AGONIST-STIMULATED CYCLIC ADP RIBOSE - ENDOGENOUS MODULATOR OF CA2-INDUCED CA2+ RELEASE IN INTESTINAL LONGITUDINAL MUSCLE()

We have previously shown that agonist-induced Ca2+ mobilization in int estinal longitudinal muscle is mediated by ryanodine-sensitive, inosit ol 1,4,5-trisphosphate-insensitive sarcoplasmic Ca2+ channels. Ca2+ re lease via these channels is triggered by agonist-stimulated Ca2+ influ x and results in Ca2+-induced Ca2+ release. The present study examined whether cyclic ADP-ribose (cADPR) is synthesized in response to stimu lation of longitudinal muscle by agonists and modulates the activity o f Ca2+ release channels. Cyclic ADPR bound with high affinity to dispe rsed longitudinal muscle cells (IC50 1.9 nM) and induced Ca2+ release (EC(50) 3.8 nM), increase in lCa(2+)](i) (EC(50) 2.0 nM), and contract ion (EC(50) 1.1 nM); cADPR had no effect on circular muscle cells. The effects of cADPR were blocked by ruthenium red, dantrolene, and the s pecific antagonist, 8-amino-cADPR, and were augmented by caffeine but not affected by heparin. The binding of cADPR and its ability to stimu late Ca2+ release were dependent on the concentration of Ca2+. Cyclic ADPR was capable of stimulating Ca2+ release at subthreshold Ca2+ conc entrations (25-100 nM) and of enhancing Ca2+-induced Ca2+ release. Lon gitudinal muscle extracts incubated with beta-NAD(+) produced a time-d ependent increase in Ca2+-mobilizing activity identified as authentic cADPR by blockade of Ca2+ release with 8-amino-cADPR and ruthenium red . Ca2+ mobilizing activity was increased by cholecystokinin octapeptid e (CCK-8) in a concentration-dependent fashion. The increase induced b y CCK-8 was suppressed by the CCK-A antagonist, L364,718, nifedipine, and guanyl-5'-yl thiophosphate. The study shows that ADP-ribosyl cycla se can be stimulated by agonists and that cADPR can act as an endogeno us modulator of Ca2+-induced Ca2+ release.