Role of extracellular Ca2+ in acetylcholine-induced repetitive Ca2+ release in submandibular gland acinar cells of the rat

The 2-aminoethoxydiphenyl borate analog, DPB161 blocks store-operated Ca2+ entry in acutely dissociated rat submandibular cells

Cellular Ca²⁺ signals play a critical role in cell physiology and pathology. In most non-excitable cells, store-operated Ca²⁺ entry (SOCE) is an important mechanism by which intracellular Ca²⁺ signaling is regulated. However, few drugs can selectively modulate SOCE. 2-Aminoethoxydiphenyl borate (2APB) and its analogs (DPB162 and DPB163) have been reported to inhibit SOCE. Here, we examined the effects of another 2-APB analog, DPB161 on SOCE in acutely-isolated rat submandibular cells. Both patch-clamp recordings and Ca²⁺ imaging showed that upon removal of extracellular Ca²⁺ ([Ca²⁺]_o=0), rat submandibular cells were unable to maintain ACh-induced Ca²⁺ oscillations, but restoration of [Ca²⁺]_o to refill Ca²⁺ stores enable recovery of these Ca²⁺ oscillations. However, addition of 50 μM DPB161 with [Ca²⁺]_o to extracellular solution prevented the refilling of Ca²⁺ store. Fura-2 Ca²⁺ imaging showed that DPB161 inhibited SOCE in a concentration-dependent manner. After depleting Ca²⁺ stores by thapsigargin treatment, bath perfusion of 1 mM Ca²⁺ induced [Ca²⁺]_i elevation in a manner that was prevented by DPB161. Collectively, these results show that the 2-APB analog DPB161 blocks SOCE in rat submandibular cells, suggesting that this compound can be developed as a pharmacological tool for the study of SOCE function and as a new therapeutic agent for treating SOCE-associated disorders.

M(3) muscarinic acetylcholine receptor plays a critical role in parasympathetic control of salivation in mice

The M(1) and M(3) subtypes are the major muscarinic acetylcholine receptors in the salivary gland and M(3) is reported to be more abundant. However, despite initial reports of salivation abnormalities in M(3)-knockout (M(3)KO) mice, it is still unclear which subtype is functionally relevant in physiological salivation. In the present study, salivary secretory function was examined using mice lacking specific subtype(s) of muscarinic receptor. The carbachol-induced [Ca(2+)](i) increase was markedly impaired in submandibular gland cells from M(3)KO mice and completely absent in those from M(1)/M(3)KO mice. This demonstrates that M(3) and M(1) play major and minor roles, respectively, in the cholinergically induced [Ca(2+)](i) increase. Two-dimensional Ca(2+)-imaging analysis revealed the patchy distribution of M(1) in submandibular gland acini, in contrast to the ubiquitous distribution of M(3). In vivo administration of a high dose of pilocarpine (10 mg kg(-1), s.c.) to M(3)KO mice caused salivation comparable to that in wild-type mice, while no salivation was induced in M(1)/M(3)KO mice, indicating that salivation in M(3)KO mice is caused by an M(1)-mediated [Ca(2+)](i) increase. In contrast, a lower dose of pilocarpine (1 mg kg(-1), s.c.) failed to induce salivation in M(3)KO mice, but induced abundant salivation in wild-type mice, indicating that M(3)-mediated salivation has a lower threshold than M(1)-mediated salivation. In addition, M(3)KO mice, but not M(1)KO mice, had difficulty in eating dry food, as shown by frequent drinking during feeding, suggesting that salivation during eating is mediated by M(3) and that M(1) plays no practical role in it. These results show that the M(3) subtype is essential for parasympathetic control of salivation and a reasonable target for the drug treatment and gene therapy of xerostomia, including Sjögren's syndrome.

Parasympathetic modulation of local acute inflammation in murine submandibular glands

The parasympathetic nervous system controls submandibular glands (SMG) functions in physiological and pathological conditions via muscarinic acetylcholine receptors (mAchR). We had previously demonstrated that IFNgamma and carbachol stimulate amylase secretion in normal murine SMG by mAchR activation. While the cytokine action depended on nitric oxide synthase activation, the effect of the agonist was mediated by prostaglandin E2 (PGE2) production. Both IFNgamma and carbachol triggered IFNgamma secretion in SMG. We here show that during local acute inflammation (LAI) induced by intraglandular injection of bacterial endotoxin, lypopolisaccharide (LPS), amylase secretion is decreased in comparison to control glands. We also observed that the muscarinic agonist carbachol stimulates in a dose-dependent manner amylase activity by M2 and M3 mAchR activation. Moreover, cyclooxygenase-2 (COX-2) activation and subsequent PGE2 liberation, in a nitric oxide independent manner, seem to be involved in M3 and M2 receptor activation by carbachol. In contrast, the addition of exogenous IFNgamma or carbachol inhibits the cytokine liberation in LAI glands.

Parasympathetic modulation of amylase secretion by IFN gamma in murine submandibular glands

IFN gamma is a pleiotropic cytokine that exerts immunologic and non-immunologic functions. We show here that at low doses (10 U/ml), it stimulates amylase secretion in murine submandibular glands (SMG) "via" muscarinic receptor activation, comparable to that produced by the muscarinic agonist carbachol. Both effects are blocked by atropine. NG-monomethyl-L-arginine (L-NMMA) and EGTA inhibited the cytokine effect on amylase secretion, involving the participation of a calcium-dependent isoform of nitric oxide synthase (NOS). We confirm NOS activation because IFN gamma stimulates nitrite production and enzyme activity in SMG. Carbachol (10(-7) M) did not modify basal nitric oxide production. In addition, both IFN gamma and carbachol increase prostaglandin E2 production in SMG, but while indomethacin potentiates IFN gamma effect on amylase secretion, it blunted amylase secretion exerted by carbachol. Thus, IFN gamma and carbachol stimulate IFN gamma secretion on SMG in a dose-dependent manner. Our results are pointing to neuroregulatory functions of IFN gamma in murine SMG, because it regulates its own levels in oral cavity, perhaps to exert a local immuno-surveillance.

Kinetics of Ca2+ release evoked by photolysis of caged InsP3 in rat submandibular cells

Quantitative time-resolved measurements of cytosolic Ca2+ release by photolysis of caged InsP3 have been made in single rat submandibular cells using patch clamp whole-cell recording to measure the Ca2+-activated Cl- and K+ currents. Photolytic release of InsP3 from caged InsP3 at 100 Joules caused transient inward (V(H) = 60 mV) and outward (V(H) = 0 mV) currents, which were nearly symmetric in their time course. The inward current was reduced when pipette Cl- concentration was decreased, and the outward current was suppressed by K+ channel blockers, indicating that they were carried by Cl- and K+, respectively. Intracellular pre-loading of the InsP3 receptor antagonist heparin or the Ca2+ chelator EGTA clearly prevented both inward and outward currents, indicating that activation of Ca2+-dependent Cl- and K+ currents underlies the inward and the outward currents. At low flash intensities, InsP3 caused Ca2+ release which normally activated the K+ and Cl- currents in a mono-transient manner. At higher intensities, however, InsP3 induced an additional delayed outward K+ current (I[K,(delay)]). I[K(delay)] was independent of the initial K+ current, independent of extracellular Ca2+, inhibited by TEA, and gradually prolongated by repeated flashes. The photolytic release of Ca2+ from caged Ca2+ did not mimic the I[K(delay)]. It is suggested that Ca2+ releases from the InsP3-sensitive pools in an InsP3 concentration-dependent manner. Low concentrations of InsP3 induce the transient Ca2+-dependent Cl- and K+ currents, which reflects the local Ca2+ release, whereas high concentrations of InsP3 induce a delayed Ca2+-dependent K+ current, which may reflect the Ca2+ wave propagation.