Age and hypohydration independently influence the peripheral vascular response to heat stress

Seven young (Y, 22-28 yr) and seven middle-aged (MA, 49-60 yr) normotensive men of similar body size, fatness, and maximal oxygen uptake (V̇O₂(max)) were exposed to a heat challenge in an environmental chamber (48°C, 15% relative humidity). Tests were performed in two hydration states: hydrated (H, 25 ml water/kg body wt 1 h before the test, 2.5 h before exercise) and hypohydrated (Hypo, after 18-20 h of water deprivation). Each test began with a 90-min rest period during which the transiently increased plasma volume and decreased osmolality after drinking in the H condition returned to base line. This period was followed by 30 min of cycle exercise at a mean intensity of 43% V̇O₂(max) and a 60-min resting recovery period with water ad libitum. Although prior drinking caused no sustained changes in plasma osmolality, Hypo increased plasma osmolality by 7-10 mosmol/kg in both groups. There were no significant age differences in water intake, urine output or osmolality, overall change in body weight, or sweating rate. In the H state, the percent change in plasma volume was less (P < 0.01) during exercise for the Y group (-5.9 ± 0.7%) than for the MA group (-9.4 ± 0.6%). Esophageal temperature (T(es)) was higher in the Hypo condition for both groups with no age-related differences. Throughout the 3-h period, mean skin temperature was higher in the Y group and significantly so (P < 0.05) in the Hypo condition. During exercise, forearm blood flow (FBF) was significantly (i.e., 50-60%) lower and mean arterial pressure was significantly (15-20 mmHg) higher in the MA group in both hydration states, yielding an elevated forearm vascular resistance compared with the Y group. Furthermore, there was no interaction between age and hydration state in these peripheral vascular responses. Estimated core-to-skin heat conductance was ~ 50% lower in the Hypo condition but was not significantly different between age groups because the higher FBF of the Y group offset their lower core-to-skin temperature gradient (thus accounting for the similar ΔT(es) in both age groups). It was concluded that age-related alterations in the peripheral circulation limit vasodilation and maintain higher arterial pressures during exercise in a warm environment. These differences are not a function of age-related differences in cardiorespiratory fitness or hydration state but appear to be a primary consequence of the aging process.