Alpha 2-adrenergic receptor-mediated regulation of growth hormone secretion in meal-fed Holstein steers.
Effects of activation or blockade of alpha 2-adrenergic receptors on serum growth hormone concentrations were studied in Holstein steers (115 +/- 4 days of age; 112 +/- 4 kg body wt). A pelleted diet was available ad libitum for 2 hr each day. Serum growth hormone concentrations were greater for 80 min immediately before feeding compared with 80 min immediately after removal of feed. Relative to saline-injected controls, activation of alpha 2-adrenergic receptors with clonidine (2 micrograms/kg body wt iv) before feeding rapidly increased serum growth hormone concentrations, but clonidine had no effect when administered after feeding. Compared with vehicle-injected controls, blockade of alpha 2-adrenergic receptors with either idazoxan (20 mg/kg body wt sc) or yohimbine (5 mg/kg body wt sc) decreased serum growth hormone concentrations before as well as after feeding. Feeding abolished the stimulatory alpha 2-adrenergic receptor-mediated increase in growth hormone secretion. Our data support the hypothesis that alpha 2-adrenergic receptor stimulation is an essential event mediating pulsatility of growth hormone secretion before feeding and is required to maintain basal concentrations after feeding.
Gaynor
P J
PJ
Department of Animal Science, Michigan State University, East Lansing 48824-1225.
Chapin
L T
LT
Lookingland
K J
KJ
Tucker
H A
HA
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D002417:Cattle / Q000502:physiology*; D003000:Clonidine / Q000494:pharmacology; D004435:Eating; D013006:Growth Hormone / Q000378:metabolism*; D008297:Male; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D013997:Time Factors
D000317:Adrenergic alpha-Antagonists; D011942:Receptors, Adrenergic, alpha; D013006:Growth Hormone; D003000:Clonidine