Calcium and its Localization in Human Lens Fibres: An Electron Tomographic Study (original) (raw)
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Changes in the distribution of lens calcium during development of x-ray cataract
Investigative Ophthalmology & Visual Science
The present study was designed to examine the possible role of calcium in the opacification of x-rayinduced cataract in rabbit. The results demonstrate that the concentration of calcium in x-rayed lenses, just prior to lens hydration (7.5 weeks postirradiation), was twice that present in contralateral control lenses. At this stage of immature cataract, the lens nucleus remained transparent and maintained a normal level of calcium, but the lens cortex, containing regions of subcapsular opacification, accumulated a level of calcium that was twice that of the control. In the completely opaque mature cataract, (8-9 weeks postx-ray), both the cortex and nucleus had gained significant amounts of calcium. As the concentration of total calcium increased in the immature x-ray cataract, the amount of the cation bound to membranes and insoluble proteins of the cytosol also increased comparably. However, the relative proportion of calcium in the various fractions remained unaltered in the immature cataract; in both control lenses and immature cataracts, 20% of the total calcium remained in the membrane pellet and 70% was located in the soluble protein fraction. Only in the mature stage of cataract was a shift in the distribution of calcium apparent, as the proportion of calcium in the soluble protein fraction increased to 90%. Although only 7% of the total calcium in a mature cataract was bound to membrane, the amount represented a fivefold increase over the control. The results of this study demonstrate that an elevation in lens calcium accompanies the opacification process in x-ray cataract. The work also suggests that changes in calcium levels are not likely to result from inactivation of Ca-ATPase. Invest Opthalmol Vis
Investigative Ophthalmology & Visual Science
Evidence based on the following three observations suggests the existence of a calcium transport system in the mammalian lens: calcium levels in the lens are lower than that measured, in the aqueous humor; calcium efflux is temperature-dependent and is reduced by inhibitors of Ca ++ transport; and there exists a calcium-activated, magnesium-dependent ATPase. In rat, bovine, dog, and rabbit lenses, the concentration of total calcium was found to be approximately 0.2 mM, at least an order of magnitude lower than that found in the aqueous humor. To determine the nature of the mechanism responsible for maintaining these low levels, calcium fluxes were measured. During the initial rapid phase of 45 Ca efflux, the rate at4° C was reduced, by 85% compared with that found at 37° C. Efflux was not altered in the absence of external Na + . Calcium efflux was reduced, however, by lanthanum and propranolol, inhibitors of Ca/Mg ATPase. The presence ofCalMg ATPase was also demonstrated in the rat, bovine, and rabbit lens and was likewise inhibited by both lanthum and propranolol.
Influence of external calcium and glucose on internal total and ionized calcium in the rat lens
The Journal of physiology, 1984
Free calcium in the rat lens, measured by ion-sensitive electrodes, is 1.8 microM while the total, measured by atomic absorption, is of the order of 600 microM. The measured free calcium concentration (pCa) varies with the depth below the surface. It is lowest in the region 100-400 micron below the capsule and again in the nucleus, while the intervening perinuclear cortex has a relatively high free calcium. In young rats (less than 16 weeks) the free calcium in the posterior and anterior cortical regions is the same, while in the older lenses the free calcium is lower in the anterior and the regional variation is greater. Rat lenses incubated in a medium of similar ionic composition to aqueous humour for 15-24 h maintained a low level of free calcium. The maintenance of low internal calcium (both free and total) was dependent on external glucose and on removing glucose the intracellular free calcium increased from 5 to 15 microM while the total calcium increased from 600 to over 100...
Connections Between Connexins, Calcium, and Cataracts in the Lens
The Journal of General Physiology, 2004
There is a good deal of evidence that the lens generates an internal micro circulatory system, which brings metabolites, like glucose, and antioxidants, like ascorbate, into the lens along the extracellular spaces between cells. Calcium also ought to be carried into the lens by this system. If so, the only path for Ca 2 ϩ to get out of the lens is to move down its electrochemical gradient into fiber cells, and then move by electrodiffusion from cell to cell through gap junctions to surface cells, where Ca-ATPase activity and Na/Ca exchange can transport it back into the aqueous or vitreous humors. The purpose of the present study was to test this calcium circulation hypothesis by studying calcium homeostasis in connexin (Cx46) knockout and (Cx46 for Cx50) knockin mouse lenses, which have different degrees of gap junction coupling. To measure intracellular calcium, FURA2 was injected into fiber cells, and the gradient in calcium concentration from center to surface was mapped in each type of lens. In wild-type lenses the coupling conductance of the mature fibers was ف 0.5 S/cm 2 of cell to cell contact, and the best fit to the calcium concentration data varied from 700 nM in the center to 300 nM at the surface. In the knockin lenses, the coupling conductance was ف 1.0 S/cm 2 and calcium varied from ف 500 nM at the center to 300 nM at the surface. Thus, when the coupling conductance doubled, the concentration gradient halved, as predicted by the model. In knockout lenses, the coupling conductance was zero, hence the efflux path was knocked out and calcium accumulated to ف 2 M in central fibers. Knockout lenses also had a dense central cataract that extended from the center to about half the radius. Others have previously shown that this cataract involves activation of a calcium-dependent protease, Lp82. We can now expand on this finding to provide a hypothesis on each step that leads to cataract formation: knockout of Cx46 causes loss of coupling of mature fiber cells; the efflux path for calcium is therefore blocked; calcium accumulates in the central cells; at concentrations above ف 1 M (from the center to about half way out of a 3-wk-old lens) Lp82 is activated; Lp82 cleaves cytoplasmic proteins (crystallins) in central cells; and the cleaved proteins aggregate and scatter light.
The Journal of physiology, 1983
Ion-sensitive micro-electrodes were used to measure the levels of intracellular free Ca2+ within the intact amphibian lens. The free [Ca2+] was found to constitute 0.4% of the total lens calcium. The pCa measured at the anterior lens surface was found to 6.59, while that at the posterior was 5.70. An 8-fold anterior/posterior Ca2+ gradient thus exists along the optical axis. The intracellular free Ca2+ could be manipulated by incubating the lens in high-Ca2+ or cA2+-free EGTA Ringer solutions. Raising the intracellular free Ca2+ to 0.22 mM caused lens opacification and cellular uncoupling; the coupling ratio was reduced from 1 in control to 0.41 in high Ca2+.
Calcium content and distribution in human cataract
Experimental Eye Research, 1982
The calcium content and distribution was measured in hrunescent cataracts from India and cataracts from the United States classified according to guidelines of the (:ooperative C:ataract Research Group (CXRG). The severity and extent of opacification correlates well with the increase in C!az+ bound to membranes and insoluble proteins separated by differential centrifugation. Thus, bound Ca*+ is approximately 30 ng/mg for the immature cataract. 100 ng/mg for the pale yellow, 200 ng/mg for the mature cataract and 300 ng/mg for the brown brunescrnt. These values can be compared to 16 ng/mg (02 mx) for a freshly excised rabbit lens.