In Vivo beta-adrenergic activation of atrial natriuretic factor (ANF) reporter expression

Isoproterenol (ISO) infusion increases ANF-mRNA levels and control of ANF expression lies at the level of transcription. In neonatal cardiomyocytes, previous investigations determined that the -125 to -100 region of the rat ANF 5' flanking region contained cis-elements critical for control of ISO induced ANF transcription. However, it is unclear if these same cis-elements regulate ANF transcription in vivo. To examine this question, reporter plasmids containing the ANF 5' flanking/promoter region were injected directly into the left ventricle. Following a recovery period, osmotic pumps were implanted to infuse vehicle or ISO (0.2 or 2.0 mg/kg/d). ISO significantly (p < .05) increased the LV/BW ratio in a dose dependent, but not a time dependent manner. ISO significantly (p < .05) increased ANF reporter expression in both a dose-dependent and time dependent manner. Injections into the midwall of the LV or into the apex did not lead to significant differences in ISO-induced ANF reporter expression. Using site-specific mutations of ANF reporter constructs, comparisons were made of ISO induced ANF transcription in vitro in neonatal cardiomyocytes and in vivo in the adult heart. Cis-elements critical for ISO activation in cultured cardiomyocytes were not essential for the increased expression of the ANF reporters in vivo. The results indicate that distinct differences in ANF transcriptional regulation exist in vivo in the adult heart as compared with neonatal cardiomyocytes, and suggest the recruitment of other signaling pathways beyond adrenergic-receptor mediated pathways.