1. Recently implanted kidneys may have decreased flexibility to adjust sodium excretion to volume challenges, since modulation by renal sympathetic nerve activity is absent. To examine this hypothesis, we studied the natriuretic response to head-out water immersion in eight patients with well-functioning renal allografts of 37 days (range 24–56 days), at a time when renal re-innervation has probably not occurred.

2. By the third hour of head-out water immersion, sodium excretion had increased equally in the patients (from 120 +21 to 204 +37 μmol/min) and in eight healthy control subjects (from 105 +9 to 191+19 μmol/min).

3. Glomerular filtration rate was 60 + 6 ml/min in the patients and 113 +6 ml/min in the control subjects, and did not change upon head-out water immersion. Estimated renal plasma flow increased upon head-out water immersion in the control group but not in the patients. Blood pressure decreased similarly in both groups. The renal vascular resistance, calculated from these data, decreased in response to head-out water immersion in the control subjects but not in the renal transplant patients.

4. Head-out water immersion suppressed plasma renin activity only in the normal group, whereas the plasma aldosterone level was suppressed in both groups. The natriuretic response in patients was associated with about 3-fold elevated plasma levels of atrial natriuretic peptide.

5. Since renal re-innervation at 37 days after transplantation is very unlikely, these data suggest that inact renal innervation is not mandatory for a normal natriuretic response to head-out water immersion in humans. However, sympathetic modulation might be involved in the decrease in renal vascular resistance and plasma renin activity normally observed during immersion.

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