Renal and hormonal responses to prolonged atrial stretch

The relationship between prolonged (6 h) atrial stretch, urine output, and plasma atrial natriuretic factor (ANF) was investigated in conscious rats. On inflation of a balloon at the superior venal caval-right atrial junction, urine volume and sodium output increased. However, this renal response was no longer apparent after the first hour, and there were no further changes in urine output when the balloon was deflated. This transient renal response was most marked during the first 30 min of balloon inflation, at which time plasma ANF was also elevated. At 6 h, plasma ANF was still elevated, although urine output had, by then, returned to normal. On deflation of the balloon, plasma ANF levels returned to normal. It is clear that the renal and hormonal responses to atrial stretch may be temporally uncoupled. This probably reflects adaptation of the atrial volume receptors in the absence of such an effect on ANF release. Although this might be interpreted to mean that ANF is not of physiological importance in fluid and electrolyte balance, a more plausible explanation is that ANF needs the backing of the reflex pathways arising from stimulation of the atrial receptors to express its natriuretic activity.