Am J Physiol Regul Integr Comp Physiol. 2022 Jul 11. doi: 10.1152/ajpregu.00090.2022. Online ahead of print.
We tested the hypothesis that, compared to normothermia, the increase in glomerular filtration rate (GFR) following an oral protein load (defined as the GFR reserve) is attenuated during moderate passive heat stress in young healthy adults. Sixteen participants (5 women; 26 ± 2 y) completed two experimental visits, heat stress or a normothermic time-control, assigned in a block-randomized crossover design. During the heat stress trial, core temperature was increased by 0.6°C in the first hour before commencing a 2-minute cold pressor test (CPT) to assess renal vasoconstrictor responses. One-hour post-CPT, subjects ingested a whey protein shake (1.2 g of protein/kg of body weight), and measurements were taken pre-, 75- and 150-min post-protein. Segmental artery vascular resistance was calculated as the quotient of Doppler ultrasound derived segmental artery blood velocity and mean arterial pressure and provided an estimate of renal vascular tone. GFR was estimated from creatinine clearance. The increase in segmental artery vascular resistance during the CPT was attenuated during heat stress (end CPT: 5.6±0.9 vs. 4.7±1.1 mmHg/cm/s, P=0.024). However, the reduction in segmental artery vascular resistance in response to an oral protein load did not differ between heat stress (at 150 min: 1.9±0.4 mmHg/cm/s) and normothermia (at 150 min: 1.8±0.5 mmHg/cm/s; P=0.979). The peak increase in creatinine clearance post-protein, independent of time, was attenuated during heat stress (+26±19 vs.+16±20 mL/min, P=0.013, n=13). GFR reserve is diminished by mild passive heat stress. Moreover, renal vasoconstrictor responses are attenuated by mild passive heat stress, but renal vasodilator responses are maintained.
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