The effect of a long-term treatment with cannabidiol-rich hemp extract oil on the adenosinergic system of the zucker diabetic fatty (ZDF) rat atrium.

Cannabidiol (CBD), the most extensively studied non-intoxicating phytocannabinoid, has been attracting a lot of interest worldwide owing to its numerous beneficial effects. The aim of this study was to explore the effect that CBD exerts on the adenosinergic system of paced left atria isolated from obese type Zucker Diabetic Fatty (ZDF) rats, maintained on diabetogenic rat chow, received 60 mg/kg/day CBD or vehicle via gavage for 4 weeks. We found that N6-cyclopentyladenosine (CPA), a relatively stable and poorly transported A1 adenosine receptor agonist, elicited a significantly weaker response in the CBD-treated group than in the vehicle-treated one. In contrast, adenosine, a quickly metabolized and transported adenosine receptor agonist, evoked a significantly stronger response in the CBD-treated group than in the vehicle-treated counterpart (excepting its highest concentrations). These results can be explained only with the adenosine transport inhibitory property of CBD (and not with its adenosine receptor agonist activity). If all the effects of CBD are attributed to the interstitial adenosine accumulation caused by CBD in the myocardium, then a significantly increased adenosinergic activation can be assumed during the long-term oral CBD treatment, suggesting a considerably enhanced adenosinergic protection in the heart. Considering that our results may have been influenced by A1 adenosine receptor downregulation due to the chronic interstitial adenosine accumulation, an adenosinergic activation smaller than it seemed cannot be excluded, but it was above the CBD-naïve level in every case. Additionally, this is the first study offering functional evidence about the adenosine transport inhibitory action of CBD in the myocardium.

A Body of Circumstantial Evidence for the Irreversible Ectonucleotidase Inhibitory Action of FSCPX, an Agent Known as a Selective Irreversible A1 Adenosine Receptor Antagonist So Far.

In previous studies using isolated, paced guinea pig left atria, we observed that FSCPX, known as a selective A1 adenosine receptor antagonist, paradoxically increased the direct negative inotropic response to A1 adenosine receptor agonists (determined using concentration/effect (E/c) curves) if NBTI, a nucleoside transport inhibitor, was present. Based on mathematical modeling, we hypothesized that FSCPX blunted the cardiac interstitial adenosine accumulation in response to nucleoside transport blockade, probably by inhibiting CD39 and/or CD73, which are the two main enzymes of the interstitial adenosine production in the heart. The goal of the present study was to test this hypothesis. In vitro CD39 and CD73 inhibitor assays were carried out; furthermore, E/c curves were constructed in isolated, paced rat and guinea pig left atria using adenosine, CHA and CPA (two A1 adenosine receptor agonists), FSCPX, NBTI and NBMPR (two nucleoside transport inhibitors), and PSB-12379 (a CD73 inhibitor), measuring the contractile force. We found that FSCPX did not show any inhibitory effect during the in vitro enzyme assays. However, we successfully reproduced the paradox effect of FSCPX in the rat model, mimicked the "paradox" effect of FSCPX with PSB-12379, and demonstrated the lipophilia of FSCPX, which could explain the negative outcome of inhibitor assays with CD39 and CD73 dissolved in a water-based solution. Taken together, these three pieces of indirect evidence are strong enough to indicate that FSCPX possesses an additional action besides the A1 adenosine receptor antagonism, which action may be the inhibition of an ectonucleotidase. Incidentally, we found that POM-1 inhibited CD73, in addition to CD39.

Accuracy and Precision of the Receptorial Responsiveness Method (RRM) in the Quantification of A1 Adenosine Receptor Agonists.

The receptorial responsiveness method (RRM) is a procedure that is based on a simple nonlinear regression while using a model with two variables (X, Y) and (at least) one parameter to be determined (cx). The model of RRM describes the co-action of two agonists that consume the same response capacity (due to the use of the same postreceptorial signaling in a biological system). While using RRM, uniquely, an acute increase in the concentration of an agonist (near the receptors) can be quantified (as cx), via evaluating E/c curves that were constructed with the same or another agonist in the same system. As this measurement is sensitive to the implementation of the curve fitting, the goal of the present study was to test RRM by combining different ways and setting options, namely: individual vs. global fitting, ordinary vs. robust fitting, and three weighting options (no weighting vs. weighting by 1/Y2 vs. weighting by 1/SD2). During the testing, RRM was used to estimate the known concentrations of stable synthetic A1 adenosine receptor agonists in isolated, paced guinea pig left atria. The estimates were then compared to the known agonist concentrations (to assess the accuracy of RRM); furthermore, the 95% confidence limits of the best-fit values were also considered (to evaluate the precision of RRM). It was found that, although the global fitting offered the most convenient way to perform RRM, the best estimates were provided by the individual fitting without any weighting, almost irrespective of the fact whether ordinary or robust fitting was chosen.

An Advanced In Silico Modelling of the Interaction between FSCPX, an Irreversible A1 Adenosine Receptor Antagonist, and NBTI, a Nucleoside Transport Inhibitor, in the Guinea Pig Atrium.

In earlier studies, we generated concentration-response (E/c) curves with CPA (N6-cyclopentyladenosine; a selective A1 adenosine receptor agonist) or adenosine, in the presence or absence of S-(2-hydroxy-5-nitrobenzyl)-6-thioinosine (NBTI, a selective nucleoside transport inhibitor), and with or without a pretreatment with 8-cyclopentyl-N3-[3-(4-(fluorosulfonyl)-benzoyloxy)propyl]-N1-propylxanthine (FSCPX, a chemical known as a selective, irreversible A1 adenosine receptor antagonist), in isolated, paced guinea pig left atria. Meanwhile, we observed a paradoxical phenomenon, i.e. the co-treatment with FSCPX and NBTI appeared to enhance the direct negative inotropic response to adenosine. In the present in silico study, we aimed to reproduce eight of these E/c curves. Four models (and two additional variants of the last model) were constructed, each one representing a set of assumptions, in order to find the model exhibiting the best fit to the ex vivo data, and to gain insight into the paradoxical phenomenon in question. We have obtained in silico evidence for an interference between effects of FSCPX and NBTI upon our ex vivo experimental setting. Regarding the mechanism of this interference, in silico evidence has been gained for the assumption that FSCPX inhibits the effect of NBTI on the level of endogenous (but not exogenous) adenosine. As an explanation, it may be hypothesized that FSCPX inhibits an enzyme participating in the interstitial adenosine formation. In addition, our results suggest that NBTI does not stop the inward adenosine flux in the guinea pig atrium completely.