Model 2. Slow liver, fast VLDL

RESULTS AND MODEL DEVELOPMENT

3.4. Model development

3.4.2. Model 2. Slow liver, fast VLDL

An

^{alternate model was proposed}

to

^account

for

the triglyceride ^kinetics

of the

YLDL2 fraction. This model was based upon observations made

by

other

workers, predominantly

in

non-human species, where

they

demonstrated that plasma

VlDl-triglyceride

turnover was more rapid than

that of the liver

^(see

Section 1.3.4.1).

To

propose

a

new model incorporating these ideas

the

apo B

data

was

examined

to look for

^evidence,

such as a ^rapidly turning

^over

component,

that

would support such

a

hypothesis.

The kinetics

of

the unbound VLDL2 apo

B

fraction

of

subject

F

^{(Figure 8)}

wefe

dominated

by a rapidly

turning

over

component and

by a minor,

^less

n

30

rapidly turning over

component, comparable

to the turnover rate of

the

bound fraction. Using these data, and

the

assumption

that a

^{precursor product}

relationship exists between the unbound and bound fractions a

^three

compartment model was constructed

to fit the

apo

B

data (Figure

22).

Three compartments were necessary

to fit

^{these data because}

of the

^presence

of

^a

slow

component

in the

unbound fraction.

It lvas

^assumed

that the

turnover

rate of this

component

was

equal

to that of the

bound

fraction, and

may

therefore

have

represented

minor

contamination

of the

unbound

fraction

by

the

bound.

Of the total

^mass associated

with the

unbound fraction

the

slow

component represented

less than one

^percent

of this

fraction,

its

impact on

the type of

model used was therefore negligible. Compared

to

^subject

K,

the

turnover

rate of the

unbound fraction

in

subject

F

was more

than

ten-fold greater (1.372

vs

0.091 h-1). The turnover rate

of

the bound fraction was ^also gre at e r.

The

fit of

this model

to

the VLDL2 apo

B

data

of

^subject

^F is

shown in Figure

23.

Note

the

discrepancies

in the initial

specific radioactivity values of

the

unbound and bound fractions. Had these been closer

at

^{zero time than the}

bound curve may have exhibited

a

more pronounced delay prior

to its

decay.

In all

^subjects direct input

of

^apo

B into

the bound fraction, other ^than

that from the

unbound fraction, was required

to

satisfy steady state conditions'

This

observation ^suggested

that

although

the

bound

fraction

appeared

to

^be

the

^product

of the

unbound fraction apo

B

was

not

derived exclusively ^from

the

unbound fraction.

1.312 M(5)=43.2 mg M(3)=42.83 mg M(4)=0.37 mg M(6)=412.0 mg lC(3)=6.726E+05 cpm IC(4)=5.871E+03 cPm IC(6)=1.5468+07 cpm U(3)=56.188 mgh U(6)=SS.Zla mglh 0.217

o.217

Figure 22.

Three compartment model used

to

describe the kinetics

of

apo

B

ⁱⁿ

unbound (comp

3

^and

4)

and bound (comp

6)

VLDL2 ^fraction

in

subject

F.

^It

was

assumed

that

when ^labelled

VLDL2

(unbound and bound) was reinjected radioactivity distributed

in

proportion

to

^apo

B

mass

in the

unbound

(M(3)

⁺

M(4)) and bound (M(6)) pools. To satisfy the steady state

conditions considerable direct

input of

apo

B into the

bound fraction was required. L(i,j)

¡- l.

t"-.-E_--_.--

.-{1.-_.-r

3

1 10

0 2 4 6 8

Hours

10

¿,

Ë'

ro'

EÊ

10

2 10

>¡

I

crlo Ë

úarl

.9 oÀ

(t)

oÀ

A

L4

Figure

^23.

Fit of

^three compartment model (Figure 22)

to

1311 unbound (^) anã bound

(tr) VLDL2

apo

B

data

of

subject

F

following reinjection

of

^labelled

YLDLZ.

In the

previous triglyceride model (Figure

20) the

turnover

rate of

^the

unbound

YLDL2 triglyceride pool

approximated

the falling ^slope of

the

specific radioactivity curve. The rising ^slope of this curve

^however

respresented

the more rapidly turning over liver triglyceride

compartment.

In

developing

a new model to

^describe

VLDL

triglyceride

kinetics it

^was

assumed

that the

rate

limiting

step

of

triglyceride metabolism was

in the

liver (Figure 24).

A

new model was developed (Figure

24)

where the rising slope of

the triglyceride specific radioactivity curve is a function of the

^rapidly

turning over

unbound compartment

and the falling part of the ^curve

^a'

function of the liver triglyceride

compartment (comp.

2).

^Evidence

of

the

slowly

turning

over liver

triglyceride

pool would

appear

as

one

of ^the

^later

exponential functions, such

as in the tail of the

specific radioactivity curve,

where evidence

of the slow

triglyceride synthesis pathway proposed

by

^Zech

et al

⁽¹⁹⁷⁹⁾

is

observed.

ll)

0.275

1.036 0.217

M(3)=1864 mg M(6)=3728 mg IC(4)=6.6E+08 dpm

Figure

^{24. YLDL2} triglyceride model

for

subject

F. In

this model the unbound

compartment

(comp 3) turns over more rapidly than that of the

^liver

"o-pa.t-ent

^(comp

2) illustrating that the liver

triglyceride compartment is

the rate limiting itep. The

turnover

rate of the

bound fraction ^(comp.

6)

^is

however comparable

to that of the liver

triglyceride compaftment.

L(ii)

¡-1.

UI

6\è

t*ra

ç

æ

Figure

25

shows

the fit of the

slow-liver model

to

YLDLZ triglyceride specific radioactivity data

for

subject

F.

Although the

fit to

^the unbound data is

good, once again

the fit to the

bound data

is

poor.

This

demonstrates that

although

the

unbound

VLDL2 triglyceride fraction turns over rapidly

^its turnover

rate is not ^fast

^enough

to

^produce

the rapid rise

observed

in

^the

specific radioactivity of the bound fraction. The faster turnover of

the

unbound

fraction

combined

with the more rapid turnover of the

^bound

fraction did however reduce the time to Tmax of the bound

specific

radioactivity curve. That the

^unbound

^{data can} be fit ^using this

^model

demonstrates

that

either

a

fast-liver

->

slow

VLDL or

slow-liver

->

fast VLDL model can

be

used

to fit

such data.

The

physiological implications

of

using

either model are significant and

will be

^{discussed later.}

tr t

I

2 4 t2 ^1A

Hours

Figure

25.

Fit of

slow

liver

triglyceride model

to

unbound

(^)

and bound

(¡) VIOfZ

triglyceride data

of

subject

F.

The dashed

line

represents

the

calculated

fit to

the bound data using the model

in

Figure 24.

è0 E

éÀ

>ì I

ctlo É6l

ú

I (Jq,

(t)È

!q)

o

>ì èo Fr

100

10

1

Â

II I

10

Two

features

of the

bound fraction kinetics prevented

this

model from

fitting the data. The first being the near

simultaneous

rise in

^specific

radioactivity

of the

unbound and bound fractions,

and

secondly

the

^complex

nature of the bound fraction curve after

reaching

its

maximum specific radioactivity.

The

complex shape

of the

bound fraction's

curve after

peak

specific

radioactivity suggested

that the

bound

fraction did not

represent a

homogeneous population

of

^particles.

The

presence

of a ^rapidly turning over

component

in the

unbound

fraction was also

observed

in the earlier

studies

of

Nestel

et al

^{(1983) in}

humans, and

Huff

and Telford (1984) who injected labelled human lipoproteins

into

minature

pigs.

Following

the

reinjection

of

^labelled

VLDL into

^humans

both

rapidly and slowly turning over components were identified

in the

VLDL

apo B

decay curves.

In

some subjects

the

unbound

fraction

decayed slowly

while in

others

the

decay

was rapid.

Assuming

that the liver

triglyceride conversion process

is the rate limiting

step

in

triglyceride metabolism there

must be, within the

unbound

fraction a population of particles

which

turnover rapidly, at a rate

comparable

to that

^observed

in subject

^F (approximately

t

^h-

^1). The fact that the

postulated

rapid

componont

in

the

unbound VLDLZ was

only

seen

in

one

of the

three subjects could

be

^readily explained

in the

unbound fraction

in two of the other

subjects,

but not

ⁱⁿ

subject F,

contains

a slowly turning over

remnant-like

fraction which

is

derived

from the

putative rapidly turning over more nascent

like

particles. In

that

case

the

^kinetics

of the

rapidly turning over component would

be

^masked.

This concept

is

developed further

in

Model

3,

below.