Effect of endotoxin-enhanced hepatic reperfusion injury on endothelium-dependent relaxation in rat aorta

We have investigated endothelial function in a two-hit model of multiple organ failure. Male Fischer rats were subjected to 20 min of partial hepatic ischemia followed by reperfusion and administration of .5 mg/kg Salmonella enteritidis endotoxin at 30 min of reperfusion. After either 4 or 24 h of reperfusion, rings of aorta were prepared and suspended in bioassay baths, contracted with phenylephrine, and examined for endothelium-dependent relaxation in response to acetylcholine and endothelium-independent relaxation using nitroglycerin. Endothelium-dependent relaxation to acetylcholine was impaired in rings from animals exposed to endotoxin-enhanced reperfusion injury at both 4 h and 24 h. At 24 h of reperfusion the EC50 for acetylcholine relaxation was significantly increased from 45 +/- 8 nM to 258 +/- 105 nM. Endothelium-independent relaxation to nitroglycerin was not affected. The 21-aminosteroid Tirilazad mesylate (U-74006F) prevented endothelial dysfunction; at 24 h of reperfusion the EC50 for acetylcholine relaxation in U-74006F-treated animals was 55 +/- 8 nM. Thus, endothelial function is impaired in this model of multiple organ failure and this impairment is prevented by Tirilazad mesylate.

Nitric oxide inhibits peroxynitrite-induced production of hydroxyeicosatetraenoic acids and F2-isoprostanes in phosphatidylcholine liposomes

Lipid peroxidation is a component of various pathologies associated with nitric oxide (NO). It has been suggested that in vivo production of peroxynitrite (ONOO-) is responsible for the detrimental effects of pathologies associated with NO. To investigate the role of NO and ONOO- in the formation of specific lipid peroxidation products, liposomes of phosphatidylcholine were incubated at 37 degrees C for 1 h with various NO sources [NO, diethylamine NONOate (DEA/NO)] or ONOO-sources [3-morpholinosydnonimine-HCl (Sin-1), ONOO-]. Gas chromatography-mass spectrometry was used to quantify the formation of hydroperoxy- and hydroxyeicosatetraenoic acids (HETEs) and F2-isoprostanes. Peroxynitrite (0.5 mm) caused significant oxidation of phosphatidylcholine liposomes as measured by the increased formation of HETEs and F2-isoprostanes. NO, either added directly to the liposomes as a bolus or delivered by slow diffusion or via the donor compound DEA/NO, caused no lipid peroxidation itself and significantly inhibited both iron- and ONOO--induced lipid peroxidation. Sin-l (1 mM), which releases both NO and superoxide, resulted in minor increases in peroxidation and did not inhibit iron-induced HETE formation. We conclude that peroxynitrite but not nitric oxide can directly cause lipid peroxidation and that NO can act as an antioxidant by terminating lipid radical chain propagation reactions.

Uptake of atrial natriuretic peptide and production of cGMP in cultured human glomerular endothelial cells

Interactions between human glomerular endothelial cells and atrial natriuretic peptide (ANP) were studied with 125I-alpha-human-ANP binding and intracellular accumulation of cGMP. Uptake for alpha-hANP (1-28 or 5-28) by homogeneous cultures of human glomerular endothelial cells was dose and time dependent with optimal uptake occurring after 30 min of incubation at 37 degrees C. Scatchard analysis of the specific binding data with a two-compartmental model identified both high (Kd = 0.3 nM)- and low (Kd = 10 nM)-affinity receptors, with a binding site density of 12,000 and 18,060 receptors per cell, respectively. alpha-hANP markedly stimulated glomerular endothelial cell-associated cGMP. After a 2-min incubation, cGMP increased 1.3-fold (from 17.88 +/- 1.29 to 23.33 +/- 3 pmol/mg of protein), in the presence of 1 nM ANP, to more than threefold (from 21 +/- .1 to 80.5 +/- 14.5 pmol/mg of protein) with 1 microM ANP (P < 0.05). In contrast, a 10 microM concentration of the clearance receptor C-ANP4-23 increased cGMP by 1.6 +/- 0.6 fold. ANP stimulation of intracellular cGMP was 100 times more sensitive in human glomerular endothelial than in mesangial cells. In comparison, higher doses of bradykinin were necessary to evoke similar responses in glomerular endothelial cells. In the presence of 10 microM bradykinin, cellular cGMP increased by 1.75 +/- 0.6-fold versus control cells. However, unlike ANP, bradykinin-stimulated cGMP synthesis was significantly inhibited by prior treatment with oxyhemoglobin (10(-5) M), an inhibitor of soluble guanylate cyclase, and NG-nitro-L-arginine (NO2Arg), a specific inhibitor of endothelial-derived relaxing factor (EDRF).(ABSTRACT TRUNCATED AT 250 WORDS)

TEA inhibits ACh-induced EDRF release: endothelial Ca(2+)-dependent K+ channels contribute to vascular tone

The effects of K(+)-channel blockers on the acetylcholine (ACh)-induced relaxation of vascular smooth muscle, intracellular free Ca2+ concentration ([Ca2+]i) elevation, and ACh-evoked outward K+ current of endothelial cells of rabbit aorta were studied using bioassay, spectrofluorimetry, and patch-clamp techniques, respectively. In bioassay experiments, ACh caused relaxation of endothelium-denuded aortic rings in a concentration-dependent manner when perfused through an endothelium-intact donor segment of aorta but not when perfused directly onto the recipient aortic ring. ACh-induced relaxation was inhibited by perfusion of tetraethylammonium ions (TEA; 5 mM) through the donor but not by perfusion directly onto the recipient segment. Glibenclamide had no effect on ACh-induced relaxation of the bioassay ring in either situation. ACh increased [Ca2+]i at the endothelial surface of aortic strips but not at the adventitial surface. TEA inhibited ACh-induced [Ca2+]i elevation, whereas glibenclamide had no effect. In patch-clamp experiments with freshly isolated endothelial cells, ACh evoked a biphasic outward current which was completely abolished by TEA (3 mM). It is concluded that Ca(2+)-dependent K+ channels are important for increasing [Ca2+]i during agonist stimulation and consequently for the synthesis/release of endothelium-derived relaxing factors (EDRFs). Furthermore, endothelial ATP-sensitive K+ channels do not contribute to ACh-induced relaxation or evoke an increase in endothelial [Ca2+]i of rabbit thoracic aorta.

Cytosolic [Ca2+] measurements in endothelium of rabbit cardiac valves using imaging fluorescence microscopy

Cytosolic Ca2+ plays a critical role in the secretion of endothelium-derived factors. A new preparation that allows fluorescence imaging of intracellular free Ca2+ concentration ([Ca2+]i) in endothelial cells of rabbit cardiac valves is described. Electron micrographs of the valves revealed no underlying smooth muscle cells that might influence endothelial cell responses or contribute to [Ca2+]i signaling. The valve leaflets, which were < 100 microns in diameter, were visualized using a specially designed chamber and a long working distance fluorescence objective. The semilunar valves (pulmonary and aortic) responded to endothelium-dependent vasodilators, including acetylcholine, with an increase in [Ca2+]i. Synchronized [Ca2+]i transients were observed in the endothelial monolayer in response to agonist stimulation in K(+)-free solutions. The ability to monitor changes in [Ca2+]i in a native endothelial monolayer provides a more realistic assessment of stimulus-response coupling within individual cells and communication between cells of native endothelium. In addition, this preparation affords an opportunity for comparative studies of endothelium-related pathophysiologies, which can be induced experimentally in animal models.

The passive calcium leak in cultured porcine aortic endothelial cells

Unidirectional 45Ca fluxes were measured to characterize the calcium-leak pathway in cultured endothelial cells of pig aorta. Lanthanum, but not other calcium channel blockers such as verapamil and diltiazem, inhibited the passive 45Ca uptake. Passive uptake of 45Ca across the plasma membrane increased progressively as extracellular pH was raised from 5.2 to 9.2. SK&F 96365, a putative inhibitor of receptor-mediated Ca2+ entry, selectively blocked ATP-induced 45Ca uptake. The Ca2+ leak pathway may play an important role in endothelial cell Ca2+ homeostasis which controls the basal and stimulated release of endothelium-derived relaxing factors (i.e., EDRF, prostacyclin) in physiological or pathological situations.

Calcium entry-dependent oscillations of cytoplasmic calcium concentration in cultured endothelial cell monolayers

Bovine endothelial cell monolayers grown to confluence and stimulated with bradykinin responded with periodic fluctuations in intracellular Ca2+ concentration ([Ca2+]i) when exposed to K(+)-free Hepes-buffered saline. The fluctuations in [Ca2+]i measured with fura-2 were synchronized among the population of cells observed and were sensitive to extracellular Ca2+ concentration ([Ca2+]o). Thapsigargin, which inhibits the endoplasmic reticular Ca2(+)-ATPase, did not inhibit the [Ca2+]i oscillations. Removal of extracellular Ca2+ or inhibition of Ca2+ entry by using La3+ or 1-(beta- [3-(4-methoxyphenyl)proproxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride (SKF 96365) abolished the [Ca2+]i oscillations in endothelial cell monolayers. The fluctuations in [Ca2+]i were therefore dependent on Ca2+ influx rather than Ca2+ mobilization from intracellular stores. Simultaneous measurements of membrane potential (Em) using the potential-sensitive bisoxonol dye bis(1,3-dibutylbarbituric acid)trimethine oxonol [Di-BAC4(3)] and [Ca2+]i using fura-2 showed that Em oscillated at the same frequency as the fluctuations in [Ca2+]i. The peak depolarization signal coincided with the maximum rate of increase in the [Ca2+]i signal. Oscillations in the Em signal were inhibited by removal of Ca2+ or by addition of 1 mM Ni2+ to the external solution. Taken together, these observations suggest that the change in Em is the consequence of oscillatory changes in a membrane conductance that also allows Ca2+ to enter the cell. Oscillations in the DiBAC4(3) signal may reflect a rhythmic entry of Ca2+ through nonselective cation channels.

Cytosolic calcium ion regulation in cultured endothelial cells

Mechanisms involved in Ca2+ homeostasis and stimulus-secretion coupling in cultured endothelial cells in response to humoral and physical stimuli include passive leak, activation of ion channels, and chemical second messengers. Calcium entry is controlled by receptor activation, passive leak, and mechanical stretch. The rate at which Ca2+ enters the cell through these pathways is dependent on the transmembrane potential which governs the electrochemical gradient for Ca2+ and which is set by participation of various K channels.

Membrane potential and Na(+)-K+ pump activity modulate resting and bradykinin-stimulated changes in cytosolic free calcium in cultured endothelial cells from bovine atria

The effects of membrane potential on resting and bradykinin-stimulated changes in [Ca2+]i were measured in fura-2 loaded cultured endothelial cells from bovine atria by spectrofluorimetry. The basal and bradykinin-stimulated release of endothelium-derived relaxing factor, monitored by bioassay methods, were dependent on extracellular Ca2+. Similarly, the plateau phase of the biphasic [Ca2+]i response to bradykinin stimulation exhibited a dependence on extracellular Ca2+, whereas the initial transient [Ca2+]i peak was refractory to the removal of extracellular Ca2+. The effect of membrane depolarization on the plateau phase of the bradykinin-induced change in [Ca2+]i was determined by varying [K+]o. The resting membrane potential measured under current clamp conditions was positively correlated with the extracellular [K+] (52 mV change/10-fold change in [K+]o). The observed decrease in resting and bradykinin-stimulated changes in [Ca2+]i upon depolarization is consistent with an ion transport mechanism where the influx is linearly related to the electrochemical gradient for Ca2+ entry (Em - ECa). The inhibition of bradykinin-stimulated Ca2+ entry by isotonic K+ was not due to the absence of extracellular Na+ since Li+ substitution did not inhibit the agonist-induced Ca2+ entry. In K(+)-free solutions and in the presence of ouabain, bradykinin evoked synchronized oscillations in [Ca2+]i in confluent endothelial cell monolayers. These [Ca2+]i oscillations between the plateau and resting [Ca2+]i levels were dependent on extracellular Ca2+ and K+ concentrations. Although the mechanism(s) underlying [Ca2+]i oscillations in vascular endothelial cells is unclear, these results suggest a role of the membrane conductance.

In vitro reactivity of digital arteries and veins to vasoconstrictive mediators in healthy horses and in horses with early laminitis

The in vitro reactivity of vasoconstrictive mediators that are implicated in acute laminitis was determined in palmar and plantar digital arteries and veins obtained from healthy horses and in palmar digital vessels of horses with early laminitis (Obel grade I). To obtain baseline reactivity data, 3 experiments were conducted, using healthy horses: (1) the reactivity of palmar and plantar digital arteries and veins to angiotensin II, norepinephrine, and 5-hydroxytryptamine (serotonin) were compared; (2) the direct effects of bacterial endotoxin on vascular reactivity were assessed; and (3) the reactivity of palmar digital arteries and veins to angiotensin II, norepinephrine, prostaglandin F2 alpha (PGF2 alpha), serotonin, and a thromboxane-endoperoxide analog (U46619) were determined. The vascular reactivity of these same 5 vasoconstrictors then was determined in horses with early laminitis and was compared with data from healthy (control) horses. Obel grade-I laminitis was experimentally induced in horses, using carbohydrate overload. Dose responses were conducted for each agent at concentrations between 10(-8)M and 10(-4)M. The potency of a drug was defined as the mean effective concentration necessary to induce 50% of maximal contraction (EC50). There were no differences in EC50 concentrations and in maximal contractions between forelimb and hind limb arteries and veins for angiotensin II, norepinephrine, and serotonin. Incubation with endotoxin had no effect on the reactivity of arteries and veins to angiotensin II, norepinephrine, and serotonin.(ABSTRACT TRUNCATED AT 250 WORDS)

Naloxone pretreatment blocks the hypotensive effects of atenolol in SHR and WKY rats

The present study examined the interaction of opioidergic systems in the hypotensive action of atenolol. Anesthetized, adult spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats were instrumented to monitor blood pressure and heart rate. Atenolol (100 micrograms/kg, i.v.) produced a decrease in blood pressure of similar magnitude in both SHR and WKY rats. However, the bradycardia was greater in SHR. Pretreatment with naloxone (0.1 mg/kg, i.v.) 15 min prior to the administration of atenolol completely prevented the hypotensive response in SHR and decreased the maximum hypotensive response by approximately 50% in WKY rats. The atenolol-induced bradycardia was unaffected in both groups of animals. Additionally, pretreatment with yohimbine, an alpha 2-receptor antagonist, inhibited the hypotensive response of atenolol in SHR. Collectively, these results suggest an interaction between opioidergic and catecholaminergic systems as a possible site of action of antihypertensive drugs.