| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Original Studies |
Department of General Surgery (G.W.M., M.E.K., H.D.B.), Department of Internal Medicine IV (W.R., H.U.H.), and Department of Radiology (P.L.P.), University Hospital, D-72076 Tübingen, Germany
Address all correspondence and requests for reprints to: Dr. G. W. Maier, Chirurgische Universitätsklinik Tübingen, Abteilung für Allgemeine Chirurgie, Hoppe-Seyler-Strasse. 3, D-72076 Tübingen, Germany.
 |
Abstract |
|---|
 Top Abstract IntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
|
Introduction |
|---|
|
TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
The PTH (1–84) metabolism, after removal of a solitary adenoma in primary hyperparathyroidism, is a suitable model for the elimination kinetics of a peptide hormone in humans. The blood supply of the adenoma can be prepared and clamped at a defined time-point, to precisely determine when hormone release from the adenoma is terminated. In addition, PTH (1–84) can be measured with high selectivity as intact hormone, by RIA. PTH (1–84) release from the remaining parathyroid glands is still suppressed immediately after adenoma removal (6), which could blur the elimination kinetics of the PTH (1–84) that stems from the adenoma. Furthermore, PTH (1–84) elimination may be calculated on the basis of a two-compartment model, with a refined mathematical analysis, even if PTH (1–84) levels are only determined in one compartment.
The aim of this study was to characterize PTH (1–84) elimination kinetics, after adenectomy, to increase the understanding of the metabolic changes that occur after primary hyperparathyroidism has been cured. In addition, we believed that this study of PTH (1–84) elimination kinetics could provide some insight into the elimination kinetics of human peptide hormones, in general.
Therefore, we precisely described the kinetics of PTH (1–84) after adenoma removal for primary hyperparathyroidism. A two-compartment model with a four-parameter kinetic model configuration was used for the calculation of PTH (1–84) half-life. To describe metabolic alterations secondary to changes in PTH (1–84) levels, we also determined ionized and total Ca and Pi levels in the peripheral blood.
|
Subjects and Methods |
|---|
|
TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
Twelve patients were studied (three males; nine females; median age, 55 yr; range, 38–67 yr) who underwent adenectomy caused by parathyroid gland adenoma. Preoperative investigations included real-time ultrasonography, which failed to localize the adenoma in three patients. Blood tests included PTH (1–84), ionized and total Ca, and Pi. The decision to perform surgery was based on elevated serum Ca and PTH (1–84) levels. In all patients, one adenoma was found intraoperatively. The diagnosis was confirmed by histologic examination of the operative specimen by standard histologic criteria. All patients gave written informed consent before their participation in the study. The study protocol was approved by the ethical committee at the University of Tübingen.
Surgical procedure
All operations were performed by the same surgeon. After a Kocher skin incision, the pretracheal muscles were dissected, the thyroid gland partly mobilised, and the neck explored on one side. If the adenoma was not found, the other side was explored similarly. As soon as the adenoma and at least one normal parathyroid gland were identified, dissection was not extended any further. Careful preparation of the adenoma was performed before clamping of its vascular pedicle. Then, 15 min later, the adenoma was removed. During this 15-min period, all surgical manipulations were suspended. The operation was terminated in accordance with standard surgical technique.
Blood sampling
Blood samples were drawn at 0800 h, 2 days and 1 day before
surgery. Further samples were taken at the actual beginning of the
operation; 15, 10, and 5 min before, and at clamping of the adenoma’s
blood supply. The next samples were drawn every 2.5 min for the
following 20 min and then every 10 min until 60 min after clamping.
Then, blood sampling was repeated at 2, 2.5, and 5 h after
clamping, and at 0800 h from the 1st to the 4th postoperative day.
All blood samples were collected in EDTA, by puncture of a peripheral
vein, except the samples that were taken 15 min before clamping, up to
2 h after clamping. These latter samples were obtained from a
radial artery catheter. PTH (1–84) levels from venous and arterial
blood vary at an average of 1.5% ± 2.5 (mean ± SD),
as has been investigated in a pilot study with 16 samples. For the
correlation between arterial and venous PTH (1–84) levels, r =
0.999 (Fig. 1
).
|
Blood samples were centrifuged immediately. Then, the plasma was frozen at -30 C and kept at this temperature until analysis. The stability of PTH (1–84) in vitro has been demonstrated previously (7). PTH (1–84) levels were determined by a two-site immunoradiometric assay (Allegro intact PTH (1–84) assay, Nichols Institute Diagnostics, San Juan Capistrano, Los Angeles, CA). This test has high selectivity for the intact PTH (1–84) molecule, because two different antibodies, specific for epitopes at both ends of the PTH (1–84) polypeptide chain, need to bind to the molecule, for detection. The normal range is 1.1–6.1 pmol/L. The intraassay variation was 8.3% at 0.5 pmol/L, 4.0% at 1 pmol/L, and 2.2% at 7.5 pmol/L (n = 10). The interassay variation was 11.0% at 0.5 pmol/L, 6.1% at 1 pmol/L, and 3.1% at 7.5 pmol/L (n = 6). The lower detection limit of the assay is 0.11 pmol/L.
Serum (S)-protein, S-albumin, S-creatinine, S-Ca, and S-Pi were determined on a Monitor Parallel (American Monitor Corporation, Indianapolis, MN). Blood-ionized Ca was determined using an ICA 2 Analyser (Radiometer, Kopenhagen, Denmark).
Calculation of elimination kinetics
To determine the half-life of PTH (1–84), the kinetic parameters were derived from the PTH (1–84) time course levels by a standard nonlinear least-square analysis using the Mathlab computing environment (The MathWorks Inc., Natick, MA). The standard two-compartment/four-parameter kinetic model configuration with constant input was used, because the curve fitting with this model was better than for a one-compartment model. This was shown by Akaike information criterion (AIC) for curve fitting: the AIC was 97.6 for a one-compartment model and 61.2 for a two-compartment model (8). The equations for the model are: 1) d/dt x1 = - k11x1 + k12x2 + Ra; and 2) d/dt x2 = - k22x2 + k21x1; with the definitions: k11 = k01 + k21; k22 = k02 + k12; and Ra: basal PTH (1–84) input from the remaining parathyroid glands, which was assumed to be equal to the nadir of PTH (1–84) levels postoperatively.
The rate equations were integrated, and the solutions of the equations were fitted with the method described above.
Statistical analysis
All data are given as mean ± SEM. Values were compared by the paired t test. P < 0.05 was taken as significant, and Bonferroni’s correction of significance for multiple testing was performed when indicated.
|
Results |
|---|
|
TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
The time course of plasma PTH (1–84) is depicted in Fig. 2. PTH (1–84) remained constant during
the days before surgery, as well as intraoperatively, while the adenoma
and its blood supply were dissected (34.2 ± 5.0 vs.
34.3 ± 4.7 pmol/L, not significant). A decrease of PTH (1–84)
had already occurred 2.5 min after clamping of the adenoma’s blood
supply (34.9 ± 4.8 vs. 23.3 ± 2.9 pmol/L,
P < 0.001). The lowest PTH (1–84) level was measured
5 h after clamping (0.96 ± 0.06 pmol/L). From the second
postoperative day on, plasma PTH (1–84) increased, day by day, until
the end of the observation period (2.3 ± 0.14 pmol/L, on
postoperative day 4).
|
Elimination kinetics for plasma PTH (1–84) were calculated by a mathematician (W. Renn), on the basis of a two-compartment model. The resulting half-lives were 3.4 ± 0.1 min and 81.7 ± 12.7 min, respectively.
Total and ionized Ca
The time courses of total and ionized Ca in the peripheral blood
are given in Figs. 3 and 4. Thirty minutes after clamping, total
Ca was decreased to 5.93 ± 0.15 vs. 5.99 ± 0.04
mmol/L at clamping (ionized Ca: 1.56 ± 0.05 vs.
1.58 ± 0.05 mmol/L at clamping, both P < 0.005).
This decrease continued until the third postoperative day (total Ca:
4.05 ± 0.04 mmol/L, ionized Ca: 1.08 ± 0.03 mmol/L). Then,
Ca levels increased again, reaching 4.14 ± 0.01 mmol/L for total
Ca and 1.14 ± 0.02 mmol/L for ionized Ca, on the fourth day after
adenectomy.
|
|
Pi levels, before and after adenoma removal, are
depicted in Fig. 4
. After surgery, Pi levels started to
increase, day by day. They had already reached a significantly higher
level at the first postoperative day, compared with preoperative
levels.
Blood creatinine, total protein, and S-albumin
All these parameters remained within normal levels throughout the
study. Data are summarized in Table 1.
|
|
Discussion |
|---|
|
TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
In the present study, plasma PTH (1–84) was already decreased, 2.5 min after clamping of the adenoma’s blood supply. After 5 h, a minimum of 2.8% of the preoperative PTH (1–84) level was reached, which is below the normal range. The elimination half-lives, calculated on the basis of a two-compartment model, were 3.4 and 81.7 min, respectively. From the first postoperative day on, PTH (1–84) levels started to increase again. Blood creatinine, total protein, and albumin remained constant throughout the observation period. Thus, a possible influence of kidney function or blood protein levels was ruled out.
The observed fall in blood PTH (1–84) levels, after clamping, is explained by interruption of PTH (1–84) release from the adenoma into the circulation while elimination continues. It has been shown in animal studies that PTH (1–84) elimination occurs mainly in the kidneys and in the liver (2, 3, 4, 5). In the kidneys, PTH (1–84) is filtered and subsequently reabsorbed in the proximal tubular cells, where it is degraded (9). The filtered PTH (1–84) does not reenter the circulation. Consequently, PTH (1–84) elimination in the kidney is proportional to its plasma concentration. This kind of elimination would follow a monoexponential curve and can be described by a one-compartment model. However, only about 40% of PTH (1–84) metabolism occurs in the kidney (4). In addition, PTH (1–84) is eliminated in the liver via two mechanisms. First, Kupffer cells degrade PTH (1–84) by an enzymatic reaction involving specific PTH (1–84) binding to a receptor (3). This reaction would follow a Michaelis-Mentgen kinetic. Second, PTH (1–84) distributes immediately into the Dissé and intracellular space (10). It is subsequently taken up by hepatocytes and modulates their glucose and amino acid metabolism (11, 12). Based on the assumption that PTH (1–84) does not reenter the circulation, the elimination by uptake into the hepatocytes would correspond to an exponential function in first approximation. In addition to PTH (1–84) elimination in the kidney and the liver, degradation in other target organs, such as the skeleton and the gut, may contribute to the overall elimination kinetics of PTH (1–84) (13). In summary, PTH (1–84) elimination is too complex to be correctly described by a one-compartment model, because then, enzymatic elimination is neglected. Therefore, we analyzed PTH (1–84) elimination kinetics on the basis of a two-compartment model.
This analysis required a complex mathematical operation, because PTH (1–84) can only be measured in one compartment, which is the peripheral blood. On the basis of our data, PTH (1–84) half-lives were calculated to be 3.4 and 81.7 min. These values were derived from the two equations that were necessary to describe PTH (1–84) elimination with a two-compartment model. The first equation and half-life describe the inital rapid fall in PTH (1–84). The second equation and its concomitant half-life represent the second part of the elimination curve that shows a slower rate of PTH (1–84) elimination. We believe that, because of this sophisticated data analysis, our description of PTH (1–84) elimination kinetics is more precise than those in previously published reports. In these few studies, an exponential decay of PTH (1–84) has been suggested, with a half-life between 2.5 and 21 min (4, 6). However, we avoided certain limitations that may have deteriorated previous investigations of PTH (1–84) elimination kinetics. First, the duration of PTH (1–84) determination after adenoma removal was comparatively short in several previous studies, ranging from 15–120 min (14, 15, 16, 17). Second, in some studies, PTH (1–84) half-life was estimated by coarse graphical analysis or was calculated by assuming oversimplifying models, rather than by precise calculation on the basis of a two-compartment model (15, 16, 18, 19). Third, a careful dissection was performed in our study, to maintain the adenoma’s blood supply until clamping. This was documented by elevated PTH (1–84) levels right until clamping time. Thus, a premature decrease of PTH (1–84) release from the adenoma into the circulation was avoided, which is likely to have occurred in some previous studies (14, 16). Fourth, we determined PTH (1–84) with a state-of-the-art RIA. Assays used in previous studies may have been less selective for PTH (1–84) and, therefore, various PTH (1–84) fragments may also have been determined (18). However, Brossard and co-workers (20) demonstrated that even the two-site RIA that we employed only has limited selectivity for PTH (1–84). In their study, approximately 20% of the assumedly determined PTH (1–84) in healthy subjects was a not-further-characterized non-PTH (1–84) fragment, which may correspond to a biologically inactive aminoterminal PTH fragment (20, 21). To our knowledge, no data are available that demonstrate whether such a non-PTH (1–84) fragment is also released from adenomas during primary hyperparathyroidism. We believe, however, that a potential non-PTH (1–84) fragment would not alter the calculated kinetics of the intact PTH (1–84) if the relative proportion of each peptide remains stable during the PTH elimination period after adenectomy. In our opinion, this is likely to be the case in patients without renal failure, as investigated in our study.
PTH (1–84) causes S-Ca to increase. Consequently, S-Ca decreases subsequent to PTH (1–84) after adenoma removal. In our study, total Ca and ionized Ca started to fall 30 min after clamping of the adenoma’s blood supply, which is about 25 min after the PTH (1–84) decrease. Then, Ca levels reached a minimum at about 3 days postoperatively. This is comparable with data from previous studies, reporting 20 h (15) to 3 days (14). PTH (1–84) regulates Ca levels via three major mechanisms. First, PTH (1–84) increases Ca reabsorption in the kidneys. Decreased plasma PTH (1–84), therefore, causes an increase in renal Ca clearance. Second, PTH (1–84) enhances 1,25(OH)2vitamin D3 synthesis in the kidney, which causes a subsequent increase in intestinal Ca absorption. This process would take several days to generate an increase in S-Ca levels. Consequently, reduced 1,25(OH)2vitamin D3 synthesis is unlikely to account for the rapid fall in S-Ca levels after adenoma removal. Third, increased Ca uptake by the skeleton may have caused the rapid fall in S-Ca. A hypothetical model of a bone surface compartment has been proposed, which would be interposed between bone and extracellular fluid (22, 23). Ca exchange between the plasma and such a reservoir may contribute to the rapid shift in Ca levels postoperatively.
PTH (1–84) regulates serum levels of Pi. Elevated PTH (1–84) levels cause a decrease in S-Pi levels via an inhibition of tubular reabsorption in the kidney, which enhances phosphate excretion in the urine (24). However, a steady state of serum phosphate levels is preserved. The action of PTH (1–84) is counteracted by increased Pi absorption from the gut and mobilization, together with Ca from the skeleton (1). After normalization of PTH (1–84) levels, excretion of Pi falls to zero within minutes (25, 26). In our study, a minimal rise in S-Pi levels occurred 1 day after normalization of plasma PTH (1–84). A slightly different time course was described by Fischer et al. (14) who saw an initial decrease of Pi, followed by an increase up to the 4th postoperative day (14). Similar to Ca, increased skeletal uptake of Pi after the PTH (1–84) decrease seems to be the most likely explanation for the slow rise in S-Pi after adenectomy (27).
We conclude from our data that the PTH (1–84) decline in the peripheral blood, after adenectomy for primary hyperparathyroidism, is characterized by a rapid initial decrease, which is followed by a prolonged phase with a lower elimination rate. This biphasic elimination pattern may also apply to other peptide hormones in man.
|
Acknowledgments |
|---|
Received February 18, 1998.
Revised July 2, 1998.
Accepted August 6, 1998.
|
References |
|---|
|
TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
|---|
This article has been cited by other articles:
 |
|
 |
|
  A. Khosravi, C. M. Cutler, M. H. Kelly, R. Chang, R. E. Royal, R. M. Sherry, F. M. Wodajo, N. S. Fedarko, and M. T. Collins Determination of the Elimination Half-Life of Fibroblast Growth Factor-23 J. Clin. Endocrinol. Metab., June 1, 2007; 92(6): 2374 - 2377. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. Bieglmayer, G. Prager, and B. Niederle Kinetic Analyses of Parathyroid Hormone Clearance as Measured by Three Rapid Immunoassays during Parathyroidectomy Clin. Chem., October 1, 2002; 48(10): 1731 - 1738. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |
| Molecular Endocrinology | Recent Prog. Horm. Res. | All Endocrine Journals |
) and ionized
(
) Ca, starting 15 min before clamping of the adenoma’s blood
supply, is shown. A decrease in both total and ionized Ca occurred 30
min after clamping (*, P < 0.005; mean ±
SEM, n = 12).
and ionized
Ca, and Pi (
, before and after adenoma removal (*,
P < 0.001; **, P < 0.01
vs. the previous day). Data are normalized to values at
the time of clamping of the adenoma’s blood supply (mean ±
SEM, n = 12). OP, Operation; pre, pre operation; post,
post operation.