Effect of an intracellular calcium antagonist (TMB-8) on carbamylcholine-induced amylase release from dispersed rat pancreatic acini

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.

Platelet-activating factor-mediated synthesis of prostaglandins in rat Kupffer cells

Synthesis of prostaglandins was stimulated in rat Kupffer cells upon challenge with platelet-activating factor (PAF). PAF-mediated synthesis of prostaglandins was inhibited by the Ca2+ ion chelator (EGTA), the Ca2+ channel antagonist (nifedipine) and U66985, a structural analogue and antagonist of the biological effects of PAF in other cellular systems. Inhibitors of protein kinase C, staurosporine and polymixin B, did not affect PAF-induced prostaglandin synthesis. Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, stimulated synthesis of prostaglandins in Kupffer cells; PAF and PMA exerted additive actions on this process. Both PAF- and PMA-stimulated prostaglandin production was inhibited by TMB-8. PAF-stimulated synthesis of prostaglandins also was inhibited upon treatment of Kupffer cells with pertussis toxin. Cholera toxin, in contrast, stimulated the production of prostaglandins in a concentration-dependent manner; cholera toxin and PAF together had an additive effect. These results suggest that PAF-induced synthesis of prostaglandins is stimulated via a specific receptor coupled to a pertussis toxin-sensitive G-protein, is dependent upon extracellular Ca2+ and is not influenced by protein Kinase C activation. Since PAF and prostaglandins are produced in the liver under conditions such as endotoxemia, PAF-mediated synthesis of these lipid autacoids may be of importance in the regulation of hepatic function during pathophysiological episodes.

Relationship between cytosolic Ca2+ concentration and amylase release in rat parotid acinar cells following muscarinic stimulation

Carbachol (CCh), a muscarinic-cholinergic agonist, increased both cytosolic free calcium concentration ([Ca2+]i) and amylase release in rat parotid acinar cells or acini in a dose-dependent manner. Treatment of acinar cells with the intracellular Ca2+ antagonist, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8), or the intracellular Ca2+ chelator, 1,2-bis(O-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid (BAPTA), strongly attenuated the increases in [Ca2+]i evoked by CCh, but amylase release from acini was not significantly suppressed by the treatment with TMB-8 or BAPTA. Low concentrations (0.02-0.5 microM) of ionomycin, a Ca2+ ionophore, caused increases in [Ca2+]i comparable to those induced by CCh, but the same concentrations had only a little effect on amylase release. The protein kinase C activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), stimulated amylase release in quantities similar to those induced by CCh, although TPA alone did not cause any change in [Ca2+]i. Combined addition of TPA and ionomycin potentiated only modestly amylase release stimulated by TPA alone. Staurosporine, a protein kinase C-inhibitor, similarly inhibited both the CCh- and TPA-induced amylase release. These results suggest that an increase in [Ca2+]i elicited by CCh does not play an essential role for inducing amylase release in rat parotid acini. Amylase release by muscarinic stimulation may be mediated mainly by activation of protein kinase C.

Differential responses of two identified neurons of the pond snail Lymnaea stagnalis to the convulsant drug pentylenetetrazol

The convulsant drug pentylenetetrazol (PTZ) induces tonic depolarization in the identified B1 neuron of Lymnaea stagnalis. Another identified neuron, right parietal dorsal 1 (RPD1), gives the opposite response: tonic hyperpolarization and cessation of spontaneous firing. These effects are mimicked by intracellular injection of calcium ions and reversed following injection of ethylene glycol-bis(beta-aminoethyl ether) N,N,N,'N'-tetraacetic acid (EGTA). Responses to PTZ are retained in the presence of extracellular cobalt ions to block calcium influx. Under voltage clamp, PTZ or injection of calcium ions induces a slow inward current in B1, which is abolished in zero sodium saline. In RPD1 the current response to PTZ or intracellular calcium ions is outward, and is blocked by potassium channel blockers. Thus, differential responses of the two neurons to PTZ appear to be mediated via increased intracellular calcium ions, leading to activation of specific ion currents in each cell type.

Ca2+ mobilization and Cl efflux in submandibular salivary cells of adult and newborn rats

These cells were isolated by enzymatic digestion from glands of 1-day-old and fully mature rats. The effects of exposure to acetylcholine on cytosolic Ca2+ and Cl efflux were studied by, respectively, spectrofluorimetry of fura-2 and the net efflux of the isotopic tracer 36Cl from preloaded cells. In both types of cells, when incubated in Ca2+-containing solutions, acetylcholine initially caused a rapid, significant increase in cytosolic Ca2+ (from approx. 90 to 480-570 nmol), followed by a slower decrease to a plateau value of 280-290 nmol. The initial peak persisted (315-339 nmol) in Ca2+-free solutions but the cytosolic Ca2+ concentrations decreased rapidly to levels below prestimulation values (30 nmol). 36Cl efflux in tracer preloaded cells incubated in Ca2+-containing medium in the presence of acetylcholine was 18% in cells of new born animals and 35% in adult cells. In Ca2+-free medium, mature cells showed a transient but significant (26%) efflux of 36Cl. Cells of 1-day-old rats did not show a net efflux of 36Cl under these conditions, but subsequent addition of Ca2+ caused a 15% reduction (i.e. efflux) in tracer content. The antagonist 3,4,5-trimethoxybenzoate-8(N,N-dithylamino)octyl ester (TMB 8), which blocks internal Ca2+ release, inhibited both the initial increase in cell Ca2+ in both types of cells and the transient efflux of 36Cl seen in mature cells when incubated in Ca2+-free solutions. At high concentrations (5 mM), LaCl3 inhibited efflux of 36Cl in mature cells but not in those of newborn rats. However, at lower concentrations (0.1 mM), which do not interfere with fluorescence spectra, LaCl3 did not inhibit the effect of acetylcholine on cell Ca2+. These results suggest that Cl efflux in adult submandibular cells is regulated by an increase in cell Ca2+ arising from release of internal Ca2+ and from influx of external Ca2+. Both of these responses are evident in cells of newborn animals but Cl efflux is either decreased or absent. This is likely to be associated with a deficiency in the Cl channels or in the coupling between Ca2+ and the channel substrate through regulatory molecules associated with phosphorylation of the channel protein.

The intracellular calcium antagonist, TMB-8, inhibits prolactin gene expression in GH3 cells

We examined the effects of the drug, TMB-8, which promotes sequestration of intracellular Ca2+, on the ability of extracellular Ca2+ to stimulate prolactin gene expression in GH3 cells. TMB-8 inhibited prolactin mRNA levels in a dose-dependent manner in the concentration range of 2.5-10 microM. Prolactin mRNA levels were increased about 18-fold by the addition of 0.1 mM CaCl2, and about 25-fold by the addition of 0.4 mM CaCl2. Addition of 10 microM TMB-8 reduced these levels to about 4-fold and 7-fold, respectively. At 10 microM TMB-8 did not effect total protein synthesis or the Ca2+-induced aggregation of the cells, indicating a selective inhibition by the drug of prolactin gene expression. Both TMB-8 and the calmodulin inhibitor, calmidazolium, reversed the effects of Ca2+ on prolactin mRNA levels in cells that had been pretreated for 2 days with 0.4 mM CaCl2.

Action of TMB-8 (8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate) on cytoplasmic free calcium in adrenal glomerulosa cell

To clarify the action of 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) on cellular calcium handling, changes in cytoplasmic free calcium concentration ([Ca2+]c) were studied in adrenal glomerulosa cell with a calcium-sensitive photoprotein, aequorin. Results of our previous study demonstrate that 100 microM TMB-8 almost completely blocks aldosterone response to angiotensin II (Biochem. J. 232 (1985) 87-92). At 50 or 100 microM, TMB-8 decreased basal [Ca2+]c significantly; however, these doses of TMB-8 had little effect on an angiotensin-induced increase in [Ca2+]c. When angiotensin-induced calcium release from an intracellular pool(s) was assessed by measuring changes in [Ca2+]c in the presence of 1 microM extracellular Ca2+, 100 microM TMB-8 had little inhibitory effect on angiotensin-induced calcium release. A higher dose of TMB-8 (250 microM) slightly inhibited calcium release. Additionally, TMB-8 did not affect exogenous arachidonic acid-induced calcium release. In contrast, 50 microM TMB-8 markedly inhibited 8 mM potassium-induced increase in [Ca2+]c. These results indicate that a major action of TMB-8 on cellular calcium is an inhibition of calcium influx but not of calcium release. We suggest that TMB-8 should not be used as an 'inhibitor of calcium release'.

Mechanism of inhibitory action of TMB-8 [8-(NN-diethylamino)octyl-3,4,5-trimethoxybenzoate] on aldosterone secretion in adrenal glomerulosa cells

The mechanism of 8-(NN-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) action was evaluated in isolated adrenal glomerulosa cells. TMB-8 inhibits both angiotensin II- and K+-stimulated aldosterone secretion in a dose-dependent manner. The ID50 for angiotensin II- and K+-stimulated aldosterone secretion is 46 and 28 microM, respectively. In spite of the fact that 100 microM-TMB-8 inhibits angiotensin II-stimulated aldosterone secretion almost completely, TMB-8 (100 microM) does not inhibit angiotensin II-induced 45Ca2+ efflux from prelabelled cells nor does it affect inositol 1,4,5-trisphosphate-induced calcium release from non-mitochondrial pool(s) in saponin-permeabilized cells. TMB-8 has no inhibitory effect on A23187-induced aldosterone secretion, but 12-O-tetradecanoylphorbol 13-acetate-induced aldosterone secretion is completely abolished. TMB-8 effectively inhibits both angiotensin II- and K+-induced increases in calcium influx but has no effect on A23187-induced calcium influx. TMB-8 inhibits the activity of protein kinase C dose-dependently. These results indicate that TMB-8 inhibits aldosterone secretion without inhibiting mobilization of calcium from an intracellular pool. The inhibitory effect of TMB-8 is due largely to an inhibition of plasma membrane calcium influx, but this drug also inhibits the activity of protein kinase C directly.

Intracellular Ca2+ levels and amylase secretion in Quin 2-loaded mouse pancreatic acini

Dispersed mouse pancreatic acini were loaded with the Ca2+-sensitive fluorescence probe Quin 2. Stimulation with carbamylcholine or cholecystokinin-octapeptide (CCK-OP) resulted in a rapid increase in Quin 2 fluorescence, which returned to a lower and sustained plateau level within 2 min of secretagogue stimulation. The magnitude of the initial rise in fluorescence intensity and of amylase secretion were closely related to the concentration of agonist used. Maximal fluorescence changes and amylase secretion were noted with 1 X 10(-5) M carbamylcholine and 1 X 10(-9) M CCK-OP, whereas both responses were half maximal in the presence of 5 X 10(-7) M carbamylcholine and approximately 1 X 10(-10) M CCK-OP. The resting cytosolic free Ca2+ concentration, as measured by the intensity of Quin 2 fluorescence, was calculated to be 1.03 +/- 0.12 X 10(-7) M. Cytosolic free Ca2+ rose to 1.3 +/- 0.3 X 10(-6) M after addition of 1 X 10(-5) M carbamylcholine and 1.25 +/- 0.21 X 10(-6) M following 1 X 10(-9) M CCK-OP. Amylase secretion, but not the Quin 2 fluorescence response, was attenuated at higher secretagogue concentrations. Both secretagogue-induced Quin 2 fluorescence and amylase secretion were inhibited by secretagogue antagonists. Removal of Ca2+ from the extracellular medium resulted in a 48.8 +/- 2.0% reduction of carbamylcholine-induced Quin 2 fluorescence. Following addition of carbamylcholine, CCK-OP was unable to stimulate a second increase in Quin 2 fluorescence without the intervening addition of a cholinergic antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)