Effects of cimaterol administration on plasma concentrations of various hormones and metabolites in friesian steers

DOMESTIC ANIMAL ENDOCRINOLOGY

Vol. 8(4):471-480,1991

EFFECTS OF CIMATEROL ADMINISTRATION ON PLASMA CONCENTRATIONS OF VARIOUS HORMONES AND METABOLITES IN FRIESIAN STEERS F. Chikhou*.**, A.P. Moloney*, F.H. Austin**, J.F. Roche** and W.J. Enright* *Teagasc, Grange Research Centre, Dunsany, Co. Meath and **Faculty of Veterinary Medicine, University College Dublin, Ballsbridge, Dublin 4, Ireland Received October 4; 1990

ABSTRACT The aim of the experiment was to determine the acute and chronic effects of the ~-agonist, cimaterol, on plasma hormone and metabolite concentrations in steers. Twelve Friesian steers (liveweight = 488 _ 3 kg) were randomly assigned to receive either 0 (control; n=6) or .09 mg cimaterol/kg body weight/day (treated; n=6). Steers were fed grass silage a d libitum. Cimaterol, dissolved in 140 ml of acidified distilled water (pH 4.2), was administered orally at 1400 hr each d. After 13 d of treatment with cimaterol or vehicle (days 1 to 13), all animals were treated with vehicle for a further 7 d (days 14 to 20). On days 1, 13 and 20, blood samples were collected at 20 min-intervals for 4 hr before and 8 hr after cimaterol or vehicle dosing. All samples were assayed for growth hormone (GH) and insulin, while samples taken at -4, -2, 0, +2, +4, +6 and +8 hr relative to dosing were assayed for thyroxine (T4), triiodothyronine (T3), cortisol, urea, glucose and non-esterified fatty acids (NEFA). Samples taken at -3 and +3 hr relative to dosing were assayed for IGF-I only. On day 1, cimaterol acutely reduced (P<.05) GH and urea concentrations (7.6 vs 2.9 __. 1.4 ng/ ml; and 6.0 vs 4.9 _+0.45 mmol/l, respectively; mean control vs mean treated _ pooled standard error of difference), and increased (P<.05) NEFA, glucose and insulin concentrations (160 vs 276 + 22 Ixmol/1, 4.1 vs 6.2 + 0.15 mmol/l and 29.9 vs 179.7 _+ 13.9 ~U/ml, respectively). Plasma IGF-I, T 3, T4 and cortisol concentrations were not altered by treatment. On day 13, cimaterol increased (P<.05) GH and NEFA concentrations (7.7 vs 14.5 _+ 1.4 ng/ ml and 202 vs 310 _+22 mEq/1, respectively) and reduced (P<.05) plasma IGF-I concentrations (1296 vs 776 _ 227 ng/ml). Seven-d withdrawal of cimaterol (day 20) returned hormone and metabolite concentrations to control values. It is concluded that : l) cimaterol acutely increased insulin, glucose and NEFA and decreased GH and urea concentrations, 2) cimaterol chronically increased GH and NEFA and decreased IGF-I concentrations, and 3) there was no residual effect of cimaterol following a 7-d withdrawal period. INTRODUCTION Treatment with ~l-adrenergic agonists (BAA), such as clenbuterol, L,644-969 or cimaterol, results in a dramatic increase in muscle accretion with a concurrent reduction in adipose tissue depots in sheep and cattle (1, 2, 3, 4, 5). However, the mechanism(s) of action of BAA have not been fully defined. Clenbuterol has been shown to enhance lipolysis and to inhibit lipogenesis both in vitro a n d in v i v o (6, 7, 8) but the effects on protein metabolism and turnover have been inconclusive. It has been suggested (9) that protein synthesis in rats was increased by clenbuterol, but others (10) have failed to demonstrate such an effect, and suggested that the protein anabolic effect of BAA may be the result of decreased protein degradation. It is well known that skeletal muscle and lipid accretion can Copyright © 1991 Butterworth-Heinemann

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CHIKHOU ET AL.

be influenced by a number of hormones and metabolites (11, 12, 13). Thus, part of the response to BAA may be due to changes in concentrations of important hormones and metabolites or sensitivity to these (14). The objective of our experiment was to determine the acute and chronic effects of cimaterol administration on blood concentrations of various hormones and metabolites in Friesian steers.

MATERIALS AND METHODS Animals and M a n a g e m e n t . Twelve Friesian steers (liveweight = 488 + 3 kg) were housed in a well-ventilated building and penned individually for 2 d before and throughout the 25-d experimental period. Steers were handled frequently before the experiment started to facilitate ease of subsequent oral administration. The animals were exposed to natural photoperiod (December) and ambient temperature (4.5 to 13 C), were fed good quality silage ad libitum and had free access to water. The silage was composed of Italian and Perennial ryegrass, had a pH of 3.85, a crude protein content of 137 g per kg dry matter (DM) and an in vitro DM digestibility of 629 g per kg. Steers received approximately 100% of energy (12.9 MCal/day) and 200% of rumen degradable protein (395 g/day) requirement for maintenance (15). Treatments. Steers were assigned at random to receive either 0 (n=6) or .09 (n=6) mg cimaterol/kg body weight/day. The dose chosen was the same as that used in a previous experiment (4) where significant effects were observed. The experiment had three phases as follows : a) a pretreatment phase of 5 d (days -4 to 0) during which all animals received oral administration of vehicle, consisting of 140 ml of distilled water with a pH of 4.2 (due to the addition of acetic acid), b) a treatment phase of 13 d (days 1 to 13) during which steers received cimaterol/vehicle orally at 1400 hr daily. Oral administration was effected with the aid of a dosing gun (Phillips 150 ml mk 1 non automatic drencher; N.J. Phillips Pty. Ltd., NSW, Australia). The cimaterol was made up by dissolving the calculated daily dose for each steer in 140 ml distilled water with the pH adjusted to 4.2 with acetic acid to allow complete solubilization, and c) a post-treatment phase of 7 d (days 14 to 20) where all steers received vehicle orally. Blood Sampling. Blood samples were collected on days 1, 13 and 20. Steers were fitted with an indwelling jugular catheter one d before sample collection. Blood samples (10 ml) were collected into EDTA (1.5 mg/ml blood) containing tubes at 20-min intervals for 12 hr, starting at 1000 hr and ending at 2200 hr. Plasma was decanted after centrifugation at 1000 x g for 20 min and was stored at -20 C until assayed. H o r m o n e and Metabolite Assays. Plasma growth hormone (GH) concentrations were assayed by a double antibody radioimmunoasssay (16). Bovine GH (NIH-GH-B 18) was used as a reference standard and guinea pig antibovine GH (NIH antibovine antiserum, code No. AFP CO 123080; 1:10,000 working dilution) as first antibody. Bovine GH (NIH-GHB 18) was iodinated by a described procedure (17). Sample volume assayed was 50 ~tl. The intra- and interassay coefficients of variation (CV; n=10) were 5.0, 2.1 and 4.9% and 14.7, 2.5 and 8.1%, respectively, for pools of plasma that contained 4.5, 9.4 and 16.4 ng GH/ml. Characteristics of plasma GH concentrations were determined using the procedure of McLeod and Craigon (18). Briefly, GH profiles were examined and the following criteria were used for defining a pulse : the assay error standard deviation (SD) was taken to be the pooled error SD after assay of 10 replications of each of the three control plasma. A single point was identified as a pulse if it exceeded the previous nadir by 6 times the SD. Two or more consecutive points were considered a pulse when each point exceeded the previous nadir by 3 times the SD in the case of two points and by 2 times the SD in the case of 3 or more points. The baseline value of GH for an animal on each sampling day was calculated as the mean of GH concentrations of those samples not involved in a pulse. The amplitude

CIMATEROL, HORMONES AND METABOLITES IN STEERS

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of a GH pulse was considered to be the highest concentration of that pulse minus the baseline value for that day. Plasma concentrations of immunoreactive insulin-like growth factor-I (IGF-I) were determined in samples collected at -180 and + 180 min relative to cimaterol/vehicle administration using the radioimmunoassay described by Spicer et al (19); >95% of binding protein activity was removed by acid-ethanol extraction (20). First antibody was supplied by NIDDK and used at a dilution of 1:18,000. Standards were supplied by Amgen Biologicals (Thousand Oaks, CA). All samples were assayed in duplicate in one assay and the intraassay CV was 11.7%. Plasma insulin was quantified by a double-antibody radioimmunoassay as described by Hampton et al (21) using bovine insulin as reference standard (99.99% pure, biological activity of 26.9 ~tU/ng; NOVOBIOLABS, Bagsvaerd, Denmark), guinea pig antiporcine insulin antisera as first antibody (1:30,000 working dilution; Scottish Antibody Production Unit, Lanarkshire, Scotland) and cellulose coated antibody (SacCell, Wellcome Ireland Ltd., Dublin, Ireland) as second antibody. The sensitivity of the assay was 3 ~U/ml. The intraand interassay CV (n=10) were 7.5, 6.2 and 8.1% and 11.8, 14.3 and 10.2%, respectively, for three pools of plasma that contained 4.3, 15.4 and 92.4 laU insulin/ml. Total thyroxine (T4) and triiodothyronine (T3), cortisol, non-esterified fatty acids (NEFA), glucose and urea concentrations in plasma were determined in samples collected at -240, -120, 0, +120, +240, +360 and +480 min relative to cimaterol/vehicle administration. Thyroxine was quantified using a double-antibody radioimmunoassay as described by Nye et al (22). The intra- and interassay CV (n=8) were 9.3, 4.3 and 4.5% and .2, .2 and .3%, respectively, for three pools of plasma that contained 61,120 and 307 ng T4/ml. Triiodothyronine was quantified using a double-antibody radioimmunoassay as described by Ratcliffe et al (23). The intra-and interassay CV (n= 10) were 3.5, 4.5 and 4.1% and 2.5, 5.3 and 4.6%, respectively, for three pools of plasma containing 1.2, 1.9 and 3.6 ng T3/ml. Cortisol was quantified using an antibody-coated tube kit (Cambridge Medical Technology, Billerica, MA). Intra- and interassay CV (n=8) were 2.8, 4.0 and 12.1% and 9.4%, 10.2 and 12.9%, respectively, for three pools of plasma containing 3.2, 10.0 and 14.6 ng cortisol/ml. Glucose and plasma urea were determined using kits obtained from Boehringer Corp. Ltd. (Dublin, Ireland). The intra- and interassay CV (n=10) for glucose were 2.3 and 3.5% and for urea were 3.4 and 5.7%, respectively. Non-esterified fatty acids were assayed using the NEFA C kit (WAKO Chemicals USA Inc., Dallas, TX) with oleic acid as standard; 1 ~Eq NEFA/I=I ~tmol oleic acid/l; as modified by McCutcheon and Bauman (24). The intra- and interassay CV (n=12) were 3.9 and 8.9%, respectively. Statistical Analyses. Statistical analyses of all data were conducted using split-split plot analyses of variance (25) for a randomized complete block design with repeated measurement, where treatment and day were the main effects. Within the main effects, means were tested using the least significant difference test (25). Data were analysed, using the same basic model, to test for differences between pre- and post dosing mean concentrations of the hormones and metabolites on each day. Data are reported as least squares means with the pooled standard error of the difference. RESULTS Plasma Hormones. Changes in GH concentrations in response to cimaterol administration are presented in Table 1. Treatment did not affect the difference between pre- and post dosing concentrations of GH on any day. On day 1, cimaterol reduced (P<.05) post dosing mean and basal plasma GH concentrations (7.6 and 3.5 ng/ml in controls vs 2.9 and 2.3 ng/ ml in cimaterol steers, respectively). Cimaterol reduced (P<.05) pulse amplitude of GH post

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dosing to only 33% that of controls. Mean GH post dosing on day 1 in cimaterol steers (2.9 ng/ml) was 20% (P<.05) of the respective value on day 13 (14.5 ng/ml). Similarly, mean GH post dosing in controls (7.7 ng/ml) was 53% (P<.05) that of cimaterol steers (14.5 ng/ ml) on day 13. On day 13, cimaterol increased (P<.05) basal GH (controls 64% that of cimaterol steers) and GH pulse amplitude (controls 53% that of cimaterol steers). Pulse frequency was not affected by treatment. On day 20, there were no differences in GH characteristics between treatments. Plasma GH profiles for a representative steer in each treatment group on days 1, 13 and 20 are presented in Figure l(a), (b) and (c), respectively. Pre- and post dosing mean plasma concentrations of insulin and IGF-I are presented in Table 2. Cimaterol increased (P<.05) post dosing insulin on day 1, but by day 13 insulin in the cimaterol steers had decreased to that of controls. Cimaterol had no effect on IGF-I on day 1. On days 13 and 20, post-dosing IGF-I was reduced (P<.05) in the cimaterol steers compared to controls. Cortisol (Table 2), T 3 and T 4 (Table 3) concentrations were not different between treatments on any sampling day. Plasma Metabolites. Effects of cimaterol on plasma glucose, NEFA and urea concentrations are presented in Table 4. Cimaterol increased (P<.05) post dosing glucose on day 1 but not on days 13 or 20. Cimaterol increased (P<.05) post dosing NEFA on days 1 and 13 but not on day 20. Cimaterol also increased pre-dosing NEFA on day 13. Cimaterol reduced (P<.05) post dosing plasma urea on day 1 but not on days 13 or 20. DISCUSSION Plasma GH was decreased within 8 hr of oral administration of cimaterol. Comparable studies on the acute effect of BAA administration on plasma GH in domestic species are few. In calves, oral administration of 25 and 2700 ~tg/kg bodyweight of the BAA P-5369 and T-3660, respectively, resulted in a reduction in plasma GH within 1 to 6 hr and 4 to 6 hr of administration, respectively (26). Similarly, in adult rats, intraventricular injection of 30 I.tg of the BAA, isoproterenol, reduced plasma GH within 15 to 30 min of administration (27). These data suggest that stimulation of the [3 adrenergic system can acutely inhibit

TABLE 1. CHARACTERISTICSOF PLASMAGROWTHHORMONE SECRETION DURING4 HR BEFORE AND 8 HR AFTER DOSING OF FRIESIAN STEERS WITHCIMATEROLOR VEHICLEON DAYS l AND 13 OF TREATMENT, AND 7 D AFTER THE END OF TREATMENT(DAY 20) A. Day of

Treatment

Treatment

Group

Overall

Basal

(ng/ml) Predosing

(ng/ml)

Post dosing Difference

Pulse Amplitude

Pulse Frequency

(ng/ml)

(number/4 hr)

PrePost PrePost dosing dosing dosing dosing

Predosing

Post dosing

1

Control Cimaterol

8.8 8.2

7.6 2.9

-1.2 -5.3

5.6 4.9

3.5 2.3

12.3 12.1

19.3 6.5

2.2 2.2

1.1 .7

13

Control Cimaterol

9.7 17.9

7.7 14.5

-2.0 -3.4

7.9 12.5

3.5 5,0

9.7 21.0

11.7 22.2

2.2 1.5

1.3 1.4

20

Control Cimaterol

10.0 14.9

6.0 8.7

-4.0 -6.2

6.9 8.3

4.4 3.1

10.2 20.0

11.0 16.8

2.3 2.0

1.2 1.3

2.2

1.4

2.3

2.8

.5

3.8

2.3

.6

.2

Significance ~ T r e a t m e n t Day

* **

NS ***

NS NS

NS *

NS *

** NS

NS NS

NS NS

NS *

Interaction

*

**

NS

NS

*

NS

***

NS

NS

SED b

N = 6 steers per treatment. Steers were blood sampled at 20-min intervals during the 12 hr period. h SED = Pooled standard error of the difference. * = P<.05; ** = P<.01; *** = P<.001: NS = not significant (P>.05).

CIMATEROL, HORMONES AND METABOLITES IN STEERS

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T i m e (h) Figure 1. Plasma growth hormone profiles in two representative Friesian steers fed either cimaterol (m m) or vehicle (D o) at 1400 hr ( indicated by arrows) and blood sampled at 20 rain intervals for 4 hr before and 8 hr after treatment on days l(a) and 13(b) of treatment, and 7 d after the end of treatment [day 20 (c)].

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CHIKHOU ET AL.

TABLE 2. PLASMA CONCENTRATIONSOF INSULIN, INSULIN-LIKEGROWTH FACTOR-I (IGF-I) AND CORTISOL BEFORE AND AFTER DOSING OF FRIESIAN STEERS WITH CIMATEROL ON DAY 1 AND nAY 13 OF TREATMENT AND 7 D AVFER CIMATEROE WITHDRAWAL (DAY 20) "~, Day of Treatment

Treatment Group

Insulin (BU/ml)

IGF-I (ng/ml)

PrePost Difference dosing dosing

Cortisol (ng/ml)

PrePost Difference dosing d o s i n g

PrePost Difference d o s i n g dosing

Control Cimaterol

26.8 14.7

29.9 .179.7

764 939

596 705

- 168

15.4

-234

15.1

8.8 18.2

-7.1

165.0

13

Control Cimaterol

9.2 4.9

14.3 9.0

5.2 4.1

567 400

648 388

81 -12

11.2 10.8

8.9 9.5

-2.3 -1.3

20

Control Cimaterol

7.5 7.8

11.2 9.5

3.7 1.7

908 659

890 615

- 18 -44

8.1 7.6

9.3 8.(1

I. I .3

SED b

5.7

13.9

14.6

165

114

159

3.7

3.2

4.5

NS ** NS

*** *** ***

*** *** ***

NS *** *

NS * NS

NS NS NS

NS NS NS

NS NS NS

NS NS NS

Significance~ Treatment Day Interaction

3.1

3.(1

"N = 6 steers per treatment. Steers were b l o o d s a m p l e d at 2 0 - m i n intervals d u r i n g the 12 hr period. IGF-I a n d cortisol c o n c e n t r a t i o n s were d e t e r m i n e d in s a m p l e s collected at - 1 8 0 and + 1 8 0 and at -240, -120, 0, + 120, + 240, + 3 6 0 a n d + 4 8 0 min, respectively relative to cimaterol administration. bSED = Pooled s t a n d a r d e r r o r o f the difference. ~* = P<.05; ** = P<.01; *** = P<.001; NS = not significant (P>.051.

TABLE 3. PLASMA CONCENTRATIONSOF THYROXINE (T 4) AND TRBODOTHYRONINE(T~) BEFORE AND AFTER DOSING OF FRIESIAN STEERS WITH CIMATEROL ON DAY 1 AND 13 OF TREATMENT AND 7 D AFTER CIMATEROt, WITHDRAWAL (DAY 20) a. Day of Treatment

Treatment Group

T~ (ng/ml)

T, (ng/ml)

Predosing

Post dosing

Difference

Predosing

Control Cimaterol

71.6 77.6

79.6 84.7

7.8 7.1

1.0 1.4

1.1 1.4

.04 .03

13

Control Cimaterol

67.0 66.8

75.4 71.1

8.4 4. I

1.0 1.5

1. I 1.3

.05 -.21

20

Control Cimaterol

63.9 53.6

76.1 61.0

12.0 7.4

1. I 1.4

1.2 1.4

.08 .05

8.7

8.6

3.0

.2

.2

.01

NS *** *

NS *** *

NS NS NS

NS NS NS

NS NS NS

NS NS NS

SED b Significance~ Treatment Day Interaction

Post dosing

Difference

aN = 6 steers per treatment. T 4 and T~ c o n c e n t r a t i o n s were determined in b l o o d samples collected at -240, -120, 0, + 1 2 0 , + 2 4 0 , + 3 6 0 a n d + 4 8 0 min relative to cimaterol administration. bSED = P o o l e d s t a n d a r d e r r o r of the difference. ¢* = P<.05; *** = P<.001; NS = not s i g n i f i c a n t (P>.051.

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TABLE 4. PLASMA CONCENTRATIONSOF GLUCOSE, NON-ESTERIFIED FATTY ACIDS (NEFA) AND UREA BEFORE AND AFTER DOSING OF FRIESIAN STEERS WITH CIMATEROL ON DAY l AND 13 OF TREATMENT AND 7 D AFTER C1MATEROL WITHDRAWAL (DAY 20) a.

Day of Treatment

Treatment Group

Glucose (mmol/l)

NEFA (BEq/I)

PrePost Difference dosing dosing

Urea (mmol/l)

PrePost Difference PrePost Difference dosing dosing dosing dosing

1

Control Cimaterol

4.3 4.3

4.1 6.2

-0.22 1.96

210 191

160 276

-53 85

4.8 4.6

6.0 4.9

1.2 .3

13

Control Cimaterol

4.1 4.0

4.2 4.1

.07 .00

266 439

202 310

-63 -129

4.6 4.2

5.3 4.7

.7 .5

20

Control Cimaterol

4.0 3.9

4.0 3.9

-0.05 -0.02

245 286

176 189

-68 -97

4.4 3.7

5.5 4.8

1.1 1.2

SED b

.12

.15

.15

39

30

27

.4

.4

.3

NS NS NS

*** *** ***

*** *** ***

* *** ***

*** *** ***

NS *** ***

NS NS NS

* NS NS

* NS NS

Significancec Treatment Day Interaction

aN = 6 steers per treatment. Glucose, NEFA and urea concentrations were determined in blood samples collected at -240, -120, 0, + 120, +240, +360 and +480 min relative to cimaterol administration. bSED = Pooled standard error of the difference. c, = P<.05; *** = P<.001; NS = not significant (P>.05).

the release of GH, but the mechanism responsible is not clearly understood. However, stimulation of the 13adrenergic receptors of the hypothalamic-peptidergic neurons stimulated somatostatin secretion from the hypothalamus which resulted in inhibition of GH secretion (28). Recent studies by Krieg et al (29) confirmed these findings and excluded the possibility of involvement of the pituitary gland directly. In contrast, O'Conner et al (30) demonstrated that dietary administration of cimaterol at 10 ppm to Iambs caused no acute (12 hr) changes in GH, despite the fact that large acute changes in glucose, insulin and NEFA were observed. Possibly, acute changes may only occur when BAA are administered acutely by oral dosing as opposed to being fed over time. In contrast to its acute effect, chronic administration of cimaterol increased GH. A chronic increase in GH in response to cimaterol treatment was observed also in lambs fed 10 ppm cimaterol for 6 wk (14) (though the authors concluded that this effect may have resulted from feed withdrawal prior to sample collection), and in bulls fed 4 ppm cimaterol for 28, 84 or 196 d (31). However, no effect on GH was observed in rats after 16 d of subcutaneous injection of 1 mg clenbuterol/kg body weight (9), in Iambs fed 10 ppm cimaterol for 3, 6 (30) and 12 wk (14), or in steers fed 33, 49.5 or 66 mg cimaterol/head/day for 85 d (4). Chronic elevation of GH in some studies may suggest that continuous exposure of the ~-adrenergic receptors of the hypothalamic-peptidergic neurons to B A A induced homologous desensitization and uncoupling of the receptor from adenylate cyclase (32) and consequent cessation of the stimulatory effect on somatostatin secretion. Concomitantly, cimaterol may have stimulated the anterior pituitary directly resulting in enhanced GH secretion. Indeed, it has been suggested that BAA may have physiological significance as GH-releasing factors distinct from the GH-releasing factor peptide (33). In the present study, insulin was increased by over 600% within 8 hr of cimaterol administration which was accompanied by a substantial increase (54%) in glucose. Our data are similar to those reported in studies with lactating sheep infused intravenously with adrena-

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CHIKHOU ET AL.

line (34), in lambs following abomasal infusion of cimaterol (35) and in calves after oral dosing with the BAA T-3660, P-5369 and Q-2636 (26, 36). The hyper-insulinemic response observed on day 1 is higher in magnitude than that observed in these other studies. This may have been due to differences in BAA and dose used. The increase in glucose may have resulted from stimulation of hepatic and skeletal muscle glycogenolysis (34), from development of insulin resistance (35) or from a sparing effect of increased NEFA turnover (37). The fact that insulin and glucose were not significantly altered by treatment on day 13 demonstrate that the acute and chronic effects of cimaterol feeding are not similar. Acute administration of cimaterol did not affect plasma IGF-I but chronic administration reduced IGF-I. These results are in agreement with studies in lambs (14) and calves (26). The reduction in IGF-I is in contrast to the increased GH observed in cimaterol steers on day 13. However, it is possible that the large decrease in insulin from day 1 to 13 in cimaterol steers may reduce IGF-I since diabetes lowers plasma IGF concentrations in the rat (38) and pancreatectomy reduces IGF-I concentrations in fetal sheep (39). T r i i o d o t h y r o n i n e , T 4 and cortisol concentrations were not affected by cimaterol, either acutely or chronically. Similarly, in lambs, neither T3, T 4 (35) nor cortisol (14) were altered by cimaterol. Also, T 3 w a s not altered in calves treated with the BAA T-3660, P-5369 and Q-2636 (26, 36). However, in another study in lambs, cimaterol chronically elevated T 3 and T 4 by 11% and 25%, respectively (14). In contrast, in cattle, cimaterol chronically reduced W3 and T 4 by an average of 22 and 18%, respectively (31). The increase in NEFA observed on days 1 and 13 of cimaterol treatment is consistant with those previously observed following acute and chronic administration of BAA (14, 36, 40). Acute elevation in NEFA suggests that treatment with cimaterol results in enhanced lipolysis while chronic elevation suggests continued lipolysis, reduced re-esterification and/ or reduced uptake of free fatty acids by adipose tissue. The effect of BAA in stimulating lipolysis by interacting with [3-receptors of adipocytes has been well documented (6, 41, 42). The observed reduction in plasma urea by cimaterol is in contrast with the findings of Eisemann et al (40) who reported no change in plasma urea in steers following either acute or chronic treatment with clenbuterol. But, our findings are in agreement with those of Quirke et al (4) who reported a dose-related reduction in plasma urea in steers fed cimaterol and Beermann et al (35) who observed a similar effect of cimaterol in sheep. The reduction in plasma urea may indicate that the amount of nitrogen released due to protein degradation is reduced or that protein synthesis is increased. The data presented in this paper demonstrate the major differences between acute and chronic effects of cimaterol on plasma concentrations of selected hormones and metabolites. The changes observed support the concept that BAA alter metabolism of several body tissues to improve lean body growth, previously observed in response to cimaterol feeding (4, 5). Our data suggest that increases in muscle protein and decreases in lipid deposition are achieved, at least in part, by the following sequence of events. Acute cimaterol administration elicits a hyper-glycemic response which in turn increases insulin secretion, which may cause the acute reduction in plasma urea. Concomitantly, cimaterol increases NEFA by binding to ~-receptors on the adipocytes. The greater supply of NEFA is used as an energy source, thereby sparing glucose and amino acids, and conserving protein. In the chronic situation, the ~-receptors in the liver and pancreas become refractory to cimaterol (therefore no effect on insulin secretion), while at the same time increasing GH. Increased GH, coupled with the continued mobilization of stored fatty acids and reduced amino acid degradation, promotes skeletal muscle growth and reduces fat accretion. Other mechanism(s) of action may also play a role in increasing lean body growth in concert with the shift in metabolism observed in our study.

CIMATEROL, HORMONES AND METABOLITES IN STEERS

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ACKNOWLEDGEMENTS/FOOTNOTES The authors are indebted to N. Hynes (UCD), A. McArthur, V. McHugh and D. Prendiville (Teagasc) for their skilled technical assistance. We thank Dr. L.J. Spicer for IGF-I assay, the National Hormone and Pituitary Program of NIDDK (Univ. of Maryland School of Med., Baltimore) for IGF-I antiserum, Dr. D. Harrington (Teagasc) for statistical analyses and M. Smith for typing the manuscript. Address all correspondence to " Dr. A.P. Moloney, Teagasc, Grange Research Centre, Dunsany, Co. Meath, Ireland.

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