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Long-distance running causes site-dependent decrease of cartilage glycosaminoglycan content in the knee joints of beagle dogs.

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ARTHRITIS & RHEUMATISM Volume 36
Number 10, October 1993, pp 1451-1459
0 1993, American College of Rheumatology
1451
LONG-DISTANCE RUNNING CAUSES
SITE-DEPENDENT DECREASE OF
CARTILAGE GLYCOSAMINOGLYCAN CONTENT
IN THE KNEE JOINTS OF BEAGLE DOGS
JAR1 AROKOSKI, ILKKA KIVIRANTA, JUKKA JURVELIN,
MARKKU TAMMI, and HEIKKI J. HELMINEN
Objective. To study the effects of a long-term
(1-year) program of running exercise (up to 40 k d d a y )
on the thickness and glycosaminoglycan (GAG) content
of articular cartilage in the knee and humeral head
cartilage of young dogs.
Methods. Samples for histologic analysis were
obtained from 12 different locations of the joints. We
conducted a detailed, area-specific analysis, measuring
the thickness of articular cartilage and analyzing the
distribution of Safranin 0 stain that binds stoichiometrically to GAG as determined by quantitative microspectrophotometry.
Results. Running exercise decreased the GAG
content of the uncalcified articular cartilage in the
weight-bearing summits of the femoral condyles by
5 4 3 % (P < 0.05), while at margins of these areas the
GAG content was equivalent to control levels. In the
lateral condyle of the femur, the reduction was most
prominent in the superficial zone (up to 28% decrease;
From the Department of Anatomy, University of Kuopio,
Kuopio, Finland.
Supported by research grants from the Finnish Research
Council for Physical Education and Sports, the Ministry of Education; the Medical Research Council of the Academy of Finland; the
North Savo Fund of the Finnish Cultural Foundation; and the
Finnish Research Foundation for Orthopaedics and Traumatology.
Jari Arokoski, MD: Department of Anatomy, University of
Kuopio; Ilkka Kiviranta, MD, PhD: Department of Surgery, Kuopio
University Hospital, Kuopio, Finland; Jukka Jurvelin, PhD: Department of Clinical Physiology, Kuopio University Hospital, Kuopio,
Finland; Markku Tammi, MD, PhD: Department of Anatomy,
University of Kuopio; Heikki J. Helminen, MD, PhD: Department
of Anatomy, University of Kuopio.
Address reprint requests to Jari Arokoski, MD, Department
of Anatomy, University of Kuopio, P.O. Box 1627, SF-70211
Kuopio, Finland.
Submitted for publication September 29, 1992; accepted in
revised form March 25. 1993.
P < 0.05), and extended into the intermediate zone
(11% decrease; P < 0.05). GAG content was also
significantlyreduced in the superficial zone at the lateral
condyle of the tibia and the head of the humerus, by
35% (P < 0.01) and 15% (P < 0.05), respectively.
Running did not alter GAG concentration in the patellofemoral region.
Conclusion. The GAG depletion caused by 40k d d a y running exercise is restricted to prominent
weight-bearing areas of the joint and begins from the
superficial cartilage without signs of degeneration. The
different degree and type of joint loading can explain the
site-dependent cartilage response to long-distance running. The loss of GAGS was possibly due to breakdown
of proteoglycans, which could not be compensated for
by neosynthesis of molecules. With time, this may affect
the condition of articular cartilage, especially if the joint
is exposed to loading for lengthy periods.
The biologic properties of articular cartilage are
highly dependent on movement and loading of the
joints (1). It is now generally agreed that unloading by
immobilization and diminished weight bearing leads to
transient or permanent atrophy of articular cartilage,
i.e., to decreased content of proteoglycans ( 2 4 , and
softening of the cartilage (5). Undoubtedly, a certain
amount of joint loading is needed to maintain normal
cartilage properties. There are conflicting opinions
about the role of increased joint loading in terms of
cartilage condition. Findings of some studies using
animal models suggest that running exercise may not
injure articular cartilage (6-9), while others have demonstrated cartilage injury (10-12). Most clinical studies
show, however, that long-distance runners and other
athletes have no more evidence of osteoarthrosis than
AROKOSKI ET AL
1452
do sedentary individuals (13-17). This question has
clinical importance due to the increasing popularity of
running and other athletic activities, and its elucidation may contribute to our understanding of the pathogenesis of osteoarthrosis.
Our earlier studies on dogs indicated that running exercise of 4 kmlday for 15 weeks significantly
increased the thickness and glycosaminoglycan (GAG)
content on the summit of the femoral cartilage of the
knee (18,19). The increase of GAGS was greatest in the
intermediate, deep, and calcified zones, while the
superficial zone did not show an increase. The increase in GAGS was associated with stiffening of
articular cartilage in these areas (20). Running exercise
of 20 kmlday for 15 weeks significantly decreased
GAG content in the superficial zone of the condylar
summit of the femur (21), while biomechanical properties in this area remained unaltered (22). The aim of
the present work was to study the effects of running
exercise of 40 km/day for 15 weeks on the thickness
and GAG content of articular cartilage in young canine
knee and humeral head cartilage, by using the quantitative microspectrophotometric method (23,24).
MATERIALS AND METHODS
Animals. Twenty female beagle dogs of pure breed,
from the National Laboratory Animal Center (Kuopio, Finland) and from Shamrock Ltd. (Hereford, UK) were used in
the experiments. There were 10 dogs in the experimental
group (runner dogs) and 10 in the control group; for each
runner dog, a littermate was used as the control. The dogs
were kept in individual stainless-steel cages (bottom area 0.9
x 1.2171, height 0.8m) in accordance with the principles
presented in the National Research Council's Guide for the
Care and Use of Laboratory Animals (25). The dogs were
fed with commercial dog food (Hankkija, Kolppi, Finland).
The daily food portion for each dog was determined based on
weekly weight measurements in the controls, to maintain
equal body weight in the runner and control groups. Water
was given ad libitum. The final body weights (mean k SD) of
the control and runner dogs were 12.4 t 0.3 kg and 12.3 t
1.0 kg, respectively.
Running program. The exercise program was started
when the runner dogs were 15 weeks old. A treadmill with
15" uphill inclination was used (26) (Table 1). During the first
40 weeks, the running distance was gradually increased until
the daily distance was 40 kmlday. Thereafter, the dogs ran 40
k d d a y for an additional 15 weeks. The control dogs were
kept in their cages throughout the experiment. Study dogs
and their sister littermates were killed at the age of 70 weeks.
The experimental design was approved by the Animal Care and Use Committee of the University of Kuopio.
The dogs performed the running exercise willingly. N o signs
of distress were indicated in the behavior of the animals or in
Table 1. Running exercise program used in the present study of
beagle dogs*
Age
(weeks)
15
16
17
18
1%23
24-21
28-3 1
3240
4146
47-54
55-70
Treadmill
speed
(kdhour)
Running
distance
(kdW
0
1.0
2.0
3.0
4.0
4.0
5.0
5.0-5.5
5.5-6.0
5.5-6.8
5.5-6.8
0
0.33
0.5
0.66
0.83-3.3
4.0
5 .O-7 .O
12.0-20.0
22.5-21.5
30.0-37.5
40.0
* The dogs ran 5 days per week on a treadmill with 15" uphill
inclination.
the daily examinations that were conducted. The dogs ate
and grew normally. Their physical condition was excellent
throughout the study. Some paw excoriations developed,
but these were cured and prevented by the application of
either elastic bandages or dog boots (26).
Preparation of samples. After completion of the 55week regimen, the dogs were anesthetized with an intravenous injection of thiopentone sodium (Hypnostan; Leiras,
Turku, Finland), and radiographs of the knee and shoulder
joints were obtained. The dogs were killed with an overdose
of the anesthetic and air insufflation. Immediately after
death, the right knee and the left shoulderjoints were opened
and dissected free. Cartilage samples for histologic study
were obtained from 11 sites on the patellar, femoral, and
tibia1 surfaces (Figure 1). The proximal head of the humerus
was stored for 12-14 hours in Ringer's solution at 4°C for
biomechanical testing of the c:artilage. After the biomechanical tests, histologic specimens were also obtained from the
humeral head tissue (Figure I).
The cartilage samples were prepared by sawing slices
perpendicular to the articular surface, using a dentist's drill
equipped with 2 cutting discs separated by a 1 mm-thick
spacer. During preparation, the specimens were kept moist
with ice-cold 0.9% NaCI. The samples were fixed for 48
hours in 4% (weightlvolume) formaldehyde in 0.07 mole/liter
sodium phosphate buffer, pH 7.0, at 4°C. Slices were decalcified with 10% (wh) EDTA (Merck, Darmstadt, Germany)
and 4% (wlv) formaldehyde in 0.1 molehiter sodium phosphate buffer, pH 7.4, for 12 days at 4°C. After dehydration,
the samples were infiltrated with paraffin and embedded in
Paraplast Plus wax (Lancer, Division Shenvood Medical,
Kildare, Ireland). Histologic sections, 3 pm thick, were cut
perpendicularly to the articular surface with an LKB 2218
HistoRange microtome (LKB-Produkter Ab, Bromma, Sweden), and dried overnight at 137°C.
Histochemical methods. The sections were deparaffinized in xylene and hydrated in a descending series of
solutions of ethanol to distilled water. Tissue sections were
stained with Safranin 0 (Fisher Scientific, Fair Lawn, NJ).
ENDURANCE TRAINING AND CARTILAGE PROTEOGLYCANS
1453
PAT
FMA
FMIM
FMI
FMIL
TML
TMI
TMM
FEMUR
TIBIA
HUMERUS
-
Figure 1. Sites of microspectrophotometric analysis in the right knee and left shoulderjoints of
the dogs. FPS and FPI = superior and inferior sections of the patellar surface of the femur; PAT
= patella; FMA and FMP = anterior and posterior sections of the medial condyle of the femur;
FMI = summit of the intermediate section of the medial condyle of the femur; FMIM and FMIL
= medial and lateral points of the intermediate section of the medial condyle of the femur; FLA
and FLP = anterior and posterior sections of the lateral condyle of the femur; FLI = summit
of the intermediate section of the lateral condyle of the femur; FLIM and FLIL = medial and
lateral points of the intermediate section of the lateral condyle of the femur; TMI = central point
of the intermediate section of the medial condyle of the tibia; TMM and TML = medial (covered
by meniscus) and lateral (not covered by meniscus) points of the medial condyle of the tibia;
TLI = central point of the intermediate section of the lateral condyle of the tibia; TLM and TLL
= medial (not covered by meniscus) and lateral (covered by meniscus) points of the lateral
condyle of the tibia; HUM = head of the humerus.
The staining solution, 0.5% (wlv) Safranin 0, was prepared
in 0.1 molelliter sodium acetate buffer at pH 4.6. After 10
minutes of staining, the sections were dehydrated in an
ascending series of ethanolic solutions, cleared in xylene,
and mounted in DPX mounting medium (Difco, West Molesey, UK). The standard periodic acid-Schiff (PAS) and
modified PAS methods were used to localize glycoproteins
and chondroitin sulfate, respectively (24). At each location,
sections of the control and the experimental joints were
stained in the same dye bath using a Varistain 24-2 staining
device (Shandon Southern Products, Runcorn, UK).
Microspectrophotometric analysis. The cationic Safranin 0 stain, which binds stoichiometrically to GAG polyanions (23), was measured with a Leitz MPV 3 microspec-
trophotometer (Leitz Wetzlar GmbH, Wetzlar, Germany)
interfaced with a Hewlett-Packard 85 A microcomputer
(Hewlett-Packard, Corvallis, OR) (23). The transmittance
measurements were fed into the microcomputer and converted into absorbance values. The intensity of the staining
was expressed in absorbance units. During analysis, the
microscope was equipped with an EF 6 3 ~ 1 0 . 8 objective
5
and
an L4Oxl0.65 condenser. The diameter of the measuring
spot and the wavelength of the light were 0.8 pm and 500 nm,
respectively, for Safranin 0-stained sections and 1.7 pm and
560 nm, respectively, for PAS-stained sections.
Two to three sections were investigated from each
location and from each dog. An acceptable degree of precision of the method (relative standard error of the mean
AROKOSKI ET AL
1454
Cartilage
thickness
(pm)
Figure 2. The microspectrophotometric analysis system. The intensity (i) of the Safranin 0 stain was determined along 3
scan lines (measuring spots), starting from the articular surface and ending in the subchondral borne. The mean absorbance
was calculated in 50-pm fractions (i, to i2& Within each fraction, 12 spot measurements, 12.5 pm apart, were made. To
characterize the overall staining property of the uncalcified and calcified cartilage, the mean absorbance values from each
fraction were summed to yield an integrated absorbance value of these compartments. The mean staining intensity in
uncalcified and calcified cartilage was calculated by dividing the integrated absorbance value by the number of fractions
measured.
<lo%) was obtained when 2-3 sections from each location
from 4 or more animals were measured (27). Absorbance
was determined in 50-pm fractions from articular surface to
subchondral bone (Figure 2). Within each fraction, 12 spot
measurements were made 12.5 p m apart, and the mean was
calculated from these measurements. The most superficial
50-pm fraction represented the superficial zone, the 50-pm
fraction in the middle of the uncalcified cartilage represented
the intermediate zone, the 50-pm fraction above the tidemark represented the deep zone, and the 50-pm fraction in
the middle of the calcified cartilage represented the calcified
zone (Figure 2).
The thickness of the uncalcified and the calcified
cartilage was measured by moving the tissue section under
microscopic control at a fixed speed of 32 pm/second on the
scanning stage. During the scan, beginning from the cartilage
surface, the time needed to cross both the tidemark line and
the cartilage-bone interface was registered within 5 pm
accuracy.
Statistical analysis. For statistical analysis, the
2-tailed nonparametric Wilcoxon matched pairs signed rank
test was used.
RESULTS
Gross findings. Macroscopically, the appearance of articular cartilage, synovium, and synovial
fluid of the joints was normal both in controls and
runners. No osteophytes were seen. Radiographically,
epiphyseal plates were closed (calcified) in the shoul-
der and knee joints, indicating that growth of the bones
had ceased. Histologicallly, cartilage surfaces were
smooth with the exception of the slightly rough surface
area of the tibial plateau cartilage not covered by the
meniscus (lateral point of the medial condyle and
medial point of the lateral condyle), which was sometimes seen in both groups.
Thickness of cartilage. At the superior section of
the patellar surface of the femur (FPS), the thickness
of uncalcified cartilage increased from 443 2 54 pm in
controls to 566 2 175 pm in runner dogs (mean 2 SD,
n = 10; P = 0.05) (see also Table 2). In general,
running had no significant effect on cartilage thickness
at the summits of femoral condyles (intermediate
section of medial femoral condyle [FMI] and intermediate section of lateral femoral condyle [FLI]) or
marginal areas of the femoral condyles (Table 2). The
total cartilage thickness increased significantly at the
lateral point of the intermediate section of the lateral
femoral condyle (FLIL) (Table 2). In the tibial
condyles, the thickness of articular cartilage after the
running program was equivalent to control levels
except at the lateral condyle of the tibia covered by
meniscus (TLL), where the thickness of the calcified
cartilage increased significantly, from 79 ? 31 pm to
1 1 1 -+ 38 pm (mean SD, n = 10; P = 0.03). At the
*
ENDURANCE TRAINING AND CARTILAGE PROTEOGLYCANS
head of the humerus, cartilage thickness was not
changed as a result of running.
Quantitative histochemistry of the carbohydrate
components. The intensity of Safranin 0 staining of the
uncalcified cartilage was significantly decreased, by
5-13%, on the summits of the femoral condyles (FMI,
FLI) (Table 2). The reduction of staining intensity was
most prominent in the superficial zone of cartilage
(23-28% decrease), but occurred also in the intermediate zone of the lateral condyle of femur (11% decrease) (Table 3 and Figure 3). At the FLI point,
Safranin 0 intensity was also decreased in the calcified
zone. At the marginal areas of the medial and lateral
Table 2. Effects of long-distance running (40 km/day) on cartilage
thickness and Safranin 0 staining intensity in different regions of the
knee and shoulder joints of beagle dogs*
Site of analysis
Cartilage thickness
Safranin 0 intensity
(no. of paired
observations) Total Uncalcified Calcified Uncalcified Calcified
Patellar surface
of femur
FPS (10)
FPI (10)
Patella (9)
Medial condyle
of femur
FMA(9)
FMIM(10)
FMI(10)
FMIL (10)
FMP(10)
Lateral
condyle
of femur
FLA (9)
FLIM (9)
FLI (9)
FLIL(9)
FLP (8)
Medial condyle
of tibia
TMM (10)
TMI (10)
TML (10)
Lateral
condy 1e
of tibia
TLM (10)
TLI (10)
TLL(10)
Humerus (10)
1.19
1.14
1.08
1.28t
1.16
1.13
0.89
1.08
0.87
0.91
0.99
I .oo
1.01
I .02
0.96
1.11
1.12
1.01
0.98
0.92
1.18
1.15
1.03
0.99
0.92
0.86
0.86
0.91
0.95
0.91
0.99
0.95
0.95$
0.88
0.83
1.03
0.93
0.97
0.92
0.90
1.08
1.06
0.94
1.19$
0.94
1.06
1.03
0.93
1.18
0.93
1.10
1.19
1.04
1.23
0.95
1.11
0.94
0.870
0.95
0.89
1.11
0.86
0.86$
0.91
1.06
0.97
1.02
0.99
0.97
1.01
0.98
0.93
1.11
1.07
0.99
1.03
1.02
1.07
I .05
0.95
0.96
0.99
1.24
0.98
0.95
1.00
1.21
0.98
I .09
0.93
1.41$
0.99
0.98
0.94$
0.96
0.95
0.92$
0.98
1 .oo
1.oo
* Values are the ratio of the mean in the runner group:mean in the
control group. The 2-tailed nonparametric Wilcoxon matched pairs
signed rank test was used for statistical analysis. See Figure 1 for
definitions.
t P = 0.05.
$. P < 0.05.
8 P < 0.01.
1455
Table 3. Effects of long-distance running (40 kdda y) on Safranin
0 staining intensity in different cartilage zones of the knee and
shoulder joints of beagle dogs*
~~
Site of analysis
(no. of paired
observations)
Patellar surface
of femur
FPS (10)
FPI (10)
Patella (9)
Medial condyle
of femur
FMA (9)
FMIM (10)
FMI (10)
FMIL (10)
FMP (10)
Lateral condyle
of femur
FLA (9)
FLIM (9)
FLI (9)
FLIL (9)
FLP (8)
Medial condyle
of tibia
TMM (10)
TMI (10)
TML (10)
Lateral condyle
of tibia
TLM (10)
TLI (10)
TLL (10)
Humerus (10)
~
Safranin 0 intensity
Superficial
zone
Intermediate
zone
Deep
zone
Calcified
zone
0.97
0.98
0.98
1.03
0.99
1.09
1.05
1.00
1.00
1.06
1.03
1.06
1.11
0.81
0.77t
0.85
0.55$
0.98
0.96
0.94
0.84t
0.84
0.90
0.95
1.01
0.97
0.86t
1.02
0.96
0.94
0.Wt
0.9 1
1.16
0.86
0.72t
0.83
0.85
1.15
0.98
0.89t
0.94
0.90
1.10
0.98
1.02
0.95
0.95
1.05
0.86
0.82t
0.91
0.94
0.93
0.83
0.91
0.95
0.95
1.02
0.98
1.02
1.07
0.97
0.95
0.66$.
0.65$
0.69t
0.857
0.94
0.99
I .oo
1 .oo
0.98
1.01
1.03
1.00
1.00
1.03
0.89t
0.99
1.02
* Values are the ratio of the mean in the runner groupmean in the
control group. The 2-tailed nonparametric Wilcoxon matched pairs
signed rank test was used for statistical analysis. See Figure 1 for
definitions.
t P < 0.05.
$. P
< 0.01.
condyles of the femur (medial and lateral points of the
intermediate section of the medial femoral condyle
[FMIM and FMIL], and medial and lateral points of
the intermediate section of the lateral femoral condyle
[FLIM and FLIL]), Safranin 0 intensity was at control
levels (Table 2). At the posterior section of the medial
femoral condyle, the staining was significantly reduced, by 45% and 14% in the superficial and deep
zones of uncalcified cartilage, respectively (Table 3).
In the patellofemoral region (FPS, inferior section of
the patellar surface of the femur, and patella), Safranin
0 intensity remained at control levels (Table 2). A
significant decrease of Safi-anin 0 staining intensity was
observed in the superficial zone of the lateral tibial
condyle, as well as in the head of the humerus (Table 3).
AROKOSKI ET AL
1456
Figure 3. Safranin 0-stained sections from the articular cartilage at the summit of the intermediate section of the lateral condyle of the femur
of a control dog (A) and a runner dog (B). The cartilage surface is intact. Loss of staining intensity was observed in the superficial and
intermediate zones of cartilage from dogs in the runner group. Bar = 80 wm.
At the FLI point, modified PAS staining, used
to demonstrate the distribution and quantity of chondroitin sulfate, was significantly reduced in the superficial and intermediate zones (Table 4). Running had
no effect on standard PAS staining properties at the
FLI (Table 4).
Table 4. Effects of long-distance running (40 k d d a y ) on cartilage
staining intensity at the summit of the intermediate section of the
lateral condyle of the femur*
Stain
Cartilage
zone
Safranin
0
Standard
PAS
Modified
PAS
Superficial
Intermediate
Deep
Calcified
0.72t
0.89t
I .02
0.92
0.91
1.03
1.06
0.70$
0.82t
0.82
0.91
0.82
* Values are the ratio of the mean in the runner group (n = 9):mean
in the control group (n = 9). The 2-tailed nonparametric Wilcoxon
matched pairs signed rank test was used for statistical analysis. PAS
= periodic acid-Schiff.
t P < 0.05.
f P < 0.01.
DISCUSSION
Our results demontstrate that running exercise
of 40 kmlday for 15 weeks decreased the GAG content
of the articular cartilage matrix in loaded, weightbearing areas. This response occurred in the superficial zone of the condylar cartilage of the femur, in the
lateral condyle of the tibia, and in the head of the
humerus. These results are in accordance with our
earlier findings in dogs. ‘The running program of 20
kndday for 15 weeks caused a significant depletion of
GAGS from the superficial zone of condylar summit
cartilage of the femur (21), while in the tibia1 condyles,
GAG content was at control levels. Observations in
the present study revealed that the GAG reduction
extended to the intermediate, and even the deep,
zones of articular cartilage, and depletion of GAG was
also seen on the lateral condyle of the tibia. GAG
content was also decreased in the calcified zone.
Concomitant with the reduction of Safranin 0 staining
at the FLI, modified PAS staining was reduced, suggesting that the chondroitin sulfate-containing proteoglycans, in particular, were decreased.
The articular cartilage response to long-
ENDURANCE TRAINING AND CARTILAGE PROTEOGLYCANS
distance running shows topographic and zonal variation, as demonstrated in our earlier studies (18,21).
The greatest effects were observed at the summits of
the femoral condyles (FMI and FLI), while at the
margins of this area (FMIM, FMIL, FLIM, and
FLIL), GAG content remained at control levels. In
contrast, in some marginal cartilage points of the
femoral and tibial condyles (FLIL), there was a significant increase in the thickness of articular cartilage.
Also at the patellar surface of the femur, no loss of
GAGS was seen and cartilage thickness (uncalcified
and calcified) was even increased, consistent with the
results of our 20-km/day running experiment (21). A
similar, and possibly compensatory, effect on osteoid
formation was observed in the subchondral bone in the
patellofemoral region after 40-km/day running (28). It
is noteworthy that, although the menisci reduce stress
on the tibial plateau and protect the articular cartilage
(29,30), the GAG content was also significantly decreased in the area covered by the menisci in the TLL.
It is very probable that the degree and type of
joint loading differs in different areas of the knee. The
summits of the femoral condyles and the area on the
humeral cartilage (humeral head) are considered to
carry the heaviest load (18,31). Peripheral and marginal areas of the femur probably carry a lighter load
than the summits of the femoral condyles also during
increased loading due to running, which can explain
the fact that the greatest effect was seen mainly in the
summit of the femoral condyles in the present study.
Obviously, the patellofemoral compartment was subjected to shearing type of forces, while the summits of
the femoral condyles and tibial condyles were exposed
to more impact and weight-bearing types of loading
during the running exercise. The shearing type of
forces during long-distance running, even if associated
with high compression as in the patellofemoral joint,
seems to be more beneficial in nature than the weightbearing type of loading. These mechanical conditions
can explain the cartilage response to long-distance
running.
Vasan found that prolonged running exercise in
dogs decreased the proteoglycan concentration in the
articular cartilage of the femoral head, although the
synthesis of proteoglycans was significantly stimulated
(12). Higher rates of proteoglycan biosynthesis and
turnover have been shown to reflect increased proteoglycan catabolism (32,33). In our study, proteoglycan
synthesis and concentration was not affected in the
femoral head cartilage, as judged by sulfate incorporation studies (34). In the knee cartilage, the rate of
1457
proteoglycan loss must have exceeded the rate of
synthesis after long-distance running, in the weightbearing areas of the tibial and femoral condyles.
Because the superficial zone was predominantly affected, release of metalloproteinases from chondrocytes or from synovial fluid may explain the enhanced
removal of proteoglycans from the tissue (35,36).
Diffusion of proteoglycans from the superficial cartilage matrix would occur more easily than diffusion
from deeper zones.
The mechanical disruption of the cartilage collagen network can also cause enhanced removal of
proteoglycans from the tissue (37,38). However, the
articular cartilage appeared to be intact at the lateral
condyle of femur, based on ordinary light microscopy
studies (Arokoski J et al: unpublished observations).
These findings and our earlier results suggest that
running exercise of young growing animals primarily
affects cartilage proteoglycan metabolism, rather than
collagen (39). Articular cartilage is probably more
vulnerable to load-induced changes in older animals,
because after vigorous running exercise, cartilage fibrillation was observed in adult dogs (12).
Biomechanical properties, including the instant, creep, and equilibrium responses of the articular
cartilage, were also determined with an in situ indentation creep test at the weight-bearing sites corresponding to those used for the present histochemical
analysis. Concomitant with the reduction of GAGS at
the lateral condyles of femoral and tibial cartilage, the
stiffness of cartilage decreased (40). This might indicate that the biomechanical properties of the tissue
deteriorated. In previous studies, the compressive
stiffness of articular cartilage has been shown to
correlate with the proteoglycan content of the tissue
(41-43).
The significant decrease in cartilage proteoglycan content in the superficial zone (in 7 of 20 sites) and
the intermediate zone (in 2 of 20 sites) resembled the
changes that have been demonstrated after immobilization of the joint (4) and those occurring in the very
early stages of natural or experimentally induced osteoarthrosis (44,45). No gross histologic difference was
observed between the control and the runner groups,
however, and the response differed according to the
joint area. The quantitative differences between runner and control animals were small, although statistically significant. It remains to be determined whether
the changes observed represent early degenerative
changes or, alternatively, adaptation of the articular
cartilage to long-distance running. In any case, run-
AROKOSKI ET AL
1458
ning training, i.e., joint loading, controls the local
proteoglycan content in articular cartilage. We do not
know if these changes are reversible. We believe that
they are, however, as long as the collagen network is
intact.
The training regimen adopted for the dogs in
this study resembles, at least to some extent, the
vigorous training programs that athletes of today undergo when they prepare for competitive sports. In
this context, the comparison of young beagles with
young human adults seems a legitimate one. The
skeletal maturity of a I-year-old beagle with recently
calcified epiphyseal lines around the knee joint is well
comparable with that of a 19-year-old human (46).
Though the load distribution in the canine knee (stifle)
and human joints probably differs, our results indicate
that regular long-distance running training has the
potential of jeopardizing the condition of articular
cartilage and, after an extended period, may even
cause osteoarthrosis. The degree of proteoglycan depletion is probably not enough for this to be the
initiating factor in the cartilage destruction. Rather, we
believe the prerequisite for the development of osteoarthrosis is the simultaneous injury of the collagen
network in the superficial zone of articular cartilage.
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site, glycosaminoglycans, causes, long, dogs, joint, knee, distance, contents, decrease, dependence, cartilage, running, beagle
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