Afferent baroreceptor discharge in pregnant rats

Abstract

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 alpha-hydroxy-dihydroprogesterone (3 alpha-OH-DHP), a major metabolite of progesterone, may contribute to this difference, because acute intravenous administration of 3 alpha-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 alpha-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 alpha-OH-DHP (220 microg/kg bolus + 22 microg x kg(-1) x min(-1) 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 alpha-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.

Effect of pregnancy and 5α-pregnan-3α-ol-20-one on atrial receptor afferent discharge in rats

“…Thus chronic administration of pregnan suppresses the response of HF atrial receptors to distension of the venoatrial junction, i.e., it mimics the effect of pregnancy on atrial volume receptor discharge. The inability of acutely administered pregnan to alter the discharge characteristics of the atrial volume receptors is consistent with the observation that this treatment also fails to modulate afferent arterial baroreceptor discharge (10).…”

Pregnancy increases baroreflex-independent GABAergic inhibition of the RVLM in rats

“…Previously our laboratory demonstrated that arterial baroreflex sympathoexcitation due to baroreceptor unloading is attenuated in near-term pregnant compared with nonpregnant rats (43,51). In response to a hypotensive challenge, fos expression is less in pregnant compared with nonpregnant rats, suggesting that neuronal activation of the RVLM during baroreceptor unloading is suppressed in pregnant rats (17).…”

“…However, arterial baroreflex sympathoinhibitory responses are well maintained (14) or augmented (43,51) in pregnant rats. Although adaptation of afferent baroreceptor discharge during prolonged pressure stimuli (10 min) appears to be more pronounced in pregnant rats (33) when afferent arterial baroreceptor discharge is assessed by using methods similar to those used in the above-mentioned baroreflex studies, the gain of arterial baroreceptor afferent discharge is not altered by pregnancy (33,43). Therefore, it is likely that alterations in central autonomic integration contribute to the suppressed baroreflex sympathoexcitation that has been reported in pregnant animals.…”

“…Direct assessment of arterial baroreflex control of renal sympathetic nerve activity (RSNA) in near-term pregnant rats (14,43,51) and rabbits (7,54,55) revealed that maximum baroreflex gain is decreased, largely due to an attenuated ability to increase RSNA above baseline levels in response to a hypotensive challenge. However, arterial baroreflex sympathoinhibitory responses are well maintained (14) or augmented (43,51) in pregnant rats.…”

“…Direct assessment of arterial baroreflex control of renal sympathetic nerve activity (RSNA) in near-term pregnant rats (14,43,51) and rabbits (7,54,55) revealed that maximum baroreflex gain is decreased, largely due to an attenuated ability to increase RSNA above baseline levels in response to a hypotensive challenge. However, arterial baroreflex sympathoinhibitory responses are well maintained (14) or augmented (43,51) in pregnant rats. Although adaptation of afferent baroreceptor discharge during prolonged pressure stimuli (10 min) appears to be more pronounced in pregnant rats (33) when afferent arterial baroreceptor discharge is assessed by using methods similar to those used in the above-mentioned baroreflex studies, the gain of arterial baroreceptor afferent discharge is not altered by pregnancy (33,43).…”