The A3 adenosine receptor agonist CP-532,903 [N6-(2,5-dichlorobenzyl)-3'-aminoadenosine-5'-N-methylcarboxamide] protects against myocardial ischemia/reperfusion injury via the sarcolemmal ATP-sensitive potassium channel

We examined the cardioprotective profile of the new A(3) adenosine receptor (AR) agonist CP-532,903 [N(6)-(2,5-dichlorobenzyl)-3'-aminoadenosine-5'-N-methylcarboxamide] in an in vivo mouse model of infarction and an isolated heart model of global ischemia/reperfusion injury. In radioligand binding and cAMP accumulation assays using human embryonic kidney 293 cells expressing recombinant mouse ARs, CP-532,903 was found to bind with high affinity to mouse A(3)ARs (K(i) = 9.0 +/- 2.5 nM) and with high selectivity versus mouse A(1)AR (100-fold) and A(2A)ARs (1000-fold). In in vivo ischemia/reperfusion experiments, pretreating mice with 30 or 100 microg/kg CP-532,903 reduced infarct size from 59.2 +/- 2.1% of the risk region in vehicle-treated mice to 42.5 +/- 2.3 and 39.0 +/- 2.9%, respectively. Likewise, treating isolated mouse hearts with CP-532,903 (10, 30, or 100 nM) concentration dependently improved recovery of contractile function after 20 min of global ischemia and 45 min of reperfusion, including developed pressure and maximal rate of contraction/relaxation. In both models of ischemia/reperfusion injury, CP-532,903 provided no benefit in studies using mice with genetic disruption of the A(3)AR gene, A(3) knockout (KO) mice. In isolated heart studies, protection provided by CP-532,903 and ischemic preconditioning induced by three brief ischemia/reperfusion cycles were lost in Kir6.2 KO mice lacking expression of the pore-forming subunit of the sarcolemmal ATP-sensitive potassium (K(ATP)) channel. Whole-cell patch-clamp recordings provided evidence that the A(3)AR is functionally coupled to the sarcolemmal K(ATP) channel in murine cardiomyocytes. We conclude that CP-532,903 is a highly selective agonist of the mouse A(3)AR that protects against ischemia/reperfusion injury by activating sarcolemmal K(ATP) channels.

The synthesis of highly potent, selective, and water-soluble agonists at the human adenosine A3 receptor

Using a combination of parallel and directed synthesis, the discovery of a highly potent and selective series of adenosine A3 agonists was achieved. High aqueous solubility, required for the intended parenteral route of administration, was achieved by the presence of one or two basic amine functional groups.

The Sulfonylurea Glipizide Does Not Inhibit Ischemic Preconditioning in Anesthetized Rabbits

The K(ATP) channel blocker glibenclamide inhibits cardioprotection afforded by ischemic preconditioning (IPC), raising concern about sulfonylurea use by patients with cardiovascular disease. We examined the effects of the widely prescribed sulfonylurea glipizide (Glucotrol XL(R) ) on IPC in anesthetized rabbits. Initially, in parallel studies in pentobarbital-anesthetized rabbits, we identified doses of glipizide (GLIP, 0.17 mg/kg + 0.12 mg/kg/h, IV) and glibenclamide (GLIB, 0.05 mg/kg + 0.03 mg/kg/h, IV) that produced steady-state, clinically relevant plasma levels of both drugs; these doses also significantly increased plasma insulin by 51 +/- 17% (GLIP) and by 57 +/- 17% (GLIB, both p < 0.05 vs. their respective baseline levels). Subsequent parallel studies in ketamine-xylazine-anesthetized rabbits examined the effects of these doses of GLIP and GLIB on IPC. Myocardial injury (30 min coronary occlusion/120 min reperfusion), either with or without IPC (5 min occlusion/10 min reperfusion) was induced midway during a 2 h infusion of vehicle (VEH), GLIP or GLIB (n = 10-11 each). Infarct area (IA) normalized to area-at-risk (%IA/AAR) was 62 +/- 3% in the VEH group, and was significantly reduced to 39 +/- 5% by IPC (p < 0.05 vs. VEH). Neither GLIP nor GLIB treatment had any effect on %IA/AAR in the absence of IPC (p > 0.05). IPC-induced cardioprotection was preserved in the GLIP + IPC treatment group (45 +/- 4%) when compared to VEH alone (p < 0.05), but was attenuated in the presence of GLIB (GLIB+IPC: 53 +/- 4% IA/AAR, p > 0.05 vs. VEH). Thus, at a clinically relevant plasma concentration, glipizide did not limit the cardioprotective effects of IPC, and is unlikely to increase the severity of cardiac ischemic injury.

Cardioprotective efficacy of zoniporide, a potent and selective inhibitor of Na+/H+ exchanger isoform 1, in an experimental model of cardiopulmonary bypass

1. We determined (1) the inhibitory potency of zoniporide against the native Na(+)/H(+) exchanger isoform 1 (NHE1) that is expressed in adult rat ventricular myocytes and platelets, and (2) the cardioprotective efficacy of zoniporide in isolated, blood-perfused adult rat hearts subjected to cardioplegic arrest, hypothermic ischaemia (150 min at 25 degrees C) and normothermic reperfusion (60 min at 37 degrees C). 2. In isolated myocytes, in which NHE1 activity was determined directly by measurement of H(+) efflux rate following intracellular acidification, zoniporide produced a dose-dependent inhibition of such activity (IC(50) 73 nm at 25 degrees C). A comparable NHE1-inhibitory potency was retained at 37 degrees C. 3. In platelets, in which the rate of cell swelling was used as a surrogate index of NHE1 activity, this was again inhibited by zoniporide (IC(50) 67 nm at 25 degrees C). 4. In the isolated heart model, administration of zoniporide (loading bolus of 1 mg kg(-1) i.v. plus continuous infusion at 1.98 mg kg(-1) h(-1) i.v.) to the support animal achieved a free plasma drug concentration of >/=1 microm. At this dose, zoniporide afforded significant cardioprotective benefit relative to vehicle treatment, with improved preservation of left ventricular end-diastolic and developed pressures and coronary perfusion pressure during reperfusion. Myocardial myeloperoxidase activity was also attenuated by zoniporide treatment, indicating reduced neutrophil accumulation. 5. These data show that zoniporide (1) is a potent inhibitor of native NHE1 activity in ventricular myocytes and platelets, and (2) affords significant cardioprotective benefit during ischaemia and reperfusion in an experimental model that mimics several distinctive features of human cardioplegic arrest with cardiopulmonary bypass.

Novel N6-substituted adenosine 5'-N-methyluronamides with high selectivity for human adenosine A3 receptors reduce ischemic myocardial injury

We recently reported the identification of a novel human adenosine A3 receptor-selective agonist, (2S,3S,4R,5R)-3-amino-5-[6-[5-chloro-2-(3-methylisoxazol-5-ylmethoxy)benzylamino]purin-9-yl]-4-hydroxytetrahydrofuran-2-carboxylic acid methylamide (CP-608,039), with 1,260-fold selectivity for the human A3 versus human A1 receptor (DeNinno et al., J Med Chem 46: 353-355, 2003). However, because the modest (20-fold) rabbit A3 receptor selectivity of CP-608,039 precludes demonstration of A3-mediated cardioprotection in rabbit models, we identified another member of this class, (2S,3S,4R,5R)-3-amino-5-[6-(2,5-dichlorobenzylamino)purin-9-yl]-4-hydroxytetrahydrofuran-2-carboxylic acid methylamide (CP-532,903), which both retained human A3 receptor selectivity (210-fold; human A3/human A1 Ki: 23/4,800 nM) and had improved rabbit A3 receptor selectivity (90-fold; rabbit A3/rabbit A1 Ki: 23/2,000 nM). Infarct size was measured in Langendorff hearts or in vivo after 30 min of regional ischemia and 120 min of reperfusion. Five-minute perfusion with CP-532,903 before ischemia-reperfusion elicited a concentration-dependent reduction in infarct size in isolated hearts (EC50: 0.97 nM; maximum reduction in infarct size: 77%, P < 0.05 vs. control). Furthermore, administration of CP-532,903 (150 nM) at reperfusion also significantly reduced infarct size by 64% (P < 0.05 vs. control), which was not different (P > or = 0.05) from the cardioprotection provided by the same concentration of drug given before ischemia. The selective rabbit A1 receptor antagonist BWA1433 did not affect CP-532,903-dependent cardioprotection. In vivo, CP-532,903 (1 mg/kg) reduced infarct size by 50% in the absence of significant hemodynamic effects (mean arterial pressure, heart rate, rate-pressure product). CP-532,903 and CP-608,039 represent a novel class of human A3 receptor-selective agonists that may prove suitable for investigation of the clinical cardioprotective efficacy of A3 receptor activation.

Cardioprotective effects of ingliforib, a novel glycogen phosphorylase inhibitor

Interventions such as glycogen depletion, which limit myocardial anaerobic glycolysis and the associated proton production, can reduce myocardial ischemic injury; thus it follows that inhibition of glycogenolysis should also be cardioprotective. Therefore, we examined whether the novel glycogen phosphorylase inhibitor 5-Chloro-N-[(1S,2R)-3-[(3R,4S)-3,4-dihydroxy-1-pyrrolidinyl)]-2-hydroxy-3-oxo-1-(phenylmethyl)propyl]-1H-indole-2-carboxamide (ingliforib; CP-368,296) could reduce infarct size in both in vitro and in vivo rabbit models of ischemia-reperfusion injury (30 min of regional ischemia, followed by 120 min of reperfusion). In Langendorff-perfused hearts, constant perfusion of ingliforib started 30 min before regional ischemia and elicited a concentration-dependent reduction in infarct size; infarct size was reduced by 69% with 10 microM ingliforib. No significant drug-induced changes were observed in either cardiac function (heart rate, left ventricular developed pressure) or coronary flow. In open-chest anesthetized rabbits, a dose of ingliforib (15 mg/kg loading dose; 23 mg.kg(-1).h(-1) infusion) selected to achieve a free plasma concentration equivalent to an estimated EC(50) in the isolated hearts (1.2 microM, 0.55 microg/ml) significantly reduced infarct size by 52%, and reduced plasma glucose and lactate concentrations. Furthermore, myocardial glycogen phosphorylase a and total glycogen phosphorylase activity were reduced by 65% and 40%, respectively, and glycogen stores were preserved in ingliforib-treated hearts. No significant change was observed in mean arterial pressure or rate-pressure product in the ingliforib group, although heart rate was modestly decreased postischemia. In conclusion, glycogen phosphorylase inhibition with ingliforib markedly reduces myocardial ischemic injury in vitro and in vivo; this may represent a viable approach for both achieving clinical cardioprotection and treating diabetic patients at increased risk of cardiovascular disease.

Isozyme-nonselective N-substituted bipiperidylcarboxamide acetyl-CoA carboxylase inhibitors reduce tissue malonyl-CoA concentrations, inhibit fatty acid synthesis, and increase fatty acid oxidation in cultured cells and in experimental animals

Inhibition of acetyl-CoA carboxylase (ACC), with its resultant inhibition of fatty acid synthesis and stimulation of fatty acid oxidation, has the potential to favorably affect the multitude of cardiovascular risk factors associated with the metabolic syndrome. To achieve maximal effectiveness, an ACC inhibitor should inhibit both the lipogenic tissue isozyme (ACC1) and the oxidative tissue isozyme (ACC2). Herein, we describe the biochemical and acute physiological properties of CP-610431, an isozyme-nonselective ACC inhibitor identified through high throughput inhibition screening, and CP-640186, an analog with improved metabolic stability. CP-610431 inhibited ACC1 and ACC2 with IC50s of approximately 50 nm. Inhibition was reversible, uncompetitive with respect to ATP, and non-competitive with respect to bicarbonate, acetyl-CoA, and citrate, indicating interaction with the enzymatic carboxyl transfer reaction. CP-610431 also inhibited fatty acid synthesis, triglyceride (TG) synthesis, TG secretion, and apolipoprotein B secretion in HepG2 cells (ACC1) with EC50s of 1.6, 1.8, 3.0, and 5.7 microm, without affecting either cholesterol synthesis or apolipoprotein CIII secretion. CP-640186, also inhibited both isozymes with IC50sof approximately 55 nm but was 2-3 times more potent than CP-610431 in inhibiting HepG2 cell fatty acid and TG synthesis. CP-640186 also stimulated fatty acid oxidation in C2C12 cells (ACC2) and in rat epitrochlearis muscle strips with EC50s of 57 nm and 1.3 microm. In rats, CP-640186 lowered hepatic, soleus muscle, quadriceps muscle, and cardiac muscle malonyl-CoA with ED50s of 55, 6, 15, and 8 mg/kg. Consequently, CP-640186 inhibited fatty acid synthesis in rats, CD1 mice, and ob/ob mice with ED50s of 13, 11, and 4 mg/kg, and stimulated rat whole body fatty acid oxidation with an ED50 of approximately 30 mg/kg. Taken together, These observations indicate that isozyme-nonselective ACC inhibition has the potential to favorably affect risk factors associated with the metabolic syndrome.

Zoniporide: a potent and selective inhibitor of the human sodium-hydrogen exchanger isoform 1 (NHE-1)

The sodium-hydrogen exchanger isoform-1 (NHE-1) plays an important role in the myocardial response to ischemia-reperfusion; inhibition of this exchanger protects against ischemic injury, including reduction in infarct size. Herein we describe a novel, potent, and highly selective NHE-1 inhibitor, zoniporide (CP-597,396; [1-(quinolin-5-yl)-5-cyclopropyl-1H-pyrazole-4-carbonyl] guanidine). Zoniporide inhibits human NHE-1 with an IC(50) of 14 nM, has >150-fold selectivity vs. other NHE isoforms, and potently inhibits ex vivo NHE-1-dependent swelling of human platelets. This compound is well tolerated in preclinical animal models, exhibits moderate plasma protein binding, has a t(1/2) of 1.5 h in monkeys, and has one major active metabolite. In both in vitro and in vivo rabbit models of myocardial ischemia-reperfusion injury, zoniporide markedly reduced infarct size without adversely affecting hemodynamics or cardiac function. In the isolated heart (Langendorff), zoniporide elicited a concentration-dependent reduction in infarct size (EC(50) = 0.25 nM). At 50 nM it reduced infarct size by 83%. This compound was 2.5-20-fold more potent than either eniporide or cariporide (EC(50)s of 0.69 and 5.11 nM, respectively), and reduced infarct size to a greater extent than eniporide. In open chest, anesthetized rabbits, zoniporide also elicited a dose-dependent reduction in infarct size (ED(50) = 0.45 mg/kg/h) and inhibited NHE-1-mediated platelet swelling (93% inhibition at 4 mg/kg/h). Furthermore, zoniporide attenuated postischemic cardiac contractile dysfunction in conscious primates, and reduced both the incidence and duration of ischemia-reperfusion-induced ventricular fibrillation in rats. Zoniporide represents a novel class of potent and selective human NHE-1 inhibitors with potential utility for providing cardioprotection in a clinical setting.

Differing cardioprotective efficacy of the Na+/Ca2+ exchanger inhibitors SEA0400 and KB-R7943

KB-R7943 and SEA0400 are Na(+)/Ca(2+) exchanger (NCX) inhibitors with differing potency and selectivity. The cardioprotective efficacy of these NCX inhibitors was examined in isolated rabbit hearts (Langendorff perfused) subjected to regional ischemia (coronary artery ligation) and reperfusion. KB-R7943 and SEA0400 elicited concentration-dependent reductions in infarct size (SEA0400 EC(50): 5.7 nM). SEA0400 was more efficacious than KB-R7943 (reduction in infarct size at 1 microM: SEA0400, 75%; KB-R7943, 40%). Treatment with either inhibitor yielded similar reductions in infarct size whether administered before or after regional ischemia. SEA0400 (1 microM) improved postischemic recovery of function (+/-dP/dt), whereas KB-R7943 impaired cardiac function at >/=1 microM. At 5-20 microM, KBR-7943 elicited rapid and profound depressions of heart rate, left ventricular developed pressure, and +/-dP/dt. Thus the ability of KB-R7943 to provide cardioprotection is modest and limited by negative effects on cardiac function, whereas the more selective NCX inhibitor SEA0400 elicits marked reductions in myocardial ischemic injury and improved +/-dP/dt. NCX inhibition represents an attractive approach for achieving clinical cardioprotection.

3'-Aminoadenosine-5'-uronamides: discovery of the first highly selective agonist at the human adenosine A3 receptor

Selective adenosine A(3) agonists have potential utility for the prevention of perioperative myocardial ischemic injury. Herein, we report on the discovery and synthesis of compound 7. This amino nucleoside agonist possesses unprecedented levels of selectivity for the human adenosine A(3) receptor.

A novel nonpeptidic caspase-3/7 inhibitor, (S)-(+)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin reduces myocardial ischemic injury

The efficacy of a novel, nonpeptidic, caspase 3/7-selective inhibitor, (S)-(+)-5-[1-(2-methoxymethylpyrrolidinyl)sulfonyl]isatin (MMPSI) for reducing ischemic injury in isolated rabbit hearts or cardiomyocytes was evaluated. MMPSI (0.1-10 microM) evoked a concentration-dependent reduction in infarct size (up to 56% vs. control; IC(50)=0.2 microM). Furthermore, apoptosis (DNA laddering, soluble nucleosomes) was reduced in the ischemic area-at-risk. MMPSI inhibited recombinant human caspase-3 with an IC(50)=1.7 microM. Apoptosis in H9c2 cells after 16-h simulated ischemia and 2-h simulated reperfusion was significantly reduced by MMPSI in a concentration-dependent manner (IC(50)=0.5 microM); similar effects were observed in isolated adult rabbit cardiomyocytes (IC(50)=1.5 microM). These data support an important role for caspase-3/7 in mediating myocardial ischemic injury. Furthermore, these data indicate that cardioprotection via caspase-3/7 inhibition is attainable via a small molecule (nonpeptidic) inhibitor, a necessary step in making this approach therapeutically viable.

Zoniporide: a potent and highly selective inhibitor of human Na(+)/H(+) exchanger-1

We evaluated the in vitro pharmacological profile of a novel, potent and highly selective Na(+)/H(+) exchanger-1 (NHE-1) inhibitor, [1-(Quinolin-5-yl)-5-cyclopropyl-1H-pyrazole-4-carbonyl]guanidine hydrochloride monohydrate (zoniporide or CP-597,396). The potency and selectivity of zoniporide were determined via inhibition of 22Na(+) uptake by PS-120 fibroblast cell lines overexpressing human NHE-1, -2 or rat NHE-3. Additionally, potency for endogenous NHE-1 was confirmed via ex vivo human platelet swelling assay (PSA), in which platelet swelling was induced by exposure to sodium propionate. The pharmacological profile of zoniporide was compared with that of eniporide and cariporide. Zoniporide inhibited 22Na(+) uptake in fibroblasts expressing human NHE-1 in a concentration-dependent manner (IC(50) = 14 nM) and was highly selective (157-fold and 15,700-fold vs. human NHE-2 and rat NHE-3, respectively). Zoniporide was 1.64- to 2.6-fold more potent at human NHE-1 than either eniporide or cariporide (IC(50) = 23 and 36 nM, respectively). Zoniporide was also more selective at inhibiting human NHE-1 vs. human NHE-2 than either eniporide or cariporide (157-fold selective compared with 27- and 49-fold, respectively). All three compounds inhibited human platelet swelling with IC(50) values in low nanomolar range. From these results, we conclude that zoniporide represents a novel, potent and highly selective NHE-1 inhibitor.

A novel sodium-hydrogen exchanger isoform-1 inhibitor, zoniporide, reduces ischemic myocardial injury in vitro and in vivo

The cardioprotective efficacy of zoniporide (CP-597,396), a novel, potent, and selective inhibitor of the sodium-hydrogen exchanger isoform 1 (NHE-1), was evaluated both in vitro and in vivo using rabbit models of myocardial ischemia-reperfusion injury. In these models, myocardial injury was elicited with 30 min of regional ischemia and 120 min of reperfusion. Zoniporide elicited a concentration-dependent reduction in infarct size (EC(50) of 0.25 nM) in the isolated heart (Langendorff) and reduced infarct size by 83% (50 nM). This compound was 2.5- to 20-fold more potent than either eniporide or cariporide (EC(50) of 0.69 and 5.11 nM, respectively), and reduced infarct size to a greater extent than eniporide (58% reduction in infarct size). In open-chest, anesthetized rabbits, zoniporide also elicited a dose-dependent reduction in infarct size (ED(50) of 0.45 mg/kg/h) and inhibited NHE-1-mediated platelet swelling (maximum inhibition 93%). Furthermore, zoniporide did not cause any in vivo hemodynamic (mean arterial pressure, heart rate, rate pressure product) changes. Zoniporide represents a novel class of potent NHE-1 inhibitors with potential utility for providing clinical cardioprotection.

Discovery of zoniporide: a potent and selective sodium-hydrogen exchanger type 1 (NHE-1) inhibitor with high aqueous solubility

Zoniporide (CP-597,396) is a potent and selective inhibitor of NHE-1, which exhibits high aqueous solubility and acceptable pharmacokinetics for intravenous administration. The discovery, synthesis, activities, and rat and dog pharmacokinetics of this compound are presented. The potency and selectivity of zoniporide may be due to the conformation that the molecule adopts due to the presence of a cyclopropyl and a 5-quinolinyl substituent on the central pyrazole ring of the molecule.

Aldose reductase inhibition alone or combined with an adenosine A(3) agonist reduces ischemic myocardial injury

This study investigated whether aldose reductase (AR) inhibition with zopolrestat, either alone or in combination with an adenosine A(3)-receptor agonist (CB-MECA), reduced myocardial ischemic injury in rabbit hearts subjected to 30 min of regional ischemia and 120 min of reperfusion. Zopolrestat reduced infarct size by up to 61%, both in vitro (2 nM to 1 microM; EC(50) = 24 nM) and in vivo (50 mg/kg). Zopolrestat reduced myocardial sorbitol concentration (index of AR activity) by >50% (control, 15.0 +/- 2.2 nmol/g; 200 nM zopolrestat, 6.7 +/- 1.3 nmol/g). A modestly cardioprotective concentration of CB-MECA (0.2 nM) allowed a 50-fold reduction in zopolrestat concentration while providing a similar reduction in infarct size (infarct area/area at risk: control, 62 +/- 2%; 1 microM zopolrestat, 24 +/- 5%; 20 nM zopolrestat plus 0.2 nM CB-MECA, 20 +/- 4%). In conclusion, AR inhibition is cardioprotective both in vitro and in vivo. Furthermore, combining zopolrestat with an A(3) agonist allows a reduction in the zopolrestat concentration while maintaining an equivalent degree of cardioprotection.

Selective activation of adenosine A3 receptors with N6-(3-chlorobenzyl)-5'-N-methylcarboxamidoadenosine (CB-MECA) provides cardioprotection via KATP channel activation

OBJECTIVE

The aim of this study was to characterize the adenosine A3 receptor agonist, N6-(3-chlorobenzyl)-5'-N-methylcarboxamidoadenosine (CB-MECA), evaluate its ability to reduce myocardial ischemia/reperfusion injury and determine the role of KATP-channel activation in A3 receptor-mediated cardioprotection.

METHODS

Binding affinities and adenylate cyclase inhibition were examined in CHO cells expressing rabbit recombinant adenosine A1 or A3 receptors. Infarct size (normalized for area-at-risk; % IA/AAR) was measured in buffer-perfused rabbit hearts exposed to 30-min regional ischemia and 120 min of reperfusion.

RESULTS

CB-MECA was 100-fold selective for A3 vs. A1 receptors (A3 Ki: 1 nM; A1 Ki: 105 nM). Five-min perfusion with CB-MECA before ischemia/reperfusion elicited a concentration-dependent reduction in infarct size (EC50: 0.3 nM). The CB-MECA-dependent cardioprotection (control: 58 +/- 2; CB-MECA: 21 +/- 3% IA/AAR) was unchanged by an A1-selective concentration of the antagonist, BWA1433, but was completely prevented (P < 0.05) by a nonselective (A1/A3) concentration (55 +/- 6% IA/AAR). The KATP channel inhibitors, glibenclamide and 5-HD, had no effect on control infarct size, yet significantly (P < 0.05) blunted the CB-MECA-dependent cardioprotection (glibenclamide: 49 +/- 6; 5-HD: 58 +/- 4% IA/AAR).

CONCLUSIONS

CB-MECA is a novel 100-fold A3 receptor-selective agonist which should prove useful for elucidating A3-dependent mechanisms in the rabbit heart. Selective stimulation of adenosine A3 receptors with CB-MECA reduces myocardial ischemia/reperfusion injury via a mechanism which involves activation of KATP channels.

Relative importance of adenosine A1 and A3 receptors in mediating physiological or pharmacological protection from ischemic myocardial injury in the rabbit heart

Although ischemic preconditioning (IP) in several species can be pharmacologically mimicked by selective adenosine A1 or A3 receptor agonists, it is currently unclear which receptor subtype (A1 and/or A3) is physiologically involved in mediating IP. To investigate this question, we determined (a) the affinity of adenosine for rabbit adenosine A1 and A3 receptors, and (b) the effects of selective rabbit A1 receptor blockade on IP and adenosine-mediated cardioprotection in a rabbit Langendorff model of myocardial ischemia-reperfusion injury. Adenosine was 19-fold selective for inhibition of N6-(4-amino-3-[125I]iodobenzyl)adenosine (125I-ABA) binding to recombinant rabbit A1 v rabbit A3 receptors (A1 Ki: 28 nm; A3 Ki 532 nm). Buffer-perfused rabbit hearts were exposed to 30 min regional ischemia and 120 min of reperfusion, and infarct size was measured by tetrazolium staining and normalized for area-at-risk (IA/AAR). Ischemic preconditioning (5 min global ischemia and 10 min reperfusion) or adenosine (20 micro M, 5 min) perfusion reduced infarct size (IA/AAR) to 17+/-3 and 14+/-2%, respectively (controls: 59+/-2%). Ischemic preconditioning and adenosine-mediated cardioprotection were completely blocked (57+/-2 and 61+/-4% IA/AAR, respectively) in the presence of a rabbit A1-selective concentration (50 nm) of the antagonist BWA1433 (rabbit A1 Ki: 3 nm; A3 Ki; 746 n m). Thus, whereas recent studies have demonstrated that selective A1 or A3 receptor agonists can both pharmacologically mimic IP, the results of the present study suggest that the adenosine-mediated component of IP in the isolated rabbit heart is preferentially mediated by adenosine A1 receptors, potentially due to adenosine's selectivity for this receptor subtype.

Expression of multiple isoforms of nitric oxide synthase in normal and atherosclerotic vessels

Atherosclerosis is associated with reduced endothelium-derived relaxing factor bioactivity. To determine whether this is due to decreased synthesis of nitric oxide synthase (NOS), we examined normal and atherosclerotic human vessels by in situ hybridization and immunocytochemistry by using probes specific for endothelial (ecNOS), inducible (iNOS), and neuronal (nNOS) NOS isoforms, ecNOS was detected in endothelial cells overlying normal human aortas, fatty streaks, and advanced atherosclerotic lesions. A comparison of the relative expression of ecNOS to von Willebrand factor on serial sections of normal and atherosclerotic vessels indicated that there was a decrease in the number of endothelial cells expressing ecNOS in advanced lesions. iNOS and nNOS were not detected in normal vessels, but widespread production of these isoforms was found in early and advanced lesions associated with macrophages, endothelial cells, and mesenchymal-appearing intimal cells. These data suggest that there is (1) a loss of ecNOS expression by endothelial cells over advanced atherosclerotic lesions and (2) a significant increase in overall NOS synthesis by other cell types in advanced lesions composed of the ecNOS, nNOS, and iNOS isoforms. We hypothesize that the increased expression of NOS and presumably NO in atherosclerotic plaques may be related to cell death and necrosis in these tissues.

Evidence for a role for both the adenosine A1 and A3 receptors in protection of isolated human atrial muscle against simulated ischaemia

OBJECTIVE

Adenosine receptor activation has been implicated in the mechanism of ischaemic preconditioning protection. Evidence suggests adenosine A1 receptor involvement, and possibly A3 receptor involvement in the rabbit. This study investigated the roles of these receptors in human preconditioning. Human A1- and A3-selective compounds were chosen based on Ki values for inhibition of N6-(4-amino-3-[125I]iodobenzyl)adenosine (125I-ABA) binding to stably expressed recombinant human A1 and A3 receptors. Cyclopentyladenosine (CPA), a 194-fold selective A1 agonist, and iodobenzylmethylcarboxamidoadenosine (IBMECA), a 10-fold selective A3 agonist were used alone and in combination with dipropylcyclopentylxanthine (DPCPX) a 62-fold selective A1 antagonist.

METHODS

Human atrial trabeculae were superfused with oxygenated Tyrode's solution. After stabilisation, muscles underwent one of 8 protocols (n = 6 per group), followed by 90 min of simulated ischaemia and 120 min of reoxygenation. The experimental endpoint was recovery of contractile function, presented as percentage baseline function.

RESULTS

5 nM CPA (52.2 +/- 3.1%), 30 nM IBMECA (49.7 +/- 3.8%) and preconditioning (55.3 +/- 2.5%) produced similar functional recoveries at 120 min of reoxygenation; significantly different to controls (27.7 +/- 1.0%; P < 0.05, ANOVA). When DPCPX (200 nM) was added prior to 5 nM CPA, protection was lost (31.8 +/- 0.9%), but when added prior to 30 nM IBMECA, muscles continued to be significantly protected (41.5 +/- 2.3%).

CONCLUSIONS

In human atrium both A1 and A3 receptor stimulation appears to mimic ischaemic preconditioning. This may represent the first evidence for A3 receptor involvement in 'pharmacological' preconditioning of human myocardium.

Selective adenosine A3 receptor stimulation reduces ischemic myocardial injury in the rabbit heart

OBJECTIVE

The aim of this study was to determine whether selective activation of the adenosine A3 receptor reduces infarct size in a Langendorff model of myocardial ischemia-reperfusion injury.

METHODS

Buffer-perfused rabbit hearts were exposed to 30 min regional ischemia and 120 min of reperfusion. Infarct size was measured by tetrazolium staining and normalized for area-at-risk (IA/AAR).

RESULTS

Preconditioning by 5 min global ischemia and 10 min reperfusion reduced infarct size (IA/AAR) to 19 +/- 4% (controls: 67 +/- 5%). Replacing global ischemia with 5 min perfusion of the rabbit A3-selective agonist, IB-MECA (A3 Ki: 2 nM; A1 Ki: 30 nM) elicited a concentration-dependent reduction in infarct size; 50 nM IB-MECA reduced IA/AAR to 24 +/- 4%. The A1-selective agonist, R-PIA (25 nM) reduced IA/AAR to a similar extent (21 +/- 6%). However, while the cardioprotective effect of R-PIA was significantly inhibited (54 +/- 7% IA/AAR) by the rabbit A1-selective antagonist, BWA1433 (50 nM), the IB-MECA-dependent cardioprotection was unaffected (28 +/- 6% IA/AAR). A non-selective (A1 vs. A3) concentration of BWA1433 (5 microM) significantly attenuated the IB-MECA-dependent cardioprotection (61 +/- 7% IA/AAR).

CONCLUSIONS

These data clearly demonstrate that selective A3 receptor activation provides cardioprotection from ischemia-reperfusion injury in the rabbit heart. Furthermore, the degree of A3-dependent cardioprotection is similar to that provided by A1 receptor stimulation or ischemic preconditioning.

Relationships between NADPH diaphorase staining and neuronal, endothelial, and inducible nitric oxide synthase and cytochrome P450 reductase immunoreactivities in guinea-pig tissues

The presence of NADPH diaphorase staining was compared with the immunohistochemical localization of four NADPH-dependent enzymes-neuronal (type I), inducible (type II), and endothelial (type III) nitric oxide synthase (NOS) and cytochrome P450 reductase. Cell types that were immunoreactive for the NADPH-dependent enzymes were also stained for NADPH diaphorase, suggesting that endothelial and neuronal NOS and cytochrome P450 reductase all show NADPH diaphorase activity in formaldehyde-fixed tissue. However, in some tissues, the presence of NADPH diaphorase staining did not coincide with the presence of any of the NADPH-dependent enzymes we examined. In vascular endothelial cells, the punctate pattern of staining observed with NADPH diaphorase histochemistry was identical to that seen following immunohistochemistry using antibodies to endothelial NOS. In enteric and pancreatic neurons and in skeletal muscle, the presence of NADPH diaphorase staining correlated with the presence of neuronal NOS. In the liver, sebaceous glands of the skin, ciliated epithelium, and a subpopulation of the cells in the subserosal glands of the trachea, zona glomerulosa of the adrenal cortex, and epithelial cells of the lacrimal and salivary glands, the presence of NADPH diaphorase staining coincided with the presence of cytochrome P450 reductase immunoreactivity. In epithelial cells of the renal tubules and zona fasciculata and zona reticularis of the adrenal cortex, NADPH diaphorase staining was observed that did not coincide with the presence of any of the enzymes. Inducible NOS was not observed in any tissue. Thus, while tissues that demonstrate immunoreactivity for neuronal and endothelial NOS also stain positively for NADPH diaphorase activity, the presence of NADPH diaphorase staining does not reliably or specifically indicate the presence of one or more NOS isoforms.

Cloning, expression and pharmacological characterization of rabbit adenosine A1 and A3 receptors

The role of adenosine A1 and A3 receptors in mediating cardioprotection has been studied predominantly in rabbits, yet the pharmacological characteristics of rabbit adenosine A1 and A3 receptor subtypes are unknown. Thus, the rabbit adenosine A3 receptor was cloned and expressed, and its pharmacology was compared with that of cloned adenosine A1 receptors. Stable transfection of rabbit A1 or A3 cDNAs in Chinese hamster ovary-K1 cells resulted in high levels of expression of each of the receptors, as demonstrated by high-affinity binding of the A1/A3 adenosine receptor agonist N6-(4-amino-3-[125I]iodobenzyl)adenosine (125I-ABA). For both receptors, binding of 125I-ABA was inhibited by the GTP analog 5'-guanylimidodiphosphate, and forskolin-stimulated cyclic AMP accumulation was inhibited by the adenosine receptor agonist (R)-phenylisopropyladenosine. The rank orders of potency of adenosine receptor agonists for inhibition of 125I-ABA binding were as follows: rabbit A1, N6-cyclopentyladenosine = (R)-phenylisopropyladenosine > N-ethylcarboxamidoadenosine > or = I-ABA > or = N6-2-(4-aminophenyl) ethyladenosine > > N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide > N6-(4-amino-3-benzyl)adenosine; rabbit A3, N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide > or = I-ABA > > N-ethylcarboxamidoadenosine > N6-2-(4-aminophenyl) ethyladenosine = N6-cyclopentyladenosine = (R)-phenylisopropyladenosine > N6-(4-amino-3-benzyl)adenosine. The adenosine receptor antagonist rank orders were as follow: rabbit A1, 8-cyclopentyl-1,3-dipropylxanthine > 1,3- dipropyl-8-(4-acrylate)phenylxanthine > or = xanthine amine congener > > 8-(p-sulfophenyl)theophylline; rabbit A3, xanthine amine congener > 1,3-dipropyl-8-(4-acrylate)phenylxanthine > or = 8-cyclopentyl-1,3-dipropylxanthine > > 8-(p-sulfophenyl)theophylline. These observations confirm the identity of the expressed proteins as A1 and A3 receptors. The results will facilitate further in-depth studies of the roles of A1 and A3 receptors in adenosine-mediated cardioprotection in rabbits, which can now be based on the appropriate recombinant rabbit A1 and A3 receptor pharmacology.

Expression of inducible nitric oxide synthase in failing and non-failing human heart

Recently, a significant activity of inducible nitric oxide synthase (iNOS) has been reported in biopsies from failing hearts due to idiopathic dilated cardiomyopathy (IDC). Thus, a potential pathophysiological role of iNOS in IDC has been stated. In order to investigate, whether iNOS expression is of pathophysiological relevance in human heart failure, we measured iNOS protein expression and cGMP content in left ventricular myocardium from non-failing and failing human hearts. Immunoblot analysis revealed iNOS protein expression in four out of six failing hearts from septic patients, whereas no iNOS-protein expression was detected in either non-failing human hearts (n = 6) or failing hearts due to IDC (n = 9), ischemic heart disease (IHD, n = 7), Becker muscular dystrophy (BMD, n = 2) and mitoxantrone-induced toxic cardiomyopathy TCM, n = 1). cGMP content was increased by 130% in septic hearts, whereas there was no cGMP increase in hearts with IDC. IHD and BMD compared to non-failing hearts. We conclude, that the induction of iNOS may play a role in contractile dysfunction observed in septic shock, but is unlikely to be of major pathophysiological importance in end-stage heart failure due to IDC, IHD, BMD and TCM.

Nitric oxide synthase isozymes antibodies

Three isozymes of nitric oxide synthase (NOS) have been identified, cDNAs isolated and sequenced, and antibodies produced against each isozyme. Isozyme I (found primarily in central and peripheral neuronal cells), II (in cytokine-induced cells), and III (in endothelial cells) show less than 58% identity in the deduced amino acid sequences from humans. Many investigators have produced isozyme-specific antibodies and used these antibodies to locate these proteins in various cells and tissues. NOS-I is constitutively expressed, and the enzymatic activity is regulated by Ca2+ and calmodulin. The anti-NOS-I antibodies have allowed investigators to characterize non-adrenergic non-cholinergic neurons as nitrergic neurons, revealed NOS-I immunoreactivity in neurons and macula densa cells of the kidney and pancreatic islet cells, human skeletal muscle, and to demonstrate that various structures within the brain and spinal cord contain NOS-I. NOS-II is not regulated by Ca2+ and has been implicated in the pathophysiology of sepsis and autoimmune diseases. The anti-NOS-II antibodies have localized this isoform to infiltrating macrophages in pancreatic islets of diabetic rats, infiltrating macrophages and myocytes of a transplant heart model in rats, various cell types in bacterially and endotoxin-treated rats, alveolar macrophages in areas of inflammation in humans, and vascular smooth muscle cells of human atherosclerotic aneurysm. Isoform III is similar to NOS-I in that it is constitutively expressed and regulated by Ca2+ and calmodulin. Anti-NOS-III antibodies have found that this isoform is relatively specific for endothelial cells.

The nitric oxide synthase inhibitor, L-NG-monomethylarginine, reduces carrageenan-induced pleurisy in the rat

Nitric oxide (NO) is a biological mediator that, when produced by the type II (inducible) nitric oxide synthase (NOS), has been implicated in the pathophysiology of inflammatory diseases. To examine this putative role of NO, pleural inflammation was elicited in rats by the intrapleural injection of carrageenan (1 mg). A pleural exudate and cellular influx developed, which peaked at 24 h and generally resolved by 72 h. The cellular influx was primarily composed of polymorphonuclear cells during the first 24 h, followed by macrophages during the subsequent 24 h. Inflammatory cell-associated NOS activity and pleural exudate nitrite (NO2-) + nitrate (NO3-) (NOx) also increased, peaking at 6 h and 24 h, respectively. Cell-associated NOS activity was calcium-independent, indicating the presence of the type II NOS isoform; NOS activity in the pleural cavity and polymorphonuclear cells influx were temporally correlated. Administration of L-NG-monomethylarginine (L-NMA) (200 mg/kg/day) attenuated the pleural exudation, cellular influx, pleural exudate NOx, and cell-associated NOS activity. The relative composition of the pleural cavity cellular infiltrate was not changed by L-NMA, indicating the influx of individual cell types were affected equally. L-Arginine (500 mg/kg/day) completely prevented the effects of L-NMA on pleural exudation and cellular influx and partially prevented the inhibition of pleural exudate NOx accumulation by L-NMA. These data implicate NO as a modulator of the pleural inflammatory response and support a future clinical role for NOS inhibitors in the treatment of inflammatory disease.

In vivo pharmacological evaluation of two novel type II (inducible) nitric oxide synthase inhibitors

Selective type II (inducible) nitric oxide synthase (NOS) inhibitors have several potential therapeutic applications, including treatment of sepsis, diabetes, and autoimmune diseases. The ability of two novel, selective inhibitors of type II NOS, S-ethylisothiourea (EIT) and 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT), to inhibit type II NOS function in vivo was studied in lipopolysaccharide (LPS) treated rats. Type II NOS activity was assessed by measuring changes in plasma nitrite and nitrate concentrations ([NOx]). Both EIT and AMT elicited a dose-dependent and > 95% inhibition of the LPS-induced increase in plasma [NOx]. The ED50 values for EIT and AMT were 0.4 and 0.2 mg/kg, respectively. In addition, the administration of LPS and either NOS inhibitor resulted in a dose-dependent increase in animal mortality; neither compound was lethal when administered alone. Pretreatment with L-arginine (but not D-arginine) prevented the mortality, while not affecting the type II NOS-dependent NO production, suggesting the toxicity may be due to inhibition of one of the other NOS isoforms (endothelial or neuronal). Thus, although EIT and AMT are potent inhibitors of type II NOS function in vivo, type II NOS inhibitors of even greater selectivity may need to be developed for therapeutic applications.

Lipopolysaccharide-induced changes in plasma nitrite and nitrate concentrations in rats and mice: pharmacological evaluation of nitric oxide synthase inhibitors

The benefits of nitric oxide synthase (NOS) inhibitors in the treatment of endotoxemia or sepsis presumably arise from inhibition of the type II (inducible) NOS. However, inasmuch as the effect of these inhibitors on NOS function in vivo is rarely assessed, NOS activity was evaluated in rats and mice by measuring changes in plasma nitrite and nitrate concentrations ([NOx]) after administration of lipopolysaccharide (LPS). In both species, [NOx] peaked at 20 hr, returning to base line by 48 to 72 hr. The ED50 values (dose that elicited a 50% inhibition of the LPS-dependent increase in [NOx] 6 hr after LPS administration) for L-NG-monomethylarginine acetate, L-NG-nitroarginine methyl ester and aminoguanidine (administered 3 hr after LPS) were 34, 21 and 19 mg/kg in the rat and 32, 5 and 4 mg/kg in the mouse. These compounds also decreased the survival of LPS-challenged animals, which in the case of L-NG-nitroarginine methyl ester was reversed by L-arginine. Dexamethasone (which prevents the induction of type II NOS) also inhibited the LPS-dependent increase in [NOx] with ED50 values of 0.05 mg/kg (rat) and 1 mg/kg (mouse), but did not lead to decreased survival. Thus, inhibition of the type I (neuronal) or type III (endothelial) NOS, rather than the type II isoform, may be a possible mechanism for the animal mortality. These models provide a simple and reproducible means for assessing the in vivo inhibition of type II NOS by various compounds.

Nitric oxide synthase activity is elevated in brain microvessels in Alzheimer's disease

The cerebral microcirculation undergoes specific biochemical changes in Alzheimer's disease. In this study, we have compared the nitric oxide synthase (NOS) activity of brain microvessels isolated from Alzheimer and control brains. L-[3H]-citrulline, the stable co-product generated with nitric oxide (NO) from L-[3H]-arginine, was measured as an indicator of NOS activity. The results indicated a significant increase in NOS activity in microvessels isolated from Alzheimer brains. In addition, using antibodies to both the endothelial and inducible NOS isoforms, we demonstrated a significant increase in enzyme level in Alzheimer-derived vessels. Elevated vascular production of NO, a potentially neurotoxic mediator in the CNS, may contribute to the susceptibility of neurons to injury and cell death in Alzheimer's disease.

Immunochemical detection of inducible NO synthase in human lung

Type II (inducible) nitric oxide synthase (NOS) may play an important role in pulmonary pathophysiology, yet it remains controversial whether human tissues are capable of expressing this protein. Therefore, a polyclonal antibody (8196) was raised against type II NOS from induced RAW 264.7 macrophages and used to investigate the expression of this enzyme in human lung tissue. Anti-type II NOS antibody did not cross-react with either neuronal (type I) or endothelial (type III) constitutive NOS, whereas a 130-kDa protein was detected in cytosol from induced macrophages or liver removed from lipopolysaccharide (25 mg/kg)-treated rats. Cells or tissues that lacked NOS activity did not express immunoreactive proteins. Similarly, in grossly normal human lung tissue, no immunoreactivity was detected with the anti-type II NOS antibody. In contrast, strong immunoreactivity was detected in alveolar macrophages present in lung tissue from a patient with bronchiectasis and acute bronchopneumonia. These data demonstrate that human alveolar macrophages are able to express type II NOS and support a role for this enzyme in pulmonary inflammatory pathophysiology.

Identification of an endothelial-like type III NO synthase in LLC-PK1 kidney epithelial cells

Porcine kidney tubular epithelial cells (LLC-PK1) produce nitric oxide or a related compound (e.g., a nitrosothiol) after stimulation with various agonists. We now report the identification and characterization of a constitutive, particulate nitric oxide (NO) synthase from LLC-PK1 cells. After partial purification on adenosine 2',5'-bisphosphate-Sepharose, the particulate NO synthase activity eluted anomalously from Superose 6 gel permeation columns near the total included volume, similar to that observed for the endothelial (type III) NO synthase. Substrate/cofactor requirements of the epithelial and endothelial NO synthases were identical, i.e., dependency on L-arginine, (6R)-5,6,7,8-tetrahydrobiopterin, FAD, calcium and calmodulin. The epithelial enzyme activity was inhibited by the arginine analogues, NG-methyl-L-arginine (100 microM) and NG-nitro-L-arginine (100 microM), as well as the calmodulin antagonists, trifluoperazine (100 microM) and calmidazolium (30 microM). Anti-type III (H32), but not anti-type I (brain, 6763-5) or anti-type II (macrophage, 8196) NO synthase antibodies, detected a single immunoreactive band in the LLC-PK1 particulate fraction of approximately 140 kDa by Western blot analysis. Finally, the presence of type III NO synthase mRNA in LLC-PK1 cells was demonstrated using the polymerase chain reaction. These data indicate that LLC-PK1 kidney epithelial cells contain type III NO synthase, which has been classically associated with the vascular endothelium.

Isoforms of nitric oxide synthase: functions in the cardiovascular system

Various cell types, including endothelial cells, can synthesize nitric oxide (NO). Three different isoforms of NO synthase have been characterized, purified and cloned. Isozyme I is present in neuronal cells of the brain (where NO may mediate synaptic plasticity), in peripheral non-adrenergic non-cholinergic (NANC) neurons (where NO acts as an atypical neurotransmitter relaxing vascular and non-vascular smooth muscle), and in various specialized epithelial cells. Macrophages can be induced with bacterial endotoxin and/or cytokines to express isozyme II. The high concentrations of NO produced by this isoform have cytostatic effects on parasitic microorganisms and tumour cells. A similar isozyme can be induced in the vascular wall (presumably in smooth muscle cells) in sepsis and during cytokine therapy. The large amounts of NO produced by this enzyme contribute to the symptoms of septic shock, such as vasodilatation and microvascular endothelial damage. Endothelial cells contain isoform III of NO synthase which seems to be unique for this cell type. Endothelium-derived NO is a physiologically significant vasodilator and inhibitor of platelet aggregation and adhesion. In addition, vascular NO can prevent leukocyte adhesion to the endothelium by interfering with the adhesion molecule CD11/CD18, and NO has also been shown to inhibit the proliferation of vascular smooth muscle cells. Hence, NO represents a protective factor against vascular damage and probably atherogenesis.

Nitric oxide synthases in neuronal cells, macrophages and endothelium are NADPH diaphorases, but represent only a fraction of total cellular NADPH diaphorase activity

NADPH diaphorase activity is used as a histochemical marker for neuronal nitric oxide (NO) synthase; however, it remains unclear whether these activities are directly correlated in all tissues. In N1E-115 neuroblastoma cells, NADPH diaphorase activity was found primarily in the particulate fraction, whereas NO synthase activity was mostly soluble. Non-induced macrophages expressed significant NADPH diaphorase activity (which was mostly particulate) but virtually no NO synthase activity. Induction of macrophages produced marked increases in both NO synthase and NADPH diaphorase activities in the soluble and particulate fractions. In endothelial cells, both NO synthase and NADPH diaphorase activities were found mostly in the particulate fraction. Purified NO synthases from brain (type I), macrophages (type II), and endothelium (type III) all showed NADPH diaphorase activity; relative activities were: macrophage > endothelium > brain. These data indicate that all known NO synthases are NADPH diaphorases; however, NO synthases represent only a fraction of total cellular NADPH diaphorase activity and these activities are not always co-localized.

Mechanism of mastoparan-induced EDRF release from pulmonary artery endothelial cells

Mastoparan is a wasp venom peptide that activates G-proteins, certain classes of which are involved in the release of endothelium-derived relaxing factor (EDRF). In the present study, we investigated whether this peptide might be a useful tool with which to elucidate the signal transduction pathways responsible for EDRF release from pulmonary artery endothelium. Mastoparan (10-50 micrograms/ml) elicited an increase in endothelial cell cytosolic free calcium concentration ([Ca2+]i) and EDRF release in a concentration-dependent manner. Both effects were dependent on Ca2+ influx, as they were inhibited by removal of extracellular Ca2+. In addition, when endothelial cells were suspended in Ca(2+)-free buffer, mastoparan inhibited ATP-induced increases in [Ca2+]i, presumably by depleting intracellular Ca2+ stores. More importantly, mastoparan also caused the release of fura-2 from dye-loaded endothelial cells, unlike ATP, which did not affect fura-2 loss. These data indicate that although mastoparan may act on G-proteins to elicit release of Ca2+ from intracellular stores, the primary mechanism of action responsible for mastoparan's ability to elicit EDRF release is an increase in cell membrane permeability followed by an influx of extracellular Ca2+.

Differential muscarinic receptor mRNA expression by freshly isolated and cultured bovine aortic endothelial cells

Endothelial cells, either in vivo or freshly isolated, respond when exposed to muscarinic agonists with an increase in cytosolic free calcium concentration ([Ca2+]i) and release of endothelium-derived relaxing factor (EDRF). When placed in culture, however, endothelial cells rapidly lose these responses, which may be related to changes in muscarinic receptor expression. Northern blot analysis of poly(A) + RNA from freshly isolated or cultured bovine aortic endothelial cells was used to address this problem. Through the use of specific cDNA probes complementary to the nonconserved regions of the m1, m2, m3, m4, and m5 muscarinic receptors, mRNA transcripts for the m1 (3.9 kb), m2 (3.8 kb), and m3 (3.1 kb) receptor subtypes were identified in freshly isolated endothelial cells, whereas m1 and m3 transcripts were identified in aortic smooth muscle. In contrast, cultured endothelial cells contained mRNA for only the m2 receptor subtype. Transcripts for the m4 or m5 receptors were not detected in either freshly isolated or cultured endothelial cells. Since m1 and m3 receptor subtypes are coupled to phospholipase C, activation of which is required for EDRF release, these observations may explain the failure of muscarinic agonists to elicit a rise in [Ca2+]i and EDRF release from cultured endothelial cells.

Mechanism of phospholipase C-induced release of EDRF from pulmonary artery endothelium

The mechanism of phospholipase (PL) C-induced release of endothelium-derived relaxing factor (EDRF) was investigated. Bovine pulmonary artery endothelial cells (BPAEC) were treated with phosphatidylinositol (PI)-selective PLCs, nonselective PLCs, or a nonselective PLD. PI-PLCs elicited PI-glycan anchor hydrolysis but did not alter either intracellular Ca2+ ([Ca2+]i) in fura-2-loaded BPAEC or EDRF production in BPAEC-vascular smooth muscle cocultures. In contrast, non-selective PLCs increased [Ca2+]i, an effect prevented by prior exposure to the PLCs, and EDRF production in a time- and concentration-dependent manner. Antibodies raised against PI-glycan anchors did not alter, while heat denaturation abolished, the PLC-dependent effects. Removal of extracellular Ca2+ with [ethylene-bis(oxyethylenenitrilo)]tetraacetic acid both prevented and reversed PLC-stimulated increases in [Ca2+]i and inhibited EDRF production. Although Mn2+ quenched PLC-induced increases in fura-2 fluorescence, high PLC concentrations elicited significant dye loss from fura-2-loaded BPAEC. We conclude that the effects of exogenous PLC on EDRF production are not dependent on release of a membrane PI-glycan-linked moiety. Rather, the PLC actions are mediated by a graded increase in cell membrane permeability, probably related to pore formation by the hemolytic activity of the enzyme, followed by an influx of extracellular Ca2+.

Comparison of spectrophotometric and biological assays for nitric oxide (NO) and endothelium-derived relaxing factor (EDRF): nonspecificity of the diazotization reaction for NO and failure to detect EDRF

Endothelium-derived relaxing factor has been tentatively identified as nitric oxide (NO) partially on the basis of chemical assays. In the present study, saline solutions that were either bubbled continuously for 30 min with NO (NO/X) or prepared using 25 ml of NO/ml (NO/25) produced equivalent relaxations of segments of rabbit aorta which had the endothelium removed. NO solutions prepared using 0.1 ml of NO/ml (NO/0.1), and 3 mM sodium nitrite (NO2-) were significantly (P less than .05) less potent vasodilators than NO/X and NO/25 (order of potency: NO/X = NO/25 greater than NO/0.1 greater than NO2-). A novel automated method was developed to monitor nitrogen oxides using continuous-flow spectrophotometric detection (diazotization reaction). The absorbance readings for the solutions were NO/X greater than NO/25 = NO/0.1 = NO2-. Argon purging of NO/X, NO/25 and NO/0.1 solutions significantly (P less than .05) reduced (44-100%) the bioactivity of these solutions in inverse proportion to the initial volume of NO used in their preparation. In contrast, the absorbance values were unchanged, indicating that the chemical assay was not correlated with the bioassay. Varying the duration of NO gassing (1-30 min) significantly (P less than .05) increased the absorbance values, while having no effect on the vascular relaxations, elicited by the solutions. The diazotization assay did not detect nitrogen oxides released from cultured endothelial cells by bradykinin, ATP, or A23187, whereas the bioassay readily detected endothelium-derived relaxing factor release.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of isolated guinea pig pulmonary venules to hypoxia and anoxia

Because small pulmonary arteries are believed to be the major site of hypoxic pulmonary vasoconstriction (HPV), pulmonary venular responses to hypoxia have received little attention. Therefore the responses of isolated guinea pig pulmonary venules to hypoxia (bath PO2, 25 Torr) and anoxia (bath PO2, 0 Torr) were characterized. Pulmonary venules [effective lumen radius (ELR), 116 +/- 2 microns] with an adherent layer of parenchyma responded to hypoxia and anoxia with a graded sustained contraction (hypoxia, 0.03 +/- 0.01; anoxia, 0.26 +/- 0.03 mN/mm), whereas paired femoral venules (ELR, 184 +/- 7 microns) contracted to anoxia only (0.05 +/- 0.02 mN/mm). Repeated challenges with hypoxia and anoxia continued to elicit sustained pulmonary venular contractions; femoral venule contractions to anoxia were not repeatable. Hypoxia- and anoxia-induced pulmonary venular contractions were calcium and pH dependent. Dissection of the parenchyma from pulmonary venules did not alter contractions to decreased PO2. Anoxic contractions of pulmonary venules were variably reduced by replacement of the bath fluid; however, the release of a contractile mediator(s) from pulmonary venules during hypoxia or anoxia was not demonstrated. Pulmonary venular responses to hypoxia and anoxia are similar to those induced by hypoxia in vivo, and results obtained from this model may be useful in predicting mechanisms of HPV.

Role of lipoxygenase, cyclooxygenase and cytochrome P-450 metabolites in contractions of isolated guinea pig pulmonary venules induced by hypoxia and anoxia

Arachidonic acid metabolites, notably leukotrienes (LTs), have been postulated to play a role in hypoxic pulmonary vasoconstriction. In the present study, we examined the contribution of arachidonic acid metabolites, via the cyclooxygenase, 5-lipoxygenase and cytochrome P-450 monooxygenase pathways, to the hypoxia (25 +/- 3 torr)- and anoxia (0 +/- 2 torr)-induced contractions of isolated pulmonary venules. Neither the cyclooxygenase inhibitors indomethacin (5 microM) or ibuprofen (10 microM) nor the 5-lipoxygenase inhibitors nordihydroguaiaretic acid (5 microM) or U 60257B (10 microM) affected the contractile responses. Similarly, the LT receptor antagonists FPL 57231 (3 microM) or LY 163443 (1 microM), at concentrations that inhibited LT-induced venular contractions, did not significantly affect the responses to hypoxia or anoxia. In fact, anoxia suppressed spontaneous LT release from the venules. The cytochrome P-450 inhibitor SKF-525A (500 microM) nonselectively depressed venular contractions induced by decreased PO2 and pharmacological agents. Induction of the cytochrome P-450 monooxygenase system with beta-naphthoflavone did not alter venular contractions induced by hypoxia or anoxia. Contractions of isolated guinea pig pulmonary venules elicited by decreased PO2 are not mediated by 5-lipoxygenase or cyclooxygenase metabolites. Furthermore, the data do not support a role for cytochrome P-450 metabolites of endogenous substrates in these contractions.

Effect of endothelial injury on the responses of isolated guinea pig pulmonary venules to reduced oxygen tension

The influence of the endothelium on pulmonary venular responses to reduced oxygen tension has not been defined. To examine this question, endothelial injury was induced in small guinea pig pulmonary artery and venule segments (effective lumen radius, 174 +/- 5 and 122 +/- 2 microns, respectively) by perfusion with either a mixture of hypoxanthine (5 mM) and xanthine oxidase (0.05 U/ml) (HX/XO) or collagenase (2 mg/ml). HX/XO significantly (p less than 0.05) reduced the relaxation of precontracted pulmonary arteries by acetylcholine (ACH), bradykinin (BK), and A-23187, and the relaxations were restored by including superoxide dismutase (40 micrograms/ml) in the HX/XO solution. However, neither HX/XO nor collagenase affected vasodilation induced by ACH, BK, and A-23187 in precontracted pulmonary venules. In contrast, HX/XO significantly (p less than 0.05) augmented the sustained contraction of pulmonary venules to hypoxia (HX/XO, 3.2 +/- 1.0 mg/mm; control, 1.0 +/- 0.5 mg/mm) and anoxia (HX/XO, 35.1 +/- 6.6 mg/mm; control, 20.3 +/- 4.0 mg/mm). Collagenase also significantly (p less than 0.05) enhanced the anoxic contractions (collagenase, 36.0 +/- 3.7 mg/mm; control, 20.9 +/- 6.8 mg/mm). Superoxide dismutase (40 micrograms/ml) and catalase (323 micrograms/ml) abolished HX-XO-induced augmentation of the hypoxic and anoxic contractions of pulmonary venules. Collagenase removed 54 +/- 8% of the venular endothelium (control, 5 +/- 1%), whereas HX/XO-exposed endothelial cells contained numerous craters. Neither gossypol (5 microM) nor methylene blue (10 microM) affected pulmonary venular contractions to reduced PO2. Endothelial damage augments the PO2-dependent contractions of the pulmonary venule, and this augmentation does not appear to be due to decreased release of endothelium-derived relaxing factor.

Effects of bradykinin and leukotrienes B4 and D4 on the bovine bronchial artery in vitro: role of the endothelium

The effects of bradykinin, leukotriene (LT) B4 and LTD4 on the bovine bronchial artery were studied using isolated, perfused vessel segments. Intrapulmonary bronchial arteries were precontracted to the EC50 of 5-hydroxytryptamine in the organ bath, after which increasing concentrations of bradykinin, LTB4 or LTD4 were perfused through the vessel lumen. In arteries with intact endothelium, bradykinin induced a concentration-dependent relaxation beginning at 1 pmol/l (p less than 0.01), whereas LTD4 and LTB4 had no significant effect. Indomethacin (10 mumol/l) significantly (p less than 0.01) inhibited bradykinin-induced relaxation with an increase in the IC50 from 0.11 +/- 0.08 nmol/l to 0.51 +/- 0.10 nmol/l (p less than 0.05). In endothelium-denuded arteries bradykinin had no observed effect, whereas LTB4 and LTD4 significantly increased tone with threshold concentrations of 50 pmol/l (p less than 0.025) and 5 pmol/l (p less than 0.05), respectively. The data indicate that in the bronchial artery: 1) bradykinin is a potent vasodilator; 2) bradykinin-induced relaxation is endothelium-dependent and only partially blocked by a cyclooxygenase inhibitor; 3) removal of the endothelium unmasks a potent contractile effect of LTB4 and LTD4.