Nitric oxide activates guanylate cyclase and increases guanosine 3':5'-cyclic monophosphate levels in various tissue preparations

The effect of inhibitors of the L-arginine/nitric oxide pathway on endotoxin-induced loss of vascular responsiveness in anaesthetized rats

1. The effects on blood pressure and on pressor responses to noradrenaline (NA), of NG-monomethyl-L-arginine (L-NMMA) and NG-nitro-L-arginine methyl ester (L-NAME), inhibitors of the L-arginine/nitric oxide pathway, were investigated in anaesthetized rats receiving an infusion of bacterial endotoxin (E. coli lipopolysaccharide, LPS). 2. Infusion of LPS (10 mg kg-1 h-1) for 50 min had no effect on mean arterial blood pressure (MABP) but induced a reduction in responsiveness to noradrenaline (100 ng-1 micrograms kg-1). L-NMMA (30 mg kg-1), but not D-NMMA, caused an increase in MABP of approximately 30 mmHg and restored responses to NA. This effect was reversed by L- but not D-arginine (100 mg kg-1). 3. In LPS-treated rats, blood pressure responses to NA were only marginally increased by the cyclooxygenase inhibitor, indomethacin (5 mg kg-1). L-NAME (1 mg kg-1) caused a similar increase in MABP and restored pressor responses to NA both in the presence and absence of indomethacin. 4. Co-infusion of vasopressin (100 ng kg-1, for 10 min) with LPS (10 mg kg-1 h-1) in order to reproduce the hypertensive effect of L-NMMA and L-NAME increased pressor responsiveness to 100 and 300 ng kg-1 NA but not to 1 microgram kg-1 NA. 5. Infusion of sodium nitroprusside (30 micrograms kg-1 min-1) decreased responsiveness to NA even when the hypotension was corrected by co-infusion of vasopressin (50 ng kg-1 min-1). 6. These results demonstrate that the restoration of vascular responsiveness to NA in LPS-treated anaesthetized rats by inhibitors of the L-arginine/nitric oxide pathway is stereospecific and reversible. Furthermore, the experiments involving indomethacin suggest that although cyclo-oxygenase products of arachidonic acid may contribute to the development of LPS-induced hyporeactivity, the effect of L-NAME is unlikely to involve inhibition of the cyclo-oxygenase pathway. Comparison of NA responsiveness during vasopressin and L-NMMA/L-NAME-induced hypertension shows that increasing the blood pressure may modify LPS-induced hyporeactivity, but cannot account for the complete restoration of responses to NA by L-NMMA and L-NAME. These observations suggest that activation of nitric oxide formation from L-arginine makes a direct contribution to the production of vascular hyporeactivity by LPS in vivo.

Evidence that an L-arginine/nitric oxide dependent elevation of tissue cyclic GMP content is involved in depression of vascular reactivity by endotoxin

1. The aim of this investigation was to study the relationship between contractile responsiveness, activation of the L-arginine pathway and tissue levels of guanosine 3':5'cyclic monophosphate (cylic GMP) in aortic rings removed from rats 4 h after intraperitoneal administration of bacterial endotoxin (E. coli. lipopolysaccharide, LPS, 20 mg kg-1). 2. LPS-treatment resulted in a reduction of the sensitivity and maximal contractile response to noradrenaline (NA). 3. Depression of the maximal contractile response was restored to control by 6-anilo-5,8-quinolinedione (LY 83583, 10 microM), which prevents activation of soluble guanylate cyclase. 4. Cyclic GMP levels in tissue from LPS-treated rats were 2 fold greater than cyclic GMP levels detected in tissue from control (saline-treated) rats. The LPS-induced increase in cyclic GMP content was observed both in the presence and absence of functional endothelium. 5. Addition of L-arginine 1 mM) to maximally contracted aortic rings produced significantly relaxation of rings from LPS-treated rats but not rings from control animals. In the LPS-treated group, addition of L-arginine was also associated with a significant increase in cyclic GMP content. L-Arginine had no effect on the cyclic GMP content of control rings. D-Arginine (1 mM) was without effect. 6. In rings from LPS-treated rats, NG-nitro-L-arginine methyl ester (L-NAME, 300 microM), an inhibitor of nitric oxide (NO) production, increased the contractile response to NA and prevented the LPS-induced increase in cyclic GMP content. In control rings, L-NAME increased the NA sensitivity only when the endothelium remained intact and reduced the cyclic GMP content of these rings to that of control endothelium-denuded rings. 7. These results demonstrate that LPS-induced hyporeactivity to NA occurs secondarily to activation of the L-arginine pathway and subsequent activation of soluble guanylate cyclase in vascular tissue. In addition they suggest that LPS induces the production of an NO-like relaxing factor in non-endothelial cells.

Neuronal NADPH diaphorase is a nitric oxide synthase

NADPH diaphorase histochemistry selectively labels a number of discrete populations of neurons throughout the nervous system. This simple and robust technique has been used in a great many experimental and neuropathological studies; however, the function of this enzyme has remained a matter of speculation. We, therefore, undertook to characterize this enzyme biochemically. With biochemical and immunochemical assays, NADPH diaphorase was purified to apparent homogeneity from rat brain by affinity chromatography and anion-exchange HPLC. Western (immunoblot) transfer and immunostaining with an antibody specific for NADPH diaphorase labeled a single protein of 150 kDa. Nitric oxide synthase was recently shown to be a 150-kDa, NADPH-dependent enzyme in brain. It is responsible for the calcium/calmodulin-dependent synthesis of the guanylyl cyclase activator nitric oxide from L-arginine. We have found that nitric oxide synthase activity and NADPH diaphorase copurify to homogeneity and that both activities could be immunoprecipitated with an antibody recognizing neuronal NADPH diaphorase. Furthermore, nitric oxide synthase was competitively inhibited by the NADPH diaphorase substrate, nitro blue tetrazolium. Thus, neuronal NADPH diaphorase is a nitric oxide synthase, and NADPH diaphorase histochemistry, therefore, provides a specific histochemical marker for neurons producing nitric oxide.

Purification of a soluble isoform of guanylyl cyclase-activating-factor synthase

The soluble form of guanylyl cyclase-activating-factor (GAF) synthase from rat cerebellum was purified to homogeneity by sequential affinity chromatographic steps on adenosine 2',5'-bisphosphate (2',5'-ADP)-Sepharose and calmodulin-agarose. Enzyme activity during purification was bioassayed by the L-arginine-, NADPH-, and Ca2+/calmodulin-dependent formation of a plasma membrane-permeable nitric oxide-like factor that stimulated soluble guanylyl cyclase in RFL-6 cells. With calmodulin and NADPH as cofactors, purified soluble GAF synthase induced an increase of 1.05 mumol of cGMP per 10(6) RFL-6 cells per 3 min per mg of protein. The coproduct of this signal-transduction pathway appeared to be L-citrulline. GAF synthase catalyzed the conversion of 107 nmol of L-arginine into L-citrulline per min per mg of protein. Based on these assays, this represents a purification of GAF synthase of approximately 10,076- and 8925-fold with recoveries of 16% and 19%, respectively. Rechromatography of the purified enzyme on Mono P (isoelectric point = 6.1 +/- 0.3), Mono Q, and Superose 12 or 6 resulted in no further purification or increase in specific activity. A Stokes radius of 7.9 +/- 0.3 nm and a sedimentation coefficient s20,w of 7.8 +/- 0.2 S were used to calculate a molecular mass of about 279 +/- 25 kDa for the native enzyme. SDS/PAGE revealed a single protein band with a molecular mass of about 155 +/- 3 kDa. These data suggest that soluble GAF synthase purified from rat cerebellum is a homodimer of 155-kDa subunits and that enzyme activity is dependent upon the presence of calmodulin.

L-arginine evokes both endothelium-dependent and -independent relaxations in L-arginine-depleted aortas of the rat

This study was designed to investigate the effects of L-arginine (the substrate for the formation of endothelium-derived nitric oxide) in vascular tissues. Rat aortic rings, with or without endothelium, were suspended in organ chambers for the measurement of isometric tension; they were contracted with phenylephrine (10(-6) M). After a short incubation period (0.5 hour) in physiological salt solution, L-arginine induced minor changes in both types of rings. In contrast, when the incubation time was increased (2, 4, 6, and 8 hours), L-arginine evoked concentration- and time-dependent relaxations in aortic rings both with and without endothelium. The relaxations were larger in rings with endothelium. The presence of L-arginine (10(-3) M) in the incubation medium inhibited subsequent relaxations evoked by the amino acid. The concentration-relaxation curves associated with acetylcholine in rings with endothelium and the curves associated with Sin-1, a spontaneous donor of nitric oxide, in rings with or without endothelium were slightly but significantly shifted to the right after a 6-hour incubation. Nitro-L-arginine (3 x 10(-5) M) and methylene blue (3 x 10(-7) M) attenuated the relaxations evoked by L-arginine in rings both with and without endothelium. Other basic amino acids (D-arginine, L-homoarginine, L-citrulline, L-lysine, and L-ornithine; all tested at 10(-3) M) either had no effect or induced small relaxations and did not affect the response to L-arginine. These observations suggest that L-arginine specifically and stereoselectively relaxes aortic rings with and without endothelium, probably by restoring the endogenous pool of the amino acid, which is likely depleted by prolonged incubation.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacology of endothelium-derived nitric oxide and nitrovasodilators

Nitric oxide is the active chemical species responsible for the vasodilator action of nitroglycerin, nitroprusside, and related nitrovasodilators. The most potent vasodilator and inhibitor of platelet aggregation known, nitric oxide was recently discovered to occur endogenously as the endothelium-derived relaxing factor. The pharmacology of endothelium-derived nitric oxide is virtually identical to that of the clinically used nitrovasodilators. Although endothelium-derived relaxing factor or endothelium-derived nitric oxide seems to be important in animals, its significance in humans still needs to be shown. We review the recent discoveries in the identification, biosynthesis, metabolism, and biologic actions of endothelium-derived nitric oxide, its significance in humans, and its relation to the clinically used nitrovasodilators.

Effect of cyclic guanosine monophosphate on hypoxic and angiotensin-II-induced pulmonary vasoconstriction

We examined, in isolated blood perfused rat lungs, the effect of the cell permeable 8-bromo derivative of cGMP on pulmonary vasoconstriction induced by either alveolar hypoxia or angiotensin II. 8-Bromo cGMP dose-dependently reduced both hypoxia-(IC50 = 2.2 X 10(-5) M) and angiotensin-II-induced pulmonary vasoconstriction (IC50 = 5.0 X 10(-5) M). This effect of 8-bromo cGMP on pulmonary vasoconstriction was not affected by cyclooxygenase blockade. M & B 22948 (0.1 mM), an inhibitor of cGMP-phosphodiesterase, reduced synergistically with 8-bromo cGMP the hypoxia or angiotensin-II-induced vasoconstriction. The cGMP-phosphodiesterase inhibitor M & B 22948, by itself, selectively reduced hypoxia-induced vasoconstriction, suggesting a modulating effect of endogenous cGMP during hypoxic vasoconstriction.

Inhibitors of cytochrome P-450 reduce cyclic GMP stimulation by glyceryl trinitrate in LLC-PK1 kidney epithelial cells

The inhibitor of cytochrome P-450 cimetidine was used to assess the role of cytochrome P-450-dependent enzymes for cyclic GMP stimulation by glyceryl trinitrate in a kidney epithelial cell line (LLC-PK1). Pretreatment of the cells with 0.1 mmol/l cimetidine markedly decreased cyclic GMP stimulation by glyceryl trinitrate (0.03-1 mumol/l). In the presence of 0.1 mmol/l cimetidine, the 14-fold cyclic GMP stimulation observed at 1 mumol/l glyceryl trinitrate was reduced by 66%. Glyceryl trinitrate-induced cyclic GMP stimulation remained unaltered by ranitidine (0.1 mmol/l), which has a much lower affinity for the cytochrome P-450 enzyme system. Another inhibitor of cytochrome P-450, miconazole (0.1 mmol/l), also attenuated glyceryl trinitrate-induced cyclic GMP stimulation. In contrast, cimetidine and miconazole did not affect cyclic GMP stimulation by sodium nitroprusside that spontaneously releases nitric oxide. These results suggest that in intact cells, glyceryl trinitrate-induced cyclic GMP stimulation is dependent on cytochrome P-450 enzymes which may be relevant for nitric oxide formation from organic nitrates.

Activation of soluble guanylate cyclase by carbon monoxide and inhibition by superoxide anion

Human platelet soluble guanylate cyclase activity was studied with respect to the function of its heme-containing regulatory subunit. As an enzyme source, the 10,000 x g supernatant was used and, since its specific activity proved to be too low for inhibition studies, also a partially purified preparation was employed. The partially purified enzyme was stimulated about 2.5-fold by carbon monoxide and this effect was abolished by illumination with visible light. Sodium nitroprusside also increased the basal activity about fourfold, which, however, is much less than the greater than 100-fold stimulation seen with the supernatant. Superoxide anions generated by the xanthine/xanthine-oxidase system were strongly inhibitory in the enriched preparation as well as in the CO-stimulated platelet supernatant (median effector concentration = 0.1 mU/ml). Unlike CO and NO, the effect of superoxide cannot be mediated through the heme-containing regulatory subunit, since heme-free enzyme, which could not be activated by NO or CO, was inhibited to the same extent as the heme-containing enzyme. Superoxide dismutase did not influence the basal activity, but resulted in a synergistic stimulation in the presence of CO. When Mn2+ replaced Mg2+ as a cofactor, the basal activity was higher but superoxide could not inhibit the enzyme, possibly due to the superoxide-dismutase-like activity of Mn2+. Superoxide turned out to be a potent and reversible inhibitor of soluble guanylate cyclase which, together with endothelium-derived relaxing factor, recently identified as NO, could form a physiologically relevant regulatory effector system.

Stimulation of cyclic GMP production in cultured endothelial cells of the pig by bradykinin, adenosine diphosphate, calcium ionophore A23187 and nitric oxide

1. The effects of bradykinin, adenosine diphosphate, calcium ionophore A23187 and nitric oxide on the production of adenosine 3':5'-cyclic monophosphate (cyclic AMP) and guanosine 3':5'-cyclic monophosphate (cyclic GMP) were investigated in cultured aortic endothelial cells of the pig. 2. Bradykinin (10(-7) M), adenosine diphosphate (3 x 10(-5) M), nitric oxide (2 x 10(-6) M) and A23187 (10(-6) M) stimulated the production of cyclic GMP. This stimulation reached a maximum within 1 min and declined rapidly with the first three agonists whereas that induced by A23187 was long-lasting. 3. These concentrations of bradykinin, A23187 and nitric oxide had no effect on cyclic AMP production. However, adenosine diphosphate (3 x 10(-5) M) slightly but significantly enhanced its production by about 1.7 fold. 4. The basal content of cyclic GMP in endothelial cells was significantly reduced by haemoglobin (10(-5) M, a scavenger of endothelium-derived relaxing factor(s] and methylene blue (10(-5) M, an inhibitor of the activation of soluble guanylate cyclase) and was significantly enhanced by superoxide dismutase (500 u ml-1, a scavenger of superoxide anions). The basal content of cyclic GMP was not affected by NG-monomethyl-L-arginine (10(-5) M, a specific inhibitor of the formation of nitric oxide from L-arginine) and was slightly but significantly increased by its D-enantiomer, NG-monomethyl-D-arginine. 5. The production of cyclic GMP stimulated by bradykinin, adenosine diphosphate, A23187 and nitric oxide was inhibited by haemoglobin (10 5M) and methylene blue (10- M) but was unaffected by superoxide dismutase (500 u ml 1)- 6. The production of cyclic GMP stimulated by bradykinin, adenosine diphosphate or A23187, but not that stimulated by nitric oxide, was significantly reduced by N0-monomethyl-L-arginine (10-M). The production of cyclic GMP evoked by nitric oxide, but not that induced by the other three agents, was enhanced significantly by N0-monomethyl-D-arginine by about 1.5 fold. 7. These data indicate that the endothelium-dependent vasodilators bradykinin, adenosine diphosphate and A23187 activate the production of cyclic GMP in endothelial cells via the synthesis of nitric oxide, which in turn stimulates the soluble guanylate cyclase.

Control of coronary vascular tone by nitric oxide

A specific difference-spectrophotometric method was used to measure nitric oxide (NO) release into the coronary effluent perfusate of isolated, constant-flow-perfused guinea pig hearts. Authentic NO applied into the coronary circulation decreased vascular resistance dose dependently and enhanced coronary release of cyclic GMP (cGMP) fivefold. Increasing oxygen tension in aqueous solutions from 150 to 700 mm Hg decreased NO half-life (5.6 seconds) by 32%. During single passage through the intact coronary system, 86% of the infused NO was converted to nitrite ions. Oxidation of NO was more than 30 times faster within the heart than in aqueous solution. Endogenously formed NO was constantly released into the coronary effluent perfusate at a rate of 161 +/- 11 pmol/min. The NO scavenger oxyhemoglobin and methylene blue increased coronary resistance and decreased cGMP release (basal release, 342 +/- 4 fmol/min), whereas superoxide dismutase reduced coronary resistance. L-Arginine (10(-5) M) slightly decreased coronary perfusion pressure and enhanced release of cGMP. NG-Monomethyl L-arginine (10(-4) M) reduced basal release of NO and cGMP by 26% and 31%, respectively, paralleled by a coronary vasoconstriction. Bradykinin in the physiological range from 5 x 10(-11) M to 10(-7) M dilated coronary resistance vessels, which was paralleled by the release of NO and cGMP. Onset of NO release preceded onset of coronary vasodilation in all cases. Upon stimulation with bradykinin, amounts of endogenously formed NO were within the same range as the dose-response curves for exogenously applied NO both for changes in coronary resistance and cGMP release. Acetylcholine (10(-5) M), ATP (10(-5) M), and serotonin (10(-8) M) increased the rate of NO and cGMP release, resulting in coronary vasodilation. Our data suggest the following: 1) NO, the most rapidly acting vasodilator presently known, is metabolized within the heart mainly to nitrite and exhibits a half-life of only 0.1 second; 2) in the unstimulated heart, basal formation of NO may play an important role in setting the resting tone of coronary resistance vessels; 3) the kinetics and quantities of NO formation suggest that NO is causally involved in the bradykinin-induced coronary vasodilation; and 4) amounts of NO formed within the heart stimulated with ATP, acetylcholine, and serotonin are effective for vasodilation.

NG-methyl-L-arginine causes endothelium-dependent contraction and inhibition of cyclic GMP formation in artery and vein

The objective of this study was to determine whether the vascular smooth muscle contractile effect of NG-methyl-L-arginine (NMA) is endothelium dependent and attributed to a decline in smooth muscle levels of cyclic GMP. Vascular smooth muscle levels of cyclic GMP are severalfold greater in endothelium-intact than in endothelium-denuded preparations because of the continuous formation and release of a lipophilic endothelium-derived chemical factor that diffuses into the underlying smooth muscle and activates cytosolic guanylate cyclase. This chemical substance, believed to be nitric oxide (NO) or a labile nitroso precursor, appears to account for the biological actions of endothelium-derived relaxing factor. NMA inhibits the formation of NO from endogenous L-arginine in endothelial cells. In the present study, NMA caused marked endothelium-dependent contraction of isolated rings of bovine pulmonary artery and vein, and this was similar to the contraction elicited by hemoglobin, an inhibitor of the relaxant action of NO. Both NMA and hemoglobin caused endothelium-dependent potentiation of contractile responses to phenylephrine in artery and vein. NMA caused endothelium-dependent decreases in the resting or basal levels of cyclic GMP in artery and vein to levels that were characteristic of those in endothelium-denuded vessels. Finally, NMA inhibited endothelium-dependent relaxant responses and cyclic GMP formation stimulated by acetylcholine and bradykinin. These observations reveal that interference with the continuous or basal generation of endothelium-derived NO in artery and vein can cause marked increases in vascular smooth muscle tone as a result of inhibition of cyclic GMP formation.

Cyclic GMP release and vasodilatation induced by EDRF and atrial natriuretic factor in the isolated perfused kidney of the rat

1. Guanosine 3':5'-cyclic monophosphate (cyclic GMP) release and vascular tone was measured in the isolated kidney of the rat perfused at constant flow with Krebs-Henseleit solution. The effects of 3 vasodilators, acetylcholine (ACh), atrial natriuretic factor (ANF) and sodium nitroprusside (SNP) on the renal release of cyclic GMP and vascular tone were examined. The ability of the endothelial-derived relaxing factor (EDRF) inhibitors, haemoglobin and gossypol, to modify vasodilatation and vasodilator-induced changes in cyclic GMP releases from the kidney was also investigated. 2. Renal cyclic GMP release was elevated 8 fold by ANF (0.01 microM), 5 fold by SNP (1 microM) and 3 fold by ACh (0.3 microM). 3. For ACh, both the increase in renal cyclic GMP release and the vasodilatation were reduced by the EDRF inhibitors, haemoglobin (1 microM) and gossypol (15 microM). For SNP, neither the increase in renal cyclic GMP release nor vasodilatation were inhibited by gossypol (15 microM). 4. For ANF, neither the increase in cyclic GMP release from the kidney nor its vasodilator activity were affected by haemoglobin (1 microM). 5. EDRF inhibitors reduced the basal release of cyclic GMP from 0.32 +/- 0.06 pmol min-1 to 0.18 +/- 0.03 pmol min-1, gossypol being more effective than haemoglobin. 6. The results are consistent with the ability of ACh to induce EDRF-mediated vasodilatation in the isolated perfused kidney of the rat. Basal EDRF release appears to contribute approximately 50% to the basal release of cyclic GMP from this preparation. The renal vasodilator action of ANF however, is independent of EDRF, although the renal vascular endothelium cannot be discounted as a site at which ANF stimulates cyclic GMP production.

Release of endothelin from the porcine aorta. Inhibition by endothelium-derived nitric oxide

This study was designed to examine whether endothelin is released from the intima of intact arteries, and whether endothelium-derived nitric oxide regulates its production. Endothelin was detected in the incubating medium of unstimulated pig aortae with, but not in those without endothelium. In preparations with endothelium, thrombin (2-6 U/ml) and the calcium ionophore A23187 (10(-6) M) stimulated the release of the peptide. The basal and thrombin-stimulated production of endothelin were prevented by the protein synthetase inhibitor cycloheximide (10(-6) M). The production of endothelin upon stimulation with thrombin (4 U/ml) was potentiated by L-NG-monomethyl arginine and methylene blue and reduced by superoxide dismutase and 8-bromo cyclic guanosine 5'-monophosphate (GMP), while the basal release of the peptide was unaffected. Thus, (a) endothelin is released from the intimal layer of intact blood vessels, both under basal conditions and after stimulation with thrombin and the calcium ionophore A23187, and (b) endothelium-derived nitric oxide released during stimulation with thrombin inhibits the production of the peptide via a cyclic GMP-dependent pathway.

Cyclic GMP stimulation by vasopressin in LLC-PK1 kidney epithelial cells is L-arginine-dependent

The L-arginine antagonist NG-monomethyl-L-arginine has been shown to inhibit nitric oxide formation from L-arginine in endothelial cells. In the present study NG-monomethyl-L-arginine was used to assess the role of L-arginine for cyclic GMP stimulation by vasopressin in a kidney epithelial cell line (LLC-PK1). Preincubation of cells with 1 mumol/l, 10 mumol/l and 100 mumol/l NG-monomethyl-L-arginine decreased cyclic GMP stimulation at 1 mumol/l vasopressin by 25%, 71% and 90%, respectively. This inhibition by NG-monomethyl-L-arginine was markedly reduced by L-arginine (2 mmol/l) but not D-arginine (2 mmol/l). Cyclic GMP stimulation by the calcium ionophore A23187 was also inhibited by NG-monomethyl-L-arginine and enantioselectively restored by L-arginine. However, NG-monomethyl-L-arginine did not affect cyclic GMP stimulation by sodium nitroprusside that spontaneously releases nitric oxide. These results suggest that, in kidney epithelial cells, vasopressin induces nitric oxide formation from L-arginine leading to activation of soluble guanylate cyclase. It is concluded that nitric oxide formation from L-arginine is not only responsible for endothelium-dependent relaxation but may be a more general pathway with regulatory function for intracellular guanylate cyclase activity.

Endothelium-derived nitric oxide: pharmacology and relationship to the actions of organic nitrate esters

Vascular smooth muscle relaxation elicited by various endogenous substances results from their interaction with vascular endothelial cells to triger the formation of endothelium-derived relaxing factor (EDRF). EDRF from pulmonary and peripheral arteries and veins and from cultured and freshly harvested aortic endothelial cells has been identified pharmacologically and chemically as nitric oxide (NO) or a labile nitroso compound. Endothelium-derived NO (EDNO) and authentic NO activate the cytoplasmic form of guanylate cyclase by heme-dependent mechanism and thereby stimulate intracellular cyclic GMP accumulation in cells including vascular smooth muscle and platelets. Cyclic GMP functions as a second messenger to cause vascular muscle relaxation and inhibition of platelet aggregation and adhesion to vascular endothelial surfaces. EDNO is synthesized from L-arginine and perhaps arginine-containing peptides by an unidentified calcium-requiring process coupled to the occupation of extracellular endothelial receptors. The biological actions of EDNO are terminated by spontaneous oxidation to NO2- and NO3-. The biological half-life of the very lipophilic EDNO is only 3-5 sec and this allows EDNO to function locally as an autacoid. Nitroglycerin and other organic nitrate esters elicit endothelium-independent relaxation after entering vascular smooth muscle cells and undergoing denitration and formation of NO. The pharmacological actions of nitroglycerin are therefore essentially the same as those of EDNO, and the endogenous NO receptor is the heme group bound to soluble guanylate cyclase. EDNO may serve a biological role to modulate local blood flow and platelet function.

Effect of endothelium on basal and on stimulated accumulation and efflux of cyclic GMP in rat isolated aorta

1. The aim of this study was to examine the possible role of the release of guanosine 3':5'-cyclic monophosphate (cyclic GMP) into the extracellular space in the regulation of rat aortic cyclic GMP content. 2. Rat aortic segments incubated in physiological solution released cyclic GMP into the medium in a time-dependent manner. This release was greatly enhanced when intact instead of tissues without endothelium were used. After 120 min of observation, a maximal 33 fold difference in extracellular cyclic GMP content was detected. 3. Treatment of rat aortic preparations with either a Ca2+-free solution or methylene blue, both conditions known to inhibit endothelium-derived relaxing factor (EDRF)-mediated responses, markedly reduced the extracellular accumulation of cyclic GMP from tissues with but not without endothelium. 4. Endothelium-dependent vasodilators such as acetylcholine (10 microM) and carbachol (10 microM) greatly increased tissue cyclic GMP content, in a time-dependent manner in rat aortic preparations with endothelium, but only slightly in tissues without. Maximal increases in intact tissues were obtained after about 1 min of agonist contact and amounted to about 35 and 15 fold respectively, thereafter tissue cyclic GMP content rapidly declined. Histamine (10 microM) elicited only minor effects on tissue cyclic GMP content of both intact preparations and those without endothelium. 5. Acetylcholine (10 microM), carbachol (10 microM) and histamine (10 microM) stimulated a time-dependent release of the cyclic nucleotide into the incubation medium from tissues with endothelium. After 120 min of observation, extracellular accumulation of cyclic GMP from intact tissues was increased by about 2.6, 6.6 and 1.7 fold respectively. Carbachol and histamine induced only minor effects on release from tissues without endothelium. 6. Sodium nitroprusside (0.3 and 10 microM), a direct activator of soluble guanylate cyclase, induced a concentration-dependent accumulation of cyclic GMP in tissues with and without endothelium that was associated with a concentration-dependent accumulation of cyclic GMP in the extracellular space. Peak tissue cyclic GMP content reached similar levels in preparations with and without endothelium, while extracellular cyclic GMP levels were about two times greater when experiments were performed with intact compared to endothelium-denuded tissues. 7. Atriopeptin II, an activator of particulate guanylate cyclase, increased tissue cyclic GMP content by about 8 and 18 fold respectively in tissues with and without endothelium. As was the case with sodium nitroprusside, atriopeptin II-stimulated release was markedly enhanced from intact tissues compared with those without endothelium. After 120 min of observation, there was a 16 fold difference in the amount of extracellular cyclic GMP.

Modulation of an electrical synapse between solitary pairs of catfish horizontal cells by dopamine and second messengers

1. Retinas from channel catfish were dissociated and the cells maintained in culture. Horizontal cells that normally receive input from cone photoreceptors were identified. The conductance of the electrical junction formed between a pair of 'cone' horizontal cells was measured by controlling the membrane voltage of each cell with a voltage clamp maintained through either a micropipette or a patch pipette. The two techniques yielded similar results. 2. Transjunctional current was measured while transjunctional voltage was stepped to values between +/- 60 mV. The current (measured 5 ms after a step) was proportional to voltage over the range tested. For steps to voltages greater than +/- 45 mV, the current exhibited a slight time-dependent decline. 3. Dopamine decreased junctional conductance in a dose-dependent fashion. A 50% reduction was obtained with 10 nM-dopamine. The D1 agonist fenoldopam (100 nM) also decreased junctional conductance. The uncoupling produced by either agent was rapid and reversible. 4. The introduction of 100 microM-cyclic AMP into one cell of a pair decreased junctional conductance by, on average, 40%. Forskolin (1-10 microM), an activator of adenylate cyclase, decreased junctional conductance 50-90%. 5. The introduction of 80 microM-cyclic GMP into one cell of a pair decreased junctional conductance by, on average, 40%. Nitroprusside (1-10 microM), an activator of guanylate cyclase, reduced junctional conductance 40-65%. 6. The introduction of a peptide inhibitor specific for the cyclic AMP-dependent protein kinase reversed a decrease in junctional conductance produced by superfusion with either dopamine (1 microM), fenoldopam (100 nM) or forskolin (5-10 microM). 7. Intracellular Ca2+ concentration was measured with the fluorescent indicator Fura-2. The intracellular Ca2+ concentration was increased by activation of a Ca2+ current. Junctional conductance remained constant as the internal Ca2+ concentration changed from 100 to 700 nM. 8. Intracellular pH was measured with the fluorescent indicator bis-carboxyethylcarboxyfluorescein. The application of acetate (2.5 mM) reduced intracellular pH by 0.2-0.3 units and decreased junctional conductance by approximately 50%. A subsequent application of fenoldopam did not alter intracellular pH, but decreased junctional conductance by more than 50%. 9. The sensitivity of the junctional conductance between isolated horizontal cells to dopamine is consistent with dopamine having a direct effect on coupling in intact retina. Dopamine regulates the activity of a cyclic AMP-dependent protein kinase which in turn modulates junctional conductance. Changes in intracellular pH and Ca2+ concentration are not involved in mediating the effect of dopamine on coupling. Cyclic GMP and intracellular pH may participate in regulatory pathways independent of that used by cyclic AMP.

Non-prostanoid endothelium-derived vasoactive factors

Endothelium-derived relaxing factor (EDRF) and endothelium-derived contracting factor (EDCF) are released by endothelial cells following stimulation by a wide range of chemical agonists acting on specific endothelial membrane receptors and by physical factors such as shear stress on the vascular wall. It has now been firmly established that EDRF is nitric oxide, whereas EDCF has been recently identified as a 21-residue peptide, endothelin. Circumstantial evidence, however, suggests that there may be more than one EDRF and/or EDCF. EDRF induces relaxation of the underlying vascular smooth muscle by enhancing intracellular cyclic GMP levels. This action is comparable to that of nitrovasodilators, and nitric oxide can, therefore, be regarded as an endogenous nitrovasodilator. The mechanism of action of endothelin is uncertain, but depends on influx of extracellular calcium. The respective roles of EDRF and EDCF in disease are still hypothetical. It is preferable to think in terms of balance (or imbalance) between these two factors which, probably, have a fundamental role, and very likely interact with each other in maintaining and regulating vascular tone in man.

Electron microscopical localization of guanylate cyclase activity in the neocortex of the guinea pig

The localization of the guanylate cyclase (GC) activity has been established in the neocortex of adult guinea pigs by means of electron microscopical histochemistry [the DMSO-method of Fujimoto et al. (1981)]. Reaction product was deposited within a population of large- and medium-sized cortical neurons as well as in the cytoplasm of a part of the dendrites of variable size and in the cytoplasm and the nuclear membrane of a number of protoplasmic astrocytes. In the perikarya of the positive neurons, the reaction precipitate was mainly located within the cisterns of the rough endoplasmic reticulum and on the nuclear membrane. In the dendrites, the reaction product was usually distributed in close contact with microtubules, microfilaments, and beneath the postsynaptic membranes of a number of axodendritic synaptic contacts. The axons and all presynaptic boutons were negative. Thus, the localization of the GC could be determined as exclusively postsynaptic. The results obtained support the view for the probable participation of cyclic GMP in the cholinergic, glutaminergic or GABAergic, or peptidergic transmitter mechanisms in the central nervous system.

Receptor-mediated release of endothelium-derived relaxing factor and prostacyclin from bovine aortic endothelial cells is coupled

Bovine aortic endothelial cells were grown on microcarrier beads and were perfused with Krebs-Ringer solution. Endothelium-derived relaxing factor (EDRF) was bioassayed on a cascade of four strips of rabbit aorta, and prostacyclin was analyzed by RIA of 6-oxo-prostaglandin F1 alpha. The endothelial cells released EDRF and prostacyclin when stimulated with bradykinin and its analogues, or with ADP, ATP, arachidonic acid, and phospholipase C (phosphatidylcholine cholinephosphohydrolase, EC 3.1.4.3). The detection of EDRF was potentiated by superoxide dismutase, and the relaxation of rabbit aortic strips induced by EDRF was antagonized by methylene blue. The release of EDRF and prostacyclin was inhibited by phorbol myristate acetate, R59022 (a diacylglycerol kinase inhibitor), and gentamycin. We suggest that the release of EDRF and prostacyclin is coupled and the initial common step is activation of a phospholipase C.

Evidence that inhibitory factor extracted from bovine retractor penis is nitrite, whose acid-activated derivative is stabilized nitric oxide

1. Unactivated extracts of bovine retractor penis (BRP) contains 3-7 microM nitrite. Acid-activation of these extracts at pH 2 for 10 min followed by neutralization generates the active form of inhibitory factor (IF; assayed by its vasodilator action on rabbit aorta), and is associated with partial loss of nitrite. 2. Increasing the time of acid-activation at pH 2 from 10 to 60 min with intermittent vortex mixing generates greater vasodilator activity and increases nitrite loss. 3. When acid-activated and neutralized extracts are incubated at 37 degrees C or 30 min or boiled for 5 min, vasodilator activity is lost and nitrite content increased. Reactivation of these samples at pH 2 for 10 min followed by neutralization leads to partial recoveries of vasodilator activity with loss in nitrite content. 4. Addition of sodium nitrite to BRP extracts increases acid-activatable vasodilator activity pro rata. 5. Acid-activation of aqueous sodium nitrite solutions results in less loss of nitrite and generation of less vasodilator activity than BRP extracts. Vasodilatation is only transient and is rapidly abolished on neutralization, whereas responses to acid-activated BRP extracts are more prolonged and activity is stable on ice. 6. Bovine aortic endothelial cells yield vasodilator activity that is indistinguishable from that isolated from BRP. It is activated by acid, stable on ice, abolished by boiling or by haemoglobin, and appears to be due to the generation of nitric oxide (NO) from nitrite. 7. The data provide confirmatory evidence that nitrite in BRP extracts is IF, that acid-activation of BRP extracts yields NO which is responsible for its vasodilator action, and that inactivation occurs by decay of NO to nitrite and nitrate. They further suggest that BRP extracts contain a NO-stabilizing agent which favours conversion of nitrite to NO. 8. The finding that bovine aortic endothelial cells yield an agent indistinguishable from IF suggests that nitrite in endothelial cells may likewise be the precursor of endothelium-derived relaxing factor (EDRF), itself identified as NO.

Effects of 8-bromo-cGMP on Ca2+ levels in vascular smooth muscle cells: possible regulation of Ca2+-ATPase by cGMP-dependent protein kinase

The effects of 8-bromo-cGMP on intracellular calcium concentrations in cultured rat aortic smooth muscle cells were studied. Both angiotensin II and depolarizing concentrations of K+ stimulated Ca2+ accumulation in the cytoplasm. The increase in Ca2+ due to angiotensin II was associated with an increase in inositol phosphates, while that due to K+ was not. Preincubation of cells with 8-bromo-cGMP (100 microM) caused an inhibition of peak Ca2+ accumulation to either angiotensin II or K+. To probe the mechanism of action of cGMP in vascular smooth muscle, the effects of cGMP-dependent protein kinase on Ca2+-ATPase from the cultured cell particulate material were investigated. Ca2+-activated ATPase was stimulated approximately equal to 2-fold by exogenous calmodulin and up to 4-fold by low concentrations of purified cGMP-dependent protein kinase. The inclusion of both calmodulin and cGMP-dependent protein kinase resulted in an additive stimulation of Ca2+-ATPase. Stimulation of Ca2+-ATPase activity was observed at all Ca2+ concentrations tested (0.01-1.0 microM). cAMP-dependent protein kinase catalytic subunit and protein kinase C were either ineffective or less effective than cGMP-dependent protein kinase in stimulating the Ca2+-ATPase from rat aortic smooth muscle cells. These results suggest a possible mechanism of action for cGMP in mediating decreases in cytosolic Ca2+ through activation of a Ca2+-ATPase and the subsequent removal of Ca2+ from the cell.

Evidence for lack of a role of cGMP in effect of alpha-hANP on aldosterone inhibition

To investigate the role of guanosine 3',5'-cyclic monophosphate (cGMP) in the inhibitory effect on aldosterone production of alpha-human atrial natriuretic polypeptide (alpha-hANP) we first compared the effects of the peptide with those of sodium nitroprusside (SNP) on the production of aldosterone and cGMP in dispersed adrenal capsular cells of rats, second, examined the effects of derivatives of cGMP on the production of aldosterone, and, third, studied the influence of potassium on the effects of alpha-hANP on the production of aldosterone and cGMP. alpha-hANP at concentrations of 3 X 10(-8) to 3 X 10(-7) M decreased the production of aldosterone in a dose-dependent manner, while markedly increasing the production of cGMP. On the other hand, although SNP at concentrations of 10(-5) to 10(-3) M increased the production of cGMP in a dose-dependent manner, it caused no significant changes in the production of aldosterone. Neither dibutyryl cGMP nor 8-bromo-cGMP affected the production of aldosterone in the adrenal cells. Although the aldosterone-inhibitory effect of alpha-hANP was lost in the potassium-free medium, the cGMP-stimulatory effect of the peptide was not altered by adding potassium to the incubation medium at concentrations of 0-5 meq/l. These results suggest that cGMP plays a minor role in the inhibitory effect of alpha-hANP on the production of aldosterone and that the production of cGMP stimulated by the peptide is not directly involved in the decrease in aldosterone production in adrenal capsular cells of rats.

The mechanisms by which haemoglobin inhibits the relaxation of rabbit aorta induced by nitrovasodilators, nitric oxide, or bovine retractor penis inhibitory factor

The mechanisms by which haemoglobin and methaemoglobin inhibit the vasodilator actions of glyceryl trinitrate, sodium azide, nitric oxide, and the bovine retractor penis inhibitory factor (IF) were studied on rabbit endothelium-denuded aortic rings. Methaemoglobin was less effective than haemoglobin against each vasodilator, it was more effective at inhibiting the relaxation to azide than that to glyceryl trinitrate. Glyceryl trinitrate was neither bound nor inactivated when passed through columns of haemoglobin-agarose or methaemoglobin-agarose. Azide was reversibly bound but less by haemoglobin-agarose than by methaemoglobin-agarose. Inhibition of the vasodilator actions of glyceryl trinitrate is not attributable therefore to a direct interaction with the haemoproteins, although a small part of the inhibition of azide-induced relaxation by methaemoglobin is likely to be due to a direct interaction. Columns of haemoglobin-agarose were more effective than columns of methaemoglobin-agarose in removing nitric oxide from solution. The greater ability of haemoglobin, compared to methaemoglobin, to inhibit vasodilatation induced by nitrovasodilators may therefore reflect the greater ability of haemoglobin to bind nitric oxide which is the active principle of the nitrovasodilators. Neither the acid-activated nor the inactive forms of IF were bound or inactivated when passed through columns of methaemoglobin-agarose. Neither form of IF was retained on passage through columns of haemoglobin-agarose, but the resulting activity in the eluates was less than control, was unstable and, unlike the original activity, decayed rapidly on ice. The greater ability of haemoglobin, compared to methaemoglobin, to inhibit vasodilatation induced by IF might therefore reflect the greater ability of haemoglobin to interact with this vasodilator and inactivate it.

Atriopeptin II-induced relaxation of rabbit aorta is potentiated by M&B 22,948 but not blocked by haemoglobin

We examined the effects of haemoglobin (which inhibits the vascular responses to stimulation of soluble guanylate cyclase) and of M&B 22,948 (which selectively inhibits cyclic GMP phosphodiesterase) on the relaxation induced in rabbit aorta by the atrial natriuretic peptide, atriopeptin II (which stimulates particulate guanylate cyclase). Pretreatment with M&B 22,948 (100 microM) produced a 2.3 fold potentiation of atriopeptin II-induced relaxation of endothelium-denuded rings of rabbit aorta. Pretreatment with haemoglobin (10 microM) had no effect on the relaxation or the 10.9 fold increase in cyclic GMP content induced by atriopeptin II in endothelium-denuded rings of rabbit aorta. The potentiation by M&B 22,948 suggests a causal role for cyclic GMP in mediating atriopeptin II-induced vasodilatation of rabbit aorta. The inability of haemoglobin to block the atriopeptin II-induced rise in cyclic GMP suggests that it does not block stimulation of particulate guanylate cyclase. Thus, it is unlikely that a ferrous haem-containing receptor site is involved in the activation of the particulate form of guanylate cyclase as it is with soluble guanylate cyclase.

Stimulation of soluble coronary arterial guanylate cyclase by SIN-1

SIN-1, a metabolite of the vasodilating drug molsidomine, was found to stimulate dose dependently (0.01-1 mM) soluble guanylate cyclase from bovine coronary arteries up to 100-fold the control value. The stimulatory effect of SIN-1 increased with rising concentrations of MnC1(2) or MgC1(2) and was diminished in the presence of methylene blue or ferricyanide. The time course of SIN-1-induced guanylate cyclase stimulation was characterized by a lag phase which was not observed after preincubation of the enzyme with SIN-1. In contrast to nitroglycerin and sodium nitroprusside, SIN-1 did not require the presence of cysteine or other thiols to stimulate guanylate cyclase. The results presented in this study provide further evidence that SIN-1 exerts its dilating effect on coronary vessels via direct stimulation of guanylate cyclase.

A comparison of the action of the endothelium-derived relaxant factor and the inhibitory factor from the bovine retractor penis on rabbit aortic smooth muscle

The dependence of relaxation of rabbit aortic strips by carbachol and by the inhibitory factor from the bovine retractor penis (BRP) on the presence of endothelium has been compared. Carbachol-induced relaxation is abolished by removing the endothelium, inhibitory factor-induced relaxation is unimpaired. The inhibitory factor, therefore, does not act by releasing an endothelium-derived relaxing factor (EDRF). The effect of inhibitors of eicosanoid metabolism on relaxation was examined. Quinacrine and nordihydroguaiaretic acid abolished the relaxant effect of carbachol and flurbiprofen had no effect. The relaxation produced by the inhibitory factor was unaffected by quinacrine and flurbiprofen while nordihydroguaiaretic acid potentiated the response. No eicosanoid appears, therefore, to be involved in the relaxant effect of the inhibitory factor from the BRP. Methylene blue, a drug reported to inhibit guanylate cyclase, in a concentration of 10 microM selectively abolished the relaxation produced by carbachol. However, at the higher concentration of 30 microM it abolished almost completely the response to inhibitory factor from the BRP and reduced inhibition by sodium nitroprusside. It is not possible from these results to exclude the possibility that the EDRF and the inhibitory factor from the BRP are chemically related.

Endothelium-dependent relaxation in rat aorta may be mediated through cyclic GMP-dependent protein phosphorylation

The action of some vascular smooth muscle relaxants depends on the presence of the endothelium. We have recently shown that relaxation may be mediated through the formation of cyclic GMP. The nitrovasodilators are another class of relaxants which exert their effects through the formation of cyclic GMP, although their relaxation is independent of the presence of the endothelium. Their relaxant properties seem to depend on free radical formation--specifically, the formation of nitric oxide. The NO-induced smooth muscle relaxation is proposed to occur through activation of guanylate cyclase and the formation of cyclic GMP. Protein phosphorylation is thought to be a common event in the pathway for many biological phenomena. Moreover, sodium nitroprusside and 8-bromo cyclic GMP induce similar patterns of protein phosphorylation in intact rat thoracic aorta. Here we report that the patterns of protein phosphorylation induced by the endothelium-dependent vasodilators and nitrovasodilators were identical. Incorporation of 32P into myosin light chain was decreased by both classes of agents. Removal of the endothelium abolished the changes in phosphorylation with the endothelium-dependent vasodilator (acetylcholine), but not those with the nitrovasodilator (sodium nitroprusside). These results suggest that endothelium-dependent vasodilators and nitrovasodilators induce relaxation through cyclic GMP-dependent protein phosphorylation and dephosphorylation of myosin light chain.

Highly purified particulate guanylate cyclase from rat lung: characterization and comparison with soluble guanylate cyclase

Guanylate cyclase was purified 1000-fold from washed rat lung particulate fractions to a final specific activity of 500 nmoles cyclic GMP produced/min/mg protein by a combination of detergent extraction and chromatography on concanavalin A-Sepharose, GTP-agarose, and blue agarose. Particulate guanylate cyclase has a molecular weight of 200 000 daltons, a Stokes radius of 48 A and a sedimentation coefficient of 9.4 while the soluble form has a molecular weight of 150 000 daltons, a Stokes radius of 44 A, and a sedimentation coefficient of 7.0. Whereas the particulate enzyme is a glycoprotein with a specific affinity for concanavalin A and wheat germ agglutinin, the soluble form of guanylate cyclase did not bind to these lectins. Purified particulate guanylate cyclase did not cross-react with a number of monoclonal antibodies generated to the soluble enzyme. While both forms of the enzyme could be regulated by the formation of mixed disulfides, the particulate enzyme was relatively insensitive to inhibition by cystine. With GTP as substrate both forms of the enzyme demonstrated typical kinetics, and with GTP analogues negative cooperativity was observed with both enzyme forms. These data support the suggestion that the two forms of guanylate cyclase possess similar catalytic sites, although their remaining structure is divergent, resulting in differences in subcellular distribution, physical characteristics, and antigenicity.

Agonist-induced endothelium-dependent relaxation in rat thoracic aorta may be mediated through cGMP

The present study investigates the hypothesis that endothelium-dependent relaxation of vascular smooth muscle may be mediated through the formation of cGMP. Relaxation of the rat thoracic aorta to acetylcholine, histamine, and Ca++ ionophore A23187 was associated with increased levels of cGMP in a time- and concentration-dependent manner, whereas cAMP levels were unaltered. Removal of the endothelium prevented relaxation to these agents and prevented the increased levels of cGMP. Removal of the endothelium after exposure to acetylcholine only partially decreased the elevated levels of cGMP, suggesting that the changes in cGMP occurred within the smooth muscle cells. Eicosatetraynoic acid, an inhibitor of lipoxygenase and cyclooxygenase, and quinacrine, an inhibitor of phospholipase, prevented and reversed acetylcholine-induced relaxation, respectively, and inhibited acetylcholine-induced increased levels of cGMP. In contrast, sodium nitroprusside-induced relaxation and increased levels of cGMP were independent of the presence of the endothelium, exposure to eicosatetraynoic acid, and quinacrine. The present results support the hypothesis that vascular smooth muscle relaxation induced by some agents is dependent on the presence of the endothelium and is mediated through the formation of an endothelial factor that increases cGMP levels in smooth muscle.

Alterations in the subcellular distribution of Guanylate cyclase and its responsiveness to nitric oxide in diethylstilbestrol-induced renal tumors

The cyclic GMP content of diethylstilbestrol-induced renal tumors in the male golden hamster was increased nearly 130-fold over that in kidney from control animals. Cyclic GMP in tumors was 91.80 +/- 19.18 pmoles cyclic GMP/mg protein compared to 0.72 +/- 0.07 in control kidneys. Cyclic AMP in tumors was also increased over control, however, to a much lesser degree (2.7-fold). In control kidneys, 84.6% of homogenate guanylate cyclase activity was recovered in the 100,000 X g supernatant fraction. Total homogenate guanylate cyclase activity from diethylstilbestrol-induced renal tumors was increased 5.5-fold over that in control kidneys and only 8.1% was associated with the 100,000 X g supernatant fraction. Neither the soluble or particulate guanylate cyclase from renal tumors could be activated by nitric oxide. The unresponsiveness of tumor guanylate cyclase to nitric oxide was independent of the cation cofactor, and not due to a shift in the dose response curve for nitric oxide. Responsiveness to nitric oxide was not restored by thiols, sugars, other proteins, or hemoglobin. Basal cyclic AMP formation by soluble guanylate cyclase from renal tumors was dramatically increased over that observed in control kidneys, and could not be increased further by nitric oxide. This is the first study of cyclic GMP and guanylate cyclase in a primary estrogen-induced tumor. The possibility that the changes observed in guanylate cyclase from diethylstilbestrol-induced renal tumors are related to in vivo activation of the enzyme by epoxide metabolites of diethylstilbestrol is discussed.

Role of lipoxygenase in the O2-dependent activation of soluble guanylate cyclase from rat lung

Guanylate cyclase activity in rat lung supernatant fractions is stimulated 3-4 fold by aerobic incubation at 30 degrees C for approx. 30 min ('O2-dependent activation'). This stimulation was blocked by 20 microM-eicosa-5,8,11,14-tetraynoic acid (ETYA), an inhibitor of lipoxygenase and cyclo-oxygenase, but not by aspirin or indomethacin, which are cyclo-oxygenase inhibitors. The enzyme activator(s) is presumed to be the fatty acid hydroperoxide(s) formed by lipoxygenase. Removal of lipoxygenase from the supernatant fraction by chromatography on Amberlite XAD-4 also prevented activation, which was restored by the addition of soya-bean lipoxygenase. Bovine serum albumin prevented O2-dependent activation or activation by soya-bean lipoxygenase, through its ability to bind the unsaturated fatty acid substrate of lipoxygenase. The lipoxygenase in the supernatant fraction is inhibited by endogenous glutathione peroxidase plus reduced glutathione (GSH); removal of GSH de-inhibits lipoxygenase and activates guanylate cyclase. This was effected by autoxidation, by cumene hydroperoxide (with GSH peroxidase) and by titration with N-ethylmaleimide (NEM). Activation by NEM was inhibited by serum albumin or ETYA, as was activation by low concentrations (less than 50 microM) of cumene hydroperoxide. Activation by higher concentrations was not so inhibited; therefore, cumene hydroperoxide can also activate by a direct effect on guanylate cyclase. A hypothesis for physiological activation is proposed.