Changes in plasma urea concentration can be used to determine protein requirements of two populations of pigs with different protein accretion rates.

The experiment had two objectives: 1) to determine the protein requirements of two strains of growing-finishing pigs based on growth performance, carcass characteristics, tissue accretion rates, and organ weights and 2) to evaluate whether protein requirements can be determined from changes in plasma urea concentration. Forty-six Gene Pool (GP) and 46 Hampshire (H) gilts with an initial BW of 28.5 kg were used. Pigs were allotted to two trials of a randomized complete block experiment with a 2 x 6 factorial arrangement of treatments. Five pigs from each strain were randomly selected and slaughtered at the beginning of each of the two trials. The remaining 72 pigs were individually penned and allotted to one of six dietary treatments (10, 13, 16, 19, 22, or 25% CP). Pigs remained on the experiment until the mean weight of a treatment group within each strain reached 115 kg (16 wk for GP and 14 wk for H), at which time all pigs of that strain were slaughtered. The only strain x protein level interactions that were detected were for carcass protein and water accretion rates. Gene Pool pigs grew less rapidly and utilized feed less efficiently than H pigs (P < .001). Average daily gain (quadratic, P < .05) and ADG/ADFI (quadratic, P < .05) were increased as protein level increased until a plateau was reached. Backfat depths were decreased (linear, P < .001) and longissimus muscle areas were increased (linear, P < .001) as protein level increased. Protein accretion rate was lower (P < .01) and fat accretion was higher (P < .01) in GP pigs than in H pigs. Protein accretion increased (quadratic, P < .001) and fat accretion decreased (linear, P < .001) with increasing dietary protein level. Examination of the response of plasma urea concentration over time suggested that GP pigs required 13% CP from 30 to 80 kg and 10% CP thereafter, whereas H pigs required 19% CP from 30 to 45 kg, 16% CP from 45 to 100 kg, and 13% CP thereafter.

The onset of maternal diabetes in swine induces alterations in the development of the fetal preadipocyte.

Diabetes induced during gestation has previously been demonstrated to increase adipose accretion in the fetal pig. The present experiment examined whether maternal diabetes alters the proliferation and differentiation of the fetal preadipocyte. Seven crossbred gilts were injected with alloxan (50 mg/kg) at d 75 of gestation to induce diabetes and seven additional gilts were injected with buffer (controls). All gilts underwent Caesarean section of d 105 of gestation. Cells obtained from adipose tissue of fetuses of diabetic swine (FDS) at d 105 of gestation demonstrated a greater (P < .05) proliferative response (57%) and higher (P < .05) rates of differentiation as determined by sn-glycerol-3-phosphate dehydrogenase (142% increase) and lipoprotein lipase (80% increase) activities than cells acquired from fetuses of control swine (FCS). Adipogenic activity of the sera from these two groups of fetuses did not differ when tested on adipose tissue from fetuses at 105 d of gestation. However, use of these sera on cells derived from normal fetuses at 75 d of gestation resulted in detection of an increase (P < .05) in adipogenic activity within the sera from FDS. This study suggests that maternal diabetes causes alterations in the preadipocyte fraction of cells within the developing adipose tissue that result in formation of more adipocytes and thus permits greater capacity for lipid accumulation in the growing fetus of the diabetic pig. These alterations in the preadipocyte result from the activity of factors that transitionally function during the latter half of gestation.

Alteration in IGF-I mRNA content of fetal swine tissues in response to maternal diabetes.

Diabetes alters the level of insulin-like growth factor I (IGF-I) mRNA in tissues of postnatal animals, but the impact of maternal diabetes or gestational diabetes on IGF-I mRNA abundance in fetal tissues has not been examined. Pregnant pigs were injected with either buffer or alloxan (50 mg/kg) at day 75 of gestation to induce diabetes. Fetal tissue samples were collected at day 105 of gestation, and IGF-I mRNA abundance (densitometric units/10 micrograms total RNA) were estimated by specific ribonuclease protection assay. Fetal glucose and IGF-I concentrations were increased 166 and 34%, respectively, by maternal diabetes. Maternal diabetes induced an increase in abundance of IGF-I mRNA in fetal skeletal muscle, liver, heart, kidney, and placenta. IGF-I mRNA levels were depressed by maternal diabetes in fetal adipose tissue and brain compared with the respective tissues from fetuses of control pigs. These data indicate that circulating levels of IGF-I and the steady-state levels of IGF-I mRNA in fetal tissues can respond to the metabolic and endocrine alterations occurring during maternal diabetes. The large variation in expression and degree of response among fetal tissues indicates that the fetus experiences tissue-specific regulation of IGF-I expression during development.

Influence of somatotropin on lipid metabolism and IGF gene expression in porcine adipose tissue.

The present study was designed to evaluate the effects of porcine somatotropin (pST) treatment (2 mg/day) and dietary fat (10%) separately and in combination on the metabolic activity of subcutaneous adipose tissue, serum adipogenic activity, and insulin-like growth factor (IGF) gene expression within adipose tissue from growing 5- to 6-mo-old barrows. This study attempted to determine how these factors might contribute to the reported changes in adiposity of treated swine. Biopsies of adipose tissue were collected after 28 days of treatment following anesthesia with thiopental sodium (15 mg/kg iv). Somatotropin inhibited in vitro glucose oxidation and lipogenesis in adipose tissue but did not affect fatty acid esterification. Adipogenic activity of serum was not altered by pST treatment. Subcutaneous adipose tissue contained mRNA for IGF-I and -II, and pST administration increased the abundance of IGF-I mRNA. Dietary fat had no effect on these variables. Thus somatotropin reduces glucose metabolism in porcine subcutaneous adipose tissue. Preadipocyte proliferation and differentiation are not affected by somatotropin through its actions on systemic factors. Dietary fat provides no additional benefit in combination with pST administration to affect accretion of adipose tissue in growing swine.

In vitro systems for the analysis of the development of adipose tissue in domestic animals.

In vitro models have been used for many years to assess processes that regulate metabolic activity of adipocytes or that regulate proliferation and differentiation of precursor cells for adipocytes (preadipocytes). The results of these studies have been useful in characterizing the role of various hormones and growth factors in regulating the cellular activities of preadipocytes and adipocytes from domestic animals. Mechanisms that regulate the proliferation and differentiation of the preadipocyte are currently being evaluated with the use of a variety of cell culture systems: matrix suspension, coverslip, micromass, clonal etc. Isolated cell culture of adipose tissue and the tools of molecular biology have allowed direct examination of the effects of potential regulatory hormones of adipogenesis upon gene expression during the development of adipose tissue in domestic animals. Much information has been collected by combining the use of in vivo animal manipulations and in vitro assay techniques to elucidate the mechanisms that affect adipocyte function or formation. The development of in vitro tools to manipulate the development of adipose tissue should result in an increase in our knowledge of the regulation of adipose tissue formation and thus should permit the development of methods to produce leaner and more efficient domestic animals.

Fatty acid metabolism by the porcine placenta.

Two experiments were performed to determine whether esterification is a major pathway of fatty acid utilization within porcine placenta and to determine what metabolic parameters may limit fatty acid transfer to the fetal pig. Maternal (endometrium) and fetal (chorioallantois) placenta were obtained by Caesarean section at d 110 of gestation in both experiments. Eight gilts were used in the first experiment. Tissue sections were incubated with palmitate at concentrations ranging from .25 to 2.0 mM. Maternal placenta metabolized palmitate at a higher rate than fetal placenta, although fetal placenta was more efficient in esterifying palmitate. Esterification composed the majority of palmitate utilization within fetal and maternal placenta. The second experiment evaluated the effect of dietary lipid on placental fatty acid metabolism and evaluated the ability of placenta to mobilize lipids. Fourteen gilts were divided into two groups of seven and fed a diet containing 15% tallow diet or a diet not supplemented with tallow (control) from d 90 to 110 of gestation. Dietary lipid had no detectable effects on lipoprotein lipase activity, [14C]palmitate metabolism, or lipolysis by the maternal or fetal placenta. Lipolytic activity of placental tissues was minimally affected by incubation with various proposed lipolytic activity of placental tissues was minimally affected by incubation with various proposed lipolytic agents. The data indicate that supply of fatty acids to the fetal pig may be limited by transfer of plasma fatty acids into the cytoplasm of placental cells or by regulatory enzymes for intermediate esterification; both types of limitations have been proposed to be influenced by fatty-acid binding proteins.

Effect of maternal fasting on fetal and placental lipid metabolism in swine.

This experiment was designed to determine whether mobilizing maternal energy stores by fasting pregnant gilts would promote fetal energy storage by altering placental and fetal lipid metabolism. Pregnant gilts were fed a 15% tallow diet from d 80 to 99 and then fed a basal high-carbohydrate diet (control) or fasted from d 100 to 110 of gestation. Caesarean section was performed on d 110. Fasting caused maternal nonesterified fatty acid (NEFA) levels to increase 7.5-fold, beta-hydroxybutyric acid (beta-HBA) levels to increase 4.8-fold triglyceride (TG) levels to decrease 1.8-fold, and no change in plasma glucose concentration compared with controls. Fasted fetuses had a 1.3-fold increase in NEFA, 1.9-fold decrease in TG, 1.5-fold decrease in glucose, and no change in beta-HBA levels compared with control fetuses. Distribution of NEFA in fetal plasma was different from distribution of NEFA in maternal plasma. Esterification of [14C]-palmitate by maternal placenta and fetal adipose tissue was reduced by fasting, but other parameters of fatty acid metabolism were unaffected. Fasting decreased lipoprotein lipase activity per milligram of protein by 33% in maternal placenta and by 44% in fetal adipose tissue. Glycogen content of fetal liver and skeletal muscle was reduced by fasting pregnant gilts, but there was no detectable effect on percentage of carcass lipid of the fetus. These data suggest that fasting mobilizes maternal fuel stores but that these stores are not effectively used by the placenta or transported to the fetus for storage.

Comparison of the effects of dietary glucose versus galactose on porcine feto-placental glucose metabolism.

The present study was conducted to determine whether dietary galactose can be used to improve glycogen and lipid accretion in fetal pigs. Pregnant gilts were fed diets containing either 24% glucose (control) or 24% galactose from d 98 to 110 of gestation. Gilts underwent abdominohysterotomy on d 110 of gestation. Slices of fetal subcutaneous adipose tissue and placenta were examined for metabolic capacity for glucose and for galactose utilization. No effects of maternal diet were evident upon glycogen content or enzyme activity of fetal semitendinosus muscle and liver. Maternal dietary galactose had no direct effects upon placental glucose oxidation or use for lipid synthesis. However, galactose supplementation of the incubation medium caused reductions in glucose oxidation (15%) and total lipid synthesis (24%) by the maternal placenta. Maternal dietary galactose caused an increase in total lipid (50%) and fatty acid synthesis (200%) from glucose in fetal subcutaneous adipose tissue; direct supplementation of galactose to the incubation medium had no effect on these parameters. The results of the present study suggest that feeding galactose to the pregnant gilt does not have direct effects upon placental metabolism or fetal glycogen storage. However, these data indicate that use of galactose in the maternal diet can result in an increase in the utilization of glucose for lipogenesis by fetal adipose tissue in swine. This effect is not a direct effect of galactose because transport across the placenta was not apparent.

Glucocorticoids and the differentiation of porcine preadipocytes.

The function of glucocorticoids in the differentiation of porcine preadipocytes was examined. Stromal-vascular cell cultures (containing preadipocytes) derived from adipose tissue of the perirenal, ham and shoulder regions of neonatal pigs were incubated in the presence of hydrocortisone at 0 to 100 ng/ml medium. Perirenal cells did not respond to hydrocortisone with an increase in enzyme expression, nor did they demonstrate growth characteristics similar to those of cultures derived from the ham or shoulder. Cultures from the shoulder and ham regions demonstrated dose-responsive increases in enzymatic expression to hydrocortisone. Enzymatic responses by cultures derived from the ham region were lower than responses by cultures from the shoulder region as measured by changes in the activities of sn-glycerol-3-phosphate dehydrogenase and lipoprotein lipase. Addition of insulin to the medium did not produce a synergistic effect with glucocorticoid on differentiation as determined by these enzymatic parameters. However, [14C]glucose metabolism by the cells in culture was synergistically increased by insulin and glucocorticoid supplementation of the medium. The ability of hydrocortisone to induce differentiation of porcine preadipocytes in vitro suggests that the changes that occur in plasma glucocorticoid concentrations during late gestation may play an important role in the rapid development of s.c. adipose tissue in the fetal pig. Secondly, the differences in culture characteristics and hormone responses of cells derived from different locations of adipose tissue formation indicate that differences may exist in the regulation of the growth and development of preadipocytes from different anatomical locations.

Insulin-like growth factor 1 induction of differentiation of porcine preadipocytes.

The role of insulin-like growth factor I (IGF-1) in the development of the porcine preadipocyte was studied. Primary cultures of stromal-vascular cells (containing preadipocytes) were derived from s.c. adipose tissue of pigs at 1 d of age by enzyme digestion and centrifugation. Cells were cultured for a total of 15 d. Cells were exposed to IGF-1 at concentrations of 0, 5, 25 or 50 ng/ml medium during one of four time periods: d 1-15, d 1-5, d 13-15, or 4 h on d 15 of culture. IGF-1 had a mitogenic effect on cells during the first three time periods as determined by coulter counting. IGF-1 induced the enzymatic differentiation of porcine preadipocytes following exposure for either the entire 15 d of culture or for only 48 h (d 13-15) after confluency had been attained (d 5). Histochemically, lipid accumulation over time paralleled changes in enzyme activity. Incubation of IGF-1 with the cell cultures during the logarithmic phase of growth (d 1-5) or for 4 h on d 15 did not affect enzyme activity. These data indicate that IGF-1 can induce the differentiation of porcine preadipocytes after the cells leave the logarithmic phase of growth through action on post-confluent events.

Alterations in adipogenic and mitogenic activity of porcine serum in response to hypophysectomy.

The importance of the pituitary in postnatal regulation of peripheral preadipocyte proliferation and differentiation was examined by hormone supplementation of hypophysectomized pig serum in primary cultures of preadipocyte and stromal-vascular cells derived from rat inguinal adipose tissue. Hypophysectomized pig serum promoted at least 25% less preadipocyte proliferation, less differentiation of sn-glycerol-3-phosphate dehydrogenase activity, and less histochemical differentiation than serum from intact pigs. Porcine GH supplementation of hypophysectomized serum-stimulated [3H]thymidine incorporation by preadipocytes and stromal cells and also histochemical differentiation of preadipocytes, but not enzymatic differentiation. Insulin-like growth factor I (IGF-I) stimulated [3H]thymidine incorporation by preadipocytes and stromal cells. Enzyme differentiation by developing cells was stimulated by IGF-I. Hydrocortisone supplementation of hypophysectomized serum inhibited [3H]thymidine incorporation and stimulated enzymatic differentiation. Thyroid hormones (T3 and T4) stimulated [3H]thymidine incorporation by preadipocytes in a dose-responsive manner when supplemented to hypophysectomized serum. Thyroid hormones stimulated differentiation of enzyme activity at the lowest concentrations examined. The mitogenic effects of GH, IGF-I, and T4 were not specific to the preadipocyte population, since the stromal-vascular cells responded in a similar manner. However, hypophysectomy resulted in a specific reduction in preadipocyte proliferation while stimulating multiplication of stromal-vascular cells. These results suggest that these hormones are nonspecific mitogens in adipose tissue, while unidentified factors of pituitary origin may be important for the specific regulation of proliferation of preadipocytes. Additionally, hypophysectomy appears to remove mitogenic inhibitors that are specific for the stromal-vascular cells.