Contractile Activity Modulates Atrial Natriuretic Factor Gene Expression in Neonatal Rat Ventricular Myocytes

[Ca2+]itransients, and the activation of Ca2+-sensitive kinases have been considered potential signaling mechanisms regulating ANF gene expression in cultured neonatal rat ventricular myocytes (NRVM). However, it is unclear whether [Ca2+]iis directly involved, or is indirectly involved by generating additional mechanical signals via contractile activity. Primary cultures of spontaneously contracting NRVM (CON), and NRVM treated for 48 h with verapamil (V, 10μm), KCl (50 mm), or 2,3-butanedione monoxime (BDM, 7.5 mm) were used to delineate the affects of contractile activityv[Ca2+]i. Verapamil, a calcium channel blocker, inhibits contraction and decreases [Ca2+]i. High [K+]ocauses membrane depolarization, loss of contraction, and elevates [Ca2+]i; whereas BDM strongly inhibits contractile activity but only modestly reduces [Ca2+]itransients. ANF production, as assessed by radioimmunoassay, was significantly reduced upon contractile arrest independently of [Ca2+]ilevels. Northern blotting analysis demonstrated that contractile arrest also reduced ANF mRNA levels. Transient transfection of a 3003 bp ANF promoter-luciferase expression plasmid in CON, V, KCl, and BDM-treated NRVM demonstrated marked down-regulation of ANF promoter activity in all of the contractile arrested myocytes. These results indicate that the activation of Ca2+-sensitive processes alone are insufficient to maintain high levels of ANF gene expression and peptide production in NRVM.