Regulation of the rainbow trout (Oncorhynchus mykiss) hepatic β2-adrenoceptor by adrenergic agonists
2003
Dugan, S.G. | Lortie, M.B. | Nickerson, J.G. | Moon, T.W.
The characteristics of hepatic β2-adrenoceptors (AR) were examined in rainbow trout (Oncorhynchus mykiss) chased once per day to exhaustion for up to 7 days or fed the repartitioning agents clenbuterol (CLEN) or ractopamine (RACT) that function in mammals as β-agonists. A one-day chase and feeding the CLEN for 37 days resulted in a significant 27% and 33% decrease, respectively, in the number of CGP-binding sites (B(max)) with no significant change in affinity (K(d)) of hepatic β2-ARs. Despite the significant decrease in β2-AR numbers with CLEN feeding, no significant differences were found for either β2-AR mRNA levels or adenylyl cyclase (ACase) activities. In addition, CLEN displayed only partial agonist activities as it was found to be more effective at blocking isoproterenol-stimulated cAMP production in isolated hepatocytes than stimulating cAMP production. The small affects of RACT may be related to its low active stereoisomer content and low affinity for the trout β2-AR. Agonist regulation of the trout hepatic β2-ARs may involve down-regulation of the receptors without affecting responsiveness.
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