Micromolar Ca2+ stimulates fusion of lipid vesicles with planar bilayers containing a calcium-binding protein

Osmotic control of bilayer fusion

We have used photography and capacitance measurement to monitor the steps in the interaction and eventual fusion of optically black lipid bilayers (BLMs), hydrostatically bulged to approximately hemispherical shape and pushed together mechanically. A necessary first step is drainage of aqueous solution from between the bilayers to allow close contact of the bilayers. The drainage can be controlled by varying the osmotic difference across the bilayers. If the differences are such as to remove water from between the bilayers, fusion occurs after a time that depends on the net osmotic difference and the area of contact. If there is an osmotic flow of water into the space between the bilayers, fusion never occurs. In the fusion process, a single central bilayer forms from the original apposed pair of bilayers. The central bilayer may later burst to allow mixing of the two volumes originally bounded by the separate bilayer; the topological equivalent of exocytosis.

Protein-mediated intermembrane contact facilitates fusion of lipid vesicles with planar bilayers

Fusion of phospholipid vesicles with planar bilayer membranes occurs provided there is an intermembrane contact, which can be mediated by phospholipid-binding proteins, even in the absence of calcium. The firm attachment phase is then followed by the osmotically-driven fusion. These results show that hydrophobic proteins (not necessarily Ca2+-binding proteins) may enhance fusion by promoting contact of membranes. Such proteins may operate synergistically with Ca2+ to reduce the threshold concentration of Ca2+ needed for fusion of biological membranes. Protein-mediated intermembrane contact resulting in fusion may play a crucial role in the regulation and catalysis of biological fusion events.

Effects of hypertonic solutions on catecholamine release from cat adrenal glands

The effects of hypertonic solutions on the spontaneous and evoked catecholamine release from isolated, perfused cat adrenal glands were studied. Hypertonic solutions enhanced three times the spontaneous release of the amines, but markedly inhibited the release evoked by nicotine (5 microM for 2 min) or high K+ (17.7 mM for 2 min). Using sucrose as osmoticant, the secretory response to high K+ was decreased to 53 and 15% of controls at 344 and 416 mosM, respectively. At 512 mosM sucrose inhibited poorly the release of catecholamines evoked by nicotine, but reduced it by 90% at 1000 mosM; sodium chloride behaved similarly to sucrose. A rise in the osmolarity of only 7.5% with choline chloride produced a complete inhibition of the K+-evoked response. These effects were not seen using isotonic choline chloride; on the contrary, isotonic choline chloride enhanced the K+ response, probably by stimulating nicotinic receptors. Since atropine did not modify this effect, it seems that secretion of catecholamines evoked by choline is mediated by nicotinic receptors. While the inhibitory effects of sucrose and NaCl were completely reversed when the tonicity of the perfusion medium was restored to its normal value (320 mosM), the effects of choline seemed to be long lasting and were reversed only partially. It is worth noting that the inhibitory effects of hyperosmotic solutions developed very fast.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between phosphatidylserine vesicles and rat brain synaptosomes

Intact rat brain synaptosomes incubated with phosphatidylserine vesicles incorporated a large amount of the phospholipid at membrane level. Chromatographic analysis showed that phosphatidylserine was not modified after the interaction. Ca2+ and an hypertonic medium favored the interaction between vesicles and synaptosomes. The phenothiazine fluphenazine blocked this effect. We suggest that the interaction of phosphatidylserine with synaptosomes could occur as a consequence of a two-step process: an initial non specific binding of the phospholipid to the membrane; a specific incorporation, via fusion, mediated by microM Ca2+ and by hypertonicity of the medium and regulated by calmodulin or calmodulin-like proteins.

Parameters affecting the fusion of unilamellar phospholipid vesicles with planar bilayer membranes

It was previously shown (Cohen, F. S., J. Zimmerberg, and A. Finkelstein, 1980, J. Gen. Physiol., 75:251-270) that multilamellar phospholipid vesicles can fuse with decane-containing phospholipid bilayer membranes. An essential requirement for fusion was an osmotic gradient across the planar membrane, with the vesicle-containing (cis) side hyperosmotic with respect to the opposite (trans) side. We now report that unilamellar vesicles will fuse with "hydrocarbon-free" membranes subject to these same osmotic conditions. Thus the same conditions that apply to fusion of multilamellar vesicles with planar bilayer membranes also apply to fusion of unilamellar vesicles with these membranes, and hydrocarbon is not required for the fusion process. If the vesicles and/or planar membrane contain negatively charged lipids, divalent cation (approximately 15 mM Ca++) is required in the cis compartment (in addition to the osmotic gradient across the membrane) to obtain substantial fusion rates. On the other hand, vesicles made from uncharged lipids readily fuse with planar phosphatidylethanolamine planar membranes in the near absence of divalent cation with just an osmotic gradient. Vesicles fuse much more readily with phosphatidylethanolamine-containing than with phosphatidylcholine-containing planar membranes. Although hydrocarbon (decane) is not required in the planar membrane for fusion, it does affect the rate of fusion and causes the fusion process to be dependent on stirring in the cis compartment.

Local application of calcium-modulating agents to a crushed goldfish optic nerve modifies visual recovery

The effects of various Ca2+-modulating agents on regeneration in the optic nerve of goldfish were determined by assaying recovery of visual function. One to three daily applications of the agents were made at the site of an optic nerve crush beginning within 3 days after the lesion. Application of calcium ionophore A-23187 significantly shortened the time required for reappearance of the startle reaction to a bright light. Some shortening of recovery time was also observed with application of high-Ca2+ Ringer's solution. A significant effect was obtained with 1% and 6% dimethylsulfoxide (DMSO). When A-23187 was combined with DMSO, a further enhancement was seen if the original DMSO effect had been weak, whereas a strong DMSO effect was reduced in the presence of A-23187. The effect of DMSO alone or DMSO in combination with A-23187 was blocked by the calcium-chelating agent EGTA. These results indicated that increased entry of Ca2+ into the regenerating axons or supporting cells may be responsible for the enhanced rate of recovery. There was no histologic evidence that the faster recovery was due to accelerated axon outgrowth, but the packing density of the regenerating axons was increased. We postulate that the recovery-enhancing agents may act by promoting axonal interactions leading to the reestablishment of the correct retinal projection, or by facilitating the function of the regenerating synaptic terminals.

Calcium-dependent exocytosis in an in vitro secretory granule plasma membrane preparation from sea urchin eggs and the effects of some inhibitors of cytoskeletal function

Egg cortical granules remain attached to the egg plasma membrane when the egg is ruptured. We present evidence that demonstrates that, when the cytoplasmic face of the egg plasma membrane is exposed to micromolar calcium concentrations, an exocytosis of the cortical granules occurs which corresponds to the cortical granule exocytosis seen when the egg is fertilized. The calcium sensitivity of the preparation is decreased by an increase in magnesium concentration and increased by a decrease in magnesium concentration. Exocytosis is inhibited by trifluoperazine (half inhibition at 6 microM), a drug that inhibits the action of the calcium-dependent regulatory protein calmodulin. Colchicine, vinblastine, nocodazole, cytochalasin B, phalloidin, N-ethylmaleimide-modified myosin subfragment 1, and antibody to actin are without effect on this in vitro exocytosis at concentrations that far exceed those required to disrupt microtubules and microfilaments. Conditions are such that penetration to the exocytotic site is optimal. It is unlikely, therefore, that either actin or tubulin participate intimately in exocytosis. Our data also exclude on quantitative grounds several other mechanisms postulated to account for the fusion of the secretory granule with the plasma membrane.

Role of calcium in triggering the release of transmitters at the neuromuscular junction

Calcium ions have a key role in triggering the release of packaged transmitter at the amphibian neuromuscular junction and of the chromaffin granules at the adrenal medulla. It is suggested that (i) proteins on the vesicle and plasma membranes are of particular importance in promoting membrane fusion and exocytosis (ii) they may be divalent cation-stimulated ATPases, which form the calcium-binding sites or have a specific calcium-binding protein in close molecular apposition (iii) these ATPases in synaptic vesicles and chromaffin granules also generate a protonmotive force which is associated with the uptake of transmitter (iv) the osmotic properties of the vesicle may be important during fission, but it is not suggested that chemiosmotic effects are involved in Ca2+-triggered fusion (v) the action of calcium is markedly co-operative (vi) the adrenal medullary cell and the n.m.j. may differ in the Ca2+-binding site; there is evidence for the involvement of calmodulin in granule-plasmalemma fusion in the chromaffin cells, but not at present (surprisingly) for a role of this Ca2+-binding protein at the n.m.j. (vii) exocytosis requires MgATP (viii) phosphorylation of the ATPase may well be involved; phosphorylation via cAMP does not seem to be involved in fusion in either system (ix) the ATPase may undergo configurational changes during exocytosis and is markedly sensitive to the physical state of its phospholipid environment and to the oxidation of its -SH groups.

Calcium dynamics at a plastic synapse in Aplysia

Because certain primitive behavioral responses in the large sea snail Aplysia have recently been linked to neurophysiological events at a synaptic level, special interest attaches to the role played by calcium ions at such synapses. Using an extended version of the model applied earlier to trace the flow of energy and information through a ganglion of the medicinal leech (Triffet & Green, 1980), the authors investigate the electropotential effects of small transient localized changes in the calcium concentration near the inner membrane surface of a neuron in the resting state. When this state is well below the firing threshold, changes in Ca2+ concentration less than 10(-8)M are shown to result only in low-level harmonic background oscillations. When the potential of the neurons is closer to threshold, however, and/or the Ca2+ concentration is of the order of 10(-8)M, easily recognizable graded potentials appear, and these grow into firing peaks when the calcium concentration is increased still further. Though no attempt is made to deal with the amplification effects dependent on calcium-vesicle interactions and the related release of transmitter molecules, a unified mechanism for the underlying calcium ion dynamics is proposed. Graded potentials of increasing size are associated with a progressive localized thickening of the inner and outer Debye layers. Moreover, the transverse and longitudinal calcium currents set up in such regions prove adequate to account for both the depletion of Ca2+ ions necessary to achieve habituation, and the increase in their concentration required for sensitization.

Osmotic swelling of phospholipid vesicles causes them to fuse with a planar phospholipid bilayer membrane

Fusion of phospholipid vesicles with planar bilayer membranes occurs if the vesicles that contact the planar membrane swell osmotically after the replacement in their medium of an impermeant solute by a permeant one. This finding directly demonstrates that osmotic swelling is a driving force for vesicle-planar membrane fusion. The method used to induce vesicle swelling and fusion may have relevance for biological systems.

Early events in calcium-induced liposome fusion

Calcium-induced interaction of liposomes composed of pure phosphatidylserine (PS) has been studied using a rapid-mixing, rapid-freeze device. Freeze-fracture electron microscopy of this material revealed that liposomes react very rapidly after addition of calcium ions. After only 10 ms (the resolution of the technique) vesicle fusion was apparent. At the same time, however, vesicles also collapsed, and appeared as aggregates of flattened membranes. This may explain controversies which have arisen over vesicle fusion studied with more indirect methods.

Stimulation of the insulin secretory mechanism following barium accumulation in pancreatic beta-cells

Electrothermal atomic absorption spectroscopy was employed for measuring barium in beta-cell-rich pancreatic islets microdissected from ob/ob-mice. Both the uptake and efflux of barium displayed two distinct phases. There was a 4-fold accumulation of barium into intracellular stores when its extracellular concentration was 0.26 mM. Unlike divalent cations with more extensive intracellular accumulation, the washout of Ba2+ was not inhibited by D-glucose. Ba2+ served as a substitute for Ca2+ both in maintaining the glucose metabolism after removal of extracellular Ca2+ and making it possible for glucose to stimulate insulin release. Furthermore, Ba2+ elicited insulin release in the absence of glucose and other secretagogues. The latter effect was reversible and was markedly potentiated under conditions known to increase the beta-cell content of cyclic AMP. It is likely that the observed actions of Ba2+ are mediated by Ca2+, since Ca2+ -dependent regulatory proteins, such as calmodulin, apparently cannot bind Ba2+ specifically.

Effects of hypertonic solutions on quantal transmitter release at the crayfish neuromuscular junction

1. The effects of a hypertonic bathing medium, containing either NaCl or melezitose, on the average number (m) and the time course of quantal release following an action potential were studied using focal extracellular recording methods at synaptic sites on the opener muscle of the crayfish leg.2. After the application of hypertonic saline, the rate of spontaneous quantal release increased but m decreased to a new level within 1 min; the extent of depression depended on the magnitude of the increase in tonicity until the osmolarity was 50% greater than the normal value of 0.43 osmol/l (Osm), but greater increases in tonicity exerted little further effect.3. The synaptic delays were increased and distributed over a longer range of time in hypertonic solutions; also, the latency between the first and second quantal releases in a multiple response to a single nerve impulse was also increased.4. Hypertonicity had no significant effect on the conduction velocity of the action potential, the independence of successive quantal releases in the same response, or the uniformity of the rise and fall of the probability of quanta release following an action potential, alpha(t).5. The time course of alpha(t) is prolonged in hypertonic solutions; this was manifested as an increase in the time constant of the exponential decline in alpha(t) from its peak value following the nerve impulse.5. When the potentiating agent 5-hydroxytryptamine (5-HT) was added to the hypertonic saline, m was increased, and the time course of alpha(t) was prolonged further than in the hypertonic solution alone; 5-HT produced no change in the time course of alpha(t) when it was added to normal saline, although m was increased.6. It is concluded that in this preparation hypertonicity decreases the rate of release of each quantum from the nerve terminal following a nerve impulse.

Ultrastructural changes during stimulation of amphibian oxyntic cells viewed by scanning and transmission electron microscopy

Amphibian oxyntic cells exposed by cryofracture were examined by field emission scanning electron microscopy. Comparisons were made between the structure thus revealed and those seen in thin-sectioned material from the same mucosas examined by transmission electron microscopy. Resting oxyntic cells had apical surfaces which were relatively smooth with some short microvilli. Apical cytoplasm was filled with smooth membrane tubules (so-called vesicotubules). Stimulation with a combination of histamine, dibutyryl cyclic AMP, and isobutylmethylxanthine (a phosphodiesterase inhibitor) led to a dramatic elaboration (i.e., increased membrane surface area) and a decrease in number of vesicotubules in the apical cytoplasm. The surface morphology of the stimulated oxyntic cell was much different from that reported for the mammalian parietal cell. Two types of surface elaboration were observed. Most commonly the surface was formed of flattened microplicae or lingulae. An irregular surface formed by the swelling of enlarged spaces near the apical surface was also observed. These new data have been used to evaluate the models which have been proposed to explain the nature of the transition from resting to stimulated morphology. A new model, which incorporates fusion of intracellular vesicotubules with each other and also with apical membrane, is proposed. The proposed fusion process may cause an increase in membrane area open to the extracellular (luminal)solution within the cell (rather than the eversion of membranes into the gastric lumen). Expansion of spaces between the microplicae may be caused by hydroosmotic pressures developed during active HCI secretion.

Ocular distribution of liposome-encapsulated epinephrine and inulin in the albino rabbit

A methodology has been developed to evaluate the disposition of liposome-encapsulated epinephrine and inulin in the conjunctival sac and selected ocular tissues. Relative to inulin, epinephrine effluxed much more rapidly from liposomes and disappeared more rapidly from the tear pool. As a result, liposomes were found to exert opposite effects on the ocular uptake of epinephrine and inulin. Over a period of 45 minutes, the concentration of epinephrine in the eye was 1.5 - 3 times lower when presented in liposomes than in aqueous solutions. This contrasted with the 3 to 15 fold increase in inulin concentration in the eye from liposomal preparations of the compound. Specifically, much of the increase in inulin concentration in the cornea can be attributed to an increase seen in the epithelium. Despite elevated inulin concentrations in the ocular tissues it bathes, the aqueous humor was devoid of inulin unless the corneal epithelium was physically removed prior to topical dosing. Collectively, these findings suggest that both the manner in which an entrapped compound interacts with the constituents of liposomes and the manner in which liposomes interact with the absorptive surfaces of the eye can significantly influence its pharmacokinetics in the eye.

Effects of temperature and lipid composition on the serum albumin-induced aggregation and fusion of small unilamellar vesicles

Small unilamellar vesicles of egg phosphatidylcholine (PC) or dimyristoylphosphatidylcholine, mixed with small unilamellar vesicles labelled with 2-(10-(1-pyrene)decanoyl)phosphatidylcholine, exhibit a constant average size and excimer to monomer (E/M) ratio for several hours when incubated at pH 3.6 at a temperature higher than the phase transition temperature (Tc) of the lipids. Addition of bovine serum albumin to this system produces a transient turbidity increase, a fast decrease in the E/M ratio, a partial loss of vesicle-entrapped [14C]sucrose and a measurable leak-in of externally added sucrose. Sepharose 4B filtration of the system demonstrates that the E/M ratio decrease is strictly paralleled by the formation of liposomes which exhibit a low E/M ratio and a hydrodynamic radius larger than that of small unilamellar vesicles. These data demonstrate that the E/M ratio decrease can be unequivocally ascribed to a vesicle-vesicle fusion process induced by serum albumin. The rate of serum-albumin induced fusion of small unilamellar vesicles is: (a) maximal at a stoichiometric ratio of approx. 2 albumins per vesicle; (b) sensitive to the nature of the lipid and; (c) not altered when human serum albumin replaces bovine serum albumin. The rate of albumin-induced fusion of dimyristoylphosphatidylcholine small unilamellar vesicles is higher below the Tc of the lipid and increases with temperature above the Tc. The formation of protein-bound aggregates with defined stoichiometries and a high local vesicle concentration, as well as changes in the local degree of hydration, are proposed to be the driving forces for the protein-induced vesicle fusion in this system.

Role of osmotic forces in exocytosis: studies of ADH-induced fusion in toad urinary bladder

Antidiuretic hormone (ADH) treatment of toad urinary bladder activates an exocytotic-like process by which intramembrane particle aggregates are transferred from membranes of elongated cytoplasmic tubules to the luminal-facing plasma membrane. We find that the number of these ADH-induced fusion events, and the number of aggregates appearing in the luminal membrane, are reduced when the luminal bathing medium is made hyperosmotic. As an apparent consequence of the inhibition of their fusion with the luminal membrane, the elongated cytoplasmic tubules become enormously swollen into large, rounded vesicles. These results are consistent with the view that osmotic forces are essential to the basic mechanism of exocytosis.

Insulin release: reconciliation of the receptor and metabolic hypotheses. Nutrient receptors in islet cells

Nutrients which stimulate insulin secretion are currently thought to initiate the series of cellular events eventually leading to insulin release either by interacting with a stereospecific receptor system (the regulatory site hypothesis) or by acting as a fuel (the substrate site hypothesis) in the pancreatic B-cell. The latter hypothesis is supported by a number of observations indicating that the capacity of nutrients to stimulate insulin release is indeed highly dependent on their capacity to increase catabolic fluxes in isolated pancreatic islets. However, these observations do not rule out the existence of nutrient receptors in islet cells. For instance, a nonmetabolized analog of L-leucine stimulates insulin release by causing allosteric activation of glutamate dehydrogenase, which should be considered, therefore, as a receptor for certain amino acids. Likewise, the increase in glycolytic flux, which is associated with the process of glucose-stimulated insulin release, is attributable not solely to a mass action phenomenon but also to the activation of phosphofructokinase by fructose 2.6-bisphosphate. The biosynthesis of this activator may involve a glucose receptor system. The fact that certain nutrient secretagogues (e.g. D-glucose and L-leucine) act in the B-cell both as substrates and enzyme activators permits reconciliation of the substrate site and regulatory site hypotheses for insulin release.