The role of insulin-like growth factor I in clenbuterol-stimulated growth in growing lambs.
We examined the role of IGF-I in muscle growth stimulated by a beta-adrenergic agonist, clenbuterol. Ewe lambs (90 d old, 20.4 kg mean live weight) were allotted to five groups. A pretreatment control group of five lambs was slaughtered immediately (0 d). The other four groups of six ewes ate freely for 38 or 80 d and were then slaughtered. Half those lambs received clenbuterol (400 micrograms.kg live weight-1.d-1) as a dietary supplement. Blood was collected at intervals from 19 d before supplementation began (0 d) until slaughter. Prerigor muscle samples were sectioned for detection of IGF-I receptors and myofibrillar ATPase activity. Carcass weights were slightly increased by treatment, whereas muscle weights (semimembranosus, gastrocnemius, and biceps femoris) were greatly increased (P < .001), up to 48% at 80 d for semimembranosus. Clenbuterol significantly decreased collagen concentration because myofibrillar proteins were preferentially produced. Collagen solubility was unaffected. Total RNA:total DNA in semimembranosus and gastrocnemius showed transcription was still stimulated between 38 and 80 d. Fiber type area analysis indicated a shift toward glycolytic metabolism, confirmed by iron measurements. However, clenbuterol did not change the portion of muscle occupied by each ATPase class, and the data indicated that type I fibers, though smaller, became relatively more numerous. In spite of significant muscle changes, plasma IGF-I was unaffected by clenbuterol. Similarly, there was no difference in the specific binding of [125I]IGF-I at slaughter between treated and control lambs. However, a response in the first few days of treatment, preceding visible hypertrophy, cannot be excluded.
Young
O A
OA
Meat Industry Research Institute of New Zealand (Inc.), Hamilton, New Zealand.
Watkins
S
S
Oldham
J M
JM
Bass
J J
JJ
D000318:Adrenergic beta-Agonists / Q000008:administration & dosage / Q000494:pharmacology*; D000818:Animals; D002976:Clenbuterol / Q000008:administration & dosage / Q000494:pharmacology*; D003094:Collagen / Q000032:analysis; D004247:DNA / Q000032:analysis; D005260:Female; D005527:Food, Fortified; D007334:Insulin-Like Growth Factor I / Q000032:analysis / Q000502:physiology*; D007457:Iodine Radioisotopes; D008460:Meat / Q000592:standards; D018485:Muscle Fibers, Skeletal / Q000737:chemistry / Q000648:ultrastructure; D018482:Muscle, Skeletal / Q000033:anatomy & histology / Q000737:chemistry / Q000187:drug effects; D009929:Organ Size / Q000502:physiology; D012313:RNA / Q000032:analysis; D011897:Random Allocation; D012756:Sheep / Q000254:growth & development* / Q000502:physiology; D013997:Time Factors
D000318:Adrenergic beta-Agonists; D007457:Iodine Radioisotopes; D012313:RNA; D007334:Insulin-Like Growth Factor I; D003094:Collagen; D004247:DNA; D002976:Clenbuterol