Growth hormone secretion and pituitary gland ...

New Zealand Journal of Agricultural Research

ISSN: 0028-8233 (Print) 1175-8775 (Online) Journal homepage: http://www.tandfonline.com/loi/tnza20

Growth hormone secretion and pituitary gland weight in suckling lambs from genetically lean and fat sheep S. M. Francis , B. A. Veenvliet , S. K. Stuart , R. P. Littlejohn & J. M. Suttie To cite this article: S. M. Francis , B. A. Veenvliet , S. K. Stuart , R. P. Littlejohn & J. M. Suttie (1998) Growth hormone secretion and pituitary gland weight in suckling lambs from genetically lean and fat sheep, New Zealand Journal of Agricultural Research, 41:3, 387-393, DOI: 10.1080/00288233.1998.9513323 To link to this article: http://dx.doi.org/10.1080/00288233.1998.9513323

Published online: 17 Mar 2010.

Submit your article to this journal

Article views: 28

View related articles

Citing articles: 2 View citing articles

Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tnza20 Download by: [181.143.9.91]

Date: 28 January 2016, At: 18:22

New Zealand Journal of Agricultural Research, 1998, Vol. 41: 387-393 0028-8233/98/4103-0387 $7.00/0 © The Royal Society of New Zealand 1998

387

Downloaded by [181.143.9.91] at 18:22 28 January 2016

Growth hormone secretion and pituitary gland weight in suckling lambs from genetically lean and fat sheep S. M. FRANCIS B. A. VEENVLIET S. K. STUART R. P. LITTLEJOHN J. M. SUTTIE* AgResearch Invermay Agricultural Centre Private Bag 50034 Mosgiel, New Zealand

GH, and pituitary gland weight occur at an early age. An investigation of foetal development patterns may be required to elucidate the relationship between these parameters. Keywords growth hormone; insulin-like growth factor I; pituitary gland; fatness; suckling lambs; carcass composition INTRODUCTION

Abstract Previous studies have found that weaned lambs from Coopworth sheep selected for low (lean) backfat depth have higher mean plasma growth hormone (GH) levels and heavier pituitary glands than those selected for high (fat) backfat depth. This study examined whether these differences between genotypes occurred in young suckling lambs. Six ewes from each genotype which were suckling a male/female set of twins were kept indoors. Four weeks after birth, ewes and lambs were blood-sampled for 6 hours at 10 minute intervals and plasma GH levels measured. Lambs were then slaughtered and carcass composition determined. Subcutaneous fat depth was lower in lean than fat lambs at four of the five different sites measured (P < 0.05). Lean lambs and ewes had greater mean and basal plasma GH concentrations and a greater amplitude of GH pulses than fat sheep (P < 0.05). Plasma IGF-I levels did not differ (P < 0.05) between genotype for either ewes or lambs. Pituitary glands were heavier (P< 0.001) in lean than fat genotype lambs (0.29 versus 0.19, SED = 0.017 g). It is concluded that differences between genotypes in body composition, plasma

*Author for correspondence A97087 Received 17 December 1997; accepted 15 April 1998

Growth hormone (GH) is involved in promoting lean growth in animals (Bengtsson et al. 1992; Etherton et al. 1993; Etherton & Smith 1991). In lambs for example, exogenous GH has been shown to stimulate growth rate and reduce carcass fat content (Bass et al. 1992). In cattle, Trenkle & Irvin (1970) showed that plasma levels of GH were negatively related to carcass fatness and positively to growth rate. It has also been shown that GH is altered in conjunction with changes in body composition in some genetic selection lines. For example, Southdown sheep selected for low backfat had higher levels of plasma GH than sheep with high backfat (Carter et al. 1989), and Althen & Gerrits (1976) and Buonomo & Klindt (1993) found similar results in pigs selected for low and high backfat. Previous work at Invermay Agricultural Centre has shown that selection for liveweight-adjusted backfat depth as described by Morris et al. (1997) is associated with changes in GH secretory parameters. Suttie et al. (1991) showed that lean genotype sheep released more GH in response to growth hormone-releasing factor than fat genotype sheep, and Francis et al. (1997) indicated that lean lambs had greater mean and basal plasma GH and altered frequency and amplitude of pulses compared with fat genotype animals. Recent work has demonstrated that lean genotype lambs have heavier pituitary glands than fat genotype sheep (Fleming et al. 1997). As all of the above research was carried

388

New Zealand Journal of Agricultural Research, 1998, Vol. 41

out in weaned male lambs, a study on suckling lambs of both sexes was initiated to examine the physiological differences between the lean and fat sheep. Specifically, the objective of this work was to determine whether the differences in GH secretion and pituitary gland weight between the lean and fat genotypes that are present in weaned lambs also occur in very young, suckling lambs and in lactating ewes.


MATERIALS AND METHODS Animals and feeding In 1995, Coopworth ewes from the lean and fat selection lines at Invermay Agricultural Centre (Morris et al. 1997) were mated to sires of the same genotype. They were grazed on pasture during the winter and scanned ultrasonically at approximately Day 97 of gestation to determine the number of foetuses. On approximately Day 116 of gestation, 34 ewes scanned with twins were brought into an open barn (14 m x 8 m) with a deep litter pad. The ewes were weighed weekly and fed 1.6 kg dry matter (DM) d~' of a diet containing chaffed lucerne hay (8.2 MJME kg' 1 DM) and sheep pellets (10.7 MJME kg""1 DM). There was an adaptation period of 14 days when the pelleted concentrate component of the diet was increased from 17 to 44%. Any animals which showed excessive weight loss (>8% of body weight) during the feed adaptation period were removed from the experiment. This experiment was approved by the Invermay Agriculture Centre Animal Ethics Committee. Trial design The experiment used 6 ewes from each of the lean and fat genotypes, each raising a set of twin lambs of one ewe and one ram lamb. There were 2 sires (n = 3 ewes per sire) within each genotype. Once the ewes had lambed, they were moved to a 3 m x 2 m individual pen and fed chaffed lucerne and sheep pellets ad libitum. Ewes and lambs were weighed weekly for the first fortnight after birth and then twice weekly for the second fortnight. The ewes' daily intake was measured by weighing food offered and any refusals. Animals were bloodsampled and the lambs slaughtered when they were aged 28 + 2 days. Because of the spread in lambing dates, blood sampling and slaughter was carried out in three groups over a 2 week period. Two days prior to this, all the animals were ultrasonically

scanned to determine fat depths at the C and GR sites and the depth (B) of the L. dorsi muscle as described by McEwan et al. (1989). Blood sampling Weekly blood samples were collected into 5 ml heparinised vacutainers (Becton Dickinson, New Jersey, USA) from ewes and lambs throughout the experiment. When the lambs were four weeks old, ewes and lambs were blood sampled for 6 hours at 10 minute intervals (4 ml from ewes and 2 ml from lambs). On the day prior to sampling, ewes had a sterile 2.1 mm x 8.3 cm cannula (Angiocath, Becton Dickinson) inserted into the jugular vein. Two hours prior to sampling, the lambs had a catheter inserted into the jugular vein. This involved positioning a sterilised, modified (the inside surface of the luer hub was drilled to produce a flush bevel) 1.25 mm x 37.5 mm needle (Nipro, Nissho Corporation, Japan) into the jugular vein. A sterile guidewire (0.89 mm x 80 cm, Kimal Scientific Products Limited, England) was then inserted through the lumen of the needle and about 10 cm into the jugular vein. Once the guide wire was in position, the needle was removed and a 25 cm length of plastic tubing (single lumen medical grade polyethylene tube with inner diameter of 1.0 mm and outer diameter of 1.5 mm; Dural Plastics & Engineering, NSW, Australia) was passed over the guidewire and positioned so that 7.5 cm of the tubing was in the vein. The tubing was sealed by inserting a sterile, blunted, thin wall 1.1 mm needle (Monoject, Sherwood Medical, St Louis, MO, USA) and capping with a one-way valve (Reflux Valve, B.Braun Melsungen AG, Germany). The tubing was sutured into place and taped behind the neck. Before sampling, all catheters were flushed with heparinised (125 000 units I"1) saline (multiparin heparin, Fisons Pharmaceuticals, NSW, Australia and 0.9% sodium chloride, Baxter Healthcare Pty Ltd, NSW, Australia). Slaughter and carcass analysis Lambs were killed by exsanguination and the weight of omental fat, kidney fat, kidney, and liver recorded. Pituitary glands were removed and weighed. The following day, measurements were made of the weight and length of the carcass, tibia length, depth (B) and width (A) of the L. dorsi muscle, and fat depths at the C, GR, S1, S2, and L3 sites as described by Kirton & Johnson (1979). Carcasses were stored at -20°C until grinding and

Francis et al.—GH and pituitary glands in suckling lambs


analysis of total fat content as described by Lord et al. (1988). Hormone assays GH was measured by radioimmunoassay (RIA) as described by Suttie et al. (1993) with rabbit antiovine GH antiserum (AFP CO 123080, NIDDK, USA) and ovine GH antigen (AFP 6762B, NIDDK, USA). For the lambs, intra- and inter-assay coefficients of variation (CV) were 18.5, 7.9, and 6.1 % and 22.8,15.8, and 9.0% for plasma pools of 1.8, 9.8, and 18.9 ng ml"1, respectively. The ewe CVs were 17.3, 7.2, and 5.9% (intra-assay) and 22.1, 15.9, and 8.5% (inter-assay) for 1.7, 9.5, and 18.7 ng ml"1 plasma pools, respectively. The GH secretory patterns were analysed by the peak detection routine PULSAR (Merriam & Wachter 1982) to determine the number of pulses of GH secretion over the six hours of sampling and the mean amplitude of those pulses. In addition, overall mean and the basal GH concentrations between pulses were calculated. Insulin-like growth factor I (IGF-I) in plasma samples was separated from the IGF binding proteins and measured by RIA as described by Francis et al. (1997). All IGF-I samples were measured in one assay and the intra-assay CVs were 4.8, 3.9, and 2.0% at 118, 396, and 970 ng ml"1, respectively. IGF-I concentrations were measured every week for the lambs and every second week for the ewes. Statistical analysis Data were analysed by analysis of variance with genotype and sire effects tested in the between-litter stratum and sex effects and their interactions tested in the within-litter stratum. All carcass and body composition measurements were covariate adjusted to a common carcass or live weight. Data are presented as means with the standard error of the difference between means (SED). Regression relationships were investigated between ewe and lamb hormone concentrations in the between-litter stratum adjusting for genotype. GH mean, basal, and amplitude were analysed on a logarithmic scale, and their backtransformed means are presented in the tables along with the standard error of the ratio (SER). Note that geometric means are significantly different when the ratio of one to another is greater than approximately the square of the SER. Statistical significance was assessed at the 5% level unless stated otherwise.

389

RESULTS Liveweight and in vivo measurements The average liveweight of the ewes prior to the blood sampling was 59.6 kg and there was no significant difference between genotypes. There were no differences between genotypes in ewe intake or liveweight gain from birth of the lamb until the end of the trial. Lean ewes had smaller liveweight-adjusted ultrasonic C (3.5 versus 5.8, SED = 0.96 mm, P < 0.05) and GR (6.7 versus 12.7, SED = 1.69 mm, P < 0.01) measurements than fat ewes. There were no differences in liveweightadjusted ultrasonic B measurement between lean and fat ewes. There were no differences between genotype or sexes in lamb birth weight, pre-slaughter-liveweight, liveweight gain from birth to slaughter, and liveweight-adjusted ultrasonic B and C measurements. However, lean lambs had lower liveweightadjusted ultrasonic GR measurement than fat lambs (4.8 versus 8.2, SED = 0.86 mm, P < 0.01). Carcass analyses The average lamb carcass weight was 5.0 kg. There were no significant differences between genotypes or sexes in carcass weight, length of the carcass or tibia, the depth or width of the L. dorsi muscle, the weight of the liver, or the depth of subcutaneous fat at the SI site. The depth of subcutaneous fat at the C, GR, S2, and L3 sites and the total body fat content were lower in lean than fat lambs (Table 1). Kidney, kidney fat, and omental fat weights did not differ between genotypes. The weight of the kidneys was greater in ram than ewe lambs (48.3 versus 46.5, SED = 0.56 g, P< 0.01) and there was a genotype x sex inter-

Table 1 Measurements of fat depots in lean and fat lambs. Data are all adjusted by covariate analysis to a common carcass weight. There were twelve lambs in each group. Data are presented as means with SED. NS, P > 0.05; *, P < 0.05; **, P < 0.01; ***, P < 0.001.

Kidney fat (g) Omental fat (g) Total fat (%) C(mm) GR (mm) SI (mm) S2 (mm) L3 (mm)

Lean

Fat

SED

Sig

85.0 54.8 13.2 1.4 5.1 0.7 2.0 2.2

100.8 64.2 17.8 2.8 9.1 1.1 4.0 4.0

17.2 6.65 1.46 0.31 1.32 0.21 0.50 0.27

NS es * ** * NS ** ***


390

action, indicating that the difference between sexes was much less in fat than lean lambs. Male lambs had less kidney fat (75.5 versus 110.3, SED = 17.35 g, P < 0.05) and omental fat (52.1 versus 66.9, SED = 6.39 g, P < 0.05) than female lambs. There were no differences between the sexes in total body fat content or any subcutaneous fat depots.

550

O

500

°

o

350 300


•

o

400

•

o

•

250

•

200

Hormone analyses Mean and basal GH concentrations and the amplitude of the GH pulses were significantly greater in lean than fat lambs and ewes (Table 2). The pulse frequency was not different between the genotypes for ewes or lambs. There were significant effects of sire within genotype for mean and basal GH and the number of GH pulses, indicating that the difference between the two sires was less in lean compared with fat lambs. Male lambs had greater mean (5.26 versus 3.50, SER = 1.18 ng ml" 1 , P< 0.05) and basal (3.01 versus 1.78, SER = 1.14 ng ml"1, P < 0.01) GH concentrations than female lambs, but there were no differences in pulse frequency or amplitude between sexes. There was a significant regression relationship of lamb mean GH on ewe mean GH with a slope of 0.82 (s.e. = 0.29). There were no differences between genotypes, sex, or their interactions for plasma IGF-I levels at any of the four samplings. However, averaging lamb data for each ewe, there was a strong relationship between lamb IGF-I (mean of the four weeks) and lamb mean plasma GH with a slope of 39 (s.e. = 9.8, P < 0.01) (Fig. 1). At the same mean GH concentration, mean plasma IGF-I was lower in lean than fat lambs (difference of 201, SED = 50.8 ng ml"1, P < 0.01). There were no differences between the ewe genotypes in the two weeks that plasma

o

450

150

G H (ng ml"')

Fig. 1 Mean plasma IGF-I (ng ml"1) of lambs for the four weeks of the trial plotted against lamb mean plasma GH (ng ml"1) at the end of the trial. Values are the mean for each pair of twins. There were six pairs of twins in each group. Lean genotype in closed circles and fat genotype in open circles.

IGF-I levels were measured, even after covariate adjustment for plasma mean GH levels. Mean values for plasma IGF-I in ewes were 405 and 494 ng ml"1 (SEM = 40 at both time points) for the second and fourth weeks after the lambs were born, respectively. Pituitary weight The pituitary gland (adjusted for carcass weight by covariate analysis) was heavier (P < 0.001) in lean than fat lambs (0.29 versus 0.19, SED = 0.017 g). There was no difference between the sexes in pituitary gland weight. DISCUSSION This trial has shown that, by four weeks of age, lean lambs have significantly less body fat, greater

Table 2 GH secretory parameters for ewes and lambs. There were twelve lambs and six ewes in each group. Mean values and SED or SER are presented. NS, P> 0.05; *,/>> 0.05;**, P> 0.01; ***,/>> 0.001. Lambs Mean (ng ml"1) Basal (ng ml"1) No. pulses per 6 h Pulse amplitude (ng ml"1) Ewes Mean (ng ml-"1) Basal (ng ml-"1) No. pulses per 6 h Pulse amplitude (ng ml"1)

Lean

Fat

6.52 3.43 6.0 8.35

2.83 1.56 6.0 5.11

6.62 4.66 5.7 6.36

3.49 2.39 6.2 3.32

SED

SER

Sig

1.18 1.12

*** *** NS *

0.71 1.21 1.28 1.36 1.0 1.26

** * NS *


391 GH secretion, and heavier pituitary glands than fat lambs. These findings agree with previous studies (Suttie et al. 1991; Francis et al. 1995a; Morris et al. 1997) which have focussed on the weaned, male lamb from the lean and fat genotypes. Suttie et al. (1993) have also shown that suckling lambs of 2 and 3 months old from the lean and fat genotype sheep differ in carcass composition and plasma GH concentrations. The research presented in this paper indicates that it is likely that the mechanisms regulating the differences in GH secretion and fat deposition between the genotypes are set up during foetal development. The association between high plasma GH levels and reduced body fatness is well known. Not only does administration of exogenous GH decrease carcass fat levels in sheep (Bass et al. 1992), but increased plasma GH has been measured in sheep (Carter et al. 1989) and pigs (Althen & Gerrits 1976; Buonomo & Klindt 1993) selected for low backfat compared with high backfat animals. Relative levels of plasma GH in weaned lambs (Suttie et al. 1991; Suttie et al. 1993; Francis et al. 1995a; Francis et al. 1997) as well as in the suckling lambs from the Invermay Coopworth lean and fat genotypes agree with those studies. Mean and basal plasma GH levels as well as the amplitude of GH pulses were higher in lean than fat genotype lambs in both weaned and suckled lambs. However, in contrast to the earlier work in weaned lambs (Francis et al. 1995a), there was no difference in pulse frequency between lean and fat suckled lambs. Suttie et al. (1993) showed that pulse frequency was higher in suckled than weaned lambs. Although the factors regulating plasma GH levels may differ between the suckling and weaned lambs, the overall result of higher mean plasma GH levels in lean than fat genotype sheep is the same. What is still not known, is the mechanism by which plasma GH levels are elevated in lean sheep and whether this is actually the cause of reduced carcass fatness in lean genotypes. The present study with suckling animals was designed to determine whether the differences in plasma GH between the lean and fat genotypes existed at an earlier age than demonstrated in previous studies with these genotypes. Certainly, this was found to be the case, although the differences in body composition between the genotypes are also still apparent, giving no indication of whether the elevated plasma GH is actually the cause of the reduced carcass fatness. The relationship between GH and body composition

appears to exist throughout life and in both sexes, since both the lactating ewes as well as male and female lambs from the lean genotype had greater GH levels and less carcass fat than fat sheep. There was a strong positive relationship between ewe and lamb mean plasma GH levels. If these measurements were expressions of the same trait, the regression would indicate that the heritability of mean plasma GH is close to 1. However, this heritability value may be over-estimated because of common environmental effects between the ewe and her lambs, as well as the large influence of one point. The relationship between ewe and lamb plasma GH levels may be physiologically important since administration of GH to pregnant sows decreased backfat depths of neonatal piglets (Kelley et al. 1995). It is thus possible that the greater GH levels of the lean ewes have an influence on the body composition of their lambs. Previous work has shown that the pituitary glands of the lean sheep are heavier than fat genotype sheep (Francis et al. 1995a). Fleming et al. (1997) showed that the pituitary concentration of GH was not different between these lean and fat sheep although the total pituitary content of GH was greater in lean sheep because of their larger pituitary glands. Recent work has demonstrated that the heavier pituitary glands of the lean sheep result from a larger number, rather than size, of pituitary cells and that there is no difference between the genotypes in the proportion of different cell types (S. M. Francis unpubl. data). The work presented in this paper shows that the difference in pituitary gland size between lean and fat genotypes exists in young suckling lambs as in the older weaned lambs. However, it is still not known whether this is a cause or effect of the differences in plasma GH levels, and the factors regulating the different levels of GH expression between the genotypes remain to be identified. Previous work in weaned lambs from these selection lines (Francis et al. 1997) indicates that other hormones such as insulin are unlikely to be involved in the phenotypic differences between the genotypes. Further research is required to clarify the relationship between heavier pituitary glands, higher GH levels, and lower body fat in lean compared with fat genotype lambs. The positive relationship between plasma IGF-I and mean plasma GH in the lambs agrees with the classical concept of GH regulating IGF-I levels. However, this differs from the work of Buonomo & Klindt (1993) who showed that plasma IGF-I concentrations are independent of plasma GH levels


392


in the early period of postnatal growth in both lean and obese lines of pigs. The lower IGF-I concentrations in lean compared with fat lambs, after correction for the GH effect, contrasts with previous work where plasma IGF-I levels were greater in lean than fat weaned lambs (Francis et al. 1995b), although in that study no adjustment was made for plasma GH levels. Buonomo & Klindt (1993) found that IGF-I was greater in lean than obese pigs during the first three days of life, but by the age of seven days the difference between the lines had disappeared. The role of IGF-I in the growth and development of fat tissue in sheep requires further study, particularly in the early post-natal period. This work has shown that carcasses are leaner, plasma GH levels are higher, and pituitary glands are heavier in suckling lambs from the lean compared with fat genotype sheep, as in the older weaned animals. It is hypothesised that these differences begin during foetal development, and future work should focus on this stage of development to further understand the relationship between plasma GH concentrations and carcass composition.

ACKNOWLEDGMENTS We wish to acknowledge John McEwan, Wendy Bain, Gordon Greer, and Alex Findlay for supplying the animals; Barbara Masters, Syd Duncan, Gordon Greer, Alex Findlay, Rob Labes, Rick Zydenbos, Rena Johnsen, and Bernadine Hill for assistance with animal feeding, weighing, and blood sampling; Jim Webster for collection of the pituitaries; Frans Laas for animal slaughter; Tim Manley for advice with hormone assays. This research was supported by the New Zealand Foundation for Research, Science and Technology.

REFERENCES Althen, T. G.; Gerrits, R. J. 1976: Pituitary and serum growth hormone levels in Duroc and Yorkshire swine genetically selected for high and low backfat. Journal of animal science 42: 14901497. Bass, J. J.; Hodgkinson, S. C ; Breier, B. H.; Carter, W. D.; Gluckman, P. D. 1992: Effects of bovine somatotrophin on insulin-like growth factor-I, insulin, growth and carcass composition of lambs. Livestock production science 31: 321-334. Bengtsson, B. A.; Brummer, R. J. M.; Eden, S.; Rosen, T.; Sjostrom, L. 1992: Effects of growth hormone on fat mass and fat distribution. Acta poediatrica supplement 383: 62-65.

Buonomo, F. C ; Klindt, J. 1993: Ontogeny of growth hormone (GH), insulin-like growth factors (IGF-I and IGF-II) and IGF binding protein-2 (IGFBP2) in genetically lean and obese swine. Domestic animal endocrinology 10: 257-265. Carter, M. L.; McCutcheon, S. N.; Purchas, R. W. 1989: Plasma metabolite and hormone concentrations as predictors of genetic merit for lean meat production in sheep: effects of metabolic challenges and fasting. New Zealand journal of agricultural research 32: 343-353. Etherton, T. D.; Smith, S. B. 1991 : Somatotropin and |3adrenergic agonists: their efficacy and mechanisms of action. Journal of animal science 69 (suppl. 2): 2-26. Etherton, T. D.; Louveau, I.; Sorensen, M. T ; Chaudhuri, S. 1993: Mechanisms by which somatotropin decreases adipose tissue growth. American journal of clinical nutrition 58 (suppl): 287S295S. Fleming, J. S.; Suttie, J. M.; Montgomery, G. W.; Gunn, J.; Stuart, S. K.; Littlejohn, R. P.; Gootwine, E. 1997: The effects of a duplication in the ovine growth hormone (GH) gene on GH expression in the pituitaries of lean and fat-selected sheep lines. Domestic animal endocrinology 14: 17-24. Francis, S. M.; Veenvliet, B. A.; Littlejohn, R. P.; Stuart, S. K.; Suttie, J. M. 1995a: Growth Hormone (GH) secretory patterns in genetically Lean and Fat sheep. Proceedings of the New Zealand Society of Animal Production 55: 272-274. Francis, S. M.; Veenvliet, B. A.; Stuart, S. K.; Littlejohn, R. P.; Suttie, J. M. 1995b: Insulin-like growth factor-I (IGF-I) mRNA and plasma concentrations in Lean and Fat genotypes of sheep. Proceedings of the Endocrine Society of Australia 38: 168. Francis, S. M.; Veenvliet, B. A.; Stuart, S. K.; Littlejohn, R. P.; Suttie, J. M. 1997: The effect of photoperiod on plasma hormone concentrations in wether lambs with genetic differences in body composition. Animal science 65: 441-450. Kelley, R. L.; Jungst, S. B.; Spencer, T. E.; Owsley, W. F.; Rahe, C. H.; Mulvaney, D. R. 1995: Maternal treatment with somatotropin alters embryonic development and early postnatal growth of pigs. Domestic animal endocrinology 12: 83-94. Kirton, A. H.; Johnson, D. L. 1979: Interrelationships between GR and other lamb carcass fatness measurements. Proceedings of the New Zealand Society of Animal Production 39: 194-201. Lord, E. A.; Fennessy, P. F.; Littlejohn, R. P. 1988: Comparison of genotype and nutritional effects on body and carcass characteristics of lambs. New Zealand journal of agricultural research 31: 13-19.

Francis et al.—GH and pituitary glands in suckling lambs


McEwan, J. C ; Clarke, J. N.; Knowler, M. A.; Wheeler, M. 1989: Ultrasonic fat depths in Romney lambs and hoggets from lines selected for different production traits. Proceedings of the New Zealand Society of Animal Production 49: 113-119. Merriam, G. R.; Wachter, K. W. 1982: Algorithms for the study of episodic hormone secretion. American journal ofphysiology 243: E310-E318. Morris, C. A.; McEwan, J. C ; Fennessy, P. R; Bain, W. E.; Greer, G. J.; Hickey, S. M. 1997: Selection for high or low backfat depth in Coopworth sheep: juvenile traits. Animal science 65: 93-103.

393

Suttie, J. M.; Lord, E. A.; Gluckman, P. D.; Fennessy, P. F.; Littlejohn, R. P. 1991: Genetically lean and fat sheep differ in their growth hormone response to growth hormone-releasing factor. Domestic animal endocrinology 8: 323-329. Suttie, J. M.; Veenvliet, B. A.; Littlejohn, R. P.; Gluckman, P. D.; Corson, I. D.; Fennessy, P. F. 1993: Growth hormone pulsatility in ram lambs of genotypes selected for fatness or leanness. Animal production 57: 119-125. Trenkle, A.; Irvin, R. 1970: Correlation of plasma hormone levels with growth and carcass characteristics of cattle. Growth 34: 313-319.