C.T. O’Connell, C.L. Chapman, S.M. Holt, S.C. Brazelton, H.N. Medved, W.A.B. Howells, K. Wiedenfeld Needham, E.L. Reed, J.R. Halliwill, FACSM, C.T. Minson, FACSM

University of Oregon, Eugene, OR

Renal vasoconstriction supports the regulation of arterial pressure. Passive heat stress induced hypohydration abates both renal vasoconstriction and increases in blood pressure during sympathetic activation. However, it is not known if this effect is due to hypohydration independent of heat stress. PURPOSE: To test the hypothesis that prolonged mild hypohydration attenuates reductions in renal artery blood velocity (RBV) and increases in renal artery vascular resistance (RVR) during exercise pressor reflex activation compared to a hydrated state (euhydration). METHODS: In a block-randomized crossover design, twenty-two healthy adults (11 females, 11 males; age: 21(3) years) completed 24 hours of fluid deprivation (HYPO) or 24 hours of normal fluid consumption (EUHY). Protocols were separated by ³72 hours. Body fluid losses were estimated via the percent change in body mass (∆BM) over 24 hours. Baseline RBV (Doppler ultrasound) and brachial artery blood pressure (electrosphygmomanometer) were measured. Participants completed 2 min static handgrip exercise (HG) followed by 2 min arterial occlusion (OCC) to activate the exercise pressor reflex. RVR was calculated as mean arterial pressure divided by RBV. Data are presented as the change from baseline (∆) with mean and 95% confidence intervals. RESULTS: Body mass was reduced in HYPO vs. EUHY [-2.5% (-2.9, -2.1) vs. 0.0% (-0.4, 0.4), P<0.0001]. Increases in mean arterial pressure during HG and OCC did not differ between conditions (P=0.8790). Reductions in RBV were attenuated in HYPO compared to EUHY at the end of HG [-0.8 cm/s (-2.5, 0.9) vs. -5.9 cm/s (-8.2, 3.7), P<0.0001] and OCC [0.7 cm/s (-0.6, 2.1) vs. -2.4 cm/s (-4.2, -0.6), P=0.0109]. Increases in RVR were also attenuated in HYPO compared to EUHY at the end of HG [0.5 mmHg/cm/s (0.4, 0.7) vs. 0.8 mmHg/cm/s (0.6, 1.1), P=0.0012] and OCC [0.2 mmHg/cm/s (0.1, 0.3) vs. 0.4 mmHg/cm/s (0.3, 0.6), P=0.0418]. CONCLUSION: Mild prolonged hypohydration attenuates both reductions in RBV and increases in RVR during exercise pressor reflex activation. Thus, these data suggest that the blunted renal vasoconstriction to sympathetic activation during passive heat stress induced hypohydration is only partially explained by hypohydration.

Supported by NIH R01HL144128 and F32HL164021.

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