title: Developmental changes in the regulation of plasma growth hormone concentrations in Holstein calves.
abstract: A study was initiated to determine whether development of a functional ruminant digestive system was associated with alterations in plasma growth hormone (GH) concentration. Holstein bull calves were fed milk or milk with grain until studied at the age of 1 month (n = 12). Calves placed on pasture with some grain supplementation were studied at the age of 3 months (n = 6) to determine plasma GH concentration in an animal with fully developed ruminant metabolism. Blood samples were taken at 10-minute intervals for 5 hours, followed by administration of bovine GH-releasing factor (0.075 micrograms/kg of body weight) and subsequent blood sample collection for 1 hour. On the following day, a blood sample was collected via jugular cannula, clonidine (10 micrograms/kg) was administered, and blood samples were subsequently obtained. Data indicated that milk-fed calves had higher mean plasma GH concentration than did either milk/grain-fed or older calves. The difference in mean plasma GH concentration was related to higher secretory pulse amplitude. Pituitary responses to bovine GH-releasing factor did not differ among the 3 groups, but response to clonidine were greater in milk-fed calves than in calves of the other groups. These data indicate that the change from a nonruminant to a ruminant-type gastrointestinal tract, perhaps attributable to subsequent changes in metabolism, may induce changes in hypothalmic function to decrease GH concentration.
authors.affiliation: Department of Physiology and Pharmacology, College of Veterinary Medicine, Auburn University, AL 36849.
authors.forename: J L
authors.initials: JL
authors.lastname: Sartin
authors.forename: R J
authors.initials: RJ
authors.lastname: Kemppainen
authors.forename: K A
authors.initials: KA
authors.lastname: Cummins
authors.forename: D N
authors.initials: DN
authors.lastname: Marple
authors.forename: J C
authors.initials: JC
authors.lastname: Williams
mesh_terms: D000375:Aging / Q000502:physiology*; D000821:Animal Feed; D000818:Animals; D002417:Cattle / Q000254:growth & development* / Q000378:metabolism; D003000:Clonidine / Q000494:pharmacology; D004063:Digestion; D002523:Edible Grain; D013006:Growth Hormone / Q000097:blood* / Q000378:metabolism; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology; D008297:Male; D008892:Milk; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism*
chemical_list: D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D003000:Clonidine