P2 purinoceptor‐activated inward currents in follicular oocytes of Xenopus laevis.

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Abstract
1. ATP, UTP and 2MeSATP (10 microM) evoked fast, transient (2‐20 s duration) and slow, sustained (60‐120 s duration) inward currents in 118 of 285 follicular Xenopus oocytes but failed to activate inward currents in defolliculated Xenopus oocytes (n = 136). 2. The potency order for nucleotides was (at 10 microM): UTP = 2MeSATP > 2ClATP = ATP > ADP > alpha, beta‐methylene‐ATP > adenosine > ITP, with beta, gamma‐methylene‐ATP, CTP and GTP inactive. This potency order is atypical for any single P2 purinoceptor subtype. 3. EC50 values (for fast and slow inward currents) were (microM): ATP, 5.6 +/‐ 2.1 and 6.7 +/‐ 3.0; UTP, 1.0 +/‐ 0.3 and 0.9 +/‐ 0.3; 2MeSATP, 0.6 +/‐ 0.2 and 9.2 +/‐ 7.4 (n = 3). Suramin inhibited fast (IC50: 65 +/‐ 20 microM; n = 7) and slow (IC50: 57 +/‐ 10 microM; n = 7) inward currents evoked by ATP. Suramin also inhibited slow inward currents produced by UTP but potentiated those caused by 2MeSATP. 4. Fast and slow inward currents evoked in response to ATP and UTP reversed at ‐12 +/‐ 3 mV (n = 3), a value close to Erev for Na(+)‐permeable ion channels present in oocytes. These same currents were reduced significantly when extracellular Na+ was substituted with choline and Erev changed to ‐30 +/‐ 5 mV (n = 3). Inward currents evoked by ATP and UTP were inhibited in a dose‐dependent manner by amiloride (10‐100 microM). 5. Fast and slow inward currents caused by 2MeSATP reversed at ‐29 +/‐ 3 mV (n = 3), a value close to Erev for Cl‐ channels in oocytes. Fast and slow responses to 2MeSATP were inhibited in a dose‐dependent manner by the Cl‐ channel blocker NPPB (10‐100 microM) and were only reduced slightly by amiloride (100 microM). 6. We conclude that two P2 purinoceptor subtypes are present on follicular Xenopus oocytes. 2MeSATP activates fast and slow Cl‐ currents (Erev = ‐30 mV) via a suramin‐insensitive P2Y purinoceptor, while UTP and ATP activate fast and slow Na(+)‐dependent currents (Erev = ‐10 mV) via a suramin‐sensitive P2U purinoceptor.