CONTROL OF INTERNAL TEMPERATURE THRESHOLD FOR ACTIVE CUTANEOUS VASODILATION BY DYNAMIC EXERCISE

Exercise induces shifts in the internal temperature threshold at which cutaneous vasodilation begins. To find whether this shift is accomplished through the vasoconstrictor system or the cutaneous active vasodilator system, two forearm sites (0.64 cm2) in each of 11 subjects were iontophoretically treated with bretylium tosylate to locally block adrenergic vasoconstrictor control. Skin blood flow was monitored by laser-Doppler flowmetry (LDF) at those sites and at two adjacent untreated sites. Mean arterial pressure (MAP) was measured noninvasively. Cutaneous vascular conductance was calculated as LDF/MAP. Forearm sweat rate was also measured in seven of the subjects by dew point hygrometry. Whole body skin temperature was raised to 38-degrees-C, and supine bicycle ergometer exercise was then performed for 7-10 min. The internal temperature at which cutaneous vasodilation began was recorded for all sites, as was the temperature at which sweating began. The same subjects also participated in studies of heat stress without exercise to obtain vasodilator and sudomotor thresholds from rest. The internal temperature thresholds for cutaneous vasodilation were higher during exercise at both bretylium-treated (36.95 +/- 0.07-degrees-C rest, 37.20 +/- 0.04-degrees-C exercise, P < 0.05) and untreated sites (36.95 +/- 0.06-degrees-C rest, 37.23 +/- 0.05-degrees-C exercise, P < 0.05). The thresholds for cutaneous vasodilation during rest or during exercise were not statistically different between untreated and bretylium-treated sites (P > 0.05). The threshold for the onset of sweating was not affected by exercise (P > 0.05). It is concluded that reflexes associated with exercise cause an elevated vasodilator threshold by delaying activation of the active vasodilator system during exercise, rather than through the adrenergic vasoconstrictor system. Lack of a similar shift in the threshold for sweating suggests separate control of sudomotor and active vasodilator function.