Four methods to recruit renal functional reserve.

The acute changes in creatinine clearance (Ccr) in response to intravenous amino acid infusion and to oral meat and milk protein meals were studied in seven healthy control subjects (Acute study). Para-aminohippurate clearance (CPAH), Ccr, inulin clearance (Cin), and plasma renin activity (PRA) were measured in seven different healthy control subjects following 1 week of low-protein diet, 0.7 g/kg per day, and again after one week of high-protein diet, 2.0 g/kg per day (Chronic study). In the acute study Ccr increased to a similar extent with each of the three stimuli, 20.1 +/- 6.2% (SD), 12.6 +/- 6.2% (SD) and 19 +/- 6.2% (SD) with amino acid infusion, 80-g meat protein and 80-g milk protein meals respectively. In the chronic study Ccr was 21%, Cin 26%, CPAH 8%, and filtration factor (FF) 14% greater during the high-protein diet than the low-protein diet, while PRA was increased by 43% on the high-protein diet. We conclude that an 80-g protein oral meal is probably sufficient to elicit the maximum acute increase in Ccr, since all three acute responses were similar, and similar in degree, to those previously reported, and may therefore be of value in measuring maximal filtration capacity. In contrast to one previous report, meat and milk proteins induced similar changes in Ccr. Thus milk protein cannot be regarded as a dietary substitute for meat in any diet designed to reduce glomerular hyperfiltration. Renin may play a part in the increase in glomerular filtration rate through the action of angiotensin II on the efferent arteriole, increasing filtration fraction.