Renal and segmental artery hemodynamics during whole body passive heating and cooling recovery.

High environmental temperatures are associated with increased risk of acute kidney injury, which may be related to reductions in renal blood flow. The susceptibility of the kidneys may be increased because of heat stress-induced changes in renal vascular resistance (RVR) to sympathetic activation. We tested the hypotheses that, compared with normothermia, increases in RVR during the cold pressor test (CPT, a sympathoexcitatory maneuver) are attenuated during passive heating and exacerbated after cooling recovery. Twenty-four healthy adults (22 ± 2 yr; 12 women, 12 men) completed CPTs at normothermic baseline, after passive heating to a rise in core temperature of ~1.2°C, and after cooling recovery when core temperature returned to ~0.2°C above normothermic baseline. Blood velocity was measured by Doppler ultrasound in the distal segment of the right renal artery (Renal, = 24 during thermal stress, = 12 during CPTs) or the middle portion of a segmental artery (Segmental, = 12). RVR was calculated as mean arterial pressure divided by renal or segmental blood velocity. RVR increased at the end of CPT during normothermic baseline in both arteries (Renal: by 1.0 ± 1.0 mmHg·cm·s, Segmental: by 2.2 ± 1.2 mmHg·cm·s, ≤ 0.03), and these increases were abolished with passive heating ( ≥ 0.76). At the end of cooling recovery, RVR in both arteries to the CPT was restored to that of normothermic baseline ( ≤ 0.17). These data show that increases in RVR to sympathetic activation during passive heating are attenuated and return to that of normothermic baseline after cooling recovery. Our data indicate that increases in renal vascular resistance to the cold pressor test (i.e., sympathetic activation) are attenuated during passive heating, but at the end of cooling recovery this response returns to that of normothermic baseline. Importantly, hemodynamic responses were assessed in arteries going to (renal artery) and within (segmental artery) the kidney, which has not been previously examined in the same study during thermal and/or sympathetic stressors.