Short and long-range forces involved in cation-induced aggregation of chromaffin granule membranes

Liposphere local anesthetic timed-release for perineural site application

PURPOSE

This investigation determines the drug delivery capacity of Lipospheres, which are drug-containing solid-filled vesicles made of triglyceride with a phospholipid outer covering, to release local anesthetic in vitro and to produce sustained peripheral nerve block in vivo.

METHODS

The local anesthetic, bupivacaine, was loaded into Lipospheres in several dosage forms, characterized, and measured for in vitro release. In rats, Lipospheres were administered into a large space between muscle layers surrounding the sciatic nerve to assess sensory and motor block in vivo.

RESULTS

The particle size of Lipospheres was determined to be between 5 and 15 microm, with over 90% surface phospholopid. Lipospheres released bupivacaine over two days under ideal sink conditions. Liposphere nerve application produced dose-dependent and reversible block. Indeed, sustained local anesthetic block (SLAB) was observed for 1-3 days in various in vivo tests: a) Hind paw withdrawal latency to noxious heat was increased over 50% for 96 hr period after application of 3.6% or 5.6% bupivacaine-Lipospheres. The 3.6% and 5.6% doses were estimated to release bupivacaine at 200 and 311 microg drug/ hr, respectively, based on release spanning 72 hr. Application of 1.6% bupivacaine-Lipospheres increased withdraw latency 25-250% but for only a 24 hr duration; b) Similarly, vocalization threshold to hind paw stimulation was increased 25-50% for 72 hr following application of 3.6% bupivacaine-Lipospheres; c) Finally, sensory blockade outlasted or equaled corresponding motor block duration for all Liposphere drug dosages.

CONCLUSIONS

Liposphere delivery of local anesthetic drugs may be well suited for site-specific pharmacotherapy of neural tissue to produce SLAB. Dose-dependent effects in duration of action may include lipophilic tissue storage.

Comparative studies on Ca2+- and Mg2+-binding of sarcoplasmic reticulum and chromaffin granule membranes

The binding of calcium and magnesium ions to sarcoplasmic reticulum (SR) and chromaffin granule membranes was comparatively studied. The SR membranes are equipped with equal quantities of binding sites for both calcium and magnesium ions. The binding sites in presence of ATP combine specifically with calcium ions, while in the absence of ATP the binding sites react unspecifically with both ions. The trace amount of magnesium present in the SR membranes preparations is sufficient to drive ATP dependent calcium accumulation. Magnesium binding, however, is not affected by ATP. The chromaffin granule membranes bind calcium and magnesium in the same concentration range as observed for the SR membranes. Magnesium binding, however, is two times higher than that of calcium binding. In the absence of ATP, calcium and magnesium ions mutually compete. In the presence of ATP, magnesium binding values increase 3-5-fold, while the calcium binding isotherm remains unchanged. The appreciable contribution of the lipid phase to ions binding has been investigated, but was found to be of minor importance in this study.

The charge distribution in the rough endoplasmic reticulum/Golgi complex transitional area investigated by microinjection of charged tracers

The distribution of microinjected ferritin, ranging in charge from anionic to highly cationic, has been used to indicate differences in surface charge on the rough endoplasmic reticulum and the Golgi complex of intact cells. Highly cationic ferritins (HCF)(HCF1, pI 7.9-9.1; HCF2, pI 8.5-9.4; and HCF3.pI 9.5-10.1) were mostly bound and caused swelling of the rough endoplasmic reticulum. Cationic ferritin (CF) (pI 7.0-8.0) and anionic ferritin (AF) (pI 4.0-4.4) caused no changes in morphology. The distribution of these ferritins in the cytoplasmic space varied with their charge. Significantly more CF was bound to surfaces than was found in the free cytoplasmic space. Conversely, there was significantly more AF in the free cytoplasmic space than close to surfaces. Therefore, the intracellular surfaces are negatively charged. Comparison of the structures in the secretory pathway showed no differences in ferritin binding to transition vesicles, rough endoplasmic reticulum, Golgi saccules or secretory vesicles. The Golgi complex beads are not distinguished by their charge. It is therefore unlikely that charge differences play a role in regulating membrane-membrane interactions in this region of the secretory pathway.

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.

The chromaffin granule as a model for membrane fusion: implications for exocytosis

Rapid freeze/freeze-fracture and thin section electron micrographic studies of the Ca2+-promoted aggregation and fusion of isolated bovine adrenal medullary chromaffin granule membranes show that the granules undergo a series of morphological changes. The contact region becomes quite extensive and the membrane curvature changes radically at the edge of the contact site. The core material retracts away from the contact site leaving an electron lucent "stripe"; however, it remains adjacent to the membrane in the non-contact areas. The pentalaminar double membrane of the contact region often shows breaks. Close examination reveals that the two granule membranes have fused and become one continuous membrane. Rearrangement of large membrane associated particles (MAPs) can be seen by freeze fracture after Ca2+-promoted granule-granule contact. The broken pentalaminar septum becomes smaller and may break down into globular structures. These observations suggest a series of reactions in which the granules first form an encounter complex, then a stable complex. The membranes within the contact region undergo lateral displacement of the proteins and phase separations of the lipids, and then fuse. Analysis of the kinetics of turbidity and fluorescence changes during granule aggregation and fusion support the main postulates of the model. The initial events of aggregation are facilitated by putative recognition proteins and K+ will promote all activities except fusion. Recent observations that several soluble proteins (synexin, and albumin) will act as fusogens are discussed in terms of the relevance to exocytosis in vivo.

Calcium-promoted resonance energy transfer between fluorescently labeled proteins during aggregation of chromaffin granule membranes

Proteins of the chromaffin granule membrane were covalently labeled in situ with sulfhydryl-specific fluorophores. Using MIANS (maleimide iodoaminonaphthyl sulfonate) as the donor and fluorescein mercury acetate or fluorescein-5-maleimide as the acceptor. Förster fluorescence resonance energy transfer (FRET) could be employed to measure the degree of inter-membrane and intra-membrane protein-protein contact upon Ca2+-induced aggregation of the membranes. The four major findings were: (1) Raising the Ca2+ concentration to approx. 500 microM causes the proteins to aggregate in the plane of the membrane. This is demonstrated by Ca2+-induced increases in the fluorescence resonance energy transfer in double labeled membranes. This effect is not protein-concentration dependent and occurs at calcium concentrations too low for granule aggregation, implying intra-membrane protein clustering or patching. To our knowledge this is the first direct demonstration of the fluid mosaic nature of subcellular organelles. (2) If two sets of granules are labeled separately, Ca2+-induced aggregation brings at least 74% of the labeled proteins into close transmembrane proximity. This effect is also observed at 10-100-fold slower rates in the absence of calcium and can be greatly reduced by depleting the granule membrane of labeled peripheral proteins. It is enhanced if the granules are aggregated by Ca2+ or K+. We conclude that (some) peripheral proteins can transfer from one membrane surface to another. (3) Aggregation of separately labeled sets of membranes by Ca2+ also produces transmembrane energy transfer since: (a) the Km for Ca2+-induced quantum transfer is in the same range as the Km for aggregation; (b) the reaction is protein-concentration dependent; (c) reversal of aggregation also (partially) reverses donor quenching. (4) A kinetic analysis of the transmembrane effect shows it to be 5-10-fold slower than aggregation itself, supporting earlier suggestions (Haynes, D.H., Kolber, M. and Morris, S.J., (1979) J. Theor. Biol. 81, 713-743) that lipid and protein rearrangements are secondary to granule membrane aggregation.

Purification and mode of action of synexin: a protein enhancing calcium-induced membrane aggregation

Synexin, a protein from the cytosol of the adrenal medulla, selectively increases the ability of Ca2+ to aggregate chromaffin granules and other membrane-bound particles. The ability of synexin to self-aggregate in the presence of Ca2+ can be employed in the purification of the protein by monitoring purification with parallel assays that utilize the aggregation of both chromaffin granule membranes and phosphatidylserine liposomes. It is shown that the enhancement of the Ca2+-induced aggregation of both liposomes and chromaffin granule membranes is a property associated with a 47,000 Mr protein. Trypsin inactivated synexin. We found that if granule membranes were well washed after trypsin treatment, they were still excellent substrates for synexin aggregation. This finding cannot be explained by extinction changes owing to synexin self-aggregation. The 47,000 Mr protein enhancement Ca2+ aggregation of phosphatidylserine liposomes containing up to 40% phosphatidylcholine, liposomes made from lipids extracted from chromaffin granule membranes, and trypsin-treated chromaffin granule membranes, thus suggesting that synexin activity in vivo may be independent of specific membrane proteins but dependent on the presence of acidic phospholipids in the membrane.

Electrokinetic properties of (Na+, K+)-ATPase vesicles as studied by laser Doppler spectroscopy

The technique of laser Doppler electrophoresis was applied for the study of the surface charge properties of (NA+, K+)-ATPase containing microsomal vesicles derived from guinea-pig kidney. The influence of pH, the screening and binding of uni- and divalent cations and the binding of ATP show: (1) one net negative charge per protein unit with a pK = 3.9; (2) deviation from the Debye relation between surface potential and ionic strength for univalent cations, with no difference in the effect of Na+ and K+; (3) Mg2+ binds with an association constant of Ka = 1.1. 10(2) M-1 while ATP binds with an apparent Ka = 1.1.10(4) M-1 for 1 mM NaCl, 0.2 mM KCI, 0.1 mM MgCl2, 0.1 mM Tris-HCl2, 0.1 mM Tris-HCl (pH 7.3). The binding is weaker at higher Mg2+ concentrations. There is no ATP binding in the absence of Mg2+. In addition, the average vesicle size derived from the linewidth of the quasielastic light scattering spectrum is 203.7 +/- 15.2 nm. In the presence of ATP a reduction in size is observed.

Experimental and theoretical considerations of mechanisms controlling cation effects on thylakoid membrane stacking and chlorophyll fluorescence

The roles of specific cation binding, charge neutralization and electrostatic screening mechanisms in controlling salt-induced stacking and chlorophyll fluorescence changes in thylakoid membranes are examined in the light of new experimental evidence and theoretical calculations of the forces between membrane surfaces. A comparison of the biphasic stacking and fluorescence phenomena generated by organic mono- and divalent cations known sterically to inhibit specific binding with the effects generated by inorganic mono- and divalent cations suggests that the observed salt-induced changes at pH greater than or equal to 7.5 are predominantly governed by the electrostatic screening mechanism in agreement with previous work (e.g. Barber, J., Mills, J.D. and Love, A. (1977) FEBS Lett. 74, 174-181). Detailed calculations of the coulombic double layer repulsive force between negatively charged membrane surfaces immersed in a mixed electrolyte of valence type Z1+/Z1-,Z2+/Z1- were performed both under the constraints of fixed surface charged density and fixed surface potential. From a close comparison of the theoretical results with new experimental data on salt-induced stacking and fluorescence changes and a consideration of the contributions of the 'hydration' repulsive force and the van der Waals attractive force, it is argued that a reduction in surface charge density alone by lateral diffusion is probably insufficient to realize membrane stacking and that an increase in the van der Waals attractive force is necessary to account for the experimental observations perhaps through the formation of protein rich domains. In view of the complexity of the thylakoid membranes, the conclusions are to be considered qualitative. Nevertheless, these calculations give support to a model in which the cation induced chlorophyll fluorescence and stacking changes can be explained by lateral diffusion of two types of pigment protein complexes in the lipid matrix of the membrane. Such diffusion gives rise to changes in energy transfer between Photosystem II and Photosystem I and also to the creation of domains having low and high electrical surface charge density.

Measurement of surface potential and surface charge densities of sarcoplasmic reticulum membranes

The binding of the anionic fluorescent probe 1-anilino-8-naphthalene-sulfonate (ANS-) was used to estimate the surface potential of fragmented sarcoplasmic reticulum (SR) derived from rabbit skeletal muscle. The method is based on the observation that ANS- is an obligatory anion whose equilibrium constant for binding membranes is proportional to the electrostatic function of membrane surface potential, exp(e psi o/kT), where psi o is the membrane surface potential, e is the electronic charge, and kT has its usual meaning. The potential measured is characteristic of the ANS- bindings of phosphatidylcholine head groups and is about one-third as large as the average surface potential predicted by the Gouy-Chapman theory. At physiological ionic strength the surface potentials, measured by ANS-, referred to as the aqueous phase bathing the surface, were in the range -10 to -15 mV. This was observed for the outside and inside surfaces of the Ca2+-ATPase-rich fraction of the SR and for both surfaces of the SR fraction rich in acidic Ca2+ binding proteins. The inside and outside surfaces were differentiated on the basis of ANS- binding kinetics observed in stopped-flow rapid mixing experiments. A mechanism by which changes in Ca2+ concentration could give rise to an electrostatic potential across the membrane and possibly result in changes in Ca2+ permeability. The dependence of the surface potential on the monovalent ion concentration in the medium was used together with the Gouy-Chapman theory to determine the lower limits for the surface charge density for the inside and outside surfaces of the two types of SR. Values for the Ca2+-ATPase rich SR fraction were between 2.9 X 10(3) and 3.8 X 10(3) esu/cm2, (0.96 X 10(-6) and 1.26 X 10(-6) C/cm2) with no appreciable transmembrane asymmetry. A small amount of asymmetry was observed in the values for the inside and outside surfaces of the fraction rich in acidic binding proteins which were ca. 6.6 X 10(3) and ca. 2.2 X 10(3) esu/cm2 (2.2 X 10(-6) and 0.73 X 10(-6) C/cm). The values could be accounted for by the known composition of negatively-charged phospholipids in the SR. The acidic Ca2+ binding proteins were shown to make at most a small contribution to the surface charge, indicating that their charge must be located at least several tens of A from the membrane surface. The experiments gave evidence for a Donnan effect on the K+ distribution in the fraction rich in acidic binding proteins. This could be accounted for by the known concentration of acidic binding proteins in this SR fraction. The equilibrium constant for ANS- was shown to be more sensitive to changes in the divalent cation concentration than to changes in the monovalent cation concentration, as predicted by the Gouy-Chapman theory. Use of these findings together with the stopped-flow rapid mixing techniques constitutes a method for rapid and continuous monitoring of changes in ion concentrations in the SR lumen.

Lipid-lipid and lipid-protein interactions in chromaffin granule membranes. A spin label ESR study

The ESR spectra of six different positional isomers of a stearic acid and three of a phosphatidylcholine spin label have been studied as a function of temperature in chromaffin granule membranes from the bovine adrenal medulla, and in bilayers formed by aqueous dispersion of the extracted membrane lipids. Only minor differences were found between the spectra of the membranes and the extracted lipid, indicating that the major portion of the membrane lipid is organized in a bilayer arrangement which is relatively unperturbed by the presence of the membrane protein. The order parameter profile of the spin label lipid chain motion is less steep over the first half of the chain than over the section toward the terminal methyl end of the chain. This 'stiffening' effect is attributed to the high proportion of cholesterol in the membrane and becomes less marked as the temperature is raised. The isotropic hyperfine splitting factors of the various positional isomers display a profile of decreasing polarity as one penetrates further into the interior of the membrane. No marked differences are observed between the effective polarities in the intact membranes and in bilayers of the extracted membrane lipids. The previously observed temperature-induced structural change occurring in the membranes at approx. 35 degrees C was found also in the extracted lipid bilayers, showing this to be a result of lipid-lipid interactions and not lipid-protein interactions in the membrane. A steroid spin label indicated a second temperature-dependent structural change occurring in the lipid bilayers at lower temperatures. This correspond to the onset of a more rapid rotation about the long axis of the lipid molecules at a temperature of approx. 10 degrees C. The lipid bilayer regions probed by the spin labels used in this study may be involved in the fusion of the chromaffin granule membrane leading to hormone release by exocytosis.

Electrokinetic properties of synaptic vesicles and synaptosomal membranes

Using the technique of electrophoretic light scattering, we have measured the electrophoretic mobilities of synaptic vesicles and synaptosomal plasma membranes isolated from guinea-pig cerebral cortex. The electrophoretic mobility of synaptic vesicles is slightly greater than that of synaptosomal plasma membranes. Ca+2 and Mg+2 reduced the mobility of both species to the same extent at physiologically relevant concentrations (0-1 mM) and near-physiologic ionic strength. The extent of the reduction was not large (approximately 6% for synaptic vesicles in the presence of 100 mM KCl) at 1 mM divalent cation concentrations. At concentrations of approximately 2 mM and higher, Ca+2 reduced the mobility of synaptic vesicles more than did Mg/2. A similar but much smaller effect was observed in the case of synaptosomal plasma membranes. The addition of 1 mM Mg+2-ATP had no effect upon synaptic vesicle mobility either in the presence or absence of the ionophores nigericin or valinomycin. These data, together with earlier work (Siegel et al., 1978, Biophys. J. 22:341-346), demonstrate that substantial reduction of the average electrostatic surface charge density is not the most important role of divalent cations in promoting close approach of secretory granules and secretory cell membranes, and that it is certainly not the Ca+2-specific step in exocytosis.