Dynamics of recovery of morphometrical variables and pQCT-derived cortical bone properties after a short-term protein restriction in maturing rats

Severe protein restriction during the post-weaning period in the rat markedly reduces femoral bone mass and produces a number of alterations in the shaft biomechanical properties. Body weight and femur length show an immediate and complete catch-up during nutritional rehabilitation. The aim of the present investigation was to assess whether the accelerated bone growth that occurs during protein rehabilitation is accompanied by recovery of cortical bone properties. The dynamics of the recovery of both material and geometric properties were thus evaluated on the femoral diaphyses in 45-day old female rats after a 10-day period of dietary protein restriction by peripheral quantitative computed tomography (pQCT). Protein starvation led to marked reduction of both body weight and femoral length (37% and 14% at day 10, respectively) which showed a complete catch-up after 30 d of protein refeeding. Protein restriction was associated with the interruption of the natural increase in cortical area (CtCSA), volumetric cortical bone mineral content (vCtBMC) and volumetric cortical bone mineral density (vCtBMD) which were 19.7, 25.8, and 14%, respectively, in malnourished than in control rats at the end of the protein starvation period. These parameters recovered completely during protein refeeding. Treatment also reduced by 30% both rectangular (xCSMI) and polar (pCSMI) moments of inertia. Although an improvement of these architectural indicators occurred with time, an approximately 20% deficit was still present at the end of the observation period (70 d), as was the bone strength index (BSI). It is concluded that protein restriction affected the adaptation of diaphyseal design which should reduce the mechanical competence of the femoral diaphysis because of an inadequate architectural distribution of cortical bone, and that the alteration did not show complete catch-up during the studied period.

Additive effects of dietary protein and energy deficiencies on mandibular growth in the weanling rat

Dietary protein restriction adversely affects mandibular growth in the weanling rat. Protein deficiency is usually accompanied by reduced food intake which, in turn, induces energy deficiency. The present study was thus designed to dissociate the effects of dietary protein and energy deficiencies on the growth of the mandible in rapidly growing rats. Four groups of Sprague-Dawley rats aged 30 days were fed a normal diet, a low-energy diet, a low protein diet, and a low-protein and low-energy diet for 20 days. Rats were sacrificed at the end of experimental period and body weight and mandibular dimensions were recorded to evaluate body growth and mandibular growth. The growth of the mandible was affected almost in the same order of magnitude by both protein and energy restrictions. When both were applied together, mandibular growth was even more severely affected. Two way analysis of variance revealed the absence of synergism between variables, indicating that the negative effects of dietary protein and energy restrictions on mandibular growth could be considered to be additive.

Erythropoietin assay in mice made polycythemic by transfusion of heterologous red cells

A simple in vivo bioassay suitable for routine testing of quality control of recombinant human erythropoietin (rHu-EPO) analogues was developed. Mice made polycythemic by intraperitoneal injection of 1.2 ml of a 80% suspension of heterologous (rat) red cells were used as assay animals and splenic 59Fe uptake as expression of the response to rHu-EPO. The assay took three days and the following schedule is proposed: 1) intraperitoneal injection of 1.2 ml of washed packed red cells obtained from donor rats, 2) subcutaneous injection of test material 4-5 h after transfusion, 3) intravenous administration of 59Fe tracer 48 h later, and 4) determination of splenic isotope uptake 6 h after injection. This method for the in vivo bioassay of rHu-EPO analogues is an economical and reliable alternative to the existing bioassays of the hormone.

Catch-up in mandibular growth after short-term dietary protein restriction in rats during the post-weaning period

Catch-up growth has been defined as growth with a velocity above the statistical limits of normality for age during a defined period of time which follows a period of impaired growth. Since no data are available on catch-up in mandibular growth, the present study was designed to estimate the dynamics of the mandibular size after short-term dietary protein restriction in rats during the post-weaning period. Weanling male rats, 22 d of age, were divided into two groups, control (C) and experimental (E). E rats were fed a protein-free diet during the first 10 d; from this time on, they were placed on a 20% protein diet, as were C rats during the entire experimental period, which lasted 70 d. Five rats from both groups were randomly selected every 10 d and sacrificed. Mandibular growth was estimated directly on the right mandible by measuring several dimensions (mandibular area, base length, mandibular height, mandibular length, alveolar length and incisor alveolar process length). Alveolar and incisor alveolar process lengths did not change with age or dietary protein. All other dimensions increased with age and were thus negatively affected by protein restriction. After growth restriction ceased, the rate of increase of all affected dimensions was above normal values and deficits were swiftly eliminated. Since age-independent dimensions compose roughly the anterior portion of the mandible, this portion of the bone was not affected by protein restriction. It was, thus, the posterior part of the mandible which stopped growth during the nutritional insult and showed catch-up during nutritional rehabilitation. In summary, the rat mandible has a high potential for catch-up during the post-weaning period, showing the ability to achieve complete catch-up in about 30 d.

[Effects of bisphosphonates on the mechanical efficiency of normal and osteopenic bones]

Bone mechanical competence (stiffness, strength) at organ level is determined by mechanical quality (intrinsic stiffness) and spatial distribution (macro-architecture) of bone material in cortical tissue (in every bone) and trabecular network (in vertebral bodies). These properties are inter-related and controlled according to mechanical usage by a feed-back mechanism known as mechanostat. Therefore, the effects on bone fragility of any treatment should be evaluated concerning the way they may have affected bone material or geometric properties as well as the mechanostatical interactions between them. Standard densitometry does not provide the necessary data, but some alternative methodologies (as peripheral quantitative computed tomography, pQCT) are being developed to complement or even substitute SPA, DPA or DXA determinations. Bisphosphonate (BP) effects on bone biomechanics have been studied only in animal models. Many sources of variation of results (type of compound, dose, mode of administration, species, race, sex, age, age since menopause, type of bone, remodeling ability of the skeleton, endocrine-metabolic status, interactions with other treatments, etc.) have been reported. In general terms, BPs are beneficial concerning cortical bone strength in purely modeling species (rodents) and trabecular strength in remodeling mammals (dogs, baboons). This positive action at organ level depends on independent improvements in bone macro-architecture (mainly affected by bone modeling) and material stiffness (chiefly affected by bone composition and remodeling). On one hand, bone macro-architecture has been positively affected by BPs in normal (not in ovariectomy (OX), steroid- or disuse-induced osteopenic) animals. On the other, bone material quality has been improved in the latter but not in the former. Mechanostatic interrelationships have been differently affected according to the compound employed. Results reported by ours and other laboratories concerning the three derivatives available nowadays in Argentina were reviewed and summarized. Pamidronate improved small rodents' cortical bone strength and geometric properties at low doses but impaired mineralization, material properties and strength at toxic doses. In normal, remodeling animals it improved mechanical properties in vertebral bodies but not in long bones. It also prevented the negative impact of OX-, steroid- or disuse-induced osteopenia in rats by improving bone material properties without affecting normal mechanostatic interrelationships. Olpadronate exerted positive effects on long-bone strength at any dose in normal rats and mice by improving cross-sectional properties and preserving both mineralization and material properties. These effects were highly dependent upon bone deformability, body weight, and mechanical usage of the limb as an evidence of an anabolic interaction induced on bone modeling and mechanostatic interrelationships. This compound also prevented the OX- or disuse-induced impairment in rat cortical long-bone strength and recovered rat cortical bone when given since 3 months after OX by improving only bone material quality. No interaction with bone mechanostat was detected in these studies. Alendronate effects on bone biomechanics in normal rats and dogs were positive only in long treatments. They were highly dependent on body weight of the animals, hence a positive interaction with bone mechanostat should be hypothesized. It also prevented the negative impact of OX in rat femurs by improving cortical material quality with no effect on cross-sectional properties, i.e., exerting an anti-catabolic interaction with bone mechanostat. The effects of all the three compounds were found positive for bone health, yet their mechanisms of action varied with type of bone and subject condition. A striking dissociation between (positive) effects on bone strength and (variable) effects on bone stiffness was repeatedly observed in these studies. Also an enla

Unexpected hypoxia-dependent erythropoietin secretion during experimental conditions not affecting tissue oxygen supply/demand ratio

Although a great deal of evidence supports the hypothesis that plasma erythropoietin (EPO) levels of mammals are related to the oxygen supply to the tissues relative to their oxygen needs, several observation millitate against its inherent simplicity. This study presents our results obtained from in vivo experiments that suggest that hypoxia-dependent EPO production can be altered by conditions which apparently do not modify the tissue oxygen supply/demand ratio. Hypoxia-dependent EPO production rate (EPO-PR), derived from plasma EPO titers and plasma EPO half-lives, were estimated in both transfused-polycythemic and normocythemic mouse models subjected to different treatments. From calculations of the O2 carrying capacity of blood and body O2 consumption, it was assumed that the tissue supply/demand ratios were similar in both experimental and control mice of the same model at the time of induction of EPO production. The following observations were worth noting: (1) EPO-PRs in transfused polycythemic mice whose erythropoietic rates were stimulated by intermittent exposure to hypobaria (0.5 atm, 18 hr/day x 3 weeks), phenylhydrazine administration (40 mg/kg at weekly intervals x 3 weeks) or repeated rh-EPO injections (1500 U/kg 3 times a week x 3 weeks) before transfusion were more than five times high than in comparabily polycythemic mice whose erythropoietic rates were not stimulated previously; and (2) EPO-PR in response to hypobaric hypoxia was 2.08 times normal in normocythemic mice with cyclophosphamide (100 mg/kg) induced depression of erythropoiesis, and 0.33 times normal in normocythemic mice with rh-EPO (400 U/kg x 2) induced enhancement of erythropoiesis. Although the results obtained in polycythemic mice are difficult to explain, those from normocythemic mice suggest the existence of a feedback mechanism between EPO-responsive cells and EPO-producing cells. Both demonstrate the existence of experimental conditions in which modulation of the hypoxia-dependent expression of the EPO gene appears to occur. This modulation would be dependent on factors other than oxygen.

Depression of stimulated erythropoietin production in mice with enhanced erythropoiesis

BACKGROUND

The reports of lower plasma erythropoietin (EPO) in anemic patients with active erythropoiesis (hyperplastic) than in comparably anemic subjects with erythroid hypoplasia have generally been interpreted as the result of EPO utilization by the target cells of the hormone. An alternative explanation could be that there is a feedback mechanism through which EPO formation by EPO-producing cells is modulated by the erythroid activity of the erythropoietic organs. The present study was thus designed to investigate EPO production during acute hypoxemia in a mouse model in which the oxygen-carrying capacity of blood, the plasma EPO level, the blood viscosity and the plasma EPO half-life are within normal values in spite of an intense stimulation of erythropoiesis.

MATERIALS AND METHODS

Adult female mice of the CF1 strain with either normal or increased rates of erythropoiesis were used in this study. Erythropoiesis was stimulated by two injections of 10 units of rhEPO given 24 h apart. All experimental determinations were performed 24 h after the second EPO injection. Erythropoiesis was measured by the percent of a tracer dose of 59Fe incorporated into the spleen. Hypobaric hypoxemia was induced by exposing mice to atmospheric air maintained at 50% atmospheric pressure for 6 h. Plasma EPO concentration was determined by RIA. Plasma disappearance of radiolabeled rhEPO was determined by i.v. injection of the hormone and sampling by cardiac puncture every hour for 6 h.

RESULTS

Administration of rhEPO to mice increased splenic 59Fe uptake significantly without affecting the hematocrit, the plasma EPO level or the plasma disappearance of radiolabeled EPO. Plasma EPO titer after 6 h of exposure to hypobaric air was about 70% lower in mice with EPO-induced stimulation of erythropoiesis than in mice with normal erythropoiesis.

CONCLUSIONS

The results of this study suggest that there is an inverse relationship between the rate of stimulated EPO production and erythropoietic marrow activity. They also suggest that the variations in plasma EPO levels during periods of rapidly increasing erythropoiesis are the reflection of a decrease in the rate of production rather than an increase in the rate of utilization by a proliferating pool of erythroid cells.

Inhibitory effect of an alpha 1-adrenergic antagonist on erythropoiesis in normoxic or hypoxic mice

The present study was undertaken to assess the effect of prazosin, a selective postsynaptic alpha 1-adrenergic receptor blocking agent, on normoxic and hypoxic mice, in order to evaluate experimentally its use in the treatment of the excessive erythrocytosis that characterizes chronic mountain sickness. The drug, injected intraperitoneally to adult mice at a dose of 400 micrograms/kg per day, induced a significant depression of the rate or erythropoiesis, as measured by red blood cell 59iron uptake, with a decrease in the hematocrit from the 3rd day. The drug also inhibited the oxygen-dependent secretion of erythropoietin (estimated by the plasma immunoreactive hormone concentration) in hypoxemic mice when injected between 0 and 2 h after initiation of the hypoxic stimulation. When injected daily into mice exposed to intermittent hypobaric hypoxia, prazosin limited the degree of polycythemia or induced a sustained decrease in the hematocrit when polycythemia was already present due to previous exposure. It is postulated that the drug, by reducing the peripheral vascular resistance seen during hypoxia, could increase renal blood flow, thus improving the renal oxygen supply and partially restoring the imbalance between gas supply and demand, which drives erythropoietin formation.

Impaired response of polycythemic mice to erythropoietin induced by protein starvation imposed after hormone administration

The present study was performed to determine the stage of the erythropoietic pathway which is affected by starvation or protein deprivation and whose manifestation is a depressed response to exogenous erythropoietin (EPO). The response to recombinant human EPO was measured in post-hypoxic polycythemic mice by determination of 59Fe uptake into red cells, spleen and femur and/or erythroid colony forming units (CFU-E) and erythroid precursor cell concentrations in femoral marrow. Experimental mice were either starved or fed one of seven different diets whose protein (casein) content ranged from 0 to 20%. All diets were isocaloric. The response of mice maintained on the standard diet (Purina Lab chow) was taken as the normal one. Starvation during the 48-hour period immediately before EPO injection had no effect on the response to the hormone. Starvation, and protein deprivation to a lesser extent, during the 48-hour period following EPO, on the other hand, significantly reduced the response. There was a progressive increase in the response as the casein content of the diet was increased. A normal response was observed when dietary casein concentration was 10%. These findings indicate that nutritional deprivation or dietary protein alterations during the period immediately following EPO injection in polycythemic mice can have detrimental effects on the erythroid response in a model in which nutritional deprivation was relatively short and acute. They also indicate that the subnormal response is not due to a decreased size of the erythroid progenitor pool available for differentiation but to deficient rates of differentiation of erythropoietic units.

Damage of tracer erythropoietin results in erroneous estimation of concentration in mouse submaxillary gland

It has been previously reported that 1) plasma erythropoietin (Epo) titer during exposure to hypobaria is lower in nephrectomized rats and mice whose submaxillary glands (SMG) were either ablated or atrophied than in nephrectomized controls whose SMG were intact and 2) that the gland shows one of the highest levels of immunoreactive Epo (iEpo) in the body. The latter observation, however, was questioned recently when it was observed that SMG extracts degrade labeled Epo used as tracer antigen in the radioimmunoassay (RIA), thus giving invalid estimates of Epo. Since this interpretation was in turn questioned, the present study was conducted to obtain more information on the subject and make these conflicting points clear. Investigation of the reported/possible degradation of Epo by SMG homogenates was conducted via polyacrylamide gel electrophoresis followed by radioautography or by a RIA in solid phase in which there was no simultaneous incubation of the tracer antigen with the SMG homogenates. It was observed that 125I-labeled rhEpo was degraded when incubated with SMG homogenates. Degradation was rapid, being evident when incubation lasted 30 minutes, and occurred in the presence of a protease inhibitor. It showed a high degree of specificity since it did not occur when Epo was incubated with kidney homogenate or normal mouse serum. SMG homogenate did not degrade labeled thyrotrophic hormone and degraded alpha interferon (IFN-alpha) only partially. When estimates of iEpo in SMG homogenate were performed in conditions of simultaneous (SI-RIA) or nonsimultaneous (NSI-RIA) incubation of the homogenate with tracer Epo, it was observed that while estimates of Epo in plasma were similar in both types of RIA and somewhat higher in kidney homogenate in the SI-RIA than in the NSI-RIA, estimates of Epo in SMG were about 60 times higher in the former than in the latter. Therefore, it could be concluded that most of the Epo detected by standard RIA in SMG homogenate does not represent true Epo because of damage of tracer Epo which determines loss of the integrity of the RIA system.

Higher erythropoietin secretion in response to cobaltous chloride in post-hypoxic than in hypertransfused polycythemic mice

BACKGROUND

We have shown previously that both erythrocyte production rate (EPR) and plasma erythropoietin (EPO) levels in response to hypoxia or to compounds able to stimulate EPO secretion are very much higher in post-hypoxic (PH) than in hypertransfused (HT) polycythemic mice with similar levels of hematocrit. Since it has been demonstrated that cobalt (Co) treatment rises renal EPO-mRNA and increases plasma EPO levels, the present study was conducted to determine whether there is a difference between PH and HT mice in relation to the erythropoietic response to Co and whether the stimulatory effect of Co on EPO secretion can be blunted by polycythemia.

METHODS

Adult female mice of the CF-1 strain were made polycythemic by either exposing them to 270 h of discontinuous hypoxia (18 h/d) in a hypobaric chamber maintained at 456 hPA (PH mice) or by injecting them with 0.8 ml of washed packed red cells on two consecutive days (HT mice). Measurement of the erythrocyte production rate (EPR) was made by RBC-59Fe uptake. Plasma EPO concentration was determined by RIA. Cobalt chloride (CoC12) was dissolved in saline and injected in doses of 4 and 8 umoles/mouse. Recombinant human EPO (HEMAX 4000, Bio Sidus SA, Argentina) was dissolved in PBS + albumin to the desired concentration.

RESULTS

By comparison with the corresponding dose-regression line for rHu-EPO, it was estimated that the responses (EPR) (measured as RBC-59Fe incorporation) of PH mice to sc injections of 4 and 8 umoles of CoC12 were equivalent to 95 and 145 mU of rHu-EPO, respectively. The response of HT mice to 4 umoles of the drug was not detectable. At the upper dose level, the response was equivalent to 52 mU of rHuEPO. Plasma immunoreactive EPO (iEPO) titers 12 h after COC12 (8 umoles) were not significantly different between normocythemic and PH mice. The observed values were significantly higher than those found in HT mice.

DISCUSSION

These findings demonstrate that EPO production in response to COC12 is depressed by polycythemia when induced by transfusion but not when induced by chronic exposure to hypobaric hypoxia. They also confirm, but not explain the nature of the conditioning effect of exposure to hypoxia which makes the mechanism controlling EPO secretion either more sensitive to EPO-secreting stimuli or unable to recognize the polycythemic state.

The concentration of dietary casein required for normal mandibular growth in the rat

To determine a suitable casein concentration for normal, undeformed mandibular growth, we placed weanling male rats on diets containing graded levels of casein between 0% and 30% for 19 days. Some weanlings were killed so that initial values could be established. Ten linear dimensions corresponding to the six skeletal units of the mandible were evaluated so that their growth rates at the end of the experimental period could be established. Other dimensions were also evaluated for study of the growth rate of the bone as a whole. The macroscopic growth of the mandible showed a sigmoidal relationship with dietary casein concentration, most of the measurements reaching a plateau at 20% casein. Within the skeletal units, four dimensions corresponding to the alveolar and symphyseal regions did not change with age and were not affected by the casein content of the diet. The remaining six dimensions-corresponding to condylar, coronoid, angular, and basal regions of the mandible-increased with age and were related positively to dietary casein concentration. Their growth patterns were not uniform, although all of them reached maximal values when the diet contained 20% casein. Therefore, deformation of the mandible appears to occur in rats fed diets with a casein concentration lower than 20%. It appears that a dietary casein concentration of 20% is required for normal, undeformed mandibular growth.

Correlation between erythropoietic activity and body growth rate in hypertransfused polycythemic growing rats as the result of an erythropoietin-dependent operating mechanism

The established relationship between erythropoietic activity and body growth rate in the polycythemic growing rat could be the result of either an erythropoietin (EPO)-dependent or an EPO-independent operating mechanism. The present study was thus undertaken to elucidate the nature of the aforementioned mechanism by assessing the ratio between plasma immunoreactive EPO (iEPO) concentration and erythropoietic activity in young hypertransfused rats for different body growth rates. Red blood cell (RBC)-59Fe uptake was about 75% in 21-day-old rats; it rapidly decreased with time when the animals were placed on a protein-free diet, approaching a level of about 1% by the 10th day of protein starvation. Over the same period plasma iEPO decreased from 55 mU/ml to 7 mU/ml. Body growth rate was 0. Following this "protein depletion period" the rats received diets containing different amounts of casein ("protein repletion period") added isocalorically to the protein-free diet to elicit a rise in body growth rate. Statistically significant relationships (p less than 0.001) were found between dietary casein concentration and body growth rate (r = 0.991), dietary casein concentration and RBC-59Fe uptake (r = 0.991), dietary casein concentration and plasma iEPO level (r = 0.992), body growth rate and RBC-59Fe (r = 0.986), and body growth rate and plasma iEPO level (r = 0.994) in hypertransfused polycythemic rats during the protein repletion period. These findings suggest that the correlation between erythropoietic activity and growth rate in the growing rat is the result of an erythropoietin-dependent operating mechanism, which appears to be independent of the ratio tissue oxygen supply/tissue oxygen demand.

Enhanced effect of increased erythrocyte production rate on plasma erythropoietin levels of mice during subsequent exposure to hypobaria

The present study was undertaken to determine plasma EPO levels in response to acute exposure to hypobaria in transfused-polycythemic mice previously exposed to different forms of erythropoietic stimulation. The erythrocyte production rate (EPR) was stimulated by one of the following conditions: (1) discontinuous exposure to 456 mb during 4 weeks, (2) one weekly injection of phenylhydrazine (PHZ) during 4 weeks for induction of a compensated hemolytic state, and 3) three weekly injections of rHuEPO at a dose level of 1500 U/Kg body weight for 4 weeks. Treated and control mice received different volumes of packed red cells in order to obtain hematocrit values around 65% in all groups. Mice from each group were exposed to 456 mb during 14 h, 4 days after transfusion. Mice were bled immediately through cardiac puncture after removal from the hypobaric chamber. Plasma EPO titer was estimated by radioimmunoassay. Two observations were worth noting: (1) plasma iEPO level was very low in mice with transfusion polycythemia exposed to hypobaria, the value of 12 +/- 1.3 mU/ml being significantly different from that of 180 +/- 18 mU/ml found in normocythemic mice similarly exposed, and (2) plasma iEPO levels during EPRs were previously stimulated by different conditions (hypobaria, PHZ and rHuEPO) were at least as high as those found in normocythemic mices similarly exposed. All these values were significantly higher than the value corresponding to polycythemic mice whose EPRs were not stimulated before exposure to hypobaria.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical performance of diaphyseal shafts and bone tissue of femurs from protein-restricted rats

The aim of this study was to define the biomechanical repercussion of a severe protein restriction on the shaft of long bones and on cortical bone tissue. Femurs from 9 rats fed a protein-free diet from the 30th to the 50th day of age showed a great reduction of bending strength and stiffness with respect to 9 controls. These alterations correlated with severe impairment in the amount and/or the architectural arrangement of bone material (volume, wall/lumen ratio, sectional inertia) and also with reduction of Ca content and modulus of elasticity of bone tissue. No changes were observed, however, in bone elastic stress. The impairment in stiffness derived from reductions in both bone mass and modulus of elasticity led to an increment in energy absorption by bone and bone tissue which in turn induced a high incidence of comminuted fractures. The data provide a biomechanical basis for the interpretation of clinical features of nutritional osteopenia in long bones.

Mandibular growth retardation in growing rats chronically exposed to hypobaria

Weanling male Wistar rats aged 21 days were divided into three groups: initial control, normobaric, and hypobaric (C, N, and H, respectively). C rats were killed three days after being weaned. H rats were placed into an altitude chamber and maintained at 456 mb (6100 m) for 14 days. N rats were maintained at sea-level conditions. Body weight, body and tail lengths, and food intake were recorded every day. Animals were killed at the end of the experimental period, and four linear dimensions of the dried mandible were measured. The amount of food eaten by the H rats during the entire exposure period was 54.6% of that consumed by N ones. Body weight gain in H rats was 32.7% of that seen in N rats. Body length was 49.0% and tail length was 56.6% of normal. All mandibular dimensions were significantly reduced in H rats when compared with N rats and were, in general, in close relation with the reduction observed in skeletal growth. Only one dimension was reduced out of proportion, which indicates some deformation of the mandible. The average daily caloric intake related to metabolic body weight (body weight 0.75) of H rats was 60% of the N value. Efficiency of protein utilization for growth was not significantly different between both groups of rats. These results indicate that chronic exposure to hypobaria induces overall skeletal and mandibular growth retardation, which appears to be the result of a diminution in food intake because of decreased appetite.

Hypoxia-induced renal and extrarenal erythropoietin production in posthypoxic or hypertransfused polycythemic rats

The erythropoietic response, measured as 59Fe uptake by RBC, and the erythropoietin content of kidney tissue after exposure to hypobaric hypoxia were determined in both posthypoxic (PH) and hypertransfused (HT) rats. Plasma Ep titers were also measured in both PH and HT nephrectomized rats treated similarly. PH and HT rats with similar degrees of polycythemia greatly differed in their responses to hypoxia, with radioiron uptake by RBC about six times higher in the former than in the latter. Kidney Ep titers were significantly enhanced after exposure to hypoxia in either normal or polycythemic rats. However, kidney Ep titers in PH rats were significantly higher than the values found in normal or HT rats. Plasma Ep titers in both PH and HT nephrectomized rats exposed to hypoxia were similar and significantly lower than the plasma values of normal or nephrectomized, nonpolycythemic rats treated similarly. These results suggest that exposure to hypoxia induces sensitization of the renal, but not extrarenal, mechanism involved in Ep production.

Comparison of erythropoietic response to erythropoietin-secreting stimuli in mice following polycythemia induced by transfusion or hypoxia

The erythropoietic response, measured as RBC-59Fe uptake, in response to either 24-h exposure to hypoxia or administration of dexamethasone, isoproterenol, testosterone, or erythropoietin, was determined in both posthypoxic (PH) and hypertransfused (HT) polycythemic mice. Highly significant differences between PH and HT mice exposed to hypoxia or injected with dexamethasone, isoproterenol, or testosterone were observed, isotope incorporation being always higher in PH than in HT mice. On the other hand, the response to erythropoietin did not show a significant difference between PH and HT mice. These results suggest that PH mice have been preconditioned by exposure to hypoxia in a way that makes them more sensitive to at least some kinds of erythropoietic stimuli. Since these stimuli have been shown by others to increase erythropoietin production, the results support our hypothesis that hypoxia induces sensitization of the erythropoietin-producing organ(s).

Enhanced erythropoiesis induced by hypoxia in hypertransfused, post-hypoxic mice

The erythropoietic response, measured as RBC-59Fe uptake after 24 h-exposure to hypoxia, were determined in posthypoxic, "acute" and "chronic" hypertransfused, and posthypoxic-hypertransfused mice. Radioiron uptake by erythrocytes was about 25 times greater in post-hypoxic than in both types of hypertransfused mice. Posthypoxic-hypertransfused mice showed an erythropoietic response to hypoxia which was not significantly different from that of posthypoxic mice and about 20 times greater than those of "acute" and "chronic" hypertransfused mice. When hypertransfused mice were exposed to hypoxia before transfusions, a linear increase in 59Fe uptake values in response to hypoxia was observed with exposures between 6 and 30 h. No further increase was observed for exposures of 46 to 216 h. These results suggest a sensitization of the erythropoietin-producing organ(s) by hypoxia.

Inhibition by dexamethasone of erythropoietin-induced amplification of the erythropoietin-responsive cell compartment

Erythropoietin-responsive cells (ERC) were reduced to very low levels in post-hypoxic polycythemic mice by intraperitoneal injection of 60 mg/kg-mouse of busulfan. Repeated injections of 5 units erythropoietin (Ep), 1, 2, and 3 days after busulfan repopulated the ERC compartment. Therefore, injection of a small test dose of Ep on day 5 induced a considerable wave of erythropoiesis. When busulfan-treated polycythemic mice received 5 mg of dexamethasone together with Ep injections they responded to the test dose of Ep with very minor increases in iron incorporation, which would indicate that a very small increase in the ERC population induced by Ep took place. These results suggest that the inhibitory action of dexamethasone on erythropoiesis is exerted not only by inhibiting the Ep-induced differentiation of ERC into proerythroblasts, but also by inhibiting the effect of the hormone on the amplification of the ERC compartment.

Influence of growth rate on the ability of hypertransfusion induced plethora to suppress erythropoiesis in the growing rat

To induce different rates of growth male rats 23 days of age were places on diets containing no protein or 20% casein for 10 days. They were then divided into three subgroups which were placed on diets containing 5%, 10% or 20% casein until the end of the experimental period (day 17). Adult rats were used as controls. Rats were hypertransfused on days 8 and 9 and RBC-59Fe uptake determined on day 17. Body weight was directly correlated with the casein content of the diet in the growing rats. RBC-radioiron incorporation was also influenced by the protein content of the diet in the young rats in spite of the fact that hematocrits were over 60% in all the animals. On the contrary, 59Fe uptake by erythrocytes was not related to diet composition in the adult rats. When radioiron incorporation values were plotted against growth rate it became evident that a close direct correlation exists between erythropoiesis and growth rate in transfused-polycythemic rats, indicating that the degree of suppression of red cell production provoked by hypertransfusion-induced plethora is dependent on the rate of growth of the animal, keeping with it an inverse relationship.

Mechanism underlying the inhibitory effect of dexamethasone on in vivo erythropoiesis

The time-response curve for RBC-59Fe uptake following i.p. injection of 5 mg of dexamethasone into normal, non-polycythemic mice, shows a maximal depression (50% of normal) at 3 days after dexamethasone with return to almost normal values by 5 days. The effect is dose-related showing a plateau with doses of dexamethasone above 3 mg. The shape of the time-response curve indicates that the more mature cells in the erythron are not affected by dexamethasone and that the major effect of the steroid must be on earlier erythroid cells. Intravenous injection of dexamethasone 33 and 48 h after i.v. injection of erythropoietin in post-hypoxic polycythemic mice has no effect on the response to erythropoietin, suggesting that the early and late erythroblasts develop normally into erythrocytes. Injection of dexamethasone 13 h after erythropoietin is also ineffective, suggesting that the final steps of differentiation from erythropoietin responsive cells (ERC) to proerythroblasts are not affected. On the contrary, injection of dexamethasone 1 h after erythropoietin reduces by 60% the effective erythropoiesis, which can be attributed to a decrease in differentiation of ERC into proerythroblasts. These results indicate that the inhibitory action of dexamethasone on erythropoiesis is exerted on cells of the erythroid line before the stage of proerythroblast is reached.

Mechanism of the decreased erythropoiesis in the water deprived rat

Radioiron uptake by erythrocytes, metabolic rate, erythropoietin formation during hypoxia and erythroid responsiveness to exogenous erythropoietin were determined in both starved and water deprived rats. The feed intake showed a marked and progressive reduction during water deprivation. The metabolic rates of rats deprived of either food of water declined progressively showing a 40% reduction 5d after water deprivation or starvation began. At this time, the 24 h red blood cells 59Fe incorporation was 85% lower in both starved and dehydrated rats than in normal rats. Plasma erythropoietin levels in response to hypoxia were approximately 50% decreased in both starved and dehydrated rats. Both polycythaemic starved and polycythaemic water deprived rats injected with human urinary erythropoietin showed a 75% decrease in 59Fe incorporation into erythrocytes when compared to control rats. It is suggested that depression of erythropoiesis during water deprivation in the rat depends on a reduced sensitivity to erythropoietin, possibly associated with decreased production of the hormone. Since water deprived rats drastically reduce feed intake it is suggested that secondary starvation is the principal cause of the decreased erythropoiesis induced in the rat by water deprivation.

Production of and response to erythropoietin in the splenectomized mouse

A reliable dose-response curve has been established by exposing normal mice to graded doses of exogenous erythropoietin (ESF). In CF1 femal mice, made polycythemic by exposure to reduced air pressure , 59 Fe incorporation in RBC following injections of ESF (ranging from 0.05 to 3.2 IRP-units) varied according to the log of the ESF dose from 2.5 to 29.0 percent. Experiments with plethorized splenectomized mice showed unequivocally that they were able to respond to ESF, although their responses were very much smaller than that of intact mice, ranging from 1.4 to 12.0 percent. ESF activity was not detected in plasma from intact mice, but was evident in the plasma of mice that had been splenectomized 4 or 10 days earlier. Under conditions of hypoxia the plasma ESF level rose rapidly during the first 48 hours and then fell . In the plasma of splenectomized, similarly treated mice, the ESF level at every interval studied was much higher and was maintained for a longer time. Re-exposure to low oxygen tension for brief periods (2-24 h) produced and erythropoietic response in both intact and splenectomized polycythemic mice, its magnitude increasing with increased exposure time. The response of splenectomized mice, however, was lower than that of intact mice in spite of the fact that the plasma ESF level in the former was higher than in the latter.

Skin acidic glycosaminoglycans in alloxan diabetic rats

Rats made diabetic by administration of alloxan showed a 36 per cent reduction in total GAG concentration. The concentration of hyaluronic acid was 59 per cent and that of chondroitin-4-sulfate 40 per cent decreased. Heparin concentration was 51 per cent increased. The other sulfated fractions did not appear to be affected. The administration of insulin almost entirely restored the level of the affected GAG to normal values, with the only exception of heparin.