Journal of Hypertension:
Arterial pressure in animal models of fetal programming: hypertensive or hyperresponsive?
Kett, Michelle M.
Department of Physiology, Monash University, Victoria, Australia
Correspondence to Dr Michelle M. Kett, Department of Physiology, Monash University, VIC 3800, Australia. Tel: +613 9905 4284; fax: +613 9905 2547; e-mail: firstname.lastname@example.org
It is generally accepted that insults in utero can dramatically alter the structure, physiology and/or metabolism of various organs of the fetus and that these changes predispose individuals to cardiovascular, metabolic and endocrine disease in adult life, a process referred to as developmental programming [1–3]. With respect to cardiovascular risks arising from fetal programming, arterial pressure is a common primary outcome measure for both clinical and experimental studies. Maternal undernutrition is a major problem in several regions of the globe with significant implications for offspring health, and thus is a common clinical and experimental focus of fetal programming studies . Clinically, low birth weight is used as a marker of maternal malnutrition and these studies have found an inverse association between birth weight and arterial pressures . However, the impact of this association is modest (2 mmHg/kg birth weight ) and not without controversy . Part of the controversy likely arises due to the dependence on birth weight as a measure of maternal undernutrition. Birth weight is impacted on by numerous factors (placental, genetic, etc) in addition to maternal nutrition and, importantly, animal studies have shown that programming can occur in the absence of low birth weight. This dependence on birth weight in examining the impact of maternal undernutrition can be averted in animal studies whereby maternal diet can be restricted generally (lower caloric intake) or specifically (e.g. low protein diet) and the timing and magnitude of undernutrition can be predetermined and controlled. Further, the use of a wide range of species can strengthen the biological significance of maternal undernutrition on the programming of adult hypertension. However, the arterial pressure findings in experimental models of maternal undernutrition have also been in dispute.
In the current issue of Journal of Hypertension, Van Abeelen et al. have addressed this dispute by performing a meta-analysis of experimental models of maternal undernutrition to examine the impact on offspring arterial pressures. They have separated the analysis into general maternal undernutrition and low protein maternal undernutrition. As stated by the authors, their findings in general support the hypothesis that maternal undernutrition programs hypertension in the offspring. However, the analysis highlights an important consideration for the measurement of arterial pressure in models of maternal malnutrition that is likely relevant for fetal programming studies in general. There is a growing body of evidence to suggest that the methodology employed to measure arterial pressure, particularly in rodents, may contribute to the elevated arterial pressures reported. Thus, this meta-analysis raises the issue of whether maternal undernutrition programs hypertension per se, or a heightened responsiveness of arterial pressure to stress.
The meta-analysis included 34 studies of maternal general undernutrition and 67 studies of low protein undernutrition. Although animal species included sheep, guinea pigs, rats and mice, the dominant species was rats (22/34 general and 63/67 low protein), and the most common technique used to measure arterial pressure was tail-cuff plethysmography in rodents. The significance of methodology is highlighted best in the outcomes for SBP. When restricted to tail cuff measurements, maternal general or low protein undernutrition resulted in SBP values that were approximately 20 mmHg greater than control animals. Yet when the analysis was restricted to direct methods (intra-arterial catheters), the values were a more modest 4–5 mmHg greater than controls. Similarly, mean arterial pressure (MAP) following low protein undernutrition was 17.5 mmHg greater than controls using tail cuff, but when restricted to radiotelemetry studies the affect disappeared. The authors argue that the lack of significant effect when the analysis is restricted to radiotelemetry studies may be due to the small number of studies. However, the absence of hypertension in low protein undernutrition models by radiotelemetry demonstrated by this meta-analysis is consistent with several additional published studies in both general and low protein models that were not included in this meta-analysis [9–11]. Indeed, Swali et al. found that although low protein offspring had significantly elevated SBP when measured by tail cuff, these same rats had significantly lower MAP when measured by radiotelemetry.
The main methods of arterial pressure measurement in animals are radiotelemetry, chronic intra-arterial catheters and tail cuff plethysmography (rodents). The degree of stress associated with each of these methods, as well as length of time over which the measurements are made, are important considerations when interpreting arterial pressure results. Telemetry is the gold standard technique for blood pressure measurement as it allows accurate, continuous measurement of SBP and DBP and heart rate, thus, also allowing analysis of circadian rhythms [12,13]. The animals are completely unrestrained in their home cages and are, therefore, not subject to the stress of handling or restraint. The downfall is that telemetry is costly, especially in the establishment phase, and thus unfortunately few studies have used this technique in the field of developmental programming [9–11,14–19]. Chronic indwelling arterial catheters are common in rodents as well as large species such as sheep and have the advantage of allowing blood sampling and, supplemented with a venous catheter, infusion of agents while blood pressure is being recorded. However, the presence of externalized catheters, and often the need to be tethered, does add an element of restraint stress to measurements in rodents, the most common model examined. Quality 24-h arterial pressure measurements can be obtained and hypertension reported  but few take advantage of the technique, taking measurements at a single time point over very short periods (1–3 h per day) during the day: the normal sleep phase for rodents. In this regard it has been shown that increasing the number of periods of recording captures circadian and other variations in arterial pressure, and that spreading these recording periods over the course of the day, greatly reduces the error and produces accurate estimates of daily MAP . Tail-cuff plethysmography is most commonly used in rodents due to the low cost and ease of use, and the possibility of long-term sequential measurements. The downfalls, however, are that only single time point SBP measurements can be made reliably, these measurements are mostly made during the day (the normal sleep phase for rodents) and, significantly, the animals must be restrained.
To this end, studies in rodent models of maternal general and low protein undernutrition and maternal dexamethasone exposure have reported SBP of offspring that are typically 10–20 mmHg, but up to 45 mmHg, greater than controls using the tail-cuff technique. However, when 24-h arterial pressure was measured by radiotelemetry, these models do not appear to demonstrate overt hypertension, indeed several groups have documented hypotension [9–11,16,22,23]. First reported by Tonkiss et al. in 1998 and consistently demonstrated since, rat offspring of intrauterine/perinatal insults have an augmented and/or prolonged response to stress, particularly restraint [13,21,22]. Tonkiss et al. found that rat offspring of maternal low protein undernutrition demonstrated a modest 4 mmHg higher nighttime DBP but normal 24-h MAP compared to controls using radiotelemetry. However, these rats had an augmented increase in both systolic (∼23 vs. 15 mmHg) and diastolic pressures (∼14 vs. 8 mmHg) during the first exposure to an olfactory stressor compared to controls . More recently, Augustyniak et al. again reported normal 24-h MAP in offspring of maternal low protein undernutrition with telemetry. Although they found the initial (∼5 min) MAP response to restraint stress similar between offspring of maternal low protein undernutrition and controls, MAP of control animals returned rapidly toward basal pressures, whereas MAP of low protein offspring remained significantly elevated over the controls for the rest of the 60-min restraint period. This finding is significant because this is the period during which tail cuff measurements will be taken. Further, this study explains the findings of Swali et al. who concluded that the elevated tail-cuff pressures they reported in low protein offspring were not secondary to restraint stress as they saw a similar response in telemetric arterial pressures during restraint and tail-cuff measures. However, they only measured the telemetric arterial pressure response to restraint stress and tail-cuff measures during this initial approximately 5-min period .
So what is the basis for the higher tail cuff pressures and response to stress, and what are the implications for future studies into the programming of hypertension? The adverse intrauterine environment is an insult on the entire fetus and the literature highlights the breadth of organ and hormonal systems that are altered in these models. Alterations to the hypothalamo–pituitary–adrenal and sympthoadrenal axes are seen to occur in models of developmental programming, as they are in human studies [24–28]. And, thus, changes in these systems are likely to contribute to the enhanced stress responses found in telemetric studies and the higher pressures recorded with tail-cuff. Therefore, an important consideration when reviewing the literature of arterial pressure effects of programming is the possibility that programming may not always lead to increased arterial pressures per se, but perhaps enhance the arterial pressure response to stress. This concept is entirely consistent with famine studies in humans, as raised by Van Abeelen et al.. These studies have generally found no affect of in-utero exposure to famine on adult arterial pressure but have reported a higher arterial pressure response to stress (see Van Abeelen, ). This enhanced response to stress is of itself an important and clinically relevant finding because, as suggested by Augustyniak et al., outside the confines of a controlled laboratory we are regularly exposed to stressful stimuli and chronic hyperresponsiveness may, in the long term, contribute to end-organ damage.
A meta-analysis of the programming effects on arterial pressure provided by Van Abeelen is long overdue. Although the meta-analysis generally supported the hypothesis that maternal undernutrition leads to higher arterial pressures in offspring, they found that methodological quality of many studies was poor, and that the results depended strongly on both the technique used to measure arterial pressure and the animal model. But probably the most significant implication for future studies into the programming of hypertension is that the different methodologies used to measure arterial pressure are measuring different aspects of arterial pressure control. Twenty-four hour unrestrained arterial pressure measurements of the like provided by radiotelemetry will provide the most robust data to answer the question of whether offspring of programming models have hypertension per se. Data obtained by tail-cuff plethysmography, that by its very nature includes elements of stress (restraint, handling), have and will continue to provide evidence that the cardiovascular control systems of offspring of programming models are hyperresponsive. Indeed, the disparate findings from the two techniques indicate that future studies need to combine 24-h measurements of arterial pressure in the absence and presence of stress stimuli.
Conflicts of interest
There are no conflicts of interest.
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