MODULATION OF BAROREFLEX SENSITIVITY AND SPECTRAL POWER OF BLOOD-PRESSURE BY HEAT-STRESS AND AGING

To investigate the effects of hyperthermia and aging on baroreceptor-h eart rate reflex sensitivity (BRS), cardiovascular parameters were rec orded during a progressive rise in core temperature in conscious matur e and senescent Fischer 344 rats. BRS was calculated from spontaneous changes in blood pressure and interbeat interval. Low- (LF, 0.01-0.20 Hz) and mid- (MF, 0.2-0.5 Hz) frequency blood pressure power were also determined. In both age groups, hyperthermia caused an increase in bl ood pressure, renal resistance, and LF but no changes in renal nerve a ctivity, whereas a tachycardia was only observed in the older rats. In creases in BRS (0.80 +/- 0.14 vs. 1.72 +/- 0.34 ms/mmHg, P < 0.05) and MF (3.10 +/- 0.55 vs. 7.81 +/- 1.89 mmHg(2), P < 0.05) and a positive correlation between BRS and MF (r = 0.50, P < 0.01) were observed wit h heating in mature but not senescent rats. These results indicate tha t LF, which increased with elevated core temperature, may be modulated by thermal stimuli. The augmented BRS in the mature group may contrib ute to the hemodynamic adjustments that occur with hyperthermia, where as the lack of an increase in BRS during heat stress in the senescent group suggests that baroreceptor reflex modulation is impaired with ag ing. The positive correlation between BRS and MF in mature rats, toget her with the lack of an increase in renal sympathetic nerve activity, indicates that MF may reflect the modulating influence of the efferent sympathetic portion of the baroreceptor reflex loop on arterial blood pressure rather than merely the activity of the peripheral sympatheti c nervous system.