Concentrations Of Dibutyryl Cyclic AMP Research Articles

Murine corticosteroid binding globulin localizes to early embryo trophoblasts

Exposures of cultured whole rat conceptuses to varying concentrations of dibutyryl cyclic AMP or isobutylmethylxanthine, alone or in combination, resulted in significant increases in rates of cytochrome P450-dependent depentylation of pentoxyphenoxazone in cell-free preparations. Lesser increases in rates of debenzylation of benzyloxyphenoxazone were also observed. In cultured whole conceptuses, basal depentylase and debenzylase activities in the visceral yolk sac were approximately sixfold higher than in the embryo. The ectoplacental cone and decidual tissues exhibited no detectable depentylase activity. Only the visceral yolk sac exhibited increased depentylase activity in response to dibutyryl cyclic AMP and isobutylmethylxanthine. Inhibitory antibodies raised against adult hepatic P450s IIB1, IIC11, and IA1 failed to significantly inhibit the yolk sac depentylase activities of noncultured conceptuses. The results suggested that the conceptal depentylation reaction may be catalyzed by a unique P450 isoform(s) that is not expressed during adult life.

Nitric oxide synthesis in rat peritoneal macrophages is induced by IgE/DNP complexes and cyclic AMP analogues. Evidence in favor of a common signaling mechanism.

The production of nitric oxide was studied in rat adherent peritoneal cells stimulated with preformed IgE/DNP-BSA complexes made of IgE obtained from a secreting hybridoma. Stimulation with complexes at equivalence induced both the production of NO and an increased expression of the mRNA of the inducible isoform of NO synthase (iNOS). Production of NO was also elicited by a rabbit polyclonal F(ab')2 anti-CD23 cross-reacting with rat CD23. Because IgE/DNP-BSA complexes did not elicit Ca2+ mobilization and genistein did not influence the production of NO, cyclic AMP was considered as an alternative signaling molecule. Combination of a suboptimal concentration of dibutyryl cyclic AMP and IgE/DNP-BSA complexes showed an additive effect on NO production, whereas this was not observed when the agonists were used at supraoptimal doses. The inhibitor of cyclic AMP-specific phosphodiesterase IV, rolipram, which acts on the enzyme isoform predominantly expressed in inflammatory cells, also induced the production of NO. Furthermore, IgE/DNP-BSA complexes increased intracellular levels of cyclic AMP. Taken together, these data indicate that stimulation of mononuclear phagocytes via the low-affinity receptor Fc epsilon RII or rising intracellular concentrations of cyclic AMP leads to an enhanced expression of iNOS. Evidence in favor of the involvement of cyclic AMP in the signaling pathway linked to Fc epsilon RII is provided by the effect of IgE/DNP-BSA complexes on intracellular cyclic AMP levels and by the additive effect produced by dibutyryl cyclic AMP on NO production elicited by IgE/DNP-BSA complexes.

Mediation of gonadotrophin-stimulated growth and differentiation of human granulosa cells by adenosine-3',5'-monophosphate: one molecule, two messages.

To determine how the second messenger adenosine-3',5'-monophosphate (cyclic AMP) is able to mediate divergent actions of FSH and LH on granulosa cell growth and differentiation in human ovaries. Human granulosa cells were cultured for 96 hours in serum-free medium 199 containing increasing doses of either FSH, LH or dibutyryl cyclic AMP. Extra and intra-cellular cyclic AMP levels were determined by radioimmunoassay. Tritiated thymidine uptake and cell number were measured as indices of cell growth, and spent medium was assayed for steroids (oestradiol and progesterone) to reflect differentiation. 'Mature' granulosa cells were aspirated from preovulatory follicles in the ovaries of clomiphene-stimulated patients undergoing laparoscopic sterilization; 'luteinized' granulosa cells were aspirated from periovulatory follicles in the ovaries of gonadotrophin-stimulated in-vitro fertilization patients. LH consistently inhibited, whereas FSH maintained or stimulated, basal granulosa cell numbers. Steroidogenesis was dose-dependently increased by both gonadotrophins, with LH having the significantly greater effect over the entire dose-response range (1-100 micrograms/l). LH also induced significantly more cyclic AMP production than FSH, both intra and extra-cellularly, providing a basis for differential post-receptor signalling via a common second messenger. Addition of dibutyryl cyclic AMP, at low concentrations (10-250 mumol/l) to the cultured cells in the absence of gonadotrophins mimicked FSH effects with stimulation/maintenance of cell numbers and moderate steroidogenesis. High concentrations of dibutyryl cyclic AMP (500-1000 mumol/l) caused a significant inhibition of cell numbers together with maximal steroidogenesis, simulating LH action. These results suggest that granulosa cell maturation in the follicular phase of the menstrual cycle (controlled by FSH) is associated with a low cyclic AMP tone that favours cell growth and expression of aromatase activity in the developing preovulatory follicle. During the early luteal phase (dominated by LH), the intracellular cycle AMP tone increases to allow maximal progesterone production and inhibition of cell growth in the corpus luteum. Thereby one second messenger can mediate divergent gonadotrophic effects on granulosa cell growth and differentiation in the human ovary.

CAMP-dependent regulation of P450-catalyzed dealkylation in rat conceptal tissues

Exposures of cultured whole rat conceptuses to varying concentrations of dibutyryl cyclic AMP or isobutylmethylxanthine, alone or in combination, resulted in significant increases in rates of cytochrome P450-dependent depentylation of pentoxyphenoxazone in cell-free preparations. Lesser increases in rates of debenzylation of benzyloxyphenoxazone were also observed. In cultured whole conceptuses, basal depentylase and debenzylase activities in the visceral yolk sac were approximately sixfold higher than in the embryo. The ectoplacental cone and decidual tissues exhibited no detectable depentylase activity. Only the visceral yolk sac exhibited increased depentylase activity in response to dibutyryl cyclic AMP and isobutylmethylxanthine. Inhibitory antibodies raised against adult hepatic P450s IIB1, IIC11, and IA1 failed to significantly inhibit the yolk sac depentylase activities of noncultured conceptuses. The results suggested that the conceptal depentylation reaction may be catalyzed by a unique P450 isoform(s) that is not expressed during adult life.

The dose dependent effect of cyclic AMP on ribonucleotide reductase in mitogen stimulated mononuclear cells

Cyclic adenosine monophosphate arrests proliferating T lymphocytes in the G1 phase of the cell cycle. Here we demonstrate that exogenous and endogenous elevations in cyclic AMP concentration result in diminished mitogen stimulation, cell cycle arrest, and decreased ribonucleotide reductase messenger RNA concentrations in peripheral blood mononuclear cells. At lower concentrations (less than 1mM) of dibutyryl cyclic AMP that do not generate cell cycle arrest there is inhibition of ribonucleotide reductase activity without decreased messenger RNA concentration for the M2 subunit of ribonucleotide reductase. However, at higher concentrations of dibutyryl cyclic AMP there is G1 cell cycle arrest and reduced M2 specific messenger RNA concentration. Thus, cyclic AMP inhibition of lymphocyte activation may occur by different mechanisms that are dose dependent.

Regulation by Dibutyryl Cyclic AMP of Carnosine Synthesis in Astroglia‐Rich Primary Cultures Kept in Serum‐Free Medium

The synthesis of carnosine (beta-Ala-His) by astroglia-rich primary cultures was much higher if the cells were cultivated in Ham's nutrient mixture F-12 than if they were grown in Dulbecco's modified Eagle's medium. Carnosine synthesis was not affected by the presence of insulin, transferrin, phorbol myristate acetate, or dexamethasone. However, dibutyryl cyclic AMP and other agents that can, directly or indirectly, activate cyclic AMP-dependent protein kinases strongly lower the rate of carnosine synthesis. The depression of carnosine synthesis was dependent on the concentration of dibutyryl cyclic AMP. The effect was maximal (approximately 80% inhibition) in cultures preincubated with 1 mM dibutyryl cyclic AMP for 4 days. The adenylate cyclase activator forskolin, the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, and 8-bromo-cyclic AMP caused the same depression as dibutyryl cyclic AMP, whereas neither butyrate nor dibutyryl cyclic GMP elicited any effect.

Synaptosomal dopamine autoreceptors: Sensitivity changes after in vitro and in vivo treatments

Dopamine (DA) synthesis in rat striatal synaptosomes was approximately doubled either by treating the animals from which the synaptosomes were obtained with reserpine, or by treating the preparations in vitro with d-amphetamine, ouabain or dibutyryl cyclic AMP. The concentration-response curve of DA synthesis inhibition by apomorphine was shifted to the right after treatment with all these compounds. The inhibitory effect of bromocriptine on DA synthesis was reduced completely after treatment with all the above compounds with the exception of dibutyryl cyclic AMP. When the inhibitory effect of bromocriptine was eliminated by treatment with reserpine or d-amphetamine, bromocriptine antagonized the inhibitory effect of apomorphine. This indicates that bromocriptine could still be bound to the DA autoreceptors and that the reduced sensitivity was due to a reduced functioning of the DA autoreceptors. The reduced sensitivity to apomorphine observed after all the above treatments was possibly due both to a reduced function of and/or to a reduced binding to the DA autoreceptors. The increase in DA synthesis produced by treatment with reserpine in vivo or with d-amphetamine or ouabain in vitro was additive to that produced by a maximally effective concentration of dibutyryl cyclic AMP in vitro, and thus mediated by a presumably non-cyclic AMP-dependent mechanism. Our results obtained with bromocriptine suggest that stimulation of the DA autoreceptors may inhibit DA synthesis by diminishing Ca2+-dependent and not cyclic AMP-dependent phosphorylation of tyrosine hydroxylase.

I. Anti-μ antibody stimulates the phosphatidylinositol cycle and immunoglobulin secretion in a human lymphoblastoid B-cell line, LA350

Within 5 min of the binding of anti-μ antibody (anti-μ) to surface IgM on LA350, a human lymphoblastoid B-cell line, there was a significantly enhanced incorporation of 32P into the phosphatidic acid (PA) and phosphatidylinositol (PI) fractions of cellular phospholipids and the magnitude of the early increase in PA was twice as great as that in PI. This anti-μ-enhanced incorporation of 32P into PA and PI required the binding of a divalent form of antibody (IgG or F(ab′) 2), was blocked by coincubation with micromolar concentrations of soluble IgM, was decreased by incubation of cells at temperatures lower than 37 °C, and was inhibited by coincubation with millimolar concentrations of dibutyryl cyclic AMP and theophylline. Similar incorporation studies with [ 3H]inositol demonstrated a selective and significant increase in labeling of PI. In LA350 labeled with [ 3H]inositol for 30 hr (equilibrium) and acutely stimulated by anti-μ, specific hydrolysis of phosphorylated PI (PI 4,5-bisphosphate) was measured by the significantly increased release at 15 min of radioactive inositol 1,4,5-trisphosphate, inositol 1,4 bisphosphate, and inositol 1-phosphate. The release of these inositol phosphates was significantly augmented by coincubation with 0.01 M LiCl which prevented their simultaneous enzymatic degradation. All of these findings are consistent with an activation of a linked series of metabolic events known as the PI cycle. In similar cell cultures anti-μ significantly stimulated the secretion of IgM by LA350 as measured at 48 hr in a reverse hemolytic plaque assay. Two other IgM-bearing human lymphoblastoid B-cell lines which gave no evidence of turnover of 32P in PA and PI in response to binding by anti-μ likewise failed to enhance their secretion of IgM. We conclude that the binding of surface IgM on LA350 by anti-μ results in the generation of a transmembrane signal which causes a rapid activation of the PI cycle which itself may play a role in the subsequent increase in IgM secretion.

Protein phosphorylation in rat mammary acini and in cytosol preparations in vitro. Phosphorylation of acetyl-CoA carboxylase is unaffected by cyclic AMP.

Phosphorylation of soluble proteins in rat mammary acinar cells was investigated. When phosphorylation proceeded in intact cells, in the presence of [32P]Pi, the major non-casein phosphoproteins, including acetyl-CoA carboxylase, were unresponsive to incubation conditions that caused major increases in the intracellular concentration of cyclic AMP. The overall 32P specific radioactivity (c.p.m./microgram of protein) of acetyl-CoA carboxylase, assessed after affinity purification of the enzyme with avidin-Sepharose, was unchanged by incubation under such conditions. Furthermore, the distribution of 32P among tryptic phosphopeptides of the enzyme, resolved by reversed-phase h.p.l.c., was not altered by cyclic AMP-increasing treatments of the acinar cells. When cytosol fractions were incubated with [gamma-32P]ATP, some phosphoproteins responded to the addition of micromolar concentrations of dibutyryl cyclic AMP or cyclic AMP by undergoing an enhancement of phosphate incorporation. In these experiments in vitro, protein phosphatase activity did not make a major contribution to the net phosphorylation of individual phosphoproteins, and acetyl-CoA carboxylase was not prominent among the phosphoproteins identified after short (less than 1 min) incubations of cytosols with [gamma-32P]ATP. The resistance of protein phosphorylation to variations in the cyclic AMP concentration in intact mammary epithelial cells, demonstrated by this work, is one of several mechanisms that ensure the pleiotropic refractoriness of those cells to agents which normally cause a stimulation of adenylate cyclase activity in hormone-sensitive cells.

The effect of dibutyryl cyclic AMP on the uptake of taurocholic acid by isolated rat liver cells

The effect of dibutyryl cyclic AMP on the uptake of taurocholic acid by isolated rat hepatocytes was studied. In the presence of low levels (10–100 μM) of the cyclic nucleotide the initial rate of uptake was increased significantly, with a peak occurring at about 20 μM. In contrast, concentrations of dibutyryl cyclic AMP between 200 μM and 1 mM caused a significant decrease in the initial rate of uptake of the bile acid by the cells. Sodium-dependent transport of taurocholic acid was found to be enhanced by 20 μM dibutyryl cyclic AMP, but sodium-independent uptake appeared to be unaffected. Inhibition by 1 mM dibutyryl cyclic AMP, however, was found to occur in both the sodium-dependent and -independent components of the transport system. The initial rate of taurocholic acid uptake in hepatocytes incubated with 1.2 mM extracellular calcium was increased compared to that in calcium-depleted cells, and this increase was entirely due to enhanced sodium-dependent transport. 1.2 mM calcium and 20 μM dibutyryl cyclic AMP together did not stimulate the uptake rate to a greater extent either treatment alone. It is conclude that calcium and low levels of dibutyryl cyclic AMP alter the rate of taurocholic acid uptake by changing the flux of sodium in the hepatocytes. The inhibitory effect of 1 mM dibutyryl cyclic AMP was not relieved by the presence of 1.2 mM calcium in the cell incubation medium. The results show that dibutyryl cyclic AMP can affect the rate of transport of bile acid into liver cells, and suggest a possible regulatory role for cyclic AMP in this process.

Cyclic AMP inhibition of phosphoinositide turnover in human neutrophils

The effect of increased intracellular levels of cyclic AMP on phosphoinositide metabolism was studied in human neutrophils stimulated with fMet-Leu-Phe. Intracellular cyclic AMP was raised by preincubation either with dibutyryl cyclic AMP and theophylline or with prostaglandin E 1. Concentrations of dibutyryl cyclic AMP and theophylline fully inhibitory for the metabolic responses inhibited phosphoinositide breakdown and phosphatidic acid formation to a large extent. The accumulation of the water-soluble inositol phosphates was also measured. In agreement with the data obtained on the phospholipids, inositol phosphate generation was found to be severely, though not completely, reduced. Treatment with dibutyryl cyclic AMP and theophylline also inhibited resynthesis of membrane inositol lipids. Treatment with prostaglandin E 1 had a similar, though less, marked effect on inositol lipid turnover, which was parallel with a smaller inhibition of metabolic responses. We therefore suggest that the elevation of intracellular cyclic AMP mainly affects neutrophil responses by inhibiting the phosphoinositide cycle.

Conditions affecting the differentiation of F9 teratocarcinoma cells: Potentiation of response by cyclic amp

F9 cells maintained in culture were shown to have a reduced ability to differentiate. The cells produced decreased amounts of alphafetoprotein when induced with retinoic acid. We show that consistent responses can be recovered after passage of F9 cells as a tumor. In addition, optimal differentiation of F9 cells to visceral endoderm may be achieved by the addition of very low concentrations of dibutyryl cyclic AMP (dbcAMP) to the medium.

Effect of glucose on induction of delta-aminolevulinic acid synthase, ferrochelatase and cytochrome P-450 hemoproteins in isolated rat hepatocytes by phenobarbital and lead.

In the present work we have been able to demonstrate that phenobarbital and lead exert an inducing effect on the biosynthesis of delta-aminolevulinic acid synthase, ferrochelatase and cytochrome P-450 hemoproteins in isolated rat hepatocytes of normal adult rats. Dibutyryl cyclic AMP enhances the induction effect produced by phenobarbital in this in vitro system. Glucose inhibits the induction of delta-aminolevulinic acid synthase and ferrochelatase. This repression effect can be reversed with increasing concentrations of dibutyryl cyclic AMP. No glucose effect was observed on the phenobarbital- and lead-mediated inductions of cytochrome P-450. he present results add more experimental evidence to support the concept that the last enzyme of the heme pathway is inducible, and as such may have a significant role in regulatory mechanisms of porphyrin and heme biosynthesis.

Effect of glucose on the induction of δ-aminolevulinic acid synthase and ferrochelatase in isolated rat hepatocytes by allylisopropylacetamide

The present work shows that allylisopropylacetamide exerts an inducing effect on δ-aminolevulinic acid synthase and ferrochelatase activities in isolated rat hepatocytes of normal adult rats. Dibutyryl cyclic AMP enhances the inducing effect produced in both enzymes. Glucose inhibits the induction of δ-aminolevulinic acid synthase and ferrochelatase in this in vitro system. A similar effect was observed with fructose and 2-deoxyglucose. No glucose effect was observed with galactose, mannose, glycerol, pyruvate and lactate. The glucose effect can be reversed with increasing concentrations of dibutyryl cyclic AMP. The simple in vitro method used in the present work promises to be a very useful tool for studies of regulatory mechanisms of porphyrin and heme biosynthesis in hepatocytes under normal and pathological conditions (hepatic porphyrias).

Persistence of precursor cells of squamous metaplasia in preneoplastic mammary outgrowth lines from mice.

The preneoplastic state is without apparent effect on the induction or prevention of epidermidalization in transplanted mammary outgrowth lines. Development of squamous metaplasia and differentiation (keratinization) were induced in organ cultures of three hyperplastic alveolar and ductular mammary outgrowth lines (D1, MH5, and MH9) that had been extensively passaged in gland-free mammary fat pads of BALB/c virgin mice. The induction was elicited by the mixture of dibutyryl cyclic AMP (0.1 mM), prostaglandins E1, E2, and B1 (each 5 micrograms/ml), and papaverine (1 microM) or by a tenfold higher concentration of dibutyryl cyclic AMP (1 mM) alone for 9 days. The retinoid 2-retinylidene-5,5-dimethyl-1,3-cyclohexanedione at 1 microM and the phorbol ester phorbol 12,13-didecanoate at 10 microM each blocked the induction process. The parameters of the induction and its prevention were analogous in many ways to those previously found with cultures of normal mammary glands of mice and humans, as well as of mouse prostate glands and chick embryo skin. The metaplastic squamous cells that developed in the cultured mammary outgrowths did not proliferate in gland-free mammary fat pads, possibly because the cells were terminally committed or because of insufficient inducers. In contrast, the alveolar and ductular epithelia in the same outgrowths have a transplantable pool of generative cells with the ability to undergo continual proliferation and development. The finding of precursor cells with the potential for epidermoid development and differentiation in the preneoplastic alveolar and ductular outgrowths, despite their extensive serial transplantations, is supportive of the existence of a common or closely associated pool of cells with the ability to develop either into squamous or alveolar mammary epithelium.

Regulation of ornithine decarboxylase activity by amino acids, cyclic AMP and luteinizing hormone in cultured mammalian cells

Any one of five amino acids (alanine, asparagine, glutamine, glycine, and serine) is an essential requirement for the induction of ornithine decarboxylase (EC 4.1.1.17) in cultured chinese hamster ovary (CHO) cells maintained with a salts glucose medium. Each of these amino acids induced a striking activation of ornithine decarboxylase in the presence of dibutyryl cyclic AMP and luteinizing hormone. The effect of the other amino acids was considerably less or negligible. The active amino acids at optimal concentrations (10 mM) induced only a 10–20 fold enhancement of enzyme activity alone, while in the presence of dibutyryl cyclic AMP, ornithine decarboxylase activity was increased 40–50 fold within 7–8 h. Of the hormones and drugs tested, luteinizing hormone resulted in the highest (300–500 fold) induction of ornithine decarboxylase with optimal concentrations of dibutyryl cyclic AMP and asparagnine. Omission of dibutyryl cyclic AMP reduced this maximal activation to one half while optimal levels of luteinizing hormone alone caused no enhancement of ornithine decarboxylase activity. The induction of ornithine decarboxylase elicited by dibutyryl cyclic AMP, amino acid and luteinizing hormone was diminished about 50% with inhibitors of RNA and protein synthesis. The specific amino acid requirements for ornithine decarboxylase induction in chinese hamster ovary cells was similar to the requirements for induction in two other transformed cell lines. Understanding the mechanism of enzyme induction requires an identification of the essential components of the regulatory system. The essential requirement for enzyme induction is one of five amino acids. The induction of ornithine decarboxylase by dibutyryl cyclic AMP and luteinizing hormone was additive in the presence of an active amino acid.

Evidence suggesting a role for cyclic nucleotides in acrosome reactions of hamster sperm in vitro.

There have been conflicting reports concerning the involvement of cyclic nucleotides in sperm capacitation. We have examined the effects of micromolar concentrations of dibutyryl cyclic AMP (Bt2cAMP) and of the phosphodiesterase inhibitors SQ20009 and ICI63,197 on hamster sperm incubated under in vitro capacitating conditions. Washed hamster sperm were incubated in a capacitation media containing bovine serum albumin, and a protein-free "motility-factor" from bovine adrenal cortex. Incubation for 3.5 hours was followed by addition of one of the compounds (0.1-10 microM) or control buffer. At the time of addition and after 30-120 minutes further incubation, sperm were examined by phase contrast microscopy. The final motility was similar to the initial motility (50-70%) and the same in incubation of controls or experimental compounds. Bt2cAMP, SQ20009, and ICI63,197 at these concentrations stimulated acrosome reactions to a statistically significant extent (P less than 0.005) compared to controls. Activation was stimulated to a varying degree by all three experimental compounds. These results suggest a role for cyclic nucleotides in capacitation and the acrosome reaction of hamster sperm.

Effect of cyclic nucleotides on mitogen: Responsiveness of lymphocytes from patients undergoing dialysis

Lymphocytes from 10 asymptomatic patients undergoing hemodialysis and from eight control subjects were repeatedly cultured with exposure to various concentrations of cyclic nucleotides and theophylline in addition to mitogen. The blastogenic response of the patients' lymphocytes was inhibited by molar concentrations of dibutyryl cyclic AMP which had much less or no inhibitory effect on the response of the control subjects' lymphocytes. This suppressive effect was not potentiated by theophylline. Cyclic GMP enhanced the proliferative response of the patients' lymphocytes as well as that of the controls. In contrast to absolute counts per minute per culture, the suppression by dibutyryl cyclic AMP of mitogen-induced blastogenesis noted in this study clearly separated the in vitro behavior of the patients' lymphocytes from that of the controls' lymphocytes and may serve as a useful marker of cellular dysfunction in such patients.

Nerve growth factor increases activity of ornithine decarboxylase in superior cervical ganglia of young rats.

Nerve growth factor produces a rapid increase in the activity of ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) in superior cervical ganglia of young rats in vivo and in vitro. Maximum activity occurs 6-7 hr after the addition of nerve growth factor. The nerve growth factor-mediated increase in ornithine decarboxylase activity in vitro can be prevented by the addition of cycloheximide, actinomycin D, or antibody to nerve growth factor. A number of other agents were tested for their ability to increase ornithine decarboxylase were tested for their ability to increase ornithine decarboxylase activity in the ganglia; only nerve growth factor, and, to a slight extent, insulin were able to raise the activity of the enzyme. High concentrations of dibutyryl cyclic AMP (10 mM) were able to mimic the effect of nerve growth factor.

Separation of mitogen-induced suppressor and helper cell activities during inhibition of interferon production by cyclic AMP

The effect of exogenous cyclic AMP on mitogen-induced suppression and enhancement of the in vitro plaque-forming cell (PFC) response and on mitogen induction of immune interferon (also called type II) in cultures was examined. Mitogen induction of immune interferon was quantitatively associated with mitogen-induced suppressor activity, and cyclic AMP blocked both the suppressor activity and the production of immune interferon in mouse (C57B1/6) spleen cell cultures. The evidence is as follows: (a) The concentrations of dibutyryl cyclic AMP that blocked T-cell mitogen (staphylococcal enterotoxin A) suppressor activity were the same as those that blocked mitogen induction of immune interferon. (b) The blocking action of dibutyryl cAMP on both the suppressor and interferon effects of mitogen was a function of the time of dibutyryl cAMP addition to cultures relative to mitogen addition. (c) A dramatic immunoenhancing effect of mitogen occurred in the presence of dibutyryl cAMP under conditions that blocked production of immune interferon. Specifically, mitogen-induced helper cell function is dramatically enhanced in the presence of dibutyryl cyclic AMP, if the mitogen is added to cultures 24 to 48 hr after SRBC and dibutyryl cyclic AMP. Dibutyryl cyclic GMP did not affect the mitogen- or cyclic AMP-induced effects under the conditions of our test system. Under the conditions described here, then, cyclic AMP appears to selectively block suppressor cell activity while allowing or aiding mitogen-induced helper cell activity. It is possible that the immune response is a reflection of the ratio of helper to suppressor activities in the system.