Chlorpromazine effects upon rabbit lens water and electrolyte balance

Calmodulin antagonists induce changes in lens permeability and transparency

Calmodulin has been shown to perform several important functions in the lens including regulation of the plasma membrane Ca(2+)-ATPase. This study investigated the effects of a variety of different CaM antagonists on rat lens membrane potential, membrane resistance, intracellular Na+ and Ca2+ content and transparency, in order to examine the role of CaM in control of lens membrane permeability. W7, calmidazolium and trifluoperazine caused biphasic changes in lens membrane electrical characteristics. Phase 1 consisted of a depolarization of the membrane potential and an increase in resistance, indicating that K+ channels were being blocked. This was confirmed by application of the K+ channel antagonist, quinine, which inhibited Phase 1 changes. Phase 2 was a further depolarization coupled with a decrease in resistance, indicating the activation of a cation conductance. Application of W7 in low Na+ medium slowed the Phase 2 depolarization and decrease in resistance indicating that Na+ is the main charge carrier through the activated conductance. The CaM inhibitors also led to a large increase in the Na+ and Ca2+ contents of the lens. W5, a less potent analogue of W7, caused a depolarization and increase in membrane resistance, but no Phase 2 changes were observed. Na+ and Ca2+ contents were similar to control lenses after 4 hours incubation in 200 microM W5. In addition, exposure to W7, TFP and calmidazolium resulted in a loss of transparency, while W5 treated lenses remained clear. It appears, therefore, that CaM is involved in control of lens membrane permeability. Loss of control of these channels leads to catastrophic changes in the intracellular ionic environment and hence opacification of the lens.

Phototoxicity of chlorpromazine on retinal pigment epithelial cells

As it is known that chlorpromazine (CPZ) can bind to melanins as well as cause ocular phototoxicity, we investigated the cytotoxic effects of UV-visible irradiation of melanotic and amelanotic retinal pigment epithelial (RPE) cells in the presence of CPZ. At low concentrations (5 micrograms/ml) of CPZ a photosensitization reaction took place which lysed the cells as measured by the release of 51Cr from cells labelled with chromium. At concentrations of CPZ less than 5 micrograms/ml, no significant cell lysis occurred when the cells were incubated at 37 degrees C in the dark. As the concentration of CPZ was increased to 25 micrograms/ml or more, high percentages of cells were lysed. When the melanotic RPE cells were exposed to different concentrations of CPZ and grown in culture, the cell growth (multiplication) diminished drastically with low concentrations (less than 2 micrograms/ml CPZ). Vitamin E decreased the cell lysis both in the dark and upon irradiation. Oxygen radical scavengers such as glutathione, B-carotene, mannitol, D-penicillamine as well as superoxide dismutase and catalase did not decrease cell lysis. The phototoxic effects of CPZ was found not to be due to stable photoproducts formed during irradiation of CPZ.

Effect of calcium on the calf lens cytoplasm

Opacification was induced in calf lens cytoplasmic extracts by addition of calcium. The sample turbidity was shown to increase with calcium molarity, incubation time and temperature and to decrease with the protein cytoplasmic concentration. Although this turbidity was enhanced when membrane fragments were left over in the cytoplasmic extracts, it did show up in the absence of any detectable vesicular fragment. Scattering techniques (X-ray and light) showed that the calcium-induced opacification is linked to enhanced light scattering, which results from the formation of additional scatterers, a few tens of nm in diameter. Additional structures were indeed visualized by freeze-fracture electron microscopy (FFEM): they appear as molecular clusters with diameters ranging from 20 to 90 nm, made of densely packed particles, with heterogeneous sizes. The turbidity expected from these clusters, as well as the expected variation of turbidity with cytoplasmic concentration, was calculated to be in agreement with the measurements. When compared with cold cataract, these results illustrate that similar opacities may result from completely different biophysical mechanisms.

Characteristics of 45Ca uptake by the rabbit lens

Lens 45Ca uptake was shown to increase when the external calcium concentration was less than 1 mM, suggesting that calcium ions themselves are able to interact with lens membranes in a way that influences the mechanism of calcium permeability. Strontium ions, added to a calcium-deficient bathing solution, were shown to attenuate the increased 45Ca uptake by the lens. There was no evidence for extracellular binding of calcium. Lens membrane permeability to calcium appeared not to be voltage-dependent since 45Ca uptake was not affected when the lens was depolarized by high potassium solutions. Calcium channel blockers were shown not to reduce the increased calcium leak resulting from exposure of the lens to a calcium-deficient medium.

Cytotoxic effects of internal calcium on lens physiology: a review

While calcium is possibly involved in cataractogenesis, it is unquestionably involved in normal lens physiology. Numerous reports have documented the many cellular processes in other tissues affected by alterations in cellular levels of calcium. The homeostasis of the lens is no less dependent on the critical balance of intracellular calcium. With advances being made in calcium-sensitive microelectrodes and pioneering studies progressing in ion channel electrophysiology, interest in calcium metabolism in the lens has been intensified. This report is an attempt to review recent findings that deal solely with biochemical changes resulting from calcium imbalances in the lens interior.

Examination of lens calcium balance using 45Ca uptake studies

The utility of 45Ca efflux studies to examine lens calcium balance is limited because of the dominant contribution of efflux from the extracellular space of the lens. On the other hand, examination of lens 45Ca uptake characteristics can generate valuable information regarding the mechanism by which calcium is largely restricted from the intracellular compartment. Such studies have enabled us to demonstrate that the access of calcium to the intracellular compartment is dependent to some extent upon the concentration of calcium in the extracellular environment. Additionally, experimental evidence speaks against extracellular binding of calcium. Finally, lens membrane permeability to calcium appears to be independent of membrane voltage.

Progress toward the establishment of nuclear magnetic resonance measurements as an index of in vivo lens functional integrity

Cataract prevention, delay, or reversal requires in vivo detection of lens changes that initiate the opacification process. The determination of lenticular biochemistry by a noninvasive methodology would represent a major step in the in vivo assessment of lens normalcy. Nuclear magnetic resonance spectroscopy appears to be a promising noninvasive technique to meet this goal. However, the correlation of lenticular "invasive" morphological and physiological data with NMR spectroscopic data in organ culture is necessary for establishing the validity of NMR spectral measurements as an index of lens functional integrity. The implementation of lenticular NMR spectroscopic studies will provide quantitative information such as intralenticular pH, Na+ and K+ ion distributions, lenticular levels of phosphorus-containing metabolites, glycolytic pathway intermediates and the end product lactate, sorbitol pathway activity, and possibly reduced glutathione levels. Development in surface coil NMR spectroscopy which enables the spatial localization of tissue metabolites and advances in the refinements required for quantitation of these results are discussed. These measurements of in vivo lenticular biochemistry will provide the information necessary for the scheduling of anticataract drug therapy as well as an ongoing method for monitoring the efficacy of such treatment.

Lens cation transport and permeability changes following exposure to hydrogen peroxide

Exposure of the rabbit lens, in vitro, to 10(-3) M-hydrogen peroxide resulted in an increase of the lens sodium, calcium and water content together with a decrease of the lens potassium content. No such changes were observed in the presence of 10(-4) or 10(-5) M-hydrogen peroxide. 10(-3) M-hydrogen peroxide was observed to diminish 86Rb uptake by 32% and to reduce the lens Na+-, K+-ATPase activity by 37%. The lens potential remained ouabain-sensitive after peroxide treatment. These findings suggest that only partial inhibition of the lens sodium pump takes place in the presence of 10(-3) M-hydrogen peroxide. Lens depolarization, increased lens conductance and an increase rate of 86Rb efflux were also observed upon exposure of the lens to 10(-3) M-hydrogen peroxide. Such alterations of 86Rb efflux and electrophysiological parameters indicate that hydrogen peroxide changes the passive permeability of lens membranes.

An analysis of 45Ca fluxes in the rabbit lens

The uptake of 45Ca by the rabbit lens reached equilibrium within 20 hr; the steady state lens/medium ratio was 0.08. This ratio is similar to that found for the lens/medium distribution of extracellular markers. Efflux of 45Ca from the lens was rapid and insensitive to iodoacetate. Mathematical analysis of 45Ca efflux curves revealed that calcium efflux from the lens could be described solely upon the basis of passive movement from the extracellular space. It is concluded that the exchangeable calcium in the normal rabbit lens is predominantly located in the extracellular space.