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Department of Medicine, University of Sydney, NSW, Australia.

1. In order to investigate the role of circulating serum factors in the altered renal haemodynamics and enhanced renal tubular transport observed in renal growth, micropuncture experiments were performed on normal animals infused with 20% plasma derived from animals in whom unilateral nephrectomy had been performed 3 days previously. 2. When animals infused with plasma from uninephrectomized animals (NxP) were compared with those infused with control plasma, the former had a higher tubular fluid flow rate measured at both the late proximal (LP; 26.7 +/- 1.6 vs. 18.4 +/- 1.4 nl min-1; P < 0.001) and early distal (ED; 14.9 +/- 1.0 vs. 7.8 +/- 1.0 nl min-1; P < 0.0001) sites, which was reflected in the final urine flow rate (16.1 +/- 3.4 vs. 5.1 +/- 0.8 microliter min-1; P < 0.005). 3. The single nephron glomerular filtration rate (SNGFR) was higher in animals infused with NxP as determined from samples taken at the LP (45.8 +/- 2.8 vs. 35.7 +/- 2.3 nl min-1; P < 0.01) and at the ED (34.5 +/- 2.5 vs. 28.1 +/- 1.8 nl min-1; P = 0.05) sites. However, this increase was not reflected in the whole kidney GFR (1.04 +/- 0.06 vs. 0.89 +/- 0.06; P = 0.07), suggestive of a preferential increase in filtration in the outer cortical nephrons. 4. Tubular Na+ transport was higher in the animals infused with NxP as evidenced by a decrease in the fractional delivery of Na+ at the ED site (4.5 +/- 0.4 vs. 6.5 +/- 0.6% of the filtered load; P < 0.05). However, in the final urine there was a significant increase in the urinary sodium excretion in animals infused with NxP (0.67 +/- 0.14 vs. 0.29 +/- 0.09%; P < 0.05) indicating that natriuresis and probably diuresis was a result of inhibition of Na+ and water transport in the late distal tubule and collecting duct. 5. In conclusion, these experiments demonstrate that circulating factors induced by a reduction in renal mass significantly alter glomerular filtration and tubular Na+ transport.