Skeletal alterations in hypophysectomized rats: II. A histomorphometric study on tibial cortical bone

Biomechanical properties of the mid-shaft femur in middle-aged hypophysectomized rats as assessed by bending test

Both stiffness and strength of bones are thought to be controlled by the "bone mechanostat". Its natural stimuli would be the strains of bone tissue (sensed by osteocytes) that are induced by both gravitational forces (body weight) and contraction of regional muscles. Body weight and muscle mass increase with age. Biomechanical performance of load-bearing bones must adapt to these growth-induced changes. Hypophysectomy in the rat slows the rate of body growth. With time, a great difference in body size is established between a hypophysectomized rat and its age-matched control, which makes it difficult to establish the real effect of pituitary ablation on bone biomechanics. The purpose of the present investigation was to compare mid-shaft femoral mechanical properties between hypophysectomized and weight-matched normal rats, which will show similar sizes and thus will be exposed to similar habitual loads. Two groups of 10 female rats each (H and C) were established. H rats were 12-month-old that had been hypophysectomized 11 months before. C rats were 2.5-month-old normals. Right femur mechanical properties were tested in 3-point bending. Structural (load-bearing capacity and stiffness), geometric (cross-sectional area, cortical sectional area, and moment of inertia), and material (modulus of elasticity and maximum elastic stress) properties were evaluated. The left femur was ashed for calcium content. Comparisons between parameters were performed by the Student's t test. Average body weight, body length, femur weight, femur length, and gastrocnemius weight were not significantly different between H and C rats. Calcium content in ashes was significantly higher in H than in C rats. Cross-sectional area, medullary area, and cross-sectional moment of inertia were higher in C rats, whereas cortical area did not differ between groups. Structural properties (diaphyseal stiffness, elastic limit, and load at fracture) were about four times higher in hypophysectomized rats, as were the bone material stiffness or Young's modulus and the maximal elastic stress (about 7×). The femur obtained from a middle-aged H rat was stronger and stiffer than the femur obtained from a young-adult C rat, both specimens showing similar size and bone mass and almost equal geometric properties. The higher than normal structural properties shown by the hypophysectomized femur were entirely due to changes in the intrinsic properties of the bone; it was thus stronger at the tissue level. The change of the femoral bone tissue was associated with a high mineral content and an unusual high modulus of elasticity and was probably due to a diminished bone and collagen turnover.

Effects of growth hormone on bone modeling and remodeling in hypophysectomized young female rats: a bone histomorphometric study

Growth hormone (GH) deficiency causes decreased bone mineral density and osteoporosis, predisposing to fractures. We investigated the mechanism of action of GH on bone modeling and remodeling in hypophysectomized (HX) female rats. Thirty female Sprague-Dawley rats at age 2 months were divided into three groups with 10 rats each: control (CON) group, HX group, and HX + GH (3 mg/kg daily s.c.) group, for a 4-week study. Hypophysectomy resulted in cessation of bone growth and decrease in cancellous bone mass. Periosteal bone formation decreased and bone turnover rate of endocortical and trabecular surfaces increased as compared to the CON group. GH administration for 4 weeks restored weight gain and bone growth and mitigated decrease in bone density after hypophysectomy. However, trabecular bone mass in the proximal tibial metaphysis remained lower in group HX + GH than in group CON. Dynamic histomorphometric analysis showed that bone modeling of periosteal bone formation and growth plate elongation was significantly higher in group HX + GH than in group HX. New bone formed beneath the growth plate was predominately woven bone in group CON and group HX + GH. Bone remodeling and modeling-remodeling mixed modes in the endocortical and PTM sites were enhanced by GH administration; both bone formation and resorption activities were significantly higher than in group HX. In conclusion, GH administration to HX rats reactivated modeling activities in modeling predominant sites and increased new bone formation. GH administration also increases remodeling activities in remodeling predominant sites, giving limited net gain in the bone mass.

Differential effects of growth hormone and alpha calcidol on trabecular and cortical bones in hypophysectomized rats

Growth hormone (GH) deficiency in children causes severe growth retardation, vitamin D deficiency, and osteopenia. We investigated whether alfacalcidol (1OHD) alone or in combination with GH can improve bone formation. Forty hypophysectomized female rats (HX) at the age of 8 wk were divided into HX, HX + 1OHD (oral 0.25 microg/kg daily), HX+GH (0.666 mg/0.2 mL SC daily) and HX+GH + 1OHD groups for a 4-wk study. Results showed that GH increased body weight, bone area, bone mineral content (BMC), and bone mineral density (BMD), whereas 1OHD only increased BMC and BMD. In cortical bone, GH increased both periosteal and endocortical bone formation resulting in a significant increase in cortical size and area in percentage, whereas 1OHD suppressed endocortical erosion surface per bone surface (ES/BS) without a significant effect on bone formation rate per bone surface (BFR/BS). In trabecular bone, GH mitigated the bone loss by increasing BFR/BS, whereas the 1OHD effect was by suppression of trabecular bone turnover in the HX rats. The combination of GH and 1OHD had no additive effect on increasing trabecular bone mass. In conclusion, GH activates new bone formation and increases bone turnover whereas 1OHD suppresses bone turnover. The combination intervention does not seem to provide any additive benefit.

Bone mineral density in young adult survivors of acute lymphoblastic leukemia

BACKGROUND

The purpose of the current study was to determine the prevalence of low bone mineral density (BMD) (ie, osteopenia) and identify factors associated with low BMD in young adult survivors of childhood acute lymphoblastic leukemia (ALL).

METHODS

Dual energy x-ray absorptiometry was used to evaluate BMD in 74 randomly selected, long-term childhood ALL survivors initially treated in Minneapolis/St. Paul, Minnesota. Growth hormone (GH)-releasing hormone-arginine stimulation testing was conducted to evaluate peak GH level, and insulin-like growth factor I (IGF-I) and other markers of endocrine functioning were also evaluated in relation to BMD.

RESULTS

The mean age at the time of interview was 30 years, and the mean time since diagnosis was 24 years. Low BMD (Z-score, < or = -1) was present in 24% of subjects, including 1 with osteoporosis. Low BMD was substantially more prevalent in men than in women and was strongly associated with short height. The mean height Z-score for those with low BMD was -1.44, compared with a height Z-score of -0.39 (P < .01) for those with normal BMD. GH insufficiency, low IGF-I Z-score, and current smoking were also suggestive risk factors for low BMD.

CONCLUSIONS

In this long-term follow-up study of childhood ALL survivors, low BMD was found to be more prevalent than expected based on population normative data, specifically in men. The health consequences of early-onset BMD problems in childhood ALL survivors need to be carefully monitored.

Effect of vitamin K2 and growth hormone on the long bones in hypophysectomized young rats: a bone histomorphometry study

The purpose of the present study was to determine whether vitamin K(2) and growth hormone (GH) had an additive effect on the long bones in hypophysectomized young rats. Forty-eight female Sprague-Dawley rats (6 weeks old) were assigned to the following five groups by the stratified weight randomization method: intact controls, hypophysectomy (HX) alone, HX + vitamin K(2) (30 mg/kg, p.o., daily), HX + GH (0.625 mg/kg, s.c., 5 days a week), and HX + vitamin K(2) + GH. The duration of the experiment was 4 weeks. HX resulted in a reduction of the cancellous bone volume/total tissue volume (BV/TV) at the proximal tibial metaphysis, as well as decreasing the total tissue area and cortical area of the tibial diaphysis. These changes resulted from a decrease of the longitudinal growth rate and the bone formation rate (BFR)/TV of cancellous bone, as well as a decrease of the periosteal BFR/bone surface (BS) and an increase of endocortical bone turnover (indicated by the BFR/BS) in cortical bone. Administration of vitamin K(2) to HX rats did not affect the cancellous BV/TV or the cortical area. On the other hand, GH completely prevented the decrease of total tissue area and cortical area in cortical bone, as well as the decrease of marrow area and endocortical circumference, by increasing the periosteal BFR/BS compared with that in intact controls and reversing the increase of endocortical bone turnover (BFR/BS). However, GH only partly improved the reduction of the cancellous BV/TV, despite an increase of the longitudinal growth rate and BFR/TV compared with those of intact controls. When administered with GH, vitamin K(2) counteracted the reduction of endocortical bone turnover (BFR/BS) and circumference caused by GH treatment, resulting in no significant difference of marrow area from that in untreated HX rats. These results suggest that, despite the lack of an obvious effect on bone parameters, vitamin K(2) normalizes the size of the marrow cavity during development of the bone marrow in young HX rats treated with GH.

Additive Effect of Vitamin K2 and Risedronate on Long Bone Mass in Hypophysectomized Young Rats

Hypophysectomy (HX) arrests bone growth and induces osteopenia in the long bones of rats. The present study investigated the combined effect of vitamin K(2) and risedronate on long bone mass in HX rats, in order to determine whether treatment with these two agents had an additive effect. Forty female Sprague-Dawley rats were hypophysectomized at 6 weeks of age by the supplier, and were shipped to our laboratory at three days after surgery along with ten intact rats that served as age-matched controls. The study was started on the day when the rats were received. Three HX rats were excluded from the study because of the failure of HX. Forty-seven rats (6 weeks old) were assigned to the following 5 groups by the stratified weight randomization method: intact controls, HX alone, HX + vitamin K(2) (30 mg/kg, p.o., daily), HX + risedronate (2.5 microg/kg, s.c., 5 days a week), and HX + vitamin K(2) + risedronate. The dosing period was 4 weeks. HX resulted in a decrease of the femoral bone area, bone mineral content (BMC) and bone mineral density (BMD), as well as a decrease in the cancellous bone mass of the proximal tibial metaphysis and the total tissue and cortical areas of the tibial diaphysis. These changes were associated with a marked reduction in the serum level of insulin like growth factor (IGF)-I and with elevation of serum alkaline phosphatase (ALP) and pyridinoline. Administration of vitamin K(2) increased the serum ALP level in HX rats, but did not affect any of the other parameters. On the other hand, risedronate ameliorated the decrease of femoral BMD and cancellous bone mass at the proximal tibial metaphysis in HX rats without affecting the serum IGF-I level, as a result of not causing a significant elevation of serum pyridinoline. Vitamin K(2) and risedronate combined had an additive effect on the femoral bone area, BMC and BMD, and the combined treatment group did not show any significant reduction of the total tissue and cortical areas at the tibial diaphysis, as well as a reduced serum pyridinoline level compared with untreated rats and an increased serum ALP level compared with untreated or risedronate-treated rats. These results suggest that risedronate had a positive effect on the BMD and cancellous bone mass of long bones in HX rats. Despite the lack of a significant effect of vitamin K(2) on bone mass parameters, it had an additive effect with risedronate on the BMC, BMD and cortical bone mass of long bones in HX rats.

Synergistic effect of vitamin K2 and prostaglandin E2 on cancellous bone mass in hypophysectomized young rats

Hypophysectomy (HX) results in cessation of bone growth and cancellous osteopenia in rats. It has been reported that prostaglandin E2 (PGE2) improves cortical and cancellous bone mass in HX rats. The purpose of the present study was to examine whether combined administration of vitamin K2 and PGE2 would have a more beneficial effect on bone than single administration of either alone in HX rats. Forty-three female Sprague-Dawley rats, 6 weeks of age, were randomized by the stratified weight method into five groups: intact controls, HX, HX + vitamin K2 (30 mg/kg, p.o., daily), HX + PGE2 (0.83 mg/kg, i.m., 5 days a week), and HX + vitamin K2 + PGE2. The duration of the experiment was 4 weeks. There was a reduction in cancellous bone volume/total tissue volume (BV/TV) of the proximal tibial metaphysis and a reduction in total tissue area and cortical area (Ct.Ar) of the tibial diaphysis. Vitamin K2 did not affect cancellous BV/TV or Ct.Ar. On the other hand, PGE2 attenuated the loss of cancellous BV/TV in association with higher bone formation rate/bone surface (BFR/BS) and eroded surface (ES)/BS compared with intact controls. PGE2 also increased percent Ct.Ar compared with nontreated HX rats as a result of attenuation of a decrease in periosteal BFR/BS. Vitamin K2 had a synergistic effect with PGE2 on cancellous BV/TV as a result of the suppression of an increase in ES/BS observed by PGE2 treatment. These results suggested that PGE2 had an anabolic action on cancellous and cortical bone and that despite no apparent effect of vitamin K2 on bone, it had a synergistic effect with PGE2 on cancellous bone mass in young HX rats.

Mandibular bone stiffening and increased bone calcium mass in rats permanently stunted by hypophysectomy

OBJECTIVE

This investigation was designed to obtain information on the changes induced by hypophysectomy on biometric parameters, bone calcium mass, and material and architectural properties during ontogenesis of the rat mandible.

DESIGN

Female Sprague-Dawley rats were hypophysectomised (HX) at 30 days of age. A "basal control group" (BC) was sacrificed on the same day surgery was performed. An "age-matched intact control group" (CON) was also included. HX and CON rats were sacrificed when aged 6 months. Body weight was monitored weekly. Mandibular growth was estimated directly on the right hemimandible by taking measurements between stable anatomical points. Its mechanical properties were determined using a three-point bending mechanical test. Load was applied transversely to the bone axis at a point immediately posterior to the posterior surface of the third molar. The left hemimandibles were ashed in a muffle furnace at 600 degrees C for 18h and the ash weight obtained. Calcium content in the ashes was determined by atomic absorption spectrophotometry. It was taken as the mandibular calcium mass. Histomorphometric studies were performed on decalcified hemimandibles: total interradicular bone, bone volume, and height of the periodontal ligament were measured.

CONCLUSIONS

Morphometric studies indicated that hypophysectomy in juvenile rats induced mandibular growth cessation, which was limited to the posterior part of the bone. Thus, the mandible maintained its juvenile proportions and showed an important deformation relative to age. In spite of the reduced bone size, both the mandibular weight and the calcium bone mass increased more than two times in ontogenia. Histomorphometric studies revealed that the interradicular bone volume was markedly increased. These findings strongly suggest that the bone that forms the mandible of the hypophysectomised rat under the conditions of the present study showed a higher than normal density. As evidenced from biomechanical studies, these bone properties, plus the significant stiffening of bone material tissue, were presumably responsible for the unnecessary and marked increment in the "load capacity" suffered by the mandible of the hypophysectomised rat during ontogenesis.

Bone's mechanostat: a 2003 update

The still-evolving mechanostat hypothesis for bones inserts tissue-level realities into the former knowledge gap between bone's organ-level and cell-level realities. It concerns load-bearing bones in postnatal free-living bony vertebrates, physiologic bone loading, and how bones adapt their strength to the mechanical loads on them. Voluntary mechanical usage determines most of the postnatal strength of healthy bones in ways that minimize nontraumatic fractures and create a bone-strength safety factor. The mechanostat hypothesis predicts 32 things that occur, including the gross anatomical bone abnormalities in osteogenesis imperfecta; it distinguishes postnatal situations from baseline conditions at birth; it distinguishes bones that carry typical voluntary loads from bones that have other chief functions; and it distinguishes traumatic from nontraumatic fractures. It provides functional definitions of mechanical bone competence, bone quality, osteopenias, and osteoporoses. It includes permissive hormonal and other effects on bones, a marrow mediator mechanism, some limitations of clinical densitometry, a cause of bone "mass" plateaus during treatment, an "adaptational lag" in some children, and some vibration effects on bones. The mechanostat hypothesis may have analogs in nonosseous skeletal organs as well.

Effects of hypophysectomy and recombinant human growth hormone on material and geometric properties and the pre- and post-yield behavior of femurs in young rats

To study the musculoskeletal effects of hypophysectomy (Hx) and a partial replacement treatment with recombinant human growth hormone (rhGH) in rats, we determined the stiffness (elastic modulus, E) and volumetric BMD (vBMD) of cortical bone; the periosteal and endosteal perimeters, area and bending moment of inertia (xCSMI) of the cross sections, and the structural stiffness and pre- and post-yield strength of the femur diaphyses by pQCT and mechanical tests, and the gastrocnemius weight of rats that were either intact (n = 9) or Hx at 15 days of age (20). The latter were otherwise untreated (Hx controls, 4) or given 0.4 (8) or 2.0 (8) IU kg(-1) day(-1), s.c., of rhGH for 45 days starting 15 days after surgery. Hx delayed musculoskeletal development (gastrocnemius weight, bone geometric properties), thus affecting the diaphyseal stiffness and strength. It also reduced the cortical vBMD through an undefined mechanism, and increased the elastic modulus of cortical bone. The Hx also affected the correlation between bone geometric and material properties (xCSMI vs. E), suggesting an antianabolic interaction with the biomechanical control of bone modeling in response to strains caused by mechanical usage. As a result, Hx reduced the stiffness, post-yield, and ultimate strength of the diaphyses. These effects should reflect changes in bone tissue microstructure, perhaps associated with crack generation and progress, but unrelated to bone mineral mass. They are compatible with the induction of a delay in collagen turnover with associated increases in fibers' diameter and crystals' size that may have resulted from the suppression of some other hormones, such as thyroid, prolactin, or other hormones regulated by ACTH. The above doses of rhGH significantly but incompletely prevented the negative Hx effects on bone and muscle development (bone geometric properties, muscle mass). However, rhGH treatment failed to prevent the demineralizing and stiffening effect of Hx on bone tissue and the unusual effects on the post-yield strength (less clearly related to muscle development than the former). Consequently, rhGH treatment tended to preserve the natural relationship between muscle function and bone geometry but not bone strength. The effects of larger rhGH doses and the interaction of other hormones with the described effects remain to be investigated. Nevertheless, these findings would deserve special attention because they challenge the prevailing view that in endocrine-metabolic bone-weakening diseases the bone matrix always has a normal composition.

Influence of growth hormone on bone marrow adipogenesis in hypophysectomized rats

The hypophysectomized rat has been used as a model to study the effects of growth hormone deficiency on bone. Here, we have investigated the influence of growth hormone administration to hypophysectomized rats (HX) for 6 wk on accumulation of triglycerides in bone marrow and on the differentiation of primary marrow stromal cells into adipocytes under in vitro conditions. We found that hypophysectomy significantly increased triglyceride concentration in bone marrow, which was attenuated by growth hormone administration. Primary bone marrow stromal cells derived from HX rats also had more adipocytes at confluence compared with growth hormone-treated hypophysectomized (GH) rats. When stimulated with 3-isobutyl-1-methylxanthine plus dexamethasone (IBMX-Dex), preadipocyte colony counts increased more significantly in GH rats. Markers of adipocyte differentiation were higher in HX than in control or GH rats at confluence. However, after stimulation with IBMX-Dex, increased expression of markers was seen in GH compared with HX rats. In conclusion, growth hormone administration to hypophysectomized rats attenuated triglyceride accumulation in bone marrow and inhibited the differentiation of stromal cells into adipocytes in vitro.

On longitudinal bone growth, short stature, and related matters: insights about cartilage physiology from the Utah paradigm

Precursor cell division in growing cartilage determines human height, the lengths of the spine and limb bones, the alignment of joints, spines and limbs, and the ratio of spinal length to limb length. That division also helps to determine the sizes and shapes of joints, apophyses and epiphyses. Ideas about what controls those facts are changing. To former views, in which mainly genetic and humoral factors controlled them, the Utah paradigm of skeletal physiology adds long-overlooked biomechanical including muscular factors. These three kinds of factors would collaborate in controlling the precursor cell division that determines the above skeletal features. Problems with that control clearly cause or help to cause many clinical disorders. Examples include short stature, gigantism, premature and delayed skeletal maturation, some changes in fracture patterns associated with puberty, joint malalignments, congenital hip dysplasia, scoliosis, limb torsions, the ball-and-socket ankle, and some skeletal abnormalities in Marfan's syndrome and the osteochondrodystrophies. The physiology such things depend on has matured sufficiently to justify a review for pediatricians, endocrinologists and other clinical specialists, and many basic scientists.

From Wolff's law to the Utah paradigm: insights about bone physiology and its clinical applications

Efforts to understand our anatomy and physiology can involve four often overlapping phases. We study what occurs, then how, then ask why, and then seek clinical applications. In that regard, in 1960 views, bone's effector cells (osteoblasts and osteoclasts) worked chiefly to maintain homeostasis under the control of nonmechanical agents, and that physiology had little to do with anatomy, biomechanics, tissue-level things, muscle, and other clinical applications. But it seems later-discovered tissue-level mechanisms and functions (including biomechanical ones, plus muscle) are the true key players in bone physiology, and homeostasis ranks below the mechanical functions. Adding that information to earlier views led to the Utah paradigm of skeletal physiology that combines varied anatomical, clinical, pathological, and basic science evidence and ideas. While it explains in a general way how strong muscles make strong bones and chronically weak muscles make weak ones, and while many anatomists know about the physiology that fact depends on, poor interdisciplinary communication left people in many other specialties unaware of it and its applications. Those applications concern 1.) healing of fractures, osteotomies, and arthrodeses; 2.) criteria that distinguish mechanically competent from incompetent bones; 3.) design criteria that should let load-bearing implants endure; 4.) how to increase bone strength during growth, and how to maintain it afterwards on earth and in microgravity situations in space; 5.) how and why healthy women only lose bone next to marrow during menopause; 6.) why normal bone functions can cause osteopenias; 7.) why whole-bone strength and bone health are different matters; 8.) why falls can cause metaphyseal and diaphyseal fractures of the radius in children, but mainly metaphyseal fractures of that bone in aged adults; 9.) which methods could best evaluate whole-bone strength, "osteopenias" and "osteoporoses"; 10.) and why most "osteoporoses" should not have bone-genetic causes and some could have extraosseous genetic causes. Clinical specialties that currently require this information include orthopaedics, endocrinology, radiology, rheumatology, pediatrics, neurology, nutrition, dentistry, and physical, space and sports medicine. Basic science specialties include absorptiometry, anatomy, anthropology, biochemistry, biomechanics, biophysics, genetics, histology, pathology, pharmacology, and cell and molecular biology. This article reviews our present general understanding of this new bone physiology and some of its clinical applications and implications. It must leave to other times, places, and people the resolution of questions about that new physiology, and to understand the many devils that should lie in its details. (Thompson D'Arcy, 1917).

Cybernetic aspects of bone modeling and remodeling, with special reference to osteoporosis and whole-bone strength

Assume mythical physiologists were taught that renal physiology and its disorders depend on "kidney cells" and their regulation by nonmechanical factors, but were taught nothing about nephrons. For decades they "knew" that idea was correct, just as Ptolemy "knew" the universe centers on our planet. But then others began to describe nephrons, their roles in renal physiology and disorders, and problems they revealed in former views, so doubts and controversies began. Today real physiologists encounter a similar situation for bone health and its disorders. A 1960 paradigm attributed such things to bone's effector cells (osteoblasts and osteoclasts) and their regulation by nonmechanical factors, without "nephron-equivalent" or biomechanical input. But both mechanical and nonmechanical factors regulate bone's nephron equivalents. Adding features of those equivalents to the 1960 views led to the Utah paradigm, which suggests problems in former views and better explanations for "osteoporosis," whole-bone strength, and other bone disorders. Such things incited controversies among current skeletal physiologists. Cybernetics concerns the relationships, mechanisms, signals, and message traffic that help to control the behavior and other features of dynamic systems. A cybernetic analysis of the bone physiology in the Utah paradigm can add many features to the 1960 paradigm that help to understand osteoporoses, other bone disorders, and whole-bone strength (and bone mass). The added features also show new and pertinent targets for the related research.

Osteoblast-like cells of the hypophysectomized rat: a model of aberrant osteoblast development

In a previous work, we demonstrated that the osteoprogenitors derived from the marrow stroma of the hypophysectomized (HX) rat demonstrate enhanced proliferative and differentiation capacities when placed in an optimal microenvironment. In this study, we sought to investigate the potential of the trabecular osteoblast-like cells of the HX rat. These cells represent a more mature pool of osteoblasts than the progenitors derived from the marrow stroma. We examined all three stages of osteoblast development using trabecular osteoblast-like cells derived from age-matched intact rats as a control. Using thymidine incorporation and cell number as indicators of proliferation, we found that these cells, like the osteoprogenitors derived from the HX rat, demonstrate augmented proliferation when placed in culture. Additionally, type I collagen expression remained at significant levels past the end stages of proliferation, at which point it is expected to be downregulated. Matrix maturation markers, such as alkaline phosphatase activity and bone sialoprotein expression, however, were significantly lower than in the controls. Mineralization potential, as measured by mineralized nodule formation, Ca(2+) content, and OPN and OCN expression, was also significantly reduced. Our results have uncovered an aberrant model of osteoblast development in which proliferation is deregulated, resulting in a minimal capacity of these cells to develop into fully differentiated mineralizing osteoblasts.

On the estrogen-bone relationship and postmenopausal bone loss: A new model

In this model of estrogen effects on bone, a postulated mediator mechanism in marrow would affect modeling and remodeling only of bone next to or close to it. That mediator mechanism could sense estrogen. In response to that hormone, it would let remodeling of bone next to marrow proceed in its conservation mode. This would minimize losses of that bone and tend to prevent an osteopenia. But acute estrogen deficiency would make that mechanism switch remodeling of bone next to marrow to its disuse mode. Meanwhile, conservation-mode remodeling would continue for haversian and subperiosteal bone. The resulting losses of bone next to marrow would expand marrow cavities, thin cortices, and reduce trabecular bone "mass," but would not reduce outside bone diameters. That scheme could explain the osteopenia that follows natural or experimental estrogen deficiency in mammalian females. If so, as estrogen secretion rises in girls at puberty they should begin accumulating more bone next to marrow. They do. Also if so, at menopause women should begin to lose that bone. They do. Those effects would exist in addition to known effects of estrogen on existing osteoclasts and osteoblasts.

Effect of hypophysectomy on the proliferation and differentiation of rat bone marrow stromal cells

Conditions such as estrogen deficiency, skeletal unloading, and aging have all been demonstrated to have various effects on the proliferation and differentiation of bone marrow stroma-derived osteoprogenitor cells. Here we have sought to examine the effects of pituitary hormone deficiency on the proliferation and the differentiation of these osteoprogenitor cells using the hypophysectomized (HX) rat as a model. In the present study, we use an in vitro culture system to examine the effects of HX on the osteogenic potential of rat bone marrow stroma. With the intact animal as a control, we used [3H]thymidine incorporation and cell number as indexes of proliferation. We also measured alkaline phosphatase enzyme activity, relative levels of osteocalcin expression with RT-PCR, and osteopontin and bone sialoprotein steady-state levels by Northern blot to delineate the effect on differentiation. Our results indicate that osteoprogenitor cells exposed to a pituitary hormone-deficient environment in vivo demonstrate an enhanced proliferative capacity and also exhibit an augmented expression of differentiation markers when exposed to an optimal environment in vitro.

Could some biomechanical effects of growth hormone help to explain its effects on bone formation and resorption?

This article suggests that growth hormone effects on bone biomechanical factors help to cause changes in osteoblastic and osteoclastic activities in gigantism and growth-hormone-deficiency states. The suggestion stands partly on the following evidence. (1) In growing mammals, acute partial disuse decreases or stops longitudinal bone growth and periosteal bone formation and increases in outside bone diameter. Yet, in metaphyseal spongiosa, bone resorption and formation continue and net bone losses usually increase. (2) The same pattern of findings occurs in very young rats 5 weeks after hypophysectomy, accompanied by cessation of body and muscle growth and also by reduced physical activity. (3) Another study reduced muscle forces on hindlimb bones by suspending rats by the tail, and found that supplemental growth hormone for 80 days increased hindlimb muscle mass but not hindlimb periosteal bone formation. (4) Such facts support the previous suggestion, which might supplement other ideas about this hormone's modes of action on bone physiology. They include the possibility that the hormone can modulate the responsiveness of bone's biologic mechanisms to mechanical and other nonmechanical influences.

Effects of estrogen and growth hormone on skeleton in the ovariectomized rat with hypophysectomy

To investigate whether growth hormone (GH) and 17beta-estradiol (E2) replacement can prevent osteopenia induced by pituitary and ovarian hormone deficiency [by hypophysectomy and ovariectomy (HX+OV)], we administered relatively low doses of GH (2.3 IU x kg(-1) x day(-1)) and E2 (100 microg x kg(-1) x wk(-1)) in experiment 1 and relatively high doses of GH (13.5 IU x kg(-1) x day(-1)) and E2 (3,500 microg x kg(-1) x wk(-1)) in experiment 2 to 2-mo-old HX+OV Sprague-Dawley rats for 6 wk. Our data show that the HX+OV of rats results in diminished periosteal bone formation, longitudinal bone growth, and decreased cancellous bone volume. Administration of either the low or high dose of GH to these rats increased their systemic growth, serum levels of osteocalcin, and cortical bone formation. Either low or high doses of GH or E2 alone only partially prevent cancellous bone loss. However, the combined treatment of GH plus E2 resulted in an additive increase in the cancellous bone mass. We conclude that the additive effect of GH plus E2 on cancellous bone is attributed to the suppressive effect of E2 on bone resorption and the anabolic effect of GH on bone formation.

Histologic evidence: growth hormone completely prevents reduction in cortical bone gain and partially prevents cancellous osteopenia in the tibia of hypophysectomized rats

BACKGROUND

In previous studies we found that the cause of bone loss in young hypophysectomized (HX) animals was due primarily to an inhibition in growth-dependent bone gain and a decrease in bone turnover. The aim of this study was to determine whether growth hormone, which has stimulatory effects on bone growth and turnover, can prevent HX-induced skeletal alterations in rats.

METHODS

Female Sprague-Dawley rats were divided into baseline control (BASAL), age-matched control (CON), HX, HX plus low-dose GH (1.5 mg/kg/d, subcutaneously), and HX plus high-dose GH (4.5 mg/kg/d) groups. The BASAL group was sacrificed at 2 months of age and the remaining groups were sacrificed after 6 weeks of treatment. Cancellous and cortical bone histomorphometry was performed on double-fluorescent-labeled 40 microm-thick sections of the proximal tibia and tibial shaft.

RESULTS

Both low- and high-dose GH prevented the HX-induced decrease of IGF-I serum levels. High-dose GH also significantly increased the body weight and the wet weight of the gastrocnemius muscle when compared to the CON groups. In the tibial shaft, the periosteal labeled surface, mineral apposition rate and bone formation rate were higher in both of the GH-treated groups than in the HX group (P < 0.05). The tissue area and cortical bone area of the high-dose GH-treated rats were greater than those of the HX rats, but did not differ from those of the CON rats. In the proximal tibia, both low- and high-dose GH prevented an HX-induced decrease in the longitudinal growth rate and growth plate width, and increased surface-based bone formation compared to the HX and CON. Cancellous bone volume, tissue-based bone formation rate, and eroded surface in both of the GH-treated groups were higher than those of the HX group, but lower than those of the BASAL and CON groups (P < 0.05). Bone architecture of the HX rats was also improved after GH treatment.

CONCLUSIONS

This study clearly demonstrates that GH replacement at the dosage of 4.5 mg/kg/d can completely prevent the HX-induced reduction in cortical bone gain in the tibial shaft, but can only partially prevent cancellous osteopenia in the proximal tibia after six weeks.

Effects of 17 beta-estradiol administration on cortical and cancellous bone of ovariectomized rats with and without hypophysectomy

Pituitary hormones are essential for bone growth and bone turnover. Hypophysectomy (HX) diminishes mitogenesis and abolishes the high bone turnover rate induced by ovariectomy (OV). It is not known whether the suppressive effect of estrogen on bone resorption is diminished or abolished by HX. The present study investigates the effects of 17 beta-estradiol (E2) (20 micrograms/wk) on cortical and cancellous bone mass and bone turnover as measured by histomorphometry in HX + OV (HO) rats. Sprague-Dawley rats at 2 months of age were OV or HO and the experiment was performed over a 6 week period. Hypophysectomy + OV (HO) resulted in a cessation of periosteal bone formation, and longitudinal bone growth and a decrease in cancellous bone volume. The tibial dry weight and tibial density were significantly lower in the HO than in the intact or OV groups. Administration of E2 to HO rats partially prevented cancellous bone loss, whereas the same dosage of E2 fully prevented cancellous bone loss in rats with OV alone. Nevertheless, cancellous bone volume was higher in the HO + E2 than in the HO-alone groups. Estradiol administration in HO rats did not suppress cancellous bone formation rate or the eroded surface as much as it did in the OV rats. The suppressive effect of E2 on periosteal bone formation rate and mineral apposition rate was also diminished in HO rats. However, factorial ANOVA showed that the effects of E2 on increasing cancellous bone volume and decreasing periosteal bone formation rate and mineral apposition rate were still significant in the HO rats. Tibial dry weight and tibial density did not differ between HO and HO + E2 groups. In conclusion, we have demonstrated that the estrogen-induced effects of preventing cancellous bone loss, of suppressing bone formation, and resorption as seen in OV rats was diminished but not abolished in HO rats.

Effect of a growth hormone treatment on bone orthotropic elasticity in dwarf rats

A refinement of the current ultrasonic elasticity technique was used to measure the orthotropic elastic properties of rat cortical bone as well as to quantify changes in elastic properties, density, and porosity of the dwarf rat cortex after a treatment with recombinant human growth hormone (rhGH). The ultrasonic elasticity technique was refined via optimized signal management of high-frequency wave propagation through cubic cortical specimens. Twenty dwarf rats (37 days old) were randomly assigned to two groups (10 rats each). The dwarf rat model (5-10% of normal GH) was given subcutaneous injections of either rhGH or saline over a 14-day treatment period. Density was measured using Archimedes technique. Porosity and other microstructural characteristics were also explored via scanning electron microscopy and image analysis. Statistical tests verified significant decreases in cortical orthotropic Young's (-26.7%) and shear (-16.7%) moduli and density (-2.42%) concomitant with an increase in porosity (+125%) after rhGH treatments to the dwarf model (p < 0.05). A change in material symmetry from orthotropy toward planar isotropy within the radial-circumferential plane after GH treatments was also noted. These results demonstrate some alteration in bone properties at this time interval. Structural implications of these changes throughout physiological loading regimens should be explored.

Ovariectomy-induced high turnover in cortical bone is dependent on pituitary hormone in rats

The aim of this study is to examine the interrelationship of pituitary and ovarian hormone deficiency on the regulation of bone growth and bone formation rate. 48 female rats, at 3 months of age, were divided into age-matched intact control, hypophysectomized (HX), ovariectomized (OV), and HX + OV groups. Ten rats were killed at 3 months of age as baseline controls, and the rest of the animals were killed 5 weeks after surgery. Serum levels of osteocalcin and dynamic histomorphometry on the periosteal surface of the tibial shaft and fifth lumbar vertebrae were measured to evaluate systemic and local bone turnover. Tibial and fourth lumbar vertebral bone area, bone mineral content, and bone density were measured by dual-energy X-ray absorptiometry (DXA). Our results confirmed that OV increased and HX suppressed systemic and periosteal bone formation parameters in both bone sites, OV increased and HX suppressed the gain in bone size and bone mass. When OV rats were HX, the serum levels of osteocalcin and periosteal bone formation parameters of the tibial shaft and the fifth lumbar vertebrae were, however, depressed and did not differ from that of the HX alone. DXA results show that the effect of OV on bone size and bone mass is also abolished by HX. In conclusion, we have demonstrated that OV increases tibial and lumbar vertebral bone formation and bone growth and this effect is pituitary hormone dependent.

Effect of ovariectomy on cancellous bone in the hypophysectomized rat

This experiment studied the effects of hypophysectomy (HX) and ovariectomy (OV) on cancellous bone in the proximal tibia and distal 5th lumbar vertebra by dynamic histomorphometry. Forty-eight female Sprague-Dawley rats, 3 months of age, were divided into age-matched control, HX, OV, and HX + OV (HO) groups. Ten rats were sacrificed at 3 months of age as baseline controls, and the rest of the animals were sacrificed 5 weeks after the surgery. While the age-matched controls, and the OV rats significantly increased in body weight compared with the baseline control rats, cancellous bone volumes in the proximal tibia and distal 5th lumbar vertebra increased in the age-matched controls and decreased in the OV rats. In the HX and HO rats, body weight equaled baseline control values, and their cancellous bone volumes were decreased with a poorer trabecular architecture in both bone sites. In all HX, OV, and HO rats, uterine weight and serum estradiol were significantly decreased. OV significantly increased longitudinal bone growth and the tissue- and surface-based bone formation and bone resorption parameters in both the proximal tibia and 5th lumbar vertebra (p < 0.05). HX alone or HO significantly decreased longitudinal bone growth and the tissue-based bone formation rate without significantly affecting surface-based bone formation and bone resorption parameters when compared with the age-matched controls. No significant differences were detected in any variables between the HX alone and HO rats.(ABSTRACT TRUNCATED AT 250 WORDS)