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.
- heart failure
- sex hormones
rates of myocardial infarction (MI) and post-MI heart failure are increasing worldwide (26). In heart failure, high levels of sympathetic drive strongly correlate with a negative prognostic outcome in the clinic (28, 44). In particular, elevated sympathetic nerve activity (SNA) specifically to the kidneys is closely associated with increased morbidity and mortality in heart failure (41, 44). Chronic elevations in renal SNA increase sodium and fluid retention and activate the renin-angiotensin system, all hallmarks of heart failure (9, 10, 41).
Elevated renal SNA in heart failure has typically been associated with an impaired arterial baroreflex function (13, 40, 53, 58, 61). Impaired arterial baroreflex function potentially leaves renal SNA more responsive to excitatory stimuli, increasing renal SNA and driving disease progression (12, 13). Most studies that have examined the arterial baroreflex control of renal SNA in heart failure have been performed in male animals and have shown that the arterial baroreflex has a reduced sensitivity (11, 38) and often also a reduced range of control (13, 31, 61). However, there is still controversy regarding the arterial baroreflex control of renal SNA in heart failure. For example, the arterial baroreflex control of renal SNA is unaltered in conscious female sheep with pacing-induced mild heart failure, measured as bursts of SNA per 100 heartbeats (48). Furthermore, in female rats and anesthetized dogs of mixed sex with either chemical- or pacing-induced heart failure, the arterial baroreflex control of renal SNA, measured directly, is unaltered (8, 50). In humans, the interpretation of muscle SNA recordings is influenced by the changes in heart rate (HR), since muscle SNA in humans is typically quantified in direct relation to HR as bursts per 100 heartbeats or unit/time (14). Therefore, when the change in arterial baroreflex control of HR in heart failure is accommodated in the muscle SNA analysis, it has been argued that baroreflex control of muscle SNA is unaltered in humans with heart failure (14). It is important to understand the factors contributing to inconsistent findings regarding arterial baroreflex function in heart failure given its potential in driving changes in renal SNA and as a possible treatment target (20).
Although previous research has used females when investigating the arterial baroreflex in heart failure (8, 48, 50), a sex-specific alteration in the arterial baroreflex control of renal SNA during the development of heart failure has not been characterized. Sex differences in arterial baroreflex regulation of SNA do exist, with healthy female rats demonstrating a reduced ability of the arterial baroreflex to increase SNA in animals (47, 15), a finding confirmed in humans using muscle SNA recordings (52). Furthermore, varying levels of ovarian hormones in females during the reproductive cycle or following menopause can significantly alter arterial baroreflex regulation of SNA (16, 18, 37, 52). It is possible that sex and sex hormone status may contribute to inconsistent findings regarding the arterial baroreflex regulation in heart failure. Based on previous studies, including those in female sheep (48), we hypothesized that, in ovary-intact females, the arterial baroreceptor reflex regulation of renal SNA will be preserved following MI.
Furthermore, it is possible that the degree of left ventricular dysfunction may also contribute to differences observed in changes in the arterial baroreflex. Given the close association between sympathetic regulation and outcome, we hypothesized that only in states of relatively severe left ventricular dysfunction would the arterial baroreflex control of renal SNA be impaired in males. As in the human heart, the size of MI is inversely correlated with left ventricular function in rats (46). Therefore, the MI model in the rat is well suited to testing the association between the state of left ventricular performance and arterial baroreflex control of renal SNA.
MATERIALS AND METHODS
Experiments were conducted in 16 male and 33 female Wistar rats and were approved by, and carried out following the guidelines of, the Animal Ethics Committee of the University of Auckland. In the current study there were nine groups representing males, females, or ovariectomized females (OVX) who had undergone sham MI or MI surgery. Rats that had an MI <25% of the left ventricular wall (based on postmortem examination of the left ventricle) were grouped into “small MI” groups. Rats that had an MI >25% of the left ventricular wall (based on postmortem examination of the left ventricle) were grouped into “large MI” groups (55).
Ovariectomy surgery was performed in female rats weighing 120–150 grams and aged 5–6 wk old as described previously (47), 2 wk before MI or sham surgery. Two weeks were allowed between ovariectomy surgery and MI surgery to allow full recovery and acclimatization before MI surgery. Furthermore, allowing a period of time between the removal of ovaries and MI better reflects the clinical situation where the majority of incidences of MI in women occur postmenopause (19). MI surgery was performed under isoflurane anesthesia (2% in oxygen) in animals aged 7–10 wk old while the rat was artificially ventilated (model 680; Harvard apparatus, Holliston, MA). At the beginning of surgery, all animals were given antibiotics (12.5 mg/kg enroflaxacin, Baytril; Bayer) and analgesia [20 μg/kg buprenorphine, Temgesic (Reckitt Benckiser), and again 24 h later]. The chest was opened via an incision through the fourth intercostal space, and the pericardium was removed. In the MI groups, MI was induced by tying off the left anterior descending coronary artery (LAD) 2–3 mm from its origin using a 6-0 silk suture. In the sham groups a suture was passed through the heart wall, but the LAD was not tied off. At the conclusion of the surgery the lungs were reinflated, and then the chest was sutured closed. As soon as the rats regained consciousness they were returned to their home cages. A heating pad was placed in the cage for 24 h after the surgery. Before the experimental day all rats were housed two to four per cage with water and food ad libitum in a room of constant temperature (22°C) with a 12:12-h light-dark cycle. Experiments were performed 6–7 wk following MI. On the day of the experiment animals were anesthetized to effect over the course of an hour with the initial dose being an intraperitoneal injection of 66 mg α-chloralose and 825 mg urethane/kg body wt, and the total average dose of anesthetic was 120 mg α-chloralose and 1,500 mg urethane/kg body wt. Sufficient anesthesia was confirmed by testing for the complete removal of the hind limb reflex, tail pinch, and blink reflex. Anesthesia was maintained throughout the experiment by intravenous infusion or intraperitoneal injection of anesthetic (10% of initial dose every hour). Body temperature was maintained at 37°C by a heating pad and heating lamp. Before beginning surgical preparation and once animals were adequately anesthetized echocardiography was performed to determine left ventricular function. Because of a malfunction in the storage of the echocardiography data, only a subset of the echocardiography data were analyzed.
Once a sufficient level of anesthesia was obtained the trachea was cannulated, and the rat was artificially ventilated (model 680; Harvard Apparatus) with inspirate gas enriched with O2 (∼50% O2) and a tidal volume of ∼3–4 ml and breathing rate of ∼70/min. The femoral artery and vein were cannulated to monitor arterial pressure and for administration of drugs, respectively. To record renal SNA a retroperitoneal incision was used to expose the left kidney. The renal nerve was identified and placed within a pair of coiled stainless steel electrodes. The electrode wires and nerve were then coated in a silicone elastomer (Kwik-sil, World Precision Instruments, Sarasota, FL). Renal SNA was wirelessly recorded via a high-gain amplifier (model TR46SP; Telemetry Research Millar, Auckland, NZ) with the signal transmitted to a receiver (TR162; Telemetry Research Millar). Renal SNA, arterial pressure, and HR (detected from the arterial pressure waveform) were recorded throughout the entire experiment.
Arterial baroreceptor reflex responses were obtained by acutely decreasing and increasing arterial pressure while simultaneously recording arterial pressure, HR, and renal SNA. Arterial pressure was lowered to ∼40 mmHg by infusing sodium nitroprusside (SNP, 5–20 μg, rate of change was 1–2 mmHg/s). Following SNP infusion and once arterial pressure had returned to baseline level, arterial pressure was increased to ∼150 mmHg by infusing phenylephrine (PE, 20–80 μg, rate of change was 1–2 mmHg/s). SNP and PE were applied sequentially, and a period of at least 5 min was allowed between each grouped infusion of SNP and PE to allow renal SNA and arterial pressure to return to baseline levels before another baroreceptor reflex response was initiated.
Heart and lung weights.
Once the experimental protocols were completed the animal was killed with an overdose of chloralose-urethane. The heart and lungs were excised and weighed, and total heart weight and lung weight were recorded for each animal.
Determination of myocardial infarct size.
Once the heart had been weighed it was frozen and then cut into three to four transmural sections ∼3 mm in thickness. The heart sections were stained using triphenyltetrazolium chloride (Sigma-Aldrich). After 20 min of incubation in the triphenyltetrazolium chloride solution, the heart sections underwent a bleach cycle in formaldehyde for 30 min. The heart sections were subsequently photographed using a digital camera (Canon SX100). The lengths of the entire left ventricular endocardial and epicardial circumferences were measured, and that segment of the endocardial and epicardial circumferences made up by the infarcted portion from each heart section was measured (Fig. 1). The percentage of the left ventricle that was infarcted was calculated from these measurements, as described previously (46). Analysis of images was performed on Image J Software.
Data collection and statistical analysis.
The original renal SNA signal was amplified and filtered between 50 and 5,000 Hz, full-wave rectified, and integrated using a low-pass filter with a 20-ms time constant. All data were sampled at 500 Hz using an analog-to-digital acquisition card (PCI-6024E; National Instruments, Austin, TX). All subsequent data analysis was performed using a data acquisition program (Universal Acquisition and Analysis, version 11; University of Auckland, Auckland, New Zealand). Two-second averages of renal SNA, mean arterial pressure, and HR were saved continuously throughout the experiment, and these were used for analysis of arterial baroreflex data.
Arterial baroreceptor reflex curves were created from the arterial pressure and renal SNA responses to SNP and PE according to the work performed by Ricketts and Head (49). In brief, a five-parameter logistic equation was used to fit the individual data points obtained from the concurrent levels of arterial pressure and renal SNA with a nonlinear regression curve. The five parameters used were the level of maximum inhibition of renal SNA during elevated arterial pressure (lower plateau), range of baroreflex-mediated change in renal SNA, level of arterial pressure at the midway between minimum and maximum levels of renal SNA (BP50), and the upper and lower curvatures. At least two baroreflex curves were created for each animal, and the subsequent values were averaged. The noise level on the integrated renal SNA signal was determined as the average over at least 20 cycles when SNA was at the lowest level between two distinct bursts of nerve activity (21).
Normalization of renal SNA was performed by analyzing the change in integrated renal SNA as a percentage change from baseline (100%), which was defined as the mean renal SNA immediately before infusion of PE or SNP. Experimentally, investigators have typically normalized the changes in renal SNA due to the technical issues surrounding whole nerve recordings. However, Guild et al. (21) have proposed that the absolute values obtained during renal nerve recordings are consistent across groups, and have therefore argued against normalization. Because results following normalization can be influenced by factors such as the baseline level, and given that baseline renal SNA was expected to be altered in heart failure, we have presented both normalized and absolute arterial baroreflex renal SNA.
Fisher's exact test was used to compare the occurrence of mortality following either a small MI or large MI following MI surgery between males, ovary-intact females, and OVX females. Two-way ANOVA with Bonferroni post hoc analysis was used for comparisons among groups for all other data. Data are shown as means ± SE. P values <0.05 were considered significant.
Survival following MI in males and females.
The majority of deaths post-MI was within the first 24 h following MI surgery. However, 2 males died 3–4 wk post-MI; postmortem indicated these animals did have very large MI. Ovary-intact female rats had reduced mortality rates following MI compared with males [mortality rate post-MI in ovary-intact females was 4 deaths from 17 that had MI surgery (24%) vs. 8/21 (38%) in males, P = 0.046] and OVX females [mortality rate post-MI in OVX females was 11/22 (50%) vs. 4/17 (24%) in ovary-intact females, P = 0.002]. There was no difference in mortality rate post-MI between OVX females and males (P > 0.05).
The body weights in the ovary-intact female groups were lower than the males in all cases (Table 1). The average body weight of the OVX sham group was lower compared with the male sham group (Table 1). Both of the OVX MI groups had greater body weights compared with the respective ovary-intact female MI groups (Table 1). Baseline mean arterial pressure and HR were not significantly different between groups (Table 1). Baseline renal SNA was significantly higher in the ovary-intact female large MI group compared with the female sham group (Table 1). No other statistically significant differences in baseline renal SNA were observed.
The male, female, and OVX large MI groups all had similar MI sizes (Table 1). As expected, MI sizes in all large MI groups were significantly greater compared with MI sizes in the respective small MI group (Table 1). Heart weight-to-body weight ratios were significantly elevated in all large MI groups, but not in small MI groups, compared with the sham groups (Table 1). Lung weight-to-body weight ratios were significantly elevated in female and OVX large MI groups but not in the male large MI group compared with the respective sham groups (Table 1).
Because of low numbers in the echocardiography analysis the sexes were grouped together into sham, small MI, and large MI. Animals with large MI displayed significantly reduced ejection fraction compared with animals with sham and small MI (Table 2). Animals with large MI, but not small MI, displayed significantly greater left ventricular end-diastolic diameter and end-systolic diameter compared with shams (Table 2). Ejection fraction and fractional shortening were significantly reduced in animals with small MI compared with sham animals (Table 2). MI size was negatively, and significantly, correlated with left ventricular ejection fraction in the animals that had echocardiography performed (R2 = 0.6179, P = 0.0041), similar to previous reports (45).
The arterial baroreceptor reflex control of normalized renal SNA (%change from baseline) in males and females with a small or large chronic MI.
Example raw recordings representing arterial baroreceptor reflex renal SNA responses are shown in Fig. 2. Arterial baroreflex curves demonstrating the relationship between arterial pressure and renal SNA in normalized units (percentage change from baseline) for male, female, and OVX groups are shown in Fig. 3. The normalized arterial baroreflex renal SNA curves were not different between the male, female, and OVX sham groups and when comparing between small MI groups and sham groups, respectively (Table 3).
In male and OVX rats with large MI, significant changes in the normalized arterial baroreflex renal SNA responses were observed. The arterial baroreflex renal SNA maximum gain in the male large MI group was reduced (−1.92 ± 0.39 vs. −3.67 ± 0.74%/mmHg in male sham group, P < 0.05, Fig. 3). The arterial baroreflex renal SNA maximum gain was not different when comparing ovary-intact and OVX female large MI groups with their respective sham groups (Fig. 3). The arterial baroreflex range of control was significantly reduced in both male and OVX large MI groups compared with their respective sham group (Table 3). The decrease in arterial baroreflex range in the male and OVX large MI groups was predominantly mediated by a significantly elevated lower plateau of the arterial baroreflex curve (lower plateau in male large MI group was 63 ± 3 vs. 37 ± 4% in the male sham group, P < 0.05, and in the OVX large MI group was 65 ± 5 vs. 35 ± 4% in the OVX sham group, P < 0.05, Fig. 3). The lower plateau of normalized arterial baroreflex curve was unaltered in the ovary-intact female large MI group (47 ± 6 vs. 41 ± 4% in the female sham group, P > 0.05) as was the arterial baroreflex range of control (70 ± 9 vs. 63 ± 5% in the female sham group, P > 0.05, Table 3). Both the ovary-intact female small- and large-MI group displayed a left shift in the baroreflex curve as represented by a reduced BP50 compared with the female sham group (Table 3).
The arterial baroreceptor reflex control of absolute renal SNA (μV) in males and females with a small or large chronic MI.
The mean absolute arterial baroreflex renal SNA responses are shown in Fig. 4. When presented in microvolts, male and OVX female arterial baroreflex responses were not different when comparing between sham, small MI, and large MI groups. In contrast, ovary-intact females with large MI displayed a significantly elevated renal SNA (μV) resting level that was associated with elevated lower and upper plateaus and increased range of the arterial baroreflex curve. Furthermore, arterial baroreflex maximum gain was enhanced in both small- and large-MI ovary-intact female groups compared with the sham group (Fig. 4).
There were no significant differences in the arterial baroreflex control of renal SNA as measured in microvolts between male and female sham groups (Table 4). The OVX sham group had an increased upper plateau and augmented range of the arterial baroreflex control of renal SNA compared with the ovary-intact female sham group (μV, Table 4). This was associated with a trend for the OVX sham group to display an elevated resting renal SNA (resting renal SNA was 9.20 ± 2.02 μV) compared with the ovary-intact female sham group (resting renal SNA was 4.15 ± 0.88 μV), although the difference did not reach statistical significance (P = 0.0977).
Relationship between arterial baroreflex control of renal SNA and ejection fraction.
When renal SNA was normalized as a percentage change from baseline a positive, significant, correlation between left ventricular ejection fraction and arterial baroreceptor reflex range of control over renal SNA was observed in males and OVX females (Fig. 5). No significant correlation was observed in ovary-intact females (Fig. 5).
When comparing the relationship between left ventricular ejection fraction and the arterial baroreflex control of absolute renal SNA (μV) no significant correlation was observed in males, ovary-intact females, or OVX females (Fig. 5).
The current study is the first to specifically and directly investigate changes and differences in the arterial baroreceptor reflex control of renal SNA between males, ovary-intact females, and OVX in MI-induced heart failure. The major findings of the current study are 1) only in male and OVX animals with relatively severe left ventricular dysfunction does the arterial baroreflex control of renal SNA become impaired, when the data are normalized as a percentage change from baseline, 2) arterial baroreflex control of renal SNA is preserved in ovary-intact females with MI-induced heart failure, when the data are normalized, and 3) the presentation of the arterial baroreflex data, whether as absolute or normalized units, can significantly impact on the interpretation of the findings. For example, when data are presented in absolute units ovary-intact females with large MI display a raised resting renal SNA and elevated lower and upper limits of the arterial baroreflex control of renal SNA alongside an augmented baroreflex sensitivity. The current findings provide evidence that help explain existing discrepancies between independent observations on the arterial baroreceptor reflex control of renal SNA post-MI.
Experimentally, ovarian hormones, particularly estrogen, have been associated with a plethora of favorable biological and physiological processes, suggesting that ovarian hormones are cardioprotective (27, 36, 39, 54, 62). This is supported by evidence in mice showing that the administration of estrogen both before and following MI in males and females reduces infarct size and protects cardiac function post-MI (4, 5). In the current study, mortality rates post-MI were lower in ovary-intact females compared with males, a finding consistent with previous studies and unlikely to be a result of bias created by differing sizes of MI (5). Ovariectomy before MI abolished the sex difference in mortality rates post-MI; therefore, our findings are consistent with previous observations suggesting that ovarian hormones are cardioprotective (4, 5).
Elevated renal norepinephrine spillover is associated with a worse outcome in heart failure patients, independent of total body, or cardiac, norepinephrine spillover (36). Furthermore, in male rats, the denervation of the renal nerves post-MI improves left ventricular performance, attenuating the development of heart failure (25, 41). Therefore, the evidence shows that elevated renal SNA can contribute to heart failure development and progression. The role the arterial baroreflex plays in driving the increase in renal sympathetic activity in heart failure remains controversial. In male rats the attenuation of the arterial baroreflex control of renal SNA in MI-induced heart failure, typically presented as a reduced gain and range, is associated with elevated renal SNA (10, 13, 40, 55, 61, 56). However, the current findings in ovary-intact females with large MI demonstrate that it is possible to have significantly elevated renal SNA in association with preserved arterial baroreflex regulation. Together with previous studies recording muscle SNA in human males, and cardiac and renal SNA in female sheep, these results challenge the concept that increases in SNA are associated with impairment of the arterial baroreflex (14, 48, 59). In addition, in the current study there was no increase in renal SNA observed in OVX with MI despite a reduced arterial baroreflex range. Therefore, an attenuated arterial baroreflex control of renal SNA is not necessarily associated with elevated renal SNA. Baroreceptor denervation alone has been suggested to have little effect on the progression of heart failure, suggesting that baroreflexes are only a contributing factor to driving the increase in renal SNA (1). Other sympathoexcitatory reflex pathways such as those driven by the cardiac and carotid body chemoreceptors would appear to act in conjunction with any impairment in baroreflex function to drive the increases in renal SNA (7, 17). A change in baroreflex function may not be essential for an increase in renal sympathetic drive in heart failure. However, our knowledge of the relationship between arterial baroreflex function and changes in renal SNA and interactions with other sympathoexcitory pathways does require further expanding.
Previous research in both anesthetized and conscious healthy rats have shown sex differences in the arterial baroreflex regulation of normalized renal SNA (15, 47). Furthermore, in conscious ovary-intact female rats the arterial baroreflex control of renal SNA varies across the estrous cycle, suggesting an effect of differing levels of circulating ovarian hormones (18). However, contradictions exist regarding the effects of sex and ovarian hormones on the arterial baroreflex regulation; for example, although previous work has demonstrated that the arterial baroreflex regulation of renal SNA varies across the estrous cycle (3, 18), both previous (24, 47) and the present evidence have shown that ovariectomy does not alter arterial baroreflex control of renal SNA. Given the potential importance of the arterial baroreflex in mediating changes in SNA in heart failure, future work investigating the mechanistic role of sex and ovarian hormones in mediating the arterial baroreflex is required.
Changes in the arterial baroreceptor reflex neural arc following chronic MI have been investigated predominantly in male subjects and include changes in the action and amount of neuromodulators (63), changes in baroreceptor sensitivity (8, 57), and changes in neural inputs from adjacent sources such as the cardiac chemoreceptor reflex (17). Although we did not investigate where in the reflex pathway ovarian hormones are acting to preserve arterial baroreflex control of renal SNA, in male rats there is strong evidence implicating the role of ANG II in mediating impaired arterial baroreflex control of renal SNA post-MI and in heart failure (11, 40, 61). Therefore, it is possible that sex differences in the interaction between the SNS and ANG II may have contributed to the current findings (6, 43, 60). Furthermore, sex-specific interactions in the heart failure setting between the SNS and various neuromodulators such as aldosterone, nitric oxide, natriuretic peptides, and vasopressin deserve future investigation (29, 35, 42, 60).
In the current study the arterial baroreflex control of renal SNA was presented in both absolute and normalized values. Normalization of renal SNA is the most commonly used form of analysis for direct recordings of renal SNA, although there appears to be no consistent method of normalization between studies (13, 40, 55, 61, 50, 56). The strong linear correlation observed in the present study in males and OVX females between left ventricular ejection fraction and arterial baroreflex range of control as measured in normalized units does suggest there is merit in presenting normalized renal SNA data. However, a change in arterial baroreflex range of normalized renal SNA deserves future investigation regarding the practicality of any changes observed, since this is an area not understood. For example, it is possible that inputs from other relevant sources such as the chemoreflex (33, 34) or centrally acting ANG II (22, 63) may excite non-baroreflex-sensitive renal efferent neurons, thereby increasing both resting renal SNA and the lower plateau of the arterial baroreflex. In the current study, it is not possible to comment on the origin and function of the renal efferent neurons being directly recorded. The current findings demonstrate the need to consider the impact of the baseline absolute renal SNA on normalized changes in renal SNA.
In ovary-intact females with large MI the association between the elevated gain of the arterial baroreflex presented in absolute microvolts and the elevated resting renal SNA may be relevant; reduced contractile function of the left ventricle following MI is compensated for, at least in part, by elevated sympathetic outflow to maintain appropriate arterial pressure. Therefore, it is possible that heightened baroreflex function in ovary-intact females post-MI may drive elevated renal SNA. Ovariectomy in females and therefore a significant reduction in circulating ovarian hormones abolished the enhanced arterial baroreflex control of renal SNA post-MI that was observed in ovary-intact females. Ovariectomy also resulted in no differences in resting renal SNA being observed between MI and sham groups. Although it is possible that the cardioprotective actions of ovarian hormones may be mediated at least in part by maintaining appropriate baroreflex regulation, further evidence is required to better understand the role of the arterial baroreflex in driving changes in renal SNA in both males and females (4, 5, 44). It is currently unknown whether the relative change from baseline or the absolute change in renal SNA best represents the physiological actions of a certain change in renal SNA, and future investigation is justified.
In the current study performed in anesthetized animals, baroreflex mechanisms are likely to have been activated to help maintain arterial pressure in the face of the depressor effects of the anesthetics, raising resting renal SNA to nearer the upper plateau of the arterial baroreflex curve (51). As such, the ability of the baroreflex to increase renal SNA was attenuated compared with previous evidence in conscious animals. Previous evidence does suggest that the arterial baroreflex control of renal SNA in male rats is not significantly altered by chloralose-urethane anesthesia compared with the conscious state; however, this has not been investigated in females (51). The increase in renal SNA following chloralose-urethane anesthesia likely mediates an increase in plasma renin levels that in part plays a role in maintaining arterial pressure via ANG II (30). This is important to consider given the actions of the renin-angiotensin system in mediating the arterial baroreflex, particularly in heart failure (11, 40, 61), and sex differences in the regulation of the SNS (6, 43, 60). In addition, chloralose-urethane anesthesia has been shown to increase plasma osmolality in rats, particularly when administered intraperitoneally as was done in the current study, which likely affects the release of hormones, such as vasopressin, that may interact with sympathetic regulatory centers (23, 32). Future investigations in conscious animals are required to further elucidate the effects of sex and ovarian hormones on changes to both resting renal SNA and the arterial baroreflex control of renal SNA following MI and the effect these have on cardiac function. Furthermore, future investigations are required to investigate the direct role of ovarian hormones in the observed changes in arterial baroreflex regulation, since it is possible that secondary changes to ovary removal, such as weight gain, may have influenced the current findings.
Acute, whole nerve recordings of SNA as performed in the current study are influenced by inherent technical limitations. The low experimental numbers, along with high variability of renal SNA signals within each group, do make comparing the absolute microvolt levels difficult. However, the significant differences observed in the current study between ovary-intact female sham and MI groups support previous reports (21) that significant differences can be observed when presenting renal SNA data as absolute units. Currently, there is no validated method for defining the actual maximum of the renal SNA being recorded in the anesthetized preparation, since methods such as the nasopharyngeal reflex for producing maximum SNA responses (21) have only been validated in conscious rabbits and not rats.
Unfortunately, in the present study, due to limitations in our ability to acquire the echocardiography data, we were unable to compare the left ventricular function between males and females post-MI and therefore cannot support previous evidence suggesting the cardioprotective actions of ovarian hormones on heart failure development and progression. However, the elevated heart weight-to-body weight ratios observed in the current study in all groups with large MI and elevated lung weight-to-body weight ratios in female groups with large MI indicate that animals with large MI, regardless of sex, were indeed showing signs of congestive heart failure.
In the current study, the HR responses in all groups were highly variable, and therefore the data have not been presented. The combined effect of anesthesia and the speed at which arterial pressure was altered to investigate the arterial baroreflex control of renal SNA is likely to have limited the ability to accurately fit a baroreflex curve to the HR data. Previous evidence in both males and females with heart failure has shown an impairment of arterial baroreflex control of HR (2, 14, 61). In heart failure, the clinical significance of changes in HR modulation does suggest that future investigation on sex-specific changes is justified. It should also be noted that relatively low numbers in the experimental groups may have contributed to lack of statistical power when comparing between groups.
Perspectives and Significance
The current findings provide important evidence that female ovarian hormones may significantly impact on the arterial baroreflex regulation of renal SNA post-MI. Furthermore, the current evidence suggests that, at least in males and OVX females, arterial baroreflex control of renal SNA and left ventricular function are positively associated so that only in animals with large MI does the arterial baroreflex control of renal SNA become significantly impaired. The findings also suggest that previous differences between independent research on the arterial baroreflex control of renal SNA post-MI and in heart failure may be due to either, or both, the type of analysis performed or the sex and state of sex hormones of the subject. Worldwide, there are increasing numbers of women presenting with cardiovascular diseases such as MI and heart failure. Therefore, it is becoming increasingly important that the physiological differences underlying the sexes are better understood, particularly regarding the sympathetic nervous system, which plays such an important role in driving disease progression.
This research was funded by the Lottery Health New Zealand and the Faculty of Medical and Health Sciences, University of Auckland Faculty Research Development Fund.
Sarah-Jane Guild and Simon C. Malpas are employees of Telemetry Research Ltd. Millar.
Author contributions: M.I.P., S.-J.G., S.C.M., and C.J.B. conception and design of research; M.I.P. performed experiments; M.I.P. and G.W. analyzed data; M.I.P., G.W., S.-J.G., S.C.M., and C.J.B. interpreted results of experiments; M.I.P. prepared figures; M.I.P. and C.J.B. drafted manuscript; M.I.P., G.A. W., S.-J.G., S.C.M., and C.J.B. edited and revised manuscript; M.I.P., S.-J.G., and C.J.B. approved final version of manuscript.
- Copyright © 2015 the American Physiological Society