Relationship of the inhibition of epinephrine-stimulated adenylate cyclase in turkey erythrocytes by adenosine to its "R-" and "P-site"-mediated effects

Adenosine as a signaling molecule in the retina: biochemical and developmental aspects

The nucleoside adenosine plays an important role as a neurotransmitter or neuromodulator in the central nervous system, including the retina. In the present paper we review compelling evidence showing that adenosine is a signaling molecule in the developing retina. In the chick retina, adenosine transporters are present since early stages of development before the appearance of adenosine A1 receptors modulating dopamine-dependent adenylate cyclase activity or A2 receptors that directly activate the enzyme. Experiments using retinal cell cultures revealed that adenosine is taken up by specific cell populations that when stimulated by depolarization or neurotransmitters such as dopamine or glutamate, release the nucleoside through calcium-dependent transporter-mediated mechanisms. The presence of adenosine in the extracellular medium and the long-term activation of adenosine receptors is able to regulate the survival of retinal neurons and blocks glutamate excitoxicity. Thus, adenosine besides working as a neurotransmitter or neuromodulator in the mature retina, is considered as an important signaling molecule during retinal development having important functions such as regulation of neuronal survival and differentiation.

The role of adenosine in insulin action coupling in rat adipocytes

The involvement of adenosine in the coupling of insulin binding to action was investigated in rat adipocytes. Reduction of endogenous adenosine levels by treatment with adenosine deaminase (ADA) had no significant effect on either basal or maximally stimulated glucose transport, but reduced the insulin sensitivity of transport stimulation. Adenosine deaminase treatment also shifted the EC50 of H2O2 stimulation of transport from 0.13 mM to 0.30 mM, and the EC50 for insulin stimulation of protein synthesis from 0.40 +/- 0.06 ng/ml to 1.30 +/- 0.25 ng/ml. Adenosine appears to be acting through the pharmacological Ri adenosine receptor subtype. The mode of action of adenosine does not seem to involve inhibition of adenylate cyclase. Adenosine also influences the kinetics of insulin action. ADA treatment slows the onset of transport stimulation by a maximal insulin concentration (10 ng/ml). Increasing the hormone level to 100 ng/ml overcomes this slowing without increasing transport further. The deactivation of glucose transport following removal of insulin is accelerated by ADA treatment. Thus, adenosine is involved both in maintaining a high efficiency of an early step in the insulin signaling process and in maintaining optimal activity of the insulin-stimulated glucose transport system.

Effects of purines and forskolin on haemolysis

Effects of adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine on the antihaemolytic action of procaine, and the effect of ATP on the antihaemolytic actions of lidocaine, dibucaine, tetracaine, pentobarbitone, and chlorpromazine were investigated in rat erythrocytes. The effects of adenosine and its analogues D-N6-phenylisopropyladenosine (D-PIA), L-N6-phenylisopropyladenosine (L-PIA), N6-cyclohexyladenosine (CHA), 2-chloroadenosine, and N6-methyladenosine, as well as the effects of cyclic-AMP (cAMP), dibutyryl cAMP, and forskolin on haemolysis were also investigated in rat erythrocytes. ATP, ADP, and AMP, but not adenosine, antagonized in a concentration-dependent manner the antihaemolytic action of procaine, and ATP was ineffective against the antihaemolytic actions of lidocaine, dibucaine, tetracaine, pentobarbitone, and chlorpromazine. Adenosine and its analogues, but not N6-methyladenosine, protected erythrocytes against hypotonic haemolysis in a concentration-dependent manner. The order of potencies was: D-PIA greater than CHA greater than L-PIA greater than adenosine greater than 2-chloroadenosine. The effect of adenosine on haemolysis was not prevented by theophylline, 8-phenyltheophylline, or 8-sulfophenyltheophylline. Dibutyryl cAMP, a stable analogue of cAMP, and forskolin, a specific activator of adenylate cyclase, mimicked the antihaemolytic action of adenosine. cAMP was less efficient than adenosine. Adenine nucleotides antagonized the antihaemolytic action of procaine, probably due to calcium-chelating properties. In contrast, adenine nucleosides have antihaemolytic properties. The possibility that the antihaemolytic effects of these substances relate to cAMP and/or to lipophilicity is discussed.

Role of adenosine as an endogenous regulator of respiration in hamster brown adipocytes

The action of endogeneous adenosine on isolated hamster brown adipocytes was examined. Adenosine production from brown adipocytes was measured after labeling of the intracellular nucleotide pool with [3H]adenine. Accumulation of [3H]adenosine in the incubation medium was maximum after 5 min of incubation and was still present after 20 min. When adenosine accumulation was prevented by addition of adenosine deaminase, the stimulatory effects of isoproterenol on oxygen uptake, lipolysis, and adenosine 3',5'-cyclic monophosphate (cAMP) generation were enhanced. However, basal rates of lipolysis and oxygen consumption and levels of cAMP were not affected on addition of adenosine deaminase. A similar potentiation of isoproterenol responses was produced by the adenosine receptor antagonist, 3-isobutyl-1-methylxanthine, present at a concentration (10 microM) which did not change basal levels of respiration or lipolysis. Addition of the adenosine analogue 2-chloroadenosine antagonized isoproterenol-stimulated respiration and lipolysis and prevented potentiation of isoproterenol responses with 3-isobutyl-1-methylxanthine. To localize the site of adenosine action, activity of adenylate cyclase in membrane preparations from brown adipocytes was measured. Isoproterenol-stimulated adenylate cyclase activity was partially inhibited by 2-chloroadenosine in a GTP-dependent manner. Addition of Na+ enhanced the inhibitory effect of 2-chloroadenosine, and 3-isobutyl-1-methylxanthine blocked it. The calculated 50% effective dose for 2-chloroadenosine inhibition was between 10 and 15 nM. These data suggest that adenosine produced by brown adipocytes is an endogenous regulator of respiration in these cells acting at the level of the adenylate cyclase enzyme.