In obesity-related hypertension, activation of the renin-angiotensin system (RAS) has been reported despite marked fluid volume expansion. Adipose tissue expresses components of the RAS and is markedly expanded in obesity. This study evaluated changes in components of the adipose and systemic RAS in diet-induced obese hypertensive rats. RAS was quantified in adipose tissue and compared with primary sources for the circulating RAS. Male Sprague-Dawley rats were fed either a low-fat (LF; 11% kcal as fat) or moderately high-fat (32% kcal as fat) diet for 11 wk. After 8 wk, rats fed the moderately high-fat diet segregated into obesity-prone (OP) and obesity-resistant (OR) groups based on their body weight gain (body weight: OR, 566 +/- 10; OP, 702 +/- 20 g; P < 0.05). Mean arterial blood pressure was increased in OP rats (LF: 97 +/- 2; OR: 97 +/- 2; OP: 105 +/- 1 mmHg; P < 0.05). Quantification of mRNA expression by real-time PCR demonstrated a selective increase (2-fold) in angiotensinogen gene expression in retroperitoneal adipose tissue from OP vs. OR and LF rats. Similarly, plasma angiotensinogen concentration was increased in OP rats (LF: 390 +/- 48; OR: 355 +/- 24; OP: 530 +/- 22 ng/ml; P < 0.05). In contrast, other components of the RAS were not altered in OP rats. Marked increases in the plasma concentrations of angiotensin peptides were observed in OP rats (angiotensin II: LF: 95 +/- 31; OR: 59 +/- 20; OP: 295 +/- 118 pg/ml; P < 0.05). These results demonstrate increased activity of the adipose and systemic RAS in obesity-related hypertension.

Previous studies in our laboratory demonstrated that rats exhibiting obesity in response to a moderately high-fat (MHF) diet developed hypertension associated with activation of the local and systemic renin-angiotensin system. In this study, we examined the effect of the angiotensin type 1 (AT(1))-receptor antagonist, losartan, on blood pressure in obesity-prone (OP) and obesity-resistant (OR) rats fed a MHF diet. Using telemetry monitoring, we characterized the evolution of blood pressure elevations during the development of obesity. Male Sprague-Dawley rats were implanted with telemetry transducers for chronic monitoring of blood pressure, and baseline measurements were obtained. Rats were then switched to the MHF diet (32% kcal as fat) and were segregated into OP and OR groups at week 5. At week 9 on the MHF diet, OP rats exhibited significantly greater 24-h mean arterial blood pressure compared with OR rats (OP: 105 +/- 4 mmHg, OR: 96 +/- 2 mmHg; P < 0.05). Elevations in blood pressure in OP rats were manifest as an increase in systolic pressure. Administration of losartan to all rats at week 9 resulted in a reduction in blood pressure; however, losartan had the greatest effect in OP rats (percent decrease in mean arterial pressure by losartan; OP: 19 +/- 4, OR: 10 +/- 2%; P < 0.05). These results demonstrate that elevations in blood pressure occur subsequent to established obesity in rats fed a high-fat diet. Moreover, these results demonstrate the ability of losartan to reverse the blood pressure increase from diet-induced obesity, supporting a primary role for the renin-angiotensin system in obesity-associated hypertension.

Adipocytes secrete a variety of factors, including angiotensinogen, the only known precursor to Angiotensin II (AngII). Recent studies suggest that adipocyte-derived angiotensinogen can contribute to circulating angiotensinogen concentrations and modulate blood pressure; however, an autocrine role for adipocyte-derived angiotensinogen and/or AngII has not been well defined. We sought to determine whether locally produced AngII influences the release of leptin from adipocytes and thus circulating leptin concentrations. In adipocytes from rats treated for 3 d with captopril demonstrating reductions in AngII release, leptin release and plasma leptin concentration were decreased. Incubation of adipocytes with AngII resulted in an increase in leptin mRNA expression and leptin release. To determine the effect of elevated systemic AngII on leptin, rats were infused with AngII (175 ng/kg.min) or saline for 1, 2, or 7 d. Plasma leptin concentration progressively declined with duration of AngII exposure. Basal and AngII-stimulated release of leptin from isolated adipocytes was initially (d 1) increased in AngII-infused rats; thereafter leptin release declined to levels less than control. To define mechanisms for declines in leptin in AngII-infused rats, we examined the effect of alpha-methyl-p-tyrosine on catecholamine turnover and plasma leptin concentration in saline- and AngII (175 ng/kg.min)-infused rats. Infusion of AngII increased catecholamine turnover in adipose tissue. Moreover, sympathetic blockade eliminated differences in plasma leptin concentration between saline- and AngII-infused rats. These results indicate that locally produced AngII directly increases leptin release from adipocytes; however, with elevations in systemic AngII, sympathetic activation counterbalances effects from locally produced AngII.

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