[Participation of cGMP in inhibitory mechanisms of the center of amphibian lymph hearts]

The effects of cAMP on generation of spontaneous rhythmic activity of lymphatic pacemaker neurons, was studied in fragments of the frog isolated perfused spinal cord. The cGMP depressed both the rhythm of the burst activity and the dibutyril-cAMP effect. The depressive effect of the cGMP was reversibly blocked with furosemide. The data obtained suggest existence of relationship between GABA-ergic inhibition of the pacemaker activity of lymphatic center and the cGMP concentration in pacemaker neurons.

In vitro action of PG F2 alpha on progesterone and cAMP synthesis in small bovine luteal cells

The action of prostaglandin F2 alpha (PG F2 alpha) on incubated small bovine luteal cells in the presence or in the absence of bovine luteinizing hormone (LH) or dibutyryl cyclic adenosine monophosphate (db cAMP) was investigated. In the absence of LH and db cAMP, PG F2 alpha stimulated progesterone synthesis at concentrations of 10 ng/ml and 100 ng/ml but had no effects at concentrations below 1 ng/ml. PG F2 alpha partially inhibited the LH or db cAMP stimulated progesterone synthesis. This inhibition was maximal for PG F2 alpha concentrations around 100 pg/ml whereas distinctly higher or lower concentrations were without effect. At the concentration of 100 pg/ml, PG F2 alpha partially inhibited the LH induced cAMP accumulation. These results demonstrate an "in vitro" action of PG F2 alpha on bovine luteal cells. They indicate that the luteolytic action of PG F2 alpha in the bovine species could involve, as already suggested for the rat, both an inhibition of the LH induced synthesis of cAMP and an inhibition of the action of cAMP.

The effect of age on somatostatin suppression of basal, growth hormone (GH)-releasing factor-stimulated, and dibutyryl adenosine 3',5'-monophosphate-stimulated GH release from rat pituitary cells in monolayer culture

To test the hypothesis that relative resistance of the somatotroph to somatostatin (SRIF) contributes to elevated circulating levels of GH in the newborn rat, we examined the effects of SRIF (0.1, 0.33, and 1 nM) on basal, human pancreatic GH-releasing factor-40 (hpGRF-40; 1 nM)-stimulated, and (Bu)2cAMP (0.5 mM)-stimulated GH release from pituitary cells of 2-day-old, 15-day-old, and adult Sprague-Dawley rats in monolayer culture. The effect of SRIF on basal GH release varied markedly with age. SRIF, in the doses studied, did not inhibit basal GH release (nanograms of GH per 10(5) cells/3 h) from pituitary cultures of 2-day-old rats. In those of 15-day-old rats, only the two higher doses of SRIF (0.33 and 1 nM) suppressed GH release. By contrast, in pituitary cell cultures of adult male and female rats, all doses of SRIF significantly inhibited basal GH release (P less than 0.001). Similarly, the degree of SRIF suppression of both hpGRF-40- and (Bu)2cAMP-stimulated GH release differed among the age groups. In pituitary cultures of 2-day-old rats, SRIF did not significantly inhibit stimulated GH release. In 15-day-old rat pituitary cells, SRIF inhibited GH release, but did not eradicate the stimulatory effect of hpGRF-40 or (Bu)2cAMP. By contrast, in pituitary cell cultures of adult male and female rats, SRIF completely abolished the stimulatory effect of both hpGRF-40 and (Bu)2cAMP. When expressed as a percentage of the control (or stimulated) value, GH release at each SRIF dose varied markedly with age (P less than 0.001). Furthermore, a similar age-associated trend was evident when, in a separate series of experiments (n = 37), we examined the suppressive effect of a single concentration of SRIF (0.33 nM) on (Bu)2cAMP (0.5 mM)-stimulated GH release in cultured pituitary cells of rats ranging in age from -1 (day 20 of gestation) to 78 days. The degree of suppression increased progressively with advancing age; GH release decreased from 82 +/- 2% (+/- SE) of stimulated values in cultured cells of perinatal rats to 20 +/- 1% of stimulated values in cultured cells of 78-day-old rats. There was a significant negative correlation between age and SRIF-inhibited GH release (r = -0.89; P less than 0.001).(ABSTRACT TRUNCATED AT 400 WORDS)

[Effects of gastric proton pump inhibitors on gastric secretion and peptic ulcers]

This is a review article on newly developed antisecretory drugs which are now called proton pump inhibitors. Gastric H+ is secreted from the secretory membrane of parietal cells into the lumen of the stomach, using energy obtained by destructing ATP with H+, K+-ATPase (proton pump). Various substituted benzimidazoles such as timoprazole, picoprazole, omeprazole, or NC-1300 potently inhibits this proton pump at pH 6.0, thereby resulting in a strong inhibition of gastric H+ secretion. This inhibition of H+ secretion lasts for a long period, ie, 1-3 days after a single oral or intraduodenal administration, both in experimental animals and humans. This long lasting activity of these compounds appears to be due to their accumulation in the parietal cells because of their low pKa values (about 4.0). Proton pump inhibitors dose-dependently inhibit the development of various experimental ulcers and accelerate healing of chronic gastric ulcers in animals. Since these compounds also potently inhibit the development of HCl X ethanol or HCl X aspirin-induced gastric ulcers in animals, they are considered to have a cytoprotective activity. Some of the compounds (e.g., omeprazole) afforded a complete healing of peptic ulcers in man when it was given once daily for 2 to 4 weeks, without any adverse effects. Therefore, these proton pump inhibitors appear to be promising drugs for the treatment of peptic ulcer diseases.

Effect of omeprazole on gastric acid secretion by the rat isolated stomach

The inhibitory effect of omeprazole has been investigated on the isolated gastric fundus from immature rats. Omeprazole (10(-7)-10(-5) M) inhibited basal acid secretion, conversely from H2-receptor antagonists, antimuscarinic compounds and calcium antagonists; the effect was mimicked only by KSCN (3 X 10(-4)-3 X 10(-2) M). Omeprazole (10(-6)-10(-5) M) caused an insurmountable antagonism of the hypersecretion induced by histamine and bethanechol, whereas it competitively antagonized the secretory response to isoprenaline and dibutyryl cAMP. Experiments carried out in low calcium media showed that calcium ions did not significantly affect the inhibitory potency of omeprazole when tested on basal acid secretion, whereas low calcium solutions enhanced the action of omeprazole against histamine-induced hypersecretion. The above data confirmed the potent antisecretory activity of omeprazole in different experimental conditions in which the common antisecretagogues are without effect and pointed out the novel site of action in the control of gastric acid secretion at intracellular level.

Growth factors suppress and phorbol esters potentiate the action of dibutyryl adenosine 3',5'-monophosphate to stimulate aromatase activity of human adipose stromal cells

In previous studies, we observed that the stimulatory effect of (Bu)2cAMP on aromatase activity of human adipose stromal cells was markedly attenuated when fetal calf serum was present in the culture medium. To determine whether growth factors may be the inhibitors of (Bu)2cAMP-stimulated aromatase activity in serum, the effects of growth factors and phorbol esters on aromatase activity of human adipose stromal cells in monolayer culture were investigated. Epidermal growth factor (EGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF) were all without effect on aromatase activity when added by themselves, but markedly inhibited aromatase activity stimulated by (Bu)2cAMP. On the other hand, nerve growth factor, multiplication-stimulating activity, relaxin, and insulin had no effect on aromatase activity, either by themselves or in the presence of (Bu)2cAMP. Thus, EGF, PDGF, and FGF can mimic the inhibitory action of fetal calf serum on (Bu)2cAMP-stimulated aromatase activity of these cells. By contrast, none of these substances was capable of mimicking the effect of serum to facilitate the stimulatory action of dexamethasone on aromatase activity of these cells. The phorbol esters phorbol-12-myristate-13-acetate, phorbol-12,13-didecanoate, and phorbol-12,13-diacetate were also capable of facilitating the action of (Bu)2cAMP to stimulate aromatase activity, but had little or no action on dexamethasone-stimulated aromatase activity or when added by themselves. It is concluded that aromatase is under multifactorial regulation in human adipose stromal cells. The activity is induced by glucocorticoids and by agents that stimulate cAMP-dependent protein kinase; the latter effect is potentiated by factors that stimulate protein kinase C, but is suppressed by growth factors such as EGF, FGF, and PDGF, whose actions are believed to be mediated by receptor-linked tyrosine kinase activity.

The role of calcium ions in the mechanism of ACTH stimulation of cortisol synthesis

Removal of free calcium ions from the incubation medium of isolated bovine adrenocortical cells with EGTA reduced basal cortisol synthesis and blocked the effects of ACTH; additional calcium restored normal steroid synthesis. Calcium channel blockers, verapamil and nitrendipine and the calmodulin antagonist, trifluoperazine inhibited ACTH-stimulated cortisol synthesis in a dose-dependent manner (IC50s of 6.2, 10 and 5.2 microM, respectively). Steroidogenic effects of dibutyryl cyclic AMP were prevented with 50 microM verapamil or trifluoperazine. Calcium ionophore A23187 at 1 microM increased cortisol synthesis 2-3 fold which was less than the normal response to ACTH. Stimulatory effects of ionophore and cyclic AMP or ACTH were not additive. ACTH-stimulation of cortisol synthesis appears to involve cyclic AMP-dependent uptake of extracellular calcium ions, possibly by a mechanism requiring calmodulin. Increases in intracellular calcium ions cannot wholly mimic ACTH actions.

Postreceptor inhibition of antral gastrin release by somatostatin

This study was performed to examine the effects of somatostatin on antral gastrin release stimulated by postreceptor increases in adenosine cyclic nucleotide. Increases in intracellular levels of cyclic adenosine monophosphate were achieved through the use of the dibutyryl derivative of cyclic adenosine monophosphate and the phosphodiesterase inhibitor theophylline. The effects of somatostatin on basal and stimulated gastrin release were examined in rat antral organ culture experiments. Inclusion of somatostatin in the culture medium (1 X 10(-8) to 1 X 10(-4) M) resulted in significant inhibition of gastrin release at somatostatin concentrations of 1 X 10(-5) and 1 X 10(-4) M: both doses of somatostatin inhibited gastrin release by approximately 52% at 30 min and by 32% at 6 h. Gastrin release stimulated by dibutyryl cyclic adenosine monophosphate was significantly inhibited by 1 X 10(-5) and 1 X 10(-4) M somatostatin to 133% and 121% at 30 min and 77% and 98% at 6 h, respectively. Gastrin release stimulated by theophylline (1 mM) was also significantly inhibited by somatostatin in doses ranging from 1 X 10(-6) to 1 X 10(-4) M. The degree of inhibition by somatostatin of dibutyryl cyclic adenosine monophosphate- and theophylline-stimulated gastrin release declined over the duration of culture. In conclusion, these results suggest that somatostatin inhibits adenosine cyclic nucleotide-stimulated gastrin release by acting at a point distal to the formation of cyclic adenosine monophosphate.

Estradiol-17 beta inhibition of lutropin and dibutyryl cyclic AMP induced steroidogenesis in intact bovine luteal cells. Absence of effect on induced protein phosphorylations

Estradiol-17 beta is known to inhibit in a dose dependent manner the lutropin-induced stimulation of progesterone synthesis in luteal cells without affecting the intracellular cyclic AMP increase produced by the hormone. The hypothesis that this inhibitory action could involve an inhibition of the cyclic AMP dependent phosphorylation of cytosolic proteins was investigated by using incubations of selected small bovine luteal cells. Doses of 10 and 100 micrograms/ml of estradiol-17 beta inhibited respectively 60 and 90% of the progesterone synthesis induced by lutropin as well as by dibutyryl cyclic AMP in small bovine luteal cells. At the concentrations of 10 and 100 micrograms/ml, estradiol-17 beta was unable to affect the cyclic AMP dependent protein kinase activation induced by lutropin. At the concentration of 10 micrograms/ml the steroid was without effect on the lutropin or dibutyryl cyclic AMP induced protein phosphorylations. However 100 micrograms/ml of estradiol-17 beta seemed to produce a slight inhibition of the induced protein phosphorylations.

Inhibition of parathyroid hormone release from dispersed bovine cells by ouabain

Ouabain, at concentrations of 3 X 10(-4)-10(-3) M, inhibited secretion of PTH approximately 50% in a freshly dispersed bovine parathyroid cell preparation. This inhibition was found with both unstimulated and secretagogue-stimulated PTH release. Reductions in PTH secretion were found at all concentrations of Ca++ tested between 0.3 mM and 2.0 mM and in the presence of the divalent cation chelators EDTA and EGTA, indicating that extracellular Ca++ is not an absolute requirement for the inhibition. The ouabain inhibition did not appear to be mediated through changes in either adenylate cyclase activity or total cellular cAMP, implying a locus distal to the generation of this cyclic nucleotide. The data suggest that transmembrane potential and/or distribution of monovalent cations across the plasma membrane is important in the maintenance of PTH secretion. The mechanisms involved in this control do not appear to involve extracellular Ca++ directly.

Inhibition of dibutyryl cyclic AMP-induced insulin release by alpha-2 adrenergic stimulation

Effects of alpha adrenergic agonists and antagonists on dibutyryl cAMP (Bt2cAMP)-induced insulin release were investigated with isolated rat pancreatic islets. Bt2cAMP (4 mM) produced 2-fold stimulation of insulin release in all the concentrations of glucose examined (3.3-16.7 mM). Clonidine but not phenylephrine inhibited the Bt2cAMP-stimulated insulin release in a concentration-dependent manner with approximate EC50 of nanomolar range. Yohimbine but not prazosin antagonized the inhibitory effect of clonidine on the Bt2-cAMP-induced insulin release. These results suggest that alpha-2 adrenergic mechanisms are involved in a step distal to cAMP generation.

Opposing actions of dibutyryl cyclic AMP and GMP on temperature in conscious guinea-pigs

Intracerebroventricular administration of dibutyryl cyclic AMP (Db-cAMP), induced hyperthermia in guinea-pigs which was not mediated through prostaglandins (PG) or norepinephrine since a prostaglandin synthesis inhibitor, indomethacin, and an alpha-adrenergic receptor blocking agent, phenoxybenzamine did not antagonize the hyperthermia. In contrast, the hyperthermic response to dibutyryl cyclic AMP was attenuated by central administration of a beta-adrenergic receptor antagonist, sotalol, indicating that cyclic AMP may be involved, through beta-adrenergic receptors, in the central regulation of heat production/conservation. Central administration of dibutyryl cyclic GMP (Db-cGMP) produced hypothermia which was not mediated via histamine H1- or H2-receptors and serotonin since the H1-receptor antagonist, mepyramine, the H2-receptor antagonist, cimetidine, and the serotonin antagonist, methysergide, had no antagonistic effects. The antagonism of hypothermia induced by dibutyryl cyclic GMP and acetylcholine + physostigmine, by central administration of a cholinergic muscarinic receptor antagonist, atropine, and not by a cholinergic nicotinic receptor antagonist, d-tubocurarine, suggests that cholinoceptive neurons and endogenous cyclic GMP may regulate heat loss through cholinergic muscarinic receptors. These results support a regulatory role in thermoregulation provided by a balance between opposing actions of cyclic AMP and cyclic GMP in guinea-pigs.

Effects of actinomycin D and cycloheximide on the differentiation of epidermal melanocytes of newborn mice

The increase in the number of epidermal melanocytes positive to the dopa reaction after birth was suppressed in C57BL/10J strain mice when injected with actinomycin D or cycloheximide. On the other hand, no change was observed in the melanoblast-melanocyte population positive to the combined dopa-premelanin reaction. The initiation of melanogenesis in the skin of newborn mice, therefore, seems to require de novo transcription and translation. The increase in the number of melanocytes after treatment with alpha-MSH or DBc-AMP was also completely suppressed when mice were injected with actinomycin D or cycloheximide. The results suggest that normal and hormone-induced differentiation of epidermal melanocytes requires de novo transcription and translation.

Blockade by haloperidol of the increase in tryptophan hydroxylase activity induced by incubation of slices of brain stem with dibutyryl cyclic AMP

Pretreatment of rat brain stem slices with dibutyryl cyclic AMP, caffeine, theophylline and 3-isobutyl-1-methylxanthine increased the activity of tryptophan hydroxylase in supernatant preparations of enzyme made from the slices. This effect does not appear to be mediated by a cyclic AMP sensitive mechanism since it was not reproduced by exposure of the slices to 8-bromo cyclic AMP, to papaverine, a nonxanthine phosphodiesterase inhibitor, or to other treatments known to raise tissue cyclic AMP levels. The ability of haloperidol to block this increase in enzyme activity is consistent with a role for calmodulin and calcium as mediators of the enzyme activation, particularly when this observation is considered in conjunction with the evidence that supernatant preparations of this enzyme was activated under phosphorylating conditions by a calcium-calmodulin dependent process [5]. Nevertheless, in view of the high concentrations of haloperidol employed in the present experiments, the possibility that this drug may produce its effects in the brain stem slices through some other action, unrelated to its ability to bind to calmodulin, should be kept in mind.

Inhibition of dibutyryl cyclic AMP induced steroidogenesis in rat adrenocortical cells by the putative calcium antagonist TMB-8

A significant proportion of the steroidogenic response of isolated rat adrenocortical cells to dibutyryl cyclic AMP does not require extracellular calcium, and this component is profoundly depressed by low concentrations of the putative calcium antagonist, TMB-8. The inhibition is reversed by either the readdition of calcium or the calcium ionophore A23187. The steroidogenic response to pregnenolone, whose mode of action does not require calcium, was not depressed by TMB-8. Corticotropin (ACTH)-induced steroidogenesis, which requires extracellular calcium, was markedly depressed by TMB-8, although enhanced cyclic AMP formation is only slightly depressed by this drug. Adrenal cortical microsomes possess an ATP-dependent 45calcium (45Ca2+) uptake system which responded to EGTA with a rapid efflux of 45Ca2+; EGTA-induced calcium efflux from this microsomal fraction was markedly reduced by a concentration of TMB-8 that blocked dibutyryl cyclic AMP-evoked steroidogenesis. TMB-8 produced a smaller but significant reduction of EGTA-facilitated 45Ca2+ efflux from a mitochondrial-enriched fraction. We interpret these results to mean that TMB-8 blocks the steroidogenic effect of dibutyryl cyclic AMP by interfering with the mobilization of a cellular pool of calcium that is probably localized to the endoplasmic reticulum. The physiological implications of these findings in relation to the complex interactions between calcium and cyclic AMP in adrenal steroidogenesis are discussed.

Cytochalasin inhibition of isolated rat gastric parietal cell function

Submicrogram concentrations (0.04-0.29 microM) of the microfilament disrupting agents cytochalasins D, E, and B (CD, CE, CB) were shown to inhibit secretagogue-stimulated 14C-aminopyrine accumulation (AP) in isolated rat gastric mucosal parietal cells. The microtubule disrupting agent colchicine had little influence on AP accumulation. Histamine- and dibutyryl cyclic AMP (DbcAMP)-stimulated AP accumulation was inhibited with an order of potency CD greater than CE approximately equal to CB. CB inhibition of these secretagogue actions was, however, only approximately 65-70% of the maximal stimulated response, whereas CD and CE caused 100% inhibition. On the other hand, carbamylcholine-stimulated AP accumulation was inhibited 100% by all cytochalasins tested with an order of potency CD approximately equal to CE greater than CB. These data are discussed in relation to acid secretagogue-induced morphological changes involving actin filament organization in parietal cells.

Specific inhibition by prostaglandins E2 and I2 of histamine-stimulated [14C]aminopyrine accumulation and cyclic adenosine monophosphate generation by isolated canine parietal cells

The effects of prostaglandins E2 and I2 on accumulation of [14C]aminopyrine and the generation of cyclic AMP by fractions of dispersed canine gastric mucosal cells, enriched in their content of parietal cells, have been studied. The parietal cell content of the fractions was enriched to between 43 and 70% using an elutriator rotor. The accumulation of [14C]aminopyrine was used as the index of parietal cell response to stimulation. Prostaglandin E2 (PGE2, 0.1 nM-0.1 mM) inhibited histamine stimulated aminopyrine uptake but did not block the response to carbachol, gastrin, or dibuturyl cyclic AMP. PGE2 did, however, inhibit aminopyrine uptake stimulated by carbachol and gastrin when the response to these agents was potentiated by histamine. PGE2 (0.1 NM-0.1 mM) inhibited histamine-stimulated cyclic AMP production in a dose-dependent fashion with maximal inhibition at 1 microM PGE2. Prostacyclin also inhibited both histamine-stimulated aminopyrine accumulation and histamine-stimulated cyclic AMP production. In the absence of added histamine, PGE2 in concentrations above 1 microM and prostacyclin in concentrations above 10 microM stimulated cyclic AMP production, probably by acting on the nonparietal cells as shown in previous studies. These present data are consistent with the hypothesis that prostaglandins E2 and I2 inhibit the response of isolated parietal cells to histamine by specifically blocking histamine-stimulated cyclic AMP production.

Inhibition of H+ secretion in frog gastric mucosa by somatostatin

Somatostatin added to the serosal bathing solution of the isolated gastric mucosa of Rana pipiens significantly inhibited pentagastrin- and histamine-stimulated H+ secretion. The decrease in H+ secretion rate was accompanied by an increase in the transmucosal potential difference and resistance. Somatostatin (10(-5) M) had no effect on the N6,O2-dibutyryl adenosine 3'-5'-cyclic monophosphate (DBcAMP)-stimulated H+ secretion rate. The mucosa exposed to somatostatin secreted H+ on stimulation by DBcAMP or histamine, but did not respond to 2.8 X 10(-7) M pentagastrin. However, pentagastrin added to the serosal solution stimulated H+ secretion after the somatostatin was washed away. Calcium inophore (3 X 10(-5) M) alone or 10(-2) M Ca2+ plus calcium ionophore temporarily increased the H+ secretion rate inhibited by somatostatin. The data suggest that somatostatin has a direct effect on the oxyntic cells in the gastric mucosa.

Failure of dibutyryl and 8-bromo-cyclic GMP to mimic the antagonistic action of carbachol on the positive inotropic effects of sympathomimetic amines in the canine isolated ventricular myocardium

Effects of carbachol, dbcGMP and 8-bromo-cyclic GMP on the positive inotropic actions of sympathomimetic amines and dbcAMP were studied on the canine isolated right ventricular myocardium. Carbachol alone did not substantially change the developed tension of the muscle, but did markedly shift the dose response curve for isoprenaline on the developed tension to the right and depressed the maximal response to the drug in a concentration dependent manner. The positive inotropic action of phenylephrine was affected by carbachol in the same manner as that of isoprenaline. DbcGMP in concentrations of 10(-3) M and higher produced a significant increase in the developed tension. The positive inotropic action of dbcGMP was partly inhibited by a beta-adrenoceptor blocking agent, pindolol. In the presence of dbcGMP, isoprenaline produced an action similar to that seen in the control experiment, but this action was not maintained on such a steady state level as in the control. This rapid decline of the effect of isoprenaline in the presence of dbcGMP was prevented by adding ascorbic acid to the organ bath. The positive inotropic actions of phenylephrine and of dbcAMP were not substantially affected by dbcGMP. 8-Bromo-cyclic GMP in a concentration of 10(-4) M did not change either the basal developed tension or the positive inotropic actions of noradrenaline and of isoprenaline. The present results indicate that these cyclic GMP derivatives are not able to mimic the antagonistic action of cholinergic stimulation on the positive inotropic action of adrenergic stimulation on the positive inotropic action of adrenergic stimulation on the canine ventricular myocardium.

Effects of prostaglandins on the action of luteinizing hormone in dispersed rat intestitial cells

Prostaglandins E1, E2, A1 and A2 at 10(-5) and 10(-4)M stimulated basal testosterone production in dispersed rat interstitial cells in vitro. They effectively inhibited steroidogenesis induced by ovine pituitary luteinizing hormone (LH) (0.2 nM), dibutyryl cyclic AMP-(DBC 0-5 mM), and cholera toxin (100 ng). PGF2 alpha (10(-3) to 10(-12)M) had no effect on either basal or hormonal or non-hormonal stimulated steroidogenesis in the cells. PGA1 and PGA2 were more effective inhibitors than PGE1 and E2. None of the PG's had any influence on 125I LH-receptor interaction. In view of this and the inhibition of DBC stimulated testosterone production, it may be suggested that the PG inhibition lies beyond the receptor-cyclic AMP formation step.

Lithium: a modulator of cyclic AMP-dependent events in lymphocytes?

Theophylline, salbutamol, isoproterenol, and dibutyryl cyclic AMP inhibited E-rosette formation by human T lymphocytes and immunoglobulin M secretion from human plaque-forming B cells and augmented T-suppressor cell activity in three patients with agammaglobulinemia. Lithium chloride increased mitogen-induced lymphocyte proliferation and inhibited suppressor cell activity. In the presence of lithium, the effects of all the drugs except dibutyryl cyclic AMP could be prevented. The data suggest a role for lithium in the modulation of cyclic AMP-dependent events in lymphocytes. Its potential role as an inhibitor of suppressor cell activity warrants further attention.