1. [Physiological role of cyclic ADP-ribose as a novel endogenous agonist of ryanodine receptor in adrenal chromaffin cells]
Toshihiro Dohi, Katsuya Morita, Shigeo Kitayama Nihon Yakurigaku Zasshi . 2002 Nov;120(1):96P-98P.
Cyclic ADP-ribose (cADPR) is suggested to be a novel messenger of ryanodine receptors (RyR) in various cellular systems. However, the regulation of its synthesis in response to cell stimulation and its functional roles are still unclear. We examined the physiological relevance of cADPR to the messenger role in stimulation-secretion coupling in bovine adrenal chromaffin cells. In digitonin-permeabilized cells, cADPR-induced Ca2+ release but not caffeine-, ryanodine-, and IP3-induced Ca2+ release was inhibited by FK506 which bind to FKBP and dissociate it from the RyR. These evidence suggesting that cADPR may be the ligand for FKBP-RyR complex, resulting in a dynamic regulation of RyR-mediated Ca2+ release. ACh causes biphasic [Ca2+]i rise, an initial transient rise followed by sustained rise, in intact cells. 8Br-cADPR, an antagonist of cADPR and FK506 specifically reduced the sustained phases of ACh-induced [Ca2+]i rise. Imperatoxin inhibitor (IpTxi) blocked the inhibitory effect of 8Br-cADPR and FK506, suggesting that cADPR contributes to sustained [Ca2+]i rise. 8Br-cADPR, FK506, and IpTxi reduced CA release in response to ACh in chromaffin cells. These results provide evidence that the synthesis of cADPR is regulated by cell stimulation, and the cADPR/Ca(2+)-induced Ca2+ release pathway forms a positive feedback to stimulation-induced secretory response in the chromaffin cells.
2. Cyclic ADP-ribose induces Ca2+ release from caffeine-insensitive Ca2+ pools in canine salivary gland cells
Y Imai, Y Akagawa, H Yamaki, K Itadani, K Morita, T Dohi, S Kitayama J Dent Res . 1998 Oct;77(10):1807-16. doi: 10.1177/00220345980770100801.
Cyclic ADP-ribose (cADPR), a novel putative messenger of the ryanodine receptor, was examined regarding its ability to mobilize Ca2+ from intracellular Ca2+ stores in isolated cells of parotid and submandibular glands of the dog. cADPR induced a rapid and transient Ca2+ release in the digitonin-permeabilized cells of salivary glands. cADPR-induced Ca2+ release was inhibited by ryanodine receptor antagonists ruthenium red, ryanodine, benzocaine, and imperatoxin inhibitor but not by the inositol 1,4,5-trisphosphate (IP3)-receptor antagonist heparin. Thapsigargin, at a concentration of 3 to 30 microM, inhibited IP3-induced Ca2+ release, while higher concentrations were required to inhibit cADPR-induced Ca2+ release. Cross-potentiation was observed between cADPR and ryanodine or SrCl2, suggesting that cADPR sensitizes the Ca2+-induced Ca2+ release mechanism. Cyclic AMP plays a stimulatory role on cADPR- and IP3-induced Ca2+ release in digitonin-permeabilized cells. Calmodulin also potentiated cADPR-induced Ca2+ release, but inhibited IP3-induced Ca2+ release. Acetylcholine and ryanodine caused the rise in intracellular free Ca2+ concentration ([Ca2+]i) in intact submandibular and parotid cells. Caffeine did not produce any increase in Ca2+ release or [Ca2+]i rise in any preparation. ADP-ribosyl cyclase activity was found in the centrifuged particulate fractions of the salivary glands. These results suggest that cADPR serves as an endogenous modulator of Ca2+ release from Ca2+ pools through a caffeine-insensitive ryanodine receptor channel, which are different from IP3-sensitive pools in canine salivary gland cells. This system is positively regulated by cyclic AMP and calmodulin.
3. Scorpion toxins targeted against the sarcoplasmic reticulum Ca(2+)-release channel of skeletal and cardiac muscle
H H Valdivia, R Coronado, M S Kirby, W J Lederer Proc Natl Acad Sci U S A . 1992 Dec 15;89(24):12185-9. doi: 10.1073/pnas.89.24.12185.
We report the purification of two peptides, called "imperatoxin inhibitor" and "imperatoxin activator," from the venom of the scorpion Pandinus imperator targeted against ryanodine receptor Ca(2+)-release channels. Imperatoxin inhibitor has a M(r) of approximately 10,500, inhibits [3H]ryanodine binding to skeletal and cardiac sarcoplasmic reticulum with an ED50 of approximately 10 nM, and blocks openings of skeletal and cardiac Ca(2+)-release channels incorporated into planar bilayers. In whole-cell recordings of cardiac myocytes, imperatoxin inhibitor decreased twitch amplitude and intracellular Ca2+ transients, suggesting a selective blockade of Ca2+ release from the sarcoplasmic reticulum. Imperatoxin activator has a M(r) of approximately 8700, stimulates [3H]ryanodine binding in skeletal but not cardiac sarcoplasmic reticulum with an ED50 of approximately 6 nM, and activates skeletal but not cardiac Ca(2+)-release channels. These ligands may serve to selectively "turn on" or "turn off" ryanodine receptors in fragmented systems and whole cells.
