Submitted on March 13, 2006
Accepted on May 15, 2006
NUCLEOTIDE REGULATION OF THE VOLTAGE-DEPENDENT NON-SELECTIVE CATION CONDUCTANCE IN MURINE COLONIC MYOCYTES
Kevin P Monaghan1, Sang Don Koh1, Seungil Ro2, Jonghun Yeom2, Burton Horowitz2, and Kenton M. Sanders3*
1 Physiology and Cell Biology, University of Nevada School of Medicine, Reno, Nevada, United States
2 Reno, Nevada, United States; Physiology and Cell Biology, University of Nevada School of Medicine, Reno, Nevada, United States
3 Dept. of Physiology & Cell Biology/352, University of Nevada School of Medicine, Reno, Nevada, United States
* To whom correspondence should be addressed. E-mail: kent{at}unr.edu.
ATP is proposed as an inhibitory neurotransmitter in the gastrointestinal (GI) tract, causing hyperpolarization and smooth muscle relaxation. ATP activates small conductance Ca2+-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, non-selective cation current (IVNSCC) and a relatively high-threshold voltage-activated (L-type) Ca2+ current (IL). Here we have investigated the effects of ATP on these currents. External application of ATP (1 mM) did not affect IVNSCC or IL in dialyzed cells. ATP (1 mM) increased IVNSCC and decreased IL in the perforated whole-cell configuration. UTP and UDP (1 mM) were more potent than ATP on IVNSCC. ADP decreased IL but had no effect on IVNSCC. The order of effectiveness was UTP=UDP>ATP>ADP. These effects were not blocked by PPADS, but the phospholipase C inhibitor, U-73122, reversed the effects of ATP on IVNSCC. ATP stimulation of IVNSCC was also reversed by protein kinase C (PKC) inhibitors chelerythrine chloride or bisindolylmaleimide I (BIM). Phorbol 12, 13-dibutyrate (PDBu) mimicked the effects of ATP. RT-PCR showed that P2Y4 is expressed by murine colonic myocytes, and this receptor is relatively insensitive to PPADS. Our data suggest that ATP activates IVNSCC and depresses IL via binding of P2Y4 receptors and stimulation of the phospholipase C/ protein kinase C pathway.
