Role Of Guanosine Research Articles

Guanosine increases global SUMO1-ylation in the hippocampus of young and aged mice and improves the short-term memory of young mice.

The nucleoside guanosine is an endogenous neuromodulator associated with neuroprotection. The roles of guanosine during aging are still not fully elucidated. Guanosine modulates SUMOylation in neurons and astrocytes in vitro, but it is not known whether guanosine can modulate SUMOylation in vivo and improve cognitive functions during aging. SUMOylation is a post-translational protein modification with potential neuroprotective roles. In this follow-up study, we investigated whether guanosine could modulate SUMOylation in vivo and behavior in young and aged mice. Young (3-month-old) and aged (24-month-old) C57BL/6 mice were treated with guanosine (8 mg/kg intraperitoneal) daily for 14 days. Starting on day 8 of treatment, the following behavioral tests were performed: open field, novel object location, Y-maze, sucrose splash test, and tail suspension test. Treatment with guanosine did not change the locomotor activity of young or aged mice in the open-field test. Treatment with guanosine improved short-term memory only for young mice but did not change the working memory of either young or aged mice, as evaluated using object recognition and the Y-maze tests, respectively. Depressive-like behaviors, such as impaired grooming evaluated through the splash test, did not change in either young or aged mice. However, young mice treated with guanosine increased their immobility time in the tail suspension test, suggesting an effect on behavioral coping strategies. Global SUMO1-ylation was significantly increased in the hippocampus of young and aged mice after 14 days of treatment with guanosine, whereas no changes were detected in the cerebral cortex of either young or aged mice. Our findings demonstrate that guanosine also targets hippocampal SUMOylation in vivo, thereby contributing to a deeper understanding of its mechanisms of action. This highlights the involvement of SUMOylation in guanosine's modulatory and neuroprotective effects.

Role of Guanosine in Glycerol Induced Acute Renal Failure in Rats

Role of Guanosine in Glycerol Induced Acute Renal Failure in Rats

Current perspectives on the antidepressant-like effects of guanosine.

Purines have a recognized importance as intercellular messengers. These molecules play an important role in the development and maintenance of the central nervous system (CNS), as well as in its response to pathological conditions. Both adenine and guanine-based purines can be released from astrocytes, where they play a relevant role as extracellular signaling molecules. Particularly, the nucleoside guanosine has been proposed as an extracellular molecule that regulates the response of CNS to damage. Following injury, this nucleoside attains high concentrations in the extracellular space, where it activates multiple signaling pathways, leading to neurotrophic and neuroprotective effects. Despite the fact that the role of guanosine in the CNS is just now being elucidated, several lines of evidence have suggested that this nucleoside protects neural tissue from damage by activating anti-apoptotic, anti-inflammatory and antioxidant mechanisms (Rathbone et al., 2008). Within this context, the protective effects of guanosine have been demonstrated against a plethora of different insults. These effects seem to be partially mediated by a specific G-protein coupled binding site, raising the possibility of existence of a putative guanosine receptor. Additionally, the trophic and neuroprotective effects of guanosine also appear to involve an interaction with the adenosinergic system, since blockage of adenosine A1 and A2A receptors can partially inhibit these pro-survival properties of guanosine (which does not act as a ligand for these receptors). The action of guanosine on both the specific G-protein coupled binding site and the adenosinergic receptors is associated with the activation of pro-survival signaling pathways such as those involving the phosphatidylinositol-3 kinase (PI3K)/Akt and mitogen-activated protein kinases (MAPKs) (Rathbone et al., 2008). Indeed, synchronization of distinct signaling pathways by this nucleoside seems to underlie most of its trophic and neuroprotective properties, including its ability to stimulate astrocytic glutamate uptake and the release of growth factors, as well as the induction of anti-inflammatory and antioxidant responses (Rathbone et al., 2008; Bettio et al., 2016a). The trophic response elicited by guanosine involves the astrocytic release of molecules with a crucial role in the reparative processes of CNS including nerve growth factor (NGF), transforming growth factor beta (TGFβ) and fibroblast growth factor 2 (FGF-2) (Bau et al., 2005; Rathbone et al., 2008; Su et al., 2009). Furthermore, this nucleoside is able to induce neural stem cell proliferation by increasing the expression of brain-derived neurotrophic factor (BDNF) (Su et al., 2013).

Evidence that Hyperprolinemia Alters Glutamatergic Homeostasis in Rat Brain: Neuroprotector Effect of Guanosine

This study investigated the effects of acute and chronic hyperprolinemia on glutamate uptake, as well as some mechanisms underlying the proline effects on glutamatergic system in rat cerebral cortex. The protective role of guanosine on effects mediated by proline was also evaluated. Results showed that acute and chronic hyperprolinemia reduced glutamate uptake, Na(+), K(+)-ATPase activity, ATP levels and increased lipoperoxidation. GLAST and GLT-1 immunocontent were increased in acute, but not in chronic hyperprolinemic rats. Our data suggest that the effects of proline on glutamate uptake may be mediated by lipid peroxidation and disruption of Na(+), K(+)-ATPase activity, but not by decreasing in glutamate transporters. This probably induces excitotoxicity and subsequent energy deficit. Guanosine was effective to prevent most of the effects promoted by proline, reinforcing its modulator role in counteracting the glutamate toxicity. However, further studies are needed to assess the modulatory effects of guanosine on experimental hyperprolinemia.

Guanine-induced inhibition of renal Na+-ATPase activity: Evidence for the involvement of the Gi protein-coupled receptor

There is some evidence to show a possible role of guanosine in the modulation of cellular function, in particular, in the neuronal system. However, nothing is known about the role of guanine in renal function. The aim of the present work was to investigate the role of guanine on modulation of Na+-ATPase activity in isolated basolateral membrane (BLM) of the renal cortex. Guanine inhibited the enzyme activity in a dose-dependent manner with maximal effect (56%) obtained at 10−6M. This effect was reversed by DPCPX (8-cyclopentyl-1,3-dipropylxanthine), an antagonist of A1 receptors, but it was not changed by 10−8M DMPX (3,7-dimethyl-1-propargylxanthine) or 10−8M MRS (2,3-diethyl-4,5-dipropyl-6-phenylpyridine-3-thiocarboxylate-5-carboxylate), antagonists of A2 and A3 receptors, respectively. Furthermore, it was observed that guanine increased [γ-35S]GTP-specific binding with the maximal effect observed at 10−6M and this effect was abolished by 10−6M GDPβS. The inhibitory effect of 10−6M guanine on Na+-ATPase activity was reversed by 10−6M GDPβS, 10−6M forskolin, 10−6M pertussis toxin and 10−8M cholera toxin. These results indicate that guanine binds to a DPCPX-sensitive receptor promoting the activation of Gi protein and leading to a decrease in cAMP level and, consequently, inhibition of BLM Na+-ATPase activity.

Guanosine Prevents Thermal Hyperalgesia in a Rat Model of Peripheral Mononeuropathy

It is well known that adenine-based purines exert multiple effects on pain transmission. Less attention has been given, however, to the antinociceptive effects of guanine-based purines. The aim of this study was to investigate the effects of intraperitoneal administration of guanosine on a rat model of peripheral mononeuropathy. Additionally, investigation of the mechanism of action of guanosine, its general toxicity and measurements of central nervous system purine levels were performed. Rats received an intraperitoneal administration of vehicle (0.1 mM NaOH) or guanosine (up to 120 mg.kg(-1)) in an acute or chronic regimen. Guanosine significantly reduced thermal hyperalgesia on the ipsilateral side of the sciatic nerve ligation. Additionally, guanosine prevented locomotor deficits and body weight loss induced by the mononeuropathy. Acute systemic administration of guanosine caused an approximately 11-fold increase on central nervous system guanosine levels, but this effect was not observed after chronic treatment. Chronic guanosine administration prevented the increase on cortical glutamate uptake but not the decrease in spinal cord glutamate uptake induced by the mononeuropathy. No significant general toxicity was observed after chronic exposure to guanosine. This study provides new evidence on the mechanism of action of guanine-based purines, with guanosine presenting antinociceptive effects against a chronic pain model. This study provides a new role for guanosine: chronic pain modulation. Guanosine presents as a new target for future drug development and might be useful for treatment of neuropathic pain.

Regulation of long-term depression by increases in [guanosine 3′,5′-cyclic monophosphate] in the hippocampal CA1 region of freely behaving rats

A role for guanosine 3′,5′-cyclic monophosphate (cGMP) and the protein kinase G (PKG) pathway in synaptic long-term depression (LTD) in the hippocampal CA1 region has been proposed, based on observations in vitro, where, for example, increases of [cGMP] result in short-term depression (STD) coupled with a reduction in presynaptic glutamate release. To date, no evidence exists to support that LTD in the intact, freely behaving animal involves these mechanisms. We examined the effect of increases of [cGMP] on basal transmission and electrically-induced STD at hippocampal CA1 synapses in vivo. We found that elevating [cGMP] dose-dependently caused a chemically-induced STD which occluded electrically-induced STD. Repeated administration of Zaprinast, an inhibitor of cGMP-degrading phosphodiesterase, resulted in persistent LTD (>24 h). Paired-pulse analysis supported a presynaptic mechanism of action. Application of an inhibitor of soluble guanylate cyclase prevented LTD induced by low-frequency stimulation (LFS), and impaired LFS-STD elicited in the presence of Zaprinast. These data suggest the involvement of cGMP in LTD in the CA1 region of freely behaving adult rats.

From RNA world to Protein: An eagle's eye view of the Role of Guanosine in Tracing the Antiquity of the Intron

(2001). From RNA world to Protein: An eagle's eye view of the Role of Guanosine in Tracing the Antiquity of the Intron. Journal of Biomolecular Structure and Dynamics: Vol. 18, No. 5, pp. 709-712.

The nitric oxide donors, SNAP and DEA/NO, exert a negative inotropic effect in rat cardiomyocytes which is independent of cyclic GMP elevation.

The role of guanosine 3',5'-cyclic monophosphate (cGMP) in the regulation of cardiac contractility remains controversial. The present study has examined the effects of high concentrations of the nitric oxide (NO) donors, S-nitroso-N-acetylpenicillamine (SNAP) and 1,1-diethyl-2-hydroxy-2-nitroso-hydrazine (DEA/NO), on cGMP levels and isoproterenol-induced increases in contractility in rat cardiomyocytes before and after selective inhibition of soluble guanylyl cyclase with 1 H -[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). In control myocytes, 100 microm SNAP or 100 microm DEA/NO increased cGMP levels by more than 15-fold at 2 and 6 min and produced marked attenuations of isoproterenol-mediated increases in maximal cell shortening over the same time period. The NO donors had no significant effect on basal cell shortening (in the absence of isoproterenol). Pretreatment of myocytes with 25 microm ODQ for 30 min resulted in a complete blockade of the SNAP- or DEA/NO-induced increases in cGMP with no reversal of negative inotropy. ODQ did not affect basal contractility, basal cGMP levels or isoproterenol-induced increases in cell shortening. Furthermore, myocytes exposed to the cGMP analog, 8-bromo-cGMP (100 microm), did not exhibit significant differences in basal contractility or isoproterenol-induced increases in cell shortening. These results suggest that attenuation of cardiac contractility by NO donors in rat cardiomyocytes occurs by a mechanism independent of increases in cGMP levels.

Cyclic Nucleotides and Phosphodiesterases in Platelets

IntroductionIt is now almost 30 years since the discovery that prostaglandin E1 (PGE1) inhibits platelet responses to aggregating agents, together with finding that the effects of this compound are mediated by adenosine 3′, 5′-cyclic monophosphate (cAMP) initiated interest in the physiological and pharmacological regulation of platelet function by other agents that increase platelet cAMP, as reviewed elsewhere.1 The most important agonists that stimulate cAMP formation in platelets have now been identified as prostacyclin (PGI2), prostaglandin D2 (PGD2), and adenosine, which exert their effects through receptors of the serpentine or seven transmembrane segment class (IP, DP and A2 receptors, respectively).2 The latter then stimulate cAMP formation by adenylyl cyclase via the GTP-dependent activation of the G-protein, Gs (Fig. 1). In the classical view, cAMP exerts its effects solely by binding to the RI and RII regulatory subunits of type I and type II cAMP-dependent protein kinases (PKA). The catalytic subunits of the kinases then dissociate and phosphorylate selected serine and threonine residues on target proteins that prevent or reverse platelet activation.2 A crucial role is played by cAMP phosphodiesterases, which degrade cAMP to 5′-AMP, thereby diminishing and terminating the effects of agonists that stimulate cAMP formation (Fig. 1). In early studies, this was demonstrated by the ability of first-generation inhibitors of cAMP phosphodiesterases, particularly the methylxanthines, to inhibit platelet aggregation and potentiate the inhibitory effects of activators of platelet adenylyl cyclase.1 Such studies provided the rationale for the subsequent development of more potent and selective phosphodiesterase inhibitors as potential antithrombotic agents.Interest in the role of guanosine 3′,5′-cyclic monophosphate (cGMP) in platelets closely followed the discovery of the inhibitory action of cAMP. An early hypothesis that cGMP might potentiate platelet aggregation was abandoned by 1978, after it was shown that some inhibitors of platelet aggregation, such as nitroprusside (NP), also increased platelet cGMP.1 It soon emerged that all nitrovasodilators release nitric oxide and activate soluble guanylyl cyclase (GC) and that the cGMP formed stimulates cGMP-dependent protein kinases (PKG) in many cells and tissues (Fig. 1), including vascular smooth muscle and platelets.3 The crucial physiological importance of this pathway was established with the identification of endothelium-derived relaxing factor (EDRF) as nitric oxide.4 cGMP phosphodiesterases play an essential role by limiting increases in cellular cGMP, and inhibition of these enzymes was found to potentiate the effects of nitric oxide and nitric oxide donors on platelets and other cells.5 The ability of cAMP and cGMP to activate distinct protein kinases led to a persistent view that these two cyclic nucleotides operate in parallel and independent ways to inhibit platelet function, cAMP mediating the effects of agonists such as PGI2, and cGMP mediating the effects of nitric oxide.2,3 However, over the last 10 years, considerable evidence has accumulated to indicate that this is not the case in platelets (or in many other cells) and that cross-talk between the cAMP and cGMP systems may occur on at least two levels, affecting both cyclic nucleotide phosphodiesterase (PDE) and protein kinase activities (Fig. 1). One of the most significant of these interactions is through the effects of cGMP on the hydrolysis of cAMP by PDEs. It is the purpose of this chapter to describe platelet PDEs and to discuss how their individual characteristics and regulation may impact platelet function and the design of useful antithrombotic agents. In addition, evidence that both cGMP and cAMP may activate PKG and that these cyclic nucleotides may exert effects in platelets that do not involve either PKA or PKG will be discussed briefly.

Role of nitric oxide and phosphodiesterase isoenzyme II for reduction of endothelial hyperpermeability

Regulation of endothelial permeability is poorly understood. Previous studies have shown that endothelial cells contain phosphodiesterase (PDE) isoenzymes II-IV and that simultaneous adenylate cyclase activation and/or PDE inhibition blocked endothelial hyperpermeability (J.Clin.Invest. 91: 1421-1428, 1993). We now focused on a possible role for guanosine 3',5'-cyclic monophosphate (cGMP)-dependent mechanisms and studied H2O2-exposed porcine pulmonary artery endothelial cell monolayers. Pretreatment of cells with different nitric oxide (NO) donors or atrial natriuretic peptide (ANP) increased endothelial cGMP-content severalfold and blocked H2O2-related effects on permeability; opposite results were obtained with a NO synthase inhibitor. Determination of cGMP degradation in nitroprusside-exposed endothelial cells identified PDE II as the major cGMP metabolizing pathway, whereas PDE III and IV contributed little or nothing. Inhibition of PDE II reduced H2O2-related endothelial hyperpermeability, an effect that could be enhanced synergistically by simultaneous guanylate cyclase activation. In summary, these studies indicate that cGMP-dependent mechanisms (NO donors, ANP, and dibutyryl-cGMP) blocked H2O2-related increases in endothelial permeability. The major cGMP degrading pathway in endothelial cells was PDE II, thereby substituting the missing PDE V in these cells. Simultaneous guanylate cyclase activation and/or PDE II inhibition may be a valuable approach to treat endothelial hyperpermeability.

Hydrogen peroxide-induced pulmonary vasodilation: role of guanosine 3',5'-cyclic monophosphate.

Hydrogen peroxide (H2O2), but not tertbutyl hydroperoxide, produces a concentration-dependent vasodilation of the pulmonary circulation in isolated saline perfused rabbit lungs when pulmonary arterial pressures (PAP) are raised with the thromboxane analogue U-46619. This vasodilation was enhanced in the presence of indomethacin, suggesting that H2O2 possesses both a prostaglandin-mediated constrictor and an additional dilator mechanism. In isolated rabbit intrapulmonary arteries the endothelium did not alter the dose-dependent relaxation of arterial rings to H2O2, and indomethacin enhanced the relaxant response of the peroxide. The decrease in PAP and relaxation of isolated pulmonary arteries observed with H2O2 was attenuated with 10 microM methylene blue, an inhibitor of soluble guanylate cyclase activation. M & B 22948, a guanosine 3',5'-cyclic monophosphate (cGMP)-selective phosphodiesterase inhibitor, enhanced the vasodilation or relaxation to the peroxide in both preparations. These changes were not endothelium dependent. Inhibition of the cGMP-associated endothelium-derived relaxant factor (EDRF) with nitro-L-arginine, did not alter relaxation of arterial rings to peroxide. Thus H2O2 appears to produce pulmonary vasodilation through the activation of guanylate cyclase and accumulation of cGMP. Both H2O2 and EDRF may function as tonic stimulators of guanylate cyclase in the pulmonary circulation and contribute to the maintenance of low basal pressures.

Interleukin 1 and endotoxin activate soluble guanylate cyclase in vascular smooth muscle

Our recent studies indicate that interleukin 1 (IL-1) and bacterial lipopolysaccharide inhibit agonist-induced contractions in rat aortic rings by an endothelium-independent mechanism. The present study investigated the role of guanosine 3',5'-cyclic monophosphate (cGMP) in the vasodilatory action of IL-1 and endotoxin. Rat aortic rings were denuded of endothelium and incubated for 3 h in physiological salt solution containing no additions, IL-1 (20 ng/ml), or endotoxin (10 micrograms/ml). Contractions induced by phenylephrine (3 x 10(-7) M) were decreased by 40 and 85% in endotoxin- and IL-1-treated rings, respectively. IL-1 increased cGMP content 2.5-fold in the absence of and 5.5-fold in the presence of 3-isobutyl-1-methylxanthine (IBMX). Endotoxin also increased cGMP content in the absence and presence of IBMX (5.5- and 25-fold, respectively). Both IL-1- and endotoxin-induced increases in cGMP occurred 3-4 h after initial exposure. The guanylate cyclase inhibitors, LY 83583 and methylene blue, each abolished IL-1- and endotoxin-induced inhibition of contraction and IL-1-induced production of cGMP. Furthermore, hemoglobin, which binds nitric oxide, completely blocked IL-1-induced increases in cGMP. We conclude that IL-1 and endotoxin inhibit vascular contraction in vitro by increasing aortic cGMP content. Studies with inhibitors suggest IL-1 and endotoxin may induce endothelium-independent production of nitric oxide or another free radical that activates soluble guanylate cyclase.

Mitochondrial metabolism of guanine nucleotides possible role of guanosine

The catabolism of intramitochondrial guanine nucleotides was examined. During 30 min incubation of rat liver mitochondria at 37°C in the presence of oligomycin and carboxyatractyloside, guanine and xanthine were formed and appeared in the medium. Under these conditions, the direct conversion of GMP to guanine by hypoxanthine—guanine phosphoribosyltransferase is suggested to be the main catabolic route within the organelles. Only very small amounts of guanosine were produced and detected both inside and outside the organelles. [14C]Guanosine and [14C]inosine were taken up by the mitochondria. Therefore, guanosine is suggested to be a precursor of intramitochondrial guanine nucleotides.

Endothelium-associated vasodilators in rat skeletal muscle microcirculation

Pharmacological probes were used to assess the possible roles of guanosine 3',5'-cyclic monophosphate (cGMP)-associated endothelium-derived relaxing factor (EDRF) in mediating microvascular responses to endogenous and exogenous agents in vivo. Pentobarbital-anesthetized rats (Wistar, 6 wk old) were prepared for in vivo microscopic observation and quantification of changes in diameter of third-order arterioles (15-25 microns) in the cremaster muscle to topical application of all agents. In indomethacin-pretreated preparations, cremasteric arteriolar dilator responses to acetylcholine, bradykinin, or ATP, but not to adenosine, histamine, or prostaglandin E2, were inhibited by hydroquinone (50 microM). Vasodilation to acetylcholine was also inhibited by methylene blue (5 microM), a blocker of guanylate cyclase activation. Constrictor responses to norepinephrine were not affected by hydroquinone or methylene blue. The inhibition of acetylcholine-induced vasodilation by hydroquinone and methylene blue was reversed by superoxide dismutase, suggesting that superoxide anion antagonized the response. On the other hand, basal arteriolar diameters or responses to acetylcholine were not affected by oxygen metabolite scavengers. Unlike in isolated arteries, vasodilator responses to the calcium ionophore A23187 or arachidonic acid were completely antagonized by cyclooxygenase inhibition. These data suggest that EDRF could be involved in the control of microvascular tone; however, significant differences exist in the stimuli that elicit dilation through this mediator in small and large blood vessels.

Guanosine 3',5'-cyclic monophosphate as a mediator of inhibition of renin release.

The role of guanosine 3',5'-cyclic monophosphate (cGMP) as an inhibitory mediator of tissue renin release was examined in two different in vitro preparations. In rat superficial cortical slices, renin release stimulated by isoproterenol (10(-5) M) was ablated by atriopeptin III (ANP, 2.1 x 10(-8) M), nitroprusside (NP, 10(-3) M), and 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP, 10(-3) and 10(-6) M). Arachidonic acid (10(-3) M)-stimulated renin release was also inhibited by ANP and 8-BrcGMP (10(-3) and 10(-6) M). Both ANP and NP increased tissue cGMP concentrations significantly (P less than 0.05), but neither had an effect on adenosine 3',5'-cyclic monophosphate (cAMP) concentrations. When methylene blue (10(-5) M), an inhibitor of guanylate cyclase, was added to slices incubated with isoproterenol and ANP, the inhibition of renin release by ANP was abolished. These results were confirmed in a preparation of isolated cultured rat juxtaglomerular cells. In these cells, isoproterenol induced a significant increase (58%, P less than 0.01) in renin release, which was inhibited by the addition of 8-BrcGMP (10(-6) M). These data demonstrate a direct inhibitory effect of ANP on isoproterenol- and arachidonic acid-induced renin release. The results with NP, 8-BrcGMP, and methylene blue suggest that cGMP is an intracellular mediator of this inhibition.

Therapeutic selectivity of vinca alkaloids: A role for guanosine 5′-triphosphate?

Panax ginseng root and cell cultures were shown to biotransform paeonol (1) into its 2-O-β-d-glucopyranoside (2). P. ginseng root cultures were also able to biotransform paeonol (1) into its 2-O-β-d-xylopyranoside (3), 2-O-β-d-glucopyranosyl(1 → 6)-β-d-glucopyranoside (4) and 2-O-β-d-xylopyranosyl(1 → 6)-β-d-glucopyranoside (5), and its demethylated derivate, 2′,4′-dihydroxyacetophenone (6). Compounds 3 and 4 are new glycosides. It is the first example that the administrated compound was converted into its xylopyranoside by plant biotransformation.

Injection of guanosine 5'-cyclic monophosphate into heart cells blocks calcium slow channels

The role of guanosine 5'-cyclic monophosphate (cGMP) in the regulation of the ionic slow channels in heart muscle is less well known than that of adenosine 3,'5'-cyclic monophosphate (cAMP). The effects of intracellular injection of cAMP and cGMP in cultured chick embryonic heart (ventricular) cells by the liposome method were studied. Injection of cAMP into the cells induced spontaneous slow action potentials that could be blocked by verapamil and nifedipine. Injection of cGMP blocked on-going slow action potentials, and this effect was reversed by increasing cAMP. Thus both cAMP and cGMP are involved in the regulation of the slow calcium channels in myocardial cells, and the two cyclic nucleotides are antagonistic.

Evidence against a role for guanosine 3′,5′-cyclic monophosphate in rat submandibular salivary gland potassium release

The role of guanosine 3′,5′-cyclic monophosphate (cGMP) in the net K + efflux following parasympathomimetic and sympathomimetic stimulation in dispersed rat submandibular cells was evaluated. In unstimulated cells, approx. 70 per cent of the total cGMP content was present in the incubation medium; the addition of 1-methyl-3-isobutylxanthine (MIX), a phosphodiesterase inhibitor, to the medium elevated both intracellular and extracellular levels of cGMP. Cholinergic receptor stimulation resulted in a rapid increase of the cGMP content in the cells and in the medium. Extracellular Ca 2+ was necessary for an increased cGMP response and MIX potentiated the cGMP response. Adrenergic-receptor activation produced a slight increase in cGMP after 5 min of stimulation but only when MIX was present in the medium. cGMP analogues (8-Br-cGMP and Bt 2cGMP) had no effect on the rate of maximal net K + efflux, on submaximal net K + efflux, or on K + re-uptake following parasympathomimetic or sympathomimetic stimulation. The dose-response relationship for a cholinergic agonist, carbamylcholine, or an adrenergic agonist, norepinephrine, was unaffected by the presence of either 8-Br-cGMP or Bt 2cGMP in the medium. MIX, at a concentration sufficient to increase intracellular and extracellular cGMP levels, had no effect on net K + efflux. These data do not support a role for cGMP in modulating rat submandibular potassium release.

Modification of radiosensitivity of mammalian cells by cyclic nucleotides

Some in vitro and in vivo studies suggest that adesosine 3′,5′-cyclic monophosphate (cyclic AMP) may be one of the important factors in determining the radiosensitivity of certain mammalian cells; however, the role of guanosine 3',5'-cyclic monophosphate (cyclic GMP) in radiosensitivity of mammalian cells is completely unknown. Recent data also suggest that the mechanism of radiation protection afforded by moderate hypoxia and SH-containing compounds may involve an alteration in the intracellular level of cyclic AMP. At least one in vivo study shows that cyclic AMP protects hair follicles and gut epithelial cells against radiation damage; however, it does not protect lymphosarcoma and breast carcinoma in mice. If a similar phenomenon is found in humans, an elevation of the intracellular level of cyclic AMP during radiation exposure may improve the effectiveness of radiation therapy in those cases where the radiation damage of normal tissue becomes the limiting factor for a continuation of the therapy program. More in vitro and in vivo studies on normal and cancer cells are needed to substantiate the role of cyclic nucleotides in radiosensitivity.