Ethanol inhibition of antigen-induced histamine release from guinea pig lung: correlation with intracellular levels of cyclic nucleotides.

The effect of ethanol on histamine release from lungs of sensitized guinea pigs was studied in conjunction with measurements of tissue concentrations of cyclic AMP and cyclic GMP. Addition of antigen $\text{in vitro}$ elicited a rapid increase in cyclic AMP and cyclic GMP and stimulated release of histamine. Ethanol (2%) inhibited antigen-induced release of histamine over 95% and completely inhibited the increase in both cyclic nucleotides. The activity of cyclic AMP-dependent protein kinase was only slightly affected by ethanol.Metiamide blocked the ovalbumin stimulated increase in cyclic AMP but not cyclic GMP. Pyrilamine did not prevent the rise in either cyclic nucleotide. This suggests that the antigen-induced rise in cyclic AMP is an indirect result of histamine released from the tissue. The inability of H₁ and H₂ receptor antagonists to affect antigen-induced elevation of cyclic GMP in sensitized lung fragments suggests that an elevation in cyclic GMP might be either a primary event in the mediator release sequence or secondary to the release of a mediator other than histamine. The ability of ethanol to inhibit mediator release might be due to its capacity to attenuate the antigen-induced elevation of cyclic GMP in sensitized lung.     

Aminopyrine uptake by guinea pig gastric mucosal cells. Mediation by cyclic AMP and interactions among secretagogues

The role of cyclic nucleotides in regulating acid secretion by dispersed mucosal cells from guinea-pig stomach was examined by measuring first the ability of histamine and carbachol to stimulate [dimethylamine-14C]aminopyrine uptake and cyclic nucleotide metabolism and secondly, the effect of exogenous cyclic nucleotides on basal and stimulated [14C]aminopyrine uptake. The [14C]aminopyrine was found in an acidic, osmotically sensitive compartment, probably associated with the initial steps in acid secretion by these cells. Although histamine increased [14C]aminopyrine uptake and cyclic AMP synthesis as expected, histamine was approx. 10-fold more potent in inducing [14C]aminopyrine uptake. This dissociation of [14C]aminopyrine uptake and cyclic AMP metabolism process was further manifested by the observation that prostaglandin E1 failed to increase [14C]aminopyrine uptake, although it did cause a rise in cellular cyclic AMP. Furthermore, prostaglandin E1 did not alter the [14C]-aminopyrine uptake caused by histamine. Carbachol was found to increase the [14C]aminopyrine uptake and also to potentiate the ability of histamine to increase [14C]aminopyrine uptake. Carbachol, however, affected neither the histamine-induced increase in cyclic AMP nor the binding of [3H]histamine to the cells. Cimetidine, a histamine H2 receptor antagonist, blocked the [14C]aminopyrine uptake induced either by histamine alone or by the potentiating combination of histamine plus carbachol. These results suggest that cyclic AMP is mediating the action of histamine on [14C]aminopyrine uptake but changes in cyclic AMP per se are not necessarily the cause for the potentiated increase in [14C]aminopyrine uptake. Furthermore, the potentiated response observed with histamine plus carbachol on [14C]aminopyrine uptake occurs at a biochemical step distal to and not obviously related to cyclic AMP generation.     

Cellular cyclic AMP in dispersed mucosal cells from guinea pig stomach.

In dispersed mucosal cells from guinea pig stomach cyclic AMP was increased 4-fold by theophylline, 5-fold by prostaglandin E2, and 10- to 15-fold by histamine. Theophylline augmented the increase in cellular cyclic AMP caused by histamine or prostaglandin E1 and the actions of histamine and prostaglandin E1 were additive. Cellular cyclic AMP was not altered by carbachol, gastrin, secretin, vasoactive intestinal peptide, glucagon, insulin or the octapeptide of cholecystokinin. Metiamide or diphenhydramine but not atropine inhibited the increase in cellular cyclic AMP caused by histamine, but did not alter the concentration of cyclic AMP in control cells or in cells incubated with theophylline or prostaglandin E1.     

Changes in cellular levels of cyclic AMP in rat mast cells during secretion of histamine induced by immunoglobulin E decapeptide and ACTH(1–24) peptide: Comparison with immunological and ionophore triggers

The role of cyclic AMP in the secretory mechanism of mast cells has been investigated by comparing the time course of changes in cellular levels of this cyclic nucleotide with the kinetics of secretion induced by basic peptides, antigen, anti-IgE and calcium ionophore. ACTH(1–24) peptide and a synthetic decapeptide representative of the sequence 497–506 within the Cε4 domain of human IgE induced a transient rise in cyclic AMP which reached approx. 150% of the resting levels by 10 s. Peptide-induced secretion of histamine was also rapid, reaching a maximum after 5–10 s. Immunological triggering of mast cells with antigen and anti-IgE raised levels of cyclic AMP to 150% of resting levels within 15 s, accompanying secretion of histamine which reached a maximum after 30 s. A relatively slower release of histamine induced by the calcium ionophore A23187 was paralleled by a significant reduction in cyclic AMP to 50% of the resting levels after 300 s. These data suggest a relationship between the accumulation of cyclic AMP in mast cells and secretion of histamine mediated by the Cε4 decapeptide and the ACTH(1–24) peptide as well as by IgE-dependent mechanisms. However, the simultaneous increase in cyclic AMP and secretion of histamine suggests that the two events may not be causally related.     

Role of cyclic AMP during histamine release. Histamine release is not directly related to increase in cyclic AMP levels in rat mast cells activated by concanavalin A, anti-IgE, antigen, prostaglandin D2 and isoproterenol

Activation of mast cells by bridging of IgE-receptors or concanavalin A (Con A) results in a rapid initial rise and fall in cyclic AMP (cAMP) levels followed by a second rise in cAMP levels and histamine release (Sullivan, T. et al. (1976) J. Immunol. 117, 713-716; Lewis, R.A. et al. (1979) J. Immunol. 123, 1663-1668; Ishizaka, T. et al. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 6812-6816). trans-4-Guanidinomethylcyclohexanecarboxylic acid 4-tert-butylphenyl ester (GMCHA-OPhBut), a strong trypsin inhibitor and an anti-allergic agent (Muramatu, M. et al. (1982) Hoppe-Seyler's Z. Physiol. Chem. 363, 203-211; Takei, M. et al. Agents Actions, in press), strongly and dose-dependently inhibited the initial and second rises in cAMP levels, and release of histamine from rat mast cells by Con A, anti-IgE and antigen. Addition of GMCHA-OPhBut after the initial rise in cAMP inhibited the second rise in cAMP and histamine release. These results suggested a possible participation of a trypsin-like proteinase, probably pH 7 tryptase present in rat mast cells, in the activation of adenylate cyclase by the above secretagogues, and the initial rise in cAMP was not directly related to the latter events. The second rise in cAMP is induced by prostaglandin D2 (PGD2), a metabolic product of arachidonic acid. PGD2 elevated the cAMP levels in mast cells whereas no histamine was secreted. GMCHA-OPhBut did not inhibit the increase in cAMP by PGD2. Therefore, the strong inhibitory effect of GMCHA-OPhBut on the second rise in cAMP might depend on the inhibition of an earlier process than the activation of adenylate cyclase by PGD2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neurohormonal receptors and cyclic AMP-binding proteins in rabbit tracheal mucosa-submucosa

Neurohormones and drugs that alter in vitro tracheal electrolyte transport and mucus glycoprotein secretion were examined for their ability to alter cyclic nucleotide accumulation in a smooth muscle-free preparation of rabbit tracheal mucosa-submucosa. cAMP levels were increased by beta-adrenergic agonists, histamine, 2-Cl-adenosine and prostaglandin E1. cGMP levels were increased by carbachol. The phosphodiesterase inhibitor isobutylmethylxanthine increased cAMP and cGMP levels and potentiated only the beta-adrenergic effects. The beta-adrenergic effects were blocked by (+/-)-propranolol and the effects of histamine by diphenhydramine, atropine and (+/-)-propranolol. Atropine blocked the carbachol effects. The isolated surface epithelium from rabbit trachea had higher basal cAMP levels and greater response to beta-adrenergic agonists and isobutylmethylxanthine than the mucosa-submucosa. Two major cAMP-binding proteins in the tracheal mucosa-submucosa were identified with the photoaffinity label 8-N3-[32P]cAMP. Agents that increased cAMP levels also decreased photoaffinity labelling, suggesting that these two cAMP-binding proteins were being occupied in the intact cell. The molecular weights of the proteins were 50 000 and 54 000 and correspond in electrophoretic mobility to the regulatory subunits of Type-I and Type-II cAMP-dependent protein kinases, respectively. The results are consistent with the hypothesis that epithelial functions in the airways are modulated by a number of agonists which increase cyclic nucleotide levels. The effects of beta-adrenergic agonists is apparently mediated by activation of adenylate cyclase and subsequent activation of cAMP-dependent protein kinases.     

Correlation of the biosynthesis of prostaglandin and cyclic AMP in monolayer cultures of rabbit articular chondrocytes

We have utilized ionophores to test whether stimulation of chondrocyte prostaglandin biosynthesis is accompanied by an increase in cyclic nucleotide levels in these cells. Radioimmunoassay of prostaglandin E2, 6-oxo-prostaglandin F1 alpha (the stable metabolite of prostaglandin I2) and prostaglandin F2 alpha showed that synthesis of each was stimulated by the divalent-cation ionophore, A23187 after short-term incubation (1-7 min) in serum-free medium. No stimulation of thromboxane B2 was detected. Two monovalent ionophores, lasalocid and monensin failed to stimulate prostaglandin biosynthesis after short-term incubation. Ionophore A23187-stimulated prostaglandin biosynthesis was variably and partially inhibited by sodium meclofenamate, indomethacin and aspirin, but not by sodium salicylate. Ionophore A23187-stimulated prostaglandin biosynthesis was accompanied by a 7.5-fold increase in cyclic AMP levels after 15 min. Sodium meclofenamate, indomethacin and aspirin which inhibited prostaglandin E2 biosynthesis also reduced cyclic AMP levels. Exogenous prostaglandin E2 (1 microgram/ml) stimulated cyclic AMP biosynthesis, which was not inhibited by aspirin. These results indicated that prostaglandins can be considered as one of the local effectors controlling cyclic AMP production in articular cartilage.     

IV. Immunologic mechanisms for release of chemical mediators of anaphylaxis from human lung tissue

The antigen-induced, IgE-dependent release of chemical mediators from human lung tissue in vitro is modulated by a variety of pharmacologic maneuvers involving alterations in the intracellular levels of cyclic nucleotides. Increase in the level of cyclic AMP inhibits the immunologic release of histamine, SRS-A and ECF-A; β-adrenergic agents, prostaglandins, cholera toxin and methylxanthines all produce accumulations of cAMP in human lung tissue. Depletion of cAMP after α-adrenergic, low-dose prostaglandin and imidazole stimulation is associated with enhancement of the release of mediators. Studies involving purified preparations of rat peritoneal mast cells confirm that alterations in the cAMP levels of a homogeneous population of target cells indeed influence histamine release in a fashion analogous to that of human lung tissue.Furthermore, cholinergic stimuli produce a marked enhancement of the antigen-induced release of mediators from human lung through an apparently independent mechanism, presumably acting through alterations in the tissue concentration of cyclic GMP. This latter observation suggests an important interaction between endogenously released parasympathetic neurohormones and the immunologic release of the chemical mediators of asthma.     

Large potentiation of agonist response in intact cells is produced by increases only in GTP-dependent adenylate cyclase activity

Treatment of cultured SV40-transformed normal rat kidney cells with the drug, 2-pyridine carboxylic acid, results in a pronounced potentiation in the ability of isoproterenol, prostaglandin E1, and cholera toxin to elevate cyclic AMP levels. With isoproterenol, the initial rate of cyclic AMP accumulation and the maximum cyclic AMP attainable are increased, and also the time of maximum cyclic AMP is prolonged. GTP-dependent adenylate cyclase activities are potentiated in crude membranes from the treated cells, but no evidence for alterations in cyclic nucleotide phosphodiesterase or release of cyclic AMP into the medium could be demonstrated. Results show that augmented adenylate cyclase activity alone, without changes in phosphodiesterase, can lead to dramatic alterations in cyclic AMP accumulation in response to cyclase agonists.     

Regulation of human basophil and lung mast cell function by cyclic adenosine monophosphate

Immunologic activation of purified human lung mast cells (HLMC) and basophils with anti-IgE induced histamine release but failed to elicit any changes in cAMP levels. In contrast, histamine release and monophasic rises in cAMP were observed in both rat peritoneal mast cells (RPMC) challenged with concanavalin A (73% enhancement over basal cAMP 20 sec after activation) and a cultured mouse bone marrow-derived mast cell (PT18 cell line) passively sensitized with dinitrophenol-specific IgE and stimulated with antigen (39% increase above basal at 15 sec). The adenylate cyclase activators isoprenaline, prostaglandin E2 (PGE2), and forskolin and the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) all induced elevations in cAMP levels in both basophils and HLMC. In basophils, PGE2 and isoprenaline produced approximately twofold increases in cAMP that were maximal at 1 min and decayed thereafter. Forskolin and IBMX produced threefold increases in cAMP that peaked 10 min after activation and persisted for up to 20 min. In HLMC, isoprenaline provoked a rapid monophasic fourfold increase in cAMP that was maximal at 1 min after addition. Levels of cAMP subsequently declined but remained significantly elevated over resting levels for up to 30 min. PGE2, forskolin, and IBMX all produced approximately threefold rises in HLMC cAMP that peaked around 5 min and persisted for 30 min. In both the basophil and HLMC, agonist-induced elevations in cAMP correlated well with the inhibition of mediator release. In basophils, the order IBMX greater than forskolin greater than PGE2 greater than isoprenaline held for both the inhibition of histamine and leukotriene C4 release and the augmentation of cAMP levels. In HLMC, individual agonists elevated cAMP levels to similar degrees and inhibited the release of histamine, leukotriene C4, and PGD2 to comparable extents, although the release of the arachidonate metabolites was generally more sensitive to the inhibitory actions of these agonists. These results suggest that elevations in cAMP, in both the basophil and HLMC, are associated with the inhibition of mediator release but not the initiation of the secretory process.     

Effects of factors released from eggs and other agents on cyclic nucleotide concentrations of sea urchin spermatozoa.

Factors released from eggs (FRE) of the sea urchin, Strongylocentrotus purpuratus, caused up to 20-fold increases in sperm cyclic AMP levels after a 1-min incubation. Putative cyclic nucleotide phosphodiesterase inhibitors such as theophylline acted in a synergistic manner with FRE to cause even greater increases in sperm cyclic AMP levels. This effect appeared to be specific for egg factors since various hormones (triiodothyronine, norepinephrine, histamine), nucleosides (adenosine, guanosine), nucleophiles (axide), anaesthetics (procaine), ionophores (X537A, A23187), metals (Mn2+) and neurotransmitters (acetylcholine) did not increase sperm cyclic AMP levels. Various mammalian tissue extracts (serum, uterus, adrenal, ovary, lung) also had no effect. We suggest that the activity which elevates the cyclic AMP of sea urchin spermatozoa is specifically associated with sea urchin eggs.     

Role of Ca2+ and cyclic nucleotides in the control of prostaglandin E production in the rat anterior pituitary gland.

In an attempt to relate changes in the intracellular concentration of prostaglandin E to the secretion process, two agents known to increase cyclic nucleotide concentrations and hormone release were added to dispersed rat anterior pituitary cells. They caused increases in teh intracellular prostaglandin E concentrations. Increasing the K+ concentration in the medium (which stimulates hormone release) caused a rapid rise in prostaglandin E concentrations. The addition of the Ca2'onophore A23187 had a similar effect. The effects of changes in the K+ and Ca2+concentrations and the addition of EDTA were measured on the redistribution of radioactivity in pituitary glands prelabelled with [3H]arachidonic acid. Elevated K+ concentrations stimulated the transfer of label to prostaglandins and free arachidonic acid, suggesting an increased phospholipase A activity. On the other hand, the absence of extracellular CaCl2 and the addition of EDTA had the opposite effect, which could be cancelled by the addition of sufficient amounts. of CaCl2. It is concluded that the addition of agents that increase membrane permeability to bivalent cations probably results in an influx of Ca2+ and this appears to result in increased phospholipase A activity, which in turn leads to an increase in prostaglandin production.     

Role of cyclic nucleotides in the long-lasting histaminergic inhibition in the mollusc,Onchidium neuron

1. The effects of histamine and presynaptic nerves on changes in cyclic nucleotide levels in the Onchidium Be-1 neurons were measured under experimental conditions in which they also produced histamine-induced hyperpolarization or histaminergic inhibition (ILD).2. Stimulation by histamine or of presynaptic nerves not only generated the hyperpolarization or ILD in Be-1, but also increased cyclic AMP and GMP levels in the Be-1, and these responses were antagonized by cimetidine.3. However, the rise in these cyclic AMP and GMP levels followed that of the hyperpolarization, though the increased hyperpolarization occurred simultaneously with the elevation of the level of another nucleotide, ATP in Be-1.4. The present results suggest that cyclic AMP is responsible for a positive-feedforward enhancement, and cyclic GMP for a negative-feedforward depression of the hyperpolarization or ILD.     

Effect of endogenous LH secretion on ovarian cyclic AMP and cyclic GMP levels in the rat

Injection of LH (2 and 10 μg) into proestrus rats increased ovarian cyclic AMP levels and concomitantly decreased the levels of cyclic GMP. When injected into diestrus rats, cyclic AMP increases were even greater, whereas cyclic GMP levels were not significantly different from controls receiving saline injections. Ovarian cyclic nucleotide levels were also examined on different days of the cycle. On the afternoon of proestrus (1700 h), the time when circulating levels of LH are at their maximum, the concentration of cyclic AMP showed a moderate but insignificant increase. At the same time, cyclic GMP levels were significantly decreased. An inverse relation between cyclic AMP and cyclic GMP levels was seen on each day of the cycle. When rats were injected with pentobarbital (35 mg/kg) on the afternoon of proestrus (1300 h) to block the LH surge, the expected increases in ovarian cyclic AMP and decreases in cyclic GMP were effectively blocked. These results indicate that ovarian cyclic AMP and cyclic GMP levels are regulated by circulating LH. The apparent differences in direction of nucleotide response to LH, suggest divergent roles for the nucleotides in ovarian function.     

Antagonism of prostaglandin-promoted pituitary cyclic amp accumulation and growth hormone secretion in vitro by 7-oxa-13-prostynoic acid

The studies reported here confirm the previously observed potent stimulus to growth hormone (GH) secretion by prostaglandin E₁ (PGE₁). Proportional increments in GH secretion were observed following $\text{in vitro}$ addition of PGE₁ over a concentration range of 10^?7 to 10^?5 M. Growth hormone secretion could not be further stimulated by higher concentrations of prostaglandin. Prostaglandin E₁ also increased cyclic AMP concentration in the pituitary explants in a proportional fashion, which correlated closely with its potency as a growth hormone secretogogue. In order to define more precisely the mechanism by which prostaglandin acts, the effects of prostaglandin antagonist, 7-oxa-13-prostynoic acid, on GH secretion and cyclic AMP accumulation were investigated. Addition of the antagonist alone had no consistent effects on GH secretion or cyclic AMP levels in the pituitary. However, the antagonist significantly reduced the stimulation of hormone release and cyclic AMP accumulation found following addition of PGE₁. Increasing the concentration of antagonist further diminished prostaglandin stimulated hormone release and nucleotide accumulation. The antagonist failed to block the stimulatory effects of theophylline and dibutyryl cyclic AMP on GH release, indicating that the inhibition observed occurred prior to intracellular accumulation of the cyclic nucleotide. These results are consistent with the hypothesis that a prostaglandin receptor on the pituitary somatotrope is linked to the adenyl cyclase-cyclic AMP system.     

On the role of cyclic AMP and cyclic GMP in steroid production by bovine cortical cells

The time course of corticotropin-induced steroidogenesis and changes in intracellular cyclic AMP and cyclic GMP levels were investigated in isolated bovine adrenocortical cells prepared by trypsin digestion. Corticotropin produced a pea a peak rise in cyclic AMP during the first 5 min of stimulation and enhanced steroid production after 15 min. Corticotropin also caused a decrease in cortical cyclic GMP at 5 min; this decrease in cyclic GMP reverted to a 2–3 fold increase at 15–30 min which gradually subsided by 60 min. A steroidogenic concentration of prostaglandin E₂ also produced an elevation in the levels of both nucleotides, but the rise in cyclic GMP preceded the rise incyclic AMP. These results suggest that the relative amount of cyclic AMP and cyclic GMP, rather than the absolute levels of cyclic AMP, may be a key factor in the regulation of steroidogenesis.     

Temporal relationship of cyclic nucleotide levels and calcium exchange to histamine-induced tension development

The purpose of these experiments was to study the temporal relationship between tension development in incubated guinea pig tracheal smooth muscle and changes in tissue levels of cAMP and cGMP, and isotopic Ca. Dose-response studies were performed with increasing concentrations of histamine both in the absence and presence of H1 receptor blockade using 10(-5) M diphenhydramine. The time course of tension development was subsequently determined in the presence of three concentrations of histamine shown to cause 50% (3 X 10(-6) M), 85% (9 X 10(-6) M), and 100% (5 X 10(-5) M) of maximal contraction. Tissue cyclic nucleotide and 45Ca levels were measured 20 sec, 1 min, and 6 min after the onset of contraction. For comparison, the influence of carbachol was also studied. Our findings demonstrate that there were no detectable alterations in tissue cAMP or cGMP levels during the initial phases of contractile change. In contrast, tissue isotopic Ca uptake increased early in histamine-induced contraction and was blocked by the H1 antagonist.     

On the role of cyclic AMP and cyclic GMP in steroid production by bovine cortical cells.

The time course of corticotropin-induced steroidogenesis and changes in intracellular cyclic AMP and cyclic GMP levels were investigated in isolated bovine adrenocortical cells prepared by trypsin digestion. Corticotropin produced a peak rise in cyclic AMP during the first 5 min of stimulation and enhanced steroid production after 15 min. Corticotropin also caused a decrease in cortical cyclic GMP at 5 min; this decrease in cyclic GMP reverted to a 2-3 fold increase at 15-30 min which gradually subsided by 60 min. A steroidogenic concentration of prostaglandin E2 also produced an elevation in the levels of both nucleotides, but the rise in cyclic GMP preceded the rise in cyclic AMP. These results suggest that the relative amounts of cyclic AMP and cyclic GMP, rather than the absolute levels of cyclic AMP, may be a key factor in the regulation of steroidogenesis.     

Prolonged prostaglandin E1 stimulation of cyclic AMP production in transformed and normal WI-38 fibroblasts.

Long-term (48-hr) incubations of either the fibroblast strain WI-38 or its SV40-transformed counterpart, WI-38-VA13-2RA, in growth medium containing 1 micron prostaglandin E1 (PGE1) resulted in a sustained production and release of cyclic AMP from the cells into the medium. Despite the steady production, intracellular levels of the nucleotide decreased, reaching steady-state values within 4 hr of the initial exposure to PGE1. These values were maintained for the remainder of the 48-hr experimental period. The steady-state levels of intracellular cyclic AMP were higher than those observed in unstimulated cells, and cyclic AMP-dependent protein phosphokinase was in a highly activated state as compared to controls. Under these conditions little change in the growth or morphology of either the normal or transformed cells was observed. In contrast, inhibition of growth, apparent cell death, and unusual morphological changes were observed in both normal and transformed cells when high concentrations of either PGE1 (10 micron) or the phosphodiesterase inhibitor 1-methyl, 3-isobutylxanthine (0.5 mM to 2 mM) were used, which was indicative of toxic effects of the drugs. It was concluded that cyclic AMP-mediated activation of protein phosphokinase does not completely inhibit growth in WI-38 cells or restore normal growth and morphology to the SV40-transformed cells.     

Cyclic nucleotides and their relationship to complement-component-C2 synthesis by human monocytes.

The time courses of changes in cyclic nucleotide levels in monocytes have been studied. Histamine and prostaglandin E2 (PGE2) produced a rapid rise in cyclic AMP (peak 15 min) levels, which returned to normal within 4h, whereas cholera toxin, NaF and phosphodiesterase inhibitors produced slow sustained rises lasting over 24h. With the exception of isobutylmethylxanthine (10 mumol X 1(-1), none of these reagents altered cyclic GMP levels. alpha 1-Adrenergic and nicotinic cholinergic receptor-ligand interactions and imidazole produced rapid and relatively short-lived falls in cyclic AMP, and rises in cyclic GMP. In contrast, prostaglandin synthetase inhibitors produced delayed but more sustained falls in cyclic AMP but no rises in cyclic GMP. Agents that increased cyclic AMP decreased complement-component-C2 production, and those that decreased cyclic AMP increased C2 production. Agents that increased cyclic GMP alone (ascorbate, nitroprusside and prostaglandin F2 alpha) did not affect C2 production. Antigen-antibody complexes that stimulate C2 synthesis produced falls in cyclic AMP and rises in cyclic GMP similar to those produced by adrenergic and cholinergic ligands. Serum-treated complexes and anaphylatoxins, which inhibited C2 production, were associated with changes in cyclic AMP similar to those produced by histamine and PGE2. These data suggest that there are two transmembrane signals involved in the regulation of C2 production by monocytes. The inhibitory signal is adenylyl cyclase activation. The stimulatory signal is not so obvious, but may be Ca2+ influx, since the time courses of changes in cyclic nucleotides produced by agents that stimulate C2 synthesis are identical, and alpha 1-adrenergic agonists cause the formation of Ca2+ channels.