Effect of atropine, ouabain, antimycin A, and A23187 on "trigger Ca2+ pool" in exocrine pancreas

45Ca2+ fluxes have been analyzed in dispersed acinar cells prepared from rat pancreas. Sudden addition of carbamylcholine (CCh) to 45Ca2+-preloaded acinar cells at quasi-steady state for 45Ca2+ resulted in a quick 45Ca2+ release followed by a slower 45Ca2+ reuptake with net accumulation of 45Ca2+. Subsequent sudden addition of atropine caused a further transient increase in cellular 45Ca2+ followed by a slow decrease to a steady-state value. 45Ca2+ release could not be evoked a second time by pancreozymin when prestimulated with CCh. However, if CCh stimulation was abolished by an interposed step of atropine, restimulation by cholecystokinin-pancreozymin was possible. Addition of A23187 or antimycin A to cells induced a fast decrease in cellular 45Ca2+. This effect was not additive to the CCh effect. In ouabain-pretreated cells, the CCh-induced sudden loss of cellular 45Ca2+ was blocked by 60%. The following slow reuptake of 45Ca2+ was blocked completely. Subsequent addition of atropine caused a fast uptake of cellular 45Ca2+ with no secondary decline. The data are consistent with the following model: acetylcholine releases Ca2+ from a cellular "trigger pool" into the cytosol located in or near the cell membrane. Then Ca2+ is extruded from the cell via Ca2+ pumps partly by a Na+-dependent Ca2+ transport system (quick phase of 45Ca2+ release). Subsequently, due to increased Ca2+ permeability of the plasma membrane as induced by acetylcholine, Ca2+ influx occurs and Ca2+ is taken up from the cytosol into intracellular Ca2+ pools (slow 45Ca2+ reuptake phase). Atropine causes refilling of the trigger Ca2+ pool and return of the increased Ca2+ permeability of the plasma membrane back to the unstimulated state.

Ca2+ control of electrolyte permeability in plasma membrane vesicles from cat pancreas

The influence of Ca2+ and other cations on electrolyte permeability has been studied in isolated membrane vesicles from cat pancreas. Ca2+ in the micromolar to millimolar concentration range, as well as Mg2+, Sr2+, Mn2+ and La3+ at a tested concentration 10(-4) M, increased Na+ permeability when applied at the vesicle inside. When added to the vesicle outside, however, they decreased Na+ permeability. Ba2+ was effective from the outside but not from the vesicle inside. When Ca2+ was present at both sides of the membrane, Na+ efflux was not affected as compared to that in the absence of Ca2+. Monovalent cations such as Rb+, Cs+, K+, Tris+ and choline+ decreased Na+ permeability when present at the vesicle outside at a concentration range of 10 to 100 mM. Increasing Na+ concentrations from 10 to 100 mM at the vesicle inside increased Na+ permeability. The temperature dependence of Na+ efflux revealed that the activation energy increased in the lower temperature range (0 to 10 degrees C) when Ca2+ was present at the outside or at both sides, but not when present at the vesicle inside only or in the absence of Ca2+. The results suggest that the Ca2+ outside effect is due to binding of calcium to negatively charged phospholipids with a consequent reduction of both fluidity and Na+ permeability of the membrane. The Ca2+-inside effect most likely involves interaction with proteins with consequent increase in Na+ permeability. The data are consistent with current hypotheses on secretagogue-induced fluid secretion in acinar cells of the pancreas according to which secretagogues elicit NaCl and fluid secretion by liberating Ca2+ from cellular membranes and by stimulating Ca2+ influx into the cell. The increased intracellular Ca2+ concentration in turn increases the contraluminal Na+ permeability which leads to NaCl influx. The luminal sodium pump finally transports Na+ ions into the lumen.

Studies on isolated subcellular components of cat pancreas. III. Alanine-sodium cotransport in isolated plasma membrane vesicles

Transport of alanine was studied in isolated plasma membrane vesicles from cat pancreas using a rapid filtration technique. The uptake is osmotically sensitive and the kinetics of L-alanine transport are biphasic showing a saturable and a nonsaturable component. The saturable component is seen only when a sodium gradient directed from the medium to the vesicular space is present. Under this condition an overshooting uptake of L-but not of D-alanine occurs. The Na+ gradient stimulated uptake of L-alanine is inhibited by L-serine and L-leucine and stimulated when the membrane vesicles had been preloaded with L-alanine, L-serine or L-leucine. The ionophore monensin inhibits stimulation of uptake caused by a sodium gradient. In the presence of valinomycin or carbonyl cyanide p-trifluoromethoxyphenylhydrazone (CFCCP), the sodium-dependent transport is augmented in vesicles preloaded with K2SO4 or H+ ions (intravesicular pH 5.5), respectively. In the presence of different anions, the Na+-dependent transport is stimulated according to increasing anionic penetration through membrane (lipid solubility). We conclude that a sodium dedpendent electrogenic amino acid transport system is present in pancreatic plasma membranes.

Studies on isolated subcellular components of cat pancreas. I. Isolation and enzymatic characterization

Pancreas of the cat was fractionated into its subcellular components by centrifugation through an exponential ficoll-sucrose density gradient in a zonal rotor. This enables a preparation of four fractions enriched in plasma membranes, endoplasmic reticulum, mitochondria and zymogen granules, respectively. The first fraction, enriched by 9- to 15-fold in the plasma membrane marker enzymes, hormone-stimulated adenylate cyclase, (Na+K+)-ATPase, and 5'-nucleotidase, is contaminated by membranes derived from endoplasmic reticulum but is virtually free from mitochondrial and zymogen-granule contamination. The second fraction from the zonal gradient shows only moderate enrichment of the above marker enzymes but contains a considerable quantity of plasma membrane marker enzymes and represents mostly rough endoplasmic reticulum. The third fraction contains the bulk of mitochondria and the fourth mainly zymogen granules as assessed by electron microscopy and marker enzymes for both mitochondria and zymogen granules, namely succinic dehydrogenase, trypsin and amylase. Further purification of the plasma membrane fractions by differential and sucrose step-gradient centrifugation yields plasma membranes enriched 40-fold in basal and hormone-stimulated adenylate cyclase and (Na+K+)-ATPase.

Calcium ion uptake induced by cholinergic and alpha-adrenergic stimulation in isolated cells of rat salivary glands

Adrenaline (10(-5) M) and carbamylcholine (10(-4) M) stimulate 45Ca2+ uptake into isolated cells of rat submandibular galnd and parotid glands. In the presence of the alpha-adrenoreceptor blocking agent phentolamine, adrenaline stimulation of 45Ca2+ uptake is abolished. The beta-adrenergic stimulant isoproterenol has no effect on 45Ca2+ uptake. Carbamylcholine induced 45Ca2+ uptake is inhibited by atropine. The Ca2+ ionophore A23187 stimulates 45Ca2+ uptake, whereas dibutyryl cyclic adenosine 3',5'-monophosphate and dibutyryl cyclic guanosine 3',5'-monophosphate have no effect on 45Ca2+ uptake. A graphical analysis of the 45Ca2+ uptake curves reveals at least two phases: a fast phase and a slow phase, both of which are stimulated by adrenaline and carbamylcholine. The 45Ca-exchangeable pool size is increased by adrenaline and carbamylcholine in both the fast and the slow phases. These results suggest that alpha-adrenergic and cholinergic agonists act by increasing the rate of Ca2+ transfer into the cells of the parotid and submandibular salivary glands most probably through an increase of the cell membrane permeability for Ca2+.

Ca++ fluxes in isolated cells of rat pancreas. effect of secretagogues and different Ca++ concentrations

Secretagogues of pancreatic enzyme secretion, the hormones pancreozymin, carbamylcholine, gastrin I, the octapeptide of pancreozymin, and caerulein as well as the Ca++ -ionophore A 23187 stimulate 45Ca efflux from isolated pancreatic cells. The non-secretagogic hormones adrenaline, isoproterenol, secretion, as well as dibutyryl cyclic adenosine 3',5'-monophosphate and dibutyryl cyclic guanosine 3',5'-monophosphate have no effect on 45Ca efflux. Atropine blocks the stimulatory effect of carbamylcholine on 45Ca efflux complately, but not that of pancreozymin. A graphical analysis of the Ca++ efflux curves reveals at least three phases: a first phase, probably derived from Ca++ bound to the plasma membrane; a second phase, possibly representing Ca++ efflux from cytosol of the cells; and a third phase, probably from mitochondria or other cellular particles. The Ca++ efflux of all phases is stimulated by pancreozymin and carbamylcholine. Ca++ efflux is not significantly effected by the presence or absence of Ca++ in the incubation medium. Metabolic inhibitors of ATP production. Antimycin A and dinitrophenol, which inhibit Ca++ uptake into mitochondria, stimulate Ca++ efflux from the isolated cells remarkably, but inhibit the slow phase of Ca++ influx, indicating the role of mitochondria as an intracellular Ca++ compartment. Measurements of the 45Ca++ influx at different Ca++ concentrations in the medium reveal saturation type kinetics, which are compatible with a carrier or channel model. The hormones mentioned above stimulate the rate of Ca++ translocation. The data suggest that secretagogues of pancreatic enzyme secretion act by increasing the rate of Ca++ transport most likely at the level of the cell membrane and that Ca++ exchange diffusion does not contribute to the 45Ca++ fluxes.

The interaction of secretin with pancreatic membranes

1. 125I-labelled secretin bound rapidly and specifically to membranes from cat pancreas. Binding of labelled hormone was competitively inhibited by unlabelled secretin in the same range of concentrations that stimulated pancreatic adenylate cyclase in these membranes. The dissociation constant of the membrane binding sites for unlabelled secretin as evaluated by these displacement experiments was 4.1-10(-9) M and the number of binding sites 1.0 pmol per mg of membrane protein. 2. Studies using different concentrations of [125I]secretin (at a constant ratio of labelled to unlabelled hormone) revealed a similar value of 4-4-10(-9) M for the dissociation constant. 3. Both the association and dissociation rate constants of [125I]secretin binding were temperature sensitive; the dissociation rate constant increased more rapidly with increase in temperature. The ratio k-1/k+1 (at 22 degrees C) gave a dissociation constant of 3.7-10(-9)M which agrees closely with the figure obtained from equilibrium data. These data indicate that 125I-labelled secretin and unlabelled secretin bind to the same binding site on pancreatic membranes, with high affinity. 4. Unlabelled secretin stimulated pancreatic adenylate cyclase with an apparent Km of 8.4-10(-9) M, while [125I]secretin apparently did not stimulate the adenylate cyclase. Together with the binding data this might suggest that different portions of the secretin molecule are responsible for binding and adenylate cyclase activation. 5. Studies on the specificity of [125I]secretin binding carried out with various peptide hormones (glucagon, human gastrin, pancreozymin and caerulein) which are all inefficient in stimulating pancreatic fluid secretin, showed that these hormones have no influence on the binding of [125I]secretin. In contrast, vasoactive intestinal polypeptide, which stimulates pancreatic fluid and bicarbonate secretion, showed a competitive inhibition of secretin binding to the plasma membrane preparation.

Calcium ion uptake in isolated pancreas cells induced by secretagogues

1. Secretagogues of pancreatic enzyme secretion: pancreozymin, carbamylcholine, gastrin I, the octapeptide of pancreozymin, caerulein and the Ca2+ ionophore A 23187 stimulate 45Ca uptake into isolated rat pancreatic cells, whereas adrenaline, isoproterenol, secretin, dibutyrylic cyclic adenosine 3',5'-monophosphate and dibutyrylic cyclic guanosine 3',5'-monophosphate have no effect on 45Ca uptake. 2. A graphical analysis of the Ca2+ uptake curves reveals at least two phases: a fast phase, probably due to binding of Ca2+ to the membrane and a slow phase representing Ca2+ transport into cells. Both phases are stimulated by pancreozymin and carbamylcholine. 3. The 45Ca-exchangeable pool size is increased by both carbamylcholine and pancreozymin, whereas a significant increase of total content of cell calcium was too small to be detected. 4. Atropine blocks the stimulatory effect of carbamylcholine completely but not that of pancreozymin. The Ca2+ antagonist D600 blocks the stimulatory effects of both carbamylcholine and pancreozymin only partially. 5. The data suggest that secretagogues of pancreatic enzyme secretion act by increasing the rate of Ca2+ transfer into the cell most probably through an increase of the cell membrane permeability for Ca2+.

The role of extracellular Ca2+ and cyclic nucleotides in the mechanism of enzyme secretion from the cat pancreas

The role of Ca2+ in protein secretion from the isolated perfused cat's pancreas, the effect of the dibutyryl analogues of cAMP and cGMP, and the interrelation of Ca2+ and the nucleotides were studied. The following results were obtained: 1. Pancreatic enzyme secretion can be elicited by CaCl2 injections into the pancreatic arteries and is linearily related to the peak Ca2+ concentration in the effluent perfusate. Different background Ca2+ concentrations in the perfusate (3 mM or 0.125 mM) do not disturb this relation, indicating that no adaptation occurs. The effect of Ca2+ injections is of the same magnitude as that evoked by the hormones pancreozymin or acetylcholine. 2. Injections of Ca2+ potentiate the effects of dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP), dibutyryl guanosine 3',5'-cyclic monophosphate (dbcGMP) or theophylline. 3. Infusion of low doses of pancreozymin increases the Ca2+ effect. The findings indicate that extracellular Ca2+ is involved in the mechanism of enzyme secretion and that Ca2+ and cyclic nucleotides have a synergistic action on the target.

[Brachmann-De Lange syndrome: apropos of 3 cases]

São relatados três casos de síndrome de Brachman-de Lange, com análise laboratorial, estudo radiológico e eletrencefalográfico. Os autores fazem revisão da literatura enquadrando seus casos nesta síndrome cuja etiologia até o momento permanece obscura. A cariotipagem feita em dois dos casos foi normal.