Effect of oral protein loading on renal hemodynamics in human pregnancy

Glomerular filtration rate (GFR) and ERPF increase approximately 50% in human pregnancy. To determine if pregnant women have additional "renal reserve," inulin and p-aminohippurate clearances (Cin, CPAH) were measured in maximally hydrated women before and after a 300-g steak meal, once during late gestation, and again > or = 3 mo postpartum. Protein loading increased Cin [106 +/- 5 (SE) to 119 +/- 4 ml/min, P < 0.003], but not CPAH (587 +/- 35 to 624 +/- 32 ml/min, NS) in the nonpregnant state, but neither clearance was altered during gestation (Cin: 156 +/- 7 to 160 +/- 9.6 ml/min, NS; CPAH: 831 +/- 36 to 899 +/- 37 ml/min, NS). A natriuresis occurred only postpartum (+142 mu eq/min, P < 0.02), which could be explained by the increased GFR alone, since indexes of filtrate delivery and reabsorption (V/GFR, CH2O/GFR, CH2O/V) and fractional sodium excretion changed little. Dopamine excretion, uninfluenced by protein, did not correlate with increments in GFR. A carbohydrate meal (time controls) had no effect on the above described parameters. We make the following conclusions. If protein and pregnancy achieve hyperfiltration by similar mechanisms, these pathways appear "exhausted" in gestation. Also, oral protein loading does not measure maximal renal reserve, since basal GFR in late gestation was substantially greater than that measured after protein feeding in nonpregnant subjects.