Identification of alpha-adrenergic receptors in catfish liver and their involvement in glucose release

This study aimed to characterize alpha-adrenergic receptors in catfish hepatocytes in which catecholamine-induced calcium transients have been observed. alpha-Adrenergic binding sites were studied in purified liver membranes using the specific alpha 1-receptor antagonist [3H]prazosin. At 22 degrees C specific binding of [3H]prazosin is saturable, reversible, and linear as a function of tissue concentration; the association reaches a maximum at 15 min, and the half-time for dissociation is about 4 min. Analysis of binding data suggests a single class of binding sites with a Kd of 1.6 nM and a Bmax of 182.1 fmol/mg protein. The relative potency of selective adrenoceptor ligands points to the presence in catfish liver of alpha-adrenergic receptors in addition to the beta-adrenergic receptor population previously characterized. Moreover, the binding data have been correlated to the glucose release from isolated hepatocytes suspended and perifused in a BioGel column, suggesting the involvement of alpha-adrenergic receptors in the glycogenolytic response to catecholamines in catfish hepatocytes.