Autonomic mechanisms underpinning the stress response in borderline hypertensive rats

Abstract

This study investigates blood pressure (BP) and heart rate (HR) short-term variability and spontaneous baroreflex functioning in adult borderline hypertensive rats and normotensive control animals kept on normal-salt diet. Arterial pulse pressure was recorded by radio telemetry. Systolic BP, diastolic BP and HR variabilities and baroreflex were assessed by spectral analysis and the sequence method, respectively. In all experimental conditions (baseline and stress), borderline hypertensive rats exhibited higher BP, increased baroreflex sensitivity and resetting, relative to control animals. Acute shaker stress (single exposure to 200 cycles min-1 shaking platform) increased BP in both strains, while chronic shaker stress (3-day exposure to shaking platform) increased systolic BP in borderline hypertensive rats alone. Low- and high-frequency HR variability increased only in control animals in response to acute and chronic shaker (single exposure to restrainer) stress. Acute restraint stress increased BP, HR, low- and high-frequency variability of BP and HR in both strains to a greater extent than acute shaker stress. Only normotensive rats exhibited a reduced ratio of low- to high-frequency HR variability, pointing to domination of vagal cardiac control. In borderline hypertensive rats, but not in control animals, chronic restraint stress (9-day exposure to restrainer) increased low- and high-frequency BP and HR variability and their ratio, indicating a shift towards sympathetic cardiovascular control. It is concluded that maintenance of BP in borderline hypertensive rats in basal conditions and during stress is associated with enhanced baroreflex sensitivity and resetting. Imbalance in sympathovagal control was evident only during exposure of borderline hypertensive rats to stressors.

Figure 1. Features of the spontaneous baroreflex in baseline conditions in borderline hypertensive rats (BHRs) and Wistar rats

Upper panel illustrates baroreflex sensitivity (line) in one Wistar rat calculated as the linear regression coefficient of the sequence averaged over all identified sequences, operating range (dashed rectangle) covering 95% of systolic blood pressure (BP)–pulse interval (PI) pairs that form sequences, and set point (filled circle) calculated as median value of all set points. Lower panel shows mean baroreflex sensitivity, operating range ± SEM and set point for 12 Wistar rats (WR; grey) and 12 BHRs (black). Note the resetting and the increase in baroreflex sensitivity of BHRs. ‡P < 0.05, ‡‡‡P < 0.001 BHRs versus WRs.

Figure 2. Blood pressure and heart rate spectral components in BHRs and Wistar rats in baseline conditions

In this and the following figures, abbreviations are as follows: BHR, borderline hypertensive rat; BP, blood pressure; DBP, diastolic BP; HF, high frequency spectral power; HR, heart rate; LF, low frequency spectral power; SBP, systolic BP; TOTAL, total spectral power; VLF, very low frequency spectral power; and WR, Wistar rat. A (absolute values), note the decrease of VLF variability in diastolic BP spectra in BHRs. B (normalized units), note redistribution of spectral frequencies towards HF variability in diastolic BP spectrum of BHRs. Values are means of 12 rats ± SEM. ‡P < 0.05 BHRs versus WRs.

Figure 3. Baroreflex parameters in BHRs and Wistar rats exposed to shaker stress

Acute stress induces resetting of the baroreflex in both rat strains. Chronic shaker stress resets the baroreflex only in BHRs. Baseline, dashed line; stress, continuous line. Values are means of six rats ± SEM (dotted line). *P < 0.05, **P < 0.01 stress versus baseline; ‡P < 0.05, ‡‡‡P < 0.001 BHRs versus WRs.

Figure 4. Effects of shaker stress on HR spectral components in BHRs and Wistar rats

A (absolute values), acute and chronic stress increases LF and HF HR variability only in Wistar rats, without affecting LF/HF HR ratio. B (normalized units), exposure of BHRs and Wistar rats to acute and chronic stress did not induce redistribution of HR spectral frequencies. Values are means of six rats ± SEM. *P < 0.05 stress versus baseline; ‡P < 0.05 BHRs versus WRs.

Figure 5. Effects of shaker stress on BP spectral components in BHRs and Wistar rats

A (absolute values), in both strains chronic shaker stress increased respiration-induced HF systolic and diastolic BP variability. B (normalized units), note a shift of spectral frequencies towards the HF band in both systolic and diastolic BP spectra of both rat strains exposed to shaker stress. Values are means of six rats ± SEM. *P < 0.05, **P < 0.01 stress versus baseline; ‡P < 0.05, ‡‡P < 0.01, ‡‡‡P < 0.001 BHRs versus WRs.

Figure 6. Baroreflex parameters in BHRs and Wistar rats exposed to restraint stress

Acute stress resets the baroreflex of Wistar rats and reduces its sensitivity and operating range. Chronic stress increases the sensitivity of BHRs with respect to baseline values and Wistar rats. Baseline, dashed line; stress, continuous line. Values are means of six rats ± SEM (dotted line). *P < 0.05, ***P < 0.001 stress versus baseline; ‡P < 0.05, ‡‡‡P < 0.001 BHRs versus WRs.

Figure 7. Effects of restraint stress on HR spectral components of BHR and Wistar rats

A (absolute values), in both strains of rats acute stress increases LF and HF HR variability. Only in Wistar rats the LF to HF ratio decreases, pointing to domination of vagal cardiac control. Chronic exposure to stress increases LF HR variability and LF to HF HR ratio only in BHRs, indicating a shift towards sympathetic cardiac control. B (normalized units), acute stress induces clustering of spectral frequencies around LF and HF only in Wistar rats. Chronic stress increases LF frequency only in BHRs. Values are means of six rats ± SEM. *P < 0.05, ***P < 0.001 stress versus baseline; ‡P < 0.05, ‡‡P < 0.01, ‡‡‡P < 0.001 BHRs versus WRs.

Figure 8. Effects of restraint stress on BP spectral components of BHRs and Wistar rats

A (absolute values), acute stress increases LF and HF systolic and diastolic BP variability in both strains of rats. Chronic exposure to stress increases LF systolic and diastolic BP variability only in BHRs, indicating an increase of sympathetic outflow to resistance blood vessels. B (normalized units), acute stress redistributes spectral frequencies around LF and HF systolic and diastolic BP variability in Wistar rats, while in BHRs the frequencies are clustered around LF variability alone. Values are means of six rats ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001 stress versus baseline; ‡P < 0.05, ‡‡P < 0.01 BHRs versus WRs.