To evaluate the mechanisms of brain natriuretic peptide (BNP) gene expression, we determined the effect of acute cardiac overload (from 30 min to 4 h) on atrial and ventricular BNP mRNA levels in normal and hypertrophied myocardium. Arginine8 vasopressin (AVP; 0.05 microgram/kg.min) and l-phenylephrine (PHE; 20 micrograms/kg.min) were infused iv to increase cardiac workload in conscious spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. At the age of 10-22 months, during the established phase of ventricular hypertrophy, baseline BNP synthesis was increased in the hypertrophic ventricular cells of SHR, as reflected by about 2-fold (P