We identified the ANP system as a novel pathway that regulates postprandial lipid oxidation. In particular, we showed that ANP attenuates the postprandial decline in lipid mobilization leading to increased FFAs, both before and after ingestion of a standardized fat-rich test meal. The increase in circulating FFAs was associated with increased lipid oxidation driving an increase in postprandial energy expenditure. ANP decreased blood pressure in the postprandial phase with minimal reflex-mediated tachycardia.
The depressor response to ANP in our study can be attributed to its well-established cardiovascular actions, for example through modulation of the renin-angiotensin system (22
). ANP decreased postprandial renin concentrations compared with placebo. Interestingly, the depressor response to ANP was more pronounced late into the postprandial phase. Possibly, ANP reduced the ability of the cardiovascular system to cope with postprandial vasodilatation in the splanchnic tract (23
). The mechanism could contribute to postprandial hypotension in the elderly (23
). Near complete adrenergic receptor blockade by propranolol does not abrogate the metabolic actions of ANP (5
), suggesting that ANP-mediated metabolic effects are not related to reflex-activated adrenergic stimulation. Vasodilatation per se has not conclusively been shown to affect lipolysis in the absence of release of a lipolytic factor or withdrawal of antilipolytic mechanisms (24
In accordance with previous studies (5
), augmented lipolysis with ANP resulted in a sustained increase in circulating FFAs throughout the experiment. The observation suggests that the increase in FFA release was not completely compensated by FFA re-esterification or lipid oxidation. Yet, compared with placebo, ANP induced an increase in postprandial lipid oxidation rate. Plasma ketone concentrations, which reflect hepatic lipid oxidation, increased sharply with ANP infusion. Circulating free carnitine concentrations have been reported to decrease with ANP infusion (26
). Carnitine is a critical factor for fatty acid intramitochondrial transport by carnitine palmityl transferase I and thus β-oxidation (27
The mechanisms increasing lipid oxidation rate with ANP are not fully understood. In human subjects, increased lipolysis can drive an increase in lipid oxidation rate. Systemic β-1 and β-2 adrenoreceptor agonist infusions increase lipolysis and lipid oxidation rate (9
). Lipolysis inhibition with acipimox reduces adrenoreceptor-mediated lipolysis and lipid oxidation. In the present study, FFA concentrations were inversely correlated with the respiratory quotient, suggesting that FFAs drive, at least in part, the ANP-mediated increase in lipid oxidation rates. The increase in FFAs may have led to secondary changes in skeletal muscle metabolism (28
). Increased postprandial insulin with unchanged systemic and muscular glucose is consistent with an ANP-induced state of insulin resistance favoring lipid rather than glucose utilization. Perhaps ANP also exerts a direct effect on fatty acid uptake and β-oxidation in peripheral tissues: ANP-induced lipolysis is mediated by cGMP formation. Recently, cGMP has been shown to increase mitochondrial biogenesis, ATP synthesis, and oxidative phosphorylation in cultured myotubes (29
). Chronic inhibition of cGMP hydrolysis with a phosphodiesterase-5 inhibitor reduced weight and fat mass through increased energy expenditure in high-fat–fed mice (30
Increased lipolysis with ANP increased circulating FFA levels throughout the experiment. Pancreatic β-cells are affected by FFAs depending on the duration of exposure. Acutely, FFAs together with an increase in glucose concentrations induce an exaggerated insulin response (31
). In accord with previous studies (6
), insulin concentrations increased with ANP infusion in the early postprandial phase. Insulin regulates lipolysis by modulating cAMP concentrations through type 3B phosphodiesterase (PDE-3B).
ANP-induced lipolysis is mediated by cGMP formation. Previously, radioligand binding assays using [123
I]-labeled ANP as the ligand showed high-affinity binding sites on human adipocytes (32
). ANP increased intracellular cGMP concentrations 187-fold, whereas cAMP concentrations remained unchanged. The increase in cGMP activates cGMP-dependent protein kinase G, which phosphorylates and activates hormone-sensitive lipase. In noncellular systems, cGMP inhibits PDE-3B (33
). However, pharmacological PDE-3B inhibition or PDE-3B stimulation through insulin did not affect ANP-induced lipolysis in isolated adipocytes (32
). In our study, the ANP-mediated lipolysis was attenuated early in the postprandial phase. In subcutaneous adipose tissue, lipolysis decreased sharply after food ingestion and increased again late into the study. Lipolysis inhibition coincided with the peak in circulating insulin concentrations. This pattern might suggest a discrepancy between in vitro and in vivo actions of ANP on metabolism.
ANP induced lipolysis in subcutaneous adipose tissue but not in skeletal muscle. The molecular mechanism for this difference is unknown. In skeletal muscle, only combined hyperinsulinemia and hyperglycemia suppress lipolytic activity (35
). Lipoprotein lipase activity, which hydrolyzes lipoproteins to FFAs and glycerol mainly in the epithelium of capillary beds, is reduced by insulin in skeletal muscle (36
). However, consistent with the present study, ANP did not increase glycerol concentrations in skeletal muscle under basal conditions (6
). This finding argues against a specific insulin effect on the ANP-mediated metabolic effect in skeletal muscle. The vastus lateralis muscle consists of mixed muscle fibers. Skeletal muscles show different degrees of lipolytic response to adrenergic stimuli according to their fiber type composition (37
). In contrast to adipose tissue, glycerol can be taken up and reused in muscle after triglyceride hydrolysis. Postprandially, the effect can account for up to 50% of the released glycerol (38
). Finally, increased blood flow could possibly wash out interstitial glycerol concentrations. We did not observe a change in local skeletal muscle blood flow with ANP. However, the ethanol ratio is a qualitative rather than quantitative method of determining blood flow and therefore not sensitive toward minor changes in blood flow (39
). We cannot rule out the possibility that we missed subtle changes in blood flow and lipolysis.
Together, our findings suggest that the natriuretic peptide system is an important regulator of postprandial metabolism. The system may be amenable to therapeutic intervention. For example, neutral endopeptidase inhibitors are in clinical development for the treatment of arterial hypertension and heart failure (40
). Neutral endopeptidase inhibition may promote lipid mobilization and oxidation. Our findings may also be important in terms of human pathophysiology, both in conditions associated with increased ANP and with decreased ANP availability. ANP availability is increased in heart failure. With induction of β-adrenoreceptor blocker therapy, natriuretic peptide release increases further (41
). Possibly, increased lipid mobilization through natriuretic peptides sustains substrate supply to the failing heart (42
). Cardiac cachexia is a complication of heart failure that worsens prognosis. The cause of cardiac cachexia is unknown. Our findings suggest that increased ANP-mediated lipid mobilization and oxidation could predispose to cardiac cachexia (43
ANP availability is decreased in obesity and states of insulin resistance presumably through upregulation of the natriuretic peptide-C clearance receptor (44
). BMI and circulating ANP and BNP concentrations are inversely correlated (46
). Reduced ANP availability may provide a pathophysiological link between obesity and arterial hypertension (44
). The novel findings presented here could have therapeutic implications for both cachexia and obesity.