Normotensive placental extracellular vesicles provide long-term protection against hypertension and cardiovascular disease.

BACKGROUND: Women with normotensive pregnancy are at a reduced risk of developing cardiovascular disease postpartum compared with those who experience hypertensive conditions during pregnancy. However, the underlying mechanisms remain poorly understood. During normotensive pregnancy, vast numbers of placental extracellular vesicles are released into the maternal circulation, which protect endothelial cells from activation and alter maternal vascular tone. We hypothesized that placental extracellular vesicles play a mechanistic role in lowering the risk of cardiovascular disease following normotensive pregnancy. OBJECTIVE: This study aimed to investigate the long-term effects of placental extracellular vesicles derived from normotensive term placentae on the cardiovascular system and explore the mechanisms underlying their biological effects. STUDY DESIGN: Spontaneously hypertensive rats were injected with placental extracellular vesicles from normotensive term pregnancies (2 mg/kg each time, n=8) or vehicle control (n=9) at 3 months of age. Blood pressure and cardiac function were regularly monitored from 3 months to 15 months of age. The response of mesenteric resistance arteries to vasoactive substances was investigated to evaluate vascular function. Cardiac remodeling, small artery remodeling, and renal function were investigated to comprehensively assess the impact of placental extracellular vesicles on cardiovascular and renal health. RESULTS: Compared with vehicle-treated control animals, rats treated with normotensive placental extracellular vesicles exhibited a significantly lower increase in blood pressure and improved cardiac function. Furthermore, the vasodilator response to the endothelium-dependent agonist acetylcholine was significantly enhanced in the normotensive placental extracellular vesicle-treated spontaneously hypertensive rats compared with the control. Moreover, treatment with placental extracellular vesicles reduced wall thickening of small renal vessels and attenuated renal fibrosis. CONCLUSION: Placental extracellular vesicles from normotensive term pregnancies have long-lasting protective effects reducing hypertension and mitigating cardiovascular damage in vivo.

Combined Arterial Hypertension and Ischemic Stroke Exaggerate Anesthesia-Related Hypotension and Cerebral Oxygenation Deficits: A Preclinical Study.

BACKGROUND: Intraoperative arterial hypotension (IOH) is a common side effect of general anesthesia (GA), associated with poor outcomes in ischemic stroke. While IOH is more prevalent with hypertension, it is unknown whether IOH may differ when GA is induced during ischemic stroke, versus other clinical settings. This is important given that many stroke patients receive GA for endovascular thrombectomy. METHODS: We evaluate the cardiovascular responses to volatile GA (isoflurane in 100% o2 ) before and during middle cerebral artery occlusion stroke in rats instrumented to record blood pressure (BP) and cerebral tissue oxygenation (p o2 ) in the projected penumbra, in clinically relevant cohorts of normotensive (Wistar rat, n = 10), treated hypertensive (spontaneously hypertensive [SH] + enalapril, n = 12), and untreated hypertensive (SH rat, n = 12). RESULTS: During baseline induction of GA, IOH was similar in normotensive, treated hypertensive, and untreated hypertensive rats during the induction phase (first 10 minutes) (-24 ± 15 vs -28 ± 22 vs -48 ± 24 mm Hg; P > .05) and across the procedure (-24 ± 13 vs -30 ± 35 vs -39 ± 27 mm Hg; P > .05). Despite the BP reduction, cerebral p o2 increased by ~50% in all groups during the procedure. When inducing GA after 2 hours, all stroke groups showed a greater magnitude IOH compared to baseline GA induction, with larger falls in treated (-79 ± 24 mm Hg; P = .0202) and untreated(-105 ± 43 mm Hg; P < .001) hypertensive rats versus normotensives (-49 ± 21 mm Hg). This was accompanied by smaller increases in cerebral p o2 in normotensive rats (19% ± 32%; P = .0144 versus no-stroke); but a decrease in cerebral p o2 in treated (-11% ± 19%; P = .0048) and untreated (-12% ± 15%; P = .0003) hypertensive rats. Sham animals (normotensive and hypertensive) showed similar magnitude and pattern of IOH when induced with GA before and after sham procedure. CONCLUSIONS: Our findings are the first demonstration that ischemic stroke per se increases the severity of IOH, particularly when combined with a prior history of hypertension; this combination appears to compromise penumbral perfusion.

The Blocking of Integrin-Mediated Interactions with Maternal Endothelial Cells Reversed the Endothelial Cell Dysfunction Induced by EVs, Derived from Preeclamptic Placentae.

Placental extracellular vesicles (EVs) have increasingly been recognized as a major mediator of feto-maternal communication. However, the cellular and molecular mechanisms of the uptake of placental EVs by recipient cells are still not well-understood. We previously reported that placental EVs target a limited number of organs in vivo. In the current study, we investigated the mechanisms underlying the uptake of placental EVs into target cells. Placental EVs were derived from explant cultures of normal or preeclamptic placentae. The mechanisms underlying the uptake of placental EVs were elucidated, using the phagocytosis or endocytosis inhibitor, trypsin-treatment or integrin-blocking peptides. The endothelial cell activation was studied using the monocyte adhesion assay after the preeclamptic EVs exposure, with and/or without treatment with the integrin blocking peptide, YIGSR. The cellular mechanism of the uptake of the placental EVs was time, concentration and energy-dependent and both the phagocytosis and endocytosis were involved in this process. Additionally, proteins on the surface of the placental EVs, including integrins, were involved in the EV uptake process. Furthermore, inhibiting the uptake of preeclamptic EVs with YIGSR, reduced the endothelial cell activation. The interaction between the placental EVs and the recipient cells is mediated by integrins, and the cellular uptake is mediated by a combination of both phagocytosis and endocytosis.

Renal denervation in male rats with heart failure improves ventricular sympathetic nerve innervation and function.

Heart failure is characterized by the loss of sympathetic innervation to the ventricles, contributing to impaired cardiac function and arrhythmogenesis. We hypothesized that renal denervation (RDx) would reverse this loss. Male Wistar rats underwent myocardial infarction (MI) or sham surgery and progressed into heart failure for 4 wk before receiving bilateral RDx or sham RDx. After additional 3 wk, left ventricular (LV) function was assessed, and ventricular sympathetic nerve fiber density was determined via histology. Post-MI heart failure rats displayed significant reductions in ventricular sympathetic innervation and tissue norepinephrine content (nerve fiber density in the LV of MI+sham RDx hearts was 0.31 ± 0.05% vs. 1.00 ± 0.10% in sham MI+sham RDx group, P < 0.05), and RDx significantly increased ventricular sympathetic innervation (0.76 ± 0.14%, P < 0.05) and tissue norepinephrine content. MI was associated with an increase in fibrosis of the noninfarcted ventricular myocardium, which was attenuated by RDx. RDx improved LV ejection fraction and end-systolic and -diastolic areas when compared with pre-RDx levels. This is the first study to show an interaction between renal nerve activity and cardiac sympathetic nerve innervation in heart failure. Our findings show denervating the renal nerves improves cardiac sympathetic innervation and function in the post-MI failing heart.

Myocardial energetics is not compromised during compensated hypertrophy in the Dahl salt-sensitive rat model of hypertension.

Salt-induced hypertension leads to development of left ventricular hypertrophy in the Dahl salt-sensitive (Dahl/SS) rat. Before progression to left ventricular failure, the heart initially undergoes a compensated hypertrophic response. We hypothesized that changes in myocardial energetics may be an early indicator of transition to failure. Dahl/SS rats and their salt-resistant consomic controls (SS-13(BN)) were placed on either a low- or high-salt diet to generate four cohorts: Dahl-SS rats on a low- (Dahl-LS) or high-salt diet (Dahl-HS), and SS-13(BN) rats on a low- (SSBN-LS) or high-salt diet (SSBN-HS). We isolated left ventricular trabeculae and characterized their mechanoenergetic performance. Our results show, at most, modest effects of salt-induced compensated hypertrophy on myocardial energetics. We found that the Dahl-HS cohort had a higher work-loop heat of activation (estimated from the intercept of the heat vs. relative afterload relationship generated from work-loop contractions) relative to the SSBN-HS cohort and a higher economy of contraction (inverse of the slope of the heat vs. active stress relation) relative to the Dahl-LS cohort. The maximum extent of shortening and maximum shortening velocity of the Dahl/SS groups were higher than those of the SS-13(BN) groups. Despite these differences, no significant effect of salt-induced hypertension was observed for either peak work output or peak mechanical efficiency during compensated hypertrophy.

Arterial baroreceptor reflex control of renal sympathetic nerve activity following chronic myocardial infarction in male, female, and ovariectomized female rats.

There is controversy regarding whether the arterial baroreflex control of renal sympathetic nerve activity (SNA) in heart failure is altered. We investigated the impact of sex and ovarian hormones on changes in the arterial baroreflex control of renal SNA following a chronic myocardial infarction (MI). Renal SNA and arterial pressure were recorded in chloralose-urethane anesthetized male, female, and ovariectomized female (OVX) Wistar rats 6-7 wk postsham or MI surgery. Animals were grouped according to MI size (sham, small and large MI). Ovary-intact females had a lower mortality rate post-MI (24%) compared with both males (38%) and OVX (50%) (P < 0.05). Males and OVX with large MI, but not small MI, displayed an impaired ability of the arterial baroreflex to inhibit renal SNA. As a result, the male large MI group (49 ± 6 vs. 84 ± 5% in male sham group) and OVX large MI group (37 ± 3 vs. 75 ± 5% in OVX sham group) displayed significantly reduced arterial baroreflex range of control of normalized renal SNA (P < 0.05). In ovary-intact females, arterial baroreflex control of normalized renal SNA was unchanged regardless of MI size. In males and OVX there was a significant, positive correlation between left ventricle (LV) ejection fraction and arterial baroreflex range of control of normalized renal SNA, but not absolute renal SNA, that was not evident in ovary-intact females. The current findings demonstrate that the arterial baroreflex control of renal SNA post-MI is preserved in ovary-intact females, and the state of left ventricular dysfunction significantly impacts on the changes in the arterial baroreflex post-MI.

Renal sympathetic nerves - what have they got to do with cardiovascular disease?

NEW FINDINGS: What is the topic of this review? This review examines the role played by renal sympathetic nerves in the regulation of cardiovascular function, focusing on changes that occur during the development of hypertension and heart failure. What advances does it highlight? While elevated levels of renal sympathetic activity are a feature of many cardiovascular diseases, the relationship is not straightforward, especially in the case of hypertension. This review highlights that before consideration of targeting the renal nerves in the clinical management of cardiovascular diseases it is essential that their role in the development of the disease is established. In recent years, with the development of new clinical techniques to target the renal nerves specifically, we have seen a renewed interest in the role of the renal sympathetic nerves in the development of cardiovascular diseases. In understanding the potential of renal nerve ablation for the treatment of cardiovascular disease, first the role played by these nerves in cardiovascular regulation must be determined. Elevated renal sympathetic activity not only has the potential to increase fluid retention but may also act in a feedforward manner to increase sympathetic activation further, increasing the workload of the heart and the potential for arrhythmias. Direct recordings of renal sympathetic nerve activity in animal models of hypertension and renal noradrenaline spillover levels in individual patients with hypertension have illustrated that hypertension is not always accompanied by an increase in renal sympathetic activity. Elevated renal sympathetic nerve activity is a feature of severe heart failure, but whether removal of the renal nerves then compromises the ability to maintain cardiac function when faced with a stressor such as sepsis remains unclear. Understanding when increased renal sympathetic drive is contributing to the progression of cardiovascular diseases such as hypertension and heart failure would appear to be the key to understanding when renal nerve ablation is likely to be of benefit.

Estradiol alters the chemosensitive cardiac afferent reflex in female rats by augmenting sympathoinhibition and attenuating sympathoexcitation.

The chemosensitive cardiac vagal and sympathetic afferent reflexes are implicated in driving pathophysiological changes in sympathetic nerve activity (SNA) in cardiovascular disease states. This study investigated the impact of sex and ovarian hormones on the chemosensitive cardiac afferent reflex. Experiments were performed in anaesthetized, sinoaortic baroreceptor denervated male, female and ovariectomized female (OVX) Wistar rats with either intact cardiac innervation or bilateral vagotomy. To investigate the chemosensitive cardiac afferent reflexes renal SNA, heart rate (HR) and arterial pressure (AP) were recorded before and following application of capsaicin onto the epicardial surface of the left ventricle. Compared to males, ovary-intact females displayed similar cardiac afferent reflex mediated changes in renal SNA albeit with a reduced maximum sympathetic reflex driven increase in renal SNA. In females, ovariectomy significantly attenuated the cardiac vagal afferent reflex mediated inhibition of renal SNA (renal SNA decreased 2 ± 17% in OVX versus -50 ± 4% in ovary-intact females, P < 0.05) and augmented cardiac sympathetic afferent reflex mediated sympathoexcitation (renal SNA increased 91 ± 11% in OVX vs 62 ± 9% in ovary-intact females, P < 0.05) so that overall increases in reflex driven sympathoexcitation were significantly enhanced. Chronic estradiol replacement, but not progesterone replacement, begun at time of ovariectomy restored cardiac afferent reflex responses to be similar as ovary-intact females. Vagal denervation eliminated all group differences. The current findings show ovariectomy in female rats, mimicking menopause in women, results in greater chemosensitive cardiac afferent reflex driven sympathoexcitation and does so, at least partly, via the loss of estradiols actions on the cardiac vagal afferent reflex pathway.

The afterload-dependent peak efficiency of the isolated working rat heart is unaffected by streptozotocin-induced diabetes.

BACKGROUND: Diabetes is known to alter the energy metabolism of the heart. Thus, it may be expected to affect the efficiency of contraction (i.e., the ratio of mechanical work output to metabolic energy input). The literature on the subject is conflicting. The majority of studies have reported a reduction of myocardial efficiency of the diabetic heart, yet a number of studies have returned a null effect. We propose that these discrepant findings can be reconciled by examining the dependence of myocardial efficiency on afterload. METHODS: We performed experiments on streptozotocin (STZ)-induced diabetic rats (7-8 weeks post-induction), subjecting their (isolated) hearts to a wide range of afterloads (40 mmHg to maximal, where aortic flow approached zero). We measured work output and oxygen consumption, and their suitably scaled ratio (i.e., myocardial efficiency). RESULTS: We found that myocardial efficiency is a complex function of afterload: its value peaks in the mid-range and decreases on either side. Diabetes reduced the maximal afterload to which the hearts could pump (105 mmHg versus 150 mmHg). Thus, at high afterloads (for example, 90 mmHg), the efficiency of the STZ heart was lower than that of the healthy heart (10.4% versus 14.5%) due to its decreased work output. Diabetes also reduced the afterload at which peak efficiency occurred (optimal afterload: 63 mmHg versus 83 mmHg). Despite these negative effects, the peak value of myocardial efficiency (14.7%) was unaffected by diabetes. CONCLUSIONS: Diabetes reduces the ability of the heart to pump at high afterloads and, consequently, reduces the afterload at which peak efficiency occurs. However, the peak efficiency of the isolated working rat heart remains unaffected by STZ-induced diabetes.

Extracellular vesicles and their effect on vascular haemodynamics: a systematic review.

Extracellular vesicles (EVs) are released from all cell types studied to date and act as intercellular communicators containing proteins, nucleic acids and lipid cargos. They have been shown to be involved in maintaining homoeostasis as well as playing a role in the development of pathology including hypertension and cardiovascular disease. It is estimated that there is 109-1010 circulating EVs/mL in the plasma of healthy individuals derived from various sources. While the effect of EVs on vascular haemodynamic parameters will be dependent on the details of the model studied, we systematically searched and summarized current literature to find patterns in how exogenously injected EVs affected vascular haemodynamics. Under homoeostatic conditions, evidence from wire and pressure myography data demonstrate that injecting isolated EVs derived from cell types found in blood and blood vessels resulted in the impairment of vasodilation in blood vessels ex vivo. Impaired vasodilation was also observed in rodents receiving intravenous injections of human plasma EVs from cardiovascular diseases including valvular heart disease, acute coronary syndrome, myocardial infarction and end stage renal disease. When EVs were derived from models of metabolic syndromes, such as diabetes, these EVs enhanced vasoconstriction responses in blood vessels ex vivo. There were fewer publications that assessed the effect of EVs in anaesthetised or conscious animals to confirm whether effects on the vasculature observed in ex vivo studies translated into alterations in vascular haemodynamics in vivo. In the available conscious animal studies, the in vivo data did not always align with the ex vivo data. This highlights the importance of in vivo work to determine the effects of EVs on the integrative vascular haemodynamics.

Forgotten Circulation: Reduced Mesenteric Venous Capacitance in Hypertensive Rats Is Improved by Decreasing Sympathetic Activity.

BACKGROUND: The mesenteric venous reservoir plays a vital role in mediating blood volume and pressure changes and is richly innervated by sympathetic nerves; however, the precise nature of venous sympathetic regulation and its role during hypertension remains unclear. We hypothesized that sympathetic drive to mesenteric veins in spontaneously hypertensive (SH) rats is raised, increasing mean circulatory filling pressure (MCFP), and impairing mesenteric capacitance. METHODS: Arterial pressure, central venous pressure, mesenteric arterial, and venous blood flow were measured simultaneously in conscious male Wistar and SH rats. MCFP was assessed using an intraatrial balloon. Hemodynamic responses to volume changes (±20%) were measured before and after ganglionic blockade and carotid body denervation. Sympathetic venoconstrictor activity was measured in situ. RESULTS: MCFP in vivo (10.8±1.6 versus 8.0±2.1 mm Hg; P=0.0005) and sympathetic venoconstrictor drive in situ (18±1 versus 10±2 µV; P<0.0001) were higher in SH rats; MCFP decreased in SH rats after hexamethonium and carotid body denervation (7.6±1.4; P<0.0001 and 8.5±1.0 mm Hg; P=0.0045). During volume changes, arterial pressure remained stable. With blood loss, net efflux of blood from the mesenteric bed was measured in both strains. However, during volume infusion, we observed net influx in Wistar (+2.3±2.6 mL/min) but efflux in SH rats (-1.0±1.0 mL/min; P=0.0032); this counterintuitive efflux was abolished by hexamethonium and carotid body denervation (+0.3±1.7 and 0.5±1.6 mL/min, respectively). CONCLUSIONS: In SH rats, excessive sympathetic venoconstriction elevates MCFP and reduces capacitance, impairing volume buffering by mesenteric veins. We propose selective targeting of mesenteric veins through sympathetic drive reduction as a novel therapeutic opportunity for hypertension.

Renal sympathetic nerve activity during asphyxia in fetal sheep.

The sympathetic nervous system (SNS) is an important mediator of fetal adaptation to life-threatening in utero challenges, such as asphyxia. Although the SNS is active well before term, SNS responses mature significantly over the last third of gestation, and its functional contribution to adaptation to asphyxia over this critical period of life remains unclear. Therefore, we examined the hypotheses that increased renal sympathetic nerve activity (RSNA) is the primary mediator of decreased renal vascular conductance (RVC) during complete umbilical cord occlusion in preterm fetal sheep (101 ± 1 days; term 147 days) and that near-term fetuses (119 ± 0 days) would have a more rapid initial vasomotor response, with a greater increase in RSNA. Causality of the relationship of RSNA and RVC was investigated using surgical (preterm) and chemical (near-term) denervation. All fetal sheep showed a significant increase in RSNA with occlusion, which was more sustained but not significantly greater near-term. The initial fall in RVC was more rapid in near-term than preterm fetal sheep and preceded the large increase in RSNA. These data suggest that although RSNA can increase as early as 0.7 gestation, it is not the primary determinant of RVC. This finding was supported by denervation studies. Interestingly, chemical denervation in near-term fetal sheep was associated with an initial fall in blood pressure, suggesting that by 0.8 gestation sympathetic innervation of nonrenal vascular beds is critical to maintain arterial blood pressure during the rapid initial adaptation to asphyxia.

Effects of sex and ovarian hormones on the initial renal sympathetic nerve activity response to myocardial infarction.

The physiological mechanisms contributing to sex differences following myocardial infarction (MI) are poorly understood. Given the strong relationship between sympathetic nerve activity (SNA) and outcome, we hypothesized there may be a sex difference in SNA responses to MI. In anaesthetized, open-chest male, female and ovariectomized (OVX) female Wistar rats, mean arterial pressure, heart rate and renal SNA were recorded in response to ligation of the left coronary artery. In males, renal SNA increased by 30 ± 6% in the first minute of coronary occlusion (P < 0.05) and remained elevated at 18 ± 7% above baseline (P < 0.05) at 2 h following MI. In response to MI, ovary-intact females displayed no change in renal SNA, whereas OVX females displayed a significant increase, similar to that seen in the males (increases of 43 ± 11% at 1 min and 21 ± 7% at 2 h post-MI, P < 0.05 versus intact females). Arterial baroreflex control of renal SNA had a smaller range in females (ovary intact and OVX) than males; no changes in arterial baroreflex responses were observed 1 h post-MI in males or females. Denervating the arterial baroreceptors abolished the renal SNA response to MI in the males, whereas in ovary-intact females and OVX females the response was unaltered. These findings suggest that ovarian hormones are able to blunt the initial sympathetic activation post-MI in females and that the importance of the arterial baroreflex in mediating initial sympathetic activation post-MI is different between the sexes.

Cardiovascular and autonomic responses to sexual activity in the rabbit.

1. Sexual intercourse is associated with an increased risk of death from arrhythmia development, myocardial infarction or stroke. It is unclear whether this increased risk is due to physical exertion alone or whether it is an inherent aspect of sexual activity itself. 2. Using a telemetric approach, we show that sexual activity is associated with transient (8-14 s) but profound increases in renal sympathetic nerve activity (RSNA; up to 22-fold that of baseline) in both male and female rabbits. This increase was significantly greater than that observed during physical exertion (three- to sixfold increase in RSNA). 3. In addition, we observed rapid transitions in male rabbits from tachycardia (422 ± 21 b.p.m.; P < 0.01) to bradycardia (186 ± 28 b.p.m.; P < 0.05) during and immediately following coitus. This suggests simultaneous activation of both the sympathetic and parasympathetic nervous systems. 4. The present study provides the first real-time insight into the extreme variation in neural and cardiovascular function occurring during sexual activity in normal healthy rabbits. Little is known about how the physiological responses to sexual activity may change under disease or drug-treatment states, and these findings may prove of use to these areas in future.

Correction to: Imaging tools for assessment of myocardial fibrosis in humans: the need for greater detail.

[This corrects the article DOI: 10.1007/s12551-020-00738-w.].

Fish oil supplementation of rats fed a high fat diet during pregnancy improves offspring insulin sensitivity.

Introduction: In rats, a maternal high-fat diet (HFD) leads to adverse metabolic changes in the adult offspring, similar to the children of mothers with obesity during pregnancy. Supplementation with a high dose of fish oil (FO) to pregnant rats fed a HFD has been shown to prevent the development of insulin resistance in adult offspring. However, the effects of supplementation at a translationally relevant dose remain unknown. Aim: To determine whether supplementation with a human-relevant dose of FO to pregnant rats can prevent the long-term adverse metabolic and cardiovascular effects of a maternal HFD on adult offspring. Methods: Female rats (N = 100, 90 days of age) were assigned to HFD (45% kcal from fat) or control diet (CD) for 14 days prior to mating and throughout pregnancy and lactation. Following mating, dams received a gel containing 0.05 ml of FO (human equivalent 2-3 ml) or a control gel on each day of pregnancy. This produced 4 groups, CD with control gel, CD with FO gel, HFD with control gel and HFD with FO gel. Plasma and tissue samples were collected at day 20 of pregnancy and postnatal day 2, 21, and 100. Adult offspring were assessed for insulin sensitivity, blood pressure, DXA scan, and 2D echocardiography. Results: There was an interaction between maternal diet and FO supplementation on insulin sensitivity (p = 0.005) and cardiac function (p < 0.01). A maternal HFD resulted in impaired insulin sensitivity in the adult offspring (p = 0.005 males, p = 0.001 females). FO supplementation in the context of a maternal HFD prevented the reduction in insulin sensitivity in offspring (p = 0.05 males, p = 0.0001 females). However, in dams consuming CD, FO supplementation led to impaired insulin sensitivity (p = 0.02 males, p = 0.001 females), greater body weight and reduced cardiac ejection fraction. Conclusion: The effects of a human-relevant dose of maternal FO on offspring outcomes were dependent on the maternal diet, so that FO was beneficial to the offspring if the mother consumed a HFD, but deleterious if the mother consumed a control diet. This study suggests that supplementation with FO should be targeted to women expected to have abnormalities of metabolism such as those with overweight and obesity.

Maturation-related changes in the pattern of renal sympathetic nerve activity from fetal life to adulthood.

Sympathetic nerve activity (SNA) has two main properties, the presence of co-ordinated bursts of activity, indicative of many nerve fibres firing at a similar time, and entrainment of the bursts to the cardiac cycle, due to inhibitory input from baroreceptors to a network of cell groups within the CNS. Although this patterning is used as a 'gold standard' for the identification of successful nerve recordings, the maturation of these basic features of SNA from fetal life to adulthood has not been investigated. Using a telemetry-based nerve amplifier, renal SNA (RSNA) was recorded in preterm (99 ± 1 days gestation; term 147 days) and near-term fetal sheep (119 ± 0 days gestation), without anaesthesia or paralysis, and contrasted with RSNA recorded in adult sheep. All three age groups showed a classic bursting pattern of RSNA and co-ordination of bursts with the cardiac cycle. However, the delay between diastole and the next peak in RSNA was longest in preterm fetuses (319 ± 1 ms), compared with near-term fetuses (250 ± 13 ms), and shortest in the adult sheep (174 ± 38 ms). This was independent of the maturational decrease in heart rate. The near-term fetuses showed a marked but sleep-state-dependent increase in resting RSNA compared with preterm fetuses. Although entrainment with the pressure pulse suggests that the intricate circuitry within the CNS is developed in the preterm fetus, the decrease in the length of the delay suggests continuing maturation of this key feature of RSNA in the last third of gestation and after birth.

Baroreflex control of renal sympathetic nerve activity and heart rate in near-term fetal sheep.

Late preterm infants, born between 34 and 36 weeks gestation, have significantly higher morbidity than neonates born at full term, which may be partly related to reduced sensitivity of the arterial baroreflex. The present study assessed baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) in near-term fetal sheep at 123 ± 1 days gestation. At this age, although fetuses are not fully mature in some respects (term is 147 days), sleep-state cycling is established [between high-voltage, low-frequency (HV) and low-voltage, high-frequency (LV) sleep], and neural myelination is similar to the term human infant. Fetal sheep were instrumented to record blood pressure (BP), HR (n = 15) and RSNA (n = 5). Blood pressure was manipulated using vasoactive drugs, phenylephrine and sodium nitroprusside. In both HV and LV sleep, phenylephrine was associated with increased arterial BP and decreased HR. In HV sleep, phenylephrine was associated with a fall in RSNA, from 124 ± 14 to 58 ± 11% (P < 0.05), but no significant change in RSNA in LV sleep. In contrast, the fall in BP after sodium nitroprusside was associated with a significant increase in HR during LV but not HV sleep, and there was no significant effect of hypotension on RSNA. These data demonstrate that in near-term fetal sheep baroreflex activity is only partly active and is highly modulated by sleep state. Critically, there was no RSNA response to marked hypotension; this finding has implications for the ability of the late preterm fetus to adapt to low BP.

A method to evaluate dynamic cerebral pressure-flow relationships in the conscious rat.

The classic dogma of cerebral autoregulation is that cerebral blood flow is steadily maintained across a wide range of perfusion pressures. This has been challenged by recent studies suggesting little to no "autoregulatory plateau" in the relationship between cerebral blood flow and blood pressure (BP). Therefore, the mechanisms underlying the cerebral pressure-flow relationship still require further understanding. Here, we present a novel approach to examine dynamic cerebral autoregulation in conscious Wistar rats (n = 16) instrumented to measure BP and internal carotid blood flow (iCBF), as an indicator of cerebral blood flow. Transient reductions in BP were induced by occluding the vena cava via inflation of a chronically implanted intravascular silicone balloon. Falls in BP were paralleled by progressive decreases in iCBF, with no evidence of a steady-state plateau. No significant changes in internal carotid vascular resistance (iCVR) were observed. In contrast, intravenous infusions of the vasoactive drug sodium nitroprusside (SNP) produced a similar fall in BP but increases in iCBF and decreases in iCVR were observed. These data suggest a considerable confounding influence of vasodilatory drugs such as SNP on cerebrovascular tone in the rat, making them unsuitable to investigate cerebral autoregulation. We demonstrate that our technique of transient vena cava occlusion produced reliable and repeatable depressor responses, highlighting the potential for our approach to permit assessment of the dynamic cerebral pressure-flow relationship over time in conscious rats.NEW & NOTEWORTHY We present a novel technique to overcome the use of vasoactive agents when studying cerebrovascular dynamics in the conscious rat. Our method of vena cava occlusion to reduce BP was associated with decreased iCBF and no change in iCVR. In contrast, comparable BP falls with intravenous SNP increased iCBF and reduced iCVR. Thus, the dynamic cerebral pressure-flow relationship shows a narrower, less level autoregulatory plateau than conventionally thought. We confirm our method allows repeatable assessment of cerebrovascular dynamics in conscious rats.

Quantifying sympathetic nerve activity: problems, pitfalls and the need for standardization.

Since the first recording of sympathetic nerve activity (SNA) early last century, numerous methods for presentation of the resulting data have developed. In this paper, we discuss the common ways of describing SNA and their application to chronic recordings. Suggestions on assessing the quality of SNA are made, including the use of arterial pressure wave-triggered averages and nasopharyngeal stimuli. Calculation of the zero level of the SNA signal from recordings during ganglionic blockade, the average level between bursts and the minimum of arterial pressure wave-triggered averages are compared and shown to be equivalent. The use of normalization between zero and maximal SNA levels to allow comparison between groups is discussed. We recommend that measured microvolt levels of integrated SNA be presented (with the zero/noise level subtracted), along with burst amplitude and frequency information whenever possible. We propose that standardization of the quantifying/reporting of SNA will allow better comparison between disease models and between research groups and ultimately allow data to be more reflective of the human situation.