CHARACTERIZATION OF THE ALPHA(1B)-ADRENOCEPTORS OF CATFISH HEPATOCYTES - FUNCTIONAL AND BINDING-STUDIES

In catfish hepatocytes (Ictalurus punctatus) alpha(1)-adrenergic activ ation by epinephrine or norepinephrine increased cytosol Ca2+ concentr ation. This effect was inhibited by alpha(1)-adrenergic antagonists wi th the potency order WB4101 > benoxathian greater than or equal to 5-m ethylurapidil > phentolamine > (+)niguldipine. Pretreatment with the i rreversible antagonist, chloroethylclonidine, reduced the alpha(1)-adr energic effect. alpha(1)-Adrenergic stimulation also increased the res ynthesis of phosphatidylinositol in whole cells. In liver membranes, a relatively small (33 fmol/mg of protein) number of sites with high af finity (K-d 100 pM) for the radioligand [I-125]HEAT was detected. Bind ing competition experiments showed the following orders of potency: (1 ) for agonists, oxymetazoline > epinephrine greater than or equal to n orepinephrine > methoxamine; (2) for antagonists, prazosin > WB4101 > benoxathian greater than or equal to 5-methylurapidil > phentolamine > (+)niguldipine. Membrane pretreatment with chloroethylclonidine marke dly reduced [I-125]HEAT binding. Good correlation was observed between the radioligand binding studies and the functional data with isolated cells. The present data suggest that the (alpha(1)-adrenoceptor prese nt in catfish liver cells belongs to the alpha(1B) subtype. (C) 1995 A cademic Press, Inc.