Vascular fluid shifts and endocrine responses to exercise in the heat

Abstract

This study examines the relationships between vascular changes and endocrine responses to prolonged exercise in the heat, associated with dehydration and rehydration by fluids of different osmolarity. Five subjects were exposed, in a 34 degrees C environment for 4 h of intermittent exercise on a cycle ergometer at 85 +/- 12 Watts (SD). Fluid regulatory hormones and cortisol were analysed in 3 experimental sessions: one without any fluid supplement (NO FLUID), and two with progressive rehydration, either by spring water (WATER) or isotonic solution (ISO), given after 70 min of exercise. Results were expressed in terms of differences between the mean values observed at the end of the exercise and the first hour values taken as references. Dehydration (NO FLUID) elicited a 4.0 +/- 0.8% (SE) decrease in plasma volume (PV) and an increase in osmolarity (8.4 +/- 3.1 mosmol X l-1). Concomitantly, plasma aldosterone (PA), renin activity (PRA), arginin vasopressin (AVP) and cortisol (PC) levels increased greatly in response to exercise in the heat (PA: 37.2 +/- 10.8 ng. 100 ml-1; PRA: 13.4 +/- 2.5 ng X ml-1 X h-1; AVP: 3.8 +/- 1.3 pg X ml-1; PC: 12.2 +/- 2.7 micrograms X 100 ml-1). Rehydration with water led to decreased osmolarity (-8.2 +/- 2.1 mosmol X l-1) with no significant changes in PV. With ISO, PV increased by 6.0 +/- 1.3% and the decrease in osmolarity was-5.8 +/- 1.8 mosmol X l-1. With both modes of rehydration, the increases in PRA, AVP and cortisol were blunted; only ISO prevented the rise in PA.(ABSTRACT TRUNCATED AT 250 WORDS)

The Induction and Decay of Short-Term Heat Acclimation

“…We emphasise that the present findings were obtained using permissive dehydration to a modest level of hypohydration (~1.8%); a level at which additional fluidregulatory stimuli are already induced (Table 2) (Brandenberger et al 1986;Brandenberger et al 1989;McConell et al 1997) and thirst becomes stimulated to limit excessive dehydration (Engell et al 1987;Sawka et al 1987;Nadel et al 1993). Whether greater dehydration and/or a longer acclimation regime would be more beneficial or detrimental is unknown and requires further investigation, but if exercise training is an important component of the acclimation then the training impulse would become reduced (especially if using the controlled hyperthermic regime).…”

“…The neuroendocrine system regulates the volume and composition of plasma volume and intracellular fluid of red blood cells, whose essential components for the retention of  Na and water are the renin-angiotensin-aldosterone system and AVP (Brandenberger et al 1986). In the present study, after 90-min exercise [aldo] p increased across DEH bouts in contrast, to a limited response after EUH acclimation and this difference was significant.…”

“…The consequence is impaired exercise tolerance and performance in temperate conditions which has previously been debated (Sawka and Noakes 2007). Dehydration during prolonged exercise in the heat increases the response of the fluid-and stress-regulatory hormones aldosterone, AVP and cortisol, as well as thirst , such that hydration is regulated at a lower level until after exercise and/or heat stress (Brandenberger et al 1986;Brandenberger et al 1989). The osmolality and volume effects of hypohydration incur fluid-regulatory responses that could partially mediate the hypervolaemia and improved fluid-regulatory efficiency that is observed with training and heat acclimation adaptations and which helps attenuate cardiovascular strain in exercise (Hopper et al 1988).…”

Effects of training for two ball games on the saliva response of adrenocortical hormones to exercise in elite sportswomen

“…Some factors such as stage in the menstrual cycle, oral contraceptive regimes (Bonen et al 1991;Murphy et al 1990) or state of hydration (Brandenberger et al 1986) may explain this disagreement. However, our results are in line with previous ®ndings in men (Remes et al 1979).…”

Plasma arginine vasopressin, atrial natriuretic peptide and brain natriuretic peptide responses to long-term field training in the heat: effects of fluid ingestion and acclimatization

“…Follenius et al (1989) have shown that the effects of ingesting isotonic solutions on AVP were similar to those of water. Brandenberger (1986) also showed that AVP responses increased with exercise and decreased with fluid intake.…”