Participation of ATP in Nonadrenergic, Noncholinergic Relaxation of Longitudinal Muscle of Wistar Rat Jejunum

Abstract

Abstract. A role of ATP in nonadrenergic, noncholinergic (NANC) relaxations was examined in the Wistar rat jejunum. Electrical field stimulation (EFS) induced NANC relaxation of longitudinal muscle of the jejunal segments in a frequency-dependent manner. A purinoceptor antagonist, adenosine 3'-phosphate 5'-phosphosulfate (A3P5PS, 100 mM) inhibited the relaxation: relaxations induced by EFS at lower or higher frequencies were either completely or partially inhibited, respectively. After the jejunal segments had been desensitized to ATP, the relaxations were decreased to the same extent as those inhibited by A3P5PS. An inhibitor of small conductance Ca 2+-activated K + channels (SK channels), apamin (100 nM), completely inhibited EFS-induced relaxations. Treatment of the segments with an inhibitor of sarcoplasmic reticulum Ca 2+ -ATPase, thapsigargin (1 mM), significantly inhibited the relaxations. The exogenous ATPinduced relaxation of longitudinal muscle occurred with a concomitant decrease in intracellular Ca 2+ levels. Apamin and thapsigargin abolished these ATP-induced responses. A3P5PS significantly inhibited the inhibitory junction potentials which were induced in the longitudinal muscle cells. In addition, apamin significantly inhibited the hyperpolarization that was induced by exogenous ATP in the cells. These findings in the Wistar rat jejunum suggest that ATP participates in the NANC relaxation via activation of SK channels induced by Ca 2+ ions that are released from the thapsigargin-sensitive store site.

P2Y receptor-mediated transient relaxation of rat longitudinal ileum preparations involves phospholipase C activation, intracellular Ca2+ release and SK channel activation

“…These results indicate that activation of PLC and subsequent IP 3 -dependent Ca 2+ release from internal stores were important steps in the intracellular pathway leading to purinergic relaxation, and these steps are upstream of SK channel activation. In agreement with previous reports [4,9,19,20] , we propose that a major pathway leading to purinergic relaxation involves P2Y 1 receptor and PLC activation, followed by IP 3 -mediated Ca 2+ release from internal stores and subsequent SK channel activation.…”

“…Similarly to atropine and L-NAME, the relaxation induced by α,β-Me-ATP (-11.4±1.5 mN, n=9) or by 2-Me-S-ADP (-7.0±1.4 mN, n=10, Table 1) was not altered by TTX. In contrast, as previously published [4,9,19,20] , SK channel inhibition by apamin (0.5 µmol/L) or UCL1684 (10 µmol/L) abolished the relaxation induced by α,β-Me-ATP and by 2-Me-S-ADP (see asterisks in Figure 2Aa and 2Ab). Thus, P2 receptor-mediated relaxation was entirely due to SK channel-activation (0±0 mN for all, Figure 2B), while the contraction following purinergic stimulation did not involve these channels ( Figure 2C).…”

“…However, there is an increasing interest in non-adrenergic non-cholinergic (NANC) neurotransmission, and the enteric nervous system (ENC) has been acknowledged to play a major role in governing smooth muscle motility [1] . Generally, nitric oxide (NO) and adenosine 5' triphosphate (ATP) are thought to be key molecules in inhibitory NANC signaling, giving rise to hyperpolarization and relaxation of GI smooth muscle [2][3][4] . While NO leads to a long-lasting inhibition of smooth muscle motility, the available data on purinergic responses are less consistent, as both excitatory and inhibitory effects have been observed and seem to depend on the subtype and cellular location of P2 purinoceptors.…”

“…While NO leads to a long-lasting inhibition of smooth muscle motility, the available data on purinergic responses are less consistent, as both excitatory and inhibitory effects have been observed and seem to depend on the subtype and cellular location of P2 purinoceptors. In addition, species differences should also be considered [2][3][4][5][6][7][8][9][10] . Seven cloned ionotropic P2X receptors that combine heteromerically, as well as eight cloned metabotropic P2Y receptors, account for the large heterogeneity of purinoceptormediated effects [11] .…”

“…Consistent with this differential expression, the ATP release and smooth muscle contraction from cholinergic fibers that are induced by the P2 agonist α,β-methylene ATP (α,β-Me-ATP) are mediated by pre-junctional P2Y and P2X receptor activation, respectively [8] . Post-junctional P2Y receptors, however, activate apamin-sensitive SK channels as a consequence of Ca 2+ release from thapsigargin-sensitive stores and thus promote relaxation [4,9,19,20] . Recently, an elegant study tested the hypothesis that spontaneous hydrolysis products of ATP, such as ADP and adenosine, mediate the biphasic effects observed following ATP administration and found that ADP and adenosine were agonists of P2Y and adenosine A 1 receptor, respectively, thereby having the opposite effects from ATP [6] .…”