Relationship between glomerular filtration rate and renal blood flow at different levels of protein-induced hyperfiltration in man

1. We studied the relationship between changes in glomerular filtration rate (GFR) determined as inulin clearance (CIn), and changes in renal blood flow (RBF), determined as p-aminohippurate clearance (CPAH), after the ingestion of a large (1.35±sem 0.04 g/kg, n = 9), moderate (1.08±0.03 g/kg, n = 10) or mild (0.55±0.02 g/kg, n = 8) protein load given as a meat meal. Control subjects (n = 10) received a carbohydrate meal. 2. CIn and CPAH increased after a protein meal. Two hours after eating the test meal, GFR levels were (mean ± sem) 160.0±13.8 (P < 0.05), 141 ± 7.69 and 127.8 ±9.07 ml/min in the groups that received a large, moderate and mild protein load, respectively. Peak CIn values after the meal were 211.6 ±sem 14.92 (P < 0.001), 177.5±10.88(P < 0.01)and 129.0±8.72 ml/min after a large, moderate and mild protein load, respectively. 3. At peak GFR levels after the meal, filtration fraction (FF) (CIn × 100/CPAH) increased significantly (P < 0.02) with the large and with the moderate protein load, but not with the mild protein load. 4. There was a significant (P < 0.001) positive relationship between increments of FF and increments of CIn, but not CPAH, whether the values were expressed as post-meal/pre-meal ratios or as absolute changes. 5. We conclude that hyperfiltration induced by a large protein load is associated with increments in GFR which are proportionally larger than the increments in RBF and, therefore, cannot be explained by simple vasodilatation. Factors capable of increasing FF operate in the physiological response to a protein meal.