Psychological stress reduces cyclic 3',5'-adenosine monophosphate levels in the cerebral cortex of conscious rats, as determined by a new cryogenic method of rapid tissue fixation.

Abstract— Cryoplates were implanted on the surface of the cortex in 32 chronic rat preparations. These devices were used both to freeze and to extract small samples of tissue. Coolant was circulated through each device by small flexible polyethylene tubes. Two series of experiments were performed. In the first, the animals were unrestrained and showed no behavioral signs of stress during the freeze fixation. The temperature responses of the cryoplates were very rapid (?632°C/s), and samples more than 1 mm thick were frozen and extracted within a few hundred ms following the onset of cooling. Each sample was analyzed for 3′.5′-adenosine monophosphate (cyclic AMP) and protein content. The results from the cryoplate group (25.6 ± 15.6pmol cyclic AMP/mg protein) were compared to those obtained from two other groups in which freeze fixation was produced by immersion in liquid nitrogen (13.6 ± 4.6pmol/mg protein) or decapitation into liquid nitrogen (18.6 + 7.6pmol/mg protein). In the second series of experiments, three types of stress (limb restraint, non-adaptation to the experimental situation, and moderate cutaneous electric shock) were induced separately in order to determine the influence of each on cortical levels of cyclic AMP. Control animals were highly adapted to the experimental situation, freely moving and not shocked. The samples from each of the stressed groups showed a statistically significant (P≤. 0.01) reduction in cyclic AMP in comparison with the level in the controls (control: 29.3 pmol/mg protein; restrained: 14.2pmol/mg protein; unadapted: 9.6pmol/mg protein; shocked: 7.1 pmol/mg protein). Thus, psychological and physical stress reduced cyclic AMP content in parietal cortex. Results from the second series of experiments suggest that the significantly higher mean and larger standard deviation of the cryoplate group in the first series are due to less psychological and physical stress being evoked by our method; different types of stress appear to account for the two different lower levels found in the immersion and decapitation groups. We believe that our method of cryogenic tissue fixation offers an improved approach to study of the neurochemical correlates of behavioral and neuroelectric events in the conscious animal.     

MICROASSAY OF ADENOSINE-3′,5′-MONOPHOSPHATE (CYCLIC AMP) IN BRAIN AND OTHER TISSUES BY THE LUCIFERIN-LUCIFERASE SYSTEM1

Abstract— A simple, sensitive and specific method for assaying cyclic AMP in various tissues is reported. Cyclic AMP was isolated from contaminating nucleotides and was converted to ATP with a phosphodiesterase-myokinase-pyruvate kinase system. The ATP was determined enzymically in a liquid scintillation counter by the firefly luciferin-luciferase technique. This procedure was capable of detecting as little as 5 × 10^?14 mol of cyclic AMP and could therefore be used for analyses on less than 1 mg of brain. The assay was reproducible and linear over a wide range of tissue concentrations. In the rat, the highest levels of cyclic AMP (2.7–4.2 pmol/mg wet wt. of tissue) were present in the pineal, heart, pituitary, thyroid, cerebellar cortex, kidney, adrenal, liver and pyloric region of the stomach; intermediate levels (1.5–2.7 pmol/mg wet wt. of tissue) were found in testis, skin, aorta, intestine, submaxillary gland, spleen, muscle and cerebral cortex, moderately low levels (1.0–1.5 pmol/mg wet wt. of tissue) were found in lung, trachea and greater curvature of the stomach; whereas low levels (0.15–0.60 pmol/mg wet wt. of tissue) were found in adipose tissue.     

The adenylate cyclase-cyclic AMP-protein kinase system in different cell populations of the guinea pig gastric mucosa

In isolated guinea pig gastric mucous and enriched parietal cells it was tested whether or not cyclic AMP in response to histamine stimulation might reach concentrations sufficiently high to activate an intracellular cyclic AMP-dependent protein kinase and thereby mediate the acid response. Although histamine stimulated parietal cell adenylate cyclase to a greater extent than mucous cell adenylate cyclase, cyclic AMP levels in response to maximal histamine stimulation reached higher levels in mucous than in parietal cells. This had to be attributed to a five times higher phosphodiesterase activity in parietal cell than in mucous cell populations. In the absence of the phosphodiesterase inhibitor isobutylmethylxanthine exposure of the cells to histamine only in mucous cells produced an increase in cyclic AMP-dependent protein kinase activity ratio, but not in parietal cells. Dibutyryl-cyclic AMP induced cyclic AMP accumulation in parietal cell populations was compared to dibutyryl-cyclic AMP induced H+ secretion, as measured by 14C-aminopyrine uptake. A maximal acid response was associated with an intracellular cyclic AMP level of approximately 300 pmol/10(6) cells, which was never reached by maximal histamine stimulation even not in the presence of the phosphodiesterase inhibitor. It is concluded that activation of the parietal cell cyclic AMP-dependent protein kinase is one way for stimulating H+ secretion, but that the acid response elicited by histamine requires another intracellular pathway.     

CYCLIC NUCLEOTIDES IN MURINE BRAIN: EFFECT OF HYPOTHERMIA ON ADENOSINE 3'',5'' MONOPHOSPHATE, GLYCOGEN PHOSPHORYLASE, GLYCOGEN SYNTHASE AND METABOLITES FOLLOWING MAXIMAL ELECTROSHOCK OR DECAPITATION

Abstract —The accumulation of adenosine-3',5'-cyclic monophosphate (cyclic AMP) has been investigated in murine brain following electroconvulsive shock and decapitation. Animals were made hypothermic (20°C) to minimize the freezing time of the brain and to delay metabolic events. Cyclic AMP concentrations were decreased in the cerebral cortex of hypothermic rats and mice. Furthermore, the changes in cyclic AMP elicited by electroconvulsive shock and decapitation were delayed. In hypothermic animals, the metabolic rate as determined by high energy phosphate use was decreased to 65% of control values. The interconversions of the active and inactive forms of glycogen phosphorylase and glycogen synthase were sufficiently retarded in hypothermic animals to correlate with changes in cyclic AMP concentrations. The conversion of phosphorylase b to a and synthase a to b occurred when cyclic AMP concentrations had increased from 2 to 5 μmol/kg, following either electroconvulsive shock or decapitation. The results indicate that cyclic AMP plays a role in regulation of glycogen metabolism in cerebral cortex.     

Influence of cyclic AMP on the growth response and anaerobic metabolism of carbon monoxide in Rhodocyclus gelatinosus

Rhodocyclus gelatinosus strain 1 (str. 1), a photoheterotrophic bacterium, used CO as an energy substrate under anaerobic CO/light conditions, and exhibited a diauxic growth response when CO was removed from the culture. Changes in the level of cyclic AMP which occurred in cells during diauxie suggested that the cyclic nucleotide operated as an intracellular control molecule. During CO/light-phase growth, intracellular cyclic AMP was 30 pmol/mg protein, and, as str. 1 adapted for photosynthetic growth after removal of CO, intracellular cyclic AMP levels decreased to 9 pmol/mg protein. Reexposure of a light culture to CO induced synthesis of CO oxidation activity (measured as CO:MV oxidoreductase). If 10 mM cyclic AMP was added with CO, the rate of synthesis of CO:MV oxidoreductase activity increased 25-fold, and str. 1 produced 1,230 units of activity (nmol CO oxidized min^-1 mg^-1 protein) after only 1 h. With cyclic AMP and no CO, no incerease in CO oxidation activity was seen. Appearance of CO oxidation activity in str. 1 represented de novo protein synthesis and was blocked with chloramphenicol. In addition to stimulating formation of CO oxidative activity, a high level of cyclic AMP in str. 1 during growth with CO appeared to influence photometabolism negatively by repressing bacteriochlorophyll formation.Abbreviations Bchl a bacteriochlorophyll a - MV methyl viologen - CO MV oxidoreductase, carbon monoxide: methyl viologen oxidoreductase     

Effect of nitrogen starvation on the level of adenosine 3′,5′-monophosphate in Anabaena variabilis

Low levels of adenosine 3′,5′-monophosphate (cyclic AMP) were detected in the cyanobacterium Anabaena variabilis using a protein binding assay and two radioisotopic labelling methods. The basal concentration of intracellular cyclic AMP ranged from 0.27 pmol/mg protein in A. variabilis Kutz grown under heterotrophic conditions to 1.0–2.7 pmol/mg protein in A. variabilis strain 377 grown autotrophically. Extracellular cyclic AMP was found to comprise as much as 90% of the total cyclic AMP in rapidly growing cultures. When A. variabilis strain 377 was starved of nitrogen, a 3–4-fold increase in intracellular cyclic AMP was observed during the 24 h period coincident with early heterocyst development.     

Calcium ions and their relation to sympathetic ganglionic transmission and to cyclic AMP levels in isolated superior cervical ganglion of the rat in vitro.

The rat superior cervical ganglion was investigated $\text{in vitro}$ by means of extracellularly recorded compound action potentials and of cyclic AMP content in relation to Ca²⁺-concentrations in the extracellular fluid. Threshold concentrations for the appearance of the potentials were lower than 1 mM. Raising the Ca²⁺-content in the Krebs solution above 40 mM led to an increase of the slow excitatory negative potential and to the appearance of the slow inhibitory positive wave, while the fast excitatory compound action potential disappeared. This effect was calcium-specific. On the other hand, cyclic AMP accumulation in the ganglion seems to be calcium-independent. While high Ca²⁺-concentrations (60 mM) in the medium led to a significant decrease of the cyclic AMP content, neurotransmitter-induced cyclic AMP accumulation occurred regardless from varying the extracellular calcium content. High external Ca²⁺ antagonized the ability of the β-adrenergic blocker propranolol in preventing isoproterenol-induced cyclic AMP accumulation to some extent. It was concluded that calcium ions are necessary for the appearance of fast and slow postganglionic compound action potentials in the ganglion. These events seemed to be independent from drug-induced changes in the ganglionic cyclic AMP content.     

Cyclic AMP in the cell cycle of Physarum polycephalum

Cyclic AMP, [³H]thymidine incorporation, and DNA content were measured in the cell cycle of Physarum polycephalum. A sensitive radioimmunoassay was employed to assay cyclic AMP so that plasmodia could be assayed individually. In contrast to previously published results (Lovely, J.R. and Threlfall, R.J. (1976) Biochem. Biophys. Res. Commun. 71, 789–795), no pre-mitotic peak of cyclic AMP was detected. In seven experiments levels of cyclic AMP showed only small changes in individual experiments and ranged from 1–6 pmol/mg protein in different experiments. When plasmodia in the immediate premitotic period were collected on the basis of nuclear mitotic morphology, no evidence of a peak of cyclic AMP was found. Light was found to increase plasmodial cyclic AMP in a rapid, transient fashion. However, the brief exposure of cell cycle samples to light during collection did not induce any apparent cell cycle specific peaks of cyclic AMP. Although the occurrence of extremely rapid transient peaks of cyclic AMP in the cell cycle cannot be ruled out, it appears likely that the P. polycephalum cell cycle can proceed normally without major changes in cyclic AMP.     

Cyclic Nucleotide Response of the Hippocampal Formation to Septal Stimulation in Naive and Kindled Rats

Rats were kindled through nonmagnetic electrodes stereotaxically implanted into the medial septum. Concentrations of cyclic AMP and cyclic GMP were measured by radioimmunoassay in seven brain regions after microwave fixation during the development and expression of kindled seizures. Hippocampal concentrations were similar to untreated controls (cyclic GMP level in the left and right hippocampus, 0.66 +/- 0.04 and 0.68 +/- 0.07 pmol/mg of protein, respectively; cyclic AMP, 9.4 +/- 0.9 and 9.6 +/- 0.8 pmol/mg of protein, respectively), in kindled animals that were not stimulated, and in naive animals in response to septal stimulation, in spite of the presence in the latter group of bilateral hippocampal afterdischarges. Animals that failed to develop kindling and kindled animals that failed to have a seizure in response to stimulation also showed no change in cyclic nucleotide concentrations in any brain region. Kindled animals that developed a seizure following stimulation showed significant elevations in levels of both cyclic GMP and cyclic AMP in hippocampus and in several other brain regions. A single naive animal that had a seizure in response to its first stimulation also appeared to have elevated concentrations of both cyclic nucleotides in hippocampus. These data suggest that the elevation in levels of both cyclic GMP and cyclic AMP during kindled seizures is associated with seizure development rather than with the generation of afterdischarges or with the kindling engram.     

Restorative effects of cyclic AMP on complex bioelectric activities of cultured fetal rodent CNS tissues after acute CA++-deprivation

Explants of fetal mouse spinal cord and cerebral cortex generate organotypic slow-wave and repetitive-spike discharges in vitro which can be abolished by agents which reduce the concentration of Ca⁺⁺ available to the tissue. Synaptically mediated discharges are rapidly blocked in Ca⁺⁺-free balanced salt solution (BSS), or in regular BSS after addition of 10^?3 M EGTA, 5–10 × 10^?3 M Mg⁺⁺, or 10^?4 M xylocaine, but simple spike potentials can still propagate. When low concentrations of cyclic AMP or dibutyryl cyclic AMP (2 × 10^?6 M) are added to the Ca ⁺⁺-free BSS or Ca⁺⁺-antagonist-BSS, a temporary (1–20 min) restoration of characteristic complex bioelectric activities occurs (or the onset of depression is delayed if cyclic AMP is initially added). Phosphodiesterase inhibitors, e.g. 10^?3 M caffeine, are also effective in restoring these blockades, whereas 5′AMP and ATP are not. Application of 10^?6 M cyclic AMP or 10^?3 M caffeine in regular BSS greatly enhances excitability of some CNS explants, resembling convulsive effects observed in CNS in situ. The data suggest that cyclic AMP can mobilize Ca⁺⁺ from membranebound Ca pools within neurons in CNS explants so as to permit Ca⁺⁺-dependent release of neurotransmitter during Ca⁺⁺ deficits. Thus, it may also be that under normal conditions, cyclic AMP can regulate the availability of Ca⁺⁺ for synaptic transmission in the central nervous system, thereby modulating the efficacy of synaptic functions.     

Effect of dibutyryl cyclic AMP on leucine incorporation in L5178Y cells II. Effect of actinomycin and heat shock

³H-leucine incorporation was assayed in L5178Y cells as a measure for protein synthesis. Protein synthesis was inhibited by short duration heat shock, actinomycin, cordycepin or a combination of these agents. Dibutyryl cyclic AMP had little or no effect upon protein synthesis in control cells. Dibutyryl cyclic AMP, however, stimulated protein synthesis in cells previously heat shocked and/or inhibited by actinomycin or cordycepin. The results have been interpreted to indicate that the cyclic nucleotide stimulated protein synthesis by influencing the metabolism of some species of RNA which may regulate translation.     

Changes in adenosine 3',5'-monophosphate induced by parathyroid hormone and theophylline in the developing rat renal cortex

To test the biochemical responsiveness of developing rat renal cortex to parathyroid hormone (PTH), intracellular concentrations of adenosine 3′,5′-monophosphate (cyclic AMP) were measured. Renal cortical slices from 10-day-, 20-day-, and 12-week-old animals contained higher concentrations of cyclic AMP when incubated in the presence of theophylline than in its absence. In the absence of theophylline, tissue from all three age groups responded to PTH with dose-dependent increases in cyclic AMP. In the presence of theophylline the response of tissue from 10-day-old animals was greater than that of 12-week-old animals.It is suggested that the differential effect of theophylline with respect to age may be the result of higher turnover rates of cyclic AMP in the young animals.     

CYCLIC AMP IN THE RAT CEREBRAL CORTEX AFTER ACTIVATION OF NORADRENALINE NEURONS OF THE LOCUS COERULEUS

The levels of cyclic AMP in the rat brain were studied in vivo following destruction or stimulation of the noradrenergic pathway originating in the locus coeruleus. After chronic lesion of the locus coeruleus no alterations in cyclic AMP content were found. Electrical stimulation of the locus coeruleus produced an elevation of cyclic AMP in the cerebral cortex of chloral hydrate anaesthetized rats of 30%. Maximal increases were found after 15–60 s stimulation at a frequency of 30–100 Hz. This maximal response was slightly inhibited by phenoxybenzamine, an α-adrenergic blocking agent, and by the β-blocker propranolol. When the α and β blockers were administered together a highly significant decrease in cyclic AMP response was observed. Pretreatment of the rats with reserpinc +α methyl-p-tyrosine prevented the cyclic AMP response. In addition to the effect in the cerebral cortex, cyclic AMP-levels were also enhanced in the hippocampus, in the striatum and in the hypothalamus. These results suggest that the locus coeruleus regulates a small fraction of cerebral cyclic AMP levels, by both α- and β-adrenergic receptors.     

Cyclic GMP and cyclic GMP-dependent protein kinase in brown adipose tissue of developing rats

In order to ascertain the possible involvement of cyclic GMP in the physiological regulation of the function and development of brown fat of the rat, we have determined its tissue concentration in vivo under a variety of conditions. The steady-state concentration of cyclic GMP in interscapular brown adipose tissue of late foetus was about 80 pmol per g fresh weight. The concentration gradually declined during the first 2 weeks after birth to reach 40 pmol/g fresh weight and then remained constant into adulthood. The cyclic GMP content of brown fat was decreased by chemical sympathectomy and was increased after complete acclimatization of the animals to the cold. The activity of cyclic GMP-dependent protein kinase was also highest in tissue from newborn and cold-acclimatized rats.Both acute cold stress and injection of norepinephrine resulted in a significant but temporary increase in the concentration of cyclic GMP in brown fat, which was followed by a depression of the concentration below values in untreated animals. The concentration of cyclic AMP showed similar pattern of changes. Injection of phenylephrine was followed by a pronounced increase in the cyclic GMP content of brown fat, with little effect upon cyclic AMP. Injection of isoproterenol raised the content of cyclic AMP but not that of cyclic GMP. The ability of norepinephrine and phenylephrine to increase the concentration of cyclic GMP was abolished by pre-treatment of the animals with phenoxybenzamine, but not by pre-treatment with propranolol. Conversely, propranolol but not phenoxybenzamine abolished the effects of norepinephrine on the cyclic AMP content of the tissue.Thus we have established the responsiveness of the cyclic GMP content of brown fat to physiological and pharmacological stimuli and have evidence of the possible participation by cyclic GMP in the α-adrenergic stimulation and in the regulation of proliferative processes in the tissue.     

Adaptive mechanisms of the noradrenergic cyclic AMP generating system in the limbic forebrain of the rat: adaptation to persistent changes in the availability of norepinephrine (NE).

Abstract— The noradrenergic cyclic AMP generating system in slices of the limbic forebrain of rats displays characteristics which are compatible with those of a central NE receptor. The cyclic AMP response to a K_max concentration of NE (concentration of NE which elicits maximal increase in the level of cyclic AMP) is significantly enhanced in slices from reserpinized animals, although the K_a value of NE (concentration of NE eliciting half-maximum response) was not significantly changed. Chemosympathectomy with 6-hydroxydopamine (6-OHDA) significantly enhanced the activity of the system to NE and isoproterenol but not to adenosine and reduced the K_a value for NE. The changes in the reactivity of the cyclic AMP generating system following 6-OHDA administration appear to be related to a decrease in the availability of NE and not to that of other neurotransmitters as protection by desipramine (DMI) of noradrenergic neurons against the neurotoxic action of 6-OHDA prevented the development of supersensitivity to NE. Conversely, and independent of the actual concentration of NE in brain tissue, a persistent increase in the availability of NE caused by prolonged MAO inhibition lead to a marked decrease in the reactivity of the cyclic AMP generating system. The results provide further evidence for a regulatory mechanism in the CNS involving the noradrenergic receptor that adapts its sensitivity to NE in a manner inversely related to the degree of its stimulation by the catecholamine.     

Extracellular Cyclic AMP—Adenosine Pathway in Isolated Adipocytes and Adipose Tissue

Objective: Our goal was to evaluate the presence and lipolytic impact of the extracellular cyclic adenosine monophosphate (AMP)–adenosine pathway in adipose tissue. Research Methods and Procedures: Sixteen miniature Yucatan swine (Sus scrofa) were used for these in vitro and in situ experiments. Four microdialysis probes were implanted into subcutaneous adipose tissue and perfused at 2 μL/min with Ringer's solution containing no addition, varying levels of cyclic AMP, 10 μM isoproterenol, or 10 μM isoproterenol plus 1 mM α,β‐methylene adenosine 5′‐diphosphate (AMPCP), a 5′‐nucleotidase inhibitor. Dialysate was assayed for AMP, adenosine, inosine, hypoxanthine, and glycerol. Freshly isolated adipocytes were incubated with buffer, 1 μM isoproterenol, or 1 μM isoproterenol plus 0.1 mM AMPCP, and extracellular levels of AMP, adenosine, inosine, hypoxanthine, and glycerol were measured. Results: Perfusion of adipose tissue with exogenous cyclic AMP caused a significant increase in AMP and adenosine appearance. Perfusion with AMPCP, in the presence or absence of isoproterenol, significantly increased the levels of AMP and glycerol, whereas it significantly reduced the level of adenosine and its metabolites. However, the AMPCP‐provoked increase in lipolysis observed in situ and in vitro was not temporally associated with a decrease in adenosine. Discussion: These data suggest the existence of a cyclic AMP—adenosine pathway in adipocytes and adipose tissue. The role of this pathway in the regulation of lipolysis remains to be clarified.     

Levels of cyclic nucleotides in mouse regional brain following 300 ms microwave inactivation.

Abstract— The uniformity and speed of inactivation of mouse brain adenylate cyclase, guanylate cyclase and cyclic nucleotide phosphodiesterase were measured after 6 kW microwave irradiation (MWR). Inactivation of enzymes was uniform throughout the brain during heating and 100% loss of activity was evident after 300 ms. MWR. For comparison of effects of inactivation times on levels of cyclic nucleotides measured in regional brain areas, cyclic AMP and cyclic GMP were estimated after 1.5 kW MWR requiring 4 s of heating and 6 kW MWR requiring 300 ms. Except for corpus striatum, uniformly lower levels of cyclic AMP were measured following 300 ms vs. 4s MWR . There was no change in cyclic GMP levels in regional brain areas after 4s vs. 300 ms MWR . Cyclic AMP and cyclic GMP were measured from the same regional brain tissue samples after 300 ms and ratios calculated. The finding of much lower cyclic AMP:cyclic GMP ratios than had previously been reported suggests that slow inactivation times provide for the measurement of regional brain cyclic nucleotide values which are not consistent with the in-vivo state.     

Species-Specific Aggregation Factor In Sponges

In dissociated single cells from the sponge Geodia cydonium, DNA synthesis is initiated after incubation with a homologous, soluble aggregation factor. During the DNA -initiation phase the cyclic AMP - and cyclic GMP levels vary drastically; the cyclic AMP content drops from 2.2 pmol/10⁶ cells to 0.3 pmol/10⁶ cells while the cyclic GMP content increases from 0.6 pmol to 3.7 pmol/10⁶ cells. the activity of neither the adenylate cyclase nor of the guanylate cyclase isolated from cells which have been incubated for different periods of time with the aggregation factor, is changed. the soluble as well as the particulate enzyme activities were checked in vitro. the cyclic nucleotide receptors have been isolated from the sponge cells and characterized with respect to their molecular weight, dissociation constant for cyclic AMP or cyclic GMP and intracellular concentration. None of these parameters are altered during aggregation factor-mediated DNA initiation. From these data it is concluded that the regulation of cyclic nucleotide levels is a consequence of a changed activity of nucleotide cyclases or of phosphodiesterases, but this is presumably not caused by a changed rate of synthesis of nucleotide cyclases or of cyclic nucleotide receptors.     

Effect of enkephalins on the energy potential of the rat liver and cerebral cortex during an early period of traumatic shock

The experiments were performed on 86 white male rats, weighing 180-220 g. The animals with traumatic shock revealed a decrease in ATP concentration and a rise in ADP and AMP levels in the brain cortex and hepatic tissue, resulting in the energy charge reduction. The injection of synthetic leu-enkephalin analog D-ala2-arg6-leu-enkephalin to animals with traumatic shock led to energy charge normalization in the brain cortex and increase in hepatic tissue. The mechanisms of enkephalin effect on the energy metabolism during shock are discussed.     

Biochemical and Molecular Responses to Loss of Renal Mass: Atpase and Cyclic Nucleotides: Cyclic Nucleotide Concentrations in Relation to Renal Growth and Hypertrophy

In rats no consistent change in the concentration of cyclic GMP or cyclic AMP concentration was found in the renal cortex between 2 hours and seven days after unilateral nephrectomy. In regenerating liver tissue, between 2 hours and seven days after removal of one-third of the liver, there were no consistent changes in cyclic GMP concentrations, but cyclic AMP concentrations were higher than in controls. During postnatal growth, no consistent changes occurred in the cyclic GMP concentration of the spleen, the testes, the kidney cortex, the renal papilla, the liver or the ventricle between two and sixty days after birth. Cyclic AMP concentration on the other hand, in all these tissues with the exception of the spleen, was depressed between the twenty-first and fortieth day after birth, i.e., at a period of rapid growth. In the spleen, the concentration of cyclic AMP increased continuously from the second to the fifth day after birth. During renal parenchymal hyperplasia induced by a large intravenous dose of folic acid two days before sacrifice, the concentration of cyclic GMP in renal cortical tissue increased consistently. A model is proposed to explain the different patterns of changes in the cyclic nucleotide concentrations found.