Sr2+ can pass through Ca2+ entry pathway activated by Ca2+ depletion, but can be hardly taken up by the Ca2+ stores in the rat salivary acinar cells

Identification of cyclic ADP-ribose-dependent mechanisms in pancreatic muscarinic Ca(2+) signaling using CD38 knockout mice

We showed that muscarinic acetylcholine (ACh)-stimulation increased the cellular content of cADPR in the pancreatic acinar cells from normal mice but not in those from CD38 knockout mice. By monitoring ACh-evoked increases in the cytosolic Ca(2+) concentration ([Ca(2+)](i)) using fura-2 microfluorimetry, we distinguished and characterized the Ca(2+) release mechanisms responsive to cADPR. The Ca(2+) response from the cells of the knockout mice (KO cells) lacked two components of the muscarinic Ca(2+) release present in wild mice. The first component inducible by the low concentration of ACh contributed to regenerative Ca(2+) spikes. This component was abolished by ryanodine treatment in the normal cells and was severely impaired in KO cells, indicating that the low ACh-induced regenerative spike responses were caused by cADPR-dependent Ca(2+) release from a pool regulated by a class of ryanodine receptors. The second component inducible by the high concentration of ACh was involved in the phasic Ca(2+) response, and it was not abolished by ryanodine treatment. Overall, we conclude that muscarinic Ca(2+) signaling in pancreatic acinar cells involves a CD38-dependent pathway responsible for two cADPR-dependent Ca(2+) release mechanisms in which the one sensitive to ryanodine plays a crucial role for the generation of repetitive Ca(2+) spikes.

Heterologous desensitization of muscarinic receptors by P2Z purinoceptors in rat parotid acinar cells

We studied the heterologous desensitization of muscarinic receptors by ATP in fura-2-loaded rat parotid acinar cells. Exposure to ATP or 3'-o-(4-benzoyl) benzoyl-ATP shortened the duration and decreased the magnitude of acetylcholine-induced Ca2+ release from intracellular Ca2+ stores in a dose-dependent manner. The shortening was observed only in an early stage of desensitization (within 20 s), whereas the decrease in the magnitude of the response was dependent upon the time the cells were exposed to the nucleotides. Atropine induced a profound shortening during the progressive decrease in the magnitude of acetylcholine-induced Ca2+ release. 3'-o-(4-Benzoyl) benzoyl-ATP did not induce an increase in the cytosolic Ca2+ concentration when the cells were incubated in the Ca2+- and Na+-free medium, but it did induce a strong desensitization of muscarinic receptors. The specific protein kinase C inhibitor bisindoylmaleimide resensitized the 3'-o-(4-benzoyl) benzoyl-ATP-treated muscarinic receptors. Phorbol 12-myristate 13-acetate potentiated the desensitization of muscarinic receptors. Ceramides that prevent the activation of phospholipase D resensitized the 3'-o-(4-benzoyl) benzoyl-ATP-treated muscarinic receptors. These results suggest that ATP, acting through P2Z purinoceptor-mediated phospholipase D, may produce a Ca2+-independent protein kinase C. Heterologous desensitization of muscarinic receptors by protein kinase C may shorten the duration and decrease the magnitude of acetylcholine-induced Ca2+ release.

Cyclic AMP induces ryanodine-sensitive Ca2+ release from microsomal vesicles of rat parotid acinar cells

The effect of cAMP on a ryanodine-sensitive Ca2+ release from microsomal vesicles of rat parotid acinar cells was studied. After a steady state of ATP-dependent 45Ca2+ uptake into the vesicles, cAMP was added to the medium with thapsigargin (TG) to block a reuptake of 45Ca2+. The addition of cAMP (1.0 mM) with TG released about 10% of the 45Ca2+ that had been taken up. The cAMP-induced 45Ca2+ release was strongly inhibited by pretreatment of the vesicles with 500 microMM ryanodine. Preincubation with cAMP (1 mM) abolished ryanodine (10 microM)-induced 45Ca2+ release. The presence of a specific inhibitor of cAMP-dependent protein kinase (PKA) H-89 (10 microMM) inhibited the cAMP-induced 45Ca2+ release. These results indicate that in rat parotid acinar cells, cAMP can activate a ryanodine-sensitive Ca2+ release mechanism in the endoplasmic reticulum and that this activation is via a PKA-dependent process.

Na+-dependent release of intracellular Ca2+ induced by purinoceptors in parotid acinar cells of the rat

In rat parotid acinar cells, ATP caused a transient increase in the intracellular Ca2+ concentration ([Ca2+]i) in the absence of external Ca2+. The ATP-induced Ca2+ response was strongly suppressed by removal of external Na+. The sequence of potency in increasing [Ca2+]i was 3'-o-(4-benzoyl) benzoyl-ATP > ATP > uridine 5'-triphosphate (UTP). Adenosine, AMP, ADP or alpha,beta-metylene ATP did not cause an increase in [Ca2+]i. The 3'-o-(4-benzoyl) benzoyl-ATP-induced increase in [Ca2+]i was abolished by removal of external Na+, but the UTP-induced response was not. The threshold external Na+ concentration required for ATP- or 3'-o-(4-benzoyl) benzoyl-ATP-induced Ca2+ release was 10-20 mM. ATP but not UTP caused a rise in the intracellular Na+ concentration ([Na+]i). Ca2+ release stimulated by caffeine or treatment with ryanodine reduced the Ca2+ release evoked by ATP. These results suggest that ATP, acting through P2Z purinoceptors, causes Na+ entry by opening cation-permeable channels, and thereafter the increase in [Na+]i triggers Ca2+ release from ryanodine-sensitive stores. UTP, acting through P2U purinoceptors, causes Ca2+ release independent of external Na+.

Depletion of ryanodine-sensitive Ca2+ store activates Ca2+ entry in rat submandibular gland acinar cells

The existence of ryanodine-sensitive Ca2+ stores and their role in the Ca2+ entry mechanism were examined in the rat submandibular gland acinar cells, using the microfluorimetry of intracellular Ca2+ concentration ([Ca2+]i). In the presence of thapsigargin, a Ca(2+)-ATPase inhibitor of inositol (1, 4, 5) triphosphate (InsP3)-sensitive Ca2+ stores, caffeine caused an increase in [Ca2+]i, which was inhibited by treatment with ryanodine (a ligand to the Ca(2+)-induced Ca2+ release channels). In the cells treated with ryanodine, 1 mM Ca2+ addition to a Ca(2+)-free solution caused a marked increase in [Ca2+]i, which was eliminated by application of Ni2+ or SK & F 96365, suggesting a Ca2+ entry triggered by ryanodine. The maximal change in the net increase in [Ca2+]i caused by the ryanodine-coupled Ca2+ entry, was 104.0 +/- 16.0 nM, which intense was caused by 10 microM ryanodine. Emptying the InsP3-sensitive stores by treatment with thapsigargin also caused Ca2+ entry, which maximally changed [Ca2+]i by 349.6 +/- 15.1 nM. Ten mumol/liter ryanodine was confirmed to cause a release of 45Ca2+ from the parotidic microsomal fraction enriched in endopalsmic reticulum. We propose that ryanodine-sensitive Ca2+ stores are present in rat submandibular gland acinar cells. We further propose that release of Ca2+ from the ryanodine-sensitive stores, which means eventually depletion of the ryanodine-sensitive Ca2+ stores, can activate the Ca2+ entry. The ability for Ca2+ entry coupled with the ryanodine-sensitive Ca2+ stores seems to be about 30% of the ability for Ca2+ entry coupled with the thapsigargin-sensitive Ca2+ stores.