Human lens membrane cation permeability increases with age

Parallel studies of the ionic balance and membrane permeability characteristics of normal human lenses were carried out in three countries (USA, England and Italy). Similar age-related changes were found in each laboratory. The lens membrane potential and resistance declined markedly with age while internal Na+ and free Ca2+ increased. There was a concomitant stimulation of Na+ and K+ transmembrane fluxes. These data indicate that in the ageing process there is an increasing contribution to membrane ion traffic from a channel, or channels, that permit Na+, K+ and Ca2+ to pass. The increase in permeability coincides exactly with the increase in optical density that occurs in the ageing human lens.

An independent evaluation of the Lens Opacities Classification System II (LOCS II). The Italian-American Cataract Study Group

The Lens Opacities Classification System II (LOCS II) has been offered for use in clinical studies of cataract. The system uses slit lamp and retroillumination photographic standards to grade lens opacities into classes of increasing severity. The authors evaluated the reproducibility and validity of LOCS II before its possible use in a natural history study of age-related cataract. The authors found excellent inter- and intraobserver reproducibility when the LOCS II standard photographs were used for clinical or photographic gradings of cataract. There was a tendency to underestimate posterior subcapsular cataracts on photographic gradings compared with slit-lamp gradings. The accuracy of the photographic gradings of posterior subcapsular opacities tended to decrease as the severity of coexisting opacities increased.

Diamide alters membrane Na+ and K+ conductances and increases internal resistance in the isolated rat lens

The voltage and conductance of the isolated rat lens were measured using a two-internal-microelectrode technique and the potassium permeability was calculated by applying Goldman theory to 86Rb efflux data. The SH oxidizing agent diamide induces a multiphasic response in lens voltage, conductance and potassium permeability. The initial response (less than or equal to 30 min) to 1 mM diamide consists of a small depolarization (approximately 10 mV) of membrane potential accompanied by a significant decrease in conductance. The 86Rb efflux and permeability data also show an initial decrease. As this initial response is abolished by TEA (20 mM) it is probably due to an inactivation of voltage-sensitive potassium channels. After 30 min exposure to 1 mM diamide both the electrical conductance and the rate of depolarization increase. The 86Rb permeability also increases. Since the conductance increase is abolished by replacing Na+ by methyl glucamine and as it is reduced by amiloride (10(-4) M) the second phase is probably due to the activation of nonspecific cation channels. The third phase is only apparent after prolonged (approximately 12 hr) incubation in 1 mM diamide and consists of a marked increase in the bulk resistance component of the lens impedance. It is suggested that this component arises from an increase in the resistance of the fibre cell gap junctions. This cellular uncoupling may be due to calcium entering the lens through the nonspecific cation channels.

Oxidative cross-linking of fodrin parallels a membrane conductance increase in the mammalian lens

An oxidative cross-linking of the lens spectrin-like protein fodrin was induced by incubating WKY-rat lenses in the presence of the SH-reagent diamide. The oxidation of fodrin was paralleled by an increase in lens membrane conductance. The time relationship between these two events as well as the reversibility of both, achieved by incubating the lens in the presence of dithiothreitol, indicate that normal permeability characteristics of the lens membranes require the integrity of the membrane attached cytoskeleton.

Vitamin A uptake from retinol-binding protein in a cell-free system from pigment epithelial cells of bovine retina. Retinol transfer from plasma retinol-binding protein to cytoplasmic retinol-binding protein with retinyl-ester formation as the intermediate step

We have investigated the steps by which retinol, released from plasma retinol-binding protein (RBP), enters the cells and is accumulated for the most part as a retinyl-ester, only a small fraction of it being present as a complex with cytoplasmic retinol-binding protein (CRBP). For this purpose, we have developed a cell-free system composed of plasma membrane-enriched fractions from bovine retinal pigment epithelium which selectively incorporates exogenous vitamin A when presented as a retinol-RBP complex. Upon incubation in the presence of [3H]retinol-RBP, isolated plasma membrane fractions take up and esterify retinol. A 4-fold reduction of total vitamin A incorporation is observed in conditions which specifically inhibit retinyl-ester formation, thus indicating that the two processes of retinol uptake and esterification are functionally coupled. Evidence is presented that retinol bound to a plasma membrane receptor sharing functional and structural similarities with CRBP is the actual substrate for esterification. Vitamin A accumulation seems to require retinol esterification to allow the recycling of a limited number of free, plasma membrane-associated, retinol receptors. Mobilization of retinol stored as a membrane-bound retinyl-ester is mediated by a membrane-associated hydrolase activity selectively controlled by the level of apo-CRBP which acts as a carrier for the released retinol. Up to 90% of membrane-bound vitamin A is released upon incubation in the presence of apo-CRBP (11 microM) with concomitant formation of retinol-CRBP. The overall process, in which retinol never needs to leave its binding proteins, allows the accumulation of vitamin A in the form of a membrane-bound retinyl-ester and its regulated mobilization as a retinol-CRBP complex.

Cytoskeleton abnormalities in human senile cataract

The cytoskeletal pattern of the most superficial layers (cortex and epithelium) of senile cataractous lenses has been analyzed by PAGE-SDS. While the nuclear type of cataract and age-matched transparent human lenses have superimposable protein patterns, lenses with cortical cataract demonstrate appreciable modifications of their cytoskeletal composition. The most evident change is the decrease of fodrin and the marked reduction or even the absence of the 98 Kd band. Fodrin may be completely removed from the water insoluble fraction (WIF) of cortical cataract by extraction in low ionic strength buffer, a treatment which only partially solubilizes this protein in transparent control lenses.

86-Rb efflux in normal and cataractous human lenses

86Rb efflux has been studied in normal lenses and in human senile cataracts. The rate constant (Ki) of the efflux gradually increases in cataractous lenses with progression of lens damage. Efflux experiments run in the presence of BaC12 suggest that a progressive activation of BaC12 inhibitable efflux routes occurs in cataractous lenses. In the final stages of opacification the ineffectiveness of BaC12 enriched or Ca++ free media on the efflux suggests that a direct disruption of the lens membranes has occurred.

The isolation by self forming gradients of Percoll of plasma membrane enriched fractions from bovine retinal pigment epithelium

A method is described for the preparation of plasma membrane enriched fractions from bovine retinal pigment epithelium (RPE) by means of differential centrifugation followed by the use of self forming gradients of Percoll. A detailed analysis of the distribution of organelle specific markers (nuclei, mitochondria, lysosomes, endoplasmic reticulum, cytosol) in the different fractions is presented. Comparison of 125I-wheat germ agglutinin (WGA) binding with more conventional plasma membrane enzyme markers demonstrates that also in RPE radiolabeled lectin is a specific and extremely sensitive marker to follow quantitatively the distribution of outer cell membranes. Results of 125I-WGA displacement experiments indicate that plasma membranes are mostly (90%) composed of right side out vesicles or sheets. On the basis of 125I-WGA radioactivity the overall recovery of plasma membranes was about 10% and purification over 15 fold. NADH cytochrome c reductase activity, which is shown to be a specific marker for endoplasmic reticulum in retinal pigment epithelium, has been utilized to evaluate microsomal contamination of the plasma membrane preparation.

Amino-acid transport and protein synthesis in the rabbit lens: absence of cryoprobe effect

Recently developed organ-culture techniques have been used to investigate the effects of cryoprobe treatment on rabbit lenses. Uptake of 14C-tyrosine into cryoprobe treated and control lenses was followed for 96 h. Lens proteins were separated by gel filtration and incorporation of label measured in the individual crystallins. The cryoprobe treatment had no measurable effect on lens water, Na+, K+ or Ca++ content, tyrosine transport or the incorporation of tyrosine into the crystallins, during the period of the experiment.

Active and passive rubidium influx in normal human lenses and in senile cataracts

The aim of this study was to investigate the mechanisms responsible for the osmotic imbalance which occurs in a majority of human senile cataracts. Active and passive influx of rubidium has been determined in vitro in human lenses both normal and cataractous. It is concluded that the active transport of cations is on the average normal in senile cataractous lenses. It is possible that the activity of the cation pump is defective in a few cataractous lenses but no direct evidence of this can be given. The results indicate that cryo-extracted lenses may be utilized in this type of study provided that cryo-treatment during surgery is kept at the lowest possible level.

Crystallization and preliminary X-ray data of human plasma retinol-binding protein

Crystals of human plasma retinol-binding protein have been obtained from 4.5 M-NaCl buffered at pH 6.8 with 20 mM-cacodylate. The crystals are trigonal with space group R3 and unit cell dimensions, referred to the hexagonal system, a = b = 104.2 A and c = 74.5 A. The crystals diffract to a resolution of 2.0 A.

Galactosemic enzyme levels in presenile cataracts

Galactose-1-phosphate uridyl transferase and galactokinase activities have been measured in the red blood cells of a group of patients with "idiopathic" presenile cataract and of a group of nondiabetic patients with senile cataract. The activity of both galactosemic enzymes was found to be within the normal range in all the patients with presenile cataract. In the group of patients with senile cataract, galactokinase activity was normal in all 24 subjects examined, and galactose-1-phosphate uridyl transferase activity was moderately reduced in 3 of 14.

Bilateral papilloedema in long term therapy with lithium carbonate

A 31-year-old white woman developed bilateral papilloedema whilst on lithium carbonate therapy for five years because of manic depressive illness. Oedema of the optic disc disappeared after lithium administration had been discontinued. To our knowledge this is the second case of this kind which has been reported in literature.

Interocular transfer of a visual aftereffect in early-onset esotropia

The interocular transfer of the tilt aftereffect was studied in strabismic patients with early-onset esotropia and in controls. Squinting subjects were divided into a microtropia subgroup with anomalous binocular vision and a large-angle esotropia subgroup with total suppression of the deviated eye. Patients with alternating microtropia and large-angle esotropia showed normal or moderately reduced interocular transfer of the aftereffect. Patients with monocular microtropia with medium- or low-degree amblyopia showed a reduced monocular visual aftereffect in the dominant eye and practically no transfer to the nondominant eye. Adaptation of the nondominant eye and transfer of the aftereffect to the dominant eye was normal. The interocular transfer of the tilt aftereffect is a poor indicator of the state of binocularity of a squinting subject, and the clinical usefulness of this test is limited.

Vertical retraction syndrome

A 14-year-old white boy, the second of 5 brothers, showed a congenital deficit of ocular motility in his left eye, characterized by limited elevation, marked retraction of the globe and narrowing of the palpebral fissure during upward gaze. The ocular movements in the other directions of gaze were normal with normal binocular vision and stereopsis. The characteristics of the motility defect and the results of the forced duction test suggest the possibility of an anomaly of superior rectus scleral insertion rather than of an innervational defect or of a muscular fibrosis.

Normal interaction of plasma retinol-binding protein from retinitis pigmentosa with bovine pigment epithelium

The interaction between retinol-binding protein and normal bovine pigment epithelium has been studied with the use of iodinated retinol-binding protein isolated from the plasma of patients with the recessive form of retinitis pigmentosa and of normal subjects. It is concluded that the capacity of the plasma carrier protein to interact with the retinol-binding protein receptor of bovine pigment epithelium is unimpaired in retinitis pigmentosa with autosomal recessive inheritance.

Binding of retinol to isolated retinal pigment epithelium in the presence and absence of retinol-binding protein

Isolated human and bovine pigment epithelium actively binds H3-retinol when vitamin A alcohol is present in the incubation medium bound to human retinol-binding protein. Pigment epithelium is unable to bind retinol present in the incubation solution as the free form, i.e., not bound to its physiologic carrier protein. It is suggested that an interaction between retinol-binding protein and the membranes of pigment epithelial cells is essential for the active transport of retinol into pigment epithelium.

Serum levels of retinol-binding protein in different genetic types of retinitis pigmentosa

The level of retinol-binding protein (RBP) was determined by a single radial immunodiffusion technique in the serum of patients with retinitis pigmentosa (RP) carefully classified according to their specific genetic type, i.e., autosomally recessive, dominant, sex-linked, and other such as those forms associated with Laurence-Moon-Bardet-Biedl or Winkelman's disease. Highly purified human normal RBP was used as a standard. In none of the patients with RP studied was the serum level of RBP found to be significantly different from that of the control subjects.