DOMESTIC ANIMAL ENDOCRINOLOGY

Vol. 8(2):323-329, 1991

GENETICALLY LEAN AND FAT SHEEP DIFFER IN THEIR GROWTH HORMONE RESPONSE TO GROWTH HORMONE-RELEASING FACTOR J.M. Suttie 1, E.A. Lord, P.D. Gluckman*, P.F. Fennessy and R.P. Littlejohn Invermay Agricultural Centre, Private Bag, Mosgiel, New Zealand and *Department of Paediatrics, University of Auckland, Private Bag, Auckland, New Zealand Received May 10, 1990

ABSTRACT The aim of this study was to compare the ovine growth hormone (oGH) responses of 5 genetically lean and 5 genetically fat 9 month old ram lambs (selected on the basis of their ultrasonic backfat thickness) given two 0.3 ~g kg-~ liveweight intravenous injections of synthetic human pancreatic GH releasing factor analogue Nle 27 hGHRF29 -NH2 (GRF-29) 150 minutes apart. Plasma oGH response curves were analysed using an exponential 2 compartmental model and comparisons made through parallel curve analysis. Plasma oGH levels over 200 ng ml-~were detected in response to GRF-29. Exponential model parameters indicated that lean lambs had a significantly higher rate of oGH release into the plasma after both consecutive GRF-29 injections, and a significantly lower rate ofoGH clearance from the plasma after the second GRF-29 injection only. Significantly smaller peak oGH responses to the second GRF-29 injection were shown by the fat lambs. These results suggest that oGH release is impaired in genetically fat lambs and that either the synthesis of releasable oGH is reduced or the inhibitory tone is greater in the fat lambs. The lean and fat sheep may provide a useful model for the study of hormonal control of factors affecting leanness and fatness. INTRODUCTION Dose-response relationships between human pancreatic growth hormone-releasing factor (GHRF) (1) and GH release have been described in rats (2), chickens (3), rabbits (4), humans (5,6), cattle (7) and goats (8) and sheep (9). It has been suggested that abnormalities of GH secretion may be associated with obesity (10). Williams et al. (11) and Kopelman et al. (12) found a reduced GH response to GHRF in obese subjects when compared with normal or control subjects. It was suggested that an impaired pituitary response to GHRF in obese subjects may be responsible, although the GH defect was reversed after dieting (11). Concern about overfatness in animal production has led to the formation of lines of sheep selected for (fat) or against (lean) their ultrasonic backfat thickness at 6-8 months of age. After 4 years of selection, lambs from the lean line have 1.86 + 0.73mm (x + standard deviation) backfat, while fat line lambs have 5.10 + 1.3mm backfat, when corrected for weight and age (13). Composition analysis of slaughtered progeny indicate that lean line lambs have 12% less fat in the carcass than fat line lambs at a carcass weight of 13.4 kg (14). The aim of these experiments was to compare the GH responses to a double injection of a potent analogue of GHRF namely NIe27hGHRF29-NH2 (GRF-29) (1) in lean and fat ram lambs. A double injection was used to evaluate the recovery capacity of oGH release following a maximal challenge from GRF-29. M A T E R I A L S AND M E T H O D S Five genetically lean (liveweight 36 + 3.6 kg) and 5 genetically fat (liveweight 37 + 4.6 kg) 9 month old ram lambs were brought indoors in April and fed a barley based pelleted diet to appetite. The lambs were penned individually with free access to water and exposed to natCopyright © 1991 by Domendo, Inc.

323

0739-7240/91/$3.00

324

SUTTIE ET AL

ural daylight and temperature variations. Immediately before blood sampling began subcutaneous fat depth was measured ultrasonically using a pulse echo ultrasound instrument (15). After a period of 3 weeks adaptation to the diet a cannula (Argyle Intramedicut, 16g, Sherwood Industries, St. Louis) was inserted into the jugular vein of each ram and manometer lines were attached thus permitting blood sampling with little disturbance to the animal. One week later the Iambs were injected twice 150 minutes apart, via the cannula, with Nle 27 hGHRH29-NH2 (GRF-29) (a gift from Dr W Vale Salk Institute La Jolla California) at a dose of 0.3 p.g kg -t liveweight. Preliminary experiments using dose levels of GRF-29 which ranged from 0.02-3.5 gg kg -j have shown that 0.3 mg kg-I was an appropriate dose. Blood samples were taken at -20, -10, 0, 5, 10, 15, 20, 25, 30, 35, 40, 60, 80, 100, 120, 130, 140, 150, 155, 160, 165, 170, 175, 180, 185, 190, 210, 230 and 250 minutes after the first GRF-29 challenge and were transferred to heparinised tubes. Plasma was recovered and stored at -20 C prior to hormone analysis.

oGH Assay Ovine growth hormone (oGH) was measured using the method of Gluckman et al (16). oGH antiserum of high potency was obtained by immunisation of a New Zealand white rabbit with oGH (NIH G-S-10). The standard used was prepared by Dr C H Li and had an immunopotency 1.7 times that of G-S-10. The sensitivity of the assay was 0.1 ng/tube take and half-maximal displacement of the radioligand was achieved by 0.8 ng per tube. The withinassay variation was 5% and the interassay variation was 11.2%.

Biometric Analysis The nature of the plasma oGH response was analysed using the compartmental model (17) described by the following equation for where plasma oGH concentration (ng ml -~) is given by y at time t (minutes): y = b + ap (e -q' - e-P')/(p-q) The plasma is thought of as a compartment which absorbs oGH at a constant rate p (ng ml 'minute ') and and from which it is cleared at a constant rate q (ng ml -t minute-~). Absorption starts at t=0 and a is the initial concentration of the releasable pool of GH expressed in units of concentration (ng ml '); a is proportional to the maximum peak plasma concentration of GH while b (ng ml -~) is the mean initial concentration of oGH in the plasma. When p = q the equation becomes y = b + a p t e pt The parameters p, q, a and b were fitted for each animal and for all appropriate groupings of animals assuming unweighted, normally distributed errors. A parallel curve analysis was then carried out, fitting each group with common non-linear parameters (p and q) but individual linear parameters (a and b). An analysis of variance then lead to the selection of the most suitable model, with due consideration taken for the weakening of assumptions caused by non-linearity. RESULTS The lean line lambs had a subcutaneous backfat thickness of 0.80mm and the fat line lambs 2.60mm, standard error of the difference (sed) = 0.43. The primary GH response data to the GRF-29 is shown in Figure 1 for all lambs. Plasma concentrations of GH robustly increased in all lambs after the injection of GRF-29. The GH was cleared from the plasma and

GH RESPONSES TO GRF-29 IN LEAN AND FAT SHEEP

"1

~.LEAN

"3

325

"8

"5

2(10.

"10

,LL

100 0. e-

"1300-

FAT

"2

"4

"6

"7

"11

200

100, O, 0

100

200

0

100

200

0

100

200

0

100

200

0

100

200

Time (minutes) Fig. 1. GH responses to GRF-29 in individual Iambs of lean (above) and fat (below) genotypes. Injections of GRF29 were given via the indwelling jugular catheter at 20 and 170 minutes after sampling began.

baseline plasma concentrations were detected about 100 minutes after the injection. Although all lambs responded to the second injection of GRF-29, this effect was greatly reduced in the fat genotype lambs. The induced pulse of GH was cleared from the plasma and baseline levels of GH were detected in the last two samples at 230 and 250 minutes after the study began. The data from the lambs was best modelled by fitting common non-linear and base parameters and individual scale parameters for each challenge of each genotype. The common parameter estimates and mean of the scale parameter estimates are given with standard errors in Table 1. Significant differences (Anova, p

Genetically lean and fat sheep differ in their growth hormone response to growth hormone-releasing factor.

The aim of this study was to compare the ovine growth hormone (oGH) responses of 5 genetically lean and 5 genetically fat 9 month old ram lambs (selec...
416KB Sizes 0 Downloads 0 Views