Linda L. Chafe

Characteristics of 24 h telemetered blood pressure in eNOS-knockout and C57Bl/6J control mice.

The purpose of the present study was to characterize in detail the 24 h blood pressure (BP) phenotype of mice lacking the gene for endothelial nitric oxide synthase (eNOS−/−) and the corresponding control strain (C57Bl/6J). Twenty‐four hour BP recordings were made in conscious 12‐ to 16‐week‐old male mice 10 days following the implantation of a BP telemeter (n= 9 per group). The BP and heart rate of both strains were markedly affected by brief locomotor activity cycles, resulting in bimodal distributions of BP and heart rate within both light and dark periods. Data from active periods were associated with the higher of the two modes, whereas data from inactive periods were associated with the lower of the two modes. In eNOS−/− mice, the 24 h average BP level was significantly elevated (+15 %, 104 ± 2 vs. 119 ± 1 mmHg), as was its daily range (+44 %), its coefficient of variation (+26 %), dark‐light difference (+48 %), and the separation of the two modes of its distribution (+41 %). Pulse pressure was also significantly greater (+23 %) in eNOS−/− mice. The 24 h heart rate level did not differ between control and eNOS−/− mice. Considerable variation was noted among previously published values of BP in eNOS−/− mice, but not in the corresponding control mice. Our results indicate that eNOS−/− mice have mild hypertension that is accompanied by more pronounced increases in BP lability and/or reactivity. Our results also demonstrate a marked effect of locomotor activity on BP in mice, which may confound short‐term measurements of BP.

Direct and indirect methods used to study arterial blood pressure.

A number of different approaches exist for assessing blood pressure in experimental animals. Here, we briefly consider the traditional indirect (rodent tail-cuff) and direct (saline-filled catheter) methods of blood pressure measurement before going on to describe our experience with blood pressure telemetry in rabbits, rats, and mice. Blood pressure telemetry offers the ability to obtain a high-fidelity recording of blood pressure continuously, for relatively long periods of time, in conscious, freely moving animals, without the limitations of restraint or anaesthesia. Since some drift in telemeter offset and sensitivity are inevitable, recalibration of the telemeter devices immediately before implantation and following explantation is essential to ensure and document the accuracy of the blood pressure measurements. For long-term implantations, verification of the calibration can be performed in vivo, at least in the case of large animals, such as rabbits. Telemetry devices suitable for small animals, such as mice, are also available now, which will facilitate the accurate characterization of blood pressure in transgenic animals. Telemeter implantation methods in mice are presently difficult, with relatively low success rates being reported. However, validation of new methods, such as the insertion of the catheter tip via the carotid artery, may make the technique more widely accessible in the near future.

Contribution of baroreceptors and chemoreceptors to ventricular hypertrophy produced by sino-aortic denervation in rats

1 To test whether sino‐aortic denervation (SAD)‐induced right ventricular hypertrophy (RVH) is a consequence of baroreceptor or chemoreceptor denervation, we compared the effects of aortic denervation (AD), carotid denervation (CD), SAD and a SAD procedure modified to spare the carotid chemoreceptors (mSAD), 6 weeks after denervation surgery in rats. A sham surgery group served as the control. 2 The blood pressure (BP) level was unaffected by AD, CD or SAD, but increased (9 %) following mSAD. The mean heart rate level was not affected. Short‐term BP variability was elevated following AD (81 %), SAD (144 %) and mSAD (146 %), but not after CD. Baroreflex heart rate responses to phenylephrine were attenuated in all denervation groups. 3 Significant RVH occurred only following CD and SAD. These procedures also produced high mortality (CD and SAD) and significant increases in right ventricular pressures and haematocrit (CD). 4 Significant left ventricular hypertrophy occurred following CD, SAD and mSAD. Normalized left ventricular weight was significantly correlated with indices of BP variability. 5 These results suggest that SAD‐induced RVH is a consequence of chemoreceptor, not baroreceptor, denervation. Our results also demonstrate that a mSAD procedure designed to spare the carotid chemoreceptors produced profound baroreflex dysfunction and significant left, but not right, ventricular hypertrophy.

PHENOTYPING THE LEVEL OF BLOOD PRESSURE BY TELEMETRY IN MICE

1 Using telemetry, arterial blood pressure (BP) can be measured directly over long periods in freely behaving animals without recent anaesthesia or surgery. In the present review, we discuss the strengths and limitations of this method and important considerations in using the method to characterize the BP level in mice. 2 A variety of informative statistics can be used to describe the BP level and we have made available a spreadsheet template for their calculation on a routine basis. The BP level is well summarized using the average value for an entire 24 h period or for the individual light and dark phases of the day. Such long‐term averages exhibit less statistical variation than those of short recording periods. In addition, averages of the dark and light phases of the day convey information concerning circadian variations of BP. 3 The frequency distribution of BP samples provides additional information concerning the range of BP values recorded over the course of the day and can be described in terms of percentiles of the distribution that correspond with the minimum and maximum BP values and their span. 4 In mice, BP can be markedly affected by locomotor activity cycles that occur frequently throughout both the light and dark phases of the day. In addition, BP is strongly affected by ambient temperature and food intake, as well as potentially by other determinants of energy balance. Consideration of these factors may help improve accuracy and precision when phenotyping the BP level in mice.

Maternal Endothelial Nitric Oxide Synthase Genotype Influences Offspring Blood Pressure and Activity in Mice

Deficiencies in maternal endothelial NO synthase (eNOS) have been associated with pregnancy complications, intrauterine growth retardation, and altered vascular function in offspring. In the present study, we investigated the influence of the maternal eNOS genotype on offspring’s blood pressure, heart rate, and locomotor activity. The effect of maternal eNOS genotype was made by comparing telemetered blood pressure and activity between 2 groups of 13- to 16-week–old male heterozygous eNOS knockout mice, 1 produced by a cross of eNOS knockout (eNOS−/−) mothers and wild-type (eNOS+/+) fathers (eNOS+/−MAT offspring, N=11), the other by a cross of eNOS+/+ mothers and eNOS−/− fathers (eNOS+/−PAT offspring, N=10). Data were also collected for homozygous eNOS−/− and eNOS+/+ mice (N=15 each). Heterozygous eNOS knockout mice exhibited blood pressures that were intermediate to the eNOS+/+ and eNOS−/− groups. Relative to eNOS+/−PAT mice, eNOS+/−MAT mice exhibited significant increases in nocturnal diastolic arterial pressure and diurnal variations (dark–light difference) in systolic, mean, and diastolic arterial pressure. In addition, indices of spontaneous nocturnal locomotor activity, including both the proportion of time spent active and the intensity of activity when active, were also significantly increased. Heart rate did not differ between the groups. Our results suggest that the maternal eNOS genotype influences both blood pressure and behavior of offspring, possibly as a consequence of developmental programming associated with intrauterine growth retardation.

Distinct rapid and slow phases of salt-induced hypertension in Dahl salt-sensitive rats

Objective To test the hypothesis that Dahl salt-sensitive (Dahl-S) rats exhibit distinct and separable phases of salt sensitivity. Methods Blood pressure (BP) telemetry was used to describe the detailed time course of salt-induced hypertension in Dahl-S rats and in hybrid rats derived from Dahl-S and Dahl salt-resistant strains. Results Switching to a high salt (4% NaCl) diet led to a biphasic increase in BP. Phase-1 reached a plateau in 4 days whereas phase-2 progressed slowly over the subsequent 5 weeks. In hybrid rats, phase-1 was present in each rat whereas phase-2 was absent in many individuals. A correlation of the amplitude of the first and second phases was of borderline significance in Dahl-S rats (P = 0.053, R2 = 0.44, n = 9) but was clearly significant in hybrid rats (P = 0.001, R2 = 0.62, n = 13) and in a combined group of Dahl-S and hybrid rats (P < 0.0001, R2 = 0.78, n = 22). Increases in BP were reversible following 1 week of high salt but progressively less so after 4 and 7 weeks. Estimation of the chronic pressure–natriuresis relationship suggests that phase-1 is attributable to a reduced slope of this relationship. In contrast, phase-2 corresponds with a further reduction in slope and a progressive and irreversible resetting of the relationship to higher BP levels. Conclusions Two phases of salt sensitivity coexist and provide distinct contributions to salt-induced hypertension in Dahl-S rats. Our data also suggest that short-term measures of salt-sensitivity may be predictive of the effect of salt on the eventual progression of salt-induced hypertension.

REDUCTION OF BLOOD PRESSURE VARIABILITY BY AMLODIPINE IN BARORECEPTOR DENERVATED RATS

To determine the effect of the calcium blocker amlodipine on the variability of mean arterial pressure (MAP), amlodipine besylate was acutely administered to sino-aortic baroreceptor-denervated (SAD) rats (0, 1, 3, 10 mg/kg s.c.), and chronically administered to SAD and sham-denervated rats (0, 50, 150, 500, and 1500 mgċkg−1 feed, 4 days per dose). Acute amlodipine administration caused significant dose-dependent reductions of the mean MAP level and short-term MAP variability at the 3 and 10 mg/kg dose levels, respectively. Chronic administration produced dose-dependent reductions in short-term MAP variability, becoming significant at the 150 and 500 mgċkg−1 feed dose level in SAD and Sham groups, respectively. Day-night differences in blood pressure were significantly attenuated or reversed at the 500 and 1500 mgċkg−1 feed dose levels. Amlodipine had little or no effect upon the 24 h MAP level, long-term MAP variability, and only modestly reduced the MAP response to hexamethonium. These results demonstrate that amlodipine can reduce MAP variability independent of changes in the mean blood pressure level.