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Life Extension in the Rotifer Mytilina brevispina
Var Redunca by the Application of Chelating Agents.
Andrew M Sincock PhD
Extensions of life span and reproductive
period were achieved in the rotifer Mytilina brevispina var redunca by
regular immersions n solutions of one of the following chelating agents, sodium
citrate, sodium tartrate, EDTA and EGTA. It was shown by means of
radiolabelling with 45 calcium that the rate of calcium accumulation in the chelation
treated rotifers was markedly lower than in the untreated controls.
Furthermore, significant quantities of calcium, which increased throughout the
life-span, were withdrawn from rotifers at chelation. These results add
further evidence to a mechanism of aging in this organism that is related to a
detrimental accumulation of calcium during the life period.
Previous experiments which have investigated the effects of
calcium on the life-span of rotifer (Lansing, 1942a,b; Sincock, 1974) have shown
that over a certain range of concentration, life is shortened by an increase in
the environmental level of calcium. Measurements of the calcium present in
rotifer (Sincock, 1974) have revealed an accumulation throughout adult life
which is more rapid and substantial at higher calcium concentrations in culture
regime. Lansing (1942b) demonstrated that immersions in sodium citrate
solutions, which he believed would remove calcium from rotifers (Lansing &
Scott, 1942), significantly increase their life expectancy. The present
study shows that life extension is produced by chelating agents other that
sodium citrate and that the calcium level in rotifers is reduced by the
chelation procedure.
Materials and Methods
Rotifers of the specie Mytilina
brevispina var redunca were cultured axenically at 24 C. on a dietary regime
of Chlamydomonas rheinhardtii as described for controls in Sincock
(1974).
In the first experiment, groups of
30 rotifers were immersed for 45 sec. either in .05% sodium citrate or a 0.25%
solution of one of the following -- sodium tartrate, ethylene diamine
tetra-acetic acid (di-sodium salt), i.e., EDTA, 1,2-di(2 aminoethoxy) ethane
tetra-acetic acid, i.e, EGTA. The treatment was given to each rotifer
individually in 0.04 ml drops of control medium before being returned to the
culture slide. Two groups wee not exposed to the chelating agents.
One of these, a parallel control, was washed three times in 0.04 ml of control
culture medium on the same days as the chelation-treated rotifers, while the
other, the control, was transferred only to fresh culture on each day along with
all other groups in the experiment.
In the second experiment direct
measurement was carried out on the calcium levels of rotifers treated with 0.5%%
sodium citrate. The rotifers were cultured in control medium supplemented
with 45 calcium in the manner described by Sincock (1974). Ten animals
from the control group were removed from their culture drops on each day of
life, washed three times in 3 ml volumes of control culture medium without 4
calcium, and each individual dried onto a separate filter disc. The same
number of animals was washed in 0.5 sodium citrate and rinsed, as previously
described, before being dried onto separate filter discs. A third series
of rotifers was washed in calcium 40 culture medium before the citrate treatment
and each animal processed in the same manner, while the citrate washings and
rinsings from each individual were dried on separate filter discs and retained
to establish the amount of calcium removed and the amount remaining in each rotifer.
Al filter discs were renewed for each individual and their activities were
measured ten times on the same day by beta-ray liquid scintillation counting.
Results
In the first experiment, the four groups
receiving treatment with a chelating agent showed significant life expectancy
and fecundity increases over the untreated controls. (Fig. 1 and Table
1). Close correlation between the survival and egg-laying values of the
parallel controls and controls indicated that the extra mechanical handling of
individuals produced no ill effects. The greatest survival increase of
75.9% was recorded for the group treated with EGTA, the most calcium specific
chelating agent in terms of log beta formation value. However, lower
survival increases of 51.7, 49.4 and 43.7% recorded for the sodium citrate,
EDTA, and sodium tartrate treated groups were not related to the log beta values
of these chelating agents (see table 3). The onset of egg-laying was
delayed in all the chelation-treated groups by one day, but the values of
subsequent reproductive periods and fecundities were all greater than in the
control group.
The results of the second experiment
(see fig. 2 and Table 2) showed that the rate of calcium accumulation in the
untreated control samples was markedly in excess of the calcium accumulated in
the citrate treated rotifers. Indeed, at Day 3, when the first counts were
obtained for the untreated controls, no counts were evident in citrate-treated
rotifers, while the counts obtained from the citrate washings indicated
100% withdrawal of the 45 calcium present in rotifers before washing
treatment. At day 9, when the last count of 1060 cpm was obtained for the
untreated controls, a count of only 198 cpm was recorded for the citrate-treated
rotifers. Taken in conjunction with the counts obtained from the sodium
citrate washings at this time (140 cpm), this corresponded to a reduction in the
fraction of calcium removed to 41%. After Day 9, the rate of calcium
accumulation in the citrate treated samples began to rise steadily.
However, even at day 11, the count recorded for this group was less than 1/3 of
the final count registered for the untreated samples, and the percentage of
calcium removed as a fraction of the total calcium present before washing
remained almost unchanged at 40%. The final count of 782 cpm recorded for
the citrate treated rotifers at day 15 was significantly lower than that of the
untreated rotifers, and it was accompanied by a reduction in the fraction of
calcium removed to 34%.
Discussion
In the first experiment, it is
very obvious that treatment with chelating agent significantly extends life-span
of the rotifer Mytilina brevispina var redunca. This extension is noticeably
marked in the case of the chelating agent having the highest affinity for
calcium in terms of its 1st Order log beta value, but the order of efficiency
of the other chelating agents is not related to their log beta values (Table
3). Indeed, if one were to relate the efficiency of calcium removal with
life extension, the inefficiency of EDTA in the face of a high 1st Order log
beta value for calcium would have to be explained in terms of additional
physical and chemical properties, possibly of a toxic nature.
The delayed onset of egg-laying
associated with subsequently increased fecundity and reproductive period in all
populations that showed life extension (Table 1) was also a feature of the group
whose life-span was extended by low calcium conditions. (Sincock, 1974).
The significance of delayed first egg production may be better understood in
terms of Lansing's (1954) pediaclones where it constituted a recognizable
feature of orthoclones whose life-spans were in the process of extending over
generations of selection. In this experiment, the delayed egg-laying
effect would have to be exhibited as a first-generation feature of a 4-day
orthoclone.
In the first experiment, only
supposition connects the extension of life with the removal of calcium.
Direct measurement of the effect of citrate washing on the calcium content of
the rotifers was therefore carried out by adding the radioisotope 45 calcium tot
he culture medium. The results (Fig. 2) clearly demonstrated that the rate
of calcium accumulation in the untreated samples greatly exceeded the rate of
accumulation by the citrate treated group. Furthermore, significant
quantities of the radionuclide 45 calcium appeared in increasing amounts
throughout life in the citrate washings of the treated rotifers, which suggests
that direct removal of the radiolabel was brought about by the chelation
procedure.
If one considers the sum of the
calcium withdrawn and the calcium present on each day of citrate treatment, the
values do not equal the corresponding values for the untreated controls after
day 3, (see table 4). This suggests that, quite apart from the calcium
removed at each chelation, there is a reduction in the rate of calcium
accumulation in the treated rotifers between days of treatment.
Furthermore, examination of the percentage of calcium removed at chelation as a
fraction of the total calcium already present in each rotifer (Table 2) shows
that there is a fall in the fraction of removable calcium from 100% to 34%
during the life-span. The inferred increase in a permanently bound calcium
fraction in the rotifer may impose a limit of efficiency of the chelating agent
in permanently offsetting the attainment of a lethal level of accumulated
calcium.
If one postulates that the given
calcium content of the control group on a given day may be taken as a measure of
its physiological age, then the physiological ages of the treated group may be
computed by comparison. For instance, the physiological age of the treated
group at Day 9 is 4 and 1/2 days, which represents a 4 and 1/2 day difference
from its chronological age. The distinction between physiological and
chronological age (fig. 3) enables a comparison to be made between the rates of
accumulation of calcium in the control and treated groups on the basis of the
calcium already present in the animal (i.e., physiological age). From this
comparison, it may be concluded that the rate of net calcium accumulation is
proportional to the physiological rather than the chronological age of the
rotifer studied.
On Day 15 of life, the physiological
age of the chelation treated rotifers is 8 days, and it must be supposed that at
this time, the rate of calcium accumulation is so high that the physiological
life-span of the rotifer is exceeded before the next chelation treatment.
Even if more frequent treatment with the chelating agent proved possible, it is
still likely that the increase in the fraction of inexchangeable calcium
referred to earlier would impose a limit on the increase of the physiological
life-span that could be obtained.
In the absence of all genetic variability
saving mutation between individuals homologated with respect to maternal age,
one may be confident that the rotifers within the finely controlled
culture conditions of the experiments exhibited a detrimental accumulation of
calcium throughout adult life that could be slowed with accompanying life
extension by treatment with chelating agents that remove calcium.
Summary
Rotifers of
the species Mytilina brevispina var redunca that had been homologated
with respect to maternal age, were cultured at 24C. n artificial saline medium
water under standard and aseptic culture conditions. Observations on the
survival times of individuals exposed by brief immersion on alternate days of
adult life to one of the following chelating agents --- EGTA, EDTA, sodium
tartrate and sodium citrate, revealed significant life extension in all
cases. The greatest life extension of 75.9 was recorded for the group
exposed to the chelating agent having the highest calcium affinity, namely
EGTA. However the order of efficacy of the other chelating agents was not
related to calcium affinities. Observations on egg production in the
chelation-treated populations revealed a single day's delay in the onset of
egg-laying but a subsequent increased fecundity and reproductive period relative
to the controls.
Direct measurement of the effect of
citrate washing on the calcium content of the rotifers cultured in the presence
of the radionucleotide 45 calcium revealed a significantly lowered rate of 45
calcium accumulation throughout the life period, which was accompanied by an
increasing amount of the radiolabel in the citrate washing medium. The sum
of the calcium accumulated and the calcium withdrawn in the
treated populations was below the corresponding day's totals of the calcium
accumulated in the untreated controls, which indicates a lower rate of
accumulation between treatments. Similarly, an increase in the fraction of
inexchangeable as evident during life in the treated rotifers,
since the percentage calcium removed as a fraction of the total calcium present
before the washings fell from 100 to 34%. A comparison of calcium
accumulation between untreated and treated samples on the basis of physiological
age revealed a rate of accumulation that was dependant on physiological rather
than chronological age.
The over-all result of the
experiments was to demonstrate that a reduction in the age related accumulation
of calcium in rotifers by the application of chelating agents with a high
affinity for calcium produced significant increases in life expectancy.
References
Lansing, A. I. Some effects of the hydrogen ion
concentration, total salt concentration , calcium and citrate on longevity and
fecundity of the rotifer. Journal of Experimental Zoology, 1942, 91,
195-211 (a)
Lansing, A. I. Increase of cortical calcium with age in the
cells of a rotifer Euchlanis dilatata, a planarian, Phagocata sp. and a toad
Bufo fowleri, as shown by the micro-incineration technique. Biological
Bulletin, 1942, 2, 228-238, (b)
Lansing, A. I. & Scott, G. H. The effect of perfusion with
sodium citrate on the content and distribution of the minerals in various cells
of the cat as shown by electron microscopy and microincineration.
Anatomical Record, 1942, 84, 91-96.
Lansing, A. I. A nongenic factor in the longevity of
rotifers. Annals of the New York academy of Sciences, 1954, 57, 455-464.
Sincock, A. M. Calcium and Aging in the Rotifer Mytilina
brevispina var redunca. Journal of Gerentology, 1974, 29 514-517.
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