EFFECT OF PHORBOL-MYRISTATE ACETATE ON ALPHA(1)-ADRENERGIC ACTION IN CELLS EXPRESSING RECOMBINANT ALPHA(1)-ADRENOCEPTOR SUBTYPES

We studied the ability of norepinephrine to stimulate [H-3]inositol tr isphosphate production and calcium mobilization in rat-1 fibroblasts s tably expressing the cloned alpha(1)-adrenergic subtypes and their sen sitivity to phorbol-12-myristate-13-acetate (PMA). It was observed tha t the three subtypes were able to activate this signal transduction pr ocess, but marked differences in efficacy were noted. The order of eff icacy was alpha(1a) > alpha(1b) > alpha(1d) in cells with similar rece ptor densities. The use of PMA blocked the alpha(1d) responses at lowe r concentrations than those needed to block the alpha(1b) action; litt le effect of PMA was observed on effects mediated through alpha(1a)-ad renoceptors. These effects of PMA were not observed in cells preincuba ted overnight with the tumor promoter, suggesting that such actions we re mediated through isoform or isoforms of protein kinase C subjected to down-regulation. The repetitive addition of norepinephrine induced desensitization of the alpha(1)-adrenergic action in cells expressing any of the three subtypes, and this effect was not altered by overnigh t pretreatment with PMA. Our data indicate that there are differences in sensitivity to PMA among the alpha(1)-adrenoceptor subtypes stably expressed in rat-1 fibroblasts.