Slow twitch soleus muscle is not protected from sarcopenia in senescent rats

Although most literature suggests a relative protection of slow twitch muscle with aging, there is limited data in senescence when muscle atrophy and functional decline markedly accelerate. To address this issue we examined age-related changes in muscle mass, contractile function, mitochondrial enzyme activities, and myosin heavy chain (MHC) expression in the slow twitch soleus (Sol) and fast twitch gastrocnemius (Gas) muscle of young adult (YA) and senescent (SEN) rats. Muscle mass declined between YA and SEN in the Sol by 35% compared to 55% in the Gas muscle. After normalizing for muscle mass, tetanic force per g of muscle declined by 58% in Sol and by 36% in Gas muscle. Time-to-peak tension was increased only in the Gas (30%), whereas time-to-half relaxation was increased by 70% in Sol and 51% in Gas. Citrate synthase and complex IV activity declined in homogenates of Sol (30–36%) and red oxidative region of Gas (46–51%), but not white glycolytic region of Gas. Strikingly, the shift away from the dominant adult MHC isoform with aging was much greater in Sol (fibers positive for MHC fast: 11 ± 2% in YA versus 36 ± 3% in SEN) than in Gas (fibers positive for MHC slow: 12 ± 1% in YA versus 26 ± 3% in SEN) muscle. Collectively, these results show that the slow twitch Sol muscle undergoes large phenotypic alterations in very old age and for several measures (tetanic tension per g, time-to-half relaxation and shift in adult MHC expression) that is of greater magnitude than fast twitch muscle, underscoring the importance of including age-related changes in slow twitch muscle in seeking potential treatments for sarcopenia.     

Fatigability and recovery of rat soleus muscle in hyperthyroidism

The effect of hyperthyroidism on the fatigue properties of the soleus muscle was investigated in rats treated with T3 (20 micrograms/100 g bw) for 14 (14 d T3) and 30 (30 d T3) days. Maximum tetanic force (Po) was identical in all groups. During 15 minutes of stimulation with 600 ms pulsetrains of 100 Hz at a rate of 60/min, Po declined by 50%, 54%, and 70% in euthyroid, 14 d T3, and 30 d T3 rats, respectively. The results were similar when indirect or direct stimulation was applied. Force recovered to 80% of Po in all groups within five minutes. Whereas relaxation rate and Ca++ transport activity were increased twofold already after 14 days of T3 treatment, myofibrillar ATPase activity (M-ATPase) was only increased in the 30 d T3 group. The decrease in phosphorylation potential ([ATP]/[ADP]f[Pi]) (PP) during stimulation was similar in euthyroid and 14 d T3 rats, but 50% larger in 30 d T3 rats. The latter indicated a higher energy consumption, presumably caused by the M-ATPase. Nevertheless, the PP during fatigue was equal in all groups. The decrease in ATP and the increase in lactate content during fatigue were larger in 14 d T3 and 30 d T3 rats as compared to euthyroid rats, but did not differ between the two hyperthyroid groups. It is concluded that the higher fatigability in the 30 d T3 group cannot be explained by impaired neuromuscular transmission, nor by shortage of energy supply.(ABSTRACT TRUNCATED AT 250 WORDS)     

Recovery of slow skeletal muscle after injury in the senescent rat

We studied the contractile, histological and biochemical characteristics of regenerating slow (soleus) muscles of aged rats and the effect of IGF-1 treatment on these parameters. Regenerating soleus muscles were studied 21 days after myotoxic injury. In senescent rats (24 month old), the in situ isometric maximal tetanic force (P0), resistance to fatigue (T20%P0) and shortening speed with an afterload of 20%P0 (SS20%P0) were lower (p<0.05) in regenerating soleus muscles as compared to uninjured controlateral soleus muscles. Moreover, the expression of type 1 myosin heavy chain (MHC-1) was decreased by injury in the soleus muscles of senescent rats (p<0.05). Furthermore, a single injection of IGF-1 (3 microg) into the soleus of senescent rats only slightly increased the level of sarcoplasmic reticulum type 2 Ca(2+)-ATPase in regenerating soleus muscles (p<0.01). Contrary to senescent animals, regenerating soleus of adult rats (10 month old) did not present significantly lower P0 and MHC-1 expression than uninjured controlateral muscles (p>0.05). In conclusion, the regeneration of a slow muscle is more uncompleted 3 weeks after myotoxic injury in senescent rats than in adult rats. It cannot be made more effective by a single injection of IGF-1 into the senescent slow muscle.     

Inhibition of insulin-stimulated xylose uptake in rat soleus muscle by cycloheximide

Cycloheximide (20-200 mg/l) did not affect basal D-[U-14C]xylose uptake by rat soleus muscle (2.4 +/- 0.2 mumol . g-1 . h-1). However, the stimulatory effect of insulin on sugar transport was progressively reduced from 375% above basal in control muscles to 170% in muscles exposed to 200 mg cycloheximide/l but above this concentration cycloheximide inhibited basal xylose uptake without further effect on the incremental effect of insulin. Cycloheximide affected the insulin dose-response curve both by depressing insulin sensitivity and by reducing the maximum stimulatory effect of the hormone. In contrast to the inhibition of insulin action, which increased progressively over the range 20-200 mg cycloheximide/l, muscle protein synthesis was inhibited maximally at a concentration of 10 mg/l. Cycloheximide also inhibited the insulinomimetic effects of anoxia, 2:4-dinitrophenol, salicylate, cooling, hydrogen peroxide, diamide, vitamin K5, hyperosmolarity and EDTA, but did not affect concanavalin A-stimulated xylose uptake. It is concluded that cycloheximide inhibits insulin-stimulated sugar transport at some late post-receptor step, and that this effect of cycloheximide is not secondary to the inhibition of protein synthesis.     

Aging affects passive stiffness and spindle function of the rat soleus muscle

Aging affects many motor functions, notably the spinal stretch reflexes and muscle spindle sensitivity. Spindle activation also depends on the elastic properties of the structures linked to the proprioceptive receptors. We have calculated a spindle efficacy index, SEI, for old rats. This index relates the spindle sensitivity, deduced from electroneurograms recording (ENG), to the passive stiffness of the muscle. Spindle sensitivity and passive incremental stiffness were calculated during ramp and hold stretches imposed on pseudo-isolated soleus muscles of control rats (aged 4 months, n=12) and old rats (aged 24 months, n=16). SEI were calculated for the dynamic and static phases of ramp (1-80 mm/s) and for hold (0.5-2mm) stretches imposed at two reference lengths: length threshold for spindle afferents discharges, L(n) (neurogram length) and slack length, L(s). The passive incremental stiffness was calculated from the peak and steady values of passive tension, measured under the stretch conditions used for the ENG recordings, and taking into account the muscle cross-sectional area. The pseudo-isolated soleus muscles were also stretched to establish the stress-strain relationship and to calculate muscle stiffness constant. The contralateral muscle was used to count muscle spindles and spindle fibers (ATPase staining) and immunostained to identify MyHC isoforms. L(n) and L(s) lengths were not significantly different in the control group, while L(n) was significantly greater than L(s) in old muscles. Under dynamic conditions, the SEI of old muscles was the same as in controls at L(s), but it was significantly lower than in controls at L(n) due to increased passive incremental stiffness under the stretch conditions used to analyze the ENG. Under static conditions, the SEI of old muscles was significantly lower than control values at all the stretch amplitudes and threshold lengths tested, due to increased passive incremental stiffness and decreased spindle sensitivity at L(s). The muscle stiffness constant values were greater in old muscles than in controls, confirming the changes in elastic properties under passive conditions due to aging. Aging also altered the intrafusal fibers: it increased the mean number of intrafusal fibers and the contents in the slow, neonatal and developmental isoforms intrafusal of MyHC have been modified. These structural modifications do not seem great enough to counteract the loss of the spindle sensitivity or the spindle efficacy under passive conditions and after the nerve was severed. However, they may help to maintain the spindle afferent message under natural conditions and under fusimotor control.     

Short-term measurement of D-xylose uptake by isolated rat soleus muscle.

The uptake of D-[U-14C]xylose by isolated rat soleus muscle was studied, using D-[1-3H]-sorbitol as an extracellular marker. Xylose uptake was limited by the diffusion of the sugar into and through the extracellular water. This could be overcome in part by allowing the test sugars to pre-equilibrate in the extracellular water at 0 degrees C, before measuring xylose uptake. It was not necessary to fill the extracellular water with the test sugars to obtain maximum rates of xylose uptake. From this it was concluded that the sugar carrier sites were located in a specific region on the plasma membrane, readily accessible to sugar carrier sites were located in a specific region on the plasma membrane, readily accessible to sugars entering the interstitial water. Pre-equilibration was more effective in the absence of insulin than in the presence of the hormone. This suggested that insulin may influence sugar uptake at some site prior to the cell membrane. Pre-incubation at 0 degrees C itself stimulated sugar transport. This effect of cooling was not influenced by insulin, nor did it appear to affect the stimulatory action of insulin on xylose transport.     

Age-related dystrophin-glycoprotein complex structure and function in the rat extensor digitorum longus and soleus muscle

This study tested the hypothesis that age-related changes in the dystrophin-glycoprotein complex (DGC) may precede age-associated alterations in muscle morphology and function. Compared to those in adult (6 month) rats, extensor digitorum longus (EDL) and soleus muscle mass was decreased in old (30 month) and very old (36 month) Fischer 344/NNiaHSD x Brown Norway/BiNia rats. The amount of dystrophin, beta-dystroglycan, and alpha-sarcoglycan increased with aging in the EDL and decreased with aging in the soleus. alpha-Dystroglycan levels were increased with aging in both muscles and displayed evidence of altered glycosylation. Immunostaining for the presence of antibody infiltration and dystrophin following increased muscle stretch suggested that the aging in the soleus was characterized by diminished membrane integrity. Together, these data suggest that aging is associated with alterations in EDL and soleus DGC protein content and localization. These results may implicate the DGC as playing a role in age-associated skeletal muscle remodeling.     

Inhibition of insulin-stimulated xylose uptake in denervated rat soleus muscle: a post-receptor effect

Summary Insulin (100 U/l) stimulated xylose uptake in rat soleus muscle from a basal value of 2.3±0.5 to 11.6±2.1 mol · g^-1 · h^-1. Denervation (section of the sciatic nerve) markedly reduced the stimulatory action of insulin (basal 1.3 ±0.4 mol · g^-1 · h^-1; insulin-stimulated 4.5±0.6 mol · g^-1 · h^-1). This effect appeared 3 days after denervation and was maximal after 5 days. Denervation also affected the insulin dose response curve; there was no effect of insulin in denervated muscle until the concentration exceeded 1 U/l. Denervation inhibited insulin-stimulated -aminoisobutyrate uptake by 77% but did not affect ¹²⁵I-insulin binding or glucose-independent activation of glycogen synthase by insulin. There was no effect of denervation on the insulinomimetic effects of concanavalin A, hydrogen peroxide, vitamin K₅, anoxia, 24-dinitrophenol, cooling, hyperosmolarity or EDTA, but the effect of diamide was inhibited. It is concluded [1] that denervation inhibits insulin-stimulated sugar transport at some early post-receptor step, and [2] that the mechanism whereby insulin activates glycogen synthase is different from the activation of the membrane transport of sugars and amino acids.     

Exercise training produces nonuniform increases in arteriolar density of rat soleus and gastrocnemius muscle

OBJECTIVE: Exercise training has been shown to increase regional blood flow capacity to muscle tissue containing fibers that experience increased activity during exercise. The purpose of this study was to test the hypothesis that the increased blood flow capacity is partially the result of increases in arteriolar density (number of arterioles/mm2 of tissue), specifically in skeletal muscle tissue, with the largest relative increase in muscle fiber activity during training bouts. METHODS: This hypothesis was tested by comparing and contrasting the effects of endurance exercise training (ET) and interval sprint training (IST) on arteriolar density in soleus muscle (S) red (Gr) and white (Gw) portions of gastrocnemius muscle of male Sprague Dawley rats. ET rats completed 10 weeks of treadmill training 30 m/min, 15% grade, 60 min/day, 5 days/week, while IST rats completed 10 weeks of IST consisting of six 2.5-min exercise bouts, with 4.5-min rest between bouts (60 m/min, 15% incline), 5 days/week. The hypothesis would be supported if ET increased arteriolar density in S and Gr and if IST increased arteriolar density in Gw. RESULTS: ET increased arteriolar density above values of sedentary rats (SED) in both the Gw (ET = 0.93 +/- 0.19 arterioles/microm2; SED = 0.44 +/- 0.09 arterioles/microm2) and Gr (ET = 0.97 +/- 0.1 arterioles/microm2; SED = 0.51 +/- 0.06 arterioles/microm2) muscles, but not in S (ET = 1.69 +/- 0.45 arterioles/microm2; SED = 1.51 +/- 0.34 arterioles/microm2) muscle. In contrast, IST did not alter arteriolar density in Gw or Gr muscle tissue. Although arterial wall thickness was greater in S (3.95 +/- 0.40 microm) and Gr (6.24 +/- 0.59 microm) than Gw (2.76 +/- 0.18 microm), neither ET or IST altered mean wall thickness in either muscle. CONCLUSION: Increases in blood flow capacity produced in Gr and Gw by ET appear to be due in part to increased arteriolar density. In contrast, increased arteriolar density does not contribute to increased blood flow capacity of Gw in IST rats.     

Effect of exercise training on antioxidant and metabolic functions in senescent rat skeletal muscle.

Aging is known to be associated with alterations of both oxidative capacity and antioxidant status in skeletal muscle. In the present investigation we compared the activity of enzymes involved in both metabolic functions and antioxidant defense capacity in young adult (5 months old) and senescent (27.5 months old) Fischer 344 rats. In addition, we studied the effects of chronic exercise training on these enzymes in the senescent skeletal muscle. Old sedentary rats had significantly lower glutathione peroxidase (GPX) activity (-22%, p less than 0.05) in the deep portion of vastus lateralis muscle (DVL) than young sedentary rats, but after a progressive 10-week treadmill training program GPX activity in DVL was significantly increased in old rats to a level higher than that seen in young sedentary rats. Superoxide dismutase and catalase activities in the DVL were not altered significantly with aging or by training. Glutathione S-transferase activity in the same muscle was elevated (p less than 0.05) with aging but unaffected by training. Citrate synthase and malate dehydrogenase activities in the DVL muscle were significantly decreased in senescence, whereas training increased these two enzyme activities by 71 and 48%, respectively (both p less than 0.05). Lactate dehydrogenase activity in the same muscle decreased with age but increased 23% (p less than 0.05) in old rats. These data indicate that while aging may significantly affect antioxidant and metabolic capacities in skeletal muscle, regular exercise can preserve functions of these enzyme systems at old age.     

Effect of insulin and leucine on protein turnover in rat soleus muscle after burn injury

To investigate the effects of thermal injury on muscle protein turnover, net protein breakdown and incorporation of leucine into protein was measured in vitro in rat soleus at 3 days following a 3 sec burn to one hindlimb. The weight gain and food consumption of the burn injured animals was similar to that of unburned animals. However, the burn caused an 11% decrease in soleus muscle weight and protein content. The levels of ATP, phosphocreatine, and the phosphocreatine/creatine ratio were all normal. Net protein breakdown from the burn-injured muscle was elevated 35% while the incorporation of leucine into protein was unchanged. Thus the increase in protein breakdown in the muscle from the burned region appears to be responsible for the loss in muscle protein. Leucine oxidation was also stimulated by burn. Since protein turnover and leucine oxidation in the contralateral muscle of the burned animal was identical to those from unburned animals, the effects appear to result from direct thermal injury to the muscle from the burned hindlimb rather than from systemic alterations in the metabolic or endocrine environment. In addition, a physiological concentration of insulin (100 μU/ml) was found to stimulate incorporation of leucine into protein and inhibit net proteolysis to the same extent in soleus from burned and unburned limbs. While lower insulin concentrations need to be tested, there appears to be no evidence for insulin resistance with respect to protein turnover under these conditions.     

Stimulation of sugar transport in rat soleus muscle by prolonged cooling at 0 °C

Summary The uptake of D-xylose by isolated rat soleus muscle (measured at 37 °C) was stimulated by prolonged cooling at 0 °C. The effect of cooling reached a maximum value after 3 h and was reversed on rewarming; reversal was temperature-dependent. Cooling stimulated xylose uptake sub-maximally compared with the effect of insulin (100 U/l). Xylose uptake in cooled muscle was further stimulated by insulin, but not by anoxia. The effect of cooling and its reversal were still demonstrable in the presence of ouabain (1 mmol/l), or when unidirectional efflux of calcium and magnesium from the muscle was induced by EDTA (5 mmol/l). The ionophore, A23187 (2.5 mg/l), depressed the effect of cooling in the presence of EDTA but not in the presence of EGTA. It is concluded that cooling disrupts an intracellular magnesium-pump and that muscle sugar transport is consequentially stimulated through an increase in cytoplasmic magnesium.     

Modulation of sodium-channel mRNA levels in rat skeletal muscle.

Action potentials in many types of excitable cells result from changes in permeability to Na ions. Although these permeability changes in nerve and muscle are mediated by voltage-gated Na channels that are functionally similar, we found that the Na-channel gene expressed in skeletal muscle is different from the genes coding for two Na channels (type I and type II) in brain. Despite the structural differences between muscle and brain Na-channel genes, a cDNA clone derived from rat brain hybridizes to skeletal muscle Na-channel mRNA of approximately 9.5 kilobases. We used this cDNA probe to measure changes in Na-channel mRNA levels in skeletal muscle during development and following denervation. By blot hybridization analysis of electrophoretically fractionated RNA, we found that Na-channel mRNA can be detected as early as embryonic day 17 and that mRNA levels increase 2-fold between birth and postnatal day 35. Denervation of adult muscle causes a further 2- to 3-fold increase in muscle Na-channel mRNA levels, suggesting that expression of Na-channel genes in fast-twitch muscle may be regulated by the state of innervation.     

Cyclic adenosine monophosphate accumulation and beta-adrenergic binding in unweighted and denervated rat soleus muscle.

Unweighting, but not denervation, of muscle reportedly "spares" insulin receptors, increasing insulin sensitivity. Unweighting also increases beta-adrenergic responses of carbohydrate metabolism. These differential characteristics were studied further by comparing cyclic adenosine monophosphate (cAMP) accumulation and beta-adrenergic binding in normal and 3-day unweighted or denervated soleus muscle. Submaximal amounts of isoproterenol, a beta-agonist, increased cAMP accumulation in vitro and in vivo (by intramuscular [IM] injection) to a greater degree (P less than .05) in unweighted muscles. Forskolin or maximal isoproterenol had similar in vitro effects in all muscles, suggesting increased beta-adrenergic sensitivity following unweighting. Increased sensitivity was confirmed by a greater receptor density (Bmax) for [125I]iodo-(-)-pindolol in particulate preparations of unweighted (420.10(-18) mol/mg muscle) than of control or denervated muscles (285.10(-18) mol/mg muscle). The three dissociation constant (Kd) values were similar (20.3 to 25.8 pmol/L). Total binding capacity (11.4 fmol/muscle) did not change during 3 days of unweighting, but diminished by 30% with denervation. This result illustrates the "sparing" and loss of receptors, respectively, in these two atrophy models. In diabetic animals, IM injection of insulin diminished cAMP accumulation in the presence of theophylline in unweighted muscle (-66% +/- 2%) more than in controls (-42% +/- 6%, P less than .001). These results show that insulin affects cAMP formation in muscle, and support a greater in vivo insulin response following unweighting atrophy. These various data support a role for lysosomal proteolysis in denervation, but not in unweighting, atrophy.     

Maintenance of heterogeneity of capillary spacing is essential for adequate oxygenation in the soleus muscle of the growing rat

OBJECTIVES: Normal muscle growth is accompanied by capillary proliferation, which usually lags behind the increase in muscle size, causing a decline in mean capillary density (CD). It is not known, however, how the capillary distribution is affected and what impact it has on the oxygenation of the muscle. METHODS: The capillarization of soleus muscles of rats (64-425 g) was determined with the method of capillary domains. As well as quantifying CD, capillary to fiber ratio (C:F), and fiber size, this method provides a measure of the heterogeneity of capillary spacing. Capillary locations were used to mathematically model oxygenation levels within the muscle. RESULTS: The increase in muscle mass was largely attributable to 5-fold increase in fiber size, accompanied by a more than 3-fold rise in C:F. The mismatch between rates of angiogenesis and muscle growth resulted in a decrease in CD. However, the heterogeneity of capillary spacing was unaffected (heterogeneity index logRSD: 0.091 +/- 0.013; mean +/- SD) as was muscle PO2, with modal values between 4 and 60 mmHg (0.5 and 8 kPa). CONCLUSIONS: Angiogenesis during normal muscle growth does not maintain CD, but with similar heterogeneity of capillary spacing it preserves the potential for adequate intramuscular oxygenation.     

Different mechanisms of increased proteolysis in atrophy induced by denervation or unweighting of rat soleus muscle

Mechanisms of accelerated proteolysis were compared in denervated and unweighted (by tail-cast suspension) soleus muscles. In vitro and in vivo proteolysis were more rapid and lysosomal latency was lower in denervated than in unweighted muscle. In vitro, lysosomotropic agents (eg, chloroquine, methylamine) did not lessen the increase in proteolysis caused by unweighting, but abolished the difference in proteolysis between denervated and unweighted muscle. Leucine methylester, an indicator of lysosome fragility, lowered latency more in denervated than in unweighted muscle. 3-Methyladenine, which inhibits phagosome formation, increased latency similarly in all muscles tested. Mersalyl, a thiol protease inhibitor, and 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8), which antagonizes sarcoplasmic reticulum release of Ca2+, reduced accelerated proteolysis caused by unweighting without diminishing the faster proteolysis due to denervation. Calcium ionophore (A23187) increased proteolysis more so in unweighted than control muscles whether or not Ca2+ was present. Different mechanisms of accelerated proteolysis were studied further by treating muscles in vivo for 24 hours with chloroquine or mersalyl. Chloroquine diminished atrophy of the denervated but not the unweighted muscle, whereas mersalyl prevented atrophy of the unweighted but not of the denervated muscle, both by inhibiting in vivo proteolysis. These results suggest that (1) atrophy of denervated, but not of unweighted, soleus muscle involves increased lysosomal proteolysis, possibly caused by greater permeability of the lysosome, and (2) cytosolic proteolysis is important in unweighting atrophy, involving some role of Ca2(+)-dependent proteolysis and/or thiol proteases.     

Mechanism of specific force deficit in the senescent rat diaphragm

Aging is associated with a decline in the maximal in vitro specific force in the rat costal diaphragm. The purpose of this study was to determine if this force deficit is associated with a decrease in the concentration of myofibrillar protein in diaphragm fibers of senescent rats. Isometric twitch and tetanic contractile properties were measured on diaphragm strips from young adult (9-month-old; n=12) and senescent (26-month-old; n=13) male specific pathogen free-barrier protected Fischer 344 rats. Maximal tetanic force (P_o) normalized to the cross-sectional area (CSA) of the in vitro diaphragm strips was 16.4% lower in the senescent diaphragms (21.03±0.4 N/cm²) compared to the young adult (25.16±0.5 N/cm²) (p<0.001). Diaphragm water content was significantly higher in the senescent group (75.9% of total wet mass) compared to the young adult (72.1% of total wet mass, p<0.05). Subtracting the contribution of water from the CSA of the diaphragm strips significantly reduced (p<0.05) the senescent specific P_o deficit (from −16.4 to −6.4%). Further, correcting P_o for the contribution of myofibrillar protein to CSA resulted in no age group differences in specific force. These data indicate that the age-related decline in diaphragm in vitro maximal specific P_o can be explained by an age-related increase in the water content of the diaphragm muscle. Future experiments are necessary to determine the mechanism(s) responsible for this observation.     

Restoration of bone mass in the severely osteopenic senescent rat

Studies in humans and rats suggest that age impairs the ability to form bone. This impairment may be due to a depletion or deficit in osteoprogenitor stem cells. Such a deficit would be expected to reduce the ability of the skeleton to respond to therapy designed to restore lost bone. This study evaluated whether severely osteopenic senescent rats are capable of responding to a potent anabolic factor in bone, prostaglandin E2 (PGE). Growing female Sprague Dawley rats were ovariectomized at 3 months and aged until the start of treatment at 23 months. Rats were treated daily with PGE (3 mg/kg sc) or vehicle for 56 days. Tibiae were harvested for bone histomorphometry and femora were obtained for mRNA analysis of bone matrix proteins. The cancellous bone area was fivefold greater in PGE-treated rats than in vehicle-treated controls and not different from age-matched ovary-intact rats. PGE approximately doubled the bone-forming surface and the mineral apposition rate and increased the bone formation rate fourfold. The increased cancellous bone area in PGE-treated rats was primarily due to an increase in osteoblasts over osteoclasts. One hundred percent of the endocortical surface and 72 +/- 9% of the periosteal surface of cortical bone was undergoing mineralization in PGE-treated rats, whereas no mineratization was evident in vehicle-treated rats. An architectural analysis of cancellous bone indicates that trabecular number and thickness were increased and separation decreased in the treated rats. Imaging by microcomputed tomography further revealed that with PGE treatment, trabeculae in the medial plane of the proximal tibial metaphysis were more robust and continuous with the endocortical surface. PGE also significantly induced message levels for the prepro-alpha (I) subunit of type I collagen (collagen), osteonectin, and osteocalcin. In summary, bone mass can be restored to severely osteopenic senescent rats, suggesting that aging does not necessarily diminish the capacity of the skeleton to form bone.