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ATP is proposed to be a major inhibitory neurotransmitter in the gastrointestinal (GI) tract, causing hyperpolarization and smooth muscle relaxation. ATP activates small-conductance Ca(2+)-activated K(+) channels that are involved in setting the resting membrane potential and causing inhibitory junction potentials. No reports are available examining the effects of ATP on voltage-dependent inward currents in GI smooth muscle cells. We previously reported two types of voltage-dependent inward currents in murine proximal colonic myocytes: a low-threshold voltage-activated, nonselective cation current (I(VNSCC)) and a relatively high-threshold voltage-activated (L-type) Ca(2+) current (I(L)). Here we have investigated the effects of ATP on these currents. External application of ATP (1 mM) did not affect I(VNSCC) or I(L) in dialyzed cells. ATP (1 mM) increased I(VNSCC) and decreased I(L) in the perforated whole-cell configuration. UTP and UDP (1 mM) were more potent than ATP on I(VNSCC). ADP decreased I(L) but had no effect on I(VNSCC). The order of effectiveness was UTP = UDP > ATP > ADP. These effects were not blocked by pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS), but the phospholipase C inhibitor U-73122 reversed the effects of ATP on I(VNSCC). ATP stimulation of I(VNSCC) was also reversed by protein kinase C (PKC) inhibitors chelerythrine chloride or bisindolylmaleimide I. Phorbol 12,13-dibutyrate mimicked the effects of ATP. RT-PCR showed that P2Y(4) is expressed by murine colonic myocytes, and this receptor is relatively insensitive to PPADS. Our data suggest that ATP activates I(VNSCC) and depresses I(L) via binding of P2Y(4) receptors and stimulation of the phospholipase C/PKC pathway.
Monaghan et al. (Thu,) studied this question.