PITUITARY PORCINE GROWTH HORMONE (pGH) AND A RECOMBINANT pGH ANALOG STIMULATE PIG GROWTH

PERFORMANCE IN A SIMILAR MANNER 1

C. M. Evock 2, T. D. Etherton 2, C. S. Chung 3 and R. E. Ivy 4

The Pennsylvania State University, University Park 16802 and Pitman-Moore Inc., Terre Haute, IN 47808

ABSTRACT

This study was conducted to establish the extent to which different doses of pi tui tary porcine growth hormone (ppGH) increase pig growth performance. Pigs were treated daily for 11 wk with 0, 35 or 70 gtg ppGH/kg BW. In addition, these effects were compared with those produced by treating pigs with 0, 35, 70 or 140 #g �9 kg BW -1 �9 d -1 of a recombinantly derived analog of porcine growth hormone (rpGH). This analog lacks the first seven amino acids at the NH2 terminus. Growth rate was increased similarly by ppGH and rpGH (the maximal increase was 19%). Feed efficiency was improved by ppGH and rpGH (the maximal response was 25%). This improvement in feed efficiency was associated with a decrease in feed intake (17% with the largest dose of rpGH). Both ppGH and rpGH decreased adipose tissue growth and increased muscle mass. Carcass lipid was decreased by 68% in pigs treated with the largest dose of rpGH. The recombinant pGH analog appeared to be less potent than ppGH in decreasing adipose tissue growth rate. All other parameters measured, however, indicated that rpGH mimicked the biological effects of ppGH (including binding to pig liver membranes and induction of insulin-like growth factor I production). Sensory panel evalua- tions indicated that neither ppGH nor rpGH affected pork palatability. Larger doses of pGH (> 70 gtg/kg BW) adversely affected pig mobili ty. This impairment in mobil i ty appears to be due to osteochondrosis. Our findings establish that the rpGH analog is equipotent to ppGH in stimulating growth performance and that pigs can be treated without any significant ad- verse effects when they are treated with less than 70 gtg of pGH �9 kg BW -1 �9 d -1 . (Key Words: Pigs, Growth, Somatotropin, Carcass Composition, Growth Factors.)

Introduction

Previous studies from our laboratory (Chung et al., 1985; Etherton et al., 1986, 1987) and others (Machlin, 1972) have established that

~Authorized for publication as paper no. 7802 in the journal series of The Pennsylvania Agric. Exp. Sta. We thank Vern Hazlett and David Hosterrnan for swine care and Jim Watkins, Donald Butts, Gerald Smeal and John Ziegler for animal slaughter and assist- ance with the collection of carcass data. We also are grateful to Karen Magri, Martin Sillence, Paul Walton and James Wiggins for assistance with hormone injec- tions and Roland Leach, Dept. of Poult. Sci., for evaluation of gross morphology of bones and cartilage.

2Dept. of Dairy and Anita. Sci., The Pennsylvania State Univ. Address reprint requests to T. D. Etherton.

3Present address: Dept. of Anim. Sci., Choongbuk National Univ., Cheongju 310, South Korea.

4Pitman-Moore Inc., Terre Haute, IN. Received November 2, 1987. Accepted March 7, 1988.

treating pigs daily with pi tui tary porcine growth hormone (ppGH) markedly enhances growth rate, improves feed efficiency, decreases adipose tissue growth and increases muscle growth. Although our studies have shown that ppGH alters growth performance and have provided information about the effects of various doses of pGH on response, the studies were conducted for only 30 to 35 d. In addition, there is no evidence that establishes that recombinant porcine growth hormone (rpGH) is equipotent to ppGH in stimulating growth performance of pigs. Because of this we con- ducted the present s tudy to establish the effects of treating pigs long-term (77 d) with either ppGH or rpGH. Pigs were treated with different doses of the hormones in order to establish a more definitive dose-response relationship than the one reported in our earlier study (Etherton et al., 1987).

1928 J. Anim. Sci. 1988.66:1928-1941

ANABOLIC EFFECTS OF pGH IN PIGS

Materials and Methods 10-

Because this is the first s tudy reported using ~. a recombinantly derived pGH in a growth trial, several experiments were conducted to deter- ~. z.5- mine the relative biological potency of rpGH vs ppGH. These studies were important because the rpGH used is not a perfect construct of .~ 5- ppGH in that it lacks the first seven amino acids ~- at the NH2 terminus of the molecule. To

2.5- compare the biological effects of ppGH and rpGH several experiments were conducted using pigs or pig tissues. We felt that using this homologous system was more relevant than 0- comparing the two proteins in a rat bioassay. The bioassays used to compare the two sources of pGH were 1) a specific pGH radioreceptor assay (RRA) using pig liver membranes (Chung and Etherton, 1986); 2) the ability of rpGH and ppGH to increase serum insulin-like growth factor I (IGF-I) concentrations in pigs after 7 d of pGH treatment (70 #g �9 kg BW -1 �9 d - l ) ; and 3) an in vitro incubation with pig adipose tissue explants to measure the relative inhibition of insulin action by pGH (which is a specific metabolic effect of pGH; Walton and Etherton, 1986; Walton et al., 1986). Finally, a comparison was made of the ability of ppGH and rpGH to bind to antiserum (guinea pig) raised against ppGH (Chung et al., 1985).

Both ppGH s and rpGH 6 were equipotent in inhibiting binding of [12SI]ppGH to liver microsomes (Figure 1). Serum IGF-I concentra- t ion was elevated comparably (data not shown) in pigs treated for 7 d with either ppGH or rpGH (blood samples were taken on d 7 at 6 h postinjection). Both rpGH and ppGH inhibited the ability of insulin to maintain lipogenic capacity in cultured pig adipose tissue (Figure 2). This effect was observed irrespective of the body weight of the pigs when the adipose tissue biopsy was taken. Although physiological concentrations of both pGH sources inhibited insulin action, rpGH was less potent than ppGH when data were pooled over the three weight groups. There also were differen~zes between ppGH and rpGH in their ability to bind to the guinea pig antiserum (the concentration of unlabeled ppGH or rpGH that half-maximally inhibited binding of tabeled ppGH was 1.3 and 2.7 ng/ml, respectively). Thus, the ppGH and rpGH used in the present study differ in their

s Lot AFP-9164-C, donated by Pitman-Moore, Inc. 6 Lot 117-043, donated by Pitman-Moore, Inc.

1929

EDso 0---~ ppGH 1.8 ng/ml ~. = rpGH 1.8 ng/ml

0 .3 3 30 I00 pGH (ng/ml)

Figure 1, Compar ison o f recombinant porcine growth hormone (rpGH) and pituitary porcine growth hormone (ppGH) in a specific pGH radioreceptor assay. The hormone preparations were incubated with pig liver microsomes and tracer ([ ms l]ppGH) for 24 h at 23 ~ C (Chung and Etherton, 1986). Microsomes were treated with 4 M MgCI 2 prior to the binding assay to remove endogenously bound pGH. The bar associated with each mean is the SE. N = 6 livers per treatment.

antigenic regions. The observations that rpGH mimicked the biological effects of ppGH in three bioassays, however, led to our hypothesis that ppGH and rpGH should stimulate growth performance in a comparable manner.

To test this hypothesis, seventy-two York- shire-Duroc barrows (27 kg BW) were randomly allocated to six t reatment groups (control; 35 /~g ppGH �9 kg BW -1 �9 d - 1 ; 7 0 / a g p p G H �9 kg BW -1 �9 d -1- 35/ /g rpGH �9 kg BW -1 �9 d -1 70 /.tg rpGH �9 kg BW - t �9 d -x ; 140/.tg rpGH - kg BW -1 �9 d - l ) . The doses selected were based on the results of a previous short-term (35 d) dose-response study (Etherton et al., 1987) in which the largest dose was 70/~g ppGH/kg BW. We decided that the largest dose in the present study would be increased twofold above that used in the previous study (Etherton et al., 1987). We did not use a dose of 140/ag ppGH/kg BW because of limited availability of the hormone. Pigs were fed a corn-soybean diet formulated to contain 18% protein (Etherton et al., 1986). Dietary protein content was increased above NRC (1979) recommendations and lysine was added (.5%) to ensure adequate amino acid availability if pGH treatment decreased feed intake. Because of space availability, pigs were housed two per pen. Pigs were treated daily between 0900 and 1000 with the respective hormone or vehicle for 77 d by i.m. injection in the extensor muscle of the neck. The ppGH and rpGH were dissolved as described by Chung et

1930 EVOCK ET AL.

800 -

600 -

400 -

200 -

0 7~ 1400]

I

period I (7Ok(J)

- - " - - - - ~ 5 0 I',~ c~---o ppGH 7,, G. ,16

I i , , "~'-~ period 2 (85 kg)

~ - ~ 5_9 • -'~..'~ o---o ppGH .9 ,~,~ rpGH 1.6

, + o o

; . . . . . . .

:E c 1400- ~ period 3 (105 kg)

1200- ~'--\ "~ o---o ppGH ~ - I000- ~',~ \ = : rpGH 1.0

800- '~"\'t" 600 -

o . 1 I I0 I00 pGH (ng/ml)

Figure 2. Effects of pituitary porcine growth hor- mone (ppGH) and recombinant porcine growth hor- mone (rpGH) on insulin action in cultured pig adipose tissue explants. Adipose tissue biopsies were obtained from six pigs at three different weights. Thin tissue explants were prepared and cultured for 48 h in the presence of 10 ng/ml of insulin, 50 ng/ml of hydro- cortisone and various concentrations of pGH (Walton and Etherton, 1986; Walton et al., 1986).

al. (1985). Stock solutions were made every 2nd d and stored at 4 ~ C.

To determine the effects of pGH on selected hormone and metabolite concentrations, blood samples were taken by venipuncture 3 h post- pGH injection on d 49. Blood samples also were collected at slaughter (d 77). The data presented

7The pigs were killed 24 h after the last pST injec- tion to comply with the requirements established by the Food and Drug Administration.

are for the d-49 samples because of the uniform time between pGH treatment and blood samp- ling. The results were essentially the same between the two sampling times, with the exception of serum pGH values. The values were much lower on d 77 because the pigs were killed 24 h after the last pGH injection 7. Serum samples were assayed for glucose, blood urea-N, insulin, pGH and IGF-I (Chung et al., 1985; Etherton et al., 1987).

On d 77, the pigs were transported to The Pennsylvania State University Meats Laboratory. Eight pigs per treatment were killed to obtain carcass information. Immediately after death, selected organs were removed quickly and weighed. The anterior pituitary was weighed and frozen for subsequent measurement of pGH concentration (Chung et al., 1985; Ether- ton, 1986). After the carcasses had chilled for 48 h, standard carcass measurements were made. Carcass composition was determined as described by Etherton et al. (1982). To deter- mine the effects of pGH on meat quality a sensory evaluation was conducted by a trained sensory panel (methods described in Table 7 footnotes).

As the experiment progressed, it became evident that some of the pigs treated with 70 or 140 /2g pGH/kg BW developed mobility prob- lems. These pigs had difficulty in getting up, limped, and, when standing, were buck-kneed. Because of these observations, blood samples were taken to measure plasma Ca and P concen- trations. In addition, when the pigs were killed the tibia and femur were examined from each pig. Samples were taken from the mid-shaft of the bones for measurement of Ca and P. Bones also were sliced longitudinally in wafers 2 to 5 mm thick in order to determine shaft thickness and examine the epiphyseal plate. The epiph- yseal growth plates and bones were examined to determine if any difference in gross morpho- logical appearance was apparent (Hill et al., 1984).

Data were analyzed by the GLM procedure of SAS (1979). Initial body weight was included in the model as a covariate for analysis of growth performance, muscle and adipose tissue mass data. Other factors in the model were source of hormone, dose of hormone and source x dose interaction. Pen was the experi- mental unit for feed/gain ratio. For all other analyses, the experimental unit was pig. Multiple regression analysis (Ryan et al., 1976)was used to generate the least squares regression equations

ANABOLIC EFFECTS OF pGH IN PIGS 1931

TABLE 1. EFFECT OF PITUITARY PORCINE GROWTH HORMONE (ppGH) AND RECOMBINANT PORCINE GROWTH HORMONE (rpGH) ON

PIG GROWTH PERFORMANCE a

pGH, ~g/kg BW

Item Source 0 35 70 140 SE

Final wt, kg ppGH 97 b 112 c 109 c 2.3 rpGH 97 b 108 c 113 c 107 c 2.8

ADG, kg ppGH .88 b 1.01 c .98 c .03 rpGH .88 h .98 c 1.05 d .99 cd .03

apigs were treated with the noted doses of pGH daily for 77 d by i.m. injection, n = 12 per treatment. b'c'dMeans in a row without a common superscript differ (P < .05).

that describe the relationship between biological response and dose. Means were separated in all analyses using linear contrasts (SAS, 1979).

Results

Both ppGH and rpGH increased ADG in a similar manner (Table 1, Figure 3a). At the low dose of pGH, ADG was increased 11%, whereas the maximal increase was 19%. Growth rate was similar among t reatment groups until wk 6 of the trial (Figure 4). After this, it was evident that growth rates diverged to the extent that pGH-treated pigs grew faster for the remainder of the experiment. Feed efficiency was markedly improved by ppGH and rpGH (Table 2, Figure 3b). There was little effect of pGH during the first 5 wk of the study (Table 2). During the remaining 6 wk ppGH and rpGH had a pro- nounced beneficial effect on feed/gain. During this period, feed efficiency was improved as much as 33%. Over the entire experiment feed efficiency was improved about 25% by the highest dose of pGH. Dose-response analyses (Figure 3b) indicated that the maximal effect of pGH occurred at approximately 90 /.tg of pGH/kg BW. The improvement in feed efficiency in part was due to a significant decrease in feed intake (Table 3). Thus, ppGH and rpGH in- creased growth rate even though feed intake was decreased.

As expected, pGH dramatically decreased fat depth and increased loin-eye area (LEA) in a dose-related manner (Table 4). There was a 40 to 50% decrease in fat depth and a 46% increase in LEA. Although backfat depth was decreased, adipose tissue growth in different anatomical regions was differentially affected by pGH. For example, pGH decreased adipose tissue growth

(measured as backfat depth) 44% over the last lumbar vertebra, 50% over the last rib and only 22% over the first rib (Table 4). Furthermore, there was a significant effect of pGH source in the regression analysis for the effects of pGH on carcass lipid and adipose tissue mass (Table 5, Figure 5a). Recombinant pGH did not decrease adipose tissue growth to the extent that ppGH did. This difference in potency paralleled the results obtained when the effects of ppGH and rpGH were compared in the adipose tissue bioassay.

Although we did not determine carcass composit ion of any pigs at the start of the experiment, it is apparent that the larger doses of pGH essentially stopped adipose tissue accretion. This is illustrated by the 68% decrease in carcass lipid observed in pigs treated with 140/ag of rpGH/kg BW (Table 5). This decrease in carcass lipid was associated with a concurrent increase in carcass water, protein and muscle (Table 5, Figure 5). Both sources of pGH stimulated muscle growth similarly.

Porcine growth hormone not only affected adipose tissue and muscle growth but also stimulated organ growth. Heart, liver and kidney weights were increased by both ppGH and rpGH (Table 6). Adrenal weight also was increased on an absolute basis (Table 6) and after adjusting for differences in body weight (data not shown).

Nothing has been published about the effects of pGH treatment on meat quality. To establish whether pGH affected meat quality, we performed a sensory evaluation. When pigs were treated for 11 wk with pGH there was a significant decrease in marbling and texture (Table 7). Firmness and color were not affected. Results from the sensory evaluation

1932 EVOCK ET AL.

(:1

1"500 t rpGH y=.88+.OO4x-.O00021x z r2=.41 (SE=.05) ppGH y= .88 + .O06x- .O0006x 2 r2=.40(SE=.05)

1'3001 o

~176 o I.I w a ~ I1~ . , ' ~ 1 7 6 1 7 6 " ' "

�9 9oo 0o

. 7 0 0 ~

. 5 0 0 ~ , , , I i I I

0 20 40 60 80 I00 120 140

4.000 rpGH y = 3.:3-.0146x + .O0006x 2 r 2 =. 70 (SE =.06) ppGH y = 3.3 -.0196x + .O0012x 2 r 2 =. 74 (SE =.06)

3.500

"~ ' " ' . . 8 ,, s .ooo ....

G)

8 2.500 - J

0

8 o

2 . 0 0 0 i , , , , , ,

0 20 40 60 80 I00 120 140 pGH (p.g/kg)

Figure 3. Effect of pituitary porcine growth hormone (ppGH) (zx) and recombinant porcine growth hormone (rpGH) (o) on ADG (a) and feed/gain (b).

ANABOLIC EFFECTS OF pGH IN PIGS 1933

o 115 115 -

I05 /! 105- /

95 o~o control / 95 -

. . . . 35 g,kgBwppG. S / 85 =,---= 70/.r 8WppG H . ' " 8 5 -

55 55

45 45

3 5 - / , l 35

b

0"

o---o cGntrol - . 35p.g/kg BW rpGH / / ?

140/.Lg/kg BWrpGH ~ '~ ~

2 5 . . . . . 2 5 t . . . . . ' ' ' ' ' { I o i o, 23456 89,o

Week of experiment Week of experiment

Figure 4. Growth curves of pigs treated daily for 77 d with pituitary porcine growth hormone (ppGH) (a) and recombinant porcine growth hormone (rpGH) (b).

pane l ind ica ted t h a t the re was a m o d e s t decrease in t enderness t h a t was grea te r for p p G H t h a n for r p G H (Table 7). Ju ic iness was no t a f fec ted b y pGH t r e a t m e n t , whereas the re was some change in f lavor. Given the numer i ca l score for the averages, however , i t was a p p a r e n t t h a t the sensory panel did no t d e t e c t any m a r k e d d i f fe rences b e t w e e n mea t f r o m con t r o l and pGH- t r ea t ed pigs (a numer i ca l score of 5 ind ica ted t h a t the sample was ne i the r l iked n o r disl iked).

The se rum me tabo l i t e s and h o r m o n e s measu red r e s p o n d e d in a m a n n e r tha t , in general , agreed w i th ou r previous s tudies (Table 8). The on ly no t ab l e excep t ions were the e x t e n t to which pGH af fec ted se rum glucose and insulin. In the p re sen t s tudy , b o t h glucose and insul in were e levated c o n s i d e r a b l y - m o r e t h a n in our previous s tudies . The p i tu i t a ry c o n t e n t of pGH was decreased in a dose-depen- d e n t m a n n e r by exogenous p p G H and r p G H (con t ro l = 7.6/ag p G H / m g p i tu i t a ry ; 35 p p G H =

TABLE 2. EFFECTS OF PITUITARY PORCINE GROWTH HORMONE (ppGH) AND RECOMBINANT PORCINE GROWTH HORMONE (rpGH) ON FEED/GAIN a

pGH, ug/kg BW

Weeks Source 0 35 70 140 SE

1 to 5 ppGH 2.4 b 2.3b 2.2 b .06 rpGH 2.4 b 2.4 b 2.3bc 2.2 c ,.07

5 to 8 ppGH 4.2 b 3.2 c 2.8 d .13 rpGH 4.2 b 3.3 c 3.0 cd 2.8 d .10

8 to 11 ppGH 4.1 b 3.0 c 2.8 c .14 rpGH 4.1 b 3.1 c 2.6 d 2.7 d .09

Overall ppGH 3.3 b 2.8 c 2.5 d .06 rpGH 3.3b 2.9 c 2.6 d 2.5 d .06

apigs were treated daily for 77 d with the noted dose of pGH, n = 12 per treatment.

b'c'dMeans in a row without a common superscript differ (P < .05).

1934 EVOCK ET AL.

TABLE 3. EFFECTS OF PITUITARY PORCINE GROWTH HORMONE (ppGH) AND RECOMBINANT PORCINE GROWTH HORMONE (rpGH) ON FEED INTAKE a

pGH,/~g/kg BW

Weeks Source 0 35 70 140 SE

1 to 5 ppGH 79.0 b 78.6 b 79.5 b 2.7 rpGH 79.0 b 79.5 b 80.0 b 74.5 b 1.8

5 to 8 ppGH 70.9 b 65.5 c 57.7 d 1.8 rpGH 70.9 b 64.6 c 62.7 c 55.5 d .9

8 to 11 ppGH 80.9 b 75.9 b 57.2 c 2.7 rpGH 80.9 b 75.4 c 7 0 . 9 c 61.8 d .9

Overall ppGH 231 b 220 b 195 c 5.5 rpGH 231 b 219 bc 213 c 192 d 3.2

avalues are kilograms of feed consumed for the time interval.

b'c'dMeans in the same row without a common superscript difrer (P < .05).

TABLE 4. EFFECTS OF PITUITARY PORCINE GROWTH HORMONE (ppGH) AND RECOMBINANT PORCINE GROWTH HORMONE (rpGH) ON

SELECTED CARCASS PARAMETERS a

pGH, ~g/kg BW

Item Source 0 35 70 140 SE

Dressing, % ppGH 72.7 f 72.1fg 70.7g .6 rpGH 72.7 f 71.5fg 70.5g 70g .6

1R fat b, cm ppGH 3.7 f 3.3fg 2.9g .23 rpGH 3.7 f 3.5 f 3.5 f 2.6g .22

LR fat c, cm ppGH 2.4 f 2.2 f 1.0g .23 rpGH 2.4 f 2.0fg 1.7g 1.2 h .23

LL fat d, cm ppGH 2.7 f 2.2 f 1.5g .18 rpGH 2.7 f 2.2fg 1.8g 1.3 h .19

Skinned ham, kg ppGH 7.6 f 9.5g 9.3g .3 rpGH 7.6 f 9.0g 9.3g 9.3g .3

LEA e, cm 2 ppGH 28 f 34g 36g 1.4 rpGH 28 f 35g 34g 41 h 1.4

aEight pigs per treatment were kilIed for determination of carcass composition.

bFat depth dorsal to first rib.

CFat depth dorsal to last rib.

dFat depth dorsal to last lumar vertebra.

eLoineye area,

f'g'hMeans in a row without a common superscript differ (P < .05).

ANABOLIC EFFECTS OF pGH 1N PIGS 1935

TABLE 5. EFFECTS OF PITUITARY PORCINE GROWTH HORMONE (ppGH) AND RECOMBINANT PORCINE GROWTH HORMONE (rpGH) ON CARCASS COMPOSITION a

pGH, ug/kg BW

Item Source 0 35 70 140 SE

H20 , % ppGH 50.4 b 56.7 c 64.8 d 1.0 rpGH 50.4 b 58.5 c 58.5 c 68.1 d 1.0

Lipid, % ppGH 34.2 b 25.1 e 15.8 d 1.3 rpGH 34.2 b 23.8 c 23.2 c 10.9 d 1.1

Protein, % ppGH 14.1 b 16.4 e 18.2 d .31 rpGH 14.1 b 16.8 e 17.3 c 19.3 d .30

Ash, % ppGH .72 b .80 bc .90 c .05 rpGH .72 b .82 b .81 b 1.03 c .04

Adipose tissue, kg ppGH 16.9 b 15.1 b 8.7 c 1.2 rpGH 16.9 b 13.5 c 13.8 c 5.7 d 1.0

Muscle, kg ppGH 34.1 b 42.5 c 42.9 c 1.1 rpGH 34.1 b 40.8 c 43.5 c 42.3 c 1.5

aEight pigs per treatment were killed for determination of carcass composition. Percentage values are on a percent of soft-tissue (muscle and adipose tissue) basis.

b'c'dMeans in a row without a common superscript differ (P < .05).

3.0; 70 ppGH = 2.1; 35 rpGH = 4.1; 70 rpGH -- 4.0 and 140 rpGH = 1.6, respectively). The mobi l i ty p rob lem no ted as the exper iment progressed was related to an increased incidence of os teochondros is (Table 9). The p redominan t lesion observed was character ized by focal deep zones of hyper t roph ied chondrocy tes prot ruding into the metaphysis . There were no differences in Ca or P concent ra t ion of bones or serum among the t r ea tment groups (data no t shown).

Discussion

The observat ions that rpGH st imulated growth performance, improved feed eff ic iency and altered carcass composi t ion provide the first evidence for pigs that a r ecombinan t ly derived growth ho rmone elicits effects similar to those of the pi tui tary-derived hormone . However , there were some differences in po tency between ppGH and rpGH. For example, rpGH was less po t en t than ppGH in decreasing adipose tissue mass and percentage carcass lipid. Thus, for this particular biological response, the difference in s tructure be tween ppGH and rpGH affected func t ion of the hormone . It is n o t e w o r t h y that the in vi tro bioassay with adipose tissue also indicated that there were po tency differences. Based on this, it appears that the in vi tro assay that measures the abil i ty of pGH to antagonize insulin act ion (Walton

and Ether ton , 1986; Walton et al., 1986) is a sensitive m e t h o d for determining the effects of s tructural modi f ica t ion of the ho rmone on biological action. For all o ther biological effects quant i f ied there were no differences be tween ppGH and rpGH (e.g., muscle mass, ADG, feed/gain, IGF-I concent ra t ion) . One explana- t ion for the difference in po tency between ppGH and rpGH on adipose tissue and no t muscle growth is that the mechanisms by which pGH affects adipose tissue growth differ f rom those affect ing muscle. Whether this is due to a difference in the s t ructure of the pGH receptor or a difference in the cascade of events leading to second messenger p roduc t ion and act ion are unknown.

Al though there is no in format ion available that indicates whether pGH receptors differ among target tissues, there is a precedent for the idea that mult iple pGH receptors occur wi thin a specific target tissue. In mouse liver membrane preparat ions, it has been shown through cross-linking studies that there are three receptor subtypes (Smith and Talamantes, 1987). Other studies with monoc lona l anti- bodies to human growth ho rmone (Thomas et al., 1987) and rabbit liver growth h o r m o n e receptor (Barnard et al., 1985) also have suggested the presence of mult iple receptor types for growth hormone . In particular, Thomas et al. (1987) suggested that the relative

1936 EVOCK ET AL.

0

45 ] rpGH y= 35.1 - .19x + .00025x 2 r 2=.85 ppGH y=54.2- 26x + . O 0 0 0 6 x 2 r2=.83

35

"~. o

= 25 8 U

15

5 0 2 0 40 6 0 80 I00 120 140

(SE. = 1.3) (SE=I.I)

C

3 0 - b

rpGH ppGH

y = 14. I + .07x - .0002 x 2 y =14.3 + .06x- .O002x 2

r 2 = .80 (SE =. :30) r 2= .80 (SE=.51)

2o

~.~ �9 o.'' ~ '''

I0 0 20 4 0 60 80 I00 120 140

pGH(p.g/kg)

Figure 5. Effect of pituitary porcine growth hormone (ppGH) (zx) and recombinant porcine growth hor- mone (rpGH) (o) on carcass lipid (a) and protein (b).

ANABOLIC EFFECTS OF pGH IN PIGS 1 9 3 7

TABLE 6. EFFECTS OF PITUITARY PORCINE GROWTH HORMONE (ppGH) AND RECOMBINANT PORCINE GROWTH HORMONE (rpGH) ON ORGAN WEIGHTS a

pGH, gg/kg BW

Item Source 0 35 70 140 SE

Adrenal, g ppGH 2.7 b 3.3bc 3.8 c .2 rpGH 2.7 b 3.4 c 3.1bc 4.1 d .2

Heart, g ppGH 300 b 352bc 407 c 17 rpGH 300 b 366 c 398 c 403 c 20

Liver, kg ppGH 1.8 b 2.1 c 2.3 d .1 rpGH 1.8 b 2.3 c 2.5 c 2.4 c .1

Kidney, g ppGH 169 b 217 b 286 c 13 rpGH 169 b 230 c 300 d 271 cd 21

aEight pigs per t rea tment were killed.

b 'e 'dMeans in a row wi thout a common superscript differ (P < .05).

TABLE 7. EFFECTS OF PITUITARY PORCINE GROWTH HORMONE (ppGH) AND RECOMBINANT PORCINE GROWTH HORMONE (rpGH) ON PORK QUALITY a

pGH, ug/kg BW

Item Source 0 35 70 140 SE

Marbling ppGH 1.5 c 1.5 c 1.0 d .15 rpGH 1.5 c 1.4 e 1.6 c 1.0 d .16

Texture ppGH 2.9 c 2.8 c 2.3d .15 rpGH 2.9 c 2.8 c 2.5 c 1.9 d .10

Firmness ppGH 2.4 a 2.4 a 2.4 a .18 rpGH 2.4 a 2.4 a 2.1a 2.4 a .17

Color ppGH 2.0 c 2.0 c 2. I c ,07 rpGH 2.0 c 1.9 c 2.0 e 2.0 c .06

Tenderness b ppGH 6.2 c 5.0 cd 4.4 d .5 rpGH 6.2 c 6.5 c 5.6 c 5.5 c .4

Flavor b ppGH 6.1 c 5.8 c 5.6 e .3 rpGH 6.1cd 6.5 c 6.2 cd 5.6 d .3

Juiciness b ppGH 5.6 c 5.4 c 5.5 c .4 rpGH 5.6 c 5.4 c 6.1 c 5.7 e .3

aMarbling, texture, firmness and color scores were determined by John Ziegler, Dept. of Food Science, The Pennsylvania State Univ. There were eight pigs per t rea tment for carcass quali ty measurements.

bTenderness, flavor and juiciness were determined by a six-member, trained sensory evaluation panel using a lO-cm line scale anchored at 0 = dislike ex t remely and at 10 cm = like extremely. Panelists scored the pork chops by making a vertical line on the horizontal scale. One-inch loin chops were cooked in a conventional oven to an internal temperature of 167~ (measured by a temperature probe).

C'dMeans in a row wi thout a common superscript differ (P < .05).

1938 EVOCK ET AL.

p r o p o r t i o n s of liver r ecep tors m a y vary ac- cord ing to the physiological s ta te of t he an imal and, perhaps , a m o n g species. The obse rva t ion t h a t the dose-response curves d i f fer wi th respec t to the m a x i m a l l y effect ive dose observed for ADG, feed /ga in and carcass c o m p o s i t i o n also argues t h a t the m e c h a n i s m s b y wh ich p G H affects g r o w t h and m e t a b o l i s m differ. In par t , th is d i f fe rence ma y be a f u n c t i o n of w h e t h e r the ef fec ts are med ia t ed d i rec t ly b y pG H or ind i rec t ly by IGF-I (F roesch e t al., 1986) .

It is i m p o r t a n t to recognize t h a t pGH is a h o r m o n e t h a t no t on ly s t imula tes g r o w t h ( somatogen ic effects) b u t also has m a r k e d m e t a b o l i c ef fec ts ( E t h e r t o n and Wal ton, 1986) . The ef fec ts o f pGH on adipose t issue g r o w t h in the pig are d i rec t ef fects n o t m e d i a t e d by IGF-I and appear to be due , in par t , to a p G H - d e p e n d e n t decrease in basal l ipogenic ra te (Walton et al., 1987) , g lucose t r a n s p o r t (K. Magri and T. E t h e r t o n , u n p u b l i s h e d d a t a ) a n d in the abi l i ty of insul in to s t imula te l ipid

TABLE 8. EFFECTS OF PITUITARY PORCINE GROWTH HORMONE (ppGH) AND RECOMBINANT PORCINE GROWTH HORMONE (rpGH) ON

SERUM METABOLITES AND HORMONES a

pGH, #g/kg BW

Item Source 0 35 70 140 SE

Glucose, mg/dl ppGH 90 e 113 f 118 f 3 rpGH 90 e 112 f 120 f 120 f 3

BUN b, mg,/dl ppGH 26 e 15.6 f 8.4g 2.5 rpGH 26 e 14.4 f 11.9 f 12.4 f 2.3

Insulin, ng/ml ppGH 1.1 e 3.0 f 5.1g .4 rpGH 1.1 e 3.0 f 4.6g 7.5 h .6

IGF-1 c, ng/ml ppGH 145 e 161 f 227g 5 rpGH 145 e 165 f 215g 265 h 7

pGH d, ng/ml ppGH 6.0 e 24.6 f 36.5g 4.0 rpGH 6.0 e 19.6 f 43.6g 94.5 h 4.5

aBlood samples were obtained by venipuncture on d 49. Blood samples were taken 3 h postinjection, n = 12. The results are similar to those obtained from blood collected at slaughter. The only exception was pGH values, which were much lower because samples were obtained 24 h after the last pGH injection.

bBlood urea-N.

Clnsulin-like growth factor 1.

dporcine growth hormone.

e'f'g'hMeans in a row without a common superscript differ (P < .05).

TABLE 9. INCIDENCE OF OSTEOCHONDROSIS IN PIGS TREATED WITH PORCINE GROWTH HORMONE (pGH) a

pGH, #g/kg BW

Item 0 35 70 140

ppGH 1/8 4/8 4/8 rpGH 1/8 3/8 3/8 5/8

aRatio presented is the number of cases per the number of pigs in the treatment. Although the values presented indicate incidence, they do not represent the severity of the lesions. Typically, the lesions were larger and more extensive for the two larger pGH doses. Mobility of pigs treated with 35 #g pST �9 kg BW -1 �9 d -1 was not af- fected. The incidence of osteochondrosis was increased significantly (P < .05) by pGH but did not differ be- tween rpGH and ppGH (P < .05). Procedure CATMOD of SAS was used.

ANABOLIC EFFECTS OF pGH IN PIGS 1939

synthesis and glucose oxidation (Walton et al., 1986, 1987). It also is likely that the decrease in energy intake contributes to the decrease in lipid synthesis. At this time it is our hypothesis, however, that the changes in adipose tissue metabolism that account for the decrease in adipose tissue growth are more the result of specific metabolic effects induced by pGH rather than strictly a function of a decrease in energy intake. Our rationale for this hypothesis is based on 1) our previous studies in which significant changes in adipose tissue growth occurred without any change in feed intake (Etherton et al., 1986, 1987) and 2) the finding that basal rates of fatty acid synthesis and insulin sensitivity of pig adipose tissue are markedly depressed after 7 d of treatment with ppGH or rpGH without any change in feed intake (Walton et al., 1987).

The effects that pGH has on muscle growth likely are the result of pGH directly and indirect- ly via IGF-I. The direct effects of pGH are probably associated with an increase in protein synthesis. Although the effects of growth hormone on protein synthesis are established in rats (Albertsson-Wikland and Isaksson, 1978; Albertsson-Wikland et al., 1979, 1980; Schwartz, 1982), there is little information available for domestic livestock. Eisemann et al. (1986) reported that whole-body protein synthesis was higher in heifers treated with bovine growth hormone. In addition to the effects that pGH may have on protein metabolism directly, it appears that IGF-I also affects protein metabolism in intact animals. In fasted rats, IGF-I inhibits protein degradation (Jacob et al., 1987). In conjunction with the effects that IGF-I may have on protein metabolism, it has been shown to increase satellite cell proliferation in culture (Dodson et al., 1985).

The finding that the effects of pGH were greater during the last 6 wk of the trial provides the first direct evidence to support the hypoth- esis (Etherton et al., 1987) that pigs that weigh more than 50 to 60 kg are more responsive to pGH than lighter pigs. From a practical perspec- tive, this indicates that pigs only need to be treated from this weight to market weight. From a scientific perspective, the mechanisms that account for this difference in responsiveness to pGH are unclear. Previously, we had specu- lated that adipose tissue responsiveness to pGH changed during growth and that this contributed to the change in response to pGH (Etherton et al., 1987). The fact that the concentration of

pGH necessary to half-maximally inhibit insulin action in adipose tissue (Figure 3) did not change appreciably as pigs grew, however, argues against this. Alternatively, other meta- bolic pathways such as lipolysis may be affected differently by pGH as pigs grow; however, there is little information available about the effects of pGH on lipolysis in the pig, or about how tissue sensitivity may change during growth.

The apparent effect of pGH on pig mobility after long-term treatment is interesting. In our previous studies we did not see this. However, those studies were shorter (30 to 35 d) and the pigs were not treated with doses so high as those used in the current study. Previously, Machlin (1972) had reported that some pigs treated with pGH developed arthritic-like symptoms. It is unlikely that this disorder is the result of inadequate dietary Ca or P because dietary content of these minerals exceeded NRC requirements. In addition, the fact that no differences in bone Ca or P were detected fur- ther supports the contention that this disorder in not the result of some impairment in bone mineralization. The clinical observations indicate that the osteochondrosis observed in the present study is similar to tibia dyschondroplasia in chickens (Freedman et al., 1985; Hargest et al., 1985a,b). This is an abnormality of the cartilage growth plate characterized by an unmineralized, nonvascularized plug of cartilage in the metaphysis. It appears that this disorder is the result of a failure of the chondrocytes to hypertrophy completely and undergo the changes necessary for mineralization and vascularization of cartilage to occur (Leach and Gay, 1987). It is not known whether this is an effect of pGH or is mediated by other physio- logical adaptations. Evidence to suggest that growth hormone is involved has been reported by Vasilatos-Younken and Leach (1986), who found that the amplitude of growth hormone secretory spikes was 50% higher in chickens with severe tibial dyschondroplastic lesions in comparison with normal birds.

It is encouraging that the sensory evaluation panel could not discern any appreciable differ- ences between rpGH- and ppGH-treated pork. Based on this, it appears that pGH treatment does not affect pork quality. However, informa- tion is needed about the effects that pGH has on meat properties such as water holding capacity and structure that are important during processing of pork.

In summary, a recombinantly derived analog

1940 EVOCK ET AL.

of p G H has been s h o w n to mimic the biological e f fec ts of p p G H in pigs t r ea t ed for 77 d. Before a rpGH-based p r o d u c t for the po rk i ndus t ry b e c o m e s a real i ty , a sys tem for del iver ing r p G H w i t h o u t daily in jec t ion is needed . Our under - s t and ing of the m e c h a n i s m s of pG H ac t ion also needs to be increased. This m a y resul t in s t rategies to e n h a n c e the biological e f fec ts of p G H and resolve the p r o b l e m s t h a t m a y arise w h e n pigs are t r ea t ed w i th large (> 70 /ag/kg BW) doses of pGH. The p resen t results, however , suggest t h a t g r o w t h p e r f o r m a n c e can be mark- edly e n h a n c e d w i t h o u t any s igni f icant adverse ef fec ts w h e n pigs are t r ea t ed wi th less t han 70 /ag of p G H �9 kg BW -1 �9 d - l .

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