Cardiovascular-sympathetic adjustments to nonexertional heat stress in mature and senescent Fischer 344 rats

The purpose of this study was to test the hypothesis that the cardiovascular-sympathetic nervous system adjustments during nonexertional heat stress are exaggerated in senescent (S, 24 mo) vs. mature (M, 12 mo) conscious unrestrained Fischer 344 rats. During two separate trials (48 h apart), each animal was exposed to an ambient temperature (T(a)) of 42°C until a colonic temperature (T(co)) of 41°C was attained and then cooled at a T(a) of 26°C until T(co) returned to the initial control level. Trial 1: heart rate (HR), mean arterial blood pressure (MAP), and arterial plasma concentrations of norepinephrine (NE), epinephrine (E), and lactate (La) were similar between the S and M groups during the baseline (control) period. The absolute increases in HR, MAP, NE, and E from the control period to the end of heating were of similar magnitudes between groups; however, La increased more in the S than M animals (P < 0.05). During recovery, the declines toward control levels for all variables were similar or even more rapid in the S vs. M animals (P < 0.05). Trial 2: the changes in HR and MAP during heating were similar to those observed in trial 1 in both groups. Generally, NE and E control levels were elevated in both groups compared with those in trial 1. The absolute increases in NE during heating were similar to trial 1 in both groups, whereas E increased to a greater extent than in trial 1 in the S animals (P < 0.05). Recovery responses were similar to trial 1 in both groups. Based on the variables measured in the present study, these results indicate that conscious, unrestrained, senescent rats do not exhibit exaggerated cardiovascular-sympathetic nervous system adjustments during or after nonexertional heat exposure compared to their younger, but mature, counterparts. Thus it appears that aging does not alter the autonomic nervous system responses to this particular form of acute physical stress in the rat.