Relationship of thermal status to productivity in heat-stressed dairy cows given recombinant bovine somatotropin.

The responses of lactating Holstein cows to daily administration of bovine somatotropin (bST) were measured at thermoneutrality (Tn) and under both constant and cycled heat-stress conditions to determine the relationship between thermal status and bST-induced shifts in milk production. All tests included a 5-d acclimation period at Tn (18 degrees C), followed by a 2-d increase in ambient temperature to 28.5 degrees C. After d 3, ambient temperature was cycled between 28.5 (day) and 25.5 degrees C (night) for 4 d. Daily injections with either 31 mg of bST or saline began on d 1 of the experiment. Milk production, feed intake, and respiratory rate (RR) were measured daily. Intraperitoneal, telemetric temperature transmitters were used for a continuous measure of core body temperature (T(core)). Blood samples were collected during each phase to evaluate the changes in serum chemistry in response to bST and heat stress. Following a 15-d recovery, cows were switched across injection treatments and the study was repeated. Milk production decreased by approximately 18.4% below the initial yield at Tn by the end of 7 d of heat challenge. Although a reduction in milk production occurred during heat stress in both groups, milk production was higher in bST-treated cows compared with control cows during periods of constant and cyclic heat. Likewise, bST treatment during the entire period increased the milk-to-feed ratio over the control level by approximately 11.3%. Plasma insulin-like growth factor 1 and serum nonesterified fatty acids accompanied the increased growth hormone level with bST treatment (approximately 122.0 and 88.8%, respectively), whereas plasma urea nitrogen was reduced by approximately 13.3% to reflect the shift to lipid metabolism. There was no difference in T(core) of the treatment and control groups at Tn. Both bST and control cows increased RR and T(core) above the Tn level by approximately 94.8 and 2.9%, respectively, during constant heat, with a greater increase in T(core) of bST-treated compared with control cows (approximately 0.6%). The increase in RR during heat stress preceded T(core) by 1 d for both groups. During cyclic heat, T(core) decreased by approximately 0.4% compared with constant heat in both the control and bST-treated groups. Bovine somatotropin treatment increased milk production similarly during the Tn and heat-stress periods, approximately 8.3% over the control; however, the bST-induced increase in milk-to-feed ratio was greatest during the continuous and cyclic heat-stress phases, approximately 16.2%. This increase occurred together with the elevation in T(core).

Effects of sustained release bovine somatotropin (sometribove) on animal health in commercial dairy herds.

The health of dairy cows given bovine somatotropin (bST) for one lactation was evaluated in 28 commercial herds located in four regions of the United States. At least six herds were in a region and at least one herd/region contained fewer than 60 cows. Cows (n = 1213) were assigned randomly to control or bST groups and were treated beginning in wk 9 to 10 of lactation and every 14 d until dry-off or d 400 of lactation. Management was according to site practices. Cows were observed for health-related signs by farm personnel daily and by the herd veterinarian biweekly. Average 305-d test-day milk yields were 932 kg greater for bST-treated cows. Pregnancy rates, days open, twinning, cystic ovaries, or abortions were unaffected by treatments. Supplementation of cows with bST had no effect on total mastitis cases, total days of mastitis, duration of mastitis, or the odds ratio of a cow to develop mastitis. Cows supplemented with bST used more medications for health events other than mastitis. This usage was associated primarily with treatments for disorders of the foot and hock. Supplemented cows had a slight increase in foot disorders. There was no effect of supplementation with bST on culling from the herd or removal from study. Overall, the results confirm that label directions for bST are adequate for safe use under field conditions. All clinical signs observed in this study occur normally in dairy herds and were managed in cows supplemented with bST.

Effect of bovine somatotropin on neutrophil functions and clinical symptoms during Streptococcus uberis mastitis.

The effect of recombinant bovine somatotropin (bST) on the chemiluminescence, diapedesis, and expression of adhesion receptors (CD11a, CD11b, CD18) of isolated polymorphonuclear leukocytes was studied. The plasma concentrations of insulin-like growth factor-I (IGF-I), bST, cortisol, and alpha-lactalbumin were also monitored. In addition, general and local clinical symptoms and the differentiation of circulating leukocytes were also studied during experimentally induced Streptococcus uberis mastitis in cows. Ten cows were infected with 500 cfu of S. uberis O140J in both left quarters. Five cows were subcutaneously treated with 500 mg of recombinant bST 7 d before and after infection, and 5 control cows received the excipient. General (fever, tachycardia, inappetance, and depression) and local symptoms (swelling, pain, firmness, and flecks in milk) were more acute, severe, and longer-lasting in control cows. Treatment with bST had no effect on chemiluminescence and diapedesis of circulating polymorphonuclear leukocytes and no effect on the expression of adhesion receptors. Recombinant bST induced significantly higher IGF-I and bST concentrations in plasma. The leukopenia observed after infection was less pronounced in the bST-treated cows, and the number of circulating band neutrophils and metamyelocytes was significantly lower in the treated group. The concentration of cortisol did not differ between both groups, but the blood concentration of alpha-lactalbumin significantly increased in both groups from 6 d after infection. These results showed that treatment with recombinant bST improves animal welfare by protecting the cows from severe local and general clinical symptoms during subsequent S. uberis mastitis, but that it has no effect on chemiluminescence, diapedesis, and the expression of adhesion receptors of circulating polymorphonuclear leukocytes.

Evaluation of the somatogenic activity of bovine placental lactogen with cell lines transfected with the bovine somatotropin receptor.

Studies have shown that bovine placental lactogen (bPL) has partial somatogenic activity in vivo even though binding results clearly indicate bPL does not cause homodimerization of the bovine somatotropin receptor (bST-R). To help understand the receptor binding versus biological activity of bovine somatotropin (bST) and bPL we have developed a homologous model system. Full length bST-R was stably transfected into a murine lymphoid cell line, Ba/F3 and a hamster kidney cell line, BHK. From both transfected cell lines, clones were isolated (Ba/F3-C1 and BHK-24) which demonstrated specific binding of bST and, or bPL. Bovine ST stimulated proliferation of the Ba/F3-C1 clonal line over a dose range of 10 to 3000 pM with an EC50 of 100 pM. A bST variant (des 1-4 bST) and porcine ST (pST) which both have approximately 10% of the binding affinity for bST-R as native bST were 1 and 10% as potent as bST in this bioassay, respectively. This suggests that affinity and biological activity are correlated for this system. Proliferation was initiated through the bST-R because addition of a monoclonal antibody which recognizes the extracellular domain of bST-R and inhibits binding of bST to its receptor, inhibited bST-stimulated mitosis. However, even though the affinity of bPL for the bST-R is similar to that of bST, bPL antagonized the proliferative action of bST with an IC50 of 1 nM. Components of the somatogenic signal transduction pathway were also evaluated in both cell lines. Addition of bST to the cell cultures increased phosphorylation of JAK2 in Ba/F3-C1 and BHK-24 cells in a dose-responsive manner but bPL failed to increase phosphorylation of JAK2 in either cell line. In summary, these data support the hypothesis that ST-R homodimerization is necessary for bioactivity in this model system but fail to explain apparent somatogenic activity of bPL in vivo.

Growth performance, endocrine, and metabolite responses of finishing hogs to porcine prolactin.

Prolactin, a member of the somatotropin-prolactin-placental lactogen gene family, increases feed intake and rate of weight gain in several species. To determine whether prolactin affects growth performance and carcass composition in swine, recombinant porcine prolactin (rpPRL) was administered to finishing hogs. Doses of 0, 2, 4, 8, and 16 mg of rpPRL/d and 4 mg of recombinant porcine somatotropin (rpST)/d were administered to groups of seven barrows and seven gilts initially weighing 75.0 +/- .2 kg for a 28-d period. Recombinant pPRL did not alter feed intake or growth rate or affect carcass composition. In addition, most growth-related blood variables did not change, although plasma IGF-I was increased in the 8 and 16 mg of rpPRL treatment groups. At slaughter, mammary development was apparent in rpPRL-treated gilts and was characterized by distended alveolar and ductal lumina and presence of secretory material. In rPST-treated hogs, feed intake was decreased 28% (P < .01), gain/feed was increased more in barrows than in gilts (59 vs 39%, treatment x sex interaction, P = .035), and growth rate was increased 22%, but in barrows only (treatment x sex interaction P = .005). Compared with those in control hogs, circulating concentrations of IGF-I, insulin, and glucose were 175, 311, and 22% higher, respectively, and of blood urea nitrogen were 62% lower in rpST-treated hogs (P < .05). These results suggest that rpPRL, at the doses administered, does not increase feed intake in finishing hogs in contrast to rats and other species.

The effect of recombinant bovine placental lactogen on induced lactation in dairy heifers.

The primary objective of this study was to determine whether bovine placental lactogen stimulated additional mammary growth as assessed by milk yield from a lactation induced by steroids. Pubertal, nonpregnant Holstein heifers (n = 23) were given daily subcutaneous injections of estradiol-17 beta (0.05 mg/kg) and progesterone (0.25 mg/kg) for 7 d to initiate mammary growth. Prolactin secretion was suppressed in all heifers via bromocriptine, which was administered until d 15. Heifers were treated with either placental lactogen (40 mg/d; n = 12) or water (control group; n = 11) for 18 d. Lactation was induced by daily injection of dexamethasone for 3 d and twice daily injections of recombinant bovine prolactin for 5 d starting on d 18. From 3 to 8 wk of lactation, milk yield of heifers treated with placental lactogen was numerically higher (22%) than the yield of control heifers, but the difference was not significant because of the high coefficient of variation. Daily injection of bovine somatotropin (d 57 to 66 of lactation) increased milk yield of both groups and stimulated a greater numerical increase in milk yield for heifers that were treated with placental lactogen. These results support the hypothesis that bovine placental lactogen is mammogenic and is one of the factors that regulates mammary growth during pregnancy.

Insulin-like growth factors-I and -II, somatotropin, prolactin, and placental lactogen are not acute effectors of lipolysis in ruminants.

The acute regulation of lipolysis in the adipose tissue of ruminants was evaluated with lactating cows (n = 4) and growing ewe lambs (n = 11). Subcutaneous adipose tissue was obtained by biopsy or at slaughter and was incubated with varying concentrations of biologically active insulin-like growth factors-I and -II (IGF-I, IGF-II), somatotropin (ST), prolactin (PRL), or placental lactogen (PL) to determine the effect of these hormones on lipolysis. Complimentary studies were conducted to examine the effects of IGF-I and IGF-II on the acute regulation of lipolysis in adipose tissue from lactating ewes and wethers under a variety of incubation conditions. Isoproterenol (ISO), a beta-adrenergic agonist known to rapidly stimulate lipolysis, was used as a positive control. Incubation with ISO for 3 hr resulted in a significant increase in the rates of lipolysis. However, there was no stimulation of lipolysis over the 3-hr incubation at any concentration or under any conditions for IGF-I or IGF-II. Furthermore, ST, PRL, or PL had no acute effects on the rates of lipolysis in adipose tissue. These data demonstrate that IGF-I, IGF-II, ST, PRL, and PL are not acute effectors of the lipolytic rate in the adipose tissue of ruminants.

Dose-response effects of recombinant bovine somatotropin (Posilac) on growth performance and body composition of two-year-old rainbow trout (Oncorhynchus mykiss).

Two hundred rainbow trout (Oncorhynchus mykiss, mean weight 301.5 g) were allotted to four treatments with five replicates in a randomized block design to determine the dose-response effects of recombinant bovine somatotropin (rbST; Posilac) on growth performance and carcass composition. Treatments were sham-injected controls (S), 10 micrograms/g BW of rbST (L), 20 micrograms/g BW of rbST (M), and 30 micrograms/g BW of rbST (H). The tanks held 135 L; water flow = 15.1 L/min; temperature = 15 degrees C. The fish were maintained in a 12-h light:dark cycle and hand-fed twice daily. The fish received a single intraperitoneal (i.p.) injection on d 0 and were weighed on d 0, 14, 28, and 56. On d 56 the fish were killed. The whole body (WBW), eviscerated carcass (EC), viscera (VIS), and reproductive organ weights and the increase in average daily body length (ADL) were determined. Recombinant bST reduced (linear, P < .004) feed intake 17.6% from d 0 to 14 and improved ADG 44.8% from d 0 to 14 (linear, P < .001) and 8.1% from d 0 to 56 (linear, P < .022). Treated groups had improved (linear, P < .001) feed efficiencies for d 0 to 28. Treatment with rbST increased final weight (linear, P < .018) and length (linear, P < .001), decreased carcass dry matter (linear, P < .001) and fat (linear, P < .001), content, increased carcass ash (linear, P < .001) and tended to increase carcass protein (linear, P < .054) content. Recombinant bST increased WBW (linear, P < .018) and EC (linear, P < .003) but decreased (linear, P < .015) testes weight. Ovary weights, VIS and overall gonadosomatic index were unaffected (P > .05) by rbST. Recombinant bST was undetectable in serum samples taken on d 56 as determined by radioimmunoassay. Overall, the improved ADG, feed efficiency, body mass, and composition indicate that administration of rbST to rainbow trout may be an efficacious method of modulating growth in fish.

Specific endometrial binding sites for bovine placental lactogen are antigenically similar to the growth hormone receptor.

Bovine liver growth hormone receptors bind both bovine growth hormone (bGH) and bovine placental lactogen (bPL) with high affinity (Kd = 1.4 x 10(-11) M and 3.0 x 10(-11) M, respectively). By contrast, the uterine endometrium of pregnant cattle has high-affinity (Kd = 8.0 x 10(-11) M) binding sites for bPL but, displays negligible binding of bGH. A polyclonal antiserum raised against the extracellular domain of the bGH receptor, was used to determine if there was antigenic similarity between the liver bGH receptor and the endometrial bPL binding site(s). On Western blots, this antiserum displayed cross-reactivity with a 180,000-mol wt protein (nonreducing conditions) in detergent-solubilized extracts of microsomal membranes from both tissues. However, different detergents (Triton X-100 for endometrium and 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonate [CHAPS] for liver) were required to solubilize the cross-reacting protein in the two tissues. The purified immunoglobulin fraction from this same antiserum also blocked binding of [125I]bPL to microsomal membrane preparations from both liver and endometrium. These results indicate that the endometrial binding site for bPL is antigenically similar to the bGH receptor and raise the possibility that it may be a modified GH receptor.

Increases in gill cytosolic corticosteroid receptor abundance and saltwater tolerance in juvenile coho salmon (Oncorhynchus kisutch) treated with growth hormone and placental lactogen.

Juvenile coho salmon (Oncorhynchus kisutch) were injected with one of two recombinant bovine hormones, growth hormone (bGH; 5.0 and 0.5 micrograms.g-1 body wt) or placental lactogen (bPL; 5.0, 0.5, and 0.05 micrograms.g-1 body wt) to determine the effect on growth, plasma cortisol concentration, cytosolic corticosteroid receptors (CR) in the gills, and the development of hypoosmoregulatory ability. One week following a single injection or six weekly injections of bGH or bPL, the fish were measured and sampled for CR concentration and Na+,K(+)-ATPase activity in the gills. Fish were also challenged with salt water (salinity 25%) for 24 hr to determine saltwater tolerance at the end of the 6-week treatment. Treatment with bPL and bGH significantly increased weight and length of the fish. The 0.05-micrograms bPL dose significantly elevated plasma cortisol concentration, whereas all other hormone treatments did not affect cortisol levels. bPL and bGH also significantly increased CR concentration and Na+,K(+)-ATPase activity in the gills. The perturbation in plasma sodium concentration was least in animals receiving the highest dose of bPL and the bGH-treated animals following transfer to seawater. An increase in cytosolic CR by bGH and bPL may increase responsiveness of the gills to cortisol and partially account for the increase in Na+,K(+)-ATPase activity and greater ability to regulate plasma sodium in seawater as exhibited by the experimental groups.

Bovine placental lactogen is a potent stimulator of growth and displays strong binding to hepatic receptor sites of coho salmon.

Juvenile coho salmon were treated with bovine placental lactogen (bPL) and bovine growth hormone (bGH) to examine the growth promoting activities of these proteins in a lower vertebrate. Fish were intraperitoneally injected either with 0.5 or 5.0 micrograms/g bPL or with 5.0 micrograms/g bGH once a week for 5 weeks. After only a single injection and 1 week of growth, the high dose of bPL stimulated a significant increase in weight and length relative to untreated fish or fish treated with a control protein, bovine serum albumin. At the end of the experiment, all hormone-treated groups were significantly larger than controls. Fish treated with 5 micrograms/g bPL gained more than three times as much weight as controls. The 5.0 micrograms/g bGH group grew at the same rate as fish treated with one-tenth this dose of bPL, indicating that bPL is a potent stimulator of growth in this species. Radioreceptor assays performed on coho salmon liver membrane preparations indicate that bPL binds with approximately 430-fold higher affinity than bGH, and some 8000-fold higher affinity than bovine prolactin. The action of bPL relative to the structure and function of salmonid pituitary hormones is discussed.

IGF-I-induced IGFBP-3 potentiates the mitogenic actions of IGF-I in mammary epithelial MD-IGF-I cells.

Limited information is available concerning the molecular and cellular mechanisms that regulate expression of insulin-like growth factor-I (IGF-I) binding proteins (IGFBPs) in bovine mammary epithelial cells. Here, we report on the autocrine mechanisms of action of IGF-I and hormonal regulation of expression of IGFBPs in bovine mammary epithelial MD-IGF-I cells which express recombinant IGF-I under the control of the glucocorticoid-inducible mouse mammary tumor virus-long terminal repeat (MMTV-LTR). Levels of IGFBP-3 mRNA and secretion of IGFBP-3 by MD-IGF-I cells were stimulated by IGF-I, insulin (INS), and IGF-I analogs but not prolactin (PRL). Conversely, parental MAC-T cells expressed little IGF-I and secreted primarily IGFBP-2 (29-32 kDa) in response to stimulation with INS, dexamethasone (DEX), or IGF-I analogs. Secretion of recombinant IGF-I caused a 26.5-fold increase in secretion of IGFBP-3, as measured by densitometric analysis of ligand blots, which was associated with a 1.7-fold increase in total DNA. Conditioned media (CM) from MD-IGF-I cells induced with DEX stimulated a 2.8-fold increase in [3H]thymidine incorporation into DNA of parental MAC-T cells, compared with uninduced cells. Moreover, inclusion of exogenous IGF-I with CM from MD-IGF-I cells triggered an additional 3.0-fold increase in label incorporation, but only a 1.6-fold increase in the presence of IGFBP-2-containing media conditions by MAC-T cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Placental lactogen and somatotropin: hormone binding to the corpus luteum and effects on the growth and functions of the ovary in heifers.

The effects of recombinant bovine placental lactogen (rbPL) and recombinant bovine somatotropin (rbST) on development of ovarian follicles and CL were tested in heifers. Estrus (day = 0) was synchronized and heifers were treated (Days 0-21) with either saline (control; n = 7), rbST (25 mg/day; n = 6), or rbPL. (50 mg/day; n = 8). Blood was collected daily for analyses of progesterone, estradiol, ST, PL, and insulin-like growth factor (IGF)-I; ultrasound was performed daily for measurement of follicles and CL. PL in plasma (mean +/- SE; ng/ml) averaged 4.1 +/- 0.2 for rbPL-treated heifers, and ST in plasma (ng/ml) averaged 2.7 +/- 0.3 for rbST-treated heifers. IGF-I in plasma (ng/ml) was increased for rbST-treated (198 +/- 10; p < 0.001) and rbPL-treated (143 +/- 9; p < 0.06) heifers compared to controls (117 +/- 9). After Day 9 of the estrous cycle, heifers treated with rbPL had larger CL (p < 0.001) and more progesterone in plasma (p < 0.001) than controls, whereas rbST-treated heifers were intermediate for these measures. Largest follicles were decreased in size (mm) throughout the estrous cycle for rbPL-treated heifers (12.9 +/- 0.4) compared to controls (14.2 +/- 0.5; p < 0.06) or heifers given rbST (14.0 +/- 0.5; p < 0.11). After Day 17 (preovulatory period), concentrations of estradiol in serum (pg/ml) were decreased for rbST-treated (2.7 +/- 0.3; p < 0.01) and rbPL-treated (2.9 +/- 0.2; p < 0.02) heifers compared to controls (3.8 +/- 0.3).(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of mammogenesis and lactogenesis by recombinant bovine placental lactogen in steroid-primed dairy heifers.

A model of induced lactation was modified to examine the effects of bovine prolactin (bPRL) and bovine placental lactogen (bPL) on mammary growth and differentiation. Thirty-two peripubertal, non-pregnant Holstein heifers were given daily s.c. injections of oestradiol (0.05 mg/kg) and progesterone (0.25 mg/kg) for 7 days to initiate mammary growth. Treatment with bromocriptine (40 mg/3 days) reduced serum PRL concentrations to approximately 25% of pretreatment levels, for the duration of the study. On the day following the last steroid injection, groups of eight heifers were given twice daily s.c. injections of either saline (negative control), recombinant bPRL (rbPRL; 80 mg/day) or recombinant bPL (rbPL; 80 and 160 mg/day) for 7 days. At the end of this period (day 15), growth and differentiation of the mammary glands were assessed. Treatment with rbPL increased total mammary DNA above control value by 50 and 60% for the 80 and 160 mg/day doses respectively. However, total DNA was not different for the control and rbPRL-treated groups. The blood serum concentration of alpha-lactalbumin was measured daily throughout the study and used as an index of mammary differentiation. Both rbPRL and rbPL stimulated mammary differentiation (i.e. induction of milk synthesis), although rbPRL appeared to be more potent than rbPL. These results indicate that rbPL is lactogenic in vivo and strongly suggest that bPL is a mammary mitogen.

Performance, clinical chemistry, and carcass responses of finishing lambs to recombinant bovine somatotropin and bovine placental lactogen.

Bovine placental lactogen (PL) is a partial somatotropin agonist in the cow and decreases urea nitrogen, indicating increased nitrogen retention. In the present study, the somatogenic effects of bovine PL (bPL; 4 and 8 mg/d) were compared with those of bovine somatotropin (bST; 4 and 8 mg/d) in finishing lambs. Measures of comparison included growth performance, carcass composition, and growth-related clinical chemistry traits. Although feed efficiency during the first 3 wk of treatment with bPL was improved by 14% (P < .05), feed efficiency for the full 6-wk treatment period did not differ from that of control lambs. Responsiveness to bPL may have been attenuated by high titer antibodies present after 2 wk of treatment. However, bPL also did not influence growth-related clinical chemistry traits during short-term (7 d) treatment, strongly suggesting that bPL was ineffective in finishing lambs at the doses tested. In contrast, bST improved 6-wk feed efficiency by an average of 17% (P < .05) and decreased feed intake by an average of 12% (P < .05). In addition, measures of carcass composition including longissimus muscle area, specific gravity of the rack, kidney and pelvic fat, and fat thickness demonstrated that bST, but not bPL, treatment decreased carcass fatness and increased carcass leanness. Treatment with bST, but not with bPL, affected IGF-I, insulin, glucose, and urea nitrogen in a dose-related manner. Thus, daily injections of bPL did not affect either performance or carcass quality, whereas performance and carcass responses of finishing lambs to bST were consistent with those reported by others.

Lactogenic hormones and extracellular matrix regulate expression of IGF-1 linked to MMTV-LTR in mammary epithelial cells.

The cell line MD-IGF-1, containing an ovine IGF-1 cDNA driven by the mouse mammary tumor virus-long terminal repeat (MMTV-LTR) promoter, was used to study expression of IGF-1 linked to the MMTV-LTR in bovine mammary epithelial cells in response to various hormonal and substratum stimuli. Acute sensitivity of the MMTV-LTR promoter to glucocorticoids and sex steroids was ascertained by transient transfection of parental MAC-T cells with an MMTV-CAT construct. Specifically, CAT activity was induced by glucocorticoids, but not by 17 beta-estradiol or progesterone. Induction of MD-IGF-1 cells with dexamethasone (DEX) alone triggered a 29.5-fold increase in secretion of recombinant IGF-1 (348.9 vs 11.8 pg/micrograms DNA), and stimulated a 1.7-fold increase in total DNA within 72 h. Growth of MD-IGF-1 cells was enhanced by exogenous IGF-1, insulin, and TGF-alpha. In contrast, TGF-beta inhibited cell proliferation, while epidermal growth factor, estrogen, progesterone, and testosterone had no effect. Extracellular matrix from the Engelbreth-Holm-Swarm (EHS) tumor, in the presence of DEX, prolactin (PRL), and insulin stimulated a 29.4-fold increase in secretion of IGF-1 (591.9 pg/microgram DNA), compared with cells in absence of hormones (20.1 pg/micrograms DNA). EHS and DEX plus PRL triggered a 63.2-fold increase in IGF-1 secretion (689.1 pg/micrograms DNA), compared with MD-IGF-1 cells cultured on plastic (10.9 pg/micrograms DNA), in the absence of hormones. These data indicate that the MMTV-LTR is regulated by both lactogenic hormones and extracellular matrix in MD-IGF-1 cells and that the MMTV-LTR may be a useful regulatory element for targeting expression of foreign proteins in bovine mammary epithelial cells.

Ligand-specific dimerization of the extracellular domain of the bovine growth hormone receptor.

The bovine growth hormone (bGH) receptor and its extracellular domain (bGHBP) bind two protein hormones with high affinity; bGH and bovine placental lactogen (bPL). However, each of these hormones bind with a different stoichiometry. bGH binds to the bGHBP in a 1:2 ratio while bPL binds in a 1:1 ratio. Scatchard analysis of saturation binding yields similar apparent dissociation constants (Kd) for bGH and bPL of 1.4 x 10(-11) M and 3.0 x 10(-11) M, respectively, for the membrane receptor and 6.8 x 10(-11) M and 4.2 x 10(-11) M, respectively, for bGHBP. In competition experiments using either liver membranes or bGHBP, bGH is 2-3-fold more effective than bPL in competing for 125I-bGH-binding sites. In similar experiments using 125I-bPL, bPL is 50-fold more effective than bGH in competing for binding sites. A rat monoclonal antibody raised against bGHBP competes effectively for 125I-bGH-binding sites, but not for 125I-bPL-binding sites. Since bPL cannot dimerize the bGHBP and yet it acts in part as a somatogen in vivo, homodimerization of the growth hormone receptor is apparently not essential for some biological responses signaled through this receptor.

Adipsin expression and growth in rats as influenced by insulin and somatotropin.

Adipsin is a molecular marker of obesity in rodents. Content of adipsin protein in blood and mRNA in adipocytes is significantly reduced in several genetic and experimentally induced obese models. It has been suggested that this reduction in adipsin is causative to obesity development. Insulin reduces adipsin expression in vitro and is negatively correlated with adipsin expression in vivo. Because bovine somatotropin (bST) opposes many actions of insulin and can reduce body fat content, we tested the hypothesis that bST enhances adipsin expression. In two experiments using 210 rats, bST and a similar hormone, bovine placental lactogen (bPL), both caused a small (14 to 27%) but statistically significant reduction in circulating adipsin protein. Because exogenous bST can increase circulating insulin we next used a diabetic model to test the bST effect on adipsin. In rats treated with streptozotocin and injected daily with insulin (STZ+I), bST had no effect on circulating adipsin. Additional variables related to growth were influenced differently by bST in normal vs. STZ+I animals. In conclusion, the drop in circulating adipsin following bST administration in normal rats is dependent upon the animals' ability to secrete insulin.

Stimulation of food intake and weight gain in mature female rats by bovine prolactin and bovine growth hormone.

Recombinant bovine prolactin (rbPRL) or bovine growth hormone (rbGH) was administered to mature female rats (10/treatment group) by daily subcutaneous injection for 10 days. Doses ranged from 7 to 5,000 micrograms/day (0.03-24 mg/kg body wt). Both rbPRL and rbGH increased body weight gain and food intake, but these parameters were increased at lower doses of rbPRL (7-63 micrograms/day) than rbGH (> 190 micrograms/day). Weight gain and food intake were maximally stimulated by 190 micrograms/day rbPRL, whereas maximal increased weight gain was obtained with the highest dose of rbGH (5,000 micrograms/day). Total carcass protein was increased by both hormones; however, protein as a percentage of body weight was unchanged. Similarly, neither rbPRL nor rbGH changed the percentage of carcass moisture. Percentage of body fat was increased by rbPRL but was decreased by rbGH. Weight of the gastrointestinal tract and kidneys was increased by both hormones, but increases were in proportion to body weight gain. These data confirm that ungulate prolactin is a hyperphagic agent in the female rat. In addition, they suggest that, while prolactin stimulates growth in mature female rats, this growth is probably not via a somatogenic mechanism.