The effect of X-irradiation on the sodium-potassium-activated adenosine triphosphatase (Na-K-ATPase) activity in the epithelium of the rat lens. A histochemical and biochemical study.

The epithelial Na-K-ATPase activity of the rat lens was studied after X-irradiation at intervals of three to ninety days. The enzyme was demonstrated histochemically by light microscopy and it was measured biochemically by a fluorometric method. Neither histochemical nor biochemical changes of Na-K-ATPase content of the lens epithelium were observed during the development of cataract. In whole-mount preparations the enzyme activity was localized in the cell membranes. However, one month after radiation a few peripheral cells had in addition a precipitated over the whole cell. The unaltered Na-K-ATPase content in the epithelium suggests that the hydration of the lens after X-irradiation is primarily caused by changes in the passive permeability properties of the cell membranes and not by a decreased capacity of the activity cation pump.     

Electron microscopic localization of adenosine triphosphatase (NaK-ATPase) activity in the rat cornea.

Sodium-potassium-activated adenosine triphosphatase (NaK-ATPase) activity was investigated histochemically in the rat cornea. The enzyme was localized in all epithelial cell membranes and in the nerve fibres of the stroma. The intercellular membranes of the flat endothelial cells were also active. When 3 × 10^?4m-ouabain was added to the preincubation and incubation solutions, a clear inhibition of the reaction was observed. The presence of 70 mm-Na⁺ and 30 mm-K⁺ in the incubation solution stimulated the histochemical enzyme reaction to a marked extent. When the substrate (ATP) was replaced by ADP or by glycerophosphates, there was no reaction as was also the case in the absence of Pb²⁺. These findings were regarded as a proof for the specificity of the technique.Methodological problems of ATPase histochemistry and the physiological role of NaK-ATPase activity in the different structures of the cornea are discussed.     

Electronmicroscopical, histochemical and biochemical findings on the Na-K-ATPase activity in the epithelium of the rat lens.

The sodium-potassium activated adenosine triphosphatase (Na-K-ATPase) activity in the epithelium of the rat lens was investigated histochemically using light and electron microscopy and biochemistry.The histochemical demonstration of Na-K-ATPase activity in the lens epithelium was modified to be highly specific and to give a precise localization. The enzyme reaction was localized on the epithelial cell membranes that are adjacent to neighbouring epithelial cell membranes, whereas the apical and basal cell membranes were negative. The reaction product was almost totally absent in the presence of the specific enzyme inhibitor (ouabain). The reaction was strongly decreased also in the absence of the specific enzyme activators (sodium and potassium). The lens capsule as well as the lens fibres were negative.Biochemical studies of the kinetic properties of the lens epithelium were characterized by studying the effect of various chemicals on Na-K-ATPase activity. The enzyme activity was increased by the Na⁺, K⁺ and Mg²⁺ and inhibited by ouabain. Enzyme activity was optimal when $\text{Mg}{}^{\mathrm{2+}}\text{ATP}$ ratio was approximately 1·4 and the pH at 7·7. Optimal activity was also obtained by using 85·0 mm-sodium and 1·0 mm-potassium in the solution.The results in this study indicate that the adjoining cell membranes of the lens epithelium contain the Na-K-ATPase activity and that these are probably responsible for the exchange of water and ions across the lens epithelium.     

The effect of near-UV light on Na-K-ATPase of the rat lens

The influence of in vitro near-UV radiation exposure on the physical state of the rat lens and on its membrane-bound Na-K-ATPase activity was investigated. Lens swelling was correlated to the appearance of opacities and the inactivation of the enzyme. The results show a significant decrease in the Na-K-ATPase activity which may be an early change leading to osmotic type cataracts. The dose-effect curves obtained for cortical and epithelial enzymes were different. Since the data do not follow a monoexponential function, the existence of two forms of Na-K-ATPase in the lens is discussed.     

In vitro production of steroid cataract in bovine lens. Part II: measurement of sodium-potassium adenosine triphosphatase activity.

Fresh calf lesnses incubated in nutritive media containing dexamethasone phosphate or ouabain in concentrations ranging from 1 X 10(-4) M to 1 X 10(-8) M developed cortical opacification and showed significant inhibition of Na(+)-K(+) ATP'ase activity. Over a 3-day incubation period the decrease in Na+-K+ ATP'ase activity correlated well with the observed decrease in light transmission. The degree of enzyme inhibition and decrease in light transmission varied directly with the concentration of dexamethasone phosphate and ouabain, with significant changes observed at 'physiologic' and 'pharmacologic' concentrations of these agents. Lenses incubated for 4 days in dexamethasone phosphate or ouabain showed substantial increases in water content as well as an increase in Na+ and a decrease in K+ concentration. These data suggest that inhibition of the cation pump may play a significant role in the formation of steroid cataract in vitro.     

Polyol pathway activity in streptozotocin-diabetic rat lens

We have examined polyol pathway kinetics in the lenses of rats made diabetic with streptozotocin. At up to 11 days after diabetes induction, the lenses were isolated and subjected to 'pulse-chase' studies: the lenses were incubated with [13C]glucose and lens metabolism followed by [13C]nuclear magnetic resonance (NMR) spectroscopy. Proton NMR spectroscopy was also performed to measure the hexose monophosphate shunt (HMPS) activity. The results showed that (1) the activity of aldose reductase increased initially and decreased after 11 days of diabetes; (2) the fructose pool increased initially but started to decline after 3 days; (3) the HMPS activity increased nearly 40% immediately after diabetes induction; and (4) the turnover rates of glucose, alpha-glycerophosphate (GP), lactate, sorbitol, and fructose were 80.8 +/- 2.6, 10.1 +/- 1.4, 47.7 +/- 3.7, 7.9 +/- 0.9 and 5.2 +/- 2.2 nmol hr-1 lens-1 (34 mg wet weight lens-1), respectively. Up to 35% of lactate appeared to derive from the polyol pathway. Further, GP was rapidly metabolized, although its fate is currently unknown. These results reveal a far more complex pattern of glucose metabolism in the diabetic lens than that in lenses incubated in high glucose.     

Changes during aging in rat lens endopeptidase activity

Protein degradation has been implicated in lens aging and cataract development. We propose that proteolytic enzymes are important in these processes. In this study, lens neutral proteinase activity and thermal stability have been measured as a function of cell age and animal age in rat lens. Epithelial, cortical, and nuclear lens regions from animals between 1 and 25 months were analyzed. Specific activity and thermal stability were found to decrease with lens cell age, that is, from epithelium to nucleus. Specific activity in the cortex increased with animal age, while specific activity in the nuclear region remained constant with animal age. In the epithelium, specific activity showed no correlation with animal age. In 15 day old rats, thermal stability curves for the cortical enzyme were linear, indicating only one form of activity hydrolyzing synthetic substrate was present. At 1.25 months of age, thermal stability curves for the cortical enzyme were non-linear indicating that multiple forms of the activity were present.     

Identification of 12(S)-hydroxyeicosatetraenoic acid in the young rat lens

Evidence is presented indicating that young rat lens has the capacity to synthesize 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE) from exogenous arachidonic acid (AA). A 9,000 xg supernatant prepared from 15 day old rat lenses, when incubated with calcium and U-[14C]-AA, generated a radiolabelled product with a retention time identical to authentic unlabelled 12-HETE in two different HPLC solvent systems. Mass spectral analysis provided evidence that the metabolite was 12-HETE while chiral studies demonstrated the exclusive presence of the 12(S) isomer. Radiolabelled 12-HETE synthesis was inhibited by preincubating the 15 day old rat lens supernatant with 0.2 Mm curcumin, a mixed cyclooxygenase/lipoxygenase inhibitor. Radiolabelled 12(S)-HETE synthetic capacity was highest in the 4 day old rat lens, the earliest time period measured, and rapidly declined with age reaching negligible levels at about 4 months. These results suggest that the young rat lens possess the biosynthetic capacity to generate radiolabelled 12(S)-HETE from U-[14]C-AA. The profile of 12-HETE synthetic activity suggests a possible regulatory function of the hydroxylated AA metabolite in the developing lens.     

Ocular hypotensive activity of the adenosine agonist (R)-phenylisopropyladenosine in rabbits.

Adenosine A1 agonists have been shown to induce a variety of pharmacological effects. In New Zealand White rabbits, the topical administration of 500 micrograms of the relatively selective adenosine A1 receptor agonist R(-) phenylisopropyladenosine (R-PIA) produced a biphasic response in IOP in the ipsilateral eye: an initial ocular hypertension (3.5 +/- 1.4 mm of Hg) at 0.5 hour, followed by significant reduction in IOP (5 to 8 mm of Hg) from 2 to 6 hours postadministration. The IOP response to 50 and 165 micrograms of R-PIA demonstrated that the ocular hypotensive response to R-PIA was dose-related; however, no initial hypertension was observed at these lower doses. The ocular response to R-PIA was primarily unilateral with only a small reduction in contralateral IOP at 1 hour observed in animals treated with 500 micrograms. No significant change in pupil diameter was observed with any dose of R-PIA. Pretreatment with the adenosine antagonist CPT (10 mg/kg; i.p.) significantly inhibited the ocular hypotensive response to R-PIA. However, pretreatment with the cyclooxygenase inhibitor indomethacin (50 mg/kg; i.p.) did not alter the change in IOP induced by R-PIA. The administration of R-PIA once a day for five days demonstrated that tolerance does not develop in rabbits with repeated administration. These data demonstrate that the adenosine A1 agonist R-PIA can lower IOP. The unilateral nature and the inhibition by CPT supports the idea that this response is mediated by adenosine receptors located in the eye.(ABSTRACT TRUNCATED AT 250 WORDS)     

Circadian rhythm of serotonin N -acetyltransferase activity in rat lens

Activity of serotonin N -acetyltransferase (NAT), a key regulatory enzyme in melatonin biosynthesis, was detected in the rat lens. NAT activity in the lens showed significant diurnal variation in vivo and in vitro, peaking during the period of darkness, when the lenses were maintained under 14 hr light/10 hr dark cycle. Cultured lenses exhibited a circadian rhythm of NAT activity when maintained under constant darkness. However, the rhythm in vitro was not entrained when the light/dark cycle was delayed 8 hr from the cycle in intact animals. These data strongly suggest that the rat lens contains a circadian clock that controls NAT activity, although the circadian clock appears to lack a photic entrainment mechanism.     

Chaperone activity in the lens

Introduction : α‐crystallin, the major protein of the eye lens, is a molecular chaperone that is able to prevent the precipitation of denatured proteins. This activity is thought to be important for the maintenance of lens transparency. Loss of the activity has been postulated to contribute to the development of cataract. The purpose of this study was to determine how chaperone activity was affected by growth and ageing of the lens. Methods : α‐crystallins were purified from nine concentric tissue layers removed from an adult bovine lens. The ability to inhibit the precipitation of ß_L‐crystallin, following thermal denaturation, was used to assess the chaperone activity of these proteins. The molar ratio of α‐crystallin/ß_L‐crystallin required to inhibit the precipitation of ß_L‐crystallin by 50 per cent was used as a measure of the affinity of the chaperone for denatured protein. Results : As evidenced by a gradual increase in the ratio, from 0.52 to 1.24, the protective ability of α‐crystallin decreased from the outside of the lens into the centre, α‐crystallin from the cortex of the lens provided greater protection against precipitation of proteins than older α‐crystallin from the nucleus. The reasons for this were investigated. Gel electrophoresis of the proteins from each concentric layer revealed an increase in degraded polypeptides from approximately one per cent in the cortex to more than nine per cent in the centre of the lens. This increase appears to be correlated with the decrease in chaperone ability. Renaturing α‐crystallin obtained from the nucleus did not increase its chaperone activity, indicating conformational changes were not responsible for the decreased activity. Phosphorylation did not appear to have any significant effect on the chaperone activity. Conclusion : The loss of chaperone activity, accompanying fibre cell compression into the centre of the lens, can be attributed to degradation of the α‐crystallin polypeptides.     

Prostaglandin biosynthesis in the rat lens

Rat lens microsomal preparations possess the capability of converting exogenous arachidonic acid into prostaglandins (PG) E2 and F2 alpha. The low, yet measurable prostaglandin biosynthetic capacity of the lens microsomes was demonstrated by radioimmunoassay (RIA) and by separation of radiolabeled products after incubation with high specific activity U-14 [C]-arachidonic acid. Maximal formation of radioimmunoassayable PGE2 and PGF2 alpha was measured after a 15-min incubation at 37 degrees C with approximately 0.5 mg protein. Prostaglandin biosynthesis was inhibited by the nonsteroidal anti-inflammatory agents aspirin (IC50 = 52 microM) and indomethacin (IC50 = 20 microM). These results unequivocally demonstrate prostaglandin biosynthesis in rat lens.