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Journal of Lipid Research, Vol 26, 327-337, Copyright © 1985 by Lipid Research, Inc.
RB Dell, GE Mott, EM Jackson, R Ramakrishnan, KD Carey, HC McGill Jr and DS Goodman
Cholesterol turnover was studied in four baboons by injecting
[14C]cholesterol 186 days and [3H]cholesterol 4 days before necropsy, and
fitting a two- or three-pool model to the resulting specific activity-time
data. At necropsy, cholesterol mass and specific activity were determined
for the total body (minus the central nervous system) and for many tissues.
A pool model permits the estimation, from the plasma specific activity-time
curve alone, of total body cholesterol within a limited range, depending
upon the extent of side pool synthesis. The principal aim of this study was
to estimate the extent of cholesterol synthesis in the side pools of the
model, by computing the amount of side pool synthesis needed to equal the
measured total body cholesterol. Central pool synthesis varied from 61 to
89% of the total cholesterol production rate. Thus, approximately 25% (11
to 39%) of the production rate arose from peripheral (pool 3 for the three-
pool, and pool 2 for the two-pool model) cholesterol synthesis. Moreover,
the finding that the measured total body cholesterol fell within the range
obtained from the kinetic analysis by using reasonable assumptions (namely,
that zero or that half the production rate occurred in the side pools),
provides evidence for the physiological validity of the model. A second aim
of this study was to explore cholesterol turnover in various tissues. A
pool model predicts that rapidly turning over tissues will have higher
specific activities at early times and lower specific activities at later
times after injection of tracer relative to slowly turning over tissues,
except where significant synthesis occurs. Tissues were ranked 1 to 17 for
3H and 17 to 1 for 14C cholesterol specific activity values. Except for the
GI tract and testis, the tissues had similar ranks for both 3H and 14C,
further validating model predictions. Results in all four baboons were
similar. Turnover rates for the different tissues loosely fell into three
groups which were turning over at fast, intermediate, and slow rates.
Finally, the magnitude of variation of cholesterol specific activity was
moderate for several distributed tissues (fat, muscle, arteries, and the
alimentary tract), but was small for liver. Cholesterol turnover in serial
biopsies of skin, muscle, and fat could, however, be fitted with a single
pool to estimate tissue turnover rates.
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Whole body and tissue cholesterol turnover in the baboon
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