Effects of acute haemorrhagic hypotension during pregnancy and lactation in conscious goats

Exposure to a dog elicits different cardiovascular and behavioral effects in pregnant and lactating goats

BACKGROUND

Heart rate and plasma cortisol concentration are often used in evaluation of physiological reactions to stress and fear, but arterial blood pressure is rarely measured in farm animals. Goats are prey animals and can be expected to react strongly to a predator, especially when they have kids. We hypothesized that exposure to a dog elicits a flight response during pregnancy and a fight response when goats have kids to defend. Arterial blood pressure and heart rate should increase in both these cases, due to a synchronized discharge of the sympathetic nervous system.

METHODS

Seven goats were exposed to a dog for 15 minutes at 12 ± 3 days before, and again at 10 ± 1 days after, parturition. Arterial blood pressure, heart rate, and activity were registered by telemetry. Behavioral data were collected during 5 minute sessions, followed by blood samples obtained via intrajugular catheters. Plasma cortisol concentration was analyzed by radioimmunoassay.

RESULTS

At the appearance of the dog, the mean arterial blood pressure of the goats increased from 90 ± 8 to 111 ± 8 mmHg (p < 0.001) during pregnancy and from 96 ± 8 to 108 ± 8 mmHg during lactation (p < 0.001). Heart rate did not change at dog exposure during lactation, but increased from 117 ± 6 to 126 ± 10 beats/min (p < 0.01) during pregnancy. Dog exposure resulted in plasma cortisol concentration increasing from 17 ± 1 to 43 ± 7 nmol/l (p < 0.01) during pregnancy and from 21 ± 1 to 49 ± 6 nmol/l (p < 0.01) during lactation. In response to the dog, goats vocalized at a higher frequency and started to ruminate later during lactation compared to pregnancy.

CONCLUSIONS

When goats were exposed to a dog during pregnancy, their heart rate, blood pressure, and plasma cortisol increased, in contrast to lactation when only their blood pressure and plasma cortisol increased. However, when they were lactating, goats vocalized more and started to ruminate later compared to when they were pregnant.

Impaired baroreflex gain during pregnancy in conscious rats: role of brain insulin

Pregnancy impairs baroreflex gain, but the mechanism is incompletely understood. To test the hypothesis that reductions in brain insulin contribute, we determined whether pregnant rats exhibit lower cerebrospinal fluid (CSF) insulin concentrations and whether intracerebroventricular infusion of insulin normalizes gain of baroreflex control of heart rate in conscious pregnant rats. CSF insulin was lower in pregnant (68 ± 21 pg/mL) compared to virgin (169 ± 25 pg/mL) rats (P < 0.05). Pregnancy reduced baroreflex gain (pregnant 2.4 ± 0.2 bpm/mm Hg, virgin 4.6 ± 0.3 bpm/mm Hg; P < 0.0001) and the maximum heart rate elicited by hypotension (pregnant 455 ± 15 bpm, virgin 507 ± 12 bpm; P = 0.01). Infusion of insulin (100 μU/min) intracerebroventricularly increased baroreflex gain in pregnant (2.4 ± 0.4 to 3.9 ± 0.5 bpm/mm Hg; P < 0.01) but not virgin (4.6 ± 0.4 to 4.2 ± 0.4 bpm/mm Hg; NS) rats. Maximum heart rate was not altered by intracerebroventricular insulin in either group. Interestingly, while in pregnant rats the baroreflex was unchanged by intracerebroventricular infusion of the artificial CSF vehicle, in virgin rats, vehicle infusion lowered baroreflex gain (4.7 ± 0.3 to 3.9 ± 0.3 bpm/mm Hg; P < 0.05) and the maximum baroreflex heart rate (495 ± 19 to 444 ± 21 bpm; P < 0.05). These data support the hypothesis that brain insulin is required to support optimal baroreflex function and that a decrease in brain insulin contributes to the fall in baroreflex gain during pregnancy.

Pregnancy and the endocrine regulation of the baroreceptor reflex

The purpose of this review is to delineate the general features of endocrine regulation of the baroreceptor reflex, as well as specific contributions during pregnancy. In contrast to the programmed changes in baroreflex function that occur in situations initiated by central command (e.g., exercise or stress), the complex endocrine milieu often associated with physiological and pathophysiological states can influence the central baroreflex neuronal circuitry via multiple sites and mechanisms, thereby producing varied changes in baroreflex function. During pregnancy, baroreflex gain is markedly attenuated, and at least two hormonal mechanisms contribute, each at different brain sites: increased levels of the neurosteroid 3alpha-hydroxy-dihydroprogesterone (3alpha-OH-DHP), acting in the rostral ventrolateral medulla (RVLM), and reduced actions of insulin in the forebrain. 3alpha-OH-DHP appears to potentiate baroreflex-independent GABAergic inhibition of premotor neurons in the RVLM, which decreases the range of sympathetic nerve activity that can be elicited by changes in arterial pressure. In contrast, reductions in the levels or actions of insulin in the brain blunt baroreflex efferent responses to increments or decrements in arterial pressure. Although plasma levels of angiotensin II are increased in pregnancy, this is not responsible for the reduction in baroreflex gain, although it may contribute to the increased level of sympathetic nerve activity in this condition. How these different hormonal effects are integrated within the brain, as well as possible interactions with additional potential neuromodulators that influence baroreflex function during pregnancy and other physiological and pathophysiological states, remains to be clearly delineated.

Pregnancy impairs baroreflex control of heart rate in rats: role of insulin sensitivity

Recent studies in rabbits suggest that insulin resistance and reduced brain insulin contribute to impaired baroreflex control of heart rate (HR) during pregnancy; however, the mechanisms are unknown. The rat model is ideal to investigate these mechanisms because much is known about rat brain baroreflex neurocircuitry and insulin receptor locations. However, it is unclear in rats whether pregnancy impairs the HR baroreflex or whether insulin resistance is involved. Therefore, this study tested the hypothesis that in rats pregnancy decreases HR baroreflex sensitivity (BRS) and that this decrease is related to concurrent decreases in insulin sensitivity (IS). BRS was quantified before, during, and after pregnancy using complementary methods: 1) spontaneous BRS (sBRS) derived from sequence method analysis of telemetric, continuous arterial pressure recordings; and 2) maximal BRS of complete sigmoidal baroreflex relationships. IS was measured (hyperinsulinemic euglycemic clamp) to determine whether BRS and IS change in parallel. sBRS was reduced at midgestation [pregnancy day 10 (P10)], returned to nonpregnant (NP) levels on P18, and fell again at late gestation (P20) (sBRS in ms/mmHg: NP, 1.66 + or - 0.04; P10, 1.17 + or - 0.11; P18, 1.55 + or - 0.12; P20, 1.31 + or - 0.05; n = 5; P < 0.05). Similar triphasic patterns were observed for both maximal BRS [in beats x min(-1) x mmHg(-1): NP, 4.45 + or - 0.52 (n = 10); P11-12, 2.76 + or - 0.11 (n = 7); P17-18, 3.79 + or - 0.14 (n = 5); P19-20, 2.32 + or - 0.40 (n = 8); P < 0.0001] and previous and current measurements of IS (in mg glucose x kg(-1) x min(-1): NP, 32 + or - 2; P19-20, 15 + or - 1; P < 0.0005). Furthermore, during pregnancy, the standard deviation (SD) of MAP increased, and the SD of HR decreased, indirectly suggesting baroreflex impairment. sBRS increased transiently during parturition, and sBRS, maximal BRS, and IS normalized 3-4 days postpartum. In conclusion, pregnancy decreases HR BRS in rats. The parallel temporal changes in BRS and IS suggest a mechanistic link.

Hemodynamic and hormonal responses to hemorrhage in conscious rabbits at mid- and late gestation

This study tests the hypothesis that conscious rabbits late in pregnancy (P), but not at midgestation (MP), are less able to maintain arterial pressure during hemorrhage. Blood volume (BV) was elevated (P < 0.05) by an average of 13 +/- 4 (MP) and 35 +/- 3% (P). Rabbits were bled in both the nonpregnant (NP) and P state at 2% of the initial BV per minute. The hemorrhage was stopped after arterial pressure decreased. In NP rabbits, arterial pressure was well maintained near control pressures of 70 +/- 2 mmHg until 38 +/- 2% of the initial BV was removed and then rapidly fell to reach a nadir at 35 +/- 2 mmHg. In contrast, in P rabbits, basal arterial pressure was lower (61 +/- 2 mmHg; P < 0.05) and gradually decreased to below control after <25% of the initial BV was removed. Moreover, the rapid hypotensive phase was triggered with a lower percent BV removal (33 +/- 2%; P < 0.05). Basal heart rate was higher during P (149 +/- 5 vs. 189 +/- 9 beats/min; P < 0.05), and reflex increases were delayed. The slope of the relationship between arterial pressure and vasopressin was not modified during P, although the line was shifted to a lower pressure (P < 0.05). Larger increases in plasma renin activity and ANG II concentration were produced during hemorrhage in P rabbits. In contrast, no differences in the changes in arterial pressure, heart rate, and vasopressin were found between NP and MP rabbits during hemorrhage, although increases in renin and ANG II were greater at MP (P < 0.05). In summary, although P conscious rabbits are less able to maintain blood pressure during hemorrhage, this change is not evident at MP. These data suggest that the factors that mediate the P-induced alterations in arterial pressure regulation are not operative until late in gestation.

Arterial baroreflex during pregnancy and renal sympathetic nerve activity during parturition in rabbits

The arterial baroreflex control of renal sympathetic nerve activity (RSNA) was evaluated in nine term pregnant (P) and 12 nonpregnant (NP) conscious New Zealand White rabbits. In an additional four P rabbits, the RSNA response to spontaneous parturition was measured. The blood pressure (BP)-RSNA relationship was generated by sequential inflations of aortic and vena caval perivascular occluders. Rest BP (P: 61 +/- 2 vs. NP: 73 +/- 2 mmHg) and the centering point of the baroreflex (P: 57 +/- 2 vs. NP: 70 +/- 2 mmHg) were lower (P < 0.05) in term pregnancy. Baroreflex range (P: 246 +/- 14% vs. NP 263 +/- 24% of rest RSNA) was not affected by pregnancy. However, maximal reflex gain was moderately depressed (-44%) in P rabbits (P: -15 +/- 1 vs. NP: -27 +/- 4% of rest RSNA/mmHg; P < 0.05) due to a significant reduction in the slope coefficient. Delivery of a fetus was associated with strong renal sympatho-excitation. Peak RSNA averaged 80 +/- 37% of smoke-elicited RSNA or 1,221 +/- 288% of rest RSNA (mean +/- SD). These results suggest that, in contrast to rat pregnancy, depressed arterial baroreflex control of RSNA in rabbit pregnancy is due primarily to a reduction in maximal gain rather than a reduction in the maximal sympathetic response to hypotension.

Haemodynamic and humoral responses to repeated hypotensive haemorrhage in conscious sheep

Haemodynamic and humoral responses to two subsequent hypotensive haemorrhages, separated by 3 hours and each followed by retransfusion, were studied in unanaesthetized sheep. Haemorrhage was induced by removal of blood from a jugular vein at a rate of 0.7 ml kg-1 min-1 until the mean systemic arterial pressure suddenly decreased by 35 mmHg or more. In addition to the mean systemic arterial pressure, the cardiac output, the mean pulmonary arterial pressure, the central venous pressure and the pulmonary capillary wedge pressure decreased in response to each haemorrhage. The recovery of the systemic and pulmonary arterial pressure was slower and/or less efficient after the second haemorrhage, due to a less pronounced increase of the vascular resistance. Relative bradycardia, in association with the abrupt fall of the mean systemic arterial pressure, was more apparent during the first haemorrhage. The plasma levels of vasopressin, renin activity and angiotensin II were increased by each blood removal, but the vasopressin response to the second haemorrhage was significantly reduced. The plasma noradrenaline concentration was slightly and transiently elevated only in response to the second haemorrhage. The concentration of neuropeptide Y-like immunoreactivity in plasma was unaffected by both haemorrhages. It is suggested that the reduced and delayed increase in the systemic vascular resistance, accompanied by impaired recovery of the arterial pressure, and the relative absence of 'bleeding bradycardia', during the second haemorrhage, were due to the diminished vasopressin response.