PLASMA-CATECHOLAMINES AND HYPERGLYCEMIA INFLUENCE THERMOREGULATION INMAN DURING PROLONGED EXERCISE IN THE HEAT

1. We manipulated plasma catecholamines (combined adrenaline and norad renaline concentrations) to three levels during prolonged exercise to determine their effect on cutaneous and forearm vascular conductance ( CVC and FVC), oesophageal temperature (T-oes) and cardiovascular respo nses. 2. On three occasions, seven endurance-trained men cycled at 65% V-O2,V-max in the heat (33.1 +/- 0.7 degrees C) for 120-150 min. Duri ng the control trial (150 min duration), 0.45% saline was intravenousl y infused (SI) starting at 30 min, at a rate that replaced a third of the fluid losses. The infusion start time and rate were identical in a ll three trials. During SI, plasma catecholamine levels increased prog ressively and were 18.2 +/- 2.7 pmol ml(-1) at 150 min. In another tri al (120 min duration), adrenaline was infused (AI) at 0.1 mu g kg(-1) min(-1) and plasma catecholamine levels were elevated 6 pmol ml(-1) ab ove SI during the 60-120 min period. In a third trial (150 min duratio n), an 18% glucose solution was infused (GI) at a rate that maintained plasma glucose levels above 11 mM and plasma catecholamine levels wer e 5.0-5.5 pmol ml(-1) lower (P < 0.05) than SI from 120-150 min. 3. He at production and sweat rate were not different during the three trial s and neither was the decline in stroke volume, cardiac output and mea n arterial pressure. 4. Soon after beginning AI, CVC decreased 15%, T- oes increased by 0.4 +/- 0.1 degrees C and heart rate increased by 6 /- 1 beats min(-1); these significant (P < 0.05) differences from SI w ere maintained throughout the bout. As a result of GI, FVC was 15% hig her than SI and T-oes and heart rate were attenuated by 0.3 +/- 0.1 de grees C and 7 +/- 1 beats min(-1) at 150 min compared with SI (P < 0.0 5). 5. In conclusion, large increases in plasma catecholamine levels c ause hyperthermia during exercise by vasoconstricting the skin. The me chanisms by which hyperglycaemia (i.e. 11 mM) attenuates hyperthermia are less clear and mag be due to others factors besides attenuation of the plasma catecholamine response to exercise.