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Diadenosine polyphosphates inhibit adenosine kinase activity but decrease levels of endogenous adenosine in rat brain

S M Delaney1, G M Blackburn, J D Geiger

  • 1Department of Pharmacology and Therapeutics, Faculty of Medicine, University of Manitoba, Winnipeg, Canada.

European Journal of Pharmacology
|July 30, 1997
PubMed
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Diadenosine polyphosphates (Ap(n)As) were investigated for their effects on brain adenosine levels. While some Ap(n)As inhibited adenosine kinase in vitro, they paradoxically decreased adenosine levels in rat striatum in vivo.

Area of Science:

  • Neuroscience
  • Biochemistry
  • Pharmacology

Background:

  • Diadenosine polyphosphates (Ap(n)As) are signaling molecules found in peripheral tissues.
  • Previous in vitro studies suggested Ap(n)As influence purine enzyme activity, specifically 5'-nucleotidase and adenosine kinase.

Purpose of the Study:

  • To test the hypothesis that Ap(n)As increase endogenous adenosine levels in the brain in vivo.
  • To investigate the in vitro effects of various Ap(n)As on key purine enzymes involved in adenosine metabolism.
  • To determine the in vivo impact of Ap(n)As on endogenous adenosine levels in the rat striatum.

Main Methods:

  • In vitro enzyme assays were performed to assess the effects of Ap(n)As on adenosine kinase, adenosine deaminase, AMP deaminase, and 5'-nucleotidase.
  • Unilateral microinjections of Ap(n)As were administered into the rat striatum.

Related Experiment Videos

  • Endogenous adenosine levels in the striatum were measured using techniques likely involving microdialysis or HPLC.

    • Main Results:

    • Ap(n)As, particularly diadenosine tetraphosphate (Ap4A), pentaphosphate (Ap5A), and hexaphosphate (Ap6A), inhibited adenosine kinase activity in vitro.
    • Inhibition of adenosine kinase by Ap4A and its analogues was uncompetitive.
    • Intrastriatal injection of Ap4A and a stable analogue (AppCH2ppA) significantly decreased striatal adenosine levels by 48% and 37%, respectively.
    • Ap5A and Ap6A did not significantly affect striatal adenosine levels.
    • Some Ap(n)As inhibited cytosolic 5'-nucleotidase and increased AMP deaminase activity.

    Conclusions:

    • Despite potent in vitro inhibition of adenosine kinase by certain Ap(n)As, these compounds did not increase, and in some cases decreased, endogenous adenosine levels in the rat striatum.
    • The in vivo effects of Ap(n)As on brain adenosine levels are complex and not solely explained by their direct inhibition of adenosine kinase.