BRAIN NATRIURETIC PEPTIDE IS INDUCED BY ALPHA(1)-ADRENERGIC AGONISTS AS A PRIMARY RESPONSE GENE IN CULTURED RAT CARDIAC MYOCYTES

To better understand the molecular basis for increased atrial natriure tic factor (ANF) and brain natriuretic peptide (BNP) expression during overload-induced cardiac hypertrophy, we studied the induction of the genes in primary myocardial cells by the alpha(1)-adrenergic agonist, phenylephrine (PE), a potent hypertrophic agent. PE augmented the tra nscription of both genes to similar extents, although the time course of mRNA accumulation differed. Increases in ANF mRNA were evident only after 6-8 h of PE exposure, when transcript levels were 2-4-fold over control. However, similar increases in BNP mRNA were observed as soon as 1 h of PE exposure. Moreover, while ANF mRNA levels continued to i ncrease through 24 h of PE treatment, maximal levels of BNP mRNA (8-10 -fold over control) were observed at 4 h, after which transcript level s declined to about 3-fold over control. The early induction of the BN P mRNA by PE was independent of protein synthesis, whereas the late in duction of both genes required protein synthesis. Interestingly, the e arly BNP induction was only partially blocked by the transcription inh ibitor, actinomycin D, indicating that, in part, the inductive effects of PE might be the result of transcript stabilization. Indeed, the BN P transcript, which was shown to possess a half-life of less than 1 h in control cells, was stabilized by the addition of PE, while the ANF transcript possessed a half-life of at least 24 h under all conditions . These data indicate that the induction of BNP by alpha(1)-adrenergic agonists has characteristics of both a primary and secondary response gene, while ANF is a typical secondary response gene. Moreover, alpha (1)-adrenergic stimulation enhances BNP expression through both transc riptional activation and transcript stabilization, while ANF expressio n is enhanced primarily transcriptionally.