Reductions in cardiac output with exercise-induced dehydration accompanied by increases in systemic and cutaneous vascular resistance. Gonz[acute]alez-Alonso, Jos[acute]e, Ricardo Mora-Rodr[acute]iguez, Paul R. Below, and Edward F. Coyle. Human Performance Laboratory, Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, TX 78712
APStracts 2:0256A, 1995.
This investigation determined whether the reductions in cardiac output (O(Q,.)) with exercise-induced dehydration were accompanied by significant increases in systemic and cutaneous vascular resistance (SVR and CVR) as well as plasma catecholamines during prolonged exercise in the heat. On two separate occasions, seven endurance -trained subjects [age 28+/-2 (SD), body wt 74.0+/-10.7 kg, maximal O2 consumption (O(V,.)O2max) 4.70+/-0.41 l/min] cycled in the heat (35 C, 48% relative humidity, wind speed 2 m/s) for 2 h, beginning at 62+/-2% O(V,.)O2max. During exercise, they randomly received either 0.2 l of a 35% carbohydrate-electrolyte (CE) solution and became dehydrated by 3.6+/-0.5 kg (i.e., 4.9% body wt loss; dehydration trial, DE) or 3.6+/-0.4 l of a 2% CE solution (i.e., identical CE intake in both trials) and replaced 95% of the fluid lost in the sweat (i.e., euhydration trial, EU). During the 10 to 120 min period of EU, O(Q,.), mean arterial pressure (MAP), SVR and CVR did not change while esophageal temperature (Tes) stabilized at 38.0+/-0.1 C after 40 min of exercise. Conversely, in DE, O(Q,.) was 18% lower and SVR was 17% higher whereas MAP was only 5% lower after 120 min compared with EU (all P < 0.05). The cutaneous circulation participated in this systemic vasoconstriction as reflected by the significantly higher CVR (126+/-16 vs. 102+/-6% of 20 min value; P < 0.05) despite the fact that Tes was 1.2 C higher at the end of the 2 h of exercise during DE compared with EU. In addition, during the 30 to 120 min period of DE, plasma norepinephrine and epinephrine concentrations increased from 9.8+/-1.4 to 17.5+/-1.3 pmol/ml (P < 0.05) and 1.0+/-0.1 to 2.1+/-0.5 pmol/ml (P = 0.07), respectively, whereas they did not increase significantly during EU. In conclusion, the significant reductions in O(Q,.) with exercise-induced dehydration are accompanied by significant increases in SVR and plasma catecholamines and an slight although significant decline in MAP. The cutaneous circulation participates in this systemic vasoconstriction as indicated by increases in CVR despite the significant hyperthermia. 

Received 1 November 1994; accepted in final form 7 June 1995.
APS Manuscript Number A1123-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  6 July 1995.