Behaviour of cyclic nucleotides and Ca2+ levels in liver tissue of rats poisoned by white phosphorus and trichlorobromomethane

The content of hepatic cyclic AMP was increased soon after intoxication by white phosphorus. Its level reached a maximum 4 h after poisoning, but in subsequent phases tended to return to normal. In contrast, the cyclic GMP concentration was altered only 24 and 36 h after treatment with the same hepatotoxin. Similar modifications of cAMP and cGMP content were also detected after poisoning by trichlorobromomethane (CBrCl3). As a consequence, an altered cGMP/cAMP ratio was found in both experimental conditions. Further, the modification of cAMP content after white phosphorus was detected prior to liver damage (steatosis and necrosis), while the highest concentration of the cyclic nucleotide in CBrCl3-poisoned rats was found when fatty liver was already evident. In addition, in phosphorus-poisoned rats, the hepatic content of Ca2+ was found to be unmodified during all phases of the intoxication, while after CBrCl3 a phasic increase of the Ca2+ level was observed at 4, 24 and 36 h.     

The inhibition by xanthine phosphodiesterase inhibitors of the induction of alkaline phosphatase activity in HeLa cells: relationship of enzyme activity to cyclic AMP concentrations.

The three xanthine derivatives, caffeine, theophylline and 3-isobutyl-1-methyl-xanthine (IBMX) produced dose-dependent increases in cyclic AMP concentrations in HeLa cells after long term treatment. Only IBMX produced increases over the first 60 minutes, with a peak of approximately 5-fold control values five to 10 minutes after the addition of the drug. About four hours after the addition of either 0.67 or 1.0 mM IBMX there was a second peak in the concentration of cyclic AMP which was at least as large and usually larger than the peak observed at five to ten minutes. Neither caffeine nor theophylline increased cyclic AMP concentrations above control values until one hour after addition of the compounds, and there was no indication of a peak in the concentration at four hours. Between 24 and 72 hours, all three compounds produced elevations in cyclic AMP levels that were steadily maintained. At any given concentration, the order of potency was IBMX greater than theophylline greater than caffeine. If the xanthine derivatives were removed from the medium after 24 hours of treatment, the cyclic AMP concentrations fell to control levels within one hour. Treatment with 5-iodo-2'-deoxyuridine (IdUrd) or hydrocortisone alone did not change the levels of cyclic AMP, nor did the presence of these inducers of alkaline phosphatase activity alter the effects of the xanthine derivations on cyclic AMP concentrations. The data showed a significant correlation between the magnitude of the increase in cycli AMP concentrations over the period from 24 to 72 hours and the degree of inhibition by the xanthine derivatives of the induction of alkaline phosphatase activity.     

Cyclic nucleotides in the denervated rat diaphragm and the effect of cyclic AMP on ornithine and adenosylmethionine decarboxylases

The concentrations of cyclic AMP and cyclic GMP were measured in the denervated rat diaphragm at various times following unilateral phrenicectomy. Cyclic AMP concentration was raised by the second day after operation, reached a peak by the third day, followed by another increase at around 10 days. By contrast, cyclic GMP concentration was decreased within a day after denervation and remained below control levels at all subsequent times studied. Epinephrine in vitro produced a comparable increase in the concentration of cyclic AMP in both normal and denervated tissue. The concentration of adenosine appeared unchanged in the denervated diaphragm by comparison with its innervated control. Activity of ornithine decarboxylase was elevated in the diaphragms of rats treated with dibutyryl cyclic AMP, but this effect could also be achieved with sodium butyrate alone. Adenosylmethionine decarboxylase activity, was unaffected after treatment with either compound. These observations and others discussed are taken to indicate a lack of direct relationship between cyclic AMP concentrations and the activity of the rate-limiting enzymes of polyamine biosynthesis in the rat diaphragm.     

Cyclic nucleotides in the denervated rat diaphragm and the effect of cyclic AMP on ornithine and adenosylmethionine decarboxylases

The concentration of cyclic AMP and cyclic GMP were measured in the denervated rat diaphragm at various times following unilateral phrenicectomy. Cyclic AMP concentration was raised by the second day after operation, reached a peak by the third day, followed by another increase at around 10 days. By contrast, cyclic GMP concentration was decreased within a day after denervation and remained below control levels at all subsequent times studied. Epinephrine in vitro produced a comparable increase in the concentration of cyclic AMP in both normal and denervated tissue. The concentration of adenosine appeared unchanged in the denervated diaphragm by comparison with its innervated contorl. Activity of ornithine decarboxylase was elevated in the diaphragms of rats treated with dibutyryl cyclic AMP, but this effect could also be achieved with sodium butyrate alone. Adenosylmethionine decarboxylase activity was unaffected after treatment with either compound. These observations and others discussed are taken to indicate a lack of direct relationship between cyclic AMP concentrations and the activity of the rate-limiting enzymes of polyamine biosynthesis in the rat diaphragm.     

Distribution of cyclic nucleotides in layers of the cerebellum after decapitation

The distribution of the cyclic nucleotides was examined in layers of the mouse cerebellum following decapitation. Cyclic AMP increased and cyclic GMP decreased in all three layers of the cerebellum examined. The increase in cyclic AMP in the granular layer was far greater than in either the molecular or white layers. In the cerebellum from control mice, the cyclic GMP concentration was highest in the molecular layer and lowest in the white layer. Even after decapitation, this cyclic GMP gradient in the cerebellum was maintained.     

Intracellular and Extracellular Cyclic Nucleotides in Wild-Type and White Collar Mutant Strains of Neurospora crassa: Temperature Dependent Efflux of Cyclic AMP from Mycelia

Cyclic AMP and cyclic GMP were released into the growth medium of mycelia of Neurospora crassa wild-type strains St.L.74A and Em5297a and by white collar-1 and white collar-2 mutant strains. After growth for 6 days at 18°C, there were 2.19 (St.L.74A), 5.83 (Em5297a), 1.38 (white collar-1), and 1.10 (white collar-2) nanomoles of cyclic AMP per gram dry weight of mycelia in the growth medium. These values corresponded to concentrations of cyclic AMP of between approximately 10 and 50 nanomolar. The corresponding values for extracellular cyclic GMP were typically less than 6% of the values for cyclic AMP. Following transfer to fresh medium, cyclic AMP efflux was demonstrated for each of the strains, and the amount of cyclic AMP exported into the fresh medium was greater at 25°C than 6°C. Intracellular cyclic AMP and cyclic GMP were also measured in each of the strains. The values for cyclic AMP were in the same range as those in the literature (approximately 0.5 to 1.5 nanomoles per gram dry weight of mycelia). However, the corresponding intracellular cyclic GMP values were less than 1% of the cyclic AMP values, i.e. more than 50 times lower than the value previously reported for the St.L.74A wild-type. Transfer of mycelia after 6 days at 18°C to fresh media and incubation for 2 hours at 25°C or 6°C did not consistently affect the intracellular level of cyclic AMP or cyclic GMP in the strains examined. We could detect no change in intracellular cyclic AMP when mycelia of the St.L.74A wild-type strain were irradiated with blue light for periods of up to 3.0 hours at 18°C, or in cyclic AMP and cyclic GMP for irradiation times of up to 1 minute at 6°C. We propose that the plasma membrane of Neurospora crassa is permeable to cyclic nucleotides, and the export of cyclic nucleotides into the growth medium may be a means of regulating intracellular levels. We conclude that three factors that affect carotenogenesis in Neurospora crassa (blue light, temperature, and the white collar mutations) have no appreciable effect on the total measurable intracellular cyclic nucleotides in this organism. There was no extracellular or intracellular cyclic AMP or cyclic GMP in the crisp-1 mutant strain, which suggested either that adenylate cyclase (which is absent in crisp-1) catalyzes the synthesis of both cyclic AMP and cyclic GMP or that the crisp-1 mutation somehow results in a deficiency of two enzymes (adenylate and guanylate cyclase).     

Circadian variations in plasma 3':5'-cyclic adenosine monophosphate and 3':5'-cyclic guanosine monophosphate of normal adults.

Circadian variations in plasma cyclic AMP and cyclic GMP were studied in thirteen male subjects (20–22 years old) under controlled invironmental condition. Plasma collections were made every six hours. Cyclic AMP and cyclic GMP were determined by radioimmunoassay. Individual values of plasma cyclic AMP at 0800 are between 13.0 and 25.8 pmole/ml, and cyclic GMP between 2.5 and 7.0 pmole/ml. Cyclic AMP demonstrated the circadian variation with the maximum level at 1400 and the minimum at 0200, and cyclic GMP with the highest level at 1400 and the lowest level at 0800.     

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.     

Changes in rat adrenal cyclic nucleotides during normal and neoplastic growth

In an attempt to correlate changes in cyclic nucleotide levels with in vivo growth of the rat adrenal gland we have measured adrenal cyclic AMP and cyclic GMP in normal, hyperplastic, and neoplastic rat adrenals. The first group of animals were subject to either unilateral adrenalectomy (ADX) or acute hypophysectomy 1 h prior to unilateral adrenalectomy (HADX). Cyclic nucleotides were measured in the contralateral adrenal post-operatively. In HADX rats cyclic GMP rose steadily throughout the 7 day study period, while ADX rats exhibited significant decreases in adrenal cyclic GMP. Cyclic AMP remained approximately 1.5 pm/mg tissue in HADX rats, while in ADX rats there was significant elevation of adrenal cyclic AMP at all time points. Cyclic GMP/cyclic AMP ratios remained constant in HADX animals; however, the growing adrenals of ADX animals exhibited depressed cyclic GMP/cyclic AMP ratios at all time periods.Adrenal hyperplasia was induced in a seond group of animals by a transplantable, corticotropin-secreting, pituitary tumor. Adrenals from age-matched animals served as controls. Adrenal cyclic AMP was significantly elevated in tumor-bearers at a time correspinding to the peak accumulation of adrenal weight, protein and DNA in these animals. In contrast, adrenal cyclic GMP in both tumor-beares and control animals fell steadily throughout the study period. Cyclic GMP/cyclic AMP ratios of control animals decreased from 2 to 3 weeks post-transplant remaining at the 3 week value during the period corresponding to rapid adrenal growth in tumor-bearers. The cyclic GMP/cyclic AMP ratio in the hyperplastic adrenals of tumor-bearers decreased steadily throughout their rapid growth period, suggesting a positive correlation between adrenal growth and depression of the cyclic GMP/cyclic AMP ratio.Cyclic nucleotide levels in neoplastic adrenals of rats bearing the transplantable adrenocortical carcinoma 494 were compared with cyclic nucleotides in normal rat adrenal glands. Cyclic AMP was not different in the two groups. However, the cyclic GMP content of neoplastic adrenals was significantly lower than that of normal adrenal tissue, causing a suppression of the cyclic GMP/cyclic AMP ratio in the neoplastic tissue. Thus, measurement of adrenal cyclic nucleotides in both hyperplastic and neoplastic rat adrenal glands suggests that adrenal growth in vivo may be characterized by a depression of the cyclic GMP/cyclic AMP ratio.     

The effect of isoproterenol and its analogs upon adenosine 3′,5′-monophosphate and guanosine 3′,5′-monophosphate levels in mouse parotid gland in vivo: Relationship to the stimulation of DNA synthesis

The ability of a large number of catecholamine analogs to stimulate DNA synthesis in the mouse parotid gland in vivo was compared to their effect on the levels of adenosine 3′,5′-monophosphate (cyclic AMP) and guanosine 3′,5′-monophosphate (cyclic GMP) in this tissue. In the normal parotid gland the level of cyclic GMP is very low (10^?9 moles/kg wet wt), being only 1/800th of the cyclic AMP concentration. Isoproterenol increases the levels of cyclic AMP and cyclic GMP 30- and 3-fold, respectively. The increase in cyclic AMP is biphasic with an apparent early maximum at 2.5 min and a main peak at 15 min while the increase in cyclic GMP is monophasic with maximum levels at 15 min. Other analogs showed a similar effect on cyclic AMP levels but the time course of increases in cyclic GMP was very variable with peak stimulation as early as 1 min in some cases. The ability of analogs to cause the accumulation of cyclic AMP was correlated with their capacity to activate adenylate cyclase in parotid extracts and to act as β-adrenergic agonists in other systems. All compounds which raised cyclic AMP levels stimulated DNA synthesis but a number of other analogs also stimulated DNA synthesis. The effects of these analogs have been correlated with their ability to raise the intracellular concentration of cyclic GMP. Cholinergic agents also cause the accumulation of cyclic GMP but the effect of the analogs does not appear to be mediated through the cholinergic system as atropine does not block their effects and cholinergic agonists do not stimulate DNA synthesis. It is suggested that cholinergic agonists and the catecholamine analogs act primarily on the duct and acinar cells, respectively.Significant with inhibitors of the rises in cyclic nucleotide levels suggest that in isoproterenol stimulation it is the rise in cyclic GMP which is the more significant event in relation to stimulation of DNA synthesis.     

Regulation of cyclic AMP and cyclic GMP levels by adrenocorticotropic hormone in cultured neurons

The effect of adrenocorticotropic hormone (ACTH) on the intracellular concentration of cyclic nucleotides was studied in cultures of neurons from embryonic chick cerebral hemispheres. Incubation of neurons with ACTH(1-24) in the presence of phosphodiesterase inhibitor isobutylmethylxanthine resulted in a sustained increase in cyclic AMP while rise in cyclic GMP level was transient. The values obtained for half-maximal stimulation were 0.5 microM and 0.03 nM for cyclic AMP and cyclic GMP respectively. Concomitantly, ACTH(1-24) stimulated guanylate cyclase activity (half-maximal stimulation at 0.02 nM). These results suggest the existence of two distinct populations of ACTH receptors in neurons and provide the first evidence that cyclic GMP does mediate the action of ACTH in neurons.     

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.     

Regulation by a β-adrenergic receptor of a Ca2+-independent adenosine 3′,5′-(cyclic)monophosphate phosphodiesterase in C6 glioma cells

The hormonal control of cyclic nucleotide phosphodiesterase (EC 3.1.4.17) activity has been studied by using as a model the isoproterenol stimulation of cyclic AMP phosphodiesterase activity in C6 glioma cells. A 2-fold increase in cyclic AMP phosphodiesterase specific activity was observed in homogenates of isoproterenol-treated cells relative to control. This increase reached a maximum 3 h after addition of isoproterenol, was selective for cyclic AMP hydrolysis, was reproduced by incubation with 8-Br cyclic AMP but not with 8-Br cyclic GMP and was limited to the soluble enzyme activity. The presence of 0.1 mM EGTA did not alter the magnitude of the increase in phosphodiesterase activity. Moreover, the calmodulin content in the cell extracts was not changed after isoproterernol. DEASE-Sephacel chromatography of the 100 000×g supernatant resolved two peaks of phosphodiesterase activity. The first peak hydrolyzed both cyclic nucleotides and was activated by Ca²⁺ and purified calmodulin. The second peak was specific for cyclic AMP but it was Ca²⁺- and calmodulin-insensitive. Isoproterenol selectively increased the specific activity of the second peak. Kinetic analysis of the cyclic AMP hydrolysis by the induced enzyme reveled a non-linear Hofstee plot with apparent K_m values of 2–5 μM. Cyclic GMP was not hydrolyzed by this enzyme in the absence or presence of calmodulin and failed to affect the kinetics of the hydrolysis of cyclic AMP. Gel filtration chromatography of the induced DEASE-Sephacel peak resolved a single peak of enzyme activity with an apparent molecular weight of 54 000.     

The content of reduced glutathione in rat liver after poisoning with white phosphorus

The content of hepatic GSH was evaluated in rats after poisoning with white phosphorus. In addition, liver damage following the administration of the hepatotoxin was assessed by determining hepatic triglyceride accumulation. Experiments in parallel were carried out in an attempt to evaluate the enhanced susceptibility of hepatic tissue to peroxidative decomposition of unsaturated lipids 'in vitro', as measured by the production of TBA-reacting substances. Our data indicate that only in the early stage of intoxication is it possible to detect a slight decrease of GSH content in the liver, while during the subsequent stages the concentration of GSH was unaffected. At 6 hours of intoxication the level of hepatic triglycerides was significantly increased. Pretreatment with GSH was followed by an amelioration of fatty infiltration, but the content of hepatic GSH was unchanged. The production of TBA-reacting products was found enhanced only at 6 hours of intoxication. These results are discussed in relation to the role of lipid peroxidation in liver injury by white phosphorus.     

Implication of guanosine 3′,5′-cyclic monophosphate,adenosine 3′,5′-cyclic monophosphate,adenosine 5′-mono-, di- and triphosphate and fructose-2,6-bisphosphate in the regulation of the glycolytic pathway in hypoxic/anoxic mussel,Mytilus galloprovincialis

The change in the content of cyclic GMP, cyclic AMP, ATP, ADP, AMP and fructose-2,6-bisphosphate that occurred in the mantle of the mussel Mytilus galloprovincialis Lmk when specimens of this mollusk were subjected to a hypoxia/anoxia situation were assessed. After the early 24 h in anaerobiosis, a clear decrease was observed in the ATP content, which remained close to that value for the rest of the time. AMP content doubled during the early 24 h in anaerobiosis and, from that time on, it remained close to that value. Fructose-2,6-bisphoshate and cyclic GMP showed a similar behavior. The levels of these compounds rose significantly during the early hours in anaerobiosis, and then fell to values similar to those of aerobiosis, remaining constant for the rest of the time. Neither ADP nor cAMP showed significant variations.     

Effects of decapitation, ether and pentobarbital on guanosine 3′,5′-phosphate and adenosine 3′,5′-phosphate levels in rat tissues

Cyclic GMP and cyclic AMP levels in eight different rat tissues were examined after animlas were immersed in liquid nitrogen. In order of decreasing concentration, cerebellu, kidney, lung and cerebral cortex contained the greatest quantities fo cyclic GMP. These tissues also contained relatively high concentrations of cyclic AMP. Compared to values in animals which were sacrificed in liquid nitrogen, levels of both nucleotides in many of the tissues examined were altered by decapitation or anesthesia with ether and pentobarbital. Decapitation increased the levels of both cyclic GMP and cyclic AMP in cerebellum, lung, heart, liver and skeletabl muscle. However, decapitation increased only cyclic AMP in cerebral cortex and kidney. Our previously reported high level of cyclic GMP in lung was attributed to ether anesthesia and surgical removal which increased the cyclic GMP content in lung, heart, testis and skeletal muscle. The effect of ether on cyclic GMP levels in lung and heart was blocked by pretreatment of animals with atropine which indicated that cholinergic agents increase cyclic GMP content in these tissues. Acetylcholine and carbachol in the presence of theophylline increased the accumulation of cyclic GMP in incubations of rat lung minces. Increases in cyclic GMP and cyclic AMP levels in cerebellum with ether anesthesia were prevented if rats were immersed in liquid nitrogen after anesthesis with ether. Anesthesia with pentobarbital decreased the levels of cyclic GMP in cerebellum and kidney and increased the nucleotide in heart, liver, testis and skeletal muscle compared to levels in tissues from animals immersed in liquid nitrogen. However, pentobarbital increased cyclic AMP levels in cerebellum and cerebral cortex and decreased the nucleotide in liver, kidney, testis and skeletal muscle. These studies provide a possible explanation for the variability in in vivo levels of cyclic GMP and cyclic AMP which have been previously reported. In addition, these studies support the hypothesis that the synthesis and degradation of cyclic AMP and cyclic GMP are regulated independently and not necessarily in a parallel or reciprocal manner. These studies also suggest that the increase accumulation of one cyclic nucleotide has no major effect on the synthesis and/or metabolism of the other; however, such interactions cannot be entirely excluded from the results of this study.     

Cyclic AMP increases the concentration of insulin receptors in cultured fibroblasts and lymphocytes.

Incubation of SV40 transformed fibroblasts with dibutyryl cyclic AMP, 8-bromo-cyclic AMP, or 1-methyl-3-isobutylxanthine (MIX), a phosphodiesterase inhibitor, produced a two-fold increase in insulin receptor concentration without an effect on receptor affinity. The increase was dose-dependent, was observed after 8 hrs of treatment, and reached a maximum level by 12 to 24 hours. Upon removal of the nucleotide, receptor number decreased towards basal level.Incubation of cultured human lymphocytes (IM-9 line) with cyclic AMP derivatives or MIX also increased the number of insulin receptors without an alteration in receptor affinity. This effect was partially blocked by inhibition of protein synthesis and was independent of changes in cell cycle. The increase in insulin receptors was a specific response to cyclic AMP as the number of receptors for human growth hormone was unaltered. Incubation with 8-bromo-cyclic GMP did not alter the level of insulin binding.     

Effects of sera types and cell density on the concentration of cyclic nucleotides in normal and simian virus transformed mouse fibroblasts

The effects of serum and cell density on the concentration of cyclic AMP, cyclic GMP in normal mouse fibroblasts cells (3T3 cells) and their Simian Virus 40 transformed derivative (SV3T3 cells) were studied. 3T3 cells grown in 10% foetal bovine serum exhibit density dependent inhibition of growth and associated with this in an increase in the concentration of cyclic AMP, a decrease in the concentration of cyclic GMP and an increase in the ratio (cyclic AMP/cyclic GMP) of the cyclic nucleotides. 3T3 cells grown in 10% newborn calf serum exhibit a higher saturation density and this is associated with a low concentration of cyclic AMP and a high concentration of cyclic GMP. SV3T3 cells grown in either 10% foetal bovine serum or 10% newborn calf serum show high saturation densities and this is associated with a low and decreasing concentration of cyclic AMP and a high concentration of cyclic GMP. When the level of the cyclic AMP in both cell lines was artificially raised by adding dibutyryl cyclic AMP and theophylline to the growth media, the cells grew to low densities.     

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

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

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

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