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The kinetics of I131 metabolism in the dog and human
The Kinetics of PI Metabolismin the Dog and Human W. R. Ruegamer’ From the Radioisotope Unit of the Veterans Administration Denver, Colorado
Hospital,
ReceivedApril 30, 1953 INTRODUCTION
Radioiodine was first used by Hamilton and Soley (1,2) in 1939as a tool for the study of iodine metabolism in the human. By means of a Geiger counter placed over the thyroid isthmus, it was discoveredthat the amount of radioiodine accumulated by the gland bore a direct relationship to its state of activity. Since this report, a number of investigators have developed improved methods for direct monitoring (3-9) and for assessingthyroid function from circulating blood levels of radioactive protein bound or hormonal iodine (10-17) and from urinary excretion measurementsof Pa1activity (18-26). In casesof renal impairment, it appears that 24-26-hr. in viva measurementsof thyroid uptake give reasonably accurate assessmentof thyroid function, and that urinary excretion data or thyroid clearance or accumulation rates may give a false picture of thyroid function (23). Not all investigators agree with this, however (26). Aside from the importance of developing rapid and accurate clinical methods for evaluating thyroid activity, severalstudies have been made of the kinetics and mechanismsof iodine clearancefrom the blood into other compartments such as the urine and thyroid. One of the first of such studies was that performed by Astwood and Stanley (8) who found that the rate of thyroid uptake as determined by direct gamma-counting was a parabolic function of time. The slope of the straight line obtained by plotting the increasing radioiodine activity against the square root of time was termed the accumulation gradient. In an excellent series of papers (4,20-22), Keating and co-workers have shown that thyroid uptake and urinary excretion of P are exponential processeswhich obey 1With the technical assistanceof T. M. Bow and A. J. Dukes. 119
120
W. R. RUEGAMER
the expression Q = Qf(l -e~‘~) where Q represents the per cent of the dose of In1 either excreted into the urine or accumulated by the thyroid in time t, Q, is the asymptotic per cent of the dose either excreted or accumulated, and T is a constant describing the rate at which 1131leaves the blood stream. Values of T calculated from these data agreed well with those obtained directly from blood det,erminations, indicating that thyroid accumulation and urinary excretion of Im are functions of the plasma iodine concentration. Myant and Pochin (10) calculated thyroid clearance in terms of the number of milliliters of plasma cleared of 113’ per hour, and concluded that the thyroid accumulation rate remains proportional to the plasma concentration. The accumulation gradient proposed by Astwood and Stanley is useful in assessing thyroid function, but it does not appear to have physiological significance and hence does not contribute to the understanding of thyroid clearance. The data of Myant and Pochin are also difficult to analyze kinetically because of the maximum through which all of the curves pass, indicating a build-up of radioiodine concentration in the blood stream due to the absorption of P3’ from the gastrointestinal tract. Although the mathematical expressions proposed by Keating et al. probably define iodine disposal correctly, the experiments were performed with humans where it is sometimes difficult to obtain frequent and complete specimens of urine, gastric juice, and blood. Because of these uncertainties and the difficulty of estimating asymptotic values, 113’ clearance experiments were performed with the dog. A kinetic theory of iodine metabolism was developed from the animal data and tested in the human. ANIMAL
EXPERIMENTS
: METHODS
A total of 45 adult mongrel dogs of the male sex were maintained under sodium pentobarbital anesthesia for periods of time ranging from 8 to 36 hr. Carrier-free iodine-131 (I’*‘), obtained from the Oak Ridge National Laboratories, was injected intravenously at a level of 5 microcuries &.)/kilogram of body weight. After a waiting period of 30 min. to allow for adequate mixing in the blood stream and to allow for the attainment of equilibrium between plasma and red cells (25), the collections of urine, gastric juice, and blood samples were started. All collections made prior to this time were discarded. The animals were catheterized, and the total urinary bladder contents together with two or three distilled water washings were coIlected at 2-hr. intervals. All animals were prepared for gastric sampling by making a stab wound at the mid-line of the rectus muscle and inserting a glass button which was held firmly in place by purse-string sutures. A lavage tube was passed through the button and into the stomach for convenient sampling. To make certain that none of the iodide secreted into the stomach would be passed
KINETICS
121
OF 113’ METABOLISM
through the pylorus and absorbed in the duodenum, the stomach was ligated at the pylorus. Since manipulation of the stomach might cause greater gastric flow, a 2.hr. waiting period was allowed before injecting the iodine. Total gastric contents plus two distilled water washings were aspirated every 2 hr. and assayed for their total 1131content. Since all animals were prepared in this manner, it is impossible to state whether ligating the pylorus or anesthetizing the animal had any effect upon the gastric rate constants. Heparinized blood samples were collected hourly from the foreleg vein, and both whole blood and plasma were digested with 2.5 N NaOH until a clear solution resulted. Glucose solution was supplied subcutaneously at 2-hr. intervals in an amount equivalent to the volume of urine removed from the animal in order to maintain a constant fluid volume. Cumulative uptake of I 13*by the thyroid was followed by direct gamma-counting with a Radiation Counter Laboratory end-window tube which had an inside diameter of 4.2 cm., was collimated 9 cm., was completely shielded with 1.4 cm. of lead (except for the window), and was mounted in a portable x-ray machine for convenient focusing. An aluminum shield (350 mg./sq. cm.) was fastened over the end of the tube holder to screen out p-radiation, and correction for body background was made according to the method developed and presented in the experimental section of this report. Uptake is expressed as per cent of the acministered dose and is computed by drawing a comparison between the counts per unit time obtained when a volume of 113’ solution equal to that given intravenously is counted at the same fixed distance (20 cm.) as the thyroid and under the same geometrical conditions. Correction for tissue absorption was made by multiplying the per cent of the administered dose by e-ud where u is the absorption coefficient in tissue (0.03) and d is the depth of the gland in the neck in centimeters. After the termination of each experiment, the thyroid gland was removed and digested in hot alkali, and the per cent of the administered dose was determined as an added check on the accuracy of the monitoring method. Tissues subjected to 1131 assay were removed immediately after sacrificing the dog with sodium pentobarbital, and were digested in hot alkali until “solubilized.” The percentage of the dose of I I31 in the urine, gastric juice, blood specimens, and thyroid and other tissues was determined by p-counting, using an adaptation of the isotope-dilution technique* to correct for self-absorption and physical decay. One milliliter samples of the extracts were plated in quadruplicate on Tracerlab planchets and fixed with a drop of 10% silver nitrate solution. To two of the platings was added 0.1 ml. of either a 10m3or 1OW dilution of the stock solution of I”J’ used in the injection, and all plates were dried thoroughly under an infrared lamp. The planchets were then counted under the same geometrical conditions, and knowing the volumes of hydrolyzate or samples, the percentage of the dose of 1131present was calculated with the aid of the following equation. Per cent dose = ‘:(y$(,)T dz’
z
where C, is the average number of counts per second for the planchets containing the sample alone, D, is the dilution factor, V. is the volume of the stock 1’31 solu2 Hlad,
C. J., Jr., unpublished
data.
122
W. R. RUEGAMER
tion added to the planchets, Vd is the volume of the dose injected, and C,’ is the average number of counts per second obtained for the planchets containing sample and stock 113’. All counts were corrected for room background.
ANIMAL
EXPERIMENTS : NESULTS
A. Experimental Since the total number of counts per second obtained over the thyroid isthmus at zero time are due primarily to body background plus room background, and since this value decreases whereas the thyroid count THROAT
AND
THIGH
DISAPPEARANCE
20
0
1. Throat thyroidectomized FIG.
and thigh dog.
OF
I-
UI
HOURS
2
4
8
disappearance
8
curves
IO
12
of P
I4
as determined
in a
increases with time, some method had to be devised for correcting for the body-background component. Experiments with thyroidectomized dogs revealed that the throat and thigh counts decrease at some complex but equal rate when the thigh counts are determined over the femoral triangle (Fig. 1). In the intact animal, therefore, one can construct a throat disappearance curve from thigh data and subtract this curve from the gross thyroid uptake curve to obtain true thyroid uptake in counts per minute (Fig. 2). Counting rate was converted to per cent administered dose by comparing this value with that obtained by counting the dose of Pa1 under the same geometrical conditions. This method of direct monitoring was used in all subsequent experiments. The average per cent dose accumulated by the thyroid per hour was calculated from
UNCORRECTED 113’ BY T”E THYROlD
UPTAKE
CORRECTED BY THE THYROID
a 6 ‘4
8 /
4
0”2
b
HOURS
i3
m
i2
14
FIG. 2. Uncorrected and corrected I la1 thyroid uptake curves for a dog. A hypothetical throat disappearance curve was constructed with a shape identical to the thigh disappearance curve but passing through the uncorrected count rate value for the throat at zero time. Cumulative thyroid uptake was then calculated by subtracting the throat disappearance curve from the gross uptake curve.
2
THYROID
I
,011 0
,
2
4
ACCUMULATION
b HOURS
8
RATE
m
12
FIG. 3. Thyroid accumulation rate curve for a dog. The y intercepts for the first and second components of the curve are K&‘z and K&‘I, respectively. The best possible curve was drawn using the’chord-area method for plotting the data. 123
124
IV.
R.
RUEGAMER
the per cent dose found in the thyroid at l- or 2-hr. intervals. Similar calculations were made for urinary excretion of P3’ except that the average per cent dose excreted per hour was determined as the product of the II31 concentration in a given urine specimen and the volume of urine excreted over this same period of collection. The chord-area method was
Fra. 4. Cumulative urine excretion and excretion rate curves for a dog. The ?/ intercepts for the first and second components of the rate curve are K&Z and K &‘I, respectively. The chord-area method was used to plot the data.
used to plot the data and to draw the best possible curve through the , respective chords. It would appear from these curves (Figs. 3 and 4) that the rate at which the thyroid accumulates and the kidney excretes P during the first 24 hr. is much greater than in succeeding hours and that once this rapid phase is completed, the accumulation or excretion proceeds at an exponential rate. Since equilibrium is reached very rapidly between plasma and red cells (25), plasma values alone could be sufEcient to study vascular dis-
KINETICS
OF
Ilal
125
METABOLISM
appearance of Pal. Curves obtained by plotting the per cent dose versus time show the same two-component forms as those obtained for thyroid accumulation and urine disposal (Fig. 5).
PLASMA
0
2
4
CLEARANCE
OF
6 0 HOURS
IO
113’
IZ
‘4
FIG. 5. Plasma clearance curve for a dog. The y intercepts of the first second components of the clearance curve were 62 and Cl, respectively.
TABLE
and
I
Pa1 Content of Various Tissues of a Dop (Injected with 5 PC. I’*1 per kilogram of body weight) Per cent dose/g. tissue Per cent dose per tissue
Tissue
Gall bladder Heart Kidney Liver Lung Muscle (leg)
(in toto)
Pancreas Spleen Stomach (cardiac end) Stomach (mid section) Stomach (pyloric end) Stomach contents Urinary bladder
0 0040 0.0026 0.0025 0.0026 0 0031 0 0026
0.067 0.55 0.30 1.65 0 57 -
0 0014 0.0029 0.0230 00200 0.0530 0.0524 00090
0 15 0 27 0.53 0 43 1.60 23 40 0.23
0 The tissues were removed for analysis immediately ficed with pentobarbital sodium (Nembutal).
after
the dog was sacri-
126
W.
R.
RUEGAMER
When the per cent dose accumulated by the thyroid was totaled with that found in the urine, a significant fraction of the dose remained unaccounted for. Certain tissues, such as the urinary bladder and stomach appeared to show a differential uptake, although this activity may have
FIG. 6. Gastric cumulative secretion and secretion rate curves for a dog. The ?/intercepts of the first and second components of the gastric secretion rate curve are K.&z and K.&I, respectively. The chord-area method was used to plot the data.
been due to contamination. The total contribution from these tissues was insignificant in any case (Table I). By contrast, however, the stomach contents were found to contain as much as 20-30 y0 of the dose, thus accounting for the remainder of the I IS’. In succeeding experiments, therefore, gastric disposal of P was followed. Cumulative gastric secretion and secretion rates were plotted against time, yielding curves like those in Fig. 6.
KINETICS
OF 113’ METABOLISM
127
The presence of such large amounts of radioiodine in the stomach led to speculation upon the mechanism involved in the transport of iodine across the stomach wall. Schiff (28) and Davenport (29) have investigated this problem with the human and have found that radioiodine is secreted as iodide into the stomach and that the concentration of iodide is independent of the rate of secretion. In our experiments, the greatest concentration of 1131was found in the pyloric end of the stomach wall where parietal cells are the most abundant (Table I). This might tend to substantiate the belief that the 1131is being secreted as HI13’. To gain further evidence for this, samples of gastric juice were collected under histamine and methacholine (Mecholyl) stimulation, and atropine inhibition, and a comparison was made between the free and total acidity of the samples and their 113’ content (Table II). Although not strictly quantitative, there does appear to be a definite correlation between the concentrations of acid and 1131secreted by the stomach under the influence of certain gastric stimulants and inhibitors. If the iodide ion were being secreted passively along with the chloride ion to satisfy the charge of the hydrogen ion, bromide, which occurs midway between chlorine and iodine in the periodic table, should behave in an analogous manner. When radioactive BrS2 was administered intravenously at a level of 5 +./kg. body weight, a qualitative correlation was observed between the percentage of the dose of Brs2 secreted and the acidity of the gastric juice. However, the amount of bromine was only a fraction of the 1131 secreted under similar conditions (Table II). B. Theoretical The fact that two-component curves were obtained for thyroid, urine, gastric juice, and plasma disposal of 1131and that the slopes of the respective components of all curves appeared to be the same, led to a mathematical formulation of the kinetics involved. Equations for expressing Ii31 metabolism are based on certain assumptions, but these appear well justified since the experimental data fit the theoretical considerations. It has been shown that the sum of the amounts of 113’found in the thyroid, stomach, urine, and estimated body fluids account for 100% of the administered dose. This may be expressed as T+U+X+PfR+B=Po=lOQ%
TABLE The Efectu EE.zzz
II
of Chemical Stimulation and Inhibztion on the Acidity and the Amounts of I’= and BT@ Secreted into the Gastric Juice ( L Dog Per ten dose Brn secreted hr.
Dog NC 1.
Total acidity
Free acidity
-
18
Control (3 hr. gastric
40
-
0.2
0.0
Gave 5 mg. Mecholyl (1 hr. gastric collection)
14.0
-
16.5
12.7
Gave 0.8 mg. atropine (1 hr. gastric collection)
1.6
-
03
0.0
-
3.3
1.2
90 --
-Control (3 hr. gastric
--
collection)
10 0
-
1.3
0.6
Gave 0.8 mg. atropine (1 hr. gastric collection)
4.0
-
0.2
0.0
-
4.5
Gave 1 mg. histamine (1 hr. gastric collection)
15.6 --
27
Control (3 hr. gastric
--
-
collection)
Gave 5 mg. Mecholyl (1 hr. gastric collection) Control (3 hr. gastric
Gave 5 mg. Mecholyl (1 hr. gastric collection)
20 --
30
0 05
0
2.8 --
0 24
30 ---
0.52 --
-
collection)
-0 21
---
28
mmolcsfhr.
collection)
Gave 10 mg. Meoholyl (1 hr. gastric collection) 19
nmoles/hr.
1.6 ---
0 07
12.0
0
8.7
--
Control Carrier bromidea (3 hr. gastric collection)
-
0.32
1.0
0
Gave 5 mg. Mecholyl (1 hr. gastric collection)
-
0.74
7.92
5.52
n Carrier potassium bromide (500 mg.) was administered animal 2 hr. before the start of the experiment. 128
intravenously
to the
KINETICS
OF 1131 METABOLISM
129
where T = the per cent dose of 1131accumulated by the thyroid, U = the S = the P = the R = the B = the PO = the
per per per per per per
cent dose excreted into the urine, cent dose secreted into the stomach, cent dose in the plasma, cent dose in the red blood cells, cent dose in all other body fluids, and cent dose in the plasma at zero time: at t = 0,
P = PO. From the work of others (lo), it can be assumed that the amounts of iodine taken up by the thyroid or excreted into the urine are functions of some rate constant for that particular organ and the plasma concentration of iodine, provided that plasma iodine remains in equilibrium with red cell iodine and that a simple permeable membrane operates between plasma and extravascular fluids. Plasma-red cell equilibrium is reached very rapidly (25), and KpsRP = KasPRwhere KP,R and K,, are proportionality constants with dimensions of time-*. If a permeable membrane exists between plasma and body fluids, then at equilibrium the rate of 113’flow into the plasma (kB.pB) is just balanced by the rate of flow into body fluids (lcp, *P) so that i&, pB = kpn=P. Disappearance from the plasma can then be expressed as
-dP -= dt
(2)
KTP + KuP + K,P + kp,~P - k&3 + KP.RP - K,.,R and the last two terms drop out since equilibrium has obtained between the plasma and red cell compartments, before the start of the experiment.
-dP = KoP - b,,B + kp,BP dt
(3)
where the constant Ko is equal to the sum of the rate constants for the thyroid, urine, and gastric juice, respectively. Ko = KT + Kv + Ke Body fluid disposal of Pa1 is given by
--dB = k,,,B - kp+P dt and the solution of Eq. (2) and (5) becomes
(4)
130
W. R. RUEGAMER
where ml
= k _
m2-
1) Ko;
K.
= m&ml
m2=k+(d~+l)K
0
2
Cl
=
p.
(K,..[
c2 = PO m2 [
khmz __--__
-
m1
(m2 - ml) (Ko
Cm2
+ k)mm - ml>
1 I
(7)
where m2 is the slope of the first and most rapid component of the plasma curve, ml is the slope of the second component, C1 and C2 are the intercepts of the two slopes of the plasma rate curve (Fig. 5) and Cl
+
c2
= PO.
Since dT = K T> p dt
!!? & = K TlC ,--“Qt + KTC2t3-’
d-u = K VP p dt
d” dt = K UlC e-m’t -I- KvC2e-““t
(8)
then T
=
KT’% -+--
&C2
ml
m2
u -_ &Cl + KuCz -ml m2 x
=
-K&I ml
+
KsC2 -m2
KT’&?-“‘t ml
KL’~e~“‘t
-
KTc~e--’ m2
KvC2ev”“’
ml
m
K&‘~eC”‘t ml
KsC2ew”“’ m2
(9)
When individual rate constants were calculated from the experimental disappearance curves and substituted into Eq. (8) for different values of 1, the resulting theoretical curves obtained for thyroid, urine, stomach, and plasma disappearance could be superimposed over their respective experimental curves.
131
KINETICS OF 1131METABOLISM HUMAN
EXPERIMENTS:
METHODS
Tile methods used for studying iodine metabolism in the human were similar to those employed in the dog with the following exceptions. Adult laboratory personnel of both sexes were administered a carrier-free tracer dose of 100 PC. Pa1 were monitored intravenously. After a waiting period of 30 min., the individuals TABLE Summary
III
of the Data Obtained from Iodine Metabolism Laboratory Personnel
=
== =
--
rhyroid CEW
Sex
Studies Made with 12 Adult
p2%i’
Total / er cent rim 1" dose" xcreted ACI 24 hr. ounted
KT
=
KU
mP
KO
:?h?
qodose y. dose JJ cc cw MM DS RV JC MS RR AS WR VM
F F F F F F F M M M M M
40 13 25. 17 0. 20. 24. 22. 54 17 21. 30.
hr.-’
46. 85. 51. 74 88. 71. 89. 65. -
98 106. 86. 103. 101. 100. 118. 95. -
59. 57. 54.
85. 97. 92.
0 080 0.028 0.045 0.037 0.00 0.028 0.037 0.066 0 072 0.038 0.031 0 060
hr.-’
hr.-’
0.079 0.21 0.11 0.22 0.15 0.19 0.16 0.17 -
0.159 0.238 0.155 0.257 0.15 0.218 0.197 0.236 -
0.12 0.12 0 18
0.158 0.151 0 240
hr.7
0.187 0.180 0.158 0.280 0.170 0.212 0.185 0 290 0 470 0.188 0.183 0 190
0.096 0.096 0.074 0.095 0 107 0.092 0.111 0 059 0 079 0 078 0.097 0 048
0.500 0.450 0.440 0.800 0.440 0.564 0.489 0 460 0 940 0.510 0.500 0 391
(1The per cent dose remaining in the general circulation was estimated from the thigh disappearance curve and added to the amounts of iodine found in the urine and thyroid. b The ml and rn2 values given for each case represent the average of the individual slopes for the thyroid, urine, and plasma disappearance curves. and a blood specimen representing zero time was obtained. At appropriate time intervals thereafter, blood samples and individual urine specimens were collected for I1al assay, and throat and thigh monitorings were made at a fixed distance of 20 cm., using the same counting system as that employed in the animal studies. Gastric samples were not collected since significant amounts of 1131do not appear to accumulate in the stomach of the “normal” unanesthetized person (28).
HUMAN
EXPERIMENTS:
RESULTS
Plasma, thyroid, and urine I I31 disappearance curves obtained from studies with laboratory personnel all exhibited the same two-component
132
W. 11. RUEGAMER
nature as those observed in the animal experiments. The curves were analyzed and found to obey the mathematical expressions derived in the previous section. Disappearance-rate constants obtained from these curves are summarized in Table III together with other pertinent data such as the percentage of the dose accounted for and the average slopes of the two components of the experimental curves. The final percentage of the dose taken up by the thyroid was checked by another monit,oring procedure used for routine clinical analysis, and the two methods were found to agree well with one another. DISCUSSION
Before the start of each experiment, a waiting period of 30 min. was allowed for adequate mixing of the injected 1131,and for the attainment of equilibrium between plasma and red cell P31. Zero time on the abscissa of Figs. l-6 therefore represents the time at which the metabolism tests were started and is actually 30 min. after the 1131injection. It is recognized that a significant amount of I 131leaves the blood stream during this time, but the disappearance rate curves become so complex during this first 30 min. that a mathematical analysis of the whole curve then becomes extremely difficult. Initial mixing in the blood stream is probably complete in 24 min. (30) and equilibrium between plasma and red cell 113’is probably reached in another 5 min. (25). If the first 30 min. is neglected, one can side-step this complication and define the remainder of the disappearance curves. Berson et al. (30) have pointed out the importance of assessing thyroid, kidney, and plasma clearances during the first 30 min. as a means of diagnosis of thyroid disorders, but it is believed that inclusion of such data in the present study would only complicate the analysis unnecessarily. Under these conditions, rate curves for plasma, thyroid, urine, and gastric disappearance all appeared to be of a two-component nature when plotted on semilogarithmic paper (Figs. 3-6). There is an initial rapid disappearance followed by a much more prolonged exponential disappearance. The slopes of the two components obtained from each of the curves in any one experiment were all found to be the same respectively. This strongly supports the theory that the rates of thyroid, urine, and gastric juice disappearance are all functions of the plasma concentration and their respective rate constants, and that -dP at
= K,P + Kc,P - K,P -I b.,P
- b,,B
KINETICS
OF
113’ METABOLISM
133
The rapid fall in plasma 1131observed during the first few hours is .apparently due to the rapid outflow of iodine into the body fluid reservoir until equilibrium is reached and kp, ,sP = Ice9pB. After equilibrium has obtained, plasma disappearance of II31 proceeds at a single exponential rate. The initial rapid drop in the plasma 1131is reflected in the disappearance curve for thyroid, urine, and gastric juice, since the amount accumulated, excreted, or secreted depends upon the plasma concentration as well as on the respective disappearance rate constants. This is also in direct support of Keating’s hypothesis that the slope r of the straight line obtained by plotting log (Qf - Q) against time either for urine or for thyroid data is the slope of the curve obtained for plasma disappearance. The sum of the disappearance-rate constants for thyroid, urine, and gastric juice was equated to another constant Ko [Eq. (7)]. Since a numerical value can be obtained for K. from plasma data directly and since numerical values can also be obtained for each disappearance constant, it is possible to compare the sum of the disappearance constants for the thyroid, urine, and gastric juice with that of K, (Table III). Actually, K, values were not obtained in the human experiments as explained previously, and in this case the sum of the urine and thyroid rate constants are shown to be numerically equal to Ko. Because of this close correlation between experimental results and those that might be expected from theoretical considerations, it is believed that the original assumptions of constant fluid volumes and of a simple permeable membrane operating between the plasma and other body fluids are valid, and that the over-all concept of 1131metabolism in the dog and in the human may be represented diagrammatically as shown in Fig. 7. This model is very similar to that of the Mayo group, but the curves obtained in the present study have perhaps more direct meaning since the gastrointestinal absorption factor and the necessity for estimating asymptotes have been eliminated. Sodium pentobarbital anesthesia may have been responsible for the accumulation of II31 in the stomach of the dog, but to prevent the iodine from being passed into the duodenum where it might be reabsorbed and placed back in the circulation, the pylorus was ligated. Stomach secretion of radioactive iodine as determined under these conditions is an exponential process. As to the mechanism, it seems fair to state only that the process appears to be intimately associated with gastric secretion as evidenced by the correlation observed between secretion and acid
134
W. R. RUEGAMER
production under chemical stimulus and inhibition, and also as evidenced by the finding that the highest 1131activity occurs in the pyloric end of the stomach wall where parietal cells are the most abundant. Radioactive bromine-82 studies afforded additional evidence that iodide is being secreted along with hydrogen ion in the same manner as the chloride ion, by demonstrating a direct relationship between the amount of Br** secreted and the acid produced under chemical stimulation. The amount of bromine (with or without carrier) secreted per hour is much less than the amount of iodine, but the increased secretion is of the same order of magnitude for both ions when gastric flow is increased. The role
R.B.C.
i_,
FIG. ‘7. A highly diagrammatic olism in the dog and human.
model portraying
the kinetics of iodine metab-
of the halogens in gastric secretion is being studied in greater detail with the aid of radioactive isotopes, and the results of these findings will be presented in a subsequent paper. ACKNOWLEDGMENTS The author is indebted to Dr. Joseph H. Holmes for his counsel and stimulating criticism and to Dr. J. R. Cann for his contribution toward the mathematical analysis of the data. SUMMARY
A kinetic study of iodine metabolism was performed with 40 dogs and 12 humans using intravenously administered radioactive carrier-free iodine-131 (P31). Because of the close correlation between the experimental data and those expected from theoretical considerations, it is believed
KINETICS OF 1131METABOLISM
135
that equilibrium between plasma and red cell Pa1 is reached in a few minutes, whereas 4 hr. is required for the attainment of equilibrium between plasma and the other body fluids. During this period, the thyroid, kidney, and stomach also clear I I31from the plasma. The outward diffusion into body fluids accounts for the rapid drop in plasma, thyroid, urine, and gastric disappearance of P since they are all functions of the plasma concentration. After equilibrium has obtained between plasma and the other body fluids, IL31disappearance into the thyroid, urine, and gastric juice proceeds at an exponential rate. Disappearance into any one of these compartments can then be expressed as the product of the individual clearance constant for that tissue (KT, K,, or I&) and the proportion of the dose present in the plasma. The P content of gastric juice was found to account for as much as 30 % of the administered dose in the anesthetized dog, and yet had no significance in the human where it was apparently absorbed rapidly from the duodenum. Gastric 1131secretion studies in the dog revealed a good qualitative correlation between the amount of 113’secreted and the acidity of the gastric juice under chemical stimulation and inhibition. REFERENCES 1. HAMILTON, J. G., AND SOLEY, M. H., Am. J. Physiol. 137, 557 (1939). 2. HAMILTON, J. G., AND SOLEY, M. H., Am. J. Physiol. 181, 135 (1940/41). 3. MYANT, N. B., HONOUR, A. J., AND POCHIN, E. E., Clin. Sci. 8, 135 (1949). 4. LUELLEN, T. J., KEATING, F. R., WILLIAMS, M. M. D., BERKSON, J., POWER M. H., AND MCCONAHEY, W. M., J. Clin. Invest. 28, 207 (1949). 5. HIDALGO, J. U., NADLER, S. B., BLOCH, T., AND NIESET, R. F., Proc. Sot. Exptl. Biol. Med. 77, 784 (1951). 6. PERLMAN, I., CHAIKOFF, I. L., AND MORTON, M. E., J. Biol. Chem. 139, 433 (1941). 7. GOMBERG, H. J., BEIERWALTES, W. H., AND LAMPE, I., Proc. Sot. Exptl. Biol. Med. 73, 405 (1950). 8. AST~OOD, E. B., AND STANLEY, M. M., Western J. Surg. Obstet. Gynecol. 66, 625 (1947). 9. QUIMBY, E. H., AND MCCUNE, D. J., Radiology 49, 201 (1947). 10. MYANT, N. B., POCHIN, E. E., AND GOLDIE, E. A. G., Clin. hi. 8, 109 (1949). 11. TAUROG, A., CHAIKOFF, I. L., AND ENTENMAN, C., Endocrinology 40,86 (197). 12. CHAIKOFF, I. L., TAUROG, A. AND REINHARDT, W. O., Endocrinology 40, 47 (1947). 13. FRIEDBERG, A. S., URELES, A., AND HERTZ, S., Proc. Sot. Exptl. BioZ. Med. 70, 679 (1949). 14. CLARK, D. E., MOE, R. H., AND ADAMS, E. E., Surg. Gynecol. Obstet. 28, 331 (1949). 15. LEBLOND, C. P., AND GROSS, J., J. CZin. EndocrinoZ. 9, 149 (1949).
136 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.
WT. R. RUEGAMER
CHAIKOFF, I. L., AND TAUROG, A., Ann. N. Y. Acad. Sci. 60, 377 (1948/50). ROLLMAN, H. S., AND PETIT, D. W. Proc. Sot. Exptl. B~ol. Med. 72,474 (1949). GREER, M. A., J. Clin. Invest. 30, 301 (1951). EDWARDS, R. R., REILLY, W. A., AND HOLRIES, R. G., Proc. Sot. Exptl. Biol. Med. 72, 158 (1949). KEATING, R. F., POWER, M. H., BERKSON, J., AND HAINES, S. F., J. Clin. Invest. 26, 1138 (1947). MCCONAHEY, W. M., KEATING, R. F., AND POWER, M. H., J. Clin. Invest. 28, 191 (1949). MCCONAHEY, W. M., KEATING, R. F., AND POWER, M. H., J. Clin. Invest. 30, 778 (1951). PERRY, W. F., AND HUGHES, F. S., J. C&z. Invest. 31, 457 (1952). ODDIE, T. H., Brit. J. Radial. 22, 261 (1949). MYANT, W. B., CORBETT, B. D., HONOUR, A. J., AND POCHIN, E. E., Clin. Sci. 9, 405 (1950). NITOWSKY, H. M., AND PUCK, T. T., J. Lab. Clin. Med. 39, 824 (1952). REYNOLDS, L., CORRIGAN, K. E., AND HAYDEN, H. S., Am. J. Roentgenol., Radium Therapy Nuclear Med. 68, 421 (1952). SCHIFF, L., STEVENS, C. D., MOLLE, W. E., STEINBERG, H., KEMPE, C. W., AND STEWART, P., J. Natl. Cancer Inst. ‘7, 349 (1946). DAVENPORT, H. W., Gastroenterology 1, 1055 (1943). BERSON, S. A., YALOW, R. S., SORRENTINO, J., AND RESWIT, B., J. Clin. Invest. 31, 141 (1952).
Hospital,
ReceivedApril 30, 1953 INTRODUCTION
Radioiodine was first used by Hamilton and Soley (1,2) in 1939as a tool for the study of iodine metabolism in the human. By means of a Geiger counter placed over the thyroid isthmus, it was discoveredthat the amount of radioiodine accumulated by the gland bore a direct relationship to its state of activity. Since this report, a number of investigators have developed improved methods for direct monitoring (3-9) and for assessingthyroid function from circulating blood levels of radioactive protein bound or hormonal iodine (10-17) and from urinary excretion measurementsof Pa1activity (18-26). In casesof renal impairment, it appears that 24-26-hr. in viva measurementsof thyroid uptake give reasonably accurate assessmentof thyroid function, and that urinary excretion data or thyroid clearance or accumulation rates may give a false picture of thyroid function (23). Not all investigators agree with this, however (26). Aside from the importance of developing rapid and accurate clinical methods for evaluating thyroid activity, severalstudies have been made of the kinetics and mechanismsof iodine clearancefrom the blood into other compartments such as the urine and thyroid. One of the first of such studies was that performed by Astwood and Stanley (8) who found that the rate of thyroid uptake as determined by direct gamma-counting was a parabolic function of time. The slope of the straight line obtained by plotting the increasing radioiodine activity against the square root of time was termed the accumulation gradient. In an excellent series of papers (4,20-22), Keating and co-workers have shown that thyroid uptake and urinary excretion of P are exponential processeswhich obey 1With the technical assistanceof T. M. Bow and A. J. Dukes. 119
120
W. R. RUEGAMER
the expression Q = Qf(l -e~‘~) where Q represents the per cent of the dose of In1 either excreted into the urine or accumulated by the thyroid in time t, Q, is the asymptotic per cent of the dose either excreted or accumulated, and T is a constant describing the rate at which 1131leaves the blood stream. Values of T calculated from these data agreed well with those obtained directly from blood det,erminations, indicating that thyroid accumulation and urinary excretion of Im are functions of the plasma iodine concentration. Myant and Pochin (10) calculated thyroid clearance in terms of the number of milliliters of plasma cleared of 113’ per hour, and concluded that the thyroid accumulation rate remains proportional to the plasma concentration. The accumulation gradient proposed by Astwood and Stanley is useful in assessing thyroid function, but it does not appear to have physiological significance and hence does not contribute to the understanding of thyroid clearance. The data of Myant and Pochin are also difficult to analyze kinetically because of the maximum through which all of the curves pass, indicating a build-up of radioiodine concentration in the blood stream due to the absorption of P3’ from the gastrointestinal tract. Although the mathematical expressions proposed by Keating et al. probably define iodine disposal correctly, the experiments were performed with humans where it is sometimes difficult to obtain frequent and complete specimens of urine, gastric juice, and blood. Because of these uncertainties and the difficulty of estimating asymptotic values, 113’ clearance experiments were performed with the dog. A kinetic theory of iodine metabolism was developed from the animal data and tested in the human. ANIMAL
EXPERIMENTS
: METHODS
A total of 45 adult mongrel dogs of the male sex were maintained under sodium pentobarbital anesthesia for periods of time ranging from 8 to 36 hr. Carrier-free iodine-131 (I’*‘), obtained from the Oak Ridge National Laboratories, was injected intravenously at a level of 5 microcuries &.)/kilogram of body weight. After a waiting period of 30 min. to allow for adequate mixing in the blood stream and to allow for the attainment of equilibrium between plasma and red cells (25), the collections of urine, gastric juice, and blood samples were started. All collections made prior to this time were discarded. The animals were catheterized, and the total urinary bladder contents together with two or three distilled water washings were coIlected at 2-hr. intervals. All animals were prepared for gastric sampling by making a stab wound at the mid-line of the rectus muscle and inserting a glass button which was held firmly in place by purse-string sutures. A lavage tube was passed through the button and into the stomach for convenient sampling. To make certain that none of the iodide secreted into the stomach would be passed
KINETICS
121
OF 113’ METABOLISM
through the pylorus and absorbed in the duodenum, the stomach was ligated at the pylorus. Since manipulation of the stomach might cause greater gastric flow, a 2.hr. waiting period was allowed before injecting the iodine. Total gastric contents plus two distilled water washings were aspirated every 2 hr. and assayed for their total 1131content. Since all animals were prepared in this manner, it is impossible to state whether ligating the pylorus or anesthetizing the animal had any effect upon the gastric rate constants. Heparinized blood samples were collected hourly from the foreleg vein, and both whole blood and plasma were digested with 2.5 N NaOH until a clear solution resulted. Glucose solution was supplied subcutaneously at 2-hr. intervals in an amount equivalent to the volume of urine removed from the animal in order to maintain a constant fluid volume. Cumulative uptake of I 13*by the thyroid was followed by direct gamma-counting with a Radiation Counter Laboratory end-window tube which had an inside diameter of 4.2 cm., was collimated 9 cm., was completely shielded with 1.4 cm. of lead (except for the window), and was mounted in a portable x-ray machine for convenient focusing. An aluminum shield (350 mg./sq. cm.) was fastened over the end of the tube holder to screen out p-radiation, and correction for body background was made according to the method developed and presented in the experimental section of this report. Uptake is expressed as per cent of the acministered dose and is computed by drawing a comparison between the counts per unit time obtained when a volume of 113’ solution equal to that given intravenously is counted at the same fixed distance (20 cm.) as the thyroid and under the same geometrical conditions. Correction for tissue absorption was made by multiplying the per cent of the administered dose by e-ud where u is the absorption coefficient in tissue (0.03) and d is the depth of the gland in the neck in centimeters. After the termination of each experiment, the thyroid gland was removed and digested in hot alkali, and the per cent of the administered dose was determined as an added check on the accuracy of the monitoring method. Tissues subjected to 1131 assay were removed immediately after sacrificing the dog with sodium pentobarbital, and were digested in hot alkali until “solubilized.” The percentage of the dose of I I31 in the urine, gastric juice, blood specimens, and thyroid and other tissues was determined by p-counting, using an adaptation of the isotope-dilution technique* to correct for self-absorption and physical decay. One milliliter samples of the extracts were plated in quadruplicate on Tracerlab planchets and fixed with a drop of 10% silver nitrate solution. To two of the platings was added 0.1 ml. of either a 10m3or 1OW dilution of the stock solution of I”J’ used in the injection, and all plates were dried thoroughly under an infrared lamp. The planchets were then counted under the same geometrical conditions, and knowing the volumes of hydrolyzate or samples, the percentage of the dose of 1131present was calculated with the aid of the following equation. Per cent dose = ‘:(y$(,)T dz’
z
where C, is the average number of counts per second for the planchets containing the sample alone, D, is the dilution factor, V. is the volume of the stock 1’31 solu2 Hlad,
C. J., Jr., unpublished
data.
122
W. R. RUEGAMER
tion added to the planchets, Vd is the volume of the dose injected, and C,’ is the average number of counts per second obtained for the planchets containing sample and stock 113’. All counts were corrected for room background.
ANIMAL
EXPERIMENTS : NESULTS
A. Experimental Since the total number of counts per second obtained over the thyroid isthmus at zero time are due primarily to body background plus room background, and since this value decreases whereas the thyroid count THROAT
AND
THIGH
DISAPPEARANCE
20
0
1. Throat thyroidectomized FIG.
and thigh dog.
OF
I-
UI
HOURS
2
4
8
disappearance
8
curves
IO
12
of P
I4
as determined
in a
increases with time, some method had to be devised for correcting for the body-background component. Experiments with thyroidectomized dogs revealed that the throat and thigh counts decrease at some complex but equal rate when the thigh counts are determined over the femoral triangle (Fig. 1). In the intact animal, therefore, one can construct a throat disappearance curve from thigh data and subtract this curve from the gross thyroid uptake curve to obtain true thyroid uptake in counts per minute (Fig. 2). Counting rate was converted to per cent administered dose by comparing this value with that obtained by counting the dose of Pa1 under the same geometrical conditions. This method of direct monitoring was used in all subsequent experiments. The average per cent dose accumulated by the thyroid per hour was calculated from
UNCORRECTED 113’ BY T”E THYROlD
UPTAKE
CORRECTED BY THE THYROID
a 6 ‘4
8 /
4
0”2
b
HOURS
i3
m
i2
14
FIG. 2. Uncorrected and corrected I la1 thyroid uptake curves for a dog. A hypothetical throat disappearance curve was constructed with a shape identical to the thigh disappearance curve but passing through the uncorrected count rate value for the throat at zero time. Cumulative thyroid uptake was then calculated by subtracting the throat disappearance curve from the gross uptake curve.
2
THYROID
I
,011 0
,
2
4
ACCUMULATION
b HOURS
8
RATE
m
12
FIG. 3. Thyroid accumulation rate curve for a dog. The y intercepts for the first and second components of the curve are K&‘z and K&‘I, respectively. The best possible curve was drawn using the’chord-area method for plotting the data. 123
124
IV.
R.
RUEGAMER
the per cent dose found in the thyroid at l- or 2-hr. intervals. Similar calculations were made for urinary excretion of P3’ except that the average per cent dose excreted per hour was determined as the product of the II31 concentration in a given urine specimen and the volume of urine excreted over this same period of collection. The chord-area method was
Fra. 4. Cumulative urine excretion and excretion rate curves for a dog. The ?/ intercepts for the first and second components of the rate curve are K&Z and K &‘I, respectively. The chord-area method was used to plot the data.
used to plot the data and to draw the best possible curve through the , respective chords. It would appear from these curves (Figs. 3 and 4) that the rate at which the thyroid accumulates and the kidney excretes P during the first 24 hr. is much greater than in succeeding hours and that once this rapid phase is completed, the accumulation or excretion proceeds at an exponential rate. Since equilibrium is reached very rapidly between plasma and red cells (25), plasma values alone could be sufEcient to study vascular dis-
KINETICS
OF
Ilal
125
METABOLISM
appearance of Pal. Curves obtained by plotting the per cent dose versus time show the same two-component forms as those obtained for thyroid accumulation and urine disposal (Fig. 5).
PLASMA
0
2
4
CLEARANCE
OF
6 0 HOURS
IO
113’
IZ
‘4
FIG. 5. Plasma clearance curve for a dog. The y intercepts of the first second components of the clearance curve were 62 and Cl, respectively.
TABLE
and
I
Pa1 Content of Various Tissues of a Dop (Injected with 5 PC. I’*1 per kilogram of body weight) Per cent dose/g. tissue Per cent dose per tissue
Tissue
Gall bladder Heart Kidney Liver Lung Muscle (leg)
(in toto)
Pancreas Spleen Stomach (cardiac end) Stomach (mid section) Stomach (pyloric end) Stomach contents Urinary bladder
0 0040 0.0026 0.0025 0.0026 0 0031 0 0026
0.067 0.55 0.30 1.65 0 57 -
0 0014 0.0029 0.0230 00200 0.0530 0.0524 00090
0 15 0 27 0.53 0 43 1.60 23 40 0.23
0 The tissues were removed for analysis immediately ficed with pentobarbital sodium (Nembutal).
after
the dog was sacri-
126
W.
R.
RUEGAMER
When the per cent dose accumulated by the thyroid was totaled with that found in the urine, a significant fraction of the dose remained unaccounted for. Certain tissues, such as the urinary bladder and stomach appeared to show a differential uptake, although this activity may have
FIG. 6. Gastric cumulative secretion and secretion rate curves for a dog. The ?/intercepts of the first and second components of the gastric secretion rate curve are K.&z and K.&I, respectively. The chord-area method was used to plot the data.
been due to contamination. The total contribution from these tissues was insignificant in any case (Table I). By contrast, however, the stomach contents were found to contain as much as 20-30 y0 of the dose, thus accounting for the remainder of the I IS’. In succeeding experiments, therefore, gastric disposal of P was followed. Cumulative gastric secretion and secretion rates were plotted against time, yielding curves like those in Fig. 6.
KINETICS
OF 113’ METABOLISM
127
The presence of such large amounts of radioiodine in the stomach led to speculation upon the mechanism involved in the transport of iodine across the stomach wall. Schiff (28) and Davenport (29) have investigated this problem with the human and have found that radioiodine is secreted as iodide into the stomach and that the concentration of iodide is independent of the rate of secretion. In our experiments, the greatest concentration of 1131was found in the pyloric end of the stomach wall where parietal cells are the most abundant (Table I). This might tend to substantiate the belief that the 1131is being secreted as HI13’. To gain further evidence for this, samples of gastric juice were collected under histamine and methacholine (Mecholyl) stimulation, and atropine inhibition, and a comparison was made between the free and total acidity of the samples and their 113’ content (Table II). Although not strictly quantitative, there does appear to be a definite correlation between the concentrations of acid and 1131secreted by the stomach under the influence of certain gastric stimulants and inhibitors. If the iodide ion were being secreted passively along with the chloride ion to satisfy the charge of the hydrogen ion, bromide, which occurs midway between chlorine and iodine in the periodic table, should behave in an analogous manner. When radioactive BrS2 was administered intravenously at a level of 5 +./kg. body weight, a qualitative correlation was observed between the percentage of the dose of Brs2 secreted and the acidity of the gastric juice. However, the amount of bromine was only a fraction of the 1131 secreted under similar conditions (Table II). B. Theoretical The fact that two-component curves were obtained for thyroid, urine, gastric juice, and plasma disposal of 1131and that the slopes of the respective components of all curves appeared to be the same, led to a mathematical formulation of the kinetics involved. Equations for expressing Ii31 metabolism are based on certain assumptions, but these appear well justified since the experimental data fit the theoretical considerations. It has been shown that the sum of the amounts of 113’found in the thyroid, stomach, urine, and estimated body fluids account for 100% of the administered dose. This may be expressed as T+U+X+PfR+B=Po=lOQ%
TABLE The Efectu EE.zzz
II
of Chemical Stimulation and Inhibztion on the Acidity and the Amounts of I’= and BT@ Secreted into the Gastric Juice ( L Dog Per ten dose Brn secreted hr.
Dog NC 1.
Total acidity
Free acidity
-
18
Control (3 hr. gastric
40
-
0.2
0.0
Gave 5 mg. Mecholyl (1 hr. gastric collection)
14.0
-
16.5
12.7
Gave 0.8 mg. atropine (1 hr. gastric collection)
1.6
-
03
0.0
-
3.3
1.2
90 --
-Control (3 hr. gastric
--
collection)
10 0
-
1.3
0.6
Gave 0.8 mg. atropine (1 hr. gastric collection)
4.0
-
0.2
0.0
-
4.5
Gave 1 mg. histamine (1 hr. gastric collection)
15.6 --
27
Control (3 hr. gastric
--
-
collection)
Gave 5 mg. Mecholyl (1 hr. gastric collection) Control (3 hr. gastric
Gave 5 mg. Mecholyl (1 hr. gastric collection)
20 --
30
0 05
0
2.8 --
0 24
30 ---
0.52 --
-
collection)
-0 21
---
28
mmolcsfhr.
collection)
Gave 10 mg. Meoholyl (1 hr. gastric collection) 19
nmoles/hr.
1.6 ---
0 07
12.0
0
8.7
--
Control Carrier bromidea (3 hr. gastric collection)
-
0.32
1.0
0
Gave 5 mg. Mecholyl (1 hr. gastric collection)
-
0.74
7.92
5.52
n Carrier potassium bromide (500 mg.) was administered animal 2 hr. before the start of the experiment. 128
intravenously
to the
KINETICS
OF 1131 METABOLISM
129
where T = the per cent dose of 1131accumulated by the thyroid, U = the S = the P = the R = the B = the PO = the
per per per per per per
cent dose excreted into the urine, cent dose secreted into the stomach, cent dose in the plasma, cent dose in the red blood cells, cent dose in all other body fluids, and cent dose in the plasma at zero time: at t = 0,
P = PO. From the work of others (lo), it can be assumed that the amounts of iodine taken up by the thyroid or excreted into the urine are functions of some rate constant for that particular organ and the plasma concentration of iodine, provided that plasma iodine remains in equilibrium with red cell iodine and that a simple permeable membrane operates between plasma and extravascular fluids. Plasma-red cell equilibrium is reached very rapidly (25), and KpsRP = KasPRwhere KP,R and K,, are proportionality constants with dimensions of time-*. If a permeable membrane exists between plasma and body fluids, then at equilibrium the rate of 113’flow into the plasma (kB.pB) is just balanced by the rate of flow into body fluids (lcp, *P) so that i&, pB = kpn=P. Disappearance from the plasma can then be expressed as
-dP -= dt
(2)
KTP + KuP + K,P + kp,~P - k&3 + KP.RP - K,.,R and the last two terms drop out since equilibrium has obtained between the plasma and red cell compartments, before the start of the experiment.
-dP = KoP - b,,B + kp,BP dt
(3)
where the constant Ko is equal to the sum of the rate constants for the thyroid, urine, and gastric juice, respectively. Ko = KT + Kv + Ke Body fluid disposal of Pa1 is given by
--dB = k,,,B - kp+P dt and the solution of Eq. (2) and (5) becomes
(4)
130
W. R. RUEGAMER
where ml
= k _
m2-
1) Ko;
K.
= m&ml
m2=k+(d~+l)K
0
2
Cl
=
p.
(K,..[
c2 = PO m2 [
khmz __--__
-
m1
(m2 - ml) (Ko
Cm2
+ k)mm - ml>
1 I
(7)
where m2 is the slope of the first and most rapid component of the plasma curve, ml is the slope of the second component, C1 and C2 are the intercepts of the two slopes of the plasma rate curve (Fig. 5) and Cl
+
c2
= PO.
Since dT = K T> p dt
!!? & = K TlC ,--“Qt + KTC2t3-’
d-u = K VP p dt
d” dt = K UlC e-m’t -I- KvC2e-““t
(8)
then T
=
KT’% -+--
&C2
ml
m2
u -_ &Cl + KuCz -ml m2 x
=
-K&I ml
+
KsC2 -m2
KT’&?-“‘t ml
KL’~e~“‘t
-
KTc~e--’ m2
KvC2ev”“’
ml
m
K&‘~eC”‘t ml
KsC2ew”“’ m2
(9)
When individual rate constants were calculated from the experimental disappearance curves and substituted into Eq. (8) for different values of 1, the resulting theoretical curves obtained for thyroid, urine, stomach, and plasma disappearance could be superimposed over their respective experimental curves.
131
KINETICS OF 1131METABOLISM HUMAN
EXPERIMENTS:
METHODS
Tile methods used for studying iodine metabolism in the human were similar to those employed in the dog with the following exceptions. Adult laboratory personnel of both sexes were administered a carrier-free tracer dose of 100 PC. Pa1 were monitored intravenously. After a waiting period of 30 min., the individuals TABLE Summary
III
of the Data Obtained from Iodine Metabolism Laboratory Personnel
=
== =
--
rhyroid CEW
Sex
Studies Made with 12 Adult
p2%i’
Total / er cent rim 1" dose" xcreted ACI 24 hr. ounted
KT
=
KU
mP
KO
:?h?
qodose y. dose JJ cc cw MM DS RV JC MS RR AS WR VM
F F F F F F F M M M M M
40 13 25. 17 0. 20. 24. 22. 54 17 21. 30.
hr.-’
46. 85. 51. 74 88. 71. 89. 65. -
98 106. 86. 103. 101. 100. 118. 95. -
59. 57. 54.
85. 97. 92.
0 080 0.028 0.045 0.037 0.00 0.028 0.037 0.066 0 072 0.038 0.031 0 060
hr.-’
hr.-’
0.079 0.21 0.11 0.22 0.15 0.19 0.16 0.17 -
0.159 0.238 0.155 0.257 0.15 0.218 0.197 0.236 -
0.12 0.12 0 18
0.158 0.151 0 240
hr.7
0.187 0.180 0.158 0.280 0.170 0.212 0.185 0 290 0 470 0.188 0.183 0 190
0.096 0.096 0.074 0.095 0 107 0.092 0.111 0 059 0 079 0 078 0.097 0 048
0.500 0.450 0.440 0.800 0.440 0.564 0.489 0 460 0 940 0.510 0.500 0 391
(1The per cent dose remaining in the general circulation was estimated from the thigh disappearance curve and added to the amounts of iodine found in the urine and thyroid. b The ml and rn2 values given for each case represent the average of the individual slopes for the thyroid, urine, and plasma disappearance curves. and a blood specimen representing zero time was obtained. At appropriate time intervals thereafter, blood samples and individual urine specimens were collected for I1al assay, and throat and thigh monitorings were made at a fixed distance of 20 cm., using the same counting system as that employed in the animal studies. Gastric samples were not collected since significant amounts of 1131do not appear to accumulate in the stomach of the “normal” unanesthetized person (28).
HUMAN
EXPERIMENTS:
RESULTS
Plasma, thyroid, and urine I I31 disappearance curves obtained from studies with laboratory personnel all exhibited the same two-component
132
W. 11. RUEGAMER
nature as those observed in the animal experiments. The curves were analyzed and found to obey the mathematical expressions derived in the previous section. Disappearance-rate constants obtained from these curves are summarized in Table III together with other pertinent data such as the percentage of the dose accounted for and the average slopes of the two components of the experimental curves. The final percentage of the dose taken up by the thyroid was checked by another monit,oring procedure used for routine clinical analysis, and the two methods were found to agree well with one another. DISCUSSION
Before the start of each experiment, a waiting period of 30 min. was allowed for adequate mixing of the injected 1131,and for the attainment of equilibrium between plasma and red cell P31. Zero time on the abscissa of Figs. l-6 therefore represents the time at which the metabolism tests were started and is actually 30 min. after the 1131injection. It is recognized that a significant amount of I 131leaves the blood stream during this time, but the disappearance rate curves become so complex during this first 30 min. that a mathematical analysis of the whole curve then becomes extremely difficult. Initial mixing in the blood stream is probably complete in 24 min. (30) and equilibrium between plasma and red cell 113’is probably reached in another 5 min. (25). If the first 30 min. is neglected, one can side-step this complication and define the remainder of the disappearance curves. Berson et al. (30) have pointed out the importance of assessing thyroid, kidney, and plasma clearances during the first 30 min. as a means of diagnosis of thyroid disorders, but it is believed that inclusion of such data in the present study would only complicate the analysis unnecessarily. Under these conditions, rate curves for plasma, thyroid, urine, and gastric disappearance all appeared to be of a two-component nature when plotted on semilogarithmic paper (Figs. 3-6). There is an initial rapid disappearance followed by a much more prolonged exponential disappearance. The slopes of the two components obtained from each of the curves in any one experiment were all found to be the same respectively. This strongly supports the theory that the rates of thyroid, urine, and gastric juice disappearance are all functions of the plasma concentration and their respective rate constants, and that -dP at
= K,P + Kc,P - K,P -I b.,P
- b,,B
KINETICS
OF
113’ METABOLISM
133
The rapid fall in plasma 1131observed during the first few hours is .apparently due to the rapid outflow of iodine into the body fluid reservoir until equilibrium is reached and kp, ,sP = Ice9pB. After equilibrium has obtained, plasma disappearance of II31 proceeds at a single exponential rate. The initial rapid drop in the plasma 1131is reflected in the disappearance curve for thyroid, urine, and gastric juice, since the amount accumulated, excreted, or secreted depends upon the plasma concentration as well as on the respective disappearance rate constants. This is also in direct support of Keating’s hypothesis that the slope r of the straight line obtained by plotting log (Qf - Q) against time either for urine or for thyroid data is the slope of the curve obtained for plasma disappearance. The sum of the disappearance-rate constants for thyroid, urine, and gastric juice was equated to another constant Ko [Eq. (7)]. Since a numerical value can be obtained for K. from plasma data directly and since numerical values can also be obtained for each disappearance constant, it is possible to compare the sum of the disappearance constants for the thyroid, urine, and gastric juice with that of K, (Table III). Actually, K, values were not obtained in the human experiments as explained previously, and in this case the sum of the urine and thyroid rate constants are shown to be numerically equal to Ko. Because of this close correlation between experimental results and those that might be expected from theoretical considerations, it is believed that the original assumptions of constant fluid volumes and of a simple permeable membrane operating between the plasma and other body fluids are valid, and that the over-all concept of 1131metabolism in the dog and in the human may be represented diagrammatically as shown in Fig. 7. This model is very similar to that of the Mayo group, but the curves obtained in the present study have perhaps more direct meaning since the gastrointestinal absorption factor and the necessity for estimating asymptotes have been eliminated. Sodium pentobarbital anesthesia may have been responsible for the accumulation of II31 in the stomach of the dog, but to prevent the iodine from being passed into the duodenum where it might be reabsorbed and placed back in the circulation, the pylorus was ligated. Stomach secretion of radioactive iodine as determined under these conditions is an exponential process. As to the mechanism, it seems fair to state only that the process appears to be intimately associated with gastric secretion as evidenced by the correlation observed between secretion and acid
134
W. R. RUEGAMER
production under chemical stimulus and inhibition, and also as evidenced by the finding that the highest 1131activity occurs in the pyloric end of the stomach wall where parietal cells are the most abundant. Radioactive bromine-82 studies afforded additional evidence that iodide is being secreted along with hydrogen ion in the same manner as the chloride ion, by demonstrating a direct relationship between the amount of Br** secreted and the acid produced under chemical stimulation. The amount of bromine (with or without carrier) secreted per hour is much less than the amount of iodine, but the increased secretion is of the same order of magnitude for both ions when gastric flow is increased. The role
R.B.C.
i_,
FIG. ‘7. A highly diagrammatic olism in the dog and human.
model portraying
the kinetics of iodine metab-
of the halogens in gastric secretion is being studied in greater detail with the aid of radioactive isotopes, and the results of these findings will be presented in a subsequent paper. ACKNOWLEDGMENTS The author is indebted to Dr. Joseph H. Holmes for his counsel and stimulating criticism and to Dr. J. R. Cann for his contribution toward the mathematical analysis of the data. SUMMARY
A kinetic study of iodine metabolism was performed with 40 dogs and 12 humans using intravenously administered radioactive carrier-free iodine-131 (P31). Because of the close correlation between the experimental data and those expected from theoretical considerations, it is believed
KINETICS OF 1131METABOLISM
135
that equilibrium between plasma and red cell Pa1 is reached in a few minutes, whereas 4 hr. is required for the attainment of equilibrium between plasma and the other body fluids. During this period, the thyroid, kidney, and stomach also clear I I31from the plasma. The outward diffusion into body fluids accounts for the rapid drop in plasma, thyroid, urine, and gastric disappearance of P since they are all functions of the plasma concentration. After equilibrium has obtained between plasma and the other body fluids, IL31disappearance into the thyroid, urine, and gastric juice proceeds at an exponential rate. Disappearance into any one of these compartments can then be expressed as the product of the individual clearance constant for that tissue (KT, K,, or I&) and the proportion of the dose present in the plasma. The P content of gastric juice was found to account for as much as 30 % of the administered dose in the anesthetized dog, and yet had no significance in the human where it was apparently absorbed rapidly from the duodenum. Gastric 1131secretion studies in the dog revealed a good qualitative correlation between the amount of 113’secreted and the acidity of the gastric juice under chemical stimulation and inhibition. REFERENCES 1. HAMILTON, J. G., AND SOLEY, M. H., Am. J. Physiol. 137, 557 (1939). 2. HAMILTON, J. G., AND SOLEY, M. H., Am. J. Physiol. 181, 135 (1940/41). 3. MYANT, N. B., HONOUR, A. J., AND POCHIN, E. E., Clin. Sci. 8, 135 (1949). 4. LUELLEN, T. J., KEATING, F. R., WILLIAMS, M. M. D., BERKSON, J., POWER M. H., AND MCCONAHEY, W. M., J. Clin. Invest. 28, 207 (1949). 5. HIDALGO, J. U., NADLER, S. B., BLOCH, T., AND NIESET, R. F., Proc. Sot. Exptl. Biol. Med. 77, 784 (1951). 6. PERLMAN, I., CHAIKOFF, I. L., AND MORTON, M. E., J. Biol. Chem. 139, 433 (1941). 7. GOMBERG, H. J., BEIERWALTES, W. H., AND LAMPE, I., Proc. Sot. Exptl. Biol. Med. 73, 405 (1950). 8. AST~OOD, E. B., AND STANLEY, M. M., Western J. Surg. Obstet. Gynecol. 66, 625 (1947). 9. QUIMBY, E. H., AND MCCUNE, D. J., Radiology 49, 201 (1947). 10. MYANT, N. B., POCHIN, E. E., AND GOLDIE, E. A. G., Clin. hi. 8, 109 (1949). 11. TAUROG, A., CHAIKOFF, I. L., AND ENTENMAN, C., Endocrinology 40,86 (197). 12. CHAIKOFF, I. L., TAUROG, A. AND REINHARDT, W. O., Endocrinology 40, 47 (1947). 13. FRIEDBERG, A. S., URELES, A., AND HERTZ, S., Proc. Sot. Exptl. BioZ. Med. 70, 679 (1949). 14. CLARK, D. E., MOE, R. H., AND ADAMS, E. E., Surg. Gynecol. Obstet. 28, 331 (1949). 15. LEBLOND, C. P., AND GROSS, J., J. CZin. EndocrinoZ. 9, 149 (1949).
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