Angiotensin II Stimulates Sympathetic Neurotransmission to Adipose Tissue

Abstract

Angiotensin II (AngII) facilitates sympathetic neurotransmission by regulating norepinephrine (NE) synthesis, release and uptake. These effects of AngII contribute to cardiovascular control. Previous studies in our laboratory demonstrated that chronic AngII infusion decreased body weight of rats. We hypothesized that AngII facilitates sympathetic neurotransmission to adipose tissue and may thereby decrease body weight. The effect of chronic AngII infusion on the NE uptake transporter and NE turnover was examined in metabolic (interscapular brown adipose tissue, ISBAT; epididymal fat, EF) and cardiovascular tissues (left ventricle, LV; kidney) of rats. To examine the uptake transporter saturation isotherms were performed using [³H]nisoxetine (NIS). At doses that lowered body weight, AngII significantly increased ISBAT [³H]NIS binding density. To quantify NE turnover, alpha-methyl-para-tyrosine (AMPT) was injected in saline-infused, AngII-infused, or saline-infused rats that were pair-fed to food intake of AngII-infused rats. AngII significantly increased the rate of NE decline in all tissues compared to saline. The rate of NE decline in EF was increased to a similar extent by AngII and by pair-feeding. In rats administered AngII and propranolol, reductions in food and water intake and body weight were eliminated. These data support the hypothesis that AngII facilitates sympathetic neurotransmission to adipose tissue. Increased sympathetic neurotransmission to adipose tissue following AngII exposure is suggested to contribute to reductions in body weight.

Keywords: body weight; catecholamine turnover; norepinephrine.

Figure 1

Chronic angiotensin II (AngII) infusion dose-dependently regulates body weight (A), food intake (B), and water intake (C). Baseline measurements of body weight, food and water intake were taken for 3 days prior to osmotic minipump implantation (arrow). Rats were administered either saline or AngII (200–600 ng/kg per minute) for 14 days by osmotic minipump. Measurements were taken daily at 10:00 am. Data are mean ± SEM from n = 8 rats/group; “*” denotes significantly different from saline.

Figure 2

Chronic angiotensin II (AngII) infusion increases plasma norepinephrine (NE concentration. AngII (200–600 ng/kg per minute) or saline were infused for 14 days by osmotic minipump. NE in plasma was extracted over alumina, followed by high-performance liquid chromatography (HPLC) with electrochemical detection for separation and quantification. Data are mean ± SEM from n = 8 rats/group; “*” denotes significantly different from control.

Figure 3

[³H]Nisoxetine (NIS) binding density is increased in brown adipose tissue, but not in left ventricle from angiotensin II (AngII)-infused rats. Rats were administered saline- or AngII (200–600 ng/kg per minute) for 14 days. ISBAT (A) and LV (B) were removed and membranes prepared for saturation isotherm binding using [³H] NIS as a ligand for the norepinephrine (NE) uptake transporter as described in methods. Data are mean ± SEM from n = 8 rats/dose of AngII; “*” denotes significantly different from saline-infused rats (P < 0.05).

Figure 4

Pair feeding demonstrates that reductions in food intake partially mediate body weight reductions of angiotensin II (AngII)-infused rats. Rats were administered saline, AngII (400 ng/kg per minute) or were pair-fed to food intake of AngII-infused rats for 14 days. Body weight (A), food intake (B), and water intake (C) of rats in each group. Data are mean ± SEM from n = 15 rats/group. *Denotes significantly different from saline-infused rats (P < 0.05).

Figure 5

Decline of norepinephrine (NE) in tissues after synthesis inhibition. Rats were administered saline, angiotensin II (AngII; 400 ng/kg per minute), or were pair-fed to the food intake of AngII-infused rats for 14 days. On the final day, rats were injected with alpha-methyl-para-tyrosine (AMPT) and tissues removed for determination of NE concentration by high-performance liquid chromatography (HPLC). The logNE concentration in tissues from rats in each group was plotted against the time after AMPT administration. Linear regression was used to determine the slope of the line for the calculation of the rate of NE decline (k = slope/0.434). The correlation coefficient (r²) from the regression analysis of each line is provided with the symbol for the respective groups.

Figure 6

Angiotensin II (AngII) infusion increases norepinephrine (NE) turnover in brown adipose tissue. Top: k, the rate of NE decline. Linear regression analysis was performed on data illustrated in Figure 5 to calculate the rate of NE decline (k = slope/0.434). Bottom: K, NE turnover rate (endogenous NE concentration × k) was increased in ISBAT from AngII-infused rats, and decreased in LV from pair-fed rats compared to saline controls. Data are mean ± SEM from n = 5 rats/group/time point; “*” denotes significantly different from saline, P < 0.05.

Figure 7

Propranolol administration prevents angiotensin II (AngII) regulation of body weight (A), food intake (B), and water intake (C). Rats were administered saline, saline + propranolol, AngII or AngII + propranolol. Arrow denotes the start of drug treatment by osmotic minipump. Data are mean ± SEM from n = 6 rats/group; “*” denotes significantly different from saline (P < 0.05).