Phosphodiesterase inhibition by a gastroprotective agent irsogladine: preferential blockade of cAMP hydrolysis

The effect of irsogladine [2,4-diamino-6-(2,5-dichlorophenyl)-s-triazine maleate], an antiulcer drug, on contents of cyclic nucleotides including cAMP and cGMP was investigated in rat stomachs. Irsogladine concentration-dependently increased cAMP content in rat glandula stomach. However, irsogladine at higher concentration (10(-5) M) was unable to further increase cAMP level in the presence of non-selective phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine, although 3-isobutyl-1-methylxanthine by itself increased cAMP level. On the other hand, irsogladine had no effect on the glandula cGMP content. Subsequently, the effect of irsogladine on the cyclic nucleotide degradation by purified bovine brain and heart PDEs was investigated. The cAMP degradation by purified bovine brain PDE was partially suppressed by PDE1 inhibitor vinpocetin, PDE2 inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride and PDE4 inhibitor rolipram but not by PDE3 inhibitor cilostamide, and completely inhibited by 3-isobutyl-1-methylxanthine, suggesting that is attributed almost exclusively to PDE1, PDE2 and PDE4. Meanwhile, cGMP degradation by purified bovine brain PDE was partially suppressed by erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride. Irsogladine preferentially inhibited the response to cAMP degradation compared with cGMP degradation by this brain PDE. The cAMP degradation by bovine heart PDE was almost completely inhibited by the combination with vinpocetine and cilostamide, indicating that is mediated almost exclusively by PDE1 and PDE3. Irsogladine suppressed this cAMP degradation measured in the presence of vinpocetine to almost the same extent as that determined in the presence of cilostamide. These results indicate that irsogladine produces the increase of intracellular cAMP content via non-selective inhibition of PDE isozymes, which may be a key mechanism involved in its gastroprotective actions.     

Metabolic pathway of magnetized fluid-induced relaxation effects on heart muscle

The effect of magnetized physiological solution (MPS) on isolated, perfused snail heart muscle contractility, (45)Ca uptake and intracellular level of cAMP, and cGMP was studied. The existence of the relaxing effect of MPS on heart muscle at room temperature (22 degrees C) and its absence in cold medium (4 degrees C) was shown. The MPS had a depressing effect on (45)Ca uptake by muscles and intracellular cAMP content and an elevating effect on intracellular cGMP level. It is suggested that the relaxing effect of MPS on heart muscle is due to the decrease of intracellular Ca ions as the result of activation of cGMP-dependent Ca efflux. The MPS induced decrease of intracellular cAMP content can be considered as a consequence of intracellular Ca loss, leading to the Na + K-ATPase reactivation, and causing the decrease of the intracellular level of ATP, serving as a substrate and positive modulator of cyclase activity.     

Cyclic nucleotides and lipolysis in rat fat cells. Interaction between calcium ionophore A23187 and FCCP,uncoupler of oxidative phosphorylation

The interaction between calcium ionophore A23187 and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) has been studied at the level of cyclic-AMP (cAMP), cyclic-GMP (cGMP) and lipolysis in isolated rat fat cells. Ionophore A23187 (1–10 μM) stimulated cGMP accumulation and glycerol release without affecting cAMP level. FCCP (1–100 μM) inhibited the effect of A23187 on cGMP level and glycerol release, but did not affect or increase cAMP. Thus a correlation exists between the changes of cGMP levels and lipolysis and a dissociation of lipolysis from cAMP.     

The role of cyclic nucleotides and prostaglandins in heart function.

A short review of the role of cyclic nucleotides and prostaglandins (PGs) in normal and pathological functions of the heart is given. Possible interrelationships of these two regulatory systems have been studied by using spontaneously beating rat atria preparations. Addition of noradrenaline (NA) to the incubate (1 . 10(-6) M) caused an increase in amplitude and frequency which was preceded and parallelled by an elevation of the tissue cAMP level. A transient increase in cGMP and PGE values was also seen. Propranolol (5 . 10(-6) M) abolished the increase in amplitude and frequency as well as in cAMP and PGE concentrations. Indomethacin (1 . 10(-5) M) inhibited the formation of PGE. The increase in cGMP was blocked by phenoxybenzamine. Interchange between beta- and alpha-receptors according as the temperature is lowered has been described earlier. Hypothermia (20 degrees C) had a positive inotropic effect on the atria and increased the tissue cAMP concentration. Loading of the atria caused an increase in cAMP without any effects on cGMP or PGs. Slight hypoxia did not change the cAMP or PG levels, but elevated the cGMP values. Arrhythmias induced by hypo- or hyperpotassemia did not modify the biochemical parameters measured. PGF2alpha (1. 10(-5) M) normalized the atrial rhythm and increased the amplitude without changing cyclic nucleotide or PG levels. PGE1 (1 . 10(-4) M) increased the amplitude of normorhythmic atria and the tissue concentration of cAMP. PGE2 was the only PG tested which stimulated the heart adenylate cyclase in vitro. There seems to be close but complicated relationships between cyclic nucleotides and PGs in the heart.     

Changes in cyclic nucleotide levels during embryonic development of chick hearts

The effects of isoproterenol, acetylcholine (Ach), and adenosine, on cyclic AMP (cAMP) and cyclic GMP (cGMP) contents were examined in chick hearts at various stages of embryonic development. The basal cAMP content was highest (87.7 +/- 1.3 pmol/mg protein) in young (3-day) embryonic chick hearts and decreased during development (9.6 +/- 0.6 pmol/mg protein in 9-19-day-old hearts). On the other hand, the cGMP content was lowest (45.5 +/- 2.3 fmol/mg protein) in young (3-day) embryonic chick hearts and increased during development (338 +/- 15.0 fmol/mg protein in 14-19-day-old hearts). Iso increased the cAMP concentration in embryonic hearts at all ages. Ach and Ado had no effect on the cAMP content at all ages. However, the Isoproterenol-induced stimulation of cAMP was inhibited by Ach and Adenosine at all ages. In young embryonic hearts, Ach and Ado increased cGMP concentration only slightly, whereas these agents caused a substantial increase in cGMP concentration in the older hearts. Thus, there was a clear age difference in the effects of Ach and Adenosine on the cGMP and cAMP concentrations. Nitroprusside and hydrogen peroxide increased cGMP concentration in older hearts (greater than 5-day-old) but not in the 3-day-old embryonic hearts. Thus, guanylate cyclase activity may be low in young (3-day-old) hearts. It summary, the cGMP level is very low in young embryonic chick hearts, and increases markedly during development. The changes in cGMP are reciprocal to those of cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Acid hydrolase activity and cyclic nucleotide contents in the rat heart during myocardial ischemia and postischemic reperfusion

The acid phosphatase and cathepsin D activities and cAMP and cGMP levels in isolated perfused rat heart were investigated during various periods of ischaemic myocardial injury and postischaemic reperfusion. The effect of phosphodiesterase inhibitor--caffeine was also studied. Free acid hydrolases activities and cyclic nucleotide content were increased under 40 and 60 min ischemia and 20 min postischaemic reperfusion. Addition of 50 microM caffeine to perfusion solution after 30 min of ischaemia resulted in increase of cAMP level, cAMP/cGMP ratio, lysosomal bound activities of acid hydrolase and decrease of free acid hydrolase activities. The obtained results suggested that defect in cAMP synthesis might be present in lysosomal membranes labilization in cardiomyocytes injured during ischaemic conditions. Addition of such agents, as caffeine, which increased heart cAMP level, may be effective in lysosomal membranes stabilization under reversible heart ischaemia and reperfusion.     

A study of signal transduction for the two diuretic peptides of Diploptera punctata

We investigated second messengers involved in the action of the CRF-related peptide Dippu-DH46 and the calcitonin-like peptide Dippu-DH31 in Diploptera punctata. Dippu-DH46 causes a dose-dependent increase in intracellular cAMP levels, its diuretic activity is mimicked by cAMP agonists, but is attenuated by Rp-cAMPS. Dippu-DH46 acts synergistically with kinins and thapsigargin; both mobilize intracellular Ca2+. Dippu-DH46 also acts synergistically with cAMP agonists, and its effect is inhibited by a PKC inhibitor, suggesting it also activates intracellular Ca2+. Dippu-DH31 has no effect on cAMP levels and its activity is not blocked by cAMP agonists. Neither peptide stimulated cGMP levels in a dose-dependent manner, nor does cGMP have any effect on fluid secretion.     

Effects of intracellular cyclic AMP and cyclic GMP levels on DNA synthesis of young-adult rat hepatocytes in primary culture.

Possible roles of dibutyryladenosine 3',5'-cyclic monophosphate (cAMP) and dibutyryl-guanosine 3',5'-cyclic monophosphate (cGMP) in regulation of hepatocyte DNA synthesis were examined using primary cultures of young-adult rat hepatocytes maintained in arginine-free medium. Throughout the experimental period, nonparenchymal cells were hardly observed in the selective medium. When epidermal growth factor (EGF) was added to the cultures, a transient increase in the intracellular cAMP level preceded the elevation of hepatocyte DNA synthesis. EGF-stimulated hepatocyte DNA synthesis was remarkably enhanced by the elevation of the intracellular cAMP level induced by treatment with cAMP alone or a combination of cAMP and theophylline, an inhibitor of cyclic nucleotide phosphodiesterase. Furthermore, the early elevation of intracellular cAMP alone, which was induced by treatment with the combination of cAMP and theophylline, caused a remarkable increase in hepatocyte DNA synthesis. On the other hand, addition of EGF to the cultures caused a rapid decrease in the intracellular cGMP level followed by an increase in hepatocyte DNA synthesis. EGF-stimulated hepatocyte DNA synthesis was severely suppressed or completely inhibited by the elevation of the intracellular cGMP level induced by treatment with cGMP alone or a combination of cGMP and dipyridamole, a specific inhibitor of cGMP phosphodiesterase. These findings indicate that cAMP and cGMP act oppositely on the regulation of DNA synthesis of young-adult rat hepatocytes in primary culture: cAMP plays a positive role, whereas cGMP plays a negative role. Also it is strongly suggested that an early elevation of the intracellular cAMP level is essential for the onset of DNA synthesis in hepatocyte primary cultures.     

The role of cyclic nucleotides in the regulation of mitotic activity in SSPE virus-infected human brain tissue.

The intracellular level of cGMP was independent of the rate of cell division in cells derived from virally infected brain tissue. The phosphodiesterase inhibitor R07-2956 (4-dimethoxybenzyl-2-imidazolidinone) increased the intracellular level of cGMP in virally infected brain cells, but it did not effect the level of cAMP. There was no correction between the increase in cGMP levels following addition of R07-2956 and changes in mitotic activity in the brain cell cultures. Experimental manipulations which increased the cAMP level were accompanied by a decreased mitotic rate indicating there was a correlation between mitotic activity and the level of cAMP in the same cells. Raising the intracellular level of cAMP by exogenous db-cAMP or cAMP or the use of other phosphodiesterase inhibitors routinely increased the level of cGMP as well. Conversely increasing the intracellular cGMP level by adding the exogenous cGMP increased the level of both cGMP and cAMP.A tissue culture system was used with the cell line derived from viral infected human brain tissue originally obtained from a patient with subacute sclerosing panencephalitis (SSPE). The intracellular levels of cAMP and cGMP were monitored by radioimmunoassay following manipulation of the system by addition of exogenous cGMP (0.05 mM), addition of exogenous db-cAMP (0.5 mM), or cAMP (0.5 mM) and the use of phosphodiesterase inhibitors: theophylline (1.0 mM), papaverine (50 μg/ml), 4-3-butoxy-4-methoxy benzyl-2-imidozalidinone (R020-1724) and R07-2956. Cell division was monitored in treated and non-treated cultures at 24 h intervals by analyzing the cell number and mitotic index.High levels of cGMP were found in cells which were not actively dividing but high levels were just as apt to be present in dividing cells. There was an inverse relationship between cell division and the level of cAMP.     

Activation of Guanylate Cyclase by Bradykinin in Rat Sensory Neurones Is Mediated by Calcium Influx: Possible Role of the Increase in Cyclic GMP

Bradykinin, which activates polymodal nociceptors, increased cyclic GMP (cGMP) in a capsaicin-sensitive population of cultured sensory neurones from rat dorsal root ganglia (DRG) by stimulating guanylate cyclase, but had no effect on cyclic AMP (cAMP). In nonneuronal cells from DRG, bradykinin increased cAMP, but not cGMP. The bradykinin-induced increase in cGMP in the neurones was completely blocked by removal of extracellular Ca2+, or by incubation of the cells with the calcium channel blockers nifedipine and verapamil. Pretreatment of the neurones with either dibutyryl cGMP or sodium nitroprusside (which elevates cGMP) inhibited bradykinin-induced formation of inositol phosphates. It is possible that cGMP could be involved in the regulation of polyphosphoinositide turnover in DRG neurones.     

Effects of radioprotectors on the cAMP and cGMP systems

Summary The sulphur-containing radioprotectors mercaptoethylamine (MEA), aminoethylisothiourea (AET), 2-aminothiazoline, 4-oxo-2-aminothiazoline, and S-S-3-oxapentane-1,5-diisothiourea, and the radioprotective biogenic amines serotonin, histamine, and dopamine, caused the elevation of cAMP content and intensified the rate of cAMP-dependent protein phosphorylation in tissues of animals following intraperitoneal injection at radioprotective doses. Biogenic amines stimulated the adenylate cyclase activity in membrane preparations from liver, spleen, and small-intestine mucosa; sulphur-containing radioprotectors caused no such effects. None of the radioprotectors affected cAMP and cGMP phosphodiesterases in vitro. AET and MEA inhibited guanylate cyclase in vitro, whereas serotonin and dopamine stimulated the enzyme. A biphasic change in the level of cGMP was observed in tissues after the administration of MEA and AET (more than 2-fold fall by 1–3 min after the administration of drug and 1.4-fold rise after 15–20 min); serotonin and dopamine caused a slow rise in the cGMP level; the cAMP/cGMP ratio in liver showed biphasic changes in level during the 20 min following injection of serotonin.The data obtained support the conclusion that the action of radioprotectors on cellular metabolism in animals may be mediated by the cAMP system. The reciprocal regulation of radioresistance by cAMP and cGMP is unlikely to exist.     

Effects of cyclic nucleotides on lipid biosynthesis in mouse mammary gland explants

The effects of dibutyryl cAMP, 1-methyl-3-isobutyl xanthine (MIX), cGMP, dibutyryl cGMP, and 8-bromo cGMP on the rate of lipid synthesis in mouse mammary gland explants were studied. Dibutyryl cAMP at 10(-4) M selectively inhibited the effect of prolactin on the rate of [14C]acetate incorporation into lipids. At 10(-3) M, dB-cAMP inhibited the effects of insulin, insulin plus cortisol, and prolactin. The phosphodiesterase inhibitor, MIX, inhibited both basal and prolactin-stimulated incorporation rates in a concentration-dependent fashion. These data suggest an inhibitory role for cAMP in the regulation of lipogenesis in the mammary gland. Cyclic GMP, db-cGMP, and 8-bromo cGMP were all without effect on either basal or prolactin-stimulated incorporation rates. Therefore, it appears that cGMP, by itself, is not involved in the regulation of lipogenesis in the mammary gland.     

Differential susceptibility to biological detergents of the particulate cGMP-stimulated phosphodiesterase from rat heart: preservation of the allosteric properties of the solubilized enzyme

The effects of various biological detergents on the particulate cGMP-stimulated cAMP phosphodiesterase activity from rat heart were investigated. When added to particulate fractions, anionic and non-ionic detergents diversely increased both cAMP and cGMP phosphodiesterase activities and slightly decreased the stimulatory effect of cGMP on cAMP hydrolysis whereas cationic detergents were rather inhibitory and drastically lowered the stimulatory effect of cGMP. Among the most efficient detergents, only sodium cholate was able to solubilize phosphodiesterase activity and preserve the stimulatory effect of cGMP on cAMP hydrolysis. Furthermore, the addition of glycerol significantly improved the conservation of the allosteric properties of the enzyme. Kinetic properties of the cholate-solubilized phosphodiesterase were quite identical to those of the membrane-bound enzyme.     

Effect of cGMP-dependent signaling system in regulation of osmotic permeability in the frog urinary bladder

In frogs' isolated urinary bladders, contribution of cytosolic guanylate cyclase and cGMP-dependent protein kinase to regulation of osmotic permeability was studied. ODQ (25-100 microM), an inhibitor of cytosolic guanylate cyclase induced an increase of vasotocin-activated osmotic permeability but had no effect on the hormone-activated transepithelial urea transport. In isolated mucosal epithelial cells ODQ (50 microM) decreased the concentration of intracellular cGMP. In these cells L-NAME (0.5 nM), an inhibitor of NO synthase, also decreased the level of cGMP whereas cAMP was significantly increased. 8-pCPT-cGMP (25 and 50 microM), a permeable cGMP analogue which selectively activates protein kinase G, inhibited vasotocin-induced increase of water transport along osmotic gradient indicating that protein kinase G is involved in regulation of water reabsorption. The data obtained show that NO/cGMP signalling system in the frog urinary bladder appears to be a negative modulator of vasotocin-activated increase of osmotic permeability.     

The effects of valproate and phenytoin on the cAMP and cGMP levels in nervous tissue

Earlier studies have demonstrated that valproic acid (VPA) and phenytoin (PHT) influence the excitability properties of crayfish axons through different mechanisms. PHT was found to antagonize the electrophysiologic effects of VPA. The purpose of the present study was to determine if the electrophysiologic effects of VPA and PHT are correlated with changes in the cellular levels of either cAMP or cGMP as these substances are known to influence membrane excitability. It was found that PHT (0.1 mM) has no effect on the levels of either cAMP or cGMP within crayfish neural tissue. VPA (4.0 mM) also has no effect on cAMP levels. However, it does significantly reduce the levels of cGMP. Pretreatment of neural tissue with PHT has been shown to eliminate the effects of VPA on membrane excitability. It was found that this pretreatment has no influence on VPA's ability to reduce cGMP levels. The effect of VPA on cGMP levels is observed in the absence of spontaneous activity. Therefore, it is concluded that the observed reduction in cGMP levels does not represent the modulation of cGMP levels that is known to accompany activity. Two experiments demonstrate that the 4-mV depolarization of membranes by VPA can not account for its effect on cGMP levels. In the first, pretreatment with PHT abolished the depolarizing effect on VPA but not its effect on cGMP. In the second, a concentration of ouabain which depolarizes crayfish neural tissue by 8-10 mV without producing spike activity had no effect on either cAMP or cGMP levels. These experiments effectively dissociate the electrophysiologic response to VPA and PHT from changes in cyclic nucleotide levels.     

Effect of delta 9-tetrahydrocannabinol on cyclic nucleotides in synchronously dividing Tetrahymena

Cyclic nucleotide levels were determined in division-synchronized Tetrahymena and the effect of delta 9-tetrahydrocannabinol (THC) on the cyclic nucleotide levels was studied. In non-drug-treated division-synchronized cells, there was no statistically significant variation in the level of cAMP and cGMP during the G2 period, preceding the first division. During the free running cell cycle (the interval of time between the first and second synchronous division) the twofold increase in the level of cAMP was statistically significant; however the variation in the level of cGMP was not statistically significant. THC caused a lowering of cAMP and cGMP levels throughout the 4-experimental treatment. The suppression of cAMP and cGMP levels altered the cyclic nucleotide pattern of the cell cycle. The cAMP pattern was changed particularly in the G2 period preceding the first synchronous division, and immediately after division during the free running cell cycle. THC treatment caused division delays of approximately 8-15 min in the onset of the first and second synchronous division. However, the duration of the free running cell cycle (110-120 min) was unchanged. The suppression of cyclic nucleotide levels resulting from THC treatment is discussed in relation to delays in the division schedule.     

Zinc-mediated inhibition of cyclic nucleotide phosphodiesterase activity and expression suppresses TNF-alpha and IL-1 beta production in monocytes by elevation of guanosine 3',5'-cyclic monophosphate

The trace element zinc affects several aspects of immune function, such as the release of proinflammatory cytokines from monocytes. We investigated the role of cyclic nucleotide signaling in zinc inhibition of LPS-induced TNF-alpha and IL-1beta release from primary human monocytes and the monocytic cell line Mono Mac1. Zinc reversibly inhibited enzyme activity of phosphodiesterase-1 (PDE-1), PDE-3, and PDE-4 in cellular lysate. It additionally reduced mRNA expression of PDE-1C, PDE-4A, and PDE-4B in intact cells. Although these PDE can also hydrolyze cAMP, only the cellular level of cGMP was increased after incubation with zinc, whereas cAMP was found to be even slightly reduced due to inhibition of its synthesis. To investigate whether an increase in cGMP alone is sufficient to inhibit cytokine release, the cGMP analogues 8-bromo-cGMP and dibutyryl cGMP as well as the NO donor S-nitrosocysteine were used. All three treatments inhibited TNF-alpha and IL-1beta release after stimulation with LPS. Inhibition of soluble guanylate cyclase-mediated cGMP synthesis with LY83583 reversed the inhibitory effect of zinc on LPS-induced cytokine release. In conclusion, inhibition of PDE by zinc abrogates the LPS-induced release of TNF-alpha and IL-1beta by increasing intracellular cGMP levels.     

Guanosine 3',5'-monophosphate and the action of alkylating agents.

The intracellular level of guanosine 3',5'-monophosphate (cGMP) has been measured in Walker carcinoma cells in tissue culture after treatment with various alkylating agents. At concentrations which caused a rise in the level of adenosine 3',5'-monophosphate (cAMP) chlorambucil and 5-(1-aziridinyl)-2,4-dinitrobenzamide (CB 1954) produced only a small (35%) elevation of cGMP, while merophan had no such effect. This suggests that any effect of cAMP will not be outweighed by an equivalent rise in cGMP. Sepcific cytosolic binding of cGMP decreased with increasing resistance of Walker cells to alkylating agents, while the dissociation constant, KD, for binding increased. This was also observed with cAMP binding which suggests that the same protein in responsible for binding both nucleotides.     

Effect of agents for the chemical prevention of radiation sickness on the cyclic nucleotide content in the bone marrow of mice

The effect of different chemical compounds on the cAMP/cGMP ratio in the bone marrow of mice and radioresistance of animals has been studied. It has been shown that all compounds possessing radioprotective properties give rise to the cAMP/cGMP ratio in the bone marrow of mice. No changes in cAMP and cGMP level were noted after the administration of nonradioprotective substances. The maximal radioprotective effect coincide in time with the largest increase of the cAMP/cGMP ratio. The injection of radioprotectors at different doses demonstrate clearly that only at radioprotective doses the increase in the cAMP/cGMP ratio takes place. The administration of some substances 2, 15 and 60 min after the irradiation of mice shows that the radioprotective effect, though modest, was evident only in one case of elevated cAMP/cGMP ratio (the injection of 2-Mercaptoethylamine 2 min after the irradiation). Evident radioprotective effect occurs at the cAMP/cGMP ratio of about 170-200%; the ratio of about 130-140% corresponds to small radioprotection.