Regulation of soluble 5'-nucleotidase I from rabbit heart

Pharmacology of adenosine receptors in the vasculature

Adenosine is widely distributed in mammals. One of the primary roles of adenosine within the cardiovascular system is to directly control the functions of both cardiac and vascular tissues. Recently, there has been considerable interest in the subclassification of adenosine receptors. Characterization of a heterogeneous population of receptors for adenosine could provide an opportunity for the development of novel compounds of therapeutic value. Adenosine is released from cells as a result of metabolism, and its release can be increased dramatically from cells that are metabolically stressed. This implies that adenosine can be released from a variety of cells throughout the body, as a result of increased metabolic rates, in concentrations that can have a profound impact on blood vessel function and, consequently, blood flow. It is recognized that the actions of this nucleoside on the vasculature are most prominent when oxygen demand is high and there is a reduction in oxygen tension at the site in question. Therefore, it is not surprising that adenosine has been shown to be an important regulator of blood vessel tone under hypoxic conditions. Furthermore, the activation of adenosine receptors on blood vessels can result in relaxation and/or contractions. The nature of the response subsequent to the activation of adenosine receptors is primarily dependent on the type of blood vessel involved and basal tone. This review will focus on the characterization of subtypes of adenosine receptors in blood vessels, as well as the effect of the stimulation of adenosine receptors on the peripheral circulation.

Evidence for increased dorsal hippocampal adenosine release and metabolism during pharmacologically induced seizures in rats

There is growing pharmacological evidence from several animal models of seizure disorder that adenosine possesses endogenous anticonvulsant activity. In order to further evaluate the role of adenosine in seizure activity, we monitored adenosine and its major biochemical metabolites inosine, xanthine, and hypoxanthine in the dorsal hippocampus by in vivo microdialysis before and during the induction of generalized seizures. Seizures were induced pharmacologically in groups of urethane-anesthetized rats by the administration of bicuculline (0.5 mg/kg, i.v.), kainic acid (12.0 mg/kg, i.v.) or pentylenetetrazol (100-250 mg/kg, i.p). Seizure activity was monitored electrophysiologically from the dorsal hippocampus. Dialysate hippocampal purine levels increased during all three seizure types. The largest increases were for the adenosine metabolites hypoxanthine and inosine, with smaller increases observed for adenosine and xanthine. Intra-hippocampal perfusion with the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl-adenine, (EHNA, 300 microM), only slightly increased basal hippocampal adenosine. Guanosine levels in the hippocampus, a purine not directly related to adenosine metabolism, were unaffected by all treatments. These findings demonstrate that an increase in hippocampal adenosine release and metabolism is associated with seizure activity and support the hypothesis that the increased adenosine levels may attenuate hippocampal seizure activity, possibly by terminating ongoing seizures and altering the pattern of subsequent seizures.

Structure-activity relationship of cytoplasmic 5'-nucleotidase substrate sites

Various 5'-nucleotidases (EC 3.1.3.5) exist in vertebrate tissues. The sequence and cDNA cloning of the membrane-bound ecto-5'-nucleotidase (e-N) and one of the cytosolic isoenzymes, IMP-preferring (c-N-II), but not the cytosolic AMP-preferring form (c-N-I), have been reported. While c-N-II has a broad tissue distribution, c-N-I is found only in vertebrate heart. The published data on substrate specificity involve mainly the naturally occurring nucleoside monophosphates, without a systematic structure-activity relationship study. In the present study we have used a series of AMP and IMP analogues to examine the structure-activity relationship for c-N-I and c-N-II in detail. The rank order of activity of the test compounds differed substantially between c-N-I and c-N-II. c-N-I and c-N-II varied with respect to the following interactions with substrate: (1) hydrogen-bond formation with the substituent in the 6-position of the purine ring (a donor-type with c-N-I and an acceptor-type with c-N-II); and (2) hydrophobic attraction of the 6-position unsubstituted purine ring (more pronounced with c-N-I than with c-N-II). No better substrate than 5'-AMP was found for c-N-I. We propose that c-N-I functions as an AMP-binding protein in the myocardial cell with an important role during ischaemic ATP breakdown when AMP accumulates rapidly.