Regulation of the black bullhead hepatic β-adrenoceptors

Black bullhead catfish (Ameiurus melas) were exposed to air for 1 h to examine the effect of an acute stress on the distribution and function of the hepatic β-adrenoceptors (β-ARs). Air exposure significantly reduced both adrenaline (ADR)- and noradrenaline (NADR)-stimulated glucose production in isolated hepatocytes with no effect on either receptor affinity (Kd) or number of binding sites (Bmax). A 24 h exposure of isolated hepatocytes to the β-agonist isoproterenol also had no significant impact on either binding parameter. Competition studies using selective agonists and antagonists suggest that the hepatic β-AR in this species is pharmacologically β2-like. However in addition to the β2-AR, molecular evidence provides support for the existence of hepatic β-ARs that phylogenetically group with the β3-ARs and the β1-ARs. Despite the presence of several potential phosphorylation sites in the third intracellular loop and cytoplasmic tail of the bullhead β2-AR, no significant changes were observed in the binding parameters. While physiological data supports the presence of only a single subtype, molecular data supports the existence of multiple β-AR subtypes in this species. The mechanisms thought to regulate mammalian β-ARs exist in the bullhead ARs reported here but these mechanisms are not as effective in this fish system as in mammals.