The effect of phospholipase A2 on chloride transport by pancreatic secretory granules

Secretory granules from the rat pancreas contain electrolyte transport pathways that may contribute to exocrine fluid production. The Cl- selective transport pathway was measured indirectly in isolated granules by ionophore-induced lysis after suspension in isotonic KCl. This Cl- transport was shown to respond to alterations in the granule membrane lipid environment. Exogenously added phospholipase A2 (PLA2) caused an increase of up to 193% in the Cl- specific transport by the isolated granules. In addition, the products of PLA2 hydrolysis, lysophospholipids and unesterified fatty acids, directly increased the rate of Cl- transport when incubated with granules in vitro. Lysophospholipids (2.0 micrograms/ml) increased the Cl- transport between 280-450% (depending on the lysophospholipid species). Similarly, free fatty acids (10 microM) increased the granule Cl- transport from 25% with capric acid (10:0) to 255% with arachidonic acid (20:4). The relative extent of stimulation by fatty acids was dependent on their carbon chain length and to a lesser extent, the degree of unsaturation. The inhibition (68%) of PLA2 promoted granule lysis by 4-acetamido-4'-isothiocyanatostilbene 2,2'-disulfonic acid (0.5 mM) also suggests that the effect is specific for the granule Cl- channel. Thus, the data show that zymogen granule Cl- transport is influenced by membrane lipids and supports a role for PLA2 in controlling electrolyte transport during stimulus-secretion coupling.

ATP-sensitive potassium transport by pancreatic secretory granule membrane

Electrolyte transport pathways in the pancreatic secretory granules may contribute to acini fluid production after fusion with the apical membrane. A component of this granule transport is a K(+)-selective pathway that has been measured indirectly by ionophore-induced lysis of the isolated secretory granules when suspended in a KCl solution. This granule membrane K+ transport was shown to be inhibited by physiological levels of ATP in a dose-dependent manner and was not reversed by ADP. The sulfonylurea tolbutamide (0.5 mM), a recognized inhibitor of ATP-sensitive K+ channels, also reduced the ionophore-dependent lysis by 46%. The ATP sensitivity of the K+ transport was influenced by pH (increased ATP sensitivity with decreasing pH) and KCl concentration (increased ATP sensitivity with increasing KCl). In addition, preincubation with phospholipase A2 (8.3 x 10(-10) g/ml) or lysophospholipids (6.7 x 10(-7) g/ml) produced a significant decrease in the granule K+ transport. However, it is not likely that this inhibition is due to a change in membrane fluidity, because fluidization with arachidonic acid or octanol did not have a comparable effect. The results then support a granule-associated K+ transport in pancreatic acinar cells and suggest that it is ATP and lysophospholipid sensitive.

Dual modulation of chloride conductance by nucleotides in pancreatic and parotid zymogen granules

The regulation of Cl- conductance by cytoplasmic nucleotides was investigated in pancreatic and parotid zymogen granules. Cl- conductance was assayed by measuring the rate of cation-ionophore-induced osmotic lysis of granules suspended in iso-osmotic salt solutions. Both inhibition and stimulation were observed, depending on the type and concentration of nucleotide. Under optimal conditions, the average inhibition measured in different preparations was 1.6-fold, whereas the average stimulation was 4.4-fold. ATP was inhibitory at 1-10 microM but stimulated Cl- conductance above 50 microM. Stimulation by ATP was more pronounced in granules with low endogenous Cl- conductance. The potency of nucleotides in terms of inhibition was ATP greater than adenosine 5'-[gamma-thio]triphosphate (ATP[S]) greater than UTP much greater than or equal to CTP much greater than or equal to GTP much greater than or equal to guanosine 5'-[gamma-thio]triphosphate (GTP[S]) much greater than or equal to ITP. The potency with respect to stimulation had the following order: adenosine 5'-[beta gamma-methylene]triphosphate (App[CH2]p) greater than ATP greater than guanosine 5'-[beta-thio]diphosphate (GDP[S]). Adenosine 5'-[beta gamma-imido]triphosphate (App[NH]p) was also stimulatory, and was more potent than ATP in the parotid granules, but less potent in the pancreatic granules. Aluminium fluoride stimulated Cl- conductance maximally at 15-30 microM-Al3+ and 10-15 mM-F. F was less effective at higher concentrations. Protein phosphorylation by kinases was apparently not involved, since the nucleotide effects (1) could be mimicked by non-hydrolysable analogues of ATP and GTP, (2) showed reversibility, and (3) were not abolished by the protein kinase inhibitors 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) or staurosporine. The data suggest the presence of at least two binding sites for nucleotides, whereby occupancy of one induces inhibition and occupancy of the other induces stimulation.

Chloride transport across the membrane of parotid secretory granules

The Cl- transport pathways in secretory granules isolated from the parotid glands of rats were characterized by the technique of ionophore-induced lysis in defined salt solutions. The granules were shown to possess a Cl- conductance that exhibited a distinct anion selectivity with a sequence I- greater than Br- greater than Cl- greater than F- greater than SO4(2-) much greater than gluconate-. This conductance could be reduced approximately 40% by the stilbene 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) from the cytoplasmic side; the half-maximal concentration for inhibition was 50 microM. Furthermore, the apparent Cl- conductance was reduced by outwardly directed granule H+ gradients and stimulated by inwardly directed gradients. An outwardly directed H+ gradient mimics the in vivo environment and may serve in a regulatory capacity, providing for a tonic inhibition of transport until the granule fuses with the luminal membrane. The granules also possessed a Cl(-)-HCO3- exchange based on electroneutrality of Cl- uptake and stimulation of this uptake by HCO3-. This pathway displayed a different anion selectivity, I- greater than Br- greater than F- greater than Cl- much greater than SO4(2-) much greater than gluconate-, and was not inhibited by SITS on the cytoplasmic side. The presence of these electrolyte transport pathways in the granule membrane is consistent with the production of primary fluid by parotid acinar cells after fusion of granules with the luminal plasma membrane.

Regulation of chloride transport in parotid secretory granules by membrane fluidity

Zymogen granule membranes contain Cl- conductance and Cl/anion exchange activities that become important for primary fluid production after fusion with the apical plasma membrane of the acinar cell. We have used steady-state fluorescence anisotropy of diphenylhexatriene derivatives and measurements of Cl- transport in isolated secretory granules to determine the contribution of membrane fluidity to the regulation of transport across the granule membrane. Secretory granules from several unstimulated glands (rat pancreas and parotid, rabbit gastric glands) were shown to have low membrane fluidity compared to plasma membranes. In addition, Cl- transport activity in different granule preparations showed a strong correlation to the membrane fluidity when measured with 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene p-toluenesulfonate (TMA-DPH), but not with 3-[p-(6-phenyl)-1,3,5-hexatrienyl)-phenyl]propionic acid (PA-DPH). These data suggest that TMA-DPH preferentially partitions into a specific lipid environment associated with, or which exerts an influence on, the Cl- transport proteins and that increases in the fluidity of this environment are associated with higher transport rates. Data from other types of plasma membranes indicate that TMA-DPH partitions much more than PA-DPH into the cytoplasmic leaflet, suggesting that this part of the granule membrane is involved in the observed fluidity changes. Furthermore, increasing the bulk membrane fluidity with the local anesthetics benzyl alcohol and n-alkanols increased the Cl- transport rates up to 10-fold. This increase was apparently through specific transporters as anion selectivity was maintained in spite of the higher absolute rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of rabbit pepsinogen granules

Pepsinogen granules were isolated from the rabbit stomach using isoosmotic Percoll density gradients, low free calcium (10(-7) M), and conditions that minimize physical damage. These granules were enriched approximately eightfold with respect to pepsinogen and were free from contamination by mitochondria and endoplasmic reticulum. Electrophoretic examination shows pepsinogen to account for approximately 80% of the Coomassie Blue-stainable intragranular protein and the granule membrane to yield a simple spectrum of proteins similar to other granule systems. In addition to purity, the isolated granules displayed a high degree of osmotic stability at physiologic conditions of pH, temperature, and ionic strength. This stability suggests strict regulation of the granule electrolyte transport pathways, which are shown to include a Cl- conductance, Cl-/anion exchange, and a K+ conductance. These transport systems in the granule membrane are consistent with the promotion of primary fluid secretion. Furthermore, granule-mediated ion transport would allow the chief cell to couple fluid secretion directly to exocytotic pepsinogen secretion and flush the enzyme from the base of oxyntic glands.

Potassium transport by pancreatic and parotid zymogen granule membranes

Zymogen granules that were stable at physiological conditions of pH, ionic strength, and temperature were isolated from the rat pancreas and parotid. The cation permeability of these granules was evaluated to characterize the mechanism of secretagogue-stimulated fluid secretion by acinar cells. Granule swelling and lysis provide a measure of the rate of cation transport, since the use of ionophore combinations such as tripropyltin and carbonyl cyanide 3-chlorophenylhydrazone (CCCP) will render cation conductance the rate-limiting step for salt influx. This technique supplies evidence for the existence of K+ conductance in the granule membrane. The pancreatic and parotid granules have a K+-selective conductance that is not inhibited by the K+ channel blockers barium, tetraethylammonium, quinidine, cesium, or 4-aminopyridine. Furthermore, the intragranular pH of pancreatic zymogen granules was measured to be approximately 6.5 and was identified as a factor that modulates the K+ conductance. Although the pancreatic and parotid granules were qualitatively identical, quantitatively the relative K+ transport rate constant was over twofold higher for the parotid than for the pancreatic granules. The zymogen granule K+ conductance may have an important role in active K+ secretion by exocrine glands, which is prominent in the parotid after stimulation with beta-adrenergic agents.

Separation of cell organelles in density gradients based on their permeability characteristics

The buoyant density of intracellular organelles is dependent in part on the nature of the buffer composition of the density gradient and the permeability characteristics of the organelle membrane to the constituents of this buffer. Therefore, knowledge of the transport properties of different organelles allows the design of density gradients useful for their purification. We have used this approach to significantly decrease mitochondrial contamination of pancreatic zymogen granules in a one-step purification procedure on a 40% Percoll density gradient. These gradients, prepared with isoosmotic sucrose, yield a narrow band of zymogen granules and mitochondria. However, by substitution of sucrose with salts to which mitochondria but not zymogen granules are permeable, the densities of mitochondria are altered to give a significant separation. For example, the incorporation of 100 mM sodium succinate in the Percoll gradient can produce a 70% reduction in mitochondrial contamination. The increased ionic strength has an additional beneficial effect on zymogen granule yield by 5-10%. The recognition and utilization of transport pathways in organelle membranes is the principal feature of this technique and should prove to be widely applicable to other isolation procedures.

Secretagogues activate chloride transport pathways in pancreatic zymogen granules

The membrane permeability of pancreatic zymogen granules was evaluated in vitro with granules isolated from rats in different secretory states: 1) untreated, 2) pretreated with a muscarinic antagonist, 3) pretreated with a muscarinic and an adrenergic antagonist, 4) pretreated as in 3 and then stimulated with the secretagogue cholecystokinin 4 min before death, and 5) pretreated as in 3 and then stimulated with the secretagogue secretin 4 min before death. Granules isolated from untreated rats had variable ionic permeabilities but in general possessed both chloride conductance and electroneutral exchange pathways with low permeabilities to alkali metal ions. In contrast, granules from animals pretreated with secretory antagonists had very low ion permeabilities to both inorganic anions, such as chloride, and alkali metal ions. Injection of the peptide secretagogues cholecystokinin or secretin resulted in a relatively fast (within 4 min) activation or induction of high chloride permeabilities through both chloride conductance and chloride/hydroxide (or chloride/bicarbonate) exchange pathways. In addition, the secretagogues increased the cation permeability of the granule membrane, which exhibited a distinct potassium selectivity. Chloride conductance has been postulated to play a major role in fluid secretion coupled to exocytosis of macromolecules [R. C. DeLisle and U. Hopfer, Am. J. Physiol. 250 (Gastrointest. Liver Physiol. 13): G489-G496, 1986]. These results demonstrate that granules may actively participate in the secretory process and suggest that some of the physiological targets in the cascade of events leading to secretion are anion and cation transporters in the zymogen granule membrane.

The inhibition and disposition of intestinal alkaline phosphatase

A primary mechanism of amino acid inhibition of intestinal alkaline phosphatase is postulated to be the formation of a dissociable enzyme-amino acid complex at an allosteric zinc site. The degree of inhibition was highly correlated with the Zn2+ stability constant of each amino acid and the inhibition was reversible by the addition of exogenous Zn2+ or by dialysis. This allosteric amino acid inhibition proved to be a useful probe of the membrane arrangement of the enzyme in the intact tissue. The catalytic site appears to face the lumen based on the poor permeability of the substrate, the accumulation of the coproducts in the luminal bath, and the response of the enzyme to luminal pH. Amino acid inhibition of alkaline phosphatase in the intact tissue was only effective in the presence of sodium; whereas sodium was not required in butanol extracted preparations which lacked the sidedness of the intact tissue. Since amino acid uptake from the intestine is sodium dependent, the allosteric inhibitory site is probably intracellular. The results suggest that the intestinal alkaline phosphatase spans the apical membrane with the catalytic site accessible from the lumen and the allosteric inhibitory site from the cytoplasm.

The response of alkaline phosphatase to osmoregulatory changes in the trout, Salmo gairdneri

The relationship between alkaline phosphatase and environmental salinity was examined in the rainbow trout and the migratory rainbow (steelhead), Salmo gairdneri. The enzyme activity in tissues involved in osmoregulation was strongly correlated with the adaptation salinity and thus to the degree of salt and fluid transport in those tissues. After transfer from freshwater to seawater, the specific activity of the enzyme increased over 260% in the intestine, decreased by 50% in kidney, and was unchanged in the liver, an organ not directly involved in osmoregulation. The sea-run steelhead trout response was similar to the nonmigratory rainbow; although, the pre-migratory transformation (smoltification) had no effect on enzyme activity. Amino acid inhibitors of alkaline phosphatase significantly reduced fluid absorption in the isolated intestine of rainbow trout, reaffirming the relationship between the enzyme and fluid movement. Electrophoretic identification of trout alkaline phosphatase isozymes, clearly distinguishes the enzyme from different tissue origins. However, from the analysis of intestinal electrophoretic patterns, osmoregulatory adjustments are not associated with the induction of new alkaline phosphatase isozymes, or in the large scale preferential stimulation of one of the two existing intestinal isozymes over the other.