Nocturnal hypertension in mice consuming a high fructose diet
Introduction
Consumption of high levels of fructose in humans and animals leads to insulin resistance, obesity, hypertension and lipid abnormalities (Basciano et al., 2005). These symptoms are characteristic of type 2 diabetes, which has reached epidemic levels in western societies. It is the cardiovascular pathologies, associated with diabetes, which are closely tied to the mortality and morbidity. Diabetics commonly have hypertension along with a nighttime, non-dipper pattern in blood pressure (BP) levels, a condition which is associated with end organ damage (Nielsen et al., 1995, Nakano et al., 1998).
There is much information on the cardiovascular and metabolic effects of a high fructose diet in animal models. Fructose-fed rats show a moderate hypertension and glucose intolerance, associated with high levels of plasma insulin, glucose, cholesterol and triglycerides (Dai and McNeill, 1995, Katovich et al., 2001, Kamide et al., 2002, Hsieh, 2005). In mice, there is evidence that fructose alters glucose metabolism and lipid levels (Nagata et al., 2004). However, there is no information on the effects of fructose consumption on the circadian pattern of blood pressure and heart rate or on autonomic function. This is an important gap in view of clinical data in diabetics which document autonomic dysfunction and attenuation of day/night BP ratio (Nielsen et al., 1995, Nakano et al., 1998).
Our study used radiotelemetry which allows for long term measurement of BP under low stress conditions. It is particularly useful for the measurement of day/night cardiovascular rhythms (Chen et al., 2006). The telemetric method is combined with autoregressive spectral analysis which is a non-invasive, statistical means for studying autonomic function in freely moving animals (Farah et al., 2004, Joaquim et al., 2004). Heart rate and blood pressure variability (HRV and BPV), estimated in time or frequency (spectral analysis) domain, are used to detect early abnormalities in autonomic modulation of the cardiovascular system (Pagani et al., 1988, Spallone et al., 1997, Fazan et al., 1999). Changes often occur before or without alterations in BP and HR. For example, reduced HRV is associated with an increased risk for sudden death in patients with chronic heart failure (Galinier et al., 2000, La Rovere et al., 2003). Moreover, in humans with diabetes mellitus, spectral analysis of HR disclosed a significant decrease in variability, which was apparent even before any changes in other cardiovascular parameters (Pagani et al., 1988). The importance of BPV in clinical pathologies was established with reports that increased BPV was associated with end-organ damage (Zanchetti and Mancia, 1987, Parati et al., 1987, Mancia et al., 1988).
In terms of the mechanisms behind the fructose-induced cardiovascular changes, there is evidence for a role of the sympathetic nervous and renin angiotensin system (RAS). Sympathectomy (adrenal medullectomy coupled with neurotoxin exposure) attenuated the development of hypertension in rats fed a high fructose diet, suggesting a role for the sympathetic nervous system (Verma et al., 1999). Fructose feeding also increased urinary catecholamine excretion and adrenergic receptor expression (Kamide et al., 2002). Evidence for a role for the RAS in fructose-induced cardiovascular changes was seen by the increased expression of Ang receptors in the vasculature and depressor effect of Ang receptor antagonists (Katovich et al., 2001, Hsieh, 2005). In mice, there is data which show that a high fructose diet caused activation of the vascular RAS (Shinozaki et al., 2004).
The purpose of this study was to explore the consequences of long-term fructose consumption on cardiovascular and metabolic parameters in mice. Using radiotelemetry for chronic cardiovascular measurements and the autoregressive spectral method for blood pressure and heart rate analysis, we tested the hypothesis that a high fructose diet causes changes in the light/dark blood pressure pattern and changes in autonomic function. We further evaluated the possible role of the sympathetic and renin angiotensin systems in the cardiovascular changes: measuring plasma Ang II, testing the effect of adrenergic blockade and quantifying brainstem tyrosine hydroxylase (TH) and Ang receptor mRNA levels.
Section snippets
General procedures
Experiments were performed in male C57BL/6 mice (Harlan Inc, Indianapolis, Ind), 24–29 g. Mice were housed individually at 22 °C with a 12:12-h light/dark cycle (0500 h–1700 h, lights on). After an acclimatization period, mice were given ad libitum access to tap water and randomly assigned to either standard pellet diet (30% protein, 58% carbohydrate and 12% fat) or a diet containing high fructose (67% carbohydrate—98% of which is fructose, 13% fat and 20% protein). After 7 weeks on the dietary
Results
Body weight, non-fasting blood glucose and plasma insulin were not different between the groups (Table 1). There were increases in plasma cholesterol and Ang II (almost four-fold), but no change in triglycerides in the fructose group (Table 1).
Glucose tolerance was attenuated as shown in the time course of the GTT and the area under the curve (AUC) (Fig. 1). For the GTT, there were significant main effects of group (con vs. fructose) [F(1.16) = 8.89, P < 0.01], time (time after glucose) [F(4.64) =
Discussion
The key findings of the present study were that a dietary model of glucose intolerance in mice was associated with nocturnal hypertension, sympathetic activation and increased plasma Ang II levels. There were also central nervous system changes, seen as increased mRNA expression of brainstem Ang AT1a receptor and TH. A mere eight weeks of consumption of a fructose-enriched diet was sufficient to induce prominent metabolic and cardiovascular changes. The fructose feeding model in mice may be
Acknowledgements
The authors acknowledge the financial support of NIH R01HL69319 (MM) and AHA Scientist Development Grant 0535201N (YC). They express thanks to the University of Cincinnati Mouse Metabolic Phenotyping Center (DK59630) and to J. Aguiar and M. Pazinne (undergraduate Brazilian research fellows supported by the Foundation for the Improvement of Postsecondary Education and Coordenação Aperfeiçoamento de Pessoal de Nível Superior- FIPSE/CAPES.
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