acetylcholine-induced relaxation in rabbit aortic rings by N omega-nitro-L-arginine but not by glybenclamide

The bimodal regulation of vascular function by superoxide anion: role of endothelium

Reactive oxygen species (ROS) are implicated in vascular homeostasis. This study investigated whether O(2) (*-), the foundationmolecule of all ROS, regulates vasomotor function. Hence, vascular reactivity was measured using rat thoracic aortas exposed to an O(2) (*-) generator (pyrogallol) which dose-dependently regulated both alpha-adrenergic agonist-mediated contractility to phenylephrine and endothelium-dependent relaxations to acetylcholine. Pyrogallol improved and attenuated responses to acetylcholine at its lower (10 nM - 1 microM) and higher (10 - 100 microM) concentrations, respectively while producing the inverse effects with phenylephrine. The endothelial inactivation by L-NAME abolished acetylcholine-induced vasodilatations but increased phenylephrine and KCl-induced vasoconstrictions regardless of the pyrogallol dose used. Relaxant responses to sodium nitroprusside, a nitric oxide donor, were not affected by pyrogallol. Other ROS i.e. peroxynitrite and H(2)O(2) that may be produced during experiments did not alter vascular functions. These findings suggest that the nature of O(2) (*-)-evoked vascular function is determined by its local concentration and the presence of a functional endothelium.

Role of potassium channels in the nitric oxide-independent vasodilator response to acetylcholine

Stimulation of vascular endothelial muscarinic receptors by acetylcholine (ACh) leads to the formation of an endothelium-derived relaxing factor (EDRF), which is generally accepted to be nitric oxide (NO). Recent evidence, however, suggests that NO may be only one of several EDRFs mediating the vasodilator response to ACh. Since this NO-independent vasodilator response to ACh has been hypothesized to be dependent upon K(+) channel activation, the current study was undertaken to investigate the role of K(+) channels in mediating the hindlimb vasodilator responses to ACh in vivo. Additionally, since variations in vascular tone can complicate the analysis of responses, the level of vascular tone was maintained at a similar level throughout the study so that responses could be compared directly. The results of the present study demonstrate that the vasodilator response to ACh possesses a significant component that is independent of NO production. The K(Ca) channel blockers charybdotoxin and apamin, but not K(+)-ATP channel blocker U37883A or the COX antagonist meclofenamate, attenuated the NO-independent component of the vasodilator response to ACh. This suggests that K(Ca) channels, but not K(+)-ATP channels or COX products, are involved in mediating the L-NAME resistant response to ACh. Further, the inhibition of the ACh vasodilator response by the K(+)-ATP opener BRL55834 suggests that the response is dependent upon membrane hyperpolarization. These data suggest that the mechanism mediating ACh responses in the hindlimb vascular bed of the rat are complex and may involve several signaling pathways.

A selective somatostatin type-2 receptor agonist inhibits neointimal thickening and enhances endothelial cell growth and morphology following aortic balloon injury in the rabbit

Somatostatin analogs have been shown to inhibit vascular smooth muscle cell (VSMC) proliferation and attenuate neointimal thickening following experimental balloon catheter injury. In this study, the effects of a selective agonist for the somatostatin receptor subtype 2, PRL-2486, on neointimal thickening and endothelial cell regrowth 2 weeks following balloon catheterization of male New Zealand White rabbits were determined. Rabbits treated 2 days prior to and 2 weeks after catheter injury with 10 microg/kg/day PRL-2486 (PRL-tx) had decreased I/M ratios (intimal area/medial area x 100; p < 0.05) but had no effect at lower (5 microg/kg/day) or higher (20 microg/kg/day) doses. PRL-tx had significantly decreased VSMC proliferation compared to untreated animals. PRL-tx increased endothelial regrowth by over 2-fold (p < 0.002) and improved endothelial cell morphology. Endothelial-dependent relaxation responses to acetylcholine were attenuated by catheter injury, and were not improved with PRL-tx. These data suggest that the PRL-2486-mediated inhibition of neointimal thickening exhibits a bell-shaped dose-response curve. This inhibition may be due in part to decreased VSMC proliferation, which may be a function of enhanced endothelial regrowth, but not the return of endothelium-dependent vascular function.

The effect of oxidative stress on endothelium-dependent and nitric oxide donor-induced relaxation: implications for nitrate tolerance

Increased inactivation of nitric oxide (NO) by superoxide has been implicated in nitrate tolerance. Here, we set out to compare the inhibitory effect of superoxide on endothelium-dependent, acetylcholine (ACh)-mediated vascular relaxation with that on the endothelium-independent effects of glyceryl trinitrate (GTN) and another NO donor drug, S-nitrosoglutathione (GSNO). Rings of thoracic aorta from adult male Wistar rats (350-450 g) were precontracted with phenylephrine (approximately EC(90)) prior to cumulative additions (10 nM/L-10 microM/L) of GTN, GSNO, or ACh. Rings were then treated with the superoxide generator pyrogallol (300 micromol/L) alone or following pretreatment with the Cu/Zn superoxide dismutase inhibitor diethyldithiocarbamate (DETCA; 100 micromol/L), and cumulative additions of the vasodilators were repeated. All experiments were conducted in the presence of catalase (3000 U/ml) to prevent accumulation of hydrogen peroxide. Relaxation to ACh was abolished by pyrogallol-derived superoxide. Relaxation to GSNO was significantly inhibited by superoxide (P < 0.05, n = 8) and was more pronounced at lower GSNO concentrations. However, GTN was relatively resistant to inhibition by superoxide with modest inhibition only occurring in rings pretreated with DETCA prior to pyrogallol (P < 0.05; n = 8). In contrast to GSNO, the inhibitory effect was more pronounced with high concentrations of GTN, suggesting that the mechanism underlying superoxide-mediated inhibition is different for the two NO donor drugs. Further experiments showed that vascular responses to ACh were not inhibited (P > 0.05, n = 6) in aortic rings made tolerant to GTN (10 micromol/L, 2-h incubation) and that treatment of vessels with the antioxidant vitamin C (1 mmol/L) successfully prevented the development of tolerance. Taken together, these results suggest that superoxide is not a major factor in tolerance in vitro and imply that the protective actions of vitamin C are unrelated to its antioxidant activity in this setting.

Endothelial cell regrowth and morphology after balloon catheter injury of alloxan-induced diabetic rabbits

Neointimal thickening after catheter injury has been reported to be influenced by the integrity of the vascular endothelium. We have previously shown that neointimal thickening is significantly reduced in alloxan-induced diabetic New Zealand White rabbits after catheter injury compared with euglycemic rabbits. In the present study, it was hypothesized that endothelial cell regrowth, morphology, and endothelium-dependent vasoreactivity after catheter injury are improved in the diabetic rabbit (glucose >/=400 mg/dl) compared with the euglycemic rabbit. Two weeks after catheter injury, the percent endothelial regrowth was significantly increased in diabetic animals compared with euglycemic animals (32.1 +/- 2 and 15.6 +/- 1, respectively; P < 0.05). The endothelial cell morphology analyzed by scanning electron microscopy was also restored 2 wk after catheter injury in thoracic aortas from the diabetic animals compared with vessels from euglycemic animals. Endothelium-dependent relaxation to ACh in vessels from diabetic and euglycemic rabbits was attenuated 2 wk after injury, and, although improved by 4 and 8 wk, relaxation remained significantly depressed. These results suggest that endothelial cell regrowth and morphology in diabetic animals was improved compared with euglycemic animals; however, endothelium-dependent vasoreactivity remained impaired. Thus the attenuated neointimal thickening seen in the diabetic rabbit may be a function of the rate and degree of regrowth rather than the normalization of ACh-induced relaxation.

Actions of neurotransmitters and other messengers on Ca2+ channels and K+ channels in smooth muscle cells

Ion channels play key roles in determining smooth muscle tone by setting the membrane potential and allowing Ca2+ influx. Perhaps not surprisingly, therefore, they also provide targets for neurotransmitters and other messengers that act on smooth muscle. Application of patch-clamp and molecular biology techniques and the use of selective pharmacology has started to provide a wealth of information on the ion channel systems of smooth muscle cells, revealing complexity and functional significance. Reviewed are the actions of messengers (e.g., noradrenaline, acetylcholine, endothelin, angiotensin II, neuropeptide Y, 5-hydroxytryptamine, histamine, adenosine, calcitonin gene-related peptide, substance P, prostacyclin, nitric oxide and oxygen) on specific types of ion channel in smooth muscle, the L-type calcium channel, and the large conductance Ca(2+)-activated, ATP-sensitive, delayed rectifier and apamin-sensitive K+ channels.

Role of NO production in NMDA receptor-mediated neurotransmitter release in cerebral cortex

L-Glutamate and norepinephrine are examples of a major excitatory neurotransmitter and a neuromodulator in the cerebral cortex, respectively. Little is known of how chemical signaling between the anatomically distinct chemical pathways occurs. Specific activation of the N-methyl-D-aspartate (NMDA) class of glutamate receptor in synaptosomal preparations from guinea pig cerebral cortex caused release of both of these chemicals, and this release was blocked by agents that inhibit nitric oxide (NO) production or remove NO from the extracellular space. Furthermore, neurotransmitter release correlated with cortical NO production after NMDA receptor stimulation. These results suggest that NO production and its extracellular movement may be links in the pathway from NMDA receptor activation to changes in chemical signaling in surrounding synaptic terminals in the cerebral cortex.