Exercise and the cutaneous circulation.
Our understanding of the control of the cutaneous circulation has increased over the past decade, but is still far from complete. There is a cutaneous vasoconstriction at the beginning of exercise that usually effectively competes with concurrent thermoregulatory drives for vasodilation. This cutaneous vasoconstrictor response, however, requires dynamic activity by a significant muscle mass as small muscle groups or isometric exercise are ineffective or nearly so. Also, exercise causes the threshold internal temperature relative to rest such that SKBF is lower during exercise than in resting conditions for a given thermal stimulus. A further influence by exercise on the cutaneous circulation is to limit the degree of cutaneous vasodilation when heat stress and exercise are combined. These three roles for exercise compete with the thermogenic role that promotes vasodilation. The previously described effects act through the adrenergic vasoconstrictor system and the separate active vasodilator system. The increase in SKBF with heat stress represents the combination of withdrawal of vasoconstrictor activity and elevation of active vasodilator activity. The vasoconstrictor effect of the initiation of exercise is accomplished strictly through enhanced vasoconstrictor activity; vasodilator withdrawal does not participate [72]. However, both the exercise-induced elevation in thermoregulatory threshold for raising SKBF and the limitation to cutaneous vasodilation during exercise are strictly functions of the active vasodilator system [69, 73, 78]. In the first case, active vasodilation is delayed until a higher (relative to rest) level of internal temperature is reached. In the second case, the plateau in SKBF during exercise in the heat is due to a similar plateau in active vasodilator activity. Exercise has also served as a tool for the study of other influences on the cutaneous circulation. The influences of alterations in body fluid volumes, osmolarity, acclimatization, hypertension, time of day, menstrual phase, and others on the control of SKBF have been assessed by using exercise as a calorigenic source. The question as to whether the nonthermoregulatory influences of exercise interact with these other influences to give a modification of the pattern of control different from what might be observed at rest is largely unanswered. Future directions for research are numerous, but several fundamental questions are outstanding. The mechanism of active cutaneous vasodilation has been elusive since its discovery and remains an exceptionally important question. Second, the sensory elements associated with exercise giving rise to the alterations in the pattern of control are unclear. This problem is made challenging by the fact that the efferent control by exercise differs between its initiation and events later in exercise.(ABSTRACT TRUNCATED AT 400 WORDS)