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 [3H]inositol trisphosphate production and calcium mobilization in rat-1 fibroblasts stably expressing the cloned alpha 1-adrenergic subtypes and their sensitivity to phorbol-12-myristate-13-acetate (PMA). It was observed that the three subtypes were able to activate this signal transduction process, but marked differences in efficacy were noted. The order of efficacy was alpha 1a > alpha 1b > alpha 1d in cells with similar receptor densities. The use of PMA blocked the alpha 1d responses at lower concentrations than those needed to block the alpha 1b action; little effect of PMA was observed on effects mediated through alpha 1a-adrenoceptors. These effects of PMA were not observed in cells preincubated overnight with the tumor promoter, suggesting that such actions were 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 overnight 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.