Potentiation by 2,2′‐pyridylisatogen tosylate of ATP‐responses at a recombinant P 2Y1 purinoceptor

1 2,2′‐Pyridylisatogen tosylate (PIT) has been reported to be an irreversible antagonist of responses to adenosine 5′‐triphosphate (ATP) at metabotropic purinoceptors (of the P 2Y family) in some smooth muscles. When a recombinant P 2Y1 purinoceptor (derived from chick brain) is expressed in Xenopus oocytes, ATP and 2‐methylthioATP (2‐MeSATP) evoke calcium‐activated chloride currents ( I cl,ca ) in a concentration‐dependent manner. The effects of PIT on these agonist responses were examined at this cloned P 2Y purinoceptor. 2 PIT (0.1–100 μ m ) failed to stimulate P 2Y , purinoceptors directly but, over a narrow concentration range (0.1–3 μ m ), caused a time‐dependent potentiation (2–5 fold) of responses to ATP. The potentiation of ATP‐responses by PIT was not caused by inhibition of oocyte ecto‐ATPase. At high concentrations (3–100 μ m ), PIT irreversibly inhibited responses to ATP with a IC 50 value of 13±9 μ m (p K B = 4.88±0.22; n = 3). PIT failed to potentiate inward currents evoked by 2‐MeSATP and only inhibited the responses to this agonist in an irreversible manner. 3 Known P 2 purinoceptor antagonists were tested for their ability to potentiate ATP‐responses at the chick P 2Y , purinoceptor. Suramin (IC 50 = 230±80 nM; n = 5) and Reactive blue‐2 (IC 50 = 580±130 nM; n = 6) reversibly inhibited but did not potentiate ATP‐responses. Coomassie brilliant blue‐G (0.1–3 μ m ) potentiated ATP‐responses in three experiments, while higher concentrations (3–100 μ m ) irreversibly inhibited ATP‐responses. The results indicated that potentiation and receptor antagonism were dissociable and not a feature common to all known P 2 purinoceptor antagonists. 4 In radioligand binding assays, PIT showed a low affinity (p K i <5) for a range of membrane receptors, including: α 1 , α 2 ‐adrenoceptors, 5‐HT 1A , 5‐HT 1B , 5‐HT 2 , 5‐HT 3 , D 1 , D 2 , muscarinic, central benzodiazepine, H 1 , μ‐opioid, dihydropyridine and batrachotoxin receptors. PIT showed some affinity (p K i = 5.3) for an adenosine (A 1 ) receptor. 5 In guinea‐pig isolated taenia caeci, PIT (12.5–50 μ m ) irreversibly antagonized relaxations to ATP (3–1000 μ m ); PIT also directly relaxed the smooth muscle and histamine was used to restore tone. Relaxations to nicotine (10–100 μ m ), evoked by stimulating intrinsic NANC nerves of taenia caeci preparations in the presence of hyoscine (0.3 μ m ) and guanethidine (17 μ m ), were not affected by PIT (50 μ m , for 25–60 min). 6 These experiments indicate that PIT causes an irreversible antagonism of ATP receptors but, for recombinant chick P 2Y , purinoceptors, this effect is preceded by potentiation of ATP agonism. The initial potentiation by PIT (and by Coomassie brilliant blue‐G) of ATP‐responses raises the possibility of designing a new class of modulatory drugs to enhance purinergic transmission at metabotropic purinoceptors.