P2x‐purinoceptors of myenteric neurones from the guinea‐pig ileum and their unusual pharmacological properties

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Abstract
Whole‐cell and outside‐out patch clamp recordings were used to characterize the physiological and pharmacological properties of the P2x‐purinoceptors of myenteric neurones from the guinea‐pig ileum. Adenosine 5′‐triphosphate (ATP) and analogues (1–3000μm) evoked a rapid inward current in > 90% of all recorded neurones. The reversal potential of this current was dependent on the extracellular sodium concentration, at +14 ± 1.9, 0 ± 1.6 and −12 ± 1 mV for 166, 83 and 42 mM of sodium, respectively. The fast activation and inactivation of this current occurred even when guanosine 5′‐triphosphate (GTP) was omitted from the pipette solution or substituted with an equimolar concentration of guanosine 5′‐o‐[2‐thiotriphosphate] (GTP‐γ‐S). Single channel currents were observed when these outside‐out membrane patches were exposed to ATP (10–30 μm). These channels have a unitary conductance of about 17 picosiemens. The rank‐order of potency of the agonists used to induce the whole‐cell currents was: ATP‐γ‐S = ATP = 2‐methylthio‐ATP (2‐Me‐S‐ATP) > > α,β‐methylene ATP = β,γ‐methylene ATP; adenosine and uridine 5′‐triphosphate (UTP) (up to 1 mM) were inactive. Pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulphonic acid (PPADS) (1–30μm) antagonized the effects of ATP (1 mM) with an IC50 of 4 μm. α,β‐Methylene ATP (100 μm) did not affect the ATP (30 μm)‐induced current. Cibacron Blue 3GA increased the ATP activated cationic current whereas Basilen Blue E‐3G had a very weak antagonistic effect (IC50 μ 3 mM). Suramin potentiated the currents induced by ATP through a mechanism that was independent of its inhibitory effect on ectonucleotidase activity, as suramin also potentiated the effect of α,β‐methylene ATP (an ATP analogue that is resistant to nucleotidases). In conclusion, the myenteric P2x‐purinoceptor shares some properties with other purinoceptors in particular with the P2times4‐ and P2X6‐purinoceptors. This receptor has also some unusual pharmacological properties suggesting that myenteric neurones express a novel subtype of P2x‐purinoceptors. The properties of this receptor, however, might be a result of the combination of two or more of the homomeric purinoceptors so far characterized.