Mobilization of intracellular calcium by methacholine and inositol 1,4,5-trisphosphate in rat parotid acinar cells

Central role of IP3R2-mediated Ca2+ oscillation in self-renewal of liver cancer stem cells elucidated by high-signal ER sensor

Ca²⁺ oscillation is a system-level property of the cellular Ca²⁺-handling machinery and encodes diverse physiological and pathological signals. The present study tests the hypothesis that Ca²⁺ oscillations play a vital role in maintaining the stemness of liver cancer stem cells (CSCs), which are postulated to be responsible for cancer initiation and progression. We found that niche factor-stimulated Ca²⁺ oscillation is a signature feature of CSC-enriched Hep-12 cells and purified α2δ1⁺ CSC fractions from hepatocellular carcinoma cell lines. In Hep-12 cells, the Ca²⁺ oscillation frequency positively correlated with the self-renewal potential. Using a newly developed high signal, endoplasmic reticulum (ER) localized Ca²⁺ sensor GCaMP-ER2, we demonstrated CSC-distinctive oscillatory ER Ca²⁺ release controlled by the type 2 inositol 1,4,5-trisphosphate receptor (IP₃R2). Knockdown of IP₃R2 severely suppressed the self-renewal capacity of liver CSCs. We propose that targeting the IP₃R2-mediated Ca²⁺ oscillation in CSCs might afford a novel, physiologically inspired anti-tumor strategy for liver cancer.

Protection by nitric oxide of morphine-induced inhibition of rat submandibular gland function

The effects of morphine, l -arginine (nitric oxide precursor) and l -NAME (nitric oxide synthesis inhibitor ) and their concurrent therapy on rat submandibular secretory function were studied. Pure submandibular saliva was collected intraorally by micro polyethylene cannula from anaesthetized rats using pilocarpine as secretagogue. Single intraperitoneal injection of morphine (6 mg kg(-1)) to rats induced significant (P< 0.01) inhibition of salivary flow rate (28%), total protein (12%) and calcium concentrations (27%). Sodium output was increased (23%, P< 0.01). Single intraperitoneal administration of l -arginine (100 mg kg(-1)) and l -NAME (10 mg kg(-1)) affected salivary gland function. Saliva flow rate was reduced by l -NAME (23%, P< 0.01). The total protein concentration of saliva was increased by l -arginine (21%, P< 0.05) and decreased by l -NAME (19%, P< 0.01). Calcium concentration of saliva was increased by l -arginine (25%, P< 0.01) and reduced by l -NAME (21%, P< 0.01). In combination treatment, l -arginine prevented (P< 0.01) morphine-induced reduction of flow rate while l -NAME potentiated it (P< 0.01). The secretion of total protein and calcium were influenced in a similar trend by concurrent therapy. l -NAME potentiated morphine-induced decrease of total protein and calcium concentrations (P< 0.01) while l -arginine restored (P< 0.01) them to levels close to control and morphine groups respectively. It is concluded that morphine inhibits salivary gland function and nitric oxide (NO) plays a positive role in this system. Also it is confirmed that morphine inhibitory effects on submandibular function are somewhat restored by l -arginine and expanded by l -NAME. The modulatory effect of the l -arginine/NO system on salivary gland function is suggested.

Alteration by cadmium of rat submandibular gland secretory function and the role of the l-arginine/nitric oxide pathway

The effects of cadmium, l-arginine (nitric oxide precursor) and N(omega)-nitro-l-arginine methyl ester (l -NAME) as a nitric oxide synthesis inhibitor and cotreatment of them on rat submandibular secretory function were studied. Pure submandibular saliva was collected intraorally by micro polyethylene cannula from anaesthetized rats using pilocarpine as secretagogue. Fourteen days treatment with 10 mg l(-1)cadmium as cadmium chloride in drinking water caused significant alterations on salivary function. Salivary flow rate, total protein concentration and amylase activity of saliva were decreased while secretion of calcium was increased by cadmium. Two weeks treatment of rats with l -arginine (2.25%) in drinking water caused an increase in submandibular gland weight. Flow rate was reduced by l-NAME. The total protein concentration of saliva was increased by l-arginine while decreased by l-NAME. Calcium concentration of saliva was reduced by l-arginine and increased by l-NAME. Cotreatment of cadmium with l-arginine prevented cadmium-induced reduction of flow rate while l-NAME cotreatment potentiated cadmium-induced reduction of flow rate. l-arginine showed a preventive effect on cadmium-induced decrease of protein concentration and reached control levels. l-arginine potentiated cadmium-induced increase of saliva calcium concentration. It is confirmed that nitric oxide (NO) has a role in salivary gland function. It is also concluded that cadmium inhibitory effects on salivary gland function are modulated by the NO system as it is observed that the cadmium inhibitory effect on submandibular gland function is diminished by l-arginine and extended by l-NAME. Considering the properties of cadmium substitution for calcium in many intracellular events, different types of alterations can be discussed.

Alteration of rat submandibulary gland secretion of protein, calcium and N-acetyl-beta-D-glucosaminidase activity by lead

1. Effects of various doses of long-term lead treatment (0.01%, 0.04% and 0.05%) on rat submandibular saliva were investigated in this study. 2. Both submandibular ducts were cannulated intraorally with polyethylene tubes and saliva was collected from anesthetized lead treated and control rats using pilocarpine as secretagogue. 3. Saliva protein concentration was found to be reduced in lead (0.04%)- and (0.05%)-treated groups. 4. Saliva calcium concentration had a significant reduction only in the lead (0.05%)-treated group. 5. The secretion of the lysosomal enzyme, N-acetyl-beta-D-glucosaminidase (NAG) in saliva decreased significantly in the lead (0.04%)- and (0.05)-treated groups. Specific activity of the enzyme showed an increase in these treated groups. 6. Good correlations were found between saliva protein concentration and NAG activity, saliva protein and calcium concentrations and calcium concentration and NAG activity. 7. There was a correlation between blood and submandibular saliva lead levels, and the saliva/blood ratio was approximately constant for all treated groups. 8. With respect to the ability of lead to substitute for calcium in several intracellular regulatory events, explanation for these alterations in submandibular saliva composition can be made.

Alteration by ouabain of rat submandibular glands function

1. Effects of various doses of intraperitoneal ouabain (1,2 and 5 mg/kg) on rat submandibular saliva were investigated in this study. 2. Potassium and calcium and their product (K+ x Ca2+) were found to be elevated in all groups. 3. Changes in salivary flow were not the major cause of the alterations in electrolytes. 4. Protein concentrations were elevated in the doses of 1 and 2 mg/kg of the drug and somewhat reduced in the dose of 5 mg/kg of ouabain but still above the base line. 5. The results show that there is an ouabain-induced close parallelism between magnesium and total protein secretion from rat submandibular glands.

On the relation of calcium channel blockers to rat parotid and submandibular glands function in vivo

1. The effects of nifedipine, verapamil and diltiazem on rat parotid and submandibular glands function were studied. 2. Nifedipine (5 mg/kg), verapamil (5 mg/kg) and diltiazem (10 mg/kg) were injected intraperitoneally 15 min before saliva collection. 3. Animals were anesthetized with 50 mg/kg of sodium pentobarbital and 8 mg/kg of pilocarpine was used as secretagogue. 4. Submandibular saliva was analyzed for flow rate, protein and calcium concentrations; and parotid saliva for calcium and amylase contents. 5. In treated groups, flow rate and calcium of submandibular saliva were significantly lower than controls. Parotid calcium in the nifedipine group was decreased and in verapamil and diltiazem groups was increased. Parotid amylase was significantly decreased in both the nifedipine and diltiazem groups. 6. It is concluded that a blockade of calcium channels in salivary glands acinar cells by CCBs causes some alterations in salivary secretions.

Effects of vinca alkaloids on rat parotid and submandibular glands function in vivo

1. Vincristine (1 mg/kg) and vinblastine (2 mg/kg) were injected intraperitoneally into the rats, 24 hr before the experiments. 2. Animals were anesthetized with 50 mg/kg of sodium pentobarbital and saliva was collected from vincristine-treated, vinblastine-treated and control animals using 8 mg/kg of pilocarpine as secretagogue. 3. Parotid saliva was analyzed for protein, amylase and Ca2+ content, and submandibular saliva for flow rate, protein and Ca2+ concentration. 4. Saliva from two treated groups was significantly lower (P < 0.01) in flow rate, amylase and protein content than that of control group. Calcium level was significantly increased (P < 0.05) in treated animals. 5. It is concluded that the antisecretory effects of vinca alkaloids may be consistent with their actions on salivary cell microtubules.

Interaction of amiloride with rat parotid muscarinic and alpha-adrenergic receptors

1. In rat parotid acini, amiloride inhibited the secretion of amylase and the efflux of calcium and rubidium in response to carbamylcholine and to norepinephrine. 2. Amiloride competitively inhibited the binding of [3H]N-methylscopolamine and [3H] is thus a competitive antagonist of muscarinic and norepinephrine alpha-adrenergic receptors. 3. Amiloride did not affect the response to substance P with respect to secretion or ion movements. 4. Thus the Na+/H+ antiporter is not involved in the short-term regulation of amylase secretion and calcium and potassium movements in rat parotid gland function.

Carbachol-induced phosphatidylcholine hydrolysis and choline efflux in rat submandibular gland involves phospholipase D activation and is modulated by protein kinase C and calcium

The role of calcium and protein kinase C activation in carbachol-induced choline efflux from submandibular glands was investigated. The participation of phospholipase D in this signal transduction pathway was demonstrated by the formation of [14C]phosphatidylethanol in [14C]lysophosphatidylcholine-labelled submandibular gland cells treated with carbachol or noradrenaline in the presence of ethanol. Chelation of the intracellular calcium with BAPTA/AM reduced the carbachol stimulated outflow of [3H]choline. The calcium ionophore A23187 in a high concentration (10 microM) increased the basal [3H]choline outflow, but decreased the carbachol-induced outflow. Removal of the extracellular calcium enhanced the carbachol-stimulated outflow, which returned to control when calcium was re-added to the medium. Activation of protein kinase C by phorbol-12,13-dibutyrate (100 nM) or 1-oleyl-2-acetyl-sn-glycerol (20 microM) was without effect per se, but enhanced the carbachol-mediated outflow of [3H]choline. Phorbol-12,13-dibutyrate in combination with 1 microM A23187 induced a small efflux of [3H]choline. A 2 h treatment with phorbol-12,13-dibutyrate (1 microM), causing down-regulation of protein kinase C, significantly decreased the carbachol-stimulated [3H]choline outflow. In conclusion, elevation of intracellular calcium levels and protein kinase C activation are of importance for the carbachol-stimulated outflow of [3H]choline. Inflow of calcium, if anything, reduces the carbachol-stimulated outflow of [3H]choline. Since phosphatidylcholine breakdown generates diacylglycerol and this could lead to activation of protein kinase C, activation of this signal transduction pathway may be important for the protein content of the saliva and for the known trophic effects of parasympathetic innervation.

Breakdown of membrane choline-phospholipids induced by endogenous and exogenous muscarinic agonist is potentiated by VIP in rat submandibular gland

The outflow of tritium from rat submandibular gland fragments, pre-labelled with [3H]choline, following electrical or pharmacological stimulation was studied. Electrical stimulation of the tissue increased the outflow of tritium in a frequency dependent manner. Atropine treatment decreased the electrically-induced release, indicating that the outflow did not reflect acetylcholine from nerve endings, but was largely brought about by postsynaptic receptors. In agreement with this hypothesis, treatment with noradrenaline or carbachol induced a dose dependent increase in tritium outflow from the gland fragments which could be blocked by prazosin or atropine, respectively. Moreover, analysis of the tissue-associated tritium revealed an incorporation primarily in the lipid fraction of the tissue (almost 80%), of which about 90% was in phosphatidylcholine, indicating that this was the source of the tritium outflow. Pre-incubation with vasoactive intestinal polypeptide (VIP), which coexists with acetylcholine in the parasympathetic neurons innervating the submandibular gland, increased the carbachol-induced tritium overflow significantly. The effect of VIP could be imitated by the adenylyl cyclase stimulator forskolin, which increased the carbachol-stimulated tritium efflux in a dose dependent manner. Taken together, our results suggests that muscarinic- and alpha 1-receptor agonists may activate a phospholipase coupled to phosphatidylcholine hydrolysis in the rat submandibular gland. Endogenous acetylcholine released from parasympathetic nerve endings appear to activate this mechanism. Furthermore, VIP treatment, and the concomitant cAMP-accumulation, potentiates the acetylcholine induced phosphatidylcholine hydrolysis, demonstrating a new type of interaction between the classical transmitter acetylcholine and the co-stored neuropeptide VIP.

Sphingosine increases inositol trisphosphate in rat parotid acinar cells by a mechanism that is independent of protein kinase C but dependent on extracellular calcium

In rat parotid acinar cells prelabelled with [3H]-inositol, sphingosine stimulated the accumulation of [3H]-inositol polyphosphates. When the cells were exposed to sphingosine, [3H]-inositol trisphosphate (InsP3) was accumulated in a time- and dose-dependent manner. When the extracellular Ca2+ was chelated by 1 mM EGTA, the effect of sphingosine on InsP3 accumulation was completely inhibited. Ionophores, A23187 and ionomycin, had no significant effect on InsP3 accumulation. An inhibitor of protein kinase C, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), failed to stimulate InsP3 accumulation. In the homogenate of parotid acinar cells, InsP3 3-kinase and 5-phosphomonoesterase activities were not affected by sphingosine. These results suggest that sphingosine activates phosphoinositide turnover by a mechanism dependent upon extracellular Ca2+, but different from that of an ionophore, and independent of protein kinase C.

Attenuation of inositol trisphosphate generation and cytosolic Ca2+ elevation in dispersed rat parotid acini stimulated simultaneously at muscarinic and alpha 1-adrenergic receptors

We have examined intracellular signalling events, peak cytosolic [Ca2+] and inositol trisphosphate levels, in rat parotid acini simultaneously stimulated with two Ca2+ mobilizing agonists, carbachol (muscarinic-cholinergic) and epinephrine (alpha 1-adrenergic). When the agonists were added together, either at sub-maximal (200 nM each, i.e. 400 nM total agonist concentration) or maximal (10 uM each, i.e. 20 uM total) stimulatory concentrations, the resulting elevations in both cytosolic [Ca2+] and inositol trisphosphate levels were not greater than those achieved when each agonist was added individually. However, with 400 nM carbachol these responses were significantly greater than those seen with either 200 nM carbachol or 200 nM carbachol + 200 nM epinephrine. The data indicate that when muscarinic and alpha 1-adrenergic receptors of rat parotid acini are simultaneously stimulated a novel regulatory mechanism is induced, which attenuates inositol trisphosphate generation and, consequently, intracellular Ca2+ release.

Activation of salivary secretion: coupling of cell volume and [Ca2+]i in single cells

High-resolution differential interference contrast microscopy and digital imaging of the fluorescent calcium indicator dye fura-2 were performed simultaneously in single rat salivary gland acinar cells to examine the effects of muscarinic stimulation on cell volume and cytoplasmic calcium concentration ([Ca2+]i). Agonist stimulation of fluid secretion is initially associated with a rapid tenfold increase in [Ca2+]i as well as a substantial cell shrinkage. Subsequent changes of cell volume in the continued presence of agonist are tightly coupled to dynamic levels of [Ca2+]i, even during [Ca2+]i oscillations. Experiments with Ca2+ chelators and ionophores showed that physiological elevations of [Ca2+]i are necessary and sufficient to cause changes in cell volume. The relation between [Ca2+]i and cell volume suggests that the latter reflects the secretory state of the acinar cell. Agonist-induced changes in [Ca2+]i, by modulating specific ion permeabilities, result in solute movement into or out of the cell. The resultant cell volume changes may be important in modulating salivary secretion.

Alterations of intracellular calcium in human non-pigmented ciliary epithelial cells of the eye

The ciliary epithelium of the eye is thought to be responsible for the active production of aqueous humor. The role of hormones and neuropeptides in mediating this process is unclear. Alterations in intracellular calcium in response to several hormones were assayed by fluorometric detection utilizing Quin2 in human SV-40 transformed non-pigmented ciliary epithelial cells grown in monolayer culture. A dose-dependent increase in intracellular calcium was found for the following drugs, which are given with their respective EC50 values: carbachol (15.7 +/- 4 microM), ATP (1.67 +/- 0.4 microM), arginine vasopressin (52 +/- 14 nM), bradykinin (2.4 +/- 0.7 nM), histamine (0.7 +/- 0.1 microM), and angiotensin II (6.4 +/- 1 nM). The following increases in calcium levels above typical resting levels of 45.9 +/- 4.6 nM were observed: 30% (0.1 microM angiotensin II), 50% (0.1 microM bradykinin, 2 microM arginine vasopressin), 100% (10 microM histamine), and 150% (1 mM carbachol, 10 microM ATP). Dopamine, KCl, phorbol esters, propranolol, epinephrine, and norepinephrine failed to increase intracellular calcium levels. The mobilization of intracellular calcium was unaffected by changes in the pH of the extracellular medium (over the pH range of 7.6 to 6.9) induced by glacial acetic, sulfuric or hydrochloric acids. Phosphatidic acid, however, did cause an elevation in intracellular calcium and is consistent with its putative role as an ionophore in other non-excitable exocrine tissues. These studies suggest a role for the hormonally induced mobilization of intracellular calcium which may underlie the secretion of aqueous humor by these cells.

Physiological localization of an agonist-sensitive pool of Ca2+ in parotid acinar cells

Muscarinic stimulation of fluid secretion by mammalian salivary acinar cells is associated with a rise in the level of intracellular free calcium ([Ca2+]i) and activation of a calcium-sensitive potassium (K+) conductance in the basolateral membrane. To test in the intact cell whether the rise of [Ca2+]i precedes activation of the K+ conductance (as expected if Ca2+ is the intracellular messenger mediating this response), [Ca2+]i and membrane voltage were measured simultaneously in carbachol-stimulated rat parotid acinar cells by using fura-2 and an intracellular microelectrode. Unexpectedly, the cells hyperpolarize (indicating activation of the K+ conductance) before fura-2 detectable [Ca2+]i begins to rise. This occurs even in Ca2+-depleted medium where intracellular stores are the only source of mobilized Ca2+. Nevertheless, when the increase in [Ca2+]i was eliminated by loading cells with the Ca2+ chelator bis(2-amino-5-methylphenoxy)ethane-N,N,N',N'-tetraacetate (Me2BAPTA) and stimulating in Ca2+-depleted medium, membrane hyperpolarization was also eliminated, indicating that a rise of [Ca2+] is required for the agonist-induced voltage response. Stimulation of Me2BAPTA-loaded cells in Ca2+-containing medium dramatically accentuates the temporal dissociation between the activation of the K+ conductance and the rise of [Ca2+]i. The data are consistent with the hypothesis that muscarinic stimulation results in a rapid localized increase in [Ca2+]i at the acinar basolateral membrane followed by a somewhat delayed increase in total [Ca2+]i. The localized increase cannot be detected by fura-2 but is sufficient to open the Ca2+-sensitive K+ channels located in the basolateral membrane. We concluded that a receptor-mobilized intracellular store of Ca2+ is localized at or near the basolateral membrane.