Fluorescence spectroscopic investigation of effect of excipients on epidermal barrier and transdermal systems

Excipients are often used in transdermal formulations to overcome the formidable barrier offered by the epidermis in order to achieve the target flux. In this study we describe the use of frequency-domain fluorescence spectroscopy to characterize the effect of two commonly used excipients, propyleneglycol monolaurate (PGML) and oleic acid on stratum corneum and in a silicone-based transdermal delivery system. Fluorescence lifetime and limiting anisotropy for the probe 1,6-diphenyl-1,3,5-hexatriene in isolated human epidermis were measured as a function of formulation treatment. The drop in lifetime ranged from 0.5 to 2.3 ns, indicative of an increased dielectric constant of the lipophilic barrier exposed to all formulations. This increase is due to increased partitioning of the polar excipients from the delivery system into the stratum corneum. The limiting anisotropy showed a drop of 0.1 in the case of the epidermis exposed to oleic acid formulation and not the PGML formulations, indicative of the different modes of action for these two excipients. The fluorescence data suggested fluidization of the silicone matrix by both oleic acid and PGML. The dynamic fluorescence measurements described in this study are a powerful way to screen formulations while gaining valuable mechanistic insight into the mode of flux enhancement in transdermal formulations.

Characterization of the stratum corneum lipid matrix using fluorescence spectroscopy

Using fluorescence techniques, we studied the dynamics of the lipid bilayer matrix of human stratum corneum (SC) and compared the results with that of distearoyl-phosphatidylcholine (DSPC). We employed a series of 9-anthroyloxy fatty acids (AF) that partitioned into the bilayer, enabling us to evaluate this structure as a function of depth within the lamellae. With AF probes, the re-orientation of the fluorophore is known to be affected by the polarity, hydrogen bonding, and rigidity of the surrounding medium, altering the emission maximum and lifetime in the excited state. In addition, we evaluated quenching, in which iodide collides with the fluorophore, revealing information on the accessibility of the fluorophore located in the bilayer. The emission and lifetime data showed that the reorientation of the fluorophore in SC was more hindered than in DSPC, indicating that SC bilayers were more rigid than DSPC bilayers. Quenching data of both SC and DSPC indicated that the deeper the fluorophore was positioned in the bilayer, the less accessible it was to iodide, pointing to a gradient in accessibility. In addition, the quenching results also showed that the SC is less accessible to iodide than in DSPC. The observed differences in bilayer rigidity and quencher accessibility between the two systems can be explained by differences in lipid composition and hydration. Whereas the DSPC bilayer consists of phospholipids, SC bilayers are composed of more anhydrous lipids like cholesterol and ceramides, which form a tight bilayer packing. In this way SC lipids exist in a relatively anhydrous and rigid environment, forming an effective diffusion barrier to water and ions.

The influence of an electric field on ion and water accessibility to stratum corneum lipid lamellae

PURPOSE

To study ion transport through stratum corneum (SC) lipid lamellae under passive and iontophoretic conditions.

METHODS

Iodide ion transport was measured by fluorescence quenching. Since the process involves diffusion of an iodide ion to the fluorophore located within the SC lamellae, the accessibility of iodide ions was measured. Moreover, the use of anthroyloxy fatty acid probes, provided information as a function of depth within the lamellae.

RESULTS

Fluorescence quenching by iodide ions increased with iontophoretic current density, suggesting increased ion accessibility within the SC lamellae. In addition, at constant current, quenching decreased as the fluorophore was located deeper within the lamellae. This gradient in ion accessibility suggests that more iodide is found near the head-group than near the core of the SC lipid lamellae. Results obtained in the absence of iodide also show increased water accessibility during iontophoresis.

CONCLUSIONS

These results show that in the presence of an applied electric field the SC lipid lamellae interior becomes more accessible to water and ions. These results imply that during iontophoresis, ion and water transport through human skin is associated, at least in part, with the SC lipid lamellae.

Activation of NMDA and muscarinic receptors induces nur-77 mRNA in hippocampal neurons

Using in situ hybridization, we show that the expression of the immediately-early gene, nur-77, increases rapidly and transiently in hippocampal neurons following activation of muscarinic and N-methyl-D-aspartate receptors and after hippocampal long-term potentiation (LTP). On the basis of these results we hypothesize that nur-77 may coordinate with other transcription factors to initiate subsequent changes in hippocampal gene expression associated with plasticity-related activation of N-methyl-D-aspartate and muscarinic receptors.

Effect of PGML excipient mixture in a transdermal system on the in vitro transport of estradiol across skin

PURPOSE

To measure the effect of a combination of excipients from a silicone based pressure sensitive adhesive (PSA) on drug transport across skin.

METHODS

Partitioning of propylene glycol monolaurate (PG-ML) from silicone PSA and a solution formulation into the stratum corneum (SC) was measured using radiolabeled PG-ML. Transport of a model drug, estradiol, as well as PG-ML across skin were measured in vitro using heat separated epidermis from human cadaver skin.

RESULTS

The PG-ML partitioning into SC showed a saturation and was independent of the formulation. The local dielectric of the lipid bilayers of the SC showed an increase as a result of PG-ML uptake, as determined by the decrease in fluorescence lifetime of a lipophilic probe incorporated into the SC. However, there was no alteration of lipid packing in SC.

CONCLUSIONS

The PG-ML and estradiol transport showed a good correlation over 3 days, suggesting that the two species are co-transported across the epidermis.

Fluorescence spectroscopy of 9-anthroyloxy fatty acids in solvents

A series of anthroyloxy fatty acid (AF) fluorescent probes, with the anthroyloxy group covalently linked at various positions along the alkyl chain, were studied in solvents exhibiting a wide range of polarity and hydrogen-bond donor (Hd) and acceptor (Ha) ability. These probes were sensitive to the solvent polarity as reflected by the Stokes' shift observed in steady state fluorescence. As determined by multi-linear regression analysis of the observed Stokes' shift and solvent parameters, such as orientation polarizability (delta f), Hd and Ha of the solvents, all the probes were sensitive to the Hd of solvents but were not affected by the Ha of solvents except the 2-AF. Due to the proximity of the polar headgroup to the fluorophore, it appears that some intramolecular hydrogen-bonding is present in 2-AF, an interaction that is sensitive to the pH of the solvent, but is less sensitive to the Hd and Ha of the solvents. Fluorescence lifetimes measured by the multi-frequency phase-modulation technique in mixtures of hexane and ethanol reflect a modified Stern-Volmer behavior suggesting the second solvent, ethanol, specifically interacts with the probe, in part through collisional quenching. Also, the lifetime data were sensitive to very low concentrations of the second solvent (0-0.1%, by vol.). The results from this study provide insight into the intrinsic differences between the different AF positions that must be taken into consideration while investigating the dynamics of lipid bilayer systems. Moreover, this study illustrates the utility and resolving power of lifetime based measurements needed for the interpretation of heterogeneous biophysical environments.

Brain-derived neurotrophic factor expression after long-term potentiation

Long-term potentiation (LTP) of perforant-path dentate granule cell synapses, in awake rats, was followed by a time-dependent expression of brain-derived neurotrophic factor (BDNF) mRNA in dentate granule cells. This BDNF expression was blocked by the N-methyl-D-aspartate (NMDA) antagonist dizocilpine maleate (MK-801), which also blocked LTP induction, and by sodium pentobarbital, which shortens LTP persistence. These results suggest that BDNF may participate in the NMDA-receptor mediated cascade of events that result in LTP stabilization.

Fluorescence anisotropy studies on the interaction of the short chain n-alkanols with stratum corneum lipid liposomes (SCLL) and distearoylphosphatidylcholine (DSPC)/distearoylphosphatidic acid (DSPA) liposomes

Previously, the action of the short chain n-alkanols (from C1 to C5) and isopropanol as possible enhancers on the transport of lipophilic and polar/ionic permeants across hairless mouse skin was investigated. In the present study, the steady-state fluorescence anisotropy was measured as a means of estimating the changes in fluidity caused by the n-alkanols at different depths in the stratum corneum lipid liposomes (SCLL). Some selected experiments with the distearoylphosphatidylcholine (DSPC)/distearoylphosphatidic acid (DSPA) liposomes were performed for relative comparisons. The effects of the n-alkanols on polarity sensitive parameters such as fluorescence lifetimes, fluorescence quantum yield ratios, and emission maxima were studied in the SCLL. The polarity of the bilayer decreased as the fluorescent probe was placed closer to the bilayer center and the n-alkanols did not alter this gradient. Assessment of the depth-dependent effects of the n-alkanols using SCLL showed that most of the significant changes in fluidity induced by the n-alkanols were observed at intermediate depths (C2-C9) and there was little or no increase in fluidity in the deep hydrophobic region close to the bilayer center. These results suggest that the short chain n-alkanols work as effective 'fluidizing' agents at the intermediate depths (C2-C9) in the bilayer.

Inhibition of antigen-induced airway and cutaneous responses by heparin: a pharmacodynamic study

We have previously shown that heparin attenuates the acute bronchoconstrictor response and immediate cutaneous reaction (ICR) to antigen in allergic sheep. In the present investigation, we studied the pharmacodynamics of the antiallergic action of heparin. Specific lung resistance (sRL) was measured in eight sheep, allergic to Ascaris suum antigen, before and 5 min after inhalation challenge with the antigen. On different experiment days, antigen challenge was repeated after pretreatment with 1) aerosol heparin (1,000 U/kg) administered < or = 20 min, 6 h, 12 h, and 24 h and 2) intravenous heparin (1,000 U/kg) administered < or = 20 min, 1 h, 6 h, and 12 h before antigen challenge. sRL increased by 374 +/- 116% (SE) above baseline with antigen alone. Both aerosol and intravenous heparin attenuated the antigen effects on sRL in a time-dependent fashion. Prolonging the lag time between pretreatment and antigen challenge decreased the inhibitory effect of aerosol heparin; delta sRL was 31 +/- 29, 99 +/- 38, 142 +/- 40, and 306 +/- 60% for < or = 20-min, 6-h, 12-h, and 24-h pretreatment protocols, respectively. In contrast, prolonging the lag time increased the inhibitory effect of intravenous heparin: delta sRL was 246 +/- 64, 66 +/- 26, and 76 +/- 32% for < or = 20 min, 1 h, and 6 h, respectively. In seven additional sheep pretreatment with intravenous heparin (1,000 U/kg) attenuated the ICR also in a time-dependent manner; the inhibitory effect of heparin on ICR to antigen was enhanced 60% by increasing the heparin pretreatment interval from 20 to 60 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between sphingosine and cholesteryl sulfate in epidermal lipids

Free sphingosine, a material with multiple and potent biological activities, is known to occur in high concentration in mammalian epidermis. In the present study, thin-layer chromatography showed that in lipid extracts of human and pig stratum corneum, sphingosine forms a relatively stable compound with endogenous cholesteryl sulfate. NMR spectrometry of sphingosine and its hydrochloride, sulfate, and mixtures with cholesteryl or dodecyl sulfate showed that interaction with the organic sulfates constituted simple salt formation. Under neutral or weakly acidic conditions, such salts were only slightly dissociated and migrated on thin-layer chromatograms as discrete compounds. Thin-layer chromatography revealed undissociated salt formation between several long-chain bases and organic sulfates, and showed that their interaction is stoichiometric. However, undissociated salts were not formed between long-chain bases and fatty acids or phosphatidic acid. Undissociated salt formation may therefore be specific for organic bases and sulfates. It was concluded that the free sphingosine in the stratum corneum may be present as its cholesteryl sulfate salt and in this form be unavailable for permeation into the viable epidermal cells.

Partition of sodium dodecyl sulfate into stratum corneum lipid liposomes

Synthetic detergents produce deleterious effects on human skin as the result of being taken up by the stratum corneum (SC). The present study aimed to determine to what extent a typical detergent enters the SC lipid lamellae, and what effect this might have on the physical properties of the lipids. These effects were studied in large unilamellar liposomes prepared from SC lipids (50% by weight of epidermal ceramides, 28% cholesterol, 17% free fatty acids, and 5% cholesteryl sulfate) by extrusion through successive polycarbonate filters of decreasing pore size, finally 400 nm. Freeze-fracture electron microscopy and light-scattering particle size analysis indicated a uniform liposome diameter averaging 230 nm. Partitioning of sodium dodecyl sulfate (SDS) into the lipid phase from aqueous buffer solutions was measured using the SC lipid liposomes and [U-14C]SDS. The partition coefficient was 416, 450, and 588 at pH 8.5 and 524, 507, and 807 at pH 7 for three different concentrations (0.1%, 0.02%, and 0.004%) of SDS. This high degree of partitioning into the liposomes is consistent with the high level of SDS partitioning seen in full SC. At the maximum, the SDS represented 18% of the liposomal lipids. Preparation of stable liposomes from SC lipids to which 10% or 20% of SDS had been added confirmed the ability of the liposomes to survive these high concentrations of surfactant. The permeability of the liposomes was enhanced as a result of SDS partitioning into the bilayers, as measured by the increased release of trapped [U-14C]glucose from these vesicles, and by their increased permeability to water in osmotic shock experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Lamellar structures formed by stratum corneum lipids in vitro: a deuterium nuclear magnetic resonance (NMR) study

Hydrated lipid mixtures consisting of stratum corneum ceramides, cholesterol, specifically deuterated palmitic acid, and cholesteryl sulfate were investigated by solid-state 2H NMR spectroscopy at different temperatures. The mole ratio of cholesterol to ceramides was varied from 1 to 0. 2H NMR spectra from these mixtures showed powder patterns with quadrupolar splittings smaller than those obtained from control mixtures containing dipalmitoylphosphatidylcholine (DPPC) instead of the ceramides. This result is attributed to the rigid amide group of the ceramides, with a planar configuration, which could prevent close packing of the alpha-methylenes of the acyl chains. There was a gradual loss of symmetry in the powder pattern as the amount of cholesterol was decreased and the amount of ceramides (or DPPC) was increased concomitantly. The loss was more pronounced in the ceramide-containing samples. This phenomenon is interpreted as a decrease in the axial reorientation rate of the alpha-deuterated palmitic acid in the bilayers, presumably caused by the increased hydrogen bonding resulting from the high amount of hydroxyl-bearing ceramides. Spectra obtained at temperatures above 60 degrees C indicated the formation of a hexagonal phase (HII) by the ceramide-containing mixtures. Spectra of the omega-deuterated palmitic acid in the mixture containing 76 mol% ceramides and no cholesterol indicated phase separation into a more rigid phase and a more mobile phase in the temperature range of 25 to 60 degrees C. The bilayer configuration of lipids at 25 degrees C was confirmed by thin-section electron microscopy.

Structures formed by epidermal lipids in vitro

The extracellular membranous structures of stratum corneum constitute the major barrier to percutaneous penetration. These intercellular lipid lamellae are devoid of phospholipids and are unique in their lipid composition. In this article, some of the recent investigations of the structures formed by the stratum corneum lipids in vitro are discussed with relevance to the barrier function of epidermis. These studies have provided some insight into the dramatic physical changes that occur in the membranous structures of the epidermis during the formation of the stratum corneum.

Deuterium NMR investigation of polymorphism in stratum corneum lipids

The intercellular lipid lamellae of stratum corneum constitute the major barrier to percutaneous penetration. Deuterium magnetic resonance and freeze-fracture electron microscopic investigation of hydrated lipid mixtures consisting of ceramides, cholesterol, palmitic acid and cholesteryl sulfate and approximating the stratum corneum intercellular lipid composition, revealed thermally induced polymorphism. The transition temperature of bilayer to hexagonal transition decreased as the ratio of cholesterol to ceramides in these mixtures was lowered. Lipid mixtures in which the stratum corneum ceramides were replaced by synthetic dipalmitoylphosphatidylcholine did not show any polymorphism throughout the temperature range used in the present study. The ability of the ceramide-containing samples to form hexagonal structures establishes a plausible mechanism for the assembly of the stratum corneum intercellular lamellae during the final stages of epidermal differentiation. Also, the bilayer to hexagonal phase transition of these nonpolar lipid mixtures could be used to enhance the penetration of drugs through skin.

Interaction between corneocytes and stratum corneum lipid liposomes in vitro

Small unilamellar vesicles were made from a mixture of epidermal ceramides (45%), cholesterol (35%), free fatty acids (15%) and cholesteryl sulfate (5%). Isolated corneocytes prepared from pig epidermis were added to the liposomes and the interaction between corneocytes and liposomes was studied by (1) thin-section electron microscopy and (2) monitoring the release of aqueous contents of the vesicles by following the fluorescence intensity of carboxyfluorescein entrapped in the vesicles. The vesicles adsorbed readily onto the corneocytes and slowly transformed into lamellar sheets. Enhanced fluorescence intensity indicated a corneocyte-induced membrane fusion process that resulted in the release of aqueous contents of the vesicles. The results suggest a cohesive role for the corneocyte cell envelope, which consists of a monomolecular layer of lipids covalently bound to the outside of a cross-linked protein envelope. This may be one of the major factors in the reassembly of extruded membranous disks into lamellar sheets which occurs during the final stages of epidermal differentiation.

Preparation of model membranes for skin permeability studies using stratum corneum lipids

Liposomes were prepared from stratum corneum lipids consisting of epidermal ceramides (55% by weight), cholesterol (25%), free fatty acids (15%), and cholesteryl sulfate (5%). Multiple lamellae were formed by air-drying the liposomal suspensions on hydrophilic filter disks, and water permeabilities through these filter-supported lamellae were measured using a diffusion cell. Ultrastructure of the lipid lamellae was characterized by scanning and thin-section electron microscopy. Water flux data and ultrastructure of the model membranes are discussed in relation to stratum corneum, the horny layer of the epidermis that constitutes the epidermal barrier.

Pharmacological profile of MDL 26,024GO: a novel antiasthmatic agent

MDL 26,024GO was shown to be an orally absorbed mediator release inhibitor in the rat PCA and PPA tests. In addition, the compound was shown to both elicit and inhibit elicitation of the Bezold-Jarisch reflex in the dog. MDL 26,024GO also significantly reduced Ascaris-induced changes in pulmonary mechanics in cynomolgus monkeys. The compound inhibited both early and late phase antigen induced-changes in Ascaris-sensitive sheep, as well as the increased airway hyperreactivity which normally follows antigen challenge. These results suggest the compound may have therapeutic potential in the treatment of asthma.

Should the patella be resurfaced in total knee arthroplasty? Efficacy of patellar resurfacing

To assess the long-term efficacy of patellar resurfacing, 100 knees were evaluated in 84 patients. The operations were performed between 1978 and 1982. The follow-up period ranged from 60 to 103 months. The diagnosis was degenerative joint disease (DJD) in 83%, rheumatoid arthritis in 12%, and miscellaneous in 5% of the knees. The implant (47 knees) and nonimplant (53 knees) groups were comparable with respect to age, body size, and length of follow-up period. The analysis revealed equivocal results. Considering all diagnostic categories combined, rest pain was marginally better in the resurfaced group (p = 0.04), but this difference resulted from an unequal distribution of subjects between mild and zero pain categories. Pain with walking, maximum walking distance, ability to climb stairs and rise from a chair, active arc of motion, extensor lag, and quadriceps strength were similar in the two groups. When the DJD group was considered separately, no significant difference emerged. There was little evidence to support a recommendation for routine patellar resurfacing in total knee arthroplasty.

Effect of epidermal acylglucosylceramides and acylceramides on the morphology of liposomes prepared from stratum corneum lipids

Epidermal acylglucosylceramides (AGC) and acylceramides (AC) cause aggregation and stacking of stratum corneum lipid liposomes formed from a lipid mixture containing epidermal ceramides (40%), cholesterol (25%), palmitic acid (25%), and cholesteryl sulfate (10%). This demonstrates the ability of these sphingolipids to hold adjacent bilayers in close apposition and their roles in the assembly of lamellar structures in the epidermis. However, AGC and AC in their hydrogenated form also caused aggregation and stacking of the stratum corneum lipid liposomes. This throws into doubt the proposed structural specificity of linoleate in the function of AGC and AC as molecular rivets in the assembly of the epidermal lamellar granules and the stratum corneum intercellular lamellae, respectively.

Fusion patterns of liposomes formed from stratum corneum lipids

Stratum corneum lipid liposomes formed from epidermal ceramides (40%), cholesterol (25%), palmitic acid (25%), and cholesteryl sulfate (10%), when exposed to hypertonic medium, form flattened liposomes. Epidermal acylglucosyl-ceramides (AGCs) and acylceramides (ACs) cause aggregation and fusion of these flattened vesicles. This could serve as a model to study (a) the fusion of membranous disks in the intercellular space of the stratum corneum and (b) the roles of AGCs and ACs in the assembly of lamellar structures in the epidermis.

The role of calcium antagonists in bronchial reactivity

Calcium (Ca) ions play an important pathophysiologic role in allergic reactions. Thus, mediator release from mast cells, synthesis of some newly formed chemical mediators, airway smooth muscle contraction, and nerve-impulse conduction are all dependent on the availability and flux of Ca ions. It is likely, therefore, that Ca antagonists would modify allergic bronchoconstriction. In vitro, Ca antagonists have been demonstrated to inhibit mediator release (histamine, slow-reacting substance of anaphylaxis, and platelet-activating factor) from mast cells, passively sensitized human lung fragments, and leukocytes. Ca antagonists have also been found to inhibit synthesis of leukotrienes in rat lungs and cyclooxygenase products in sheep, possibly by inactivating phospholipase A2 and/or 5-lipoxygenase. In addition, nifedipine, verapamil, and gallopamil have demonstrated inhibition of airway smooth muscle contractions to histamine, carbachol, and antigen in various species. In vivo effects of Ca antagonists are variable, depending on the species, experimental design, the stimulus or the agonist, and the Ca antagonist used. Animal studies have demonstrated the inhibition of histamine, methacholine, citric acid, and prostaglandin F2 alpha-induced bronchoconstriction in guinea pigs and dogs by intravenous nifedipine. In contrast, verapamil inhibited antigen-induced bronchoconstriction in allergic sheep without any effect on histamine- and carbachol-induced responses. Ca antagonists (nifedipine and verapamil) have been of limited value in human subjects and generally have no significant bronchodilating activity. Both nifedipine and verapamil prevent the exercise-induced asthma and partly attenuate the histamine and methacholine-induced bronchoconstriction. Oral nifedipine is generally more effective than oral verapamil against acute antigen-induced bronchoconstriction; however, this efficacy may be limited by systemic side effects. Inhaled Ca antagonists may be more effective and free of systemic side effects, as demonstrated by greater efficacy of inhaled verapamil. A new Ca antagonist, gallopamil (a methoxy derivative of verapamil), is being investigated as an aerosol, and preliminary studies in animals and humans have found it fourfold to seventeenfold more potent than verapamil. In sheep, gallopamil has been found to attenuate histamine, carbachol, and platelet-activating factor-induced bronchoconstriction, as well as to inhibit early and late-phase allergic airway responses. Studies in human subjects have also demonstrated the inhibition of antigen-induced bronchoconstriction by inhaled gallopamil, with efficacy comparable or better than cromolyn sodium.(ABSTRACT TRUNCATED AT 400 WORDS)

Essential fatty acids and epidermal integrity

The intercellular spaces of the stratum corneum contain multilamellar lipid sheets derived from the extruded contents of lamellar granules. In the absence of linoleic acid, lamellar granules appear empty, and only fragmentary extracellular sheets are found. This defective differentiation is attributable to substitution of oleate for linoleate in O-acylsphingolipids. Normally, linoleate is ester-linked to 30- to 34-carbon omega-hydroxyacids, which, in turn, are amide-linked to sphingosine. Acylglucosylceramides, bearing a beta-D-glucosyl moiety on the sphingosine, may provide the driving force for lamellar granule assembly. The omega-hydroxyacyl chains are long enough to span a lipid bilayer, while the linoleate inserts into an adjacent bilayer. This interaction could promote assembly of lamellar granules. It has also been proposed that acylceramides may stabilize the extracellular sheets by a similar mechanism. In addition, the horny cell has been found to possess a covalently bound lipid envelope consisting principally of omega-hydroxyacylsphingosines derived from O-acylsphingolipids.

Glucosylsterol and acylglucosylsterol of snake epidermis: structure determination

The structures of two classes of glycolipids, acylglucosylsterol and glucosylsterol, from snake epidermis have been determined by chemical, spectroscopic, and gas-liquid chromatographic methods. The acylglucosylsterol consists of a glucose molecule attached to cholesterol and an ester-linked fatty acid on carbon 6 of the sugar. The major ester-linked fatty acids are palmitic, stearic, and oleic. The glucosylsterol consists of glucose attached to cholesterol. These unusual glycolipids may play a role in epidermal barrier function in snakes and it is concluded that these sterol glycosides may remain in the reptiles as a vestige of a more primitive barrier-forming mechanism.

Skin lipids: an update

The stratum corneum lipids, responsible for the epidermal water barrier, consist principally of ceramides, cholesterol, cholesteryl sulfate, and free fatty acids. These lipids are arranged in multiple intercellular lamellae that provide an efficient water barrier because of the crystalline array of the straight and predominantly saturated lipid chains. Interlamellar linkages provided by lipids based on 30-carbon omega-hydroxyacids may be responsible for holding together the intercellular lamellae as well as for assembly of the lamellar granules of the granular cells. The normally ordered exfoliation of corneocytes as they arrive at the surface seems to require hydrolysis of the cholesteryl sulfate to free cholesterol. The sebaceous glands secrete continuously, producing sebum that consists predominantly of triglycerides, wax esters, and squalene. High rates of sebum production per sebocyte result in low levels of linoleate in the sebaceous esters, subjecting the follicular epithelium to essential fatty acid deficiency and the characteristic hyperkeratosis that results in comedo formation. Suppression of sebum production by drugs elevates sebum linoleate concentration and relieves follicular hyperkeratosis. Thus, sebum continues to be a prime suspect in the crime of acne. Low levels of sebaceous gland activity are not correlated with the occurrence of dry skin.

Stratum corneum lipid liposomes: calcium-induced transformation into lamellar sheets

The epidermal water barrier in mammalian stratum corneum is formed of broad lamellar sheets of lipids consisting principally of ceramides (40%), cholesterol (25%), cholesteryl sulfate (10%), and free fatty acids (25%). Such lipid mixtures have been shown to form lipid bilayers in the form of small, unilamellar liposomes when sonicated at 80 degrees C in water containing Tris buffer and 100 mM NaCl. In the present study it is shown that such liposomes are slowly transformed into large unilamellar liposomes and then into broad lamellar sheets after the addition of stoichiometric amounts of calcium chloride. The presence of free fatty acids was a necessary condition for this calcium-induced fusion. These observations may provide a useful analogy for the transformation of flattened liposomes into broad lamellar sheets that occurs during transition of epidermal granular cells into corneocytes.

Preparation of liposomes from stratum corneum lipids

Mammalian stratum corneum contains multiple intercellular lipid bilayers that constitute the epidermal water barrier. Unlike all other biologic membranes, the epidermal lamellae do not contain phospholipids, as a result of which the ability of the stratum corneum lipid mixture to form bilayers has been questioned. In the present study, a lipid mixture containing only epidermal ceramides (40%), cholesterol (25%), palmitic acid (25%), and cholesteryl sulfate (10%), approximating the composition of stratum corneum lipids, formed stable, unilamellar liposomes when sonicated at 80 degrees C in buffer containing 100 mM NaCl, 5 mM Tris, and 1 mM EDTA at pH 7.5. The size and form of the liposomes were studied by both freeze fracture and negative staining electron microscopy. Lipid mixtures from which either the palmitic acid or the cholesteryl sulfate were omitted were still capable of forming similar liposomes, but a mixture of ceramides and cholesterol, or ceramides alone, were incapable of forming liposomes. The results indicate that lipid mixtures similar to those found in stratum corneum are capable of forming bilayers at physiologic pH.

Lipids of chicken epidermis

The lipids from chicken epidermis were analyzed by a combination of quantitative thin-layer and gas-liquid chromatography and by chemical and spectroscopic methods. The lipid groups present included wax diesters (34%), triglycerides (32%), sterols (11%), phospholipids (11%), nonphosphorus-containing sphingolipids (3%), beta-D-glucosylsterols (3%), 6-O-acyl-beta-D-glucosylsterols (2%), steryl esters (1%), cholesteryl sulfate (1%), and free fatty acids (1%). The major phospholipids were phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin, and the sphingolipids included ceramides, glucosylceramides, O-acylceramides, and O-acylglucosylceramides. Glucosylsterols and acylglucosylsterols have not been found in mammalian skin, and may be relevant to the evolutionary history of the epidermal water barrier. The wax diesters contained mainly 16-, 18-, and 20-carbon saturated fatty acids esterified to 20- through 24-carbon threo and erythro 2,3-diols, while the chicken epidermal triglycerides contained some very long-chain (26-40 carbon) saturated fatty acids. These wax diesters and unusual triglycerides may be of significance in human health.

Migration of pollutants in groundwater. III. Numerical dispersion reduction with cartesian coordinates

The partial differential equation governing the movement of a decomposing pollutant undergoing 2-dimensional flow in a saturated aquifer is examined. The analytical solution of the equation is usually not possible, and use of mesh numerical integration techniques causes excessive numerical dispersion to arise from the advection term. We apply two asymmetrical upwind formulas to approximate the advection term. These markedly reduce numerical dispersion without requiring the use of coordinate systems obtained by conformal mapping. Flow within a right angle and flow toward a sink in an otherwise uniform field are analyzed to illustrate the methods.

Skin surface lipids of the mink

Skin surface lipids from mink (Mustela vison) were collected in acetone and analyzed by thin-layer chromatography and gas chromatography. The principal components were wax monoesters (92%), cholesteryl esters (5%), free fatty acids (1%), fatty alcohols (1%) and cholesterol (1%). The fatty acids and alcohols contained in these lipids were composed principally of homologous series of straight chained omega 7-unsaturated structures (C16-C24), accompanied by lesser proportions of homologous series of saturated (C14-C22) and omega 9-unsaturated (C18-C22) structures.

Uropygiols: confirmation of structure by proton magnetic resonance

Lipids were extracted from excised uropygial glands of domestic chickens and the wax diesters were isolated by preparative thin-layer chromatography (TLC). The diesters were hydrolyzed and the liberated diols were resolved by boric acid TLC into two fractions. These were investigated by proton magnetic resonance at 360 MHz of the free diols and of their acetonide derivatives. The results showed that the cis and trans acetonides, formed from the erythro and threo isomers of the diols, respectively, could be distinguished by the degree of magnetic nonequivalence of the two acetonide methyl groups in each molecule. On the presumption that the cis isomer should show the greater nonequivalence of the methyl groups, this configuration was assigned to the acetonides of these diols which had the lesser TLC mobility on boric acid/silica gel. This agrees with the assignment of configuration made by earlier workers on the basis of the relative TLC mobility of the diol isomers on boric acid/silica gel, but was contrary to a previous assignment based on gas-liquid chromatographic (GLC) retention times. We conclude that the erythro isomers of the diols are characterized by lower mobility on boric acid TLC, as well as on silica gel TLC, and form acetonides that have longer retention times on GLC, and greater nonequivalence of the acetonide methyl groups in the NMR spectrum, than do the acetonides of the threo isomers.

Linoleate-rich acylglucosylceramides of pig epidermis: structure determination by proton magnetic resonance

The structure of the linoleate-rich acylglycosylceramides isolated from pig epidermis has been reinvestigated. Gas-liquid chromatographic analysis of the alditol acetates produced from the sugar component indicated that 90% of the hexose is glucose while the remaining 10% is galactose. The predominance of the beta-D-glucosyl group was confirmed by 360 MHz proton magnetic resonance spectroscopy. The magnetic resonance method was also used to prove that the ester-linked linoleic acid is actually attached to the omega-hydroxyl group of the long chain hydroxyacid, not to the sugar as had been reported previously. A key spectral feature supporting this new structural assignment was a triplet at 3.82 ppm, which indicates methylene protons between another methylene and an ester linkage. After saponification, this signal moved to 3.33 ppm, a chemical shift expected for a methylene bearing a free hydroxyl group. Furthermore, all of the sugar ring protons could be accounted for both before and after acetylation. No evidence was found to suggest that an ester is attached to the sugar ring in the native material. It is concluded that the principal porcine epidermal acylglycosylceramide is 1-beta-D-glucosyl-N-(omega-O-linoleoyl)-triacontanoylsphingosine.

Differences between inhaled and intravenous carbachol in detecting O3-induced airway effects

The response of specific lung resistance (SRL) to inhalation of 5 and 10 mg/ml carbachol was compared with the response of SRL to intravenous infusion of 2 and 5 micrograms/kg carbachol before and after a 2-day exposure to 0.5 ppm ozone (O3) in eight conscious sheep. Airway reactivity was defined as the slope of the dose-response curve and airway sensitivity as the largest increase in SRL after carbachol challenge. O3 exposure did not alter mean airway reactivity or mean airway sensitivity as determined by inhalation challenge. In contrast, O3 exposure significantly increased mean airway reactivity by 34% (P less than 0.01) and mean airway sensitivity by 31% (P less than 0.01) as assessed by intravenous challenge. The failure of O3 exposure to enhance responsiveness to inhaled carbachol may have been related to decreased airway penetration of the aerosol, possibly due to mucus hypersecretion. However, O3 exposure may have had a direct effect on the airway smooth muscle, thereby explaining the increased response to infused carbachol.