Characterization of the α<SUB>1B</SUB>-Adrenoceptors of Catfish Hepatocytes: Functional and Binding Studies

In catfish hepatocytes (Ictalurus punctatus) α₁-adrenergic activation by epinephrine or norepinephrine increased cytosol Ca²⁺ concentration. This effect was inhibited by α₁-adrenergic antagonists with the potency order WB4101 > benoxathian  5-methylurapidil > phentolamine > (+)niguldipine. Pretreatment with the irreversible antagonist, chloroethylclonidine, reduced the α₁-adrenergic effect. α₁-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 (K_d 100 pM) for the radioligand [¹²⁵I]HEAT was detected. Binding competition experiments showed the following orders of potency: (1) for agonists, oxymetazoline > epinephrine  norepinephrine > methoxamine; (2) for antagonists, prazosin > WB4101 > benoxathian  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 α₁-adrenoceptor present in catfish liver cells belongs to the α_1B subtype. Copyright 1995, 1999 Academic Press