Adenosine blocks calcium entry in activated neutrophils and binds to flunarizine-sensitive calcium channels

Purines and neutrophil leukocytes

1. Adenosine is a normal constituent of all body fluids and its levels are raised, for example, by hypoxia and ischemia. In addition, both adenosine and ATP can be released by endothelial cells and neutrophils in response to physiologic stimulation. 2. Human neutrophil leukocytes possess multiple adenosine receptors and P2 purinoceptors. 3. ATP can increase intracellular Ca2+ levels in neutrophils, cause degranulation and enzyme release, potentiate the oxidative burst and enhance their adhesion to the endothelium. ATP is broken down to adenosine by ecto-enzymes. Via A1 receptors, adenosine can increase neutrophil chemotaxis and, via A2A receptors, it can decrease the oxidative burst, degranulation and adhesion to endothelium. 4. Adenosine and adenine nucleotides are important endogenous modulators of neutrophil functions, and drugs may exert important actions via purinoceptors on neutrophil leukocytes.

The effect of flunarizine on central nervous system oxygen toxicity in rats

The toxicity of hyperbaric oxygen in the central nervous system is expressed by generalized tonic-clonic seizures. In the search for drugs effective against these seizures, we tested flunarizine, a calcium antagonist known to have antiepileptic properties and only minimal cardiovascular side effects. 49 rats with chronic cortical electrodes were injected i.p. with six different doses of flunarizine (10-300 mg/kg) or vehicle, before exposure to 0.5 MPa oxygen. Two doses of flunarizine and vehicle were given to rats exposed to oxygen with 5% CO2 at an absolute pressure of 0.5 MPa. EEG and spectral analysis of background EEG activity were monitored. The duration of the latent period before the appearance of electrical discharges in the EEG was used as an index of oxygen toxicity. There was no statistical difference between the duration of the latent periods for the seven groups treated by flunarizine or by vehicle on exposure to 0.5 MPa pure oxygen (P = 0.9 in ANOVA), but on exposure to oxygen with CO2, there was significant prolongation of the latent periods in comparison with vehicle (P < 0.001). Our results suggest that on exposure to hyperbaric oxygen, the antiepileptic effect of flunarizine might be masked, probably by its cerebral antivasoconstrictive effect.

Adenosine inhibits divalent cation influx across human neutrophil plasma membrane via surface adenosine A2 receptors

Adenosine and its analogues inhibited increases in divalent cation influx stimulated by platelet-activating factor (PAF) and formyl-methionyl-leucyl-phenylalanine (FMLP) in a dose-dependent fashion. This effect was antagonized by theophylline, an adenosine receptor antagonist. When extracellular adenosine was removed by adenosine deaminase, the effect of adenosine was completely abolished. Two adenosine analogues with different affinities for adenosine receptor subtypes, 5'-N-ethylcarboxamideadenosine (NECA) and L-N6-phenylisopropyladenosine (PIA), also inhibited divalent cation influx, NECA being more potent than PIA. These results suggest that adenosine and its analogues inhibit divalent cation influx across neutrophil plasma membranes via surface adenosine A2 receptors. Adenosine had little effect on the initial peaks of intracellular free calcium rises induced by chemoattractants, but it inhibited the subsequent rise in free calcium. Since calcium influx through the divalent cation channels or neutrophil plasma membranes is responsible for maintaining free calcium concentration following the initial peaks, we suggest that adenosine modulates neutrophil function by interfering with this calcium influx.

Pharmacodynamics of salmon calcitonin in humans: new markers of pharmacological activity

In order to define the pharmacodynamic profile of salmon calcitonin (sCT) in humans, several markers of the biological activity of the drug have been studied, namely cAMP, adenosine and pO2 in venous blood, and the cytosolic free calcium level in circulating cells. Different dosages and routes of administration (1.5 IU.kg-1 and 0.75 IU kg-1 IM, and 1.5 IU.kg-1 via nasal spray) were compared. sCT caused an increase in cAMP, adenosine and pO2, and a decrease in cytosolic free calcium in neutrophils, lymphocytes and platelets. The peak times of all these parameters ranged between 109 and 182 min, and 101 and 168 min after IM and nasal spray administration respectively. There was greater variability in the values after IM than nasal spray of administration of sCT. It is concluded that adenosine and pO2 in venous blood, and cytosolic free calcium in circulating cells are valuable markers of the activity of sCT and that sCT decreases the cytosolic free calcium level in neutrophils, lymphocytes and platelets. Pharmacodynamic analysis of the biological effects of the drug is highly reliable and valuable in predicting its pharmacological profile. sCT administration via a nasal spray is able to produce significant biological effects, although they are less marked than after IM dosing.