Testosterone and its precursors and metabolites enhance guanylate cyclase activity.

Both testosterone and cyclic GMP stimulate DNA synthesis. Because cyclic GMP and testosterone seem to have similar actions, the objective of this investigation was to determine if testosterone and its precursors might have part of their mechanism of action through stimulation of guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2], the enzyme that catalyzes the formation of cyclic GMP from GTP. The precursors--namely, progesterone, pregnenolone, 17 alpha-progesterone, 17 alpha-hydroxypregnenolone, androstenedione, and dehydroepiandrosterone--caused a 2- to 3 1/2-fold enhancement of guanylate cyclase activity in rat liver, kidney, skeletal muscle, and ventral prostate at a concentration of 1 microM. These precursors are generated from cholesterol, which had no effect itself on guanylate cyclase activity. Testosterone, 19-nortestosterone, 17-methyltestosterone, and 5 alpha-dihydrotestosterone enhanced guanylate cyclase activity 2- to 5-fold in the same tissues at 1 microM. Etiocholanolone, androsterone, and epiandrosterone, metabolites of testosterone metabolism, enhanced guanylate cyclase activity 1 1/2- to 2-fold at this same concentration. Dose-response relationships revealed that testosterone and its precursors and metabolites had their maximal effect at 1 microM but still had some effect at 0.001 microM. The data in this investigation suggest that the guanylate cyclase-cyclic GMP system plays a role in the mechanism of action of testosterone and its precursors.     

Regulation by guanosine 3'':5''-cyclic monophosphate of phospholipid methylation during chemotaxis in Dictyostelium discoideum.

In Dictyostelium discoideum, the chemoattractant cyclic AMP activates the enzyme guanylate cyclase, giving a brief up to 10-fold increase in the intracellular cyclic GMP content. The addition of physiological cyclic GMP concentrations to a homogenate of D. discoideum cells markedly increased the incorporation of the 3H-labeled methyl group from S-adenosyl-L-[methyl-3H]methionine into mono- and dimethylated phosphatidylethanolamine and phosphatidylcholine. Lipid methylation was inhibited by S-adenosyl-L-homocysteine, which inhibits transmethylation. When whole cells prelabeled with L-[methyl-3H]methionine were exposed to cyclic AMP, a rapid transient increase in the amount of [methyl-3H]phosphatidylcholine was observed. The time course of [methyl-3H]phosphatidylcholine formation agrees with its being mediated by the intracellular increase in cyclic GMP originating during chemotactic stimulation. Addition of the 8-Br derivative of cyclic GMP to whole cells also increased the levels of labeled phosphatidylcholine. It is therefore likely that cyclic GMP contributes to chemotaxis by regulating membrane function via phospholipid methylation.     

Activation of guanylate cyclase by superoxide dismutase and hydroxyl radical: a physiological regulator of guanosine 3'',5''-monophosphate formation.

Partially purified soluble rat liver guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2] was activated by superoxide dismutase (superoxide: superoxide oxidoreductase, EC 1.15.1.1). This activation was prevented with KCN or glutathione, inhibitors of superoxide dismutase. Guanylate cyclase preparations formed superoxide ion. Activation by superoxide dismutase was further enhanced by the addition of nitrate reductase. Although guanylate cyclase activity was much greater with Mn2+ than with Mg2+ as sole cation cofactor, activation with superoxide dismutase was not observed when Mn2+ was included in incubations. Catalase also decreased the activation induced with superoxide dismutase. Thus, activation required the formation of both superoxide ion and H2O2 in incubations. Activation of guanylate cyclase could not be achieved by the addition of H2O2 alone. Scavengers of hydroxyl radicals prevented the activation. It is proposed that superoxide ion and hydrogen peroxide can lead to the formation of hydroxyl radicals that activate guanylate cyclase. This mechanism of activation can explain numerous observations of altered guanylate cyclase activity and cyclic GMP accumulation in tissues with oxidizing and reducing agents. This mechanism will also permit physiological regulation of guanylate cyclase and cyclic GMP formation when there is altered redox or free radical formation in tissues in response to hormones, other agents, and processes.     

Immunohistochemical localization of guanylate cyclase within neurons of rat brain.

The immunohistochemical localization of guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2] has been examined in rat neocortex, caudate-putamen, and cerebellum by using specific monoclonal antibodies. Immunofluorescence could be seen within somata and proximal dendrites of neurons in the these regions. A nuclear immunofluorescence reaction to guanylate cyclase was characteristically absent. The staining pattern for guanylate cyclase was coincident with previously described localizations of cyclic GMP immunofluorescence within medium spiny neurons of the caudate-putamen and pyramidal cells of the neocortex. Cerebellar guanylate cyclase immunoreactivity was primarily confined to Purkinje cells and their primary dendrites, similar to the pattern reported for cyclic GMP-dependent protein kinase localization. Guanylate cyclase immunofluorescence was abolished when the monoclonal antibodies were exposed to purified enzyme prior to incubation of the tissue slices or when control antibody was substituted for the primary antibody. Immunohistochemical localization of cyclic AMP in these same tissues was readily distinguished from that of guanylate cyclase or cyclic GMP, showing uniform fluorescence throughout the cell bodies of neurons and glial elements.     

Antihypertrophic actions of the natriuretic peptides in adult rat cardiomyocytes: importance of cyclic GMP

Atrial natriuretic peptide (ANP) prevents hypertrophy of neonatal cardiomyocytes. However, whether this effect is retained in the adult phenotype or if other members of the natriuretic peptide family exhibit similar antihypertrophic properties, has not been elucidated. OBJECTIVE: Our objective was to examine whether the natriuretic peptides protect against adult cardiomyocyte hypertrophy in vitro. METHODS: Adult rat cardiomyocytes were incubated with angiotensin II (Ang II)+/-ANP, B-type (BNP) or C-type (CNP) natriuretic peptides for determination of [3H]phenylalanine incorporation, c-fos mRNA expression and cyclic GMP. The effects of 8-bromo-cyclic GMP (cyclic GMP analogue), HS-142-1 (particulate guanylyl cyclase inhibitor) and KT5823 (cyclic GMP-dependent protein kinase inhibitor) were also investigated. RESULTS: Ang II-stimulated increases in markers of hypertrophy, [3H]phenylalanine incorporation (to 136+/-3% of control, n=9) and c-fos mRNA expression (4.3+/-1.4-fold, n=5), were completely prevented by each of ANP, BNP or CNP. This protective action was accompanied by increased cardiomyocyte cyclic GMP. Inhibitory actions on [3H]phenylalanine incorporation were mimicked by 8-bromo-cyclic GMP, and were abolished by HS-142-1. KT5823 blocked the response to BNP and CNP, but not to ANP. CONCLUSION: ANP prevents hypertrophy of adult rat cardiomyocytes. This protective action is shared by BNP and CNP and involves activation of particulate guanylyl cyclase receptors. Antihypertrophic effects of BNP and CNP are mediated through cyclic GMP-dependent protein kinase, but ANP can activate additional pathways independent of cyclic GMP to prevent adult cardiomyocte hypertrophy. These novel findings are of interest particularly since BNP appears to exert antifibrotic rather than antihypertrophic actions in vivo, while CNP is thought to act at least in part via the endothelium.     

Plant growth-promoting hormones activate mammalian guanylate cyclase activity

In vivo injections of plant growth-promoting hormones increase the growth of animals as well as plants. Plant growth-promoting hormones and positive plant growth regulators are known to increase RNA and protein synthesis. Since cyclic GMP also increases RNA and protein synthesis, the object of the present investigation was to determine whether physiological levels of plant growth-promoting hormones and positive plant growth regulators have part of their mechanism(s) of action through stimulation of the guanylate cyclase (EC 4.6.1.2)-cyclic GMP system. Representatives of the three classes of growth-promoting hormones were investigated. Thus, auxins (indole-3-acetic acid, indole-3-butyric acid, beta-naphthoxyacetic acid, and 2,4,5-trichlorophenoxy acetic acid), gibberellins (gibberellic acid), and cytokinins [N6-benzyl adenine, kinetin (6-furfuryl aminopurine), and beta-(2-furyl) acrylic acid] all increased rat lung, small intestine, liver, and renal cortex guanylate cyclase activity 2- to 4-fold at the 1 microM concentration. Dose response curves revealed that maximal stimulation of guanylate cyclase by these plant growth regulators was at 1 microM; there was no augmented cyclase activity at 1 nM. The guanylate cyclase cationic cofactor manganese was not essential for augmentation of guanylate cyclase by these plant growth-promoting regulators. The antioxidant butylated hydroxytoluene did not block the enhancement of guanylate cyclase by these plant growth-promoting factors. These data suggest that guanylate cyclase may play a role in the mechanism of action of plant growth-promoting hormones and even of positive plant regulators at the cellular level.     

Acute antihypertrophic actions of bradykinin in the rat heart: importance of cyclic GMP

The antihypertrophic action of angiotensin (Ang)-converting enzyme (ACE) inhibitors in the heart is attributed in part to potentiation of bradykinin. Bradykinin prevents hypertrophy of cultured cardiomyocytes by releasing nitric oxide (NO) from endothelial cells, which increases cardiomyocyte guanosine 3'5'-cyclic monophosphate (cyclic GMP). It is unknown whether cyclic GMP is essential for the action of bradykinin, or whether findings in isolated cardiomyocytes apply in whole hearts, in the presence of other cell types and mechanical/dynamic activity. We now examine the contribution of cyclic GMP to the antihypertrophic action of bradykinin in cardiomyocytes and perfused hearts. In adult rat isolated cardiomyocytes cocultured with bovine aortic endothelial cells, the inhibitory action of bradykinin (10 micromol/L) against Ang II (1 micromol/L)-induced [3H]phenylalanine incorporation was abolished by the soluble guanylyl cyclase inhibitor [1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (10 micromol/L). In Langendorff-perfused rat hearts, Ang II (10 nmol/L)-induced increases in [3H]phenylalanine incorporation and atrial natriuretic peptide mRNA expression were prevented by bradykinin (100 nmol/L), the NO donor sodium nitroprusside (3 micromol/L), and the ACE inhibitor ramiprilat (100 nmol/L). The acute antihypertrophic action of bradykinin was accompanied by increased left ventricular cyclic GMP, and the ramiprilat effect was attenuated by HOE 140 (1 micromol/L, a B2-kinin receptor antagonist) or [1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (100 nmol/L). In conclusion, bradykinin exerts a direct inhibitory action against the acute hypertrophic response to Ang II in rat isolated hearts, and elevation of cardiomyocyte cyclic GMP may be an important antihypertrophic mechanism used by bradykinin and ramiprilat in the heart.     

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

Nitric oxide gas (NO) increased guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2] activity in soluble and particulate preparations from various tissues. The effect was dose-dependent and was observed with all tissue preparations examined. The extent of activation was variable among different tissue preparations and was greatest (19- to 33-fold) with supernatant fractions of homogenates from liver, lung, tracheal smooth muscle, heart, kidney, cerebral cortex, and cerebellum. Smaller effects (5- to 14-fold) were observed with supernatant fractions from skeletal muscle, spleen, intestinal muscle, adrenal, and epididymal fat. Activation was also observed with partially purified preparations of guanylate cyclase. Activation of rat liver supernatant preparations was augmented slightly with reducing agents, decreased with some oxidizing agents, and greater in a nitrogen than in an oxygen atmosphere. After activation with NO, guanylate cyclase activity decreased with a half-life of 3-4 at 4 degrees but re-exposure to NO resulted in reactivation of preparations. Sodium azide, sodium nitrite, hydroxylamine, and sodium nitroprusside also increased guanylate cyclase activity as reported previously. NO alone and in combination with these agents produced approximately the same degree of maximal activation, suggesting that all of these agents act through a similar mechanism. NO also increased the accumulation of cyclic GMP but not cyclic AMP in incubations of minces from various rat tissues. We propose that various nitro compounds and those capable of forming NO in incubations activate guanylate cyclase through a similar but undefined mechanism. These effects may explain the high activities of guanylate cyclase in certain tissues (e.g., lung and intestinal mucosa) that are exposed to environmental nitro compounds.     

Purification of soluble guanylate cyclase from rat liver

Soluble guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2] has been purified from rat liver and exhibited a single protein band on polyacrylamide gels coincident with activity and indicative of a molecular weight of 150,000. The apparent specific activity of the purified enzyme was 276 nmol of cyclic GMP formed per mg per min with Mn²⁺ as the cation cofactor and 23.8 nmol of cyclic GMP formed per mg per min with Mg²⁺. This represented 9200-fold and 7400-fold purifications of Mn²⁺ and Mg²⁺ activities, respectively. The specific activity of soluble guanylate cyclase was not constant with protein concentration. At all stages of purification, increasing the enzyme concentration in the guanylate cyclase assay increased the apparent specific activity of the preparation. The purified enzyme could be activated by nitroprusside, nitric oxide, arachidonate, linoleate, oleate, and superoxide dismutase. However, the degree of activation was dependent upon the concentration of enzyme protein assayed.     

Effect of in vitro organic nitrate tolerance on relaxation, cyclic GMP accumulation, and guanylate cyclase activation by glyceryl trinitrate and the enantiomers of isoidide dinitrate

Previously, it was shown that the D enantiomer of isoidide dinitrate was 10-fold more potent than the L enantiomer and 10-fold less potent than glyceryl trinitrate for stimulating cyclic GMP accumulation and relaxation of isolated rat aorta. In the present study, these organic nitrates were tested for their ability to induce tolerance to organic nitrate-induced relaxation, cyclic GMP accumulation, and guanylate cyclase activation in rat aorta in vitro. To compensate for the differences in vasodilator potency, tolerance was induced by incubating isolated rat aorta with concentrations of organic nitrates 1,000-fold greater than the EC50 for relaxation. Under these conditions, the EC50 for relaxation was increased significantly for each organic nitrate and to a similar degree on subsequent reexposure. These data suggest that the potential for inducing in vitro tolerance to relaxation was the same for the three organic nitrates tested. When activation of soluble guanylate cyclase by these compounds was assessed, the enantiomers of isoidide dinitrate were equipotent, but less potent than glyceryl trinitrate, suggesting that the site of enantioselectivity is not guanylate cyclase itself. In blood vessels made tolerant to organic nitrates by pretreatment with glyceryl trinitrate, vasodilator activity, cyclic GMP accumulation, and guanylate cyclase activation were attenuated on reexposure to each organic nitrate. In addition, differences in the potency of the three organic nitrates and the enantioselectivity of isoidide dinitrate for relaxation were abolished in tolerant tissue, whereas the potency difference between glyceryl trinitrate and isoidide dinitrate for activation of guanylate cyclase was unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of protein kinase a in human hepatocyte DNA synthesis

The cellular mechanisms associated with the replicative response of hepatocytes to growth factor stimulation is incompletely understood. Murine hepatocyte DNA synthesis is altered by cyclic AMP, suggesting that protein kinase A is involved in the cellular mechanisms associated with liver growth. The purpose of this study was to evaluate the role of protein kinase A in human hepatocyte DNA synthesis. Human hepatocytes were isolated and maintained in primary culture on rat tail collagen. DNA synthesis was evaluated by determining [³H]thymidine incorporation. Human hepatocytes between 24 and 96 hr following harvest increased DNA synthesis in response to epidermal growth factor but not in response to glucagon, a stimulant of adenyl cyclase, or dibutyryl cyclic AMP. Mitogen-stimulated DNA synthesis was decreased by dibutyryl cyclic AMP. Cyclic AMP isomers that block or stimulate the effect of cyclic AMP on protein kinase A did not significantly alter resting or mitogen-stimulated human hepatocyte DNA synthesis. The results suggest that increased protein kinase A activity does not produce human hepatocyte replicative DNA synthesis.Supported in part by USPHS DK27695.     

Products of reaction catalyzed by purified rat liver guanylate cyclase determined by 31p NMR spectroscopy.

Products of the reactions catalyzed by highly purified preparations of soluble guanylate cyclase [GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2] from rat liver were identified and quantified with 31P NMR spectroscopy. Utilization of this technique necessitated modification of the standard assay conditions; higher concentrations of enzyme and substrate (2 mM), Mg2+ instead of Mn2+, and longer incubation times (up to 46 hr) at 30 degrees C were used. Revision of our reported procedure for purificaton of guanylate cyclase [Tsai, S-C., Manganiello, V. C. & Vaughan, M. (1978) J. Biol. Chem. 253, 8452-8457] to include chromatography on Ultrogel AcA34 and agarose-hexane-GTP provided an enzyme with specific activity higher than in our earlier preparations. 31P NMR spectra obtained during incubation of this enzyme showed that the rates of GTP disappearance and cyclic GMP (cGMP) accumulation were constant for approximately 16 hr. They indicated, however, that the preparations were contaminated with inorganic pyrophosphatase. This was removed by preparative electrophoresis, yielding enzyme with specific activities (900-1300 nmol/min per mg of protein) higher than those reported for guanylate cyclases from rat liver or lung. With this preparation, cGMP and PPi were the only products of GTP detected, consistent with the assumption that the guanylate and adenylate cyclase reactions are analogous.     

Guanylin: an endogenous activator of intestinal guanylate cyclase.

Intestinal guanylate cyclase mediates the action of the heat-stable enterotoxin to cause a decrease in intestinal fluid absorption and to increase chloride secretion, ultimately causing diarrhea. An endogenous ligand that acts on this guanylate cyclase has not previously been found. To search for a potential endogenous ligand, we utilized T84 cells, a human colon carcinoma-derived cell line, in culture as a bioassay. This cell line selectively responds to the toxin in a very sensitive manner with an increase in intracellular cyclic GMP. In the present study, we describe the purification and structure of a peptide from rat jejunum that activates this enzyme. This peptide, which we have termed guanylin, is composed of 15 amino acids and has the following amino acid sequence, PNTCEICAYAACTGC, as determined by automated Edman degradation sequence analysis and electrospray mass spectrometry. Analysis of the amino acid sequence of this peptide reveals a high degree of homology with heat-stable enterotoxins. Solid-phase synthesis of this peptide confirmed that it stimulates increases in T84 cyclic GMP levels. Guanylin required oxidation for expression of bioactivity and subsequent reduction of the oxidized peptide eliminated the effect on cyclic GMP, indicating a requirement for cysteine disulfide bond formation. Synthetic guanylin also displaces heat-stable enterotoxin binding to cultured T84 cells. Based on these data, we propose that guanylin is an activator of intestinal guanylate cyclase and that it stimulates this enzyme through the same receptor binding region as the heat-stable enterotoxins.     

Pancreastatin, a chromogranin A-derived peptide, inhibits DNA and protein synthesis by producing nitric oxide in HTC rat hepatoma cells

BACKGROUND/AIMS: Pancreastatin, a chromogranin A-derived peptide, has a counter-regulatory effect on insulin action. We have previously characterized pancreastatin receptor and signalling in rat liver and HTC hepatoma cells. A G alpha(q/11)-PLC-beta pathway leads to an increase in [Ca2+]i, PKC and mitogen activated protein kinase (MAPK) activation. These data suggested that pancreastatin might have a role in growth and proliferation, similar to other calcium-mobilizing hormones. METHODS: DNA and protein synthesis were measured as [3H]-thymidine and [3H]-leucine incorporation. Nitric oxide (NO) was determined by the Griess method and cGMP production was quantified by enzyme-linked immunoassay. RESULTS: Contrary to the expected results, we have found that pancreastatin inhibits protein and DNA synthesis in HTC hepatoma cells. On the other hand, when the activity of NO synthase was inhibited by N-monomethyl-L-arginine (NMLA), the inhibitory effect of pancreastatin on DNA and protein synthesis was not only reverted, but a dose-dependent stimulatory effect was observed, probably due to MAPK activation, since it was prevented by PD98059. These data strongly suggested the role of NO in the inhibitory effect of pancreastatin on protein and DNA synthesis, which is overcoming the effect on MAPK activation. Moreover, pancreastatin dose-dependently increased NO production in parallel to cyclic guanosine monophosphate (cGMP). Both effects were prevented by NMLA. Finally, an indirect effect of pancreastatin through the induction of apoptosis was ruled out. CONCLUSIONS: Therefore, the NO and the cGMP produced by the NO-activated guanylate cyclase may mediate the dose-dependent inhibitory effect of pancreastatin on growth and proliferation in HTC hepatoma cells.     

Inhibition of Replication in Functional Mouse Adrenal Tumor Cells by Adrenocorticotropic Hormone Mediated by Adenosine 3′:5′-Cyclic Monophosphate

Adrenocorticotropic hormone (ACTH) inhibited replication in functional adrenal tumor cells with a concomitant stimulation of steroidogenesis and a characteristic change of morphology from a flattened to a spherical type. [(3)H]Thymidine incorporation into DNA was inhibited by about 50% 6 hours after ACTH treatment. Both cyclic AMP and dibutyryl cyclic AMP inhibited [(3)H]thymidine incorporation and caused the characteristic morphological change noted with ACTH. The extent of stimulation of steroidogenesis and the amount of inhibition of [(3)H]thymidine incorporation in response to various doses of ACTH were closely related and both were in parallel with the concentration of cyclic AMP in the cells. Cyclic GMP and cyclic IMP did not inhibit [(3)H]thymidine incorporation significantly, and did not change the morphology of the cells. AMP inhibited [(3)H]thymidine incorporation into DNA and caused the characteristic morphological change. However, AMP did not increase the cyclic AMP content of the cells. CMP, GMP, and UMP showed a significant inhibition of [(3)H]thymidine incorporation into DNA, but the extent of the inhibition was much less than that with AMP. These nucleotides did not change the morphology of the cells.     

The interaction of signal transduction pathways in FRTL5 thyroid follicular cells: studies with stable expression of beta 2-adrenergic receptors.

Multiple signal transduction pathways interact in FRTL5 cells to promote thyroid follicular cell differentiated function and cell proliferation. In these cells, TSH is a tissue-specific mitogen that promotes DNA synthesis primarily through activation of adenylate cyclase. To further test the role of adenylate cyclase in regulating cell growth and differentiated function we have introduced into FRTL5 the human beta 2-adrenergic receptor (BAR) complementary DNA and have studied the ability of isoproterenol, alone and in combination with insulin-like growth factor I (IGF-I), to stimulate cAMP accumulation, iodide transport, [3H]thymidine incorporation into DNA, and cell growth. Wild-type FRTL5 were infected with a PLJ retroviral construct containing the BAR in either a sense (FRTL BAR) or antisense (FRTL RBAR) orientation, and cell populations were selected on the basis of resistance to the antibiotic geneticin. FRTL BAR expressed approximately 1.3 x 10(5) high affinity binding sites per cell for the beta 2-specific ligand, CGP-12177, while neither FRTL5 wild-type nor RBAR cells demonstrated any specific binding. FRTL BAR had significantly higher levels of intracellular cAMP, [3H]thymidine incorporation, and iodide uptake in the absence of added isoproterenol than FRTL RBAR or wild-type cells. In FRTL BAR, but not RBAR cells, isoproterenol stimulated a dose-dependent accumulation of cAMP, iodide uptake, [3H]thymidine incorporation, and cell growth. FRTL BAR and RBAR cells were equally responsive to TSH and to IGF-I. Isoproterenol enhanced the ability of IGF-I to stimulate [3H]thymidine incorporation in BAR but not RBAR cells. Isoproterenol partially inhibited the ability of TSH to stimulate cAMP generation and DNA synthesis. These studies demonstrate that activation of adenylate cyclase through the BAR introduced into FRTL5 cells by retroviral infection reproduces the range of biological effects in these cells stimulated by TSH and suggest that activation of adenylate cyclase is sufficient to stimulate thyroid differentiated function and cell growth. FRTL BAR cells will provide an interesting model system with which to study the heterologous regulation of both TSH and BARs through activation of a common signal transduction pathway, adenylate cyclase.     

Heat-stable enterotoxin of Escherichia coli: in vitro effects on guanylate cyclase activity, cyclic GMP concentration, and ion transport in small intestine.

A partially purified preparation of the heat-stable enterotoxin of Escherichia coli caused a rapid and persistent increase in electric potential difference and short-circuit current when added in vitro to the luminal surface of isolated rabbit ileal mucosa. As little as 1 ng/ml produced an easily detectable response. Under short-circuit condition, the enterotoxin abolished net Cl- absorption; this change was half that produced by theophylline, which stimulated net secretion. The enterotoxin did not change cyclic AMP concentration but caused large and persistent increases in cyclic GMP concentration. The electrical and nucleotide responses exhibited similar and unusually broad concentration-dependences and maximal effects could not be demonstrated. Theophylline elevated cyclic GMP concentration 3-fold both in the presence and absense of the enterotoxin, suggesting no effect of the toxin on cyclic GMP phosphodiesterase. Guanylate cyclase [GTP pyrophosphatelyase(cyclizing); EC 4.6.1.2] activity in a crude membrane fraction from intestinal epithelial cells was stimulated 7-fold by the enterotoxin. These results suggest that guanylate cyclase stimulation is the basis for the toxin's diarrheagenic effect.     

Cyclic GMP affecting the tracheal nonadrenergic noncholinergic inhibitory system

An association between guanosine 3',5'-monophosphate (cyclic GMP) and the nonadrenergic noncholinergic inhibitory system (NANCIS) has been demonstrated in the isolated bovine tissue (Bowman and Drummond, 1984). In order to investigate this association in the guinea pig trachea, we used cyclic GMP derivatives, guanylate cyclase activators (N-methylhydroxylamine (NMH) and nitroglycerin (NG)] and inhibitors [oxyhemoglobin (HbO2) and methylene blue (MB)]. Under general anesthesia paralysis, the animals were ventilated and hourly injected with atropine (0.2 mg/kg) and propranolol (1 mg/kg). Cervical segment of the trachea was converted to a closed tracheal pouch and then filled with Kreb's solution augmented with atropine (1 microM) and propranolol (3.5 microM). A decrease in the pouch pressure (Pp) reflected NANCIS nerve transmural stimulation (TS)--or drug-induced relaxation. Pharmacological agents were applied intravenously. At 2-11 min after injection, NMH and NG decreased baseline Pp and reduced TS-induced relaxation. NMH, which is more potent than NG in activating particulate guanylate cyclase activity, potentiated the TS-induced relaxation at high frequencies, but NG did not. HBO2 inhibited the TS-induced relaxation at high but not at low frequencies. In contrast, MB inhibited the relaxation at low but not high frequencies. The results suggest that activation of particulate or membrane bound guanylate cyclase potentiates NANCIS-induced decrease in Pp. Therefore, there is a possible association between cyclic GMP and the NANCIS in the guinea pig trachea.     

Ovarian cyclic GMP concentration and guanylate cyclase activity in immature rats after treatment with pregnant mare serum gonadotrophin

We have previously reported that the LH-induced decrease in the concentration of ovarian cyclic GMP (cGMP) in the rabbit was accompanied by a drop in ovarian guanylate cyclase activity. The present experiments were carried out to see if the increase in cGMP concentration that occurs in immature rat ovaries after stimulation with pregnant mare serum gonadotrophin (PMSG) is also accompanied by changes in guanylate cyclase activity. Total ovarian cGMP, along with ovarian weight, was found to be increased at 16 h after PMSG treatment. Ovarian concentrations of cGMP, however, increased only after that period (at 20, 24 and 48 h) and the increase was progressive. Guanylate cyclase activity was found in both the cytosol and 100 000 g particulate fractions of the immature rat ovaries. Forty-three hours after PMSG treatment, activity in the particulate fraction was found to be significantly increased. This increase in guanylate cyclase activity was also found at 20 h but not at 16 h. Thus, the increase in ovarian cGMP concentration in immature rats after PMSG treatment was accompanied by increased guanylate cyclase activity.