Purinergic modulation of pacemaker Ca2+ activity in interstitial cells of Cajal

Purinoceptors are widely distributed throughout the body, and are thought to have important contributions to numerous functions. In this study, we characterised the contribution of purinoceptors to the mechanisms underlying spontaneous rhythmicity of the gastro-intestinal tracts. Using cell cluster preparations (100-200 μm diameter) obtained from murine ileum, we measured spontaneous intracellular Ca2+([Ca2+]i) oscillations in the presence of nifedipine, as an index of pacemaker [Ca2+]i activity in interstitial cells of Cajal (ICCs, c-Kit-immunopositive cells), the pacemaker cells for gastrointestinal motility. This small preparation also contained smooth muscle and enteric neurones. Using various purinoceptor agonists and an antagonist, we characterised both TTX-sensitive and insensitive modulations of pacemaker [Ca2+]i activity in ICCs. Continuous application of either ATP, ATPγS, suramin or α,β-methylene ATP (α,β-meATP) suppressed pacemaker [Ca2+]i activity. The inhibitory effect of α,β-meATP was completely abolished by a prior application of TTX. On the other hand, even in the presence of TTX, continuous application of 2-methylthio ATP (2-MeSATP) at concentrations greater than 30 μM caused a prompt rise followed by a slow decline of the baseline [Ca2+]i, and pacemaker [Ca2+]i oscillations were gradually suppressed during the decline. Neither UTP nor α,β-meATP at high concentrations (30-100 μM) produced a similar [Ca2+]i response. These results suggest that the TTX-resistant, direct purinergic modulation of pacemaker [Ca2+]i activity in ICCs is mediated via P2X purinoceptors distinct from those involved in TTX-sensitive modulation. The slow decline may be attributed to desensitisation of these purinoceptors. The possible involvement of other purinoceptors is also discussed.