Hemorrhage decreases arterial pressure sooner in pregnant compared with nonpregnant dogs: role of baroreflex

The Hypo-Osmolality of Pregnancy Does Not Alter Baroreceptor Responses to Acute Changes in Osmolality

Reducing extracellular osmolality by 10-12 mOsm/L reduces baroreceptor firing in vitro in tissues from male rats. Pregnancy is associated with a similar reduction in plasma osmolality (pOsm) as well as with alterations in baroreceptor firing and reduced baroreceptor reflex activity. Mechanisms for reduced baroreflex activity are not yet understood, but they have important implications for maintenance of cardiovascular homeostasis in the pregnant female. Thus, this study was designed to test whether changes in plasma osmolality alter baroreceptor discharge during pregnancy. Late-gestation pregnant and virgin control rats were anesthetized, femoral vessels were cannulated for measurement of arterial pressure and drug infusion, and the aortic depressor nerve, containing baroreceptor afferents, was isolated and prepared for recording. Plasma osmolality was measured before and 30 min after graded intraperitoneal injections of NaCl (50-1500 mOsm/L). Arterial pressure, heart rate, and aortic depressor nerve activity (ADNA) were measured continuously before and after injections. A 50 mOsm/L NaCl injection significantly decreased pOsm but did not alter ADNA in either group. Likewise, 1200 mOsm/L NaCl injections significantly increased pOsm and had no effect on ADNA in either group. The 1500 mOsm salt load significantly increased pOsm and ADNA in pregnant rats, and in virgin animals, it increased pOsm but evoked a paradoxical decrease in ADNA. The authors’ studies indicate that pOsm has minimal effects on baroreceptor activity, which are not significantly different during gestation. Their findings suggest that in vitro data from male rats indicating a correlation between pOsm and baroreceptor discharge are not applicable in anesthetized intact female animals.

Visceral pain decreases tolerance to blood loss in conscious female but not male rabbits

Pain is a component of traumatic blood loss, yet little is known about how pain alters the response to blood loss in conscious animals. We evaluated the effects of colorectal distension on the cardiorespiratory response to blood loss in six male and six female conscious, chronically instrumented New Zealand White rabbits. The goal of these experiments was to test the hypotheses that 1) colorectal distension would increase tolerance to hemorrhage (i.e., increase the blood loss required to decrease mean arterial pressure <or= 40 mmHg); and 2) the increase in tolerance would be similar in male and female rabbits. For hemorrhage, venous blood was withdrawn until mean arterial pressure decreased to <or=40 mmHg. Conscious rabbits underwent three treatments in a balanced design: a control hemorrhage, hemorrhage with a colorectal balloon present but not inflated (sham CRD), and hemorrhage in the presence of colorectal distension (CRD). Colorectal distension reproducibly increased mean arterial pressure, decreased respiratory rate, and did not change heart rate. There was no difference in control blood loss between males (21.8 +/- 0.3 ml/kg) and females (21.6 +/- 0.3 ml/kg). However, although CRD blood loss did not change in males (22.8 +/- 0.3 ml/kg), it was significantly less than control in females (19.1 +/- 0.3 ml/kg; P = 0.004). Thus, in conscious rabbits, colorectal distension alters cardiovascular control during hemorrhage. Furthermore, colorectal distension did not improve tolerance to blood loss in males or females as hypothesized but instead decreased tolerance to blood loss only in females.

CNS effects of ovarian hormones and metabolites on neural control of circulation

Pregnant women often experience orthostatic hypotension, and pregnancy is associated with increased susceptibility to hemorrhagic hypotension. Experiments evaluating arterial baroreflex control of efferent sympathetic nerve activity in virgin and term-pregnant rats revealed that arterial baroreflex sympathoexcitation is attenuated, while sympathoinhibitory responses are well-maintained or potentiated. Following a hypotensive challenge, pregnant animals exhibit attenuated Fos expression in the rostral ventrolateral medulla (RVLM), suggesting that unloading of arterial baroreceptors results in less excitation of presympathetic neurons in the brain stem. Other experiments, in which afferent baroreceptor discharge was recorded, suggest that this was not due to differences in afferent baoreceptor function. GABAergic mechanisms are responsible for tonic inhibition of sympathoexcitatory neurons in the RVLM and the major metabolite of progesterone, 3 alpha-OH-dihydro-progesterone (3 alpha-OH-DHP), which is elevated in pregnancy, is the most potent endogenous positive modulator of CNS GABAA receptor function. Additional experiments revealed that acutely administered 3 alpha-OH-DHP, either intravenously or directly into the RVLM, mimicked the effects of pregnancy on baroreflex control of efferent sympathetic nerve activity and potentiated pressure sensitivity of spinally projecting RVLM neurons. Preliminary experiments using semiquantitative RT-PCR, evaluated the relative expression of three subunits (alpha 1-3) of the GABAA receptor, and suggest that chronic exposure to elevated levels of ovarian hormones can result to changes in GABAA receptor subunit composition. It is likely that changes in control of sympathetic outflow in pregnancy are related to complex interactions between genomic and nongenomic actions of ovarian hormones and metabolites.

Pregnancy alters cortisol feedback inhibition of stimulated ACTH: studies in adrenalectomized ewes

These studies test the hypothesis that pregnancy alters the feedback effects of cortisol on stimulated ACTH secretion. Ewes were sham-operated (Sham), or adrenalectomized (ADX) at approximately 108 days gestation and replaced with aldosterone (3 microg x kg(-1) x day(-1)) and with cortisol at either of two doses (ADX + 0.6 and ADX + 1 mg x kg(-1) x day(-1)); ewes were studied during pregnancy and postpartum. Mean cortisol levels produced in ADX ewes were similar to normal pregnant ewes (ADX+1) or nonpregnant ewes (ADX+0.6), respectively. Plasma ACTH concentrations in response to infusion of nitroprusside were significantly increased in the pregnant ADX+0.6 ewes (1,159 +/- 258 pg/ml) relative to pregnant Sham ewes (461 +/- 117 pg/ml) or the ADX+1 ewes (442 +/- 215 pg/ml) or the same ewes postpartum (151 +/- 69 pg/ml). Plasma ACTH concentrations were not significantly different among the groups postpartum. Increasing plasma cortisol to 20-30 ng/ml for 24 h before hypotension produced similar inhibition of ACTH in all groups. Pregnancy appears to decrease the effectiveness of low concentrations of cortisol to inhibit ACTH responses to hypotension.

Effects of human pregnancy on cardiac autonomic function above and below the ventilatory threshold

This study examined the effects of human pregnancy on heart rate variability (HRV), spontaneous baroreflex (SBR) sensitivity, and plasma catecholamines at rest and during exercise. Subjects were 14 healthy, physically active pregnant women (PG; mean gestational age = 33.9 +/- 1.0 wk). Results were compared with an age-matched nonpregnant control group (NPG; n = 14) with similar characteristics. The electrocardiographic R-wave-R-wave interval and systolic blood pressure (via finger plethysmograph) were measured on a beat-to-beat basis at rest and during upright cycling at 60 and 110% of the ventilatory threshold (T(vent)). Parasympathetic nervous system (PNS) modulation (as reflected by HRV high-frequency/total power and SBR slope) was significantly reduced at rest in the PG vs. the NPG. During exercise, PNS modulation decreased significantly in both groups, but the magnitude of PNS withdrawal from rest to 110% T(vent) was smaller in the PG vs. NPG. Sympathetic nervous system (SNS) modulation (reflected by the low-frequency power-to-high-frequency power ratio) increased above resting values at 60 and 110% T(vent) in the NPG. SNS modulation at 110% T(vent) was significantly lower in the PG compared with the NPG. Plasma norepinephrine and epinephrine levels were also lower at 110% T(vent) in the PG. It was concluded that healthy pregnant women exhibit lower PNS modulation at rest and blunted SNS modulation during exercise above T(vent) in late gestation.

Baroreceptor afferent discharge in the pregnant rat

Pregnancy is associated with blunted reflex responses to cardiac and arterial baroreceptor stimulation. We tested the hypothesis that arterial baroreceptor afferent discharge is attenuated in response to a pressure stimulus in pregnant rats. Multifiber aortic depressor nerve activity (ADNA), mean arterial pressure (MAP), and heart rate were measured in anesthetized (pentobarbital sodium, 35 mg/kg ip) late-pregnant and virgin rats in response to increases ¿phenylephrine (PE), 1.5-24 microg. kg(-1). min(-1) and 1-16 microg/kg and decreases ¿sodium nitroprusside (SNP), 5-80 microg. kg(-1). min(-1) and 0.05-16 microg/kg in MAP. Resting MAP was lower in pregnant rats, but changes in MAP were similar to those in virgin rats during both PE and SNP administration. ADNA was significantly attenuated in pregnant animals during both PE and SNP infusions (P < 0.05) due to a more rapid adaptation to the pressure stimulus. Bolus drug administration evoked similar changes in MAP and ADNA in both groups; however, the maximum decrease in ADNA was achieved at the lowest dose of SNP in pregnant rats. Thus baroreceptor afferent discharge is attenuated in pregnant rats, and this involves a more rapid adaptation to a pressure stimulus.