Osmotic lysis of bovine chromaffin granules in isotonic solutions of salts of weak organic acids. Release of catecholamines, ATP, dopamine beta-hydroxylase, and enkephalin-like material

Punctate appearance of dopamine-beta-hydroxylase on the chromaffin cell surface reflects the fusion of individual chromaffin granules upon exocytosis

A secretion from cultured bovine chromaffin cells was stimulated to examine the pattern of exocytotic fusion on the plasma membrane. Confocal microscopy revealed that dopamine-beta-hydroxylase immunofluorescence in intact cells stimulated for 20s with the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium was almost entirely punctate and evenly distributed on the cell surface. The basis for the fine, punctate appearance of dopamine-beta-hydroxylase was investigated. Dopamine-beta-hydroxylase presentation on the surface of permeabilized cells stimulated with 1-30 microM Ca2+ was punctate and similar to that on the plasma membrane of intact cells. The fluorescence intensities of both surface dopamine-beta-hydroxylase sites and internal chromaffin granules were estimated by computerized digital image analysis. The surface area of punctate surface dopamine-beta-hydroxylase (0.218 +/- 0.013 microm2, mean +/- S.E.M.) is similar to the surface area of a 0.28 microm diameter chromaffin granule (0.25 microm2). The average fluorescence intensity integrated over the area of the surface spots was 25-30% of the average chromaffin granule intensity, a fraction that is similar to the published values of 40-50% of the dopamine-beta-hydroxylase in the chromaffin granule being membrane bound. The surface density of the spots is consistent with the number of granules undergoing exocytosis. The spots do not tend to be clumped. The key conclusions from this work are that each individual punctate site of dopamine-beta-hydroxylase represents the fusion of a single chromaffin granule and that the distribution of dopamine-beta-hydroxylase spots over the cell surface is extensive and random, suggesting that each individual granule associates with its own release site.

Effects of trypsin on secretion stimulated by micromolar Ca2+ and phorbol ester in digitonin-permeabilized adrenal chromaffin cells

1. Catecholamine secretion from digitonin-treated chromaffin cells is stimulated directly by micromolar Ca2+ in the medium. The permeabilized cells are leaky to proteins. 2. In this study trypsin (30-50 micrograms/ml) added to cells after digitonin treatment completely inhibited subsequent Ca2+-dependent catecholamine secretion. The same concentrations of trypsin did not inhibit secretion from permeabilized cells if trypsin was present only prior to cell permeabilization. 3. The data indicate that trypsin entered digitonin-treated chromaffin cells which were capable of undergoing secretion and that an intracellular, trypsin-sensitive protein is involved in secretion. Chymotrypsin was less potent but had effects similar to those of trypsin. 4. The enhancement of Ca2+-dependent secretion from permeabilized chromaffin cells induced by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) was inhibited by trypsin added simultaneously with Ca2+ to permeabilized cells at concentrations (3-10 micrograms/ml) which had little or no effect on Ca2+-dependent secretion from cells untreated with TPA. Ca2+-dependent secretion in TPA-treated cells was reduced by trypsin only to the level that would have occurred in cells not treated with TPA. Trypsin reduced the large TPA-induced increment of membrane-bound protein kinase C.

Amine uptake into intact mast cell granules in vitro

Histamine, the principal amine of rat peritoneal mast cells, is taken up into isolated granules with intact membranes. Uptake is pH- and concentration-dependent and is not stimulated by the addition of Mg2+-ATP. The saturable uptake has a Km of 91.1 microM and a Vmax of 95.4 pmol (mg of protein)-1 min-1. Uptake is abolished by 5 mM ammonium ion. 5-HT, the other endogenous amine of the granules, and dopamine and tyramine, which do not occur naturally in rat mast cells, each competitively inhibits [3H]-histamine uptake with Ki's close to 1 microM. Reserpine, a putative amine carrier blocker, inhibits uptake at nanomolar concentrations. At high concentrations, uptake of [3H]-5-HT is nonsaturable; at low concentrations, a saturable component is observed with a Km of 1.6 microM. Uptake of [3H]-5-HT is not enhanced by Mg2+-ATP. It is pH-dependent but with a lower apparent pKa than that of histamine. [3H]-5-HT uptake can be completely inhibited by ammonium ions. Amine inhibition of [3H]-5-HT gives nonlinear Dixon plots, and high concentrations of the competing amines or reserpine cannot completely block uptake. We propose a model consistent with these results in which amine uptake occurs by several distinct saturable transport systems. According to the model, histamine is transported by a single system, which also transports 5-HT and dopamine. 5-HT and dopamine are transported by one or more other systems.

Plasma membrane and chromaffin granule characteristics in digitonin-treated chromaffin cells

Digitonin permeabilizes the plasma membranes of bovine chromaffin cells to Ca2+, ATP, and proteins and allows micromolar Ca2+ in the medium to stimulate directly catecholamine secretion. In the present study the effects of digitonin (20 microM) on the plasma membrane and on intracellular chromaffin granules were further characterized. Cells with surface membrane labeled with [3H]galactosyl moieties retained label during incubation with digitonin. The inability of digitonin-treated cells to shrink in hyperosmotic solutions of various compositions indicated that tetrasaccharides and smaller molecules freely entered the cells. ATP stimulated [3H]norepinephrine uptake into digitonin-treated chromaffin cells fivefold. The stimulated [3H]norepinephrine uptake was inhibited by 1 microM reserpine, 30 microM NH4+, or 1 microM carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). The data indicate that [3H]norepinephrine was taken up into the intracellular storage granules by the ATP-induced H+ electrochemical gradient across the granule membrane. Reduction of the medium osmolality from 310 mOs to 100 mOs was required to release approximately 50% of the catecholamine from chromaffin granules with digitonin-treated chromaffin cells which indicates a similar osmotic stability to that in intact cells. Chromaffin granules in vitro lost catecholamine when the digitonin concentration was 3 microM or greater. Catecholamine released into the medium by micromolar Ca2+ from digitonin-treated chromaffin cells that had subsequently been washed free of digitonin could not be pelleted in the centrifuge and was not accompanied by release of membrane-bound dopamine-beta-hydroxylase. The studies demonstrate that 20 microM of digitonin caused profound changes in the chromaffin cell plasma membrane permeability but had little effect on intracellular chromaffin granule stability and function. It is likely that the intracellular chromaffin granules were not directly exposed to significant concentrations of digitonin. Furthermore, the data indicate that during catecholamine release induced by micromolar Ca2+, the granule membrane was retained by the cells and that catecholamine release did not result from release of intact granules into the extracellular medium.

Osmotic fragility of chromaffin granules prepared under isoosmotic or hyperosmotic conditions and localization of acetylcholinesterase

In chromaffin cells of the adrenal medulla, catecholamines are stored in secretory granules. Different methods have been described to purify chromaffin granules. In the present study, storage granules were prepared using isoosmotic self-generating Percoll gradients or hyperosmotic sucrose gradients, and a comparison of their physical properties in response to osmotic changes was made. Catecholamines, dopamine beta-hydroxylase activity and protein were detected both in the external medium and in the granule fraction according to the medium osmolality. Suspension turbidity was used as a measure of organelle integrity. Acetylcholinesterase activity was found to be associated with both isoosmotically and hyperosomotically prepared granules. The total acetylcholinesterase activity was determined after adding Triton X-100 to the assay medium. When adrenal medullary tissue was homogenized in buffers containing echothiopate, an inhibitor of acetylcholinesterase, only 15-20% of enzyme activity was inhibited, excluding the possibility that main granule acetylcholinesterase could be due to contamination by plasma membrane fragments, endoplasmic reticulum and Golgi membranes. When granules were suspended in hypoosmotic buffers, a soluble acetylcholinesterase form was released into the external medium, while an insoluble acetylcholinesterase form was still found associated with the membrane fraction. Soluble acetylcholinesterase was found to be released differently than soluble dopamine beta-hydroxylase, indicating that acetylcholinesterase may be associated with a more osmotically resistant granule population.

Characteristics and determinants of osmotic lysis in chromaffin granules

(1) Using isolated bovine chromaffin granules, we demonstrate that osmotic lysis is not a random process and establish the osmotic pressure dependence of osmotic lysis in chromaffin granules, the so-called osmotic fragility curve. (2) We show by measuring the release of constituents of the granule core and correlating these with changes in spectroscopic parameters (turbidity and endogenous catecholamine fluorescence), that the latter can be safely used to measure lysis. (3) Within a particular granule population, noradrenaline granules lyse at higher osmolarities than adrenaline granules, suggesting a higher core osmolarity of the noradrenaline granules. (4) The size distribution of chromaffin granules as a function of lysis was determined by the use of whole mount electron microscopy. It is shown that the mean size of chromaffin granules decreases as a function of lysis. (5) On the basis of theoretical considerations three alternative models of the sequence of osmotic lysis in chromaffin granules are proposed. The experimental results best support a model which postulates that during partial osmotic lysis, granule membranes reseal into smaller vesicles after graded release of contents. The osmotic fragility would represent several cycles of lysis and resealing and would not be a reflection of the distribution of osmotic pressures in the granule population.