The effect of genetic selection for milk yield on the response to growth hormone secretagogues in immature cattle.
Eighty 4-month-old calves of both sexes and of two selected lines differing by 70 kg in their predicted total yield of milk fat and protein were injected intravenously with three of four GH secretagogues: these were, per kg liveweight; (i) 0.2 micrograms human GH-releasing factor (1-29) (GRF), (ii) 0.2 micrograms TRH, (iii) a combination of (i) and (ii), and (iv) 0.1 g arginine hydrochloride. The response of GH was measured for 2 h following administration. Geometric mean concentration of the 5-, 10-, 15-, and 20-min samples following GRF, TRH and their combination were 29.3, 19.5 and 156 micrograms/l compared with baseline means of 6.5, 10.0 and 12.6 micrograms/l respectively, and for arginine (in which the mean response included the 30-min instead of the 5-min sample) 14.6 micrograms/l compared with a baseline of 8.31 micrograms/l. The line selected for greater yield responded more to each secretagogue by 1.53-fold following GRF (P less than 0.01), 1.34-fold following TRH (P less than 0.05), 1.11-fold following the combination (P greater than 0.01) and 1.26-fold following arginine (P less than 0.1). Females responded 2.3-fold more than males following GRF administration (P less than 0.001), only 1.2-fold more following TRH (P greater than 0.1), but less (0.63-fold) than males when GRF was combined with TRH (P less than 0.05). For all secretagogues the concentration of GH before administration was important in determining the size of response (P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)
Løvendahl
P
P
AFRC Institute of Animal Physiology and Genetics Research, Edinburgh Research Station, Roslin, Midlothian.
Angus
K D
KD
Woolliams
J A
JA
D000818:Animals; D001120:Arginine / Q000494:pharmacology; D002417:Cattle / Q000502:physiology*; D005260:Female; D013006:Growth Hormone / Q000097:blood / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D007774:Lactation; D008297:Male; D008892:Milk / Q000378:metabolism*; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism; D012641:Selection, Genetic; D012741:Sexual Maturation / Q000502:physiology*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology
D013973:Thyrotropin-Releasing Hormone; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D001120:Arginine