Renal Functional Reserve Is Attenuated During Mild Passive Heat Stress

Abstract

PURPOSE: Whether glomerular filtration rate (GFR) is reduced with passive heat stress remains equivocal, with some studies showing no change in GFR and others showing a reduction. One explanation for these disparate findings is that GFR is only reduced when the magnitude of physiological challenge exceeds the capacity to recruit additional nephrons to sustain GFR. Thus, renal functional reserve, which is defined as the ability to increase GFR, typically in response to a protein load, is likely reduced with passive heat stress. However, this has never been reported. Therefore, we tested the hypothesis that renal functional reserve is reduced during passive heat stress. METHODS: Twelve healthy adults (26 ± 1 y, 3 females) completed two visits where they underwent passive heat stress or a normothermic time control trial in a randomized crossover design. In the heat stress trial, core temperature was increased ~0.8 °C over the first two hours and then subjects ingested a whey protein shake (1.2 g of protein/kg of body weight). Serum and urinary creatinine were measured pre-protein and 150 min post-protein ingestion. GFR was assessed via creatinine clearance (urinary creatinine multiplied by urine flow rate divided by serum creatinine). Data are presented as mean ± SD. RESULTS: The percent change in core temperature (0.8 ± 0.1 vs 0.3 ± 0.2 °C, P < 0.01) and body fluid loss (3.0 ± 0.9% vs. 1.5 ± 0.4%, P < 0.01) were greater in heat stress. Urinary creatinine was higher in heat stress pre- (96.2 ± 77.1 vs 36.9 ± 14.4 mg/dL, P = 0.05) and post- (191.7 ± 77.0 vs. 97.9 ± 47.7 mg/dL, P < 0.01) protein. Serum creatinine did not differ pre-protein between heat stress (1.00 ± 0.19 mg/dL) and normothermia (1.01 ± 0.17 mg/dL, P = 0.98), but was higher during heat stress post-protein (1.04 ± 0.22 vs. 0.96 ± 0.18 mg/dL, P = 0.02). Creatinine clearance did not differ between heat stress (135.1 ± 27.7 mg/dL) and normothermia (120.4 ± 33.1 mg/dL, P = 0.84) pre-protein and increased with protein in both trials (P = 0.04). However, increases in creatinine clearance with protein ingestion were attenuated with heat stress (17.1 ± 17.4 mg/dL) compared to normothermia (65.1 ± 66.7 mg/dL, P = 0.01). CONCLUSIONS: Mild passive heat stress reduces renal functional reserve, suggesting that heat stress reduces the ability to recruit nephrons to support GFR.Supported by R01OH011528.