Inhibitory effects of mepacrine and eicosatetraynoic acid on cyclic GMP elevations caused by calcium and hormonal factors in rat ductus deferens

Leukotriene D4 receptors are not negatively coupled to adenylyl cyclase in guinea-pig lung parenchyma

1. The possibility that receptors for the peptide-containing leukotrienes may be negatively coupled to adenylyl cyclase in guinea-pig lung parenchyma was investigated by comparing the effect of leukotriene D4 (LTD4) on the intracellular cyclic nucleotide (cyclic AMP and cyclic GMP) content and on the activity of cyclic AMP-dependent protein kinase (PKA). In addition, the potential association between changes in the cyclic nucleotide content and the ability of LTD4 to increase lung parenchymal tone was also evaluated. 2. Non-cumulative challenge of parenchymal lung strips with LTD4 elicited concentration-dependent contractions (pD2 = 8.23) that were paralleled by concentration-related increases in the intracellular level of cyclic AMP and cyclic GMP, and in the activation state of PKA (Kact = 33 nM). Temporally, these biochemical effects of LTD4 were transient, peaking after approximately 5 min drug contact thereafter decaying, despite the continued generation of tone. Both the biochemical and mechanical effects of LTD4 were antagonized by the LTD4-receptor blocking drug, ICI 198,615 (1 microM for 60 min), indicating that they were receptor-mediated events. 3. Challenge of guinea-pig lung with LTD4 (200 nM; EC100 for tension generation) stimulated a 150 and 70 fold increase in the elaboration of thromboxane B2 (TXB2) and 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) respectively, relative to that generated spontaneously. 4. Pretreatment of lung strips for 60 min with an irreversible inhibitor of cyclo-oxygenase, flurbiprofen,at a concentration (8 microM) that abolished both basal and LTD4 (200 nM)-induced TXB2 and 6-keto-PGF1alpha release, relaxed rapidly the spontaneous tone of the tissues, reduced the cyclic AMP content by ~50%and lowered the PKA activity ratio from 29% to 17%. In addition, flurbiprofen abolished the ability of LTD4 (200 nM) to increase the cyclic AMP content and to activate PKA. Functionally, the magnitude of LTD4 (200 nM)-induced tone and the increase in cyclic GMP content were attenuated by approximately 20% and 50% respectively in flurbiprofen-treated tissues.5. In flurbiprofen-treated tissues, isoprenaline (10 microM for 10 min) increased the cyclic AMP content(from 4 to 27 pmol mg-1 protein) and activated PKA (from 15% to 26%). Preincubation (30 s or 5 min)of lung with LTD4 (200 nM) did not inhibit (or enhance) these isoprenaline-induced effects.6. Pretreatment of lung strips for 60 min with the thromboxane synthetase inhibitor, dazmegrel (10 microM),relaxed the spontaneous tone of the tissues, abolished the LTD4 (200 nM)-stimulated release of TXB2 and significantly enhanced (~two fold) the elaboration of 6-keto-PGF1alpha. In addition, dazmegrel attenuated (by ~50%) LTD4 (200 nM)-induced cyclic GMP accumulation but approximately doubled both the cyclic AMP content and PKA activity ratio. LTD4-induced contractions, in contrast, were not affected by dazmegrel.7. EP 092 (1 microM for 60 min), a selective TP-receptor blocking drug, had no effect on spontaneous tone,eicosanoid formation or on the cyclic GMP content of guinea-pig lung parenchymal strips. Likewise,EP 092 exerted no significant mechancial effect in lung challenged with LTD4 (200 nM) although it did potentiate, to a small extent, the ability of LTD4 (200 nM) to increase the cyclic AMP content.8. It is concluded that LTD4 can increase the intracellular level of cyclic AMP in guinea-pig parenchyma and activate PKA by a leukotriene-receptor-mediated mechanism sensitive to ICI 198,615. However,these biochemical actions of LTD4 are induced indirectly by an arachidonic acid-derived cyclo-oxygenase product(s) other than TXA2. Thus, contrary to reports of other investigators, no evidence was found to corroborate the finding that stimulation of leukotriene receptors on guinea-pig lung parenchyma results in a rapid lowering of the cyclic AMP content even in cyclo-oxygenase-blocked tissues. These data,therefore, do not support the hypothesis that leukotriene-induced tension generation is dependent upon a prior reduction in the cyclic AMP content.

Sequence homologies between guanylyl cyclases and structural analogies to other signal-transducing proteins

The cyclic GMP-forming enzyme guanylyl cyclase exists in cytosolic and in membrane-bound forms differing in structure and regulations. Determination of the primary structures of the guanylyl cyclases revealed that the cytosolic enzyme form consists of two similar subunits and that membrane-bound guanylyl cyclases represent enzyme forms in which the catalytic part is located in an intracellular, C-terminal domain and is regulated by an extracellular, N-terminal receptor domain. A domain of 250 amino acids conserved in all guanylyl cyclases appears to be required for the formation of cyclic nucleotide, as this homologous domain is also found in the cytosolic regions of the adenylyl cyclase. The general structures of guanylyl cyclases shows similarities with other signal transducing enzymes such as protein-tyrosine phosphatases and protein-tyrosine kinases, which also exist in cytosolic and receptor-linked forms.

Contractile action of heat-labile toxin of Bordetella parapertussis on aortic smooth muscles of pigs

Using both vascular smooth muscle strips (VSMS) and cultured cells (VSMC) from aortas of pigs, the contractile action of Bordetella heat-labile toxin (HLT) purified from B. parapertussis was studied in an attempt to elucidate the mechanisms of its action. HLT induced contractions in VSMC in parallel with the increase of Ca2+-influx. The HLT-induced Ca2+-influx and contraction were not influenced by verapamil or diltiazem, though a certain extension of the lag period was seen. The contractile action of HLT on VSMS and VSMC was not influenced either by diltiazem or quinacrine; that on VSMC was not influenced by prednisolone, indomethacin, aspirin, CV-3988, FPL-55712, ruthenium red, or TEAC. On VSMS, prednisolone caused the extension of lag period following HLT exposure. The action of HLT on VSMS was inhibited by TMB-8, whereas that on VSMC was not though the extension of lag period was seen. The HLT-induced contraction in both VSMS and VSMC was completely inhibited by H-7. The contraction in VSMS, but not in VSMC, was inhibited by H-8. HLT did not induce specific activation of the protein kinases in VSMC. The addition of cGMP or cAMP brought about relaxation in the HLT-exposed VSMS contracting in maximum. HLT caused a significant increase in permeability of VSMC membrane to trypan blue, accompanied with contraction. Both HLT-induced contraction and increase in permeability were inhibited by dextran of M.W. 8,000, but not of M.W. 5,000. These results suggested that HLT acted on vascular smooth muscle cells by damaging the membrane permeability, but not by disturbing the known cascades or systems for physiological contractions, resulting in the increase in Ca2+-influx and then contractions.

Effect of a stimulant of guanylate cyclase, sin 1, on calcium movements and phospholipase C activation in thrombin-stimulated human platelets

The effects of sin 1, a metabolite of an antianginal agent, molsidomine, were investigated on human platelet activation induced by thrombin. This drug promoted a slight inhibition of serotonin release in a medium containing 1 mM Ca2+ or 1 mM EGTA (from 63% to 46% and from 57% to 41% of total serotonin secretion, respectively, with the highest dose used). Under these conditions, Ca2+ movements, monitored by quin 2 fluorescence, were markedly impaired. The most pronounced effect was towards Ca2+ influx, which presented a rapid inhibition with low doses. In the presence of external calcium, thrombin raised cytoplasmic free Ca2+ concentration from 100 nM to 1277 nM. This was reduced to 466 nM and 175 nM with 10(-7) M and 10(-4) M sin 1, respectively. Ca2+ mobilization from internal stores was less inhibited, since cytoplasmic free Ca2+ movements, sin 1 was tested on [32P] phosphatidic acid synthesis resulting from phospholipase C activation induced by thrombin. Phosphatidic acid labelling displayed a maximal inhibition of 43-50% with the highest doses of sin 1 (10(-4) M-10(-3) M) with or without Ca2+ in the incubation medium. However, this effect appeared much more sensitive to sin 1 in the presence of external Ca2+ (25% at 10(-7) M sin 1 with external Ca2+ against 12% at the same sin 1 concentration with EGTA). This discrepancy might be explained by the difference of cGMP level obtained when platelets were treated by sin 1 in the presence or in the absence of Ca2+ in the medium. This study shows that the major target of sin 1 via cGMP is not platelet phospholipase C as previously described, but inhibition of Ca2+ influx through plasma membrane.

Blockade of receptor-mediated cyclic GMP formation by hydroxyeicosatetraenoic acid

Receptor-mediated cyclic GMP formation in N1E-115 murine neuroblastoma cells appears to involve oxidative metabolism of arachidonic acid. Evidence in support of this includes the blockade of this response by lipoxygenase inhibitors, e.g., eicosatetraynoic acid (ETYA) or other metabolic perturbants, e.g., methylene blue. It was recently discovered that the lipoxygenase products 15-hydroxyeicosatetraenoic (15-HETE) acid and 12-HETE, like ETYA, were inhibitors of M1 muscarinic receptor-mediated cyclic GMP formation. In the present report, the effects of monoHETEs are explored in more detail, particularly with regard to the function of the muscarinic receptor. Like 12-HETE and 15-HETE (IC50 = 13 and 11 microM, respectively), 5-HETE inhibited the cyclic GMP response to the muscarinic receptor (IC50 = 10 microM). All three of these monoHETEs were shown also to be inhibitors of the cyclic GMP responses to receptors stimulated by carbachol, histamine, thrombin, neurotensin, and bradykinin. 15-HETE was shown to inhibit the muscarinic receptor-mediated response in a complex manner (apparent noncompetitive and uncompetitive components; IC50 = 18 and 2 microM, respectively). 15-HETE did not inhibit either the M1 muscarinic receptor-stimulated release of [3H]inositol phosphates from cellular phospholipids or the M2 muscarinic receptor-mediated inhibition of hormone (prostaglandin E1)-induced AMP formation. It seemed possible that the monoHETEs could enter into biochemical pathways for arachidonate in N1E-115 cells. [3H]Arachidonate and the three [3H]-monoHETEs all rapidly labeled the membrane lipids of intact N1E-115 cells, with each [3H]eicosanoid producing a unique labeling profile. [3H]15-HETE labeling was noteworthy in that 85% of the label found in the phospholipids was in phosphatidylinositol (PI;t1/2 to steady state = 3 min). Exogenous 15-HETE inhibited the labeling of PI by [3H]arachidonate (IC50 = 28 microM) and elevated unesterified [3H]arachidonate levels. Thus, the mechanism of blockade of receptor-mediated cyclic GMP responses by monoHETEs is likely to be more complex than the simple inhibition of cytosolic mechanisms, e.g., generation of a putative second messenger by lipoxygenase, and may involve also alterations of membrane function accompanying the redistributions of esterified arachidonate.

Induction of prostacyclin biosynthesis is closely associated with increased guanosine 3',5'-cyclic monophosphate accumulation in cultured human endothelium

Stimuli of prostacyclin (PGI2) biosynthesis such as thrombin, bradykinin, histamine, and A23187 increase guanosine 3',5'-cyclic monophosphate (cyclic GMP) levels in primary monolayer cultures of human umbilical vein endothelium by about twofold. This effect is dependent on the presence of extracellular Ca2+. Increases of about tenfold are observed when cyclic GMP phosphodiesterase activity is inhibited, which suggests that the observed increases in cyclic GMP involve the activation of guanylate cyclase. Activation of guanylate cyclase appears to involve an early event in the induction of PGI2 biosynthesis, as neither arachidonic acid nor its metabolites stimulate cyclic GMP accumulation. Although activators of guanylate cyclase such as atriopeptin III, sodium nitroprusside, and tert-butylhydroperoxide increase cyclic GMP levels by approximately 2-3-fold, they do not stimulate or modulate PGI2 production. We conclude that cyclic GMP does not play a primary role in mediating the induction or regulation of PGI2 biosynthesis in vascular endothelium.

Modulation of guanylate cyclase by lipoxygenase inhibitors

Drugs that inhibit endothelium-dependent relaxation were tested to determine their effect on soluble guanylate cyclase purified from dog aorta. Basal, arachidonic acid (10(-5) M)-stimulated, and nitroprusside (5 X 10(-5) M)-stimulated guanylate cyclase activities were inhibited by methylene blue and the lipoxygenase inhibitors nordihydroguaiaretic acid and eicosatetraynoic acid. The effective inhibitory doses were in the range of those that have been reported to inhibit endothelium-dependent relaxation. Other compounds known to inhibit endothelium-dependent relaxation had little or no effect on guanylate cyclase activity. Basal guanylate cyclase activity was more resistant to inhibition than were activated states of the enzyme. The data suggest that reported inhibition of endothelium-dependent relaxation by some lipoxygenase inhibitors may be the result, at least in part, of their direct effect on guanylate cyclase activity.

See-saw signal processing in pinealocytes involves reciprocal changes in the alpha 1-adrenergic component of the cyclic GMP response and the beta-adrenergic component of the cyclic AMP response

Pineal cyclic AMP and cyclic GMP are regulated by norepinephrine (NE) acting through alpha 1- and beta-adrenoceptors. beta-Adrenergic stimulation appears to be an absolute requirement and alpha 1-adrenergic activation amplifies beta-adrenergic stimulation of the cyclic AMP response 10-fold and the cyclic GMP response 100-fold, respectively. Chronic deprivation of adrenergic stimulation, due to exposure to constant light (LL) or by surgical denervation, enhances the cyclic AMP response and diminishes the cyclic GMP response as compared to control animals in a 10:14 light/dark (LD) cycle. This phenomenon is termed see-saw signal processing. In the current study we find these changes do not reflect shifts in the time course or Ka of these responses. Dose-response studies indicate the beta-adrenergic component of cyclic AMP stimulation is enhanced and the alpha 1-adrenergic component of cyclic GMP stimulation is diminished in LL pinealocytes. Several observations indicate these changes may reflect alterations in Ca2+-sensitive postreceptor mechanisms.

Activation of soluble guanylate cyclase by arachidonic acid and 15-lipoxygenase products

The activity of soluble guanylate cyclase can be increased by exposure of the enzyme to arachidonic acid or to some oxidized metabolites of the fatty acid. We have tried to determine whether activation of the enzyme by arachidonate requires that the fatty acid be converted to an oxidized metabolite, either by a possible trace contaminant of a lipoxygenase or by guanylate cyclase itself, which contains a heme moiety. Soluble guanylate cyclase purified from bovine lung was activated 4-6-fold by arachidonic acid. This activation was not dependent on the presence of oxygen in the incubation medium. No detectable metabolites of arachidonic acid were formed during incubation with soluble guanylate cyclase. Addition of soybean lipoxygenase to the incubation did not increase activation by arachidonic acid. The inhibitors of lipoxygenase activity, nordihydroguaiaretic acid and eicosatetraynoic acid, had direct effects on soluble guanylate cyclase and interfered with its activation by arachidonate, whereas another lipoxygenase inhibitor, BW 755 C, did not. The data suggest that arachidonic acid increases the activity of guanylate cyclase by direct interaction with the enzyme rather than by being converted to an active metabolite.

Characterization of intestinal brush border guanylate cyclase activation by Escherichia coli heat-stable enterotoxin

Intestinal brush border guanylate cyclase was previously reported to be activated by the Escherichia coli enterotoxin (STa). This system was reexamined in order to develop a hypothesis for the mechanism of activation. The extent of activation was previously underestimated, since by using sodium azide to inhibit competing reactions and ethylene glycol bis(beta-aminoethyl ether) N,N-tetraacetic acid to chelate Ca2+, which is inhibitory, maximal activations of 30- to 50-fold were obtained. Ca2+ inhibition was only partially relieved by the calmodulin inhibitor calmidazolium. Inhibitors of the O2-dependent activation of soluble guanylate cyclase had no effect on STa activation; hence, it was concluded that STa activation did not involve arachidonate release and oxidation. STa was able to further increase activity already elevated by the nonionic detergent Lubrol PX. The membrane-active agent filipin, which was previously reported to inhibit both basal and agonist-stimulated adenylate cyclase, did not inhibit STa activation of guanylate cyclase. Digitonin, another cholesterol binder, inhibited STa activation at low concentrations, which disappeared at higher concentrations. Both of these agents stimulated basal activity. Dimethyl sulfoxide produced a concentration-dependent inhibition of STa activation, while increasing basal activity 7-fold. Ethanol inhibited both basal and STa-stimulated activity, with the former being more affected. Benzyl alcohol, like ethanol, a "fluidizer" of cell membranes, also inhibited both basal and activated enzymes. We concluded that STa directly activates this guanylate cyclase and, because of the differential effects of inhibitors on basal and STa-stimulated activity, propose a receptor-mediated mechanism.

Mechanisms of adenosine triphosphate-, thrombin-, and trypsin-induced relaxation of rat thoracic aorta

The mechanisms by which adenosine triphosphate, thrombin, and trypsin cause relaxation of vascular smooth muscle were investigated. Relaxation of the rat thoracic aorta with adenosine triphosphate, thrombin, and/or trypsin was associated with increased levels of cyclic guanosine monophosphate in both time- and concentration-dependent manners. Thrombin and trypsin did not alter cyclic adenosine monophosphate levels, whereas adenosine triphosphate increased cyclic adenosine monophosphate levels after significant relaxation occurred. Removal of the endothelium abolished adenosine triphosphate-, thrombin-, and trypsin-induced relaxation and the associated increased levels of cyclic nucleotides. Relaxation due to these agents was also inhibited by exposure to nordihydroguaiaretic acid, a lipoxygenase inhibitor, and eicosatetraynoic acid, a lipoxygenase and cyclooxygenase inhibitor. Indomethacin, a cyclooxygenase inhibitor, potentiated relaxation to these agents, whereas the increased levels of cyclic nucleotides due to adenosine triphosphate were unaltered. Bromophenacyl bromide, a phospholipase A2 inhibitor, decreased relaxation due to adenosine triphosphate, thrombin, and trypsin and the associated increased levels of cyclic nucleotides. Removal of extracellular calcium, which also presumably inhibits phospholipase A2, prevented the elevated levels of cyclic nucleotides and the inhibitory effects of adenosine triphosphate and trypsin on contraction. In contrast, sodium nitroprusside-induced relaxation and/or increased levels of cyclic guanosine monophosphate were unaltered by nordihydroguaiaretic acid, eicosatetraynoic acid, bromophenacyl bromide, and removal of extracellular calcium. After incubation of intact tissue with 32P-orthophosphate, the patterns of protein phosphorylation caused by adenosine triphosphate, thrombin, and trypsin were indistinguishable from those of acetylcholine, sodium nitroprusside and 8-bromo cyclic guanosine monophosphate. All these agents dephosphorylated myosin light chain. Thus, the present study supports the hypothesis that relaxation induced by adenosine triphosphate, thrombin, and trypsin is mediated through the formation of an endothelial factor which elevates cyclic guanosine monophosphate levels and causes cyclic guanosine monophosphate-dependent protein phosphorylation and dephosphorylation of myosin light chain.

Neurotransmitter receptors mediate cyclic GMP formation by involvement of arachidonic acid and lipoxygenase

Evidence is presented that has led us to abandon the hypothesis that receptor-mediated cyclic GMP formation in cultured nerve cells occurs via the influx of extracellular calcium ions and an increase in the cytosolic free calcium ion concentration. While the cyclic GMP response is absolutely dependent on the presence of Ca2+, there is no increase in free intracellular Ca2+ subsequent to agonist stimulation. Instead, we have found that muscarinic or histamine H1 receptor stimulation elicits the release of arachidonic acid through a quinacrine-sensitive mechanism, possibly phospholipase A2. Inhibition of the release or metabolism of arachidonate by the lipoxygenase pathway prevents receptor-mediated cyclic GMP formation. We hypothesize that neurotransmitter receptors that mediate cyclic GMP synthesis function by releasing arachidonic acid and that an oxidative metabolite of arachidonic acid then stimulates soluble guanylate cyclase.

Effects of fatty acids on activity of cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver

Effects of fatty acids, prostaglandins, and phospholipids on the activity of purified cGMP-stimulated cyclic nucleotide phosphodiesterase from calf liver were investigated. Prostaglandins A2, E1, E2, F1 alpha, and F2 alpha, thromboxane B2, and most phospholipids were without effect; lysophosphatidylcholine was a potent inhibitor. Several saturated fatty acids (carbon chain length 14-24), at concentrations up to 1 mM, had little or no effect on hydrolysis of 0.5 microM [3H]cGMP or 0.5 microM [3H]cAMP with or without 1 microM cGMP. In general, unsaturated fatty acids were inhibitory, except for myristoleic and palmitoleic acids which increased hydrolysis of 0.5 microM [3H]cAMP. The extent of inhibition by cis-isomers correlated with the number of double bonds. Increasing concentrations of palmitoleic acid from 10 to 100 microM increased hydrolysis of [3H]cAMP with maximal activation (60%) at 100 microM; higher concentrations were inhibitory. Palmitoleic acid inhibited cGMP hydrolysis and cGMP-stimulated cAMP hydrolysis with IC50 values of 110 and 75 microM, respectively. Inhibitory effects of palmitoleic acid were completely or partially prevented by equimolar alpha-tocopherol. Palmitelaidic acid, the trans isomer, had only slightly inhibitory effects. The effects of palmitoleic acid (100 microM) were dependent on substrate concentration. Activation was maximal with 1 microM [3H]cAMP and was reduced with increasing substrate; with greater than 10 microM cAMP, palmitoleic had no effect. Inhibition of cGMP hydrolysis was maximal at 2.5 microM cGMP and was reduced with increasing cGMP; at greater than 100 microM cGMP palmitoleic acid increased hydrolysis slightly. Palmitoleic acid did not affect apparent Km or Vmax for cAMP hydrolysis, but increased the apparent Km (from 17 to 60 microM) and Vmax for cGMP hydrolysis with little or no effect on the Hill coefficient for either substrate. These results suggest that certain hydrophobic domains play an important role in modifying the catalytic specificity of the cGMP-stimulated phosphodiesterase for cAMP and cGMP.

Inhibition of agonist-induced degradation of muscarinic cholinergic receptor by quinacrine and tetracaine--possible involvement of phospholipase A2 in receptor degradation

Muscarinic cholinergic receptors (mAChR) are degraded on the addition of agonists through energy- and temperature-dependent processes, probably with clustering and endocytosis. Pretreatment of guinea-pig vas deferens with 0.5 mM quinacrine or 5 mM tetracaine, phospholipase A2 (PLase A2) inhibitors, inhibited the ACh-induced degradation of mAChR in the smooth muscle and kept mAChR on the surface membrane, while cocaine and procaine were not effective. On pretreatment with quinacrine or tetracaine the PLase A2 activity in the smooth muscle decreased continuously during culture without change in the contractile response of the tissue. Pretreatment with cocaine and procaine which had no significant effect on the down regulation of mAChR did not inhibit PLase A2 activity. However, activation of PLase A2 by long-term culture of the muscle with ACh and formation of endogenous inhibitor of PLase A2 were not observed under our experimental conditions. The participation of PLase A2 in the agonist-induced degradation of mAChR is discussed in the light of these findings.

Calmodulin, phospholipase, and exocytosis. p-Bromophenacyl bromide inhibits but mepacrine stimulates secretion in rat mast cells

p-Bromophenacyl bromide (10-50 microM), a reagent that reacts with and inhibits phospholipase A2, completely suppressed exocytotic responses in rat serosal mast cells elicited by A23187. In contrast, mepacrine (0.01-0.1 mM), an antimalarial drug which has likewise been reported to inhibit phospholipase and additionally calmodulin, did not inhibit exocytosis elicited by concanavalin A (con A) or ionophore A23187. Rather, mepacrine (0.1-0.5 mM) alone increased histamine release up to 70%. Light microscopic observations following ruthenium red staining demonstrated the mepacrine-induced response to be exocytosis. Like the response to compound 48/80, that elicited by mepacrine was maximal within 40 s and was inhibited by phosphatidylserine. In the absence of extracellular calcium, secretion elicited by mepacrine and by con A was inhibited by 87 and 90%, respectively, whereas that elicited by 48/80 was unaffected. Incubation of mast cells in the presence of 2 mM EDTA for 2 h inhibited responses to 48/809 by 76% and nearly abolished those to mepacrine and to con A. 5,8,11,14-Eicosatetraynoic acid (50 and 100 microM) inhibited secretion elicited by con A but not that evoked by mepacrine or polymyxin B. The phenothiazines thioridazine (10-50 microM) and chlorpromazine (25-125 microM) inhibited secretory responses to mepacrine, to 48/80 and to con A. Both phenothiazines inhibited secretion elicited by con A more effectively than that elicited by mepacrine or 48/80. The results indicate that mepacrine, like con A, elicits exocytosis in mast cells by drawing on extracellular sources of calcium, but the early events of secretion initiated by these two secretagogues are pharmacologically distinct.(ABSTRACT TRUNCATED AT 250 WORDS)

Thrombin stimulation of guanosine 3',5'-monophosphate formation in murine neuroblastoma cells (clone N1E-115)

Thrombin, the central regulatory enzyme in coagulation, when incubated in nanomolar concentrations with murine neuroblastoma cells produced a rapid and marked increase in tritiated guanosine 3',5'-monophosphate (cyclic GMP) formation that was blocked by hirudin and competitively antagonized by dansylarginine N-(3-ethyl-1,5-pentanediyl)amide. Diisopropylphosphofluoridate-inactivated thrombin as well as the serine protease trypsin were markedly less potent and less effective than alpha-thrombin in producing this effect. Thrombin-stimulated cyclic GMP formation was inhibited by mepacrine and nordihydroguaiaretic acid but unaffected by indomethacin, suggesting that lipoxygenase metabolites of arachidonic acid are involved in the response. These results suggest that a thrombin-like protease in the brain may be involved with the function of neurons or that thrombin interactions with nerve cells, such as those following cerebral hemorrhage or other trauma of the central nervous system, may be important in the subsequent neuropathology.

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.

Soluble guanylate cyclase activation by nitric oxide and its reversal. Involvement of sulfhydryl group oxidation and reduction

Pre-incubation of either crude or purified nitric oxide-stimulated soluble lung guanylate cyclase resulted in a temperature-dependent decay of enzyme activity. The decay of nitric oxide-stimulated activity during pre-incubation was associated with a reduced responsiveness of the enzyme to reactivation by a second exposure to nitric oxide. This loss of enzyme responsiveness to reactivation by nitric oxide was greater with purified guanylate cyclase than with the crude enzyme and was highly dependent upon the nitric oxide dose. The addition of dithiothreitol or other thiols to nitric oxide-stimulated enzyme markedly accelerated the decay of activity in a dose-dependent manner. In addition, thiols prevented the loss of responsiveness of guanylate cyclase to reactivation by nitric oxide. Nitric oxide-stimulated enzyme activity was, therefore, reversed by the addition of thiol reducing agents. The addition of the thiol oxidizing agents, diamide or oxidized glutathione, to nitric oxide-stimulated guanylate cyclase caused a rapid and irreversible loss of activity. The effects of diamide or oxidized glutathione on the crude enzyme were prevented by excess dithiothreitol. Dithiothreitol did not prevent the destruction of purified nitric oxide-stimulated guanylate cyclase activity by diamide or oxidized glutathione, however. The results suggest that nitric oxide activation and its reversal are linked to the reversible oxidation and reduction, respectively, of sulfhydryl groups on guanylate cyclase which are involved in enzyme activation. The results further suggest the existence of a second class of sulfhydryl groups involved in the maintenance of enzyme activity.

Contribution of lipoxygenase and cyclooxygenase metabolites in the modulation of cyclic nucleotides in the guinea pig myometrium. Differential effects of carbachol and ionophore A23187

Accumulation of cyclic GMP in estradiol-treated immature guinea pig myometrium was enhanced by carbachol, ionophore A23187, unsaturated fatty acids and their hydroperoxides. Cyclic AMP content was elevated only by arachidonic acid, A23187 and PGI2. Eicosatetraynoic acid (TYA), but not indomethacin prevented all cyclic GMP responses. The effects of A23187 and arachidonate on cyclic AMP were accompanied by a parallel increase (2-3 fold) in the generation of PGI2 by the myometrium. Both events were similarly reduced by indomethacin, TYA, 15-hydroperoxyarachidonic acid and tranylcypromine, suggesting that PGI2 was involved. Omission of Ca2+ or addition of mepacrine or p-bromophenacylbromide abolished the stimulatory effects of A23187 and carbachol on cyclic GMP as well as the A23187-induced elevations in both PGI2 and cyclic AMP generation. Thus, with both exogenous arachidonate as well as with endogenous fatty acid, released through an apparent phospholipase A2-induced activation process, the lipoxygenase pathway was associated with an activation of the cyclic GMP system and the cyclooxygenase pathway, via PGI2 generation, with an activation of the cyclic AMP system. Carbachol failed to alter both cyclic AMP content and the release of PGI2 suggesting a cholinergic receptor-mediated fatty acid release process, selectively coupled to the lipoxygenase route.

Sulfhydryl group involvement in the modulation of guanosine 3',5'-monophosphate metabolism by nitric oxide, norepinephrine, pyruvate and t-butyl hydroperoxide in minced rat lung

The cyclic GMP content of rat lung mice was increased nearly 50-fold within 4 sec following exposure to nitric oxide. This rapid increase in cyclic GMP accumulation was prevented by 10 mM, but not 1 mM, dithiothreitol which itself caused a slower yet massive (100-fold) increase in the cyclic GMP content of lung mince. Tissue cyclic GMP following nitric oxide exposure declined rapidly even in the presence of the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine. The decline in cyclic GMP was accelerated by the thiol oxidant diamide (1 mM). The cyclic GMP content of lung mince was also increased by norepinephrine, pyruvate and t-butyl hydroperoxide. Diamide blocked cyclic GMP accumulation in response to these other agents as well as that caused by nitric oxide or dithiothreitol. The results suggest that sulfhydryl group modification may be a common pathway for the enhancement of cyclic GMP synthesis in tissues by a variety of stimuli.

Effects of intravenous methylprednisolone on spinal cord lipid peroxidation and Na+ + K+)-ATPase activity. Dose-response analysis during 1st hour after contusion injury in the cat

✓ The present study was undertaken to examine the ability of a single large intravenous dose of methylprednisolone (15, 30, or 60 mg/kg) to attenuate lipid peroxidation and enhance (Na+ + K+)-ATPase activity during the 1st hour after a 400 gm-cm injury to the cat spinal cord. The contusion injury was associated with a rise in the concentration of fluorescent lipid peroxy products in the injured segment at 1 hour. In addition, the accumulation of cyclic guanosine 3″,5″-monophosphate (cyclic GMP), which was used as a new index of injury-induced free radical reactions, in the injured spinal segment was twice control levels. The injury-induced increase in fluorescence and cyclic GMP content in the contused spinal segment at 1 hour was completely prevented by the administration of 15 or 30 mg/kg of methylprednisolone at 30 minutes after injury. A 60-mg/kg dose, however, did not prevent the elevation in cyclic GMP. A concomitant examination of the acute effects of glucocorticoid administration on (Na+ + K+)-ATPase activity in the injured cord revealed a striking increase of enzyme activity after the 30-mg/kg dose, but a depression in activity with the 60-mg/kg dose. These results demonstrate that a single massive dose of methylprednisolone can beneficially reduce free-radical reactions and lipid peroxidation as well as enhance the activity of neuronal (Na+ + K+)-ATPase during the early phase after spinal cord contusion. The requirement for doses to be in the range of 15 to 30 mg/kg in order to produce these neurochemical changes is consistent with other studies that have demonstrated significantly greater recovery and tissue preservation in spinal cord-injured animals treated with comparable doses of methylprednisolone soon after injury. These findings suggest the need for a rigorous approach to glucocorticoid therapy in central nervous system trauma.

Inhibition of Escherichia coli heat-stable enterotoxin effects on intestinal guanylate cyclase and fluid secretion by quinacrine

Enterotoxigenic Escherichia coli may produce a heat-stable enterotoxin (ST) that causes diarrheal disease in humans and in animals ST activates particulate guanylate cyclase in intestinal mucosal cells and causes intestinal fluid secretion. In this study, we examined the effects of quinacrine on ST activation of guanylate cyclase and ST-mediated intestinal fluid secretion. Quinacrine significantly reduced ST activation of particulate guanylate cyclase in rat intestinal tissue. Additionally, quinacrine reduced ST-mediated fluid secretion in a rat intestinal loop assay (P less than 0.05). In the suckling mouse model, subcutaneous quinacrine (0.1 mumole/mouse) reduced ST-induced fluid secretion at a submaximally effective dose of the toxin, but it did not reduce ST-mediated fluid secretion at a near maximally effective dose. Quinacrine (0.1 mumole/mouse) did not significantly reduce intestinal fluid secretion induced by the analog of cyclic GMP, 8-bromo cyclic GMP. However, at a higher concentration of quinacrine (1 mumole/mouse), significant inhibition of 8-bromo cyclic GMP-induced secretion was observed. Inhibition by the antimalarial agent quinacrine of ST-induced fluid secretion, by a block prior to guanylate cyclase activation, suggests a possible role for a phospholipase early in the sequence of events of ST activation of guanylate cyclase. The results suggest that ST may activate membrane phospholipases prior to ST activation of guanylate cyclase.

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.

Mepacrine inhibition of bradykinin-induced contractions of the rabbit ear vein

Bradykinin induced concentration-dependent contractions in spiral strip preparations of the central vein of rabbit's ears. These contractions require the presence of Ca2+ in the extracellular medium. Mepacrine (2.1 to 42.3 mumol/l) significantly reduced these contractions. Since indomethacin (28 mumol/l) did not reduce the contractions produced by bradykinin, the effect of mepacrine is not explained by inhibition of prostaglandin synthesis. Procaine (10(-7) to 10(-4) mol/l) did not reduce the contractions produced by bradykinin and thus is does not seem that mepacrine acts through a non-specific, local anaesthetic effect. Mepacrine reduced the phosphatidate-mediated transfer of 45Ca from an aqueous phase to an organic phase. It has recently been proposed that phosphatidate may be the Ca2+ ionophore in smooth muscle. However, in the present experiments procaine was about as potent as mepacrine in inhibiting phosphatidate-mediated 45Ca transfer, and since it did not reduce the contractile responses these results do not support the concept that mepacrine inhibits contractions by reducing the Ca2+ influx via a phosphatidate ionophore.

Increase of cGMP and accumulation of 45Ca2+ evoked by drugs acting on sodium or potassium channels

Cyclic guanosine monophosphate (cGMP) and 45Ca2+ accumulation were measured in mouse cerebellar slices after treatment with compounds known to affect ion channels in excitable membranes. Scorpion toxin and sea anemone toxin II raise cGMP and 45Ca2+ contents. Both toxins are known to keep the activated sodium channel open. 4-Aminopyridine and tetraethylammonium, which block potassium conductance, also increase cGMP and promote dose-dependently the 45Ca2+ accumulation. The effects of these 4 drugs but not the effects of depolarizing K+-concentrations on the accumulation of cGMP and 45Ca2+ are inhibited by tetrodotoxin. The cyclic GMP content of non-excitable cells, such as isolated hepatocytes from rat and guinea pig, is not affected in a comparable manner. We conclude that in excitable cells, sodium influx triggers an increase of intracellular free calcium and in that way a rise of cGMP. This effect is independent of the ion channel primarily affected. Conversely, the concentration of cGMP might serve as an indicator of intracellular free calcium.

Relationship between contraction and cyclic GMP levels of guinea pig taenia coli

The relationship between isometric tension and the increase in cyclic GMP level associated with the initial phase of drug-induced contraction of taenia coli was studied. Maximal contraction induced by 100 micrometers carbachol or 124 mM KCl occurred in 30 sec and was associated with an increase in cyclic GMP levels of at least ten fold. However, 100 micrometers serotonin or low concentrations of carbachol (30 microM) produced nearly maximal contraction but only 10--20% of the maximal elevation in cyclic GMP induced by high concentrations of KCl. Furthermore, in the presence of a low concentration of Ca2+, or verapamil, both 124 mM KCl and 100 micrometers carbachol increased cyclic GMP with little elevation in isometric tension. Thus, large contractions were obtained with only small increases in cyclic GMP, and under other conditions, large increases in cyclic GMP were obtained with little increase in isometric tension. The results suggest that the initial increase in cyclic GMP level caused by these agents is not directly dependent on Ca2+ in the cellular pool containing the contractile proteins.

Alpha-adrenoceptor-mediated modulation of 5-hydroxytryptamine release from rat brain cortex slices

Slices of rat brain cortex preincubated with 3H-5-hydroxytryptamine were superfused with physiological salt solution and stimulated electrically, or they were superfused with Ca2+-free solution containing 25 mM K+ and stimulated by introduction of 1.3 mM CaCl2 for 2 min. 1. The electrically evoked 3H overflow was decreased by noradrenaline and increased by phentolamine in a concentration-dependent manner. 2. Phentolamine caused a parallel shift to the right of the concentration-response curve of noradrenaline for its inhibitory effect on impulse-evoked 3H overflow; by contrast, it left the inhibitory effect of unlabelled 5-hydroxytryptamine on the electrically evoked 3H overflow unaffected. 3. Propranolol did not alter the inhibitory effect of noradrenaline on impulse-induced 3H overflow. 4. The increasing effect of phentolamine on the electrically evoked 3H overflow was not modified by paroxetine. 5. Cocaine (at a concentration which almost completely blocks the uptake of 3H-noradrenaline but only partially that of 3H-5-hydroxytryptamine into cortex slices) decreased impulse-evoked 3H overflow. This effect was abolished by phentolamine. 6. In the presence of tetrodotoxin throughout superfusion, the Ca2+-evoked 3H-overflow from slices superfused with Ca2+-free solution was inhibited by noradrenaline and increased by phentolamine. These findings suggest that the terminal serotoninergic fibers of rat brain cortex possess alpha-adrenoceptors, activation of which by exogenous or endogenous noradrenaline leads to inhibition of the release of 5-hydroxytryptamine.