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Plasma Hormones and Expression of Growth Hormone Receptor and Insulin-Like Growth Factor-I mRNA in Hepatic Tissue of Periparturient Dairy Cows¹

R. P. Radcliff^*, B. L. McCormack^*, B. A. Crooker and M. C. Lucy^*

^* Department of Animal Sciences, University of Missouri, Columbia 65211
Department of Animal Science, University of Minnesota, St. Paul 55108

Corresponding author: M. C. Lucy; e-mail: lucym{at}missouri.edu.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
Growth hormone plays a central role in the change in nutrient metabolism that occurs during the initiation of lactation. The actions of growth hormone are mediated by the growth hormone receptor (GHR) whose mRNA is present in three alternatively spliced forms (GHR 1A, 1B, and 1C). Liver-specific GHR 1A mRNA is transiently decreased around parturition, but the exact timing of the decline is not known. Our objective was to generate a daily profile for total GHR (GHRtot; all GHR transcripts), GHR 1A, and IGF-I mRNA expression in liver of periparturient Holstein cows and evaluate these daily mRNA profiles relative to daily profiles for periparturient hormones and metabolites. Liver biopsies and blood samples (n = 139) were collected from 65 Holstein cows at the University of Missouri Dairy Farm. At least two cows were sampled on each day from 14 d before to 14 d after parturition. Total cellular RNA was isolated and reverse transcribed to cDNA. Target cDNA were measured by quantitative real-time polymerase chain reaction. Plasma was assayed for progesterone, estradiol, insulin, growth hormone, IGF-I, glucose, and nonesterified fatty acids. The GHR 1A mRNA declined 2 d before parturition, was lowest 3 to 4 d after parturition, and then increased. The IGF-I mRNA declined 1 d after parturition, was lowest 2 to 5 d after parturition and then increased. Total GHR mRNA was not affected by day. The decrease in GHR 1A mRNA was associated with a decrease in progesterone and an increase in estradiol shortly before parturition. A detailed profile of GHR 1A, IGF-I, and GHRtot mRNA expression during the periparturient period was provided. The decreases in GHR 1A and IGF-I during the transition period occurred immediately before (GHR 1A) or shortly after (IGF-I) parturition. Rapid changes in placental and ovarian steroids before parturition were coincident with changes in GHR 1A mRNA.

Key Words: growth hormone receptor • liver • dairy cattle

Abbreviation key: GH = growth hormone, GHR = growth hormone receptor, GHRtot = total GHR, qRT-PCR = quantitative real-time polymerase chain reaction, RIA = radioimmunoassay

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
The change in nutrient metabolism that is required to support lactation in high producing dairy cattle is controlled by hormones that coordinate a variety of processes including the mobilization of fatty acids from adipose tissue and the synthesis of glucose from gluconeogenic precursors in liver. Growth hormone (GH) plays a central role in this process (Lucy et al., 2001). The actions of GH are mediated via the GH receptor (GHR) and there are at least three alternative forms of GHR mRNA in cattle (GHR 1A, 1B, and 1C). We previously reported that GHR 1A is only expressed in the adult liver (Lucy et al., 1998). The existence of GHR 1A mRNA in the liver may be a mechanism for maintaining a high level of GHR expression in liver where GH has multiple actions (Etherton and Bauman, 1998).

There is a natural decline in blood IGF-I concentrations in cattle at parturition (McGuire et al., 1992). Expression of GHR 1A and IGF-I mRNA in liver also declines at parturition, while GHR 1B mRNA expression is unchanged (Kobayashi et al., 1999a). Within 3 wk after parturition, however, expression of GHR 1A and IGF-I mRNA return to periparturient levels. The decrease in GHR 1A expression in early lactation cows coincides with a period of liver refractoriness to GH when GH-dependent IGF-I synthesis and secretion are decreased (Vicini et al., 1991).

Our previous work demonstrated a decrease in both GHR 1A and IGF-I mRNA expression at parturition, but we only sampled cows on d -14, 0, and 14 to 21. A more detailed daily profile for GHR and IGF-I mRNA is needed to fully understand GHR physiology during this period. Furthermore, it may be possible to infer metabolic or endocrine mechanisms controlling GHR from coincident daily changes in mRNA and blood hormones and metabolites. The objective of the present study, therefore, was to generate a daily profile for total GHR (GHRtot; all GHR transcripts), GHR 1A and IGF-I mRNA expression in liver of periparturient Holstein cows and to evaluate these daily mRNA profiles relative to daily profiles for periparturient hormones and metabolites.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
Animals
Multiparous periparturient Holstein cows (n = 65) at the University of Missouri Foremost Dairy farm were used. Cows were maintained on a dirt lot before parturition. The dry cow diet was a TMR based on corn silage, grass hay, ground corn, soybean meal, and vitamin/mineral premix. After parturition, cows were moved to the milking herd, managed according to the farm’s standard operating practices, and fed a TMR consisting of corn silage, alfalfa hay, alfalfa haylage, ground corn, soybean meal, whole cottonseeds, and vitamin/mineral premix. Diets were balanced to meet or exceed the requirements for dry or lactating cattle (National Research Council, 1989). The University of Missouri Animal Care and Use Committee approved experimental procedures.

Liver Biopsies
Cows were randomly assigned to have their liver biopsied on a specific day relative to expected parturition date and were biopsied approximately weekly thereafter. It was impossible to predict the day of parturition (a random event). Therefore, the actual day of sampling relative to parturition for each cow was random. After parturition, samples obtained before parturition were retrospectively assigned their actual day relative to parturition. After collecting the majority of samples, specific sampling days for individual cows were shifted by 1 or 2 d to complete collection on specific days. Biopsy samples were collected from 14 d before parturition through 14 d after parturition. Samples collected before d -14 were not analyzed. The average number of biopsies per cow was 2.1 ± 0.9 (range 1 to 4), and the average interval between biopsies was 8.6 ± 3.4 d (range 5 to 23). Cows that developed periparturient diseases were not biopsied after disease diagnosis. Liver biopsies were performed according to previously published methods (Kobayashi et al., 1999a) with slight modifications. Biopsy samples (approximately 120 mg of liver tissue) were collected with a 14-gauge Tru-Cut Biopsy needle (Allegiance, McGaw Park, IL). Briefly, a 36-cm² area between the 10th and 11th ribs was shaved and washed with 7.5% povidone iodine solution and then soaked with 70% ethanol. The area was anesthetized with 5 ml of lidocaine HCl (Abbott Laboratories, North Chicago, IL). A scalpel blade was used to make a 3-mm stab incision in the skin. The biopsy needle was then inserted through the intercostal muscle and into the liver. Liver tissue was placed immediately into a screw cap tube and frozen in liquid nitrogen. Samples were stored at -80°C until total cellular RNA was isolated. A total of 139 liver biopsies were collected from 65 cows between May and December 2001 and used in this study.

Blood Samples
Immediately before each liver biopsy, a single blood sample was collected into an evacuated glass tube containing EDTA (Vacutainer, Becton Dickinson and Co., Franklin Lake, UT) via coccygeal venipuncture. Blood samples were stored on ice for transport to the laboratory and immediately centrifuged at 2500 x g for 20 min. Plasma was transferred to a polypropylene tube and frozen at -20°C until assayed.

RNA Isolation
Total cellular RNA was isolated from liver samples using the TRIZOL procedure (Invitrogen Life Technologies, Carlsbad, CA). After isolation, RNA was dissolved in sterile water treated with 0.1% (vol/vol) diethylpyrocarbonate. Concentrations of RNA were determined by measuring absorbance at 260 nm and the purity of RNA was determined by calculating the ratio of absorbencies at 260 and 280 nm, and by electrophoresis of an RNA aliquot (2.5 µg) from each sample through a 1% agarose gel in Tris-borate/EDTA buffer (0.09 M Tris-borate and 0.002 M EDTA) with ethidium bromide (0.5 µg/ml). Isolated RNA was stored at -80°C until quantitative real time PCR (qRT-PCR) was performed.

Quantitative Real Time PCR
Total cellular RNA (2.1 µg) was used for cDNA synthesis using the GIBCOBRL SUPERSCRIPT First-Strand RT-PCR kit (Invitrogen Life Technologies). Probe and primer sets for bovine GHR 1A, bovine IGF–I, and bovine cyclophilin I (Table 1) were designed using the Primer Express Software (Applied Biosystems, Foster City, CA). The GHR 1A and IGF-I probes were labeled at the 5'-end with the reporter dye, FAM, and at the 3'-end with the quencher dye, TAMRA. The 5'- and 3'-end of the bovine cyclophilin I probe was labeled with VIC and TAMRA, respectively. Amplification mixes (25 µl) contained 2.5 µl of cDNA (25 ng), 500 nM of forward primer (2.5 µl), 500 nM of reverse primer (2.5 µl), 100 nM of probe (0.025 µl), 12.5 µl of Taqman Universal PCR Master Mix (Applied Biosystems), and 5 µl of RNAse free distilled water. An equal volume of water (no template control), internal controls (low, medium, and high), and standard curves were run in separate wells on the same plate.

Hormone Assays
Plasma GH concentrations were quantified using a validated homologous double antibody radioimmunoassay (RIA; Gorewit, 1981). Recombinantly derived bovine GH (SV-3001-B, Pharmacia Inc., Kalamazoo, MI) was used as the standard and as the iodinated trace (Cohick et al., 1989). Before use, the first antibody (rabbit anti-oGH2; AFP C0123080; a gift from the National Hormone and Pituitary Program, A. F. Parlow, Scientific Director) was diluted 1:20,000 and the second antibody (goat anti-rabbit; lot # 35318; Pel-Freez, Rogers, AK) was diluted 1:75. Samples were analyzed in triplicate. The minimal detectable concentration of GH was 0.7 ng/ml of standard or sample added to the assay tubes. Intra- and interassay CV were 6.2 and 7.7%, respectively.

Plasma IGF-I concentrations were quantified using a validated homologous double antibody RIA (Johnson et al., 1996) that was modified so that the first antibody (APF-4892898) and trace were added to the assay tubes and incubated for 24 h before the addition of the second antibody (goat anti-rabbit). Recombinantly derived human IGF-I (H-5555, lot C00219; Bachem, King of Prussia, PA) was used as the standard and as the iodinated trace (Cohick et al., 1989). ¹²⁵I was purchased from Perkin Elmer Life Sciences (Boston, MA; NEZ-033H). For iodination, the amount of IGF-I was reduced (1 µg), and reaction time was increased (5 min) relative to the original procedure. Samples were analyzed in triplicate. The minimal detectable concentration of IGF-I was 0.2 ng/ml of standard or sample added to the assay tubes. Intra- and interassay CV were 5.8 and 4.6%, respectively.

Plasma insulin (McGuire et al., 1995), progesterone (Kirby et al., 1996) and estradiol (Kirby et al., 1996) were quantified by RIA. Insulin and progesterone were each quantified in a single assay (intraassay CV of 7.2 and 5.1%, respectively). Estradiol was quantified in three assays (intra- and interassay CV were 12.8 and 16.6%, respectively). Plasma glucose (Sigma, St. Louis, MO) and NEFA (Waco Pure Chemical Industries Ltd., Osaka, Japan) were quantified using commercially available colorimetric assay kits (intraassay CV were 4.6 and 1.8%, respectively).

Statistical Analysis
The amount of GHR 1A, IGF-I, GHRtot, and cyclophilin mRNA and the blood concentrations of hormones and metabolites were analyzed by using Proc Mixed of SAS (SAS, 1999). The model included the effect of day relative to parturition. Cow was included as a random variable. In addition, the mRNA data collected on specific days relative to parturition were grouped into weeks (d -14 to -8 [wk -2], d -7 to 0 [wk -1], d 1 to 7 [wk 1] and d 8 to 14 [wk 2]) and analyzed for the effect of week. Weekly means were separated by Duncan’s multiple-range test. Data are presented as least squares means ± standard error of the least squares mean. Significance was inferred as P < 0.05 unless stated otherwise.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
The amount of GHR 1A mRNA declined from 2 wk before parturition to 1 wk after parturition and then increased (P < 0.05; Figure 1). The amount of IGF-I mRNA was also affected by week (P < 0.05), and the IGF-I expression pattern was similar to that of GHR 1A. Expression of GHRtot mRNA decreased (P < 0.05) from 2 wk before to 1 wk before parturition. Expression of cyclophilin mRNA increased (P < 0.05) from 2 wk to 1 wk before parturition and then remained relatively constant until 2 wk after parturition.

View larger version (31K): [in this window] [in a new window]   Figure 1. Growth hormone receptor (GHR) 1A, IGF-I, total GHR (GHRtot), and cyclophilin (cyclo) mRNA in liver of dairy cows on consecutive weeks relative to parturition. Bars represent the least squares mean ± standard error of mean for each week relative to parturition (d 0). Within mRNA, bars with different letters differ P < 0.05.

View larger version (28K): [in this window] [in a new window]   Figure 2. Daily means for growth hormone receptor (GHR) 1A (standard error of mean [SEM] = 267 fg/25 ng of reverse transcribed RNA [RT]) and IGF-I mRNA (SEM = 31 fg/25 ng RT) expressed in liver and plasma IGF-I (SEM = 19.4 ng/ml) of periparturient cows from 14 d before through 14 d after parturition.

View larger version (14K): [in this window] [in a new window]   Figure 3. Growth hormone receptor (GHR) 1A mRNA expression (fg/25 ng of reverse transcribed RNA [RT]) in liver of five individual cows that were biopsied 3 times across the 29-d study period.

View larger version (28K): [in this window] [in a new window]   Figure 4. Daily means for plasma growth hormone (GH; standard error of mean [SEM] = 1.4 ng/ml) and nonesterified fatty acids (NEFA; SEM = 0.12 µEq/L) of periparturient cows from 14 d before through 14 d after parturition. Liver growth hormone receptor 1A mRNA from Figure 2 is shown for reference.

View larger version (27K): [in this window] [in a new window]   Figure 5. Daily means for plasma estradiol (standard error of mean [SEM] = 16.8 pg/ml) and progesterone (SEM = 0.5 ng/ml) in periparturient cows from 14 d before through 14 d after parturition. Liver growth hormone receptor 1A mRNA from Figure 2 is shown for reference.

View larger version (28K): [in this window] [in a new window]   Figure 6. Daily means for plasma glucose (standard error of mean [SEM] = 6.0 mg/dL), and insulin (SEM = 0.2 ng/ml) of periparturient cows from 14 d before through 14 d after parturition. Liver growth hormone receptor 1A mRNA from Figure 2 is shown for reference.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

Blood insulin concentrations decreased as well during the periparturient period. The changes in insulin were similar to the changes in IGF-I mRNA and blood IGF-I concentrations. Butler et al. (2003) recently reported that GHR 1A and IGF-I mRNA were increased in early postpartum cows subjected to a hyperinsulinemic-euglycemic clamp. The coordinated changes in insulin and IGF-I that we observed may reflect the dependence of IGF-I on insulin in postpartum dairy cows. The relationship may ultimately depend on the effects of insulin on GHR 1A in liver because the GHR 1A is controlled by insulin and GH controls IGF-I synthesis and secretion. Although insulin may control the recrudescence of the GHR 1A and IGF-I after parturition, changes in blood insulin probably do not explain the decrease in GHR 1A before parturition because GHR 1A mRNA declined before the postpartum decrease in insulin.

The changes in GHR 1A appear to coordinate early phases of nutrient partitioning. The decrease in GHR 1A leads to a decrease in IGF-I mRNA and a decrease in blood IGF-I concentrations. Blood GH concentrations are elevated during the first week after parturition because blood IGF-I concentrations are low (reduced negative feedback on GH). The increase in blood GH concentrations promotes lipolysis, which releases NEFA into blood (Figure 3). The NEFA may be oxidized in liver or extra-hepatic tissues or may be incorporated directly into milk fat (Etherton and Bauman, 1998). The loss of GHR 1A from liver, therefore, may be an endocrine mechanism to ensure lipid mobilization during lactogenesis.

In this study, as well as our previous studies (Kobayashi et al., 1999c), we noted some variability in GHR 1A mRNA expression across cows. When plots of individual cows were examined (Figure 3), we observed the general pattern of GHR 1A mRNA expression (high prepartum and low during the week after parturition followed by a slight increase later postpartum). However, some individual cows (e.g., cow 374 of Figure 3), had remarkably high levels of GHR 1A relative to other individual cows. The metabolic consequences of GHR 1A variability are unclear. We would predict that cows with extremely high GHR 1A would be more responsive to GH. Conversely, cows with extremely low GHR 1A may be somewhat insensitive to GH. Regardless of the prepartum variability, all cows had low GHR 1A during the week after parturition. The low GHR 1A may be a critical component for nutrient partitioning in early-lactation cows.

Something must trigger the large periparturient decrease in GHR 1A mRNA. The present study provides correlative evidence to address this question. Blood progesterone concentrations decreased 3 to 4 d before parturition. Blood estradiol concentrations underwent a sharp rise when blood progesterone concentrations decreased. The decrease in GHR 1A seemed to coincide with the large increase in blood estradiol before parturition. Changes in blood progesterone, estradiol, and glucocorticoids initiate parturition in cattle (Head, 1999; Tucker, 2000). We have not critically tested the effects of steroid hormones on GHR 1A. Neither short-term feed restriction (mimicking the natural decrease in feed intake around parturition) nor acute epinephrine challenge (mimicking the increase in epinephrine around parturition; Kobayashi et al., 2002) decreased GHR 1A. Larges doses of glucocorticoids where ineffective for decreasing GHR 1A as well (Kobayashi et al., 1999b). Byatt et al. (1997) reported a dramatic decrease in IGF-I when glucocorticoids were given following estradiol and progesterone treatment in an induced lactation model. A sequence of steroid hormones given in a manner that will induce lactation may, therefore, be necessary to recapitulate the changes in GHR 1A and IGF-I that we observed in periparturient dairy cattle. This hypothesis should be tested in future experiments.

The total amount of GHR (GHRtot) underwent a modest decline during the periparturient period (Figure 1). Our previous analyses suggested that GHR 1A mRNA accounts for 70% of liver GHR (Kobayashi et al., 1999a, 1999c). We expected to detect a larger change in total GHR when GHR 1A decreased because we had thought GHR 1A comprised the bulk of liver GHR mRNA. There are two possible explanations for our observation. First, alternative GHR mRNA (1B and [or] 1C) may be up regulated when GHR 1A mRNA is down regulated. The increase in alternative GHR would preclude a change in GHRtot when GHR 1A decreased. A second possibility is that our ribonuclease protection assay overestimated the amount of GHR 1A relative to GHR 1B/1C. If the relative amount of GHR 1A is lower, then we may not detect a large change in GHRtot.

In summary, the GHR 1A mRNA expression in liver decreased gradually during the 2 wk before parturition, remained low for about 7 d after parturition and then increased. The decrease in liver IGF-I mRNA and plasma IGF-I concentrations followed the decline in GHR 1A mRNA expression. The decrease in GHR 1A mRNA was associated with a decrease in progesterone and an increase in estradiol shortly before parturition. Additional studies will be necessary to determine the periparturient hormones that control GHR 1A expression.

	ACKNOWLEDGEMENTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 
The authors thank W. R. Butler of Cornell University (Ithaca, NY) for measuring plasma insulin concentrations.

	FOOTNOTES

 
¹ This research was supported by the National Research Initiative Competitive Grants Program. USDA CSREES 00-35206-9536 awarded to M. C. Lucy.

Received for publication March 25, 2003. Accepted for publication April 22, 2003.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGEMENTS REFERENCES

 


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