Cholesterol modifies water concentration and dynamics in phospholipid bilayers: a fluorescence study using Laurdan probe.

The effect of cholesterol on the gel, the liquid-crystalline, and mixed phospholipid phases has been studied using the fluorescence properties of 2-dimethylamino-6-lauroylnaphthalene (Laurdan). Laurdan sensitivity to the polarity and to the dynamics of its environment reveals that cholesterol affects phospholipid bilayers in the gel phase by expelling water and by increasing the amount of dipolar relaxation. In the liquid-crystalline phase, the effect of cholesterol is a reduction of both water concentration and amount of dipolar relaxation. Detailed studies of Laurdan excitation and emission spectral contours as a function of cholesterol concentration show that there are some cholesterol concentrations at which Laurdan spectral properties changes discontinuously. These peculiar cholesterol concentrations are in agreement with recent observations of other workers showing the formation of local order in the liquid-crystalline phase of phospholipids upon addition of phospholipid derivatives of pyrene. A local organization of phospholipids around cholesterol molecule seems to be produced by the presence of peculiar concentrations of cholesterol itself. This local organization is stable enough to be observed during the excited state lifetime of Laurdan of approximately 5-6 ns.

Influence of cholesterol on phospholipid bilayers phase domains as detected by Laurdan fluorescence.

Coexisting gel and liquid-crystalline phospholipid phase domains can be observed in synthetic phospholipid vesicles during the transition from one phase to the other and, in vesicles of mixed phospholipids, at intermediate temperatures between the transitions of the different phospholipids. The presence of cholesterol perturbs the dynamic properties of both phases to such an extent as to prevent the detection of coexisting phases. 6-Lauroyl-2-dimethylaminopahthalene (Laurdan) fluorescence offers the unique advantage of well resolvable spectral parameters in the two phospholipid phases that can be used for the detection and quantitation of coexisting gel and liquid-crystalline domains. From Laurdan fluorescence excitation and emission spectra, the generalized polarization spectra and values were calculated. By the generalized polarization phospholipid phase domain coexistence can be detected, and each phase can be quantitated. In the same phospholipid vesicles where without cholesterol domain coexistence can be detected, above 15 mol% and, remarkably, at physiological cholesterol concentrations, > or = 30 mol%, no separate Laurdan fluorescence signals characteristic of distinct domains can be observed. Consequences of our results on the possible size and dynamics of phospholipid phase domains and their biological relevance are discussed.

Absence of lipid gel-phase domains in seven mammalian cell lines and in four primary cell types.

Fluorescence properties of 6-lauroyl-2-dimethylaminonaphthalene (Laurdan) are used to explore gel and liquid-crystalline phase domains coexistence in membranes of various cell types and in erythrocyte ghosts. Experiments and simulations were performed using liposomes composed of equimolar gel and liquid-crystalline phases in the absence and in the presence of 30 mol% cholesterol. In this model system two distinct coexisting phases can be easily recognized in the absence of cholesterol. When cholesterol is added to this phospholipid mixture, Laurdan parameters characteristic of the gel and of the liquid-crystalline phase are no longer resolvable. Coexisting domains of gel and liquid-crystalline phase were not detected in any of the examined cell membranes as judged by Laurdan excitation and emission Generalized Polarization (GP) spectra. Both in liposomes and in cell membranes, the behaviour of GP values as a function of excitation and emission wavelength corresponds to a homogeneous liquid-crystalline phase, despite the absolute GP values being relatively high, closer to the values observed in gel phase phospholipids. The presence of cholesterol appears to be the major cause for the homogeneity of phospholipids' dynamical properties in natural membranes, properties that appear close to the liquid-ordered phase state, defined to describe model systems with cholesterol concentration > or = 30 mol%.

Congenital basilar impression: correlated neurological syndromes.

A series of 8 cases operated on for symptomatic basilar impression associated with occipitalization of the atlas is reported (with or without atlantoaxial dislocation). Symptoms of onset (such as the frequent association between nuchal pain and vertigo) are emphasized and analyzed in relation to the pathogenetic mechanism that underlies the multiform symptomatology of the basilar impression. The diagnostic workup for basilar impression foresees X-rays, magnetic resonance imaging and computed tomography. The most important diagnostic problem is that of considering the possible existence of such a pathology in the presence of very common symptoms such as nuchal pain and vertigo. The surgical treatment has certainly been useful both to improve and to stabilize the symptomatology mainly when there is atlantoaxial dislocation. In fact in these cases the symptomatology is more severe and progressive for the alteration of the transverse ligament of the atlas secondary to abnormal mechanical stimuli.

Warning signs in subarachnoid hemorrhage: a cooperative study.

The present study is part of a survey of subarachnoid hemorrhage cases observed in 16 neurosurgical and 8 neurological departments in Italy between June 1985 and June 1986. Warning signs preceding major hemorrhage were analyzed in 364 patients with intracranial aneurysms confirmed by angiography and reliable clinical history. Seventy-four (20.3%) had warning signs. Clinical features of premonitory warning signs were compared with symptoms of 78 patients without a history of minor leak and clinical grade 1 (according to the criteria of Hunt & Hess) at admission. Symptoms of warning signs are generally clear enough to be considered a misdiagnosis of intracranial aneurysm. Thunderclap headache described as severe, unusual and sudden was the main symptom in every case though the higher frequency of focal of diffuse signs in groups with a correct diagnosis attracted more careful attention in referral and diagnostic-therapeutic management. Improving the identification of minor leak and defining diagnostic strategy are discussed.