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1 Department of Veterinary Biosciences, Ohio State University, Columbus, Ohio 43210; and 2 Department of Veterinary Biomedical Sciences and Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri 65211-3300
The baroreflex function curve is shifted to lower operating
pressures, efferent sympathoexcitatory responses are attenuated, and
sympathoinhibitory responses are potentiated in pregnant compared with
virgin rats. It has been proposed that during pregnancy, elevated
levels of 3
-hydroxy-dihydroprogesterone (3-OH-DHP), a major
metabolite of progesterone, may contribute to this difference, because
acute intravenous administration of 3-OH-DHP to virgin female rats
mimics the effects of pregnancy on the baroreflex. To determine whether
changes in the afferent limb might contribute to these baroreflex
responses, the effects of pregnancy and 3-OH-DHP on aortic depressor
nerve activity were assessed in the current study. Baroreceptor
discharge curves were obtained in Inactin-anesthetized rats by
recording aortic nerve activity during ramp increases and decreases in
mean arterial pressure (MAP) [intravenous phenylephrine and
nitroprusside infusion] before [(control, C) 15 min (E1), and
30 min (E2) after 3-OH-DHP (220 µg/kg bolus + 22 µg · kg
1 · min1 infusion
iv)]. Baseline blood pressure was significantly lower in pregnant
(109 ± 4.4 mmHg) compared with virgin (122 ± 2.8 mmHg) rats. The only significant difference in the baroreceptor discharge curves was a decrease in curve midpoint in pregnant rats (virgin = 140 ± 2.7 vs. pregnant = 124 ± 3.6 mmHg). 3-OH-DHP
had no effect on afferent baroreceptor discharge curves in either
virgin or pregnant groups. These results suggest that
pressure-dependent baroreceptor resetting may contribute to a shift in
the baroreflex curve to lower operating pressures, but cannot
completely explain differences in baroreflex function between virgin
and pregnant animals.
aortic depressor nerve; 3-hydroxy-dihydroprogesterone; neurosteroid; baroreflex; pregnancy
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