EFFECTS OF ALPHA(1)-RECEPTOR BLOCKADE ON THE CARDIOVASCULAR AND THERMOREGULATORY RESPONSES TO SEVERE EXERCISE IN THE HEAT

Up to an internal temperature of 38-degrees-C, nonacral skin blood flo w increases with mild exercise in a warm environment. While this appea rs to be due predominately to an active vasodilator system and not to altered vasoconstrictor activity, it is uncertain if more intense exer cise affects this vasodilator response. Consequently, six fit (VO2pk = 62.8 +/- 2.2 ml . kg-1 . min-1, xBAR +/- SEM) college aged men perfor med 30 min bouts of cycle ergometry exercise at 70% VO2pk in a hot env ironment (35-degrees-C, 55% rh) while under the influence of either an alpha1-receptor blocker (prazosin, PRAZ) or a placebo (CTL). Resting rectal temperature (T(re)) was lower (p = 0.002) and heart rate (HR) h igher (p = 0.018) during PRAZ. Neither resting nor exercise forearm bl ood flow (FBF, plethysmography) and forearm vascular conductance (FVC) differed between PRAZ and CTL. Exercise T(re) and mean skin temperatu re (T(sk)BAR) did not differ significantly between treatments, rising to 38.7 and 36.4-degrees-C, respectively, by the 30th min of exercise. Exercise HR was 4 beats . min-1 higher (p = 0.016) with PRAZ. Mean ar terial pressure (MAP) did not differ significantly between drug treatm ents, As FBF, FVC, T(re), T(sk) and MAP were similar between treatment s while only HR was significantly affected by PRAZ, it appears that al pha1-receptor blockade does not alter the thermoregulatory responses t o severe exercise in the heat. However, the elevated HR in the presenc e of a stable MAP suggests that central venous return may have been at tenuated during PRAZ.