P2x-purinoceptors of myenteric neurones from the guinea-pig ileum and their unusual pharmacological properties
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1. 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. 2. Adenosine 5'-triphosphate (ATP) and analogues (1-3000 microM) 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-gamma-S). Single channel currents were observed when these outside-out membrane patches were exposed to ATP (10-30 microM). These channels have a unitary conductance of about 17 picosiemens. 3. The rank-order of potency of the agonists used to induce the whole-cell currents was: ATP-gamma-S = ATP = 2-methylthio-ATP (2-Me-S-ATP) > > alpha, beta-methylene ATP = beta, gamma-methylene ATP; adenosine and uridine 5'-triphosphate (UTP) (up to 1 mM) were inactive. 4. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (1-30 microM) antagonized the effects of ATP (1 mM) with an IC50 of 4 microM. alpha, beta-Methylene ATP (100 microM) did not affect the ATP (30 microM)-induced current. Cibacron Blue 3GA increased the ATP activated cationic current whereas Basilen Blue E-3G had a very weak antagonistic effect (IC50 > or = 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 alpha, beta-methylene ATP (an ATP analogue that is resistant to nucleotidases). 5. In conclusion, the myenteric P2x-purinoceptor shares some properties with other purinoceptors in particular with the P2x4- 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.
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