Chronic hormone replacement therapy alters thermoregulatory and vasomotor function in postmenopausal women

This investigation examined effects of chronic (≤2 yr) hormone replacement therapy (HRT), both estrogen replacement therapy (ERT) and estrogen plus progesterone therapy (E+P), on core temperature and skin blood flow responses of postmenopausal women. Twenty-five postmenopausal women [9 not on HRT (NO), 8 on ERT, 8 on E+P] exercised on a cycle ergometer for 1 h at an ambient temperature of 36°C. Cutaneous vascular conductance (CVC) was monitored by laser-Doppler flowmetry, and forearm vascular conductance (FVC) was measured by using venous occlusion plethysmography. Iontophoresis of bretylium tosylate was performed before exercise to block local vasoconstrictor (VC) activity at one skin site on the forearm. Rectal temperature (T(re)) was ~0.5°C lower for the ERT group (P < 0.01) compared with E+P and NO groups at rest and throughout exercise. FVC: mean body temperature (T(b)) and CVC: T(b) curves were shifted ~0.5°C leftward for the ERT group (P < 0.0001). Baseline CVC was significantly higher in the ERT group (P < 0.05), but there was no interaction between bretylium treatment and groups once exercise was initiated. These results suggest that 1) chronic ERT likely acts centrally to decrease T(re), 2) ERT lowers the T(re) at which heat-loss effector mechanisms are initiated, primarily by actions on active cutaneous vasodilation, and 3) addition of exogenous progestins in HRT effectively blocks these effects.