Modification of active cutaneous vasodilation by oral contraceptive hormones.

It is not clear whether the altered thermoregulatory reflex control of the cutaneous circulation seen among phases of the menstrual cycle also occurs with the synthetic estrogen and progesterone in oral contraceptive pills and whether any such modifications include altered control of the cutaneous active vasodilator system. To address these questions, we conducted controlled whole body heating experiments in seven women at the end of the third week of hormone pills (HH) and at the end of the week of placebo/no pills (LH). A water-perfused suit was used to control body temperature. Laser Doppler flowmetry was used to monitor cutaneous blood flow at a control site and at a site at which noradrenergic vasoconstrictor control had been eliminated by iontophoresis of bretylium (BT), isolating the active cutaneous vasodilator system. The oral temperature (Tor) thresholds for cutaneous vasodilation were higher in HH at both control [37.09 +/- 0.12 vs. 36.83 +/- 0.07 degrees C (LH), P < 0.01] and BT-treated [37. 19 +/- 0.05 vs. 36.88 +/- 0.12 degrees C (LH), P < 0.01] sites. The Tor threshold for sweating was similarly shifted (HH: 37.15 +/- 0.11 degrees C vs. LH: 36.94 +/- 0.11 degrees C, P < 0.01). A rightward shift in the relationship of heart rate to Tor was seen in HH. The sensitivities (slopes of the responses vs. Tor) did not differ statistically between phases. The similar threshold shifts at control and BT-treated sites suggest that the hormones shift the function of the active vasodilator system to higher internal temperatures. The similarity of the shifts among thermoregulatory effectors suggests a centrally mediated action of these hormones.