Effect of acute vascular fluid volume expansion on erythrocyte sodium transport in essential hypertension. 
Evidence exists that volume expansion is associated with the appearance of a circulating sodium transport inhibitor.  We have evaluated intra-arterial blood pressure (BP), central venous pressure (CVP), plasma renin activity (PRA), intraerythrocyte sodium content, erythrocyte sodium influx and rate constant of sodium efflux in 10 untreated primary hypertensive men (WHO stages I and II).  The investigations were done during baseline conditions and after rapid intravenous infusion of 1 litre of saline (0.9% NaCl solution) over 15-20 min.  Volume expansion caused an increase in CVP by 6.0 +/- 0.5 cmH2O (p less than 0.01), while BP only exhibited a slight increase.  No significant changes in intraerythrocyte sodium content, sodium influx, sodium efflux rate constant or PRA were found after volume expansion compared to baseline.  All patients with low normal PRA experienced a decrease in sodium efflux rate constant after volume expansion.  We found a positive correlation between baseline PRA and change in sodium efflux rate constant after volume expansion (r = 0.62, p less than 0.05).  At baseline the relationship between PRA and intraerythrocyte sodium content nearly reached statistical significance (r = 0.63, p = 0.054).  These results may indicate that acute volume expansion influences the release of a circulating factor, modulating sodium transport in low-renin hypertension.