Biomedical subjects
K Hightower
Publications and source records attributed to K Hightower.
The role of calcium in UVB-induced damage in irradiated ocular lenses.
The purpose of this study was to evaluate the role of altered calcium homeostasis in the development of irreversible membrane damage in the UVB-irradiated ocular lens. In particular, experiments were designed to determine whether restricting calcium influx could prevent membrane damage that typically leads to ion imbalances and lens opacification following short-term exposure to ultraviolet light (UVB). The influx of calcium was reduced by culturing lenses in a low-calcium culture medium containing 0.3 mM Ca2+ rather than physiological concentrations of 1.6 mM. This low-calcium protocol retarded calcium accumulation in UVB-irradiated lenses for 2 days of culture, and opacification was delayed by 24 h. Loss of transparency did occur during the second day of culture, but more slowly than in irradiated lenses cultured in normal-calcium medium. Membrane damage was assessed by evaluating loss in cation transport activity, assessed by measuring 86Rb uptake into cultured lenses. Uptake was markedly inhibited in UVB-irradiated lenses and low-calcium culture did not prevent this inhibition of cation transport, a finding that explains why low-calcium protocol did not help maintain sodium homeostasis in irradiated lenses. Inhibition of cation transport and sodium accumulation eventually caused lens hydration and light scattering during extended culture in the absence of significant calcium elevation. Additional experiments were done to establish whether initial damage sustained by membranes could be repaired through the biosynthesis of new membrane proteins. Incorporation of 14C-histidine in membranes of the UVB-exposed lens was measured to assess membrane synthesis essential for repairing membrane damage. The rate of membrane protein synthesis, assessed by measuring incorporation of labeled amino acids, declined in UVB cataract, despite the prevention of calcium accumulation. These results suggest that one explanation for irreversible gain in sodium and calcium content accompanying opacification is the inability of lenses to replenish damaged membrane proteins comprising ion channels or transporters.
Selenite-induced damage to lens membranes.
The present study focuses on three aspects of membrane damage to explain selenite-induced loss of ion homeostasis: membrane transport processes, i.e. cation pump; biosynthesis of membrane proteins and membrane permeability. Cation pump activity, assessed by both 86Rb uptake and Na-K-ATPase activity in the epithelium, was observed to decline gradually after exposure to selenite and subsequent culture for 2 days in a selenite-free medium. In fact, the major loss of transport and ATPase activity occurred during culture of lenses after transfer from selenite to a selenite-free medium. The delay between selenite presentation to the lens and final inhibition of the cation pump (47%) corresponds to the delay in the observed loss of Na-K-ATPase activity (50%). Initial loss of cation transport and Na-K-ATPase activity may be due to the oxidative capacity of selenite. Oxidation, however, might not explain the delayed, progressive loss of transport activity after selenite removal. A plausible cause for this sustained loss might be a depleted supply of Na-K-ATPase due to impaired biosynthesis. Evidence for such a possibility comes from the observation that the rate of synthesis of total membrane protein is impaired by 44% in selenite-treated lenses. Membrane permeability to Na+ was not affected at the end of day 1, a conclusion based on the following observation: ouabain-treated lenses exposed to selenite did not gain any more Na+ than did ouabain-treated lenses. With the pump blocked by ouabain in both groups of lenses, the passive influx of Na+ was unchanged by selenite, indicating little damage to membrane permeability.(ABSTRACT TRUNCATED AT 250 WORDS)
Comparative effect of UVA and UVB on cultured rabbit lens.
Effects on lens physiology of UVB and UVA used separately and sequentially were investigated using 4 week old rabbit lenses in organ culture. Narrowband UVB at 0.3 J/cm2 = joules/lens (1 h exposure) has little effect on sodium and calcium concentrations in the lens interior or transparency of lenses subsequently cultured for 20 h after a 1 h exposure. With an incident energy of 3 J/cm2 of broadband UVB (295-330 nm), lenses become opaque and slightly swollen with significant ion imbalances during culture over a 1 day period. In contrast, lenses exposed to approximately 6-24 J/cm2 of UVA (330-400 nm) remain transparent after 1 day of culture. Extended culture up to 4 days reveals no signs of opacification. Ion homeostasis and normal lens hydration are also maintained in UVA-irradiated lenses. The presence of 95% oxygen during UVA irradiation is also without effect. Broadband UVA irradiation is damaging, however, if lenses are first exposed to subthreshold doses of narrowband UVB (307 +/- 5 nm) irradiation, viz. 0.3 J/cm2. Thus, sequential UVB/UVA irradiation at subthreshold doses causes impaired active cation transport and accumulation of sodium and calcium accompanying lens opacification.
Mechanisms involved in cataract development following near-ultraviolet radiation of cultured lenses.
Cultured rabbit lenses were irradiated with UV (311 nm peak; 295-340 nm) for 30 to 60 min. The entire spectrum lies in the near-UV, the major component is UVB, with a minor portion (25%) of UVA, and is henceforth referred to as near-UV(B). Posterior irradiation caused no cataract and no significant ionic imbalances compared to anterior irradiation, which caused opacification and marked changes in sodium and calcium concentrations. Anterior irradiation also resulted in reduced Na/K-ATPase activity in the epithelium. ATPase activity was not immediately inhibited; rather, only after culture was enzyme activity reduced. The concentration of reduced glutathione (GSH) decreased rapidly in the epithelium and more slowly in the underlying lens fibers. Loss of GSH was more rapid and extensive when irradiation occurred in the presence of oxygen. Irradiation under anaerobic conditions resulted in opacification but was considerably less extensive than when irradiation of lenses occurred in the presence of 7% oxygen. Near-UV(B) damage following anaerobic irradiation and 20 hrs of culture resulted in an increase in sodium levels and loss of GSH; calcium levels were not significantly elevated. Since irradiation of tryptophan solutions produced small amounts of hydrogen peroxide, the possibility of hydrogen peroxide-mediated damage was investigated but no role could be substantiated. Peroxide detoxification by the epithelium of near-UV(B) cataracts was observed, as measured by its ability to eliminate hydrogen peroxide added as a bolus.
Physiological effects of UVB irradiation on cultured rabbit lens.
The purpose of this study was to investigate the physiological effects of irradiation in the spectral range 295-340 nm in cultured rabbit lenses. Ultraviolet B cataract was produced in lenses exposed to low levels of irradiation, 1-2 mW/cm2. Opacification was assessed by laser transmittance measurements. The changes observed during lens culture after a 1 hr dose (4 J/cm2) include a gradual increase in hydration, sodium concentration, and calcium levels. Loss in membrane voltage and a rise in 36Cl accumulation indicate that membrane permeability was increased. The cation pump was impaired within 20 hr of irradiation, as concluded by an observed fall in 22Na efflux. Availability of glucose for cation transport was diminished based on the reduced rate of uptake of tritiated 3-o-methylglucose in irradiated lenses, but this reduced accumulation was observed much later than was sodium elevation. Ionic imbalances and opacification required less than 1 d of culture for 4-12-wk-old lenses and required nearly 7 d of culture for 100-wk-old lenses.