Effect of age on renal blood flow during exercise.

The present study examined the effect of age on the control of renal blood flow (RBF; PAH clearance) and renal vascular conductance (RVC = RBF/mean arterial pressure) during and after a bout of dynamic exercise in a warm environment. Six healthy fit older men (O; 67 +/- 1 years) and 6 young men (Y; 24 +/- 2 years) were matched for body size, adiposity, and maximal oxygen uptake (VO2max). Subjects exercised at approximately 50% of VO2max for 90 minutes in an environment of 30 degrees C, 60% humidity on each of 4 consecutive days, with data collected on days 1 and 4. There was no effect of repeated days of exercise on RBF or RVC, despite a 4% expansion of blood volume in Y (< 1% in O). On each day, resting RBF was significantly lower in O (e.g., Y = 1127 +/- 67, O = 852 +/- 114 mL/min on day 1; p < 0.05). During exercise, Y decreased RBF to a significantly (p < 0.05) greater extent [-508 (-45%) and -365 (-36%) mL/min on days 1 and 4, respectively] than the O [-98 (-12%) and -83 (-12%) mL/min]. RVC followed a similar pattern, decreasing by 52% and 37% during exercise for Y vs only 15% and 13% for O. The relationships between delta RBF and HR and delta RBF and plasma norepinephrine concentration were independent of age, implying similar sympathetic control during exercise. During recovery, RBF and RVC increased as expected in Y, but continued to decrease in O, falling significantly below exercise values (p < 0.05). Compared to young men, fit healthy older men redistribute less blood flow away from the kidneys during dynamic exercise in the heat, an effect which appears to result from the existence of a smaller resting RBF rather than differential sympathetic control. On the other hand, chronological age seems to be associated with altered control of RBF and RVC during recovery from exercise.