Tissue distribution and metabolism of guanosine in rats following intraperitoneal injection

Guanosine has long been known as an endogenous purine nucleoside deeply involved in the modulation of several intracellular processes, especially G-protein activity. More recently, it has been reported to act as an extracellular signaling molecule released from neurons and, more markedly, from astrocytes either in basal conditions or after different kinds of stimulation including hypoxia. Moreover, in vivo studies have shown that guanosine plays an important role as both a neuroprotective and neurotrophic agent in the central nervous system. Specific high-affinity binding sites for this nucleoside have been found on membrane preparations from rat brain. The present study was undertaken to investigate the distribution and metabolic profiles of guanosine after administering the nucleoside to gain a better understanding of the biological effects of this potential drug candidate. Rats were given an intraperitonal (i.p.) injection of 2, 4, 8 or 16 mg/kg of guanosine combined with 0.05% of [3H]guanosine. Plasma samples were collected 7.5, 15, 30, 60 and 90 min after the guanosine-mixture administration and analyzed by either a liquid scintillation counter or by HPLC connected to a UV and to an on-line radiochemical detector to measure the levels of guanosine and its metabolic products guanine, xanthine and uric acid. The levels of guanosine, guanine and xanthine were also measured in brain, lung, heart, kidney and liver tissue homogenates at the defined time points after the injection of 8 mg/kg of the guanosine-mixture. We found that the levels of radioactivity in plasma increased linearly in a dose- and time-dependent manner. Guanosine was widely distributed in all tissues examined in the present study, at almost twice its usual levels. In addition, guanine levels dramatically increased in all the organs. Interestingly, enzymatic analysis of the plasma samples showed the presence of a soluble purine nucleoside phosphorylase, a key enzyme in the purine salvage pathway and nucleoside catabolism. Since guanosine has been shown to be neuroprotective and astrocytes have been reported to play critical roles in mediating neuronal survival and functions in different neurodegenerative disorders, we also performed uptake and release experiments using primary cultures of rat brain astrocytes. Astrocytes actively and specifically take up both guanosine and guanine and release the respective nucleotides which, in turn, undergo breakdown extracellularly. The high levels of guanine found in the culture medium following astrocyte preloading with guanosine indicates that guanine should also be considered as a potential signaling molecule able to mediate, at least in part, the biological effects that, until now, have been ascribed to guanosine. These findings indicate that newly synthesized guanosine/guanine-based analogues could represent a novel therapeutic approach to neurodegenerative disorders and support the evidence that astrocytes could represent novel, promising therapeutic targets in brain diseases. Copyright © by BIOLIFE, s.a.s

Purine nucleosides protect injured neurons and stimulate neuronal regeneration by intracellular and membrane receptor-mediated mechanisms

Like adenine-based purines, extracellular nonadenine-based purines have a multitude of trophic effects on the growth, differentiation, and survival of target cells. The nonadenine-based purines, which include guanosine, inosine, and GTP, apparently exert their trophic effects by interacting with both intercellular targets as well as those on the cell surface. Specifically, guanosine and inosine target the protein kinase N-kinase, in promoting remarkable nerve process extension, even in long tracts of the central nervous system after injury. In contrast, GTP may exert its effects via a cell surface receptor coupled to the release of calcium from internal stores. In other cases trophic effects may be mediated by the enhancement of release of adenine-based purines by guanosine. Additionally, evidence is presented for the existence of a high-affinity binding site for guanosine with receptor-like characteristics on the plasma membranes of astrocytes and brain tissue. This site may be G-protein-coupled and exert its effects through activation of the MAP kinase cascade. One effect apparently mediated through this mechanism is the production and release by astrocytes of trophic protein growth factors such as NGF and TGFβ. These have substantial neuroprotective effects. Additionally, this pathway is apparently involved in modulating the expression of P2Y1 and P2Y2 receptors in response to extracellular guanosine. Extracellular nonadenine-based purines can interact with other growth factors, but these interactions are not always synergistic. For example, combinations of guanosine and FGF are antagonistic and reduce the growth of microvascular cells in vitro. Some of the properties of the nonadenine-based purines likely derive from their unique intracellular metabolism in which conversion of guanine to xanthine is the final catabolic step. This step is catalyzed by guanase, the activity of which varies markedly in different brain regions, raising the possibility that guanine or guanosine are involved in neurotransmission. Together these data suggest several potentially useful pharmacological approaches involving nonadenine-based purines to modulate trophic effects in the central nervous syste

Do non-steroidal anti-inflammatory drugs or smoking predispose to Helicobacter pylori infection?

Susceptibility to Helicobacter pylori infection is a poorly understood phenomenon. This study was undertaken to establish whether either smoking or chronic non-steroidal anti-inflammatory drug (NSAID) consumption might in some way predispose to H. pylori infection and hence lead to peptic ulceration. Serological evidence of H. pylori infection was assessed in 100 consecutive subjects receiving NSAIDs without any evidence of gastrointestinal upset and 100 matched controls. All subjects had a full assessment of their smoking habits. Sixty-three per cent of patients taking NSAIDs compared to 51% of controls had evidence of H. pylori infection (NS). Smoking habit also had no effect on H. pylori colonization. The ulcerogenic potential of NSAIDs and smoking does not appear to be mediated via a prediposition to H. pylori infection