Characterization of the alpha 1B-adrenoceptors of catfish hepatocytes: functional and binding studies

In catfish hepatocytes (Ictalurus punctatus) alpha 1-adrenergic activation by epinephrine or norepinephrine increased cytosol Ca2+ concentration. This effect was inhibited by alpha 1-adrenergic antagonists with the potency order WB4101 > benoxathian > or = 5-methylurapidil > phentolamine > (+)niguldipine. Pretreatment with the irreversible antagonist, chloroethylclonidine, reduced the alpha 1-adrenergic effect. alpha 1-Adrenergic stimulation also increased the resynthesis of phosphatidylinositol in whole cells. In liver membranes, a relatively small (33 fmol/mg of protein) number of sites with high affinity (Kd 100 pM) for the radioligand [125I]HEAT was detected. Binding competition experiments showed the following orders of potency: (1) for agonists, oxymetazoline > epinephrine > or = norepinephrine > methoxamine; (2) for antagonists, prazosin > WB4101 > benoxathian > or = 5-methylurapidil > phentolamine > (+)niguldipine. Membrane pretreatment with chloroethylclonidine markedly reduced [125I]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 present in catfish liver cells belongs to the alpha 1B subtype.