Contribution of nonthermal baroreceptor influence on postexercise core temperature regulation

Purpose. The objective of the thesis was to examine the role of nonthermal regulatory influences on thermoregulatory responses during the postexercise period. Specifically, study #1 examined the effect of head down tilt (HDT), and by extension the role of baroreceptors, on prolonged postexercise heat loss and hemodynamic responses in males. Study #2 examined the effect of 15° HDT on cardiovascular and thermal responses in females. In particular the role of baroreceptors on cutaneous vascular conductance (CVC) and sweating during extended recovery from dynamic exercise. Study #3 examined the effect of exercise intensity on hemodynamic and thermal responses in females. Methods. Study #1 and #2- seven male subjects and seven females subjects respectively, performed the following three experimental protocols: (1) 60 min in the upright-seated (URS) posture followed by 60 min in the 15° head-down tilt position (HDT); (2) 15 min of cycle ergometry at 75% of their pre-determined V˙O2 peak followed by 60 min recovery in the URS posture; or (3) 15 min of cycle ergometry at 75% of their pre-determined V˙O2peak followed by 60 min recovery in the 15° HDT position. Study #3- Seven females subjects performed the following 3 experimental protocols on a cycle ergometer: exercise at (1) 85% V˙O2peak; (2) 75% V˙O 2peak; or (3) 55% of their pre determined V˙O2peak followed by 60 min recovery in the upright-seated position. Mean skin temperature (Tsk), esophageal temperature (Tes), skin blood flow (SkBF), sweat rate, cardiac output (CO), stroke volume (SV), heart rate (HR), total peripheral resistance (TPR), and mean arterial pressure (MAP) were measured in all three experiments. Results. Study #1- During recovery from exercise, a significantly greater MAP, SV, CVC and sweat rate and a significantly lower HR were found with HDT in comparison to URS posture (p≤0.05). Subsequently a significantly lower Tes was observed with HDT after 15-min of recovery onwards (p≤0.05). At the end of 60-min recovery, Tes remained significantly elevated above baseline with URS (p≤0.05), however Tes returned to baseline with HDT. Study #2- During recovery from exercise, a significantly greater MAP, SV CVC and sweat rate and a significantly lower HR were found with HDT in comparison to URS recovery posture (P≤0.05). Subsequently a significantly lower Tes was observed with HDT from 12-min till the end of the recovery period (P≤0.05). At the end of 60-min recovery, Tes remained significantly elevated above baseline with URS recovery posture (P≤0.05), however Tes returned to baseline with HDT. Study #3- as exercise intensity increased MAP, SV, sweating and CVC significantly decreased and HR significantly increased (P≤0.05). Subsequently the magnitude and duration of Tes also increased with exercise intensity (P≤0.05). At the end of 60-min recovery, Tes remained significantly elevated above baseline following exercise at 85% VO2peak and 75% VO 2peak (P≤0.05) but returned to baseline values following exercise at 55% V˙O2peak (P≥0.05). Conclusion. Study #1&2- extended recovery from dynamic exercise in the 15° HDT position attenuates the reduction in CVC and sweating thereby significantly increasing the rate of esophageal temperature decay compared to recovery in the URS posture. Study #3- in association with an increase in postexercise hypotension as exercise intensity increases esophageal temperature remains elevated for a prolonged period of time and CVC and sweat rate return to pre-exercise values more rapidly