Atrial natriuretic factor release during volume expansion in the spontaneously hypertensive rat--effect of long-term hydralazine treatment

We tested the ability of spontaneously hypertensive rats (SHR) to release atrial natriuretic factor (ANF) during acute volume loading and its relationship to right and left atrial pressures. The effect of decreasing cardiac afterload by hypotensive treatment was also investigated. Fourteen to 15-week-old SHR and Wistar-Kyoto (WKY) rats were treated with hydralazine (12 mg/kg.day p.o.) for a period of 4 weeks. Untreated rats served as controls. The systolic blood pressure (BP) of SHR decreased to normotensive levels and cardiac hypertrophy was also reduced. Isotonic, iso-oncotic volume expansion (VE) was performed 3 times as 10% increments and at 15-min intervals. Despite greater changes in left-ventricular end-diastolic pressures (LVEDP) and to a lesser extent in central venous pressure (CVP) in SHR controls, the increases in plasma ANF (N-terminal concentrations) induced by VE were of a similar magnitude in both SHR and WKY control rats. The LVEDP and ANF C-terminal elevations were of a lower magnitude in treated SHR. Auricular ANF concentrations, measured at the end of VE, were lower in the left and higher in the right atrium in SHR versus WKY. It is concluded that despite a lower distensibility of their left atrium, ANF release is not impaired in SHR upon a VE. This adequate ANF release is obtained through higher increases in atrial pressures.