PPNDS is an agonist, not an antagonist, for the ATP receptor of Paramecium

Abstract

SUMMARY Paramecium represents a simple, eukaryotic model system to study the cellular effects of some neuroactive drugs. They respond to the agonistβ,γ-methylene ATP with a transient depolarizing receptor potential,Ca2+-based action potentials and repetitive bouts of forward and backward swimming called `avoiding reactions' (AR). In vivo[32P]ATP binding assays showed saturable [32P]ATP binding with an apparent Kd of approximately 23 nmol l-1. Prolonged (15 min) exposure to 25 μmol l-1β,γ-methylene ATP caused behavioral adaptation and losses of AR,ATP receptor potentials and [32P]ATP binding. While screening various ATP receptor inhibitors, we found that the P2X1 `antagonist'pyridoxal-phosphate naphthylazo-nitro-disulfate (PPNDS) is actually an agonist, producing the same responses as β,γ-methylene ATP.[32P]ATP binding assays suggest that both agonists may bind to the same site as [32P]ATP. Cross-adaptation is also seen between PPNDS and β,γ-methylene ATP in terms of losses in AR, depolarizing receptor potentials and [32P]ATP binding. We conclude that the inhibition caused by PPNDS in Paramecium is due to agonist-induced desensitization. Either this represents a unique new class of ATP receptors,in which PPNDS is an agonist instead of an antagonist, or PPNDS (and other drugs like it) may actually be an agonist in many other cell types in which prolonged exposure is necessary for inhibition.

Structural Bases of Norovirus RNA Dependent RNA Polymerase Inhibition by Novel Suramin-Related Compounds

“…Inspection of the crystal structures showed that 6 binds to a site located in the polymerase thumb domain, more specifically in a cleft along the exit path for the newly synthesized RNA. This site doesn't match the previously identified Suramin binding site, but corresponds to the Pyridoxal-5′-phosphate-6-(2′-naphthylazo-6′-nitro-4′,8′-disulphonate; PPNDS) [32], [33] binding site [34](Tarantino et al in press). Notably, PPNDS is also a molecule composed of two parts: the naphthalene disulphonate head and the pyridoxal phosphate group, linked by an azo bridge.…”

Activation and Regulation of Purinergic P2X Receptor Channels

“…The naphthylazo derivative of the PPADS family, PPNDS, inhibits rP2X1R at nanomolar concentrations, with an IC 50 of 15 nM (Lambrecht et al, 2000b) but behaves as an agonist for the ATP receptor in Paramecium spp. (Wood and Hennessey, 2003). Two other PPADS derivatives, MRS2159 and MRS2220, are highly selective for ATP-induced responses in rP2X1R, and are ectonucleotidase resistant (Jacobson et al, 1998;Kim et al, 2001).…”

Ca2+ oscillations mediated by exogenous GTP in Paramecium cells: assessment of possible Ca2+ sources

“…From this, the occurrence of surface receptors for GTP has been inferred in Paramecium [25][26][27] and Tetrahymena [23,28,29]. The typical receptor property, i.e., adaptation [30,31], has also been observed with GTP in Paramecium [25] and Tetrahymena [23].…”

“…The fact that GTP causes the phenomena described, while ATP or UTP do not [26], remains undisputed, although non-hydrolysable ATP analogues work equally well at very low concentrations [27]. Binding studies strongly suggest the occurrence of purinergic receptors in Paramecium [27] and Tetrahymena [29].…”

“…Binding studies strongly suggest the occurrence of purinergic receptors in Paramecium [27] and Tetrahymena [29]. In the future, molecular cloning and modelling may specify such receptors in more detail.…”